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Recent Advances in Electrocatalysts for Oxygen Reduction Reaction

Shao, Minhua; Chang, Qiaowan; Dodelet, Jean-Pol; Chenitz, Regis

Chemical reviews , v. 116, (6), 23 March 2016, Pages 3594-3657

Pre-published version

https://doi.org/10.1021/acs.chemrev.5b00462

American Chemical Society (ACS)

© 2016 American Chemical Society

http://hdl.handle.net/1783.1/77094

Recent Advances in Electrocatalysts for Oxygen Reduction ReactionMinhua Shao,*,† Qiaowan Chang,† Jean-Pol Dodelet,‡ and Regis Chenitz‡

†Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Clear Water Bay,Kowloon, Hong Kong‡INRS-Energie, Materiaux et Telecommunications, 1650, boulevard Lionel Boulet, Varennes, Quebec J3X 1S2, Canada

ABSTRACT: The recent advances in electrocatalysis for oxygen reduction reaction(ORR) for proton exchange membrane fuel cells (PEMFCs) are thoroughly reviewed.This comprehensive Review focuses on the low- and non-platinum electrocatalystsincluding advanced platinum alloys, core−shell structures, palladium-based catalysts,metal oxides and chalcogenides, carbon-based non-noble metal catalysts, and metal-freecatalysts. The recent development of ORR electrocatalysts with novel structures andcompositions is highlighted. The understandings of the correlation between the activityand the shape, size, composition, and synthesis method are summarized. For the carbon-based materials, their performance and stability in fuel cells and comparisons with thoseof platinum are documented. The research directions as well as perspectives on thefurther development of more active and less expensive electrocatalysts are provided.

CONTENTS

1. Introduction A2. ORR Activity Screening Technique Based on

Rotating Disk Electrode B3. ORR on Pure Pt Surfaces E

3.1. Structure Effect on Bulk Single Crystals E3.2. Particle Size/Shape Effect E

4. Pt Alloys F4.1. Pt−Late Transition Metal Alloys F

4.1.1. Activity Enhancement Mechanisms andSurface Segregation F

4.1.2. Ternary and Quaternary Pt Alloys G4.1.3. Particle Size Effect G

4.2. Pt−Early Transition Metal Alloys G4.3. Ordered Pt Alloys H4.4. Pt Alloys with NSTF Structure H4.5. Porous Pt Alloys I4.6. Shape-Controlled Pt Alloys I

4.6.1. Synthesis I4.6.2. ORR Activity J

4.7. Nanowires, Nanorods, and Nanotubes L5. Core−Shell Structures L

5.1. Cu-Mediated Deposition L5.1.1. Core Material Effect L5.1.2. Core Structure Effect M5.1.3. Scale-Up and MEA Testing M

5.2. Chemical Reduction O5.3. Dealloying Q5.4. Other Methods S

6. Pd-Based Electrocatalysts S6.1. Structure Dependence S6.2. Pd Alloys T

7. Metal Oxides, Nitrides, Oxynitrides, and Carboni-trides U7.1. Metal Oxides U

7.2. Metal Nitrides and Oxynitrides V7.3. Metal Carbonitrides V

8. Metal Chalcogenides V8.1. Noble Metal-Based Chalcogenides V8.2. Non-noble Metal-Based Chalcogenides X

9. Carbon-Based Non-noble Metal and Metal-FreeCatalysts X9.1. Initial Performance in H2/O2 PEM Fuel Cells Y9.2. Synthesis of Non-noble Metal Catalysts AA9.3. Initial Performance in H2/Air PEM Fuel Cells AD9.4. Durability of Non-noble Metal Catalysts AF9.5. Origin of Activity Loss in Fuel Cells AL9.6. Comparison with Pt AN

10. Conclusions AOAuthor Information AP

Corresponding Author APNotes APBiographies AP

Acknowledgments AQReferences AQ

1. INTRODUCTION

Similar to batteries, fuel cells convert chemical energy of fueland oxidant into electric energy. Yet unlike batteries, they donot need recharging as long as fuel and oxidant are con-tinuously supplied. When hydrogen is fed as fuel, the fuel cellonly generates electricity, water, and some heat. As comparedto thermal engines, the advantages of fuel cells are highefficiency, no environmental pollution, and unlimited sourcesof reactants. Therefore, fuel cells are expected to come intowidespread commercial use in the areas of transportation,

Received: August 7, 2015

Review

pubs.acs.org/CR

© XXXX American Chemical Society A DOI: 10.1021/acs.chemrev.5b00462Chem. Rev. XXXX, XXX, XXX−XXX

This is the Pre-Published Version

stationary, and portable power generation, and thus will helpsolve the global problems of energy supply and clean environ-ment. Among all of the existing fuel cells, the proton exchangemembrane fuel cell (PEMFC) has been actively developed foruse in vehicles, portable electronics, and combined heat andpower (CHP) systems due to its simplicity, low working tem-perature, high power density, and quick start-up. PEMFCs areespecially well suited as the main power sources for automobilesand buses. Fuel cell vehicles (FCVs) have been considered asone of the final solutions for automotive business and haveprofound advantages over battery powered electric vehicles(EVs). Indeed, the first mass produced FCVs, the Toyota Mirai(“future” in Japanese), have been commercially sold in Japansince 2014 and are going to be available in North America in2015 at a price of ∼57 000 US dollars. One of the main reasonsfor the high sale price of the Mirai is the high Pt loading in thefuel cell stacks.At the anode of a PEMFC, hydrogen is oxidized to produce

electrons and protons that are transferred to the cathodethrough an external circuit and the proton exchange membrane,respectively (H2 → 2H+ + 2e−). At the cathode, oxygen isreduced by reaction with protons and electrons to producewater (1/2O2 + 2H+ + 2e− → H2O). Both the anode and thecathode electrodes consist of highly dispersed Pt-basednanoparticles on carbon black to promote the reaction ratesof the hydrogen oxidation reaction (HOR) and the oxygenreduction reaction (ORR). The reaction rate of HOR on Ptis extremely fast so that the Pt loading at the anode can bereduced to less than 0.05 mg cm−2. On the other hand, at thecathode, the sluggish reaction kinetics of ORR even on the bestPt-based catalyst requires much higher Pt loading (∼0.4 mg cm−2)to achieve a desirable fuel cell performance. Pt is a scarce andexpensive metal as shown in Figure 1.1 Therefore, reducing itsloading or even completely replacing it with an abundant andcheap metal would be advantageous.Recent intensive research efforts have led to the development

of less expensive and more abundant electrocatalysts forPEMFCs. These include advanced Pt alloys, core−shell struc-tures, transition metal oxides and chalcogenides, and carbon-based non-noble metal composite catalysts. Some of the progresshas been summarized in several reviews.2−11 This Review aimsto summarize recent advances in the past eight years of electro-catalysis in oxygen reduction in acidic media, with a particular

focus on low- and nonplatinum electrocatalysts including advancedPt alloys, core−shell structures, carbon-based non-noble metalcatalysts, Pd-based catalysts, metal oxides, and chalcogenides. Theprogress of catalyst supports has been reviewed thoroughlyrecently12,13 and will not be included here.

2. ORR ACTIVITY SCREENING TECHNIQUE BASED ONROTATING DISK ELECTRODE

Ideally, newly developed ORR catalysts should be evaluated in afuel cell environment and compared to the benchmark, forinstance, the state-of-the-art Pt/C. In most cases, this approachis impractical because the membrane electrode assembly(MEA) fabrication and test require special skills, equipment,and abundant materials. Fast screening techniques are moresuitable to characterize the electrochemical behaviors of newlydeveloped materials at the lab scale. Rotating disk electrode(RDE) with a porous catalyst layer has been the most widelyused technique to characterize the supported catalysts in liquidelectrolytes since it was proposed by Stonehart and Ross at theUnited Technologies Corp. (UTC) in 1976.14 The generalrecipe of fabricating the thin film-RDE commonly used todaywas developed by Gloaguen et al. in 1994.15 The history of thedevelopment of this technique was summarized by Schmidt andGasteiger.16 Catalyst powders are typically dispersed in a water/alcohol mixture forming a uniform ink, which is then depositedon glassy carbon electrodes to form catalyst films. To mitigatethe mass transfer effect during ORR activity measurements,glassy carbon electrodes are rotated to increase the mass transferrates of O2 at the electrode surface. The intrinsic activity (kineticcurrent without mass transfer effect) of the catalysts can bederived according to the Koutecky−Levich equation:

= + = + * ϖ−j j j j nFAC D v1 1 1 1 1

0.62k l,c k 0 02/3 1/6 1/2

(1)

where j, jk, and jl,c are the measured current density, kineticcurrent density, and diffusion-limited current density, respec-tively. The diffusion-limited current is determined by the numberof electrons transferred (n), the Faraday constant (F), the elec-trode’s geometric area (A), the concentration of dissolved O2 insolution (C0*), the diffusion coefficient of O2 (D0), the kineticviscosity of the solution (v), and the rotation speed of the electrode(ϖ). jk is generally extrapolated from the Koutecky−Levich

Figure 1. Price of the elements (in $/kg) versus their annual production (in kg/yr).1Reprinted with permission from ref 1. Copyright 2012 RoyalSociety of Chemistry.

Chemical Reviews Review

DOI: 10.1021/acs.chemrev.5b00462Chem. Rev. XXXX, XXX, XXX−XXX

B


plot (j−1 vs ω−1/2) at various ration speeds. Alternatively, it canalso be derived by using the measured diffusion-limited currentat a single rotation speed (typically 1600 rpm). In practice,catalyst films usually consist of Nafion ionomer as a binder tokeep the catalysts on the RDE when the electrode is rotated.Diluted Nafion solution can be added into the powder/solventmixture during the ink preparation step or dropped on thecatalyst film to form a thin Nafion layer. In either case, theNafion content should be as low as possible to minimize extradiffusion resistance of O2 and IR drop caused by the Nafionfilm.17,18 As a general rule, the Nafion film should not exceed0.2 μm when it is casted on the top of the catalyst layer, or thecontent of solid Nafion is less than 20 wt % in the catalyst filmwhen it is mixed in the ink. It is important to bear in mind thatthe Koutecky−Levich equation is based on smooth electrodesurfaces under laminar flow hydrodynamics.19,20 Therefore,the quality of a given catalyst film has a great impact on theaccuracy of kinetic current calculation in the RDE measure-ments. A good catalyst film is thin, uniform, and smooth. Thickfilms lead to increased mass-transport resistance through thefilm and incomplete utilization of the catalyst. For Pt-basedcatalysts, the thickness of the catalyst layer is mainly deter-mined by the amount of carbon black. For catalysts withdifferent Pt loadings, it was recommended that the weight ofcarbon black stayed constant on the RDE so that the thick-nesses of the catalyst films were roughly the same.21 It isobvious that irregularly built-up films (nonuniform coverage,very rough surface, etc.) must be avoided in RDE measure-ments because the Koutecky−Levich equation is not validanymore under these conditions.To obtain reproducible results in RDE measurements, the

first task is to prepare a well-dispersed ink. Because of differentsurface properties of carbon black (and other noncarbonsupports) and catalyst nanoparticles, it is difficult to design auniversal recipe for all catalysts. In addition to water, alcohol(typically isopropanol and ethanol) is usually added into the inkto help wet the carbon surfaces. The alcohol/water ratios arehighly dependent on the types of the carbons and catalysts, andpretreatment conditions (annealing, acid treatment, etc.), andshould be tuned for different catalysts. Even with a fine ink, auniform catalyst film is not always guaranteed. The dry condi-tions play a great role in the film quality. Recently, Garsanyet al.22,23 were able to reproducibly fabricate good qualitycatalyst films by rotating the glassy carbon electrode at 700 rpmwith the ink droplet on the top. The standard deviations in theelectrochemcial active area (ECA) and ORR measurementswere much smaller with catalyst films made by the rotationaldrying method, and the activities were also 70% higher thanthose measured with ununiformly covered films. Ke et al.24

also developed an “intermittently microcontact-coating fine-droplets” method to uniformly cover the electrode with morethan 3000 tiny droplets (3 nL per droplet) to overcome thereproducibility issue. Similarly to previous work, higher acti-vities were obtained with catalyst films prepared by the fine-droplets methods. Shinozaki et al.25 found that drying underthe isopropanol gas environment could also produce a high-quality film and in turn good activity in ORR measurement. Inthe same study, the Nafion ionomer that has been commonlyused as the binder in the catalyst film was confirmed tonegatively affect the measured activity. The contribution of theblock effect from Nation ionomer to the specific activity at0.9 V for Pt/C was about 0.15 mA cm−2

Pt.

In addition to the ink and film qualities, other importantfactors include the purity of the electrolyte, the potentialscanning rate, the flow rate of O2, and the position of thereference electrode.26 It is well-known that ORR of Pt-basedcatalysts is very sensitive to the anions in the electrolytes.27

Even the purest HClO4 available on the market has a tinyamount of Cl− in it, which will definitely impact the ORRpolarization curves. Shinozaki et al.26 found the specific activityof poly-Pt electrode measured in a regular ACS grade HClO4solution was 3 times lower than that measured in a high-purityVeritas Doubly Distilled (GFS) solution. The specific adsorptionof anions in turn is responsible for the scanning rate-dependentactivity in RDE measurements.26,28 Fast scanning rates result inhigher ORR activities due to fewer amounts of accumulations ofpoison species and oxide formation.29 A scanning rate of 10 or20 mV has been recommended.28 The background currentsincluding the capacitive currents and Faradaic currents associatedwith H adsorption/desorption and oxide formation/reductionprocesses are recommended to be subtracted from the ORRpolarization curves, especially for the high carbon loadings onthe electrode and scanning rates. The IR drop in the RDEmeasurement not only depends on the concentration of theelectrolytes and temperature, but the position of the referenceelectrode. The Ohmic resistance and distance of Luggin capillaryfrom the electrode surface follow Newman’s disk model.20

Fortunately, the resistance does not increase significantly from2 mm (28.1 Ω) to 20 mm (30.4 Ω) as measured in a 0.1 MHClO4 solution at room temperature.30 Therefore, a slightchange of the reference position does not affect the polarizationcurve much. The IR drop is proportional to the current, that is,larger at the higher currents close to the diffusion-limited current.The IR drop at half-wave potential for a 5 mm diameter RDE at1600 rpm (jl,c ≈ 1.2 mA) is 18 mV assuming a 30 Ω resis-tance, causing a considerable error in the calculation of kineticcurrent.30 Thus, it is strongly recommended to correct the IRdrops especially when the measured ORR curves are far awayfrom the benchmark curve. Taken together, it is important to listall of the measurement conditions and data analysis methodwhen reporting the activity data.Table 1 lists the ECAs and activities of two commonly used

benchmark Pt/C catalysts (50% Pt on Ketjen black fromTanaka Kikinzoku Kogyo (TKK), and 20% Pt on Vulcan fromE-TEK) measured by different groups with different dataprocessing approaches. Without background current and IRdrop corrections, the Pt mass and specific activities of 50%Pt/C at 0.9 V are around 0.22 A mg−1 and 0.25 mA cm−2,respectively. For 20% Pt/Vul, they are around 0.2 A mg−1 and0.35 mA cm−2. After corrections, these values increase ingeneral but need more data to confirm the absolute values.Finally, the ECA calculation is not as straightforward as

one thinks. Most of the ECA values in the literatures werecalculated from the hydrogen adsorption/desorption (HUPD)charges for Pt-based catalysts.31,32 In a standard practice ofHUPD charge calculation, a constant double layer current wasextrapolated into the hydrogen adsorption region. However, itwas pointed out by Mayrhofer et al.33and Binninger et al.34 thatthe double layer current was not a constant for the high surfacearea catalysts due to the influence from the high surface areasupports. The effect from the supports was particularly signi-ficant when they were metal oxides. Thus, the CV in thehydrogen region has to be corrected by the capacity from thesupport, which can be obtained by recording the CV in a COsaturated electrolyte. Some recent studies found that the areas



C


derived from the HUPD were underestimated especially for Ptalloys,35−37 and the estimations from CO stripping and Cuunderpotential deposion (UPD) charges were more accurate.37

In the case of Cu UPD, the small interference on the UPDcharges from the coadsorption of anions (SO4

2−, HSO4−, etc.)

can be compensated by conducting rotating ring disk electrode

(RRDE) experiments.38 The hydrogen adsorption was signi-ficantly suppressed in Pt alloys due to the alternated electronicproperties of Pt surfaces resulting in an underestimation ofECA by nearly 50% using HUPD. In addition to the alloyingeffect, the shape/structure of the nanoparticles also made thecalculation more complicated. The surface area could be

Figure 2. Cathodic current density of the ORR at 0.90 V against the step atom density in 0.1 M HClO4: surfaces with (111) terrace (A), surfaceswith (100) terrace (B). The value of n shows the number of terrace atomic rows. Possible active sites for the ORR on the surfaces with (111) terrace(C). Solid and broken rectangulars show the (111) terrace edge and the (111) terrace atomic row neighboring to the edge, respectively. Circles,triangles, and squares present the position of on-top, 3-fold, and bridged sites, respectively.52 Reprinted with permission from ref 52. Copyright 2013Elsevier.

Table 1. ECAs and Activities of Benchmark Pt/C Catalysts at 0.9 V Taken from the Literaturea

catalysts ECA (m2 g−1) MA (A mg−1) SA (mA cm−2) background correction IR drop compensation scanning rate (mV s−1) refs

∼50% Pt/C (TKK) 80 0.21b 0.26 N N 20 2896 0.22 0.23 N N 10 3972 0.23 0.31 N N 10 4085 0.20 0.24 N N 10 4172 0.06 0.08 N N 5 4279 0.40 0.51 Y N 10 2478 0.28 0.36 Y N 10 43

105c 0.27 0.29 Y N 10 4480d 0.43 0.54 Y Y 20 2391 0.51 0.60 Y Y 25 45101 0.87 0.86 Y Y 20 25

∼20% Pt/Val (E-TEK) 61d 0.31 0.51 Y N 20 4672 0.19 0.26 N N 20 28

aAssuming a charge density of 210 μC cm−2 for H adsorption, in 0.1 M HClO4 solution, room temperature, and 1600 rpm unless otherwisementioned. bAt 60 °C. cCharge density was not disclosed. dAssuming 200 μC cm−2.



D


underestimated by more than 2 times using HUPD on octahedralshape Pt alloy nanocrystals.37 Thus, caution has to be takenin calculating the ECAs of noble metal-based catalysts withvarious compositions and structures.

3. ORR ON PURE Pt SURFACES

3.1. Structure Effect on Bulk Single Crystals

The ORR behaviors of low index Pt surfaces, that is, Pt(111),Pt(100), and Pt(110), have been extensively studied. Theresults were summarized in several comprehensive reviews27,47

and are not discussed here. The principal conclusion is that theactivity of ORR in a weakly adsorbed electrolyte, such asHClO4 solution, follows the order of Pt(100) ≪ Pt(111) ≈Pt(110). Feliu et al.48−51 and Hoshi et al.52 systematicallystudied the structural effects of high index Pt surfaces andconcluded that the ORR activities were highly dependent onthe orientation of the steps and terraces on the surfaces. It wasfound that the activity increased with the increase of terracedensity (or the decrease of terrace width) on high indexplanes n(hkl)-(mno) except for the n(110)-(111) surfaces(n represents the number of terrace atomic rows, (hkl) and(mno) present the structures of the terrace and step,respectively).48−51 In a later study, Hoshi et al.52 confirmedthis general trend on (111) terrace except for the surface withthe number of the terrace atomic rows n = 2, as shown inFigure 2A. In contrast, the ORR activity does not depend onthe step density on (100) terrace as shown in Figure 2B. Theactive sites on high index surfaces with (111) terrace wereproposed to locate at the (111) terrace edge and its neigh-boring terrace row, as illustrated in Figure 2C.52 The mechanismof higher ORR activity on high index planes, however, has notbeen well understood.53 These results may be importantguidelines in the development of more active pure Pt catalysts,which should possess a stepped surface instead of a smooth(111) surface because Pt(111) is less active than its vicinalstepped surfaces. According to Figure 2A, Pt nanocrystals withhigh index facets such as (221) = 4(111)-(111) and (331) =3(111)-(111) are expected to exhibit activities over 3 times higherthan (111). Attempts to synthesize such high index facets Ptnanocrystals have been taken.54−58 The stability of these speciallydesigned structures during potential cycling is a big concern.3.2. Particle Size/Shape Effect

Inspired from the ORR activity trend obtained on Pt singlecrystals, that is, high index planes > (111) > (100), efforts havebeen taken to synthesize shape-controlled nanostructures tooptimize the structure effect. Octahedral Pt nanoparticlesbound by (111) facets were found to be more active in ORRthan cubic ones bound by (100) facets, consistent with the bulksingle crystal work.35 Nanoparticles with high-index planesincluding tetrahexahedron (hk0), trapexezohedron (hkk), andtrisoctahedron (hhk) with at least one Miller index being largerthan unity have demonstrated higher activity than (111) or(100).48,49,59−61 This activity enhancement was assigned to thehigh density of low-coordinated atoms situated on steps, ledges,and kinks.51,62 The main issue with the shape-controlled Ptnanoparticles is their stability under ORR condition as theytend to evolve to thermodynamically equilibrated shape.63

The Pt particle size effect on ORR has been a long-standingproblem that has yet to be solved.63−89 The structural depen-dence activity observed on bulk Pt single crystals has been usedto predicate the particle size and shape effects on the ORR.90

As the particle size changes from 5 to 1 nm, the distributions of

(111) and (100) terrace sites decrease dramatically, while thelow coordination number edges and kinks become predom-inant sites in the surface. Because of the much stronger oxygenbinding energies on the later, the ORR activity is expected to belower than that of large particles. As a result, the specific activityof Pt nanoparticles decreases sharply when they are smallerthan 3 nm, as observed by many groups, and a typical result isshown in Figure 3A. In an attempt to minimize the effects from

size distribution of different samples and errors in RDEmeasurements, the activity of Pt particles ranging from 1 to 5nm was measured using one Pt/C thin film electrode withdifferent sizes being synthesized by layer-by-layer growth usinga Cu-UPD-Pt-replacement method.41 The maximum massactivity was observed around 2.2 nm (Figure 3A). Similar con-clusions have been drawn on the basis of density functionaltheoretical calculations.91−94

On the other hand, other researchers argued that the specificactivity does not depend on the particle size, even in the range

Figure 3. (A) Specific (blue ◆) and mass (red ■) activities as afunction of Pt particle size in a 0.1 M HClO4 solution at 0.93 Vwithout background or IR correction. Scanning rate = 10 mV s−1. Theparticle size was controlled by a Cu-UPD-Pt-replacement method.41

The specific (blue ◇) and mass (red □) activities of state-of-the-artPt/C from TKK (TEC10E50E, 46.7 wt %) with an average particlesize of 2.5 nm were also included for comparison. (B) Specific andmass activities as a function of electrochemical active area (ECA) indifferent electrolytes measured at room temperature at 0.9 V. Activitieswere analyzed from the IR compensated positive-going sweeps at50 mV s−1, after subtraction of the capacitive background.95 Reprintedwith permission from ref 41. Copyright 2011 American ChemicalSociety. Reprinted with permission from ref 95. Copyright 2011American Chemical Society.



E


of 1−5 nm. Nesselberger et al.95 found that the difference inspecific activity between carbon supported Pt particles withvarious size between 1 and 5 nm was very small, and the massactivity increased linearly with increasing catalyst dispersionregardless of the electrolytes used, as shown in Figure 3B. Thistrend certainly cannot be explained by the ideal single crystalfacet distribution model. A possible reason can be the change ofthe effective reaction pathway of the ORR with the particle size.Because of the increase of oxygen binding energy on the surfaceof the smaller Pt particles, the rate-determining step maychange from the first proton and electron transfer on largeparticles to O−O bond breaking on particles smaller than3 nm.93,95 Other studies implied that the specific activity doesnot depend on the particle size but on the interparticledistance.78−81,96,97 Nesselberger et al.96 prepared and loadedsmall Pt clusters (0.6, 0.8, and 2.3 nm) on glassy carbonelectrodes using a mass-selection technique. The interparticledistance was well controlled by adjusting the coverage of Ptclusters on the electrode. It was found that the ORR activitiesof these well-defined Pt clusters increased with decreasingthe interparticle distance. Decreasing the interparticle distanceoverlapped the electric double layers between neighboringparticles, leading to a potential drop within the compact layerand consequently weaker adsorption energy on Pt surfaces. Thisparticle proximity effect has been also observed on Pt particles(∼2 nm) supported on high surface area carbon with various Ptloadings.97 Fabbri et al.98 confirmed that the dispersion of Ptnanoparticles on carbon support significantly affected the furtherreduction of H2O2. The H2O2 yield increased dramatically asthe interparticle distance increased from extended layer to well-dispersed particles.Pt nanoclusters (less than 1 nm in diameter) that are not

normally used in fuel cells have shown some interesting activitytrend.88,96,99−101 For instance, Pt12 (∼0.9 nm) clusters wereover 10 times more active than that of 2.5 nm Pt/C (TKK).88

By just adding one Pt atom to form Pt13, the ORR activitydecreased by more than 2 times.99 This significant activity losswas attributed to the structural evolution from C2v Pt12 toicosahedral Pt13, which had a much stronger oxygen bindingenergy than that of the former. By adding more Pt atoms to theicosahedral Pt13 core, the resulting Pt17 and Pt19 with edgestructure showed higher activity than Pt13.

101 This result furtherconfirmed that the stable icosahedral Pt13 structure was notactive toward ORR.Because of a lack of image and probe techniques to directly

identify the arrangement of surface atoms of a Pt particle, it isalmost impossible to establish a real relationship between theORR activity and the shape/size of nanoparticles. Even on thesurface of a well-defined nanostructure, an octahedron, forexample, TEM images reveal that there are numerous defects/steps instead of smooth terraces.63,102 Angelopoulos et al.91

tried to correlate the surface active sites with the ORR activityon Pt nanoparticles in the size range of 1.8−6.9 nm using theBi and Ge specific adsorption technique. It was concludedthat the predominant active sites were (110) and (311) ratherthan (111) terraces atoms that were historically believedto be the main active sites for ORR. The role of the steppedsurface atoms in a Pt nanoparticle needs to be further investi-gated to fully understand the size and shape effects on theORR.

4. Pt ALLOYS

4.1. Pt−Late Transition Metal Alloys

Since the discovery of Pt alloys as superior ORR catalysts forfuel cells at UTC in the 1980s,103−105 they have attracted greatattention and been considered as the second generation fuelcell catalysts after pure Pt.28,106−111 Indeed, they have beenused in the UTC’s stationary PAFCs and the Toyota MiraiFCVs due to their higher activity and durability than pure Pt.The membrane and ionomer contamination caused by thetransition metals dissolved during fuel cell operation delayedtheir applications in PEMFCs in early years. With proper post-treatment (acid washing, for example) of Pt alloys, their betteractivity and durability than Pt/C in PEM fuel cells have beenconfirmed.28

4.1.1. Activity Enhancement Mechanisms and SurfaceSegregation. Various reasons have been proposed to explainthe higher ORR activity of Pt alloys. They include compressivestrains due to shorter Pt−Pt bond distances,112−114 highersurface roughness caused by the transition metal dissolution,115

downshifting the d-band center of Pt116 or changing the d-bandvacancy117 due to strain and ligand effects, delayed formationof oxide species,33,118 etc. It has been generally agreed that aPt-skin like surface is formed during the initial acid treatmentand potential cycling. The structural and electronic effects oftransition metals in the core and subsurfaces play a significantrole in weakening the adsorption of oxygen containing species.The lower coverage of these oxygen containing species hasbeen thought to be beneficial to enhance the ORR activitybecause they poison the active sites.This argument has been challenged recently by a few

studies.119−121 Using the electrochemical quartz crystal micro-balance (EQCM), Omura et al.119 found that the coverage ofoxygen containing species on Pt-skin/Pt3Co thin film washigher than that on pure Pt in the potential range of 0.86−0.96 V(RHE). This observation opposed the conventional model ofdelayed formation of oxides on Pt alloys as the primary reasonfor the ORR activity enhancement. The higher oxide coverageand faster oxide growth rate on PtCo than Pt were also observedby Huang et al. in their potential-hold measurements (both RDEand MEA).120 These results implied that other mechanisms playa role in determining the activity of Pt surface besides the oxidecoverage.The activity enhancement of Pt alloys originates from the

transition metals. Thus, the type and amount of the transitionmetals certainly have pronounced effects on the ORR activity,which was summarized by Wang et al. recently.5 PtM alloysconsisting of different metals M (M = Co,122−137

Ni , 123 , 133 , 138−142 , 115 , 128 , 133 , 136 , 137 , 143−146Fe ,147−150

Cu,123,133,145,151−159 Ag,160−163 Au,161,164−169 Pd,161,170−174

Cr,175,176 Mo,177 Mn,178 Al,179 etc.) have been synthesized,and their activities have been compared. Among them, Co, Ni,and Fe have been studied more intensively due to their superioractivities. Stamenkovic et al.116found that the activities of theas-sputtered polycrystalline films followed the order of Pt <Pt3Ti < Pt3V < Pt3Ni < Pt3Fe ≈ Pt3Co. A recent study of Hanet al. indicated that the ORR activity and stability of Pt alloyscorrelated with the dissolution potentials of the alloyingelements.180 A low dissolution potential of a transition metalresulted in a high ORR activity but low chemical stability.During electrochemical measurements, the surfaces of Pt alloysbecame pure Pt due to dissolution of non-noble metals (notedas Pt-skeleton). After a mild thermal annealing at 1000 K,



F


Pt atoms segregated to the surface, while non-noble metalatoms moved to the sublayers resulting in a Pt-skin type sur-face. The Pt specific activity trends of the Pt-skin and -skeletonwere slightly different with the former following the order ofPt < Pt3Ti < Pt3V < Pt3Fe < Pt3Ni < Pt3Co. The specific activityenhancement factor of annealed Pt3Co was ∼5, while it was3 for the nonannealed one. This result emphasized the impor-tance of the post treatment of Pt alloys in tuning the ORR activityby engineering the surface structures of Pt alloys.28,169,181,182

The Pt atom segregation was also observed on Pt3Conanoparticles (5 nm) by annealing the acid washed samples at727 °C.183,184 The Pt-rich surface and Co-rich layer beneath itwere confirmed by scanning transmission electron microscopy(STEM). The reduction of the Pt−Pt bond distance among thePt surface atoms might be the main reason for the enhancedORR activity. During thermal annealing, the particle size of Ptalloys and the properties of carbon support might be changed,which could have a negative impact on the fuel cell perfor-mance. Mayrhofer et al.185 developed a CO annealing methodeither in a gas or in a liquid phase. By annealing the Pt3Co/C inCO stream at 200 °C, or electrochemical cycling in a COsaturated alkaline solution, Pt atoms could segregate to the toplayer due to a higher adsorption enthalpy of CO on Pt than onCo. During the CO annealing in the liquid cell, there was noCo dissolution into the electrolyte. After CO annealing, thespecific activity was increased by ∼50% due to the surfacereconstruction. Ciapina et al. found that CO-induced surfacesegregation could also occur in acid solution.181 A similar Ptsegregation phenomenon was observed in Pt3Au

186 andPtAu169 nanoparticles. Recently, the post-treatment of Pt−NiNPs has been studied intensively.187−190 In addition to thesurface segregation, thermal annealing can also induce thereorientation of the surface. Chung et al. found that the ratioof (111) facet to all surface atoms was 35% after annealing in amixture of H2 and Ar at 700 °C, while this value was only 25%at 300 °C.189 The Pt- and (111)-enriched surface of theformer resulted in a 2-fold activity enhancement over thelatter. The composition of the subsurface of the alloy alsohas a significant impact on the ORR activity. With the assis-tance of theoretical calculations, Gao and Muller discoveredthat Pt3Ni(111) with the first three layers and the fourth layerbeing pure Pt and Ni, respectively, had the highest activity.The ORR activity of the structure with the first and secondlayers being pure Pt and Ni, respectively, was 3 orders ofmagnitude lower.191

4.1.2. Ternary and Quaternary Pt Alloys. Besidesbimetallic alloys, ternary,151,178,192−222 quaternary,178 andquinary223 alloys also have been studied in recent years.Various approaches, such as combinatorial high throughputscreening,224 DFT calculations,196 and facile synthesis strat-egy,178,225 have been applied to optimize compositions ofalloys. The combinations of Pt−MN (M, N = Fe, Co, Ni, Ti, V,Sn, Cr, Mn, Mo, Ag, Au, Pd, Ir) have been synthesized andevaluated by different groups. Because of the possiblesynergetic effects, the activity and stability of the ternary alloysmight be higher than the corresponding binary ones.133,225,226

4.1.3. Particle Size Effect. The particle size effect of Ptalloys on ORR activity is even more complicated than that ofpure Pt. In addition to effects purely from size, other param-eters like composition, degree of alloying, annealing tempera-ture, and shape also play roles in determining the activity.227−229

It is difficult to make a meaningful conclusion if other parameters(annealing temperature, composition, etc.) are also changed in

the same time besides particle size. Wang and co-workers227

synthesized Pt3Co nanoparticles with different sizes from3 to 9 nm via an organic solvothermal method with synthesistemperatures between 225 and 145 °C. No further thermalannealing was conducted except for removing surfactants at185 °C in an oxygen environment. The specific activity of Pt3Coincreased with increasing particle size as smaller nanoparticleswere oxidized at a lower potential, which led to a strongeradsorption of oxygenated species and thus a lower ORR activity.The size-dependent mass activities presented volcano-shapebehavior, and the maximum mass activity was found around4.5 nm due to the two opposite trends in specific surface areaand specific activity with particle size (Figure 4). On the contrast,

Loukrakpam et al.230 found that the mass activity of Pt3Co/Cdecreased with particle size increasing from 3 to 8 nm. In theirstudy, the nanoparticles were synthesized via a similarsolvothermal method but annealed in 7% H2 at 400 °C. Thetrend, however, was not observed on Pt3Ni/C, the mass activityof which increased gradually up to 8 nm.Several studies focused on the size effect of Pt alloys after a

heat treatment.200,228,231,232 With an increase in annealing tem-perature, the particle size and the degree of alloying increase.Wang et al.228 annealed the Pt3Co/C with an initial particle sizeof 4.5 nm from 300 to 800 °C. The sintering did not occur untilthe temperature was 500 °C or higher, while the specific activitycontinuously increased with annealing temperature. It wasbelieved that the activity enhancement below 500 °C wascaused by surface smoothing, removing surface defects, and Ptsegregation. Wanjala and co-workers200 studied the thermaltreatment results of PtCoNi with the temperature range from400 to 926 °C. The particle size did not change dramaticallybetween 400 and 800 °C (within 1 nm), while the latticeconstant decreased by 2%. Both the specific and the massactivities of the catalysts increased with temperature. Theenhanced activity toward ORR could be ascribed to the latticeshrinkage.4.2. Pt−Early Transition Metal Alloys

By alloying with some early transition metals or rare earths,such as Y,233−238 Sc,235,239 Hf,235 La,240−244 Ce,241,243,245 Ga,246

and Gd,247 the activity and stability of Pt can be significantlyimproved. In some studies, their activities were even higherthan that of Pt late-transition metal alloys (Pt−Co, Pt−Ni,

Figure 4. Specific and mass activities of Pt3Co/C at 0.9 V withdifferent particle sizes measured in 0.1 M HClO4 solutions at ascanning rate of 20 mV s−1 and rotation speed of 1600 rpm.227

Reprinted with permission from ref 227. Copyright 2009 AmericanChemical Society.



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Pt−Fe, etc.). Sung and co-workers117 found that the activitiesof Pt3M (M = Y, Zr, Ti, Ni, and Co) films followed the order ofPt3Ti < Pt < Pt3Zr < Pt3Co < Pt3Ni < Pt3Y. In another studycarried out by the same group, Pt3La was also found to show ahigh activity.113,242 Malacrida et al.,241 however, demonstratedthat the activity of Pt3La was only comparable to that of Pt.Instead, Pt5La, Pt5Ce, Pt5Gd, and Pt3Y had much higheractivity than that of Pt3La, as shown in Figure 5. The activity

enhancement of Pt5La and Pt5Ce over polycrystalline Pt isabout 3.5,241 while for Pt5Gd and Pt3Y, the enhancement iseven higher, about 5.247,248 A thick Pt overlayer (∼3 mono-layers) was formed on the Pt5M by removing La, Ce, and Gd intop layers. This thick Pt overlayer played a significant role inthe enhanced activity and stability by protecting active ele-ments, while La continued dissolving from Pt3La withoutforming a stable core−shell structure.241DFT work suggested that the enhanced activity of Pt−Y or

La was due to the ligand effect from the early transition metalsin the sublayers.248,249 The pure Pt overlayer is ∼1 nm thick,235

which excludes the possibility that the enhanced activity is fromligand effect.250 Recently, Stephens et al. proposed that thecompressive strain effect might be the reason for the superioractivity in Pt−Y, −Ce, and −Gd.235,241 It was found thatdespite a larger atomic radius of Y than Pt, Pt−Y alloys havesmaller Pt−Pt bond distances than pure Pt, resulting incompressive strains. The favorable negative heats of formationof Pt−early transition metal alloys lead to energy barriers in thediffusion of transition metals from the core to the surfaces ofcatalysts and improve their stability under fuel cell operationconditions.4.3. Ordered Pt Alloys

The study of Pt intermetallic (ordered) alloys started in thelate 1980s and developed slowly in the 1990s for the lack ofresearch interest due to the poor performance of orderedPtCr251 and PtCo252,253 in both PAFC and PEMFC. Recently,Pt intermetallic alloys have been revisited and argued to bebetter electrocatalysts than their corresponding disordered onesfor ORR, in the aspect of both activity and stability.254 Manyordered compositions, including PtCo,255−257 PtFe,258−262

PtNi,263,264 PtCu,265 PtAl,266 PtZn,267,268 PtFeCo,269,270

PtFeCo,271 PtIrCo,272 and PtAuCu,273 have been synthesizedsuccessfully and evaluated for ORR. The ordered structureswere usually achieved during high temperature annealing. Forinstance, PtFe intermetallic alloys could be formed at high

temperature with excess NaCl,258 SiO2,261 or MgO coating260

annealed in an inert262 or reducing259 atmosphere. It was foundthat Pt3Fe is more stable than PtFe. Wang et al.255 obtained theordered Pt3Co/C by annealing the mixture of Pt/C with Cosalts in a H2/N2 mixed atmosphere at 700 °C. The activity andstability of ordered Pt3Co/C were found to be higher than thatof disordered Pt3Co/C prepared at 400 °C. They believed thatthat the unique structure consisting of a Pt-rich shell and astable intermetallic Pt3Co core were responsible for its superiorperformance. Even starting with a transition metal enrichedcomposition, the ordered structure still can be maintained usinga proper post-treatment method. Wang et al.265 found that theordered Cu3Pt structure was maintained as the core, and a Ptthin layer formed as Cu atoms leached from the near surfacelayers during potential cycling. This unique core−shell struc-ture had higher activity that that prepared by acid washingduring which the ordered structure was completed destroyed.Theoretical studies have been carried out to explain thedifferent electrochemical properties between ordered anddisordered Pt alloys.266,270 The results suggested that activityenhancement might be due to the stronger Pt−metal covalentbond and more negative formation heat of ordered Pt alloys ascompared to that of the disordered one. The claim of higheractivity with an ordered structure is not conclusive because theabsolute activities of ordered Pt alloys reported so far were notthat high and the portion of ordered structure based on theXRD data was unknown. A systematic comparison betweenfully ordered and disordered Pt alloys with the same com-position, particle size, morphology, and annealing temperaturehas to be conducted.

4.4. Pt Alloys with NSTF Structure

Another unique category of Pt alloys is based on 3M’snanostructured thin film (NSTF). Pt alloys were sputtered onnonconductive polymer (N,N-di(3,5-xylyl)perylene-3,4:9,10bis(dicarboximide)) whiskers forming a continuous catalystfilm. Such core−shell structure catalysts eliminate carboncorrosion issues and contact resistance between carbon andcatalyst that would lead to poor utilization and degradation ofthe catalyst layer. More importantly, the unique thin filmstructure significantly reduces the population of the lowcoordination number atoms and hence increases the specificactivity. It was found that the fully formed Pt-based whiskerettefilms consist of pyramid-like pillars with a cross-section of∼6 nm. The surface of the catalyst film was dominated by the(111) facets with four extended {111} side facets truncated by asingle {100} facet.274 This structure has been demonstratedto have higher chemical and electrochemical stability, and5−10-fold higher specific activity of ORR as compared to thatof carbon supported highly dispersed nanoparticles.275,276

Various Pt−M (M = transition metals) including binary andternary compositions have been prepared and tested for ORR.For instance, Pt−Co,4,277 Pt−Ni,277−281 Pt−Ir,282 Pt−Co−Ni,Pt−Co−Mn,197 Pt−Co−Fe,277 etc., all showed higher specificactivity than conventional catalysts. The compositions andratios of Pt to the transition metals played a significant role inthe ORR activity. Pt3Ni7 with a lattice constant of 0.371 nmshowed the highest activity. The severe Ni dissolution in acid,however, led to an increased flooding of the NSTF cathode dueto an excess of Ni cations in the membrane enhancing thenet water transport across the membrane from anode tocathode.277 Post-treatment such as acid washing may minimizethe negative effect from the Ni cations. Another approach is to

Figure 5. Specific activities of Pt−early transition metal alloy thin filmsas compared to polycrystalline Pt at 0.9 V.113 Reprinted withpermission from ref 113. Copyright 2012 Royal Society of Chemistry.



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form a Pt-skin type surface with low Ni content. Van der Vlietet al.279 annealed the Pt3Ni NSTF film at relatively low tem-perature. At 300 °C, the organic whiskers started to evaporateand were completely removed at 400 °C. Meanwhile, the low-coordinated sites were diminished with (111) facets prevailedon the surface, as shown in Figure 6A−C. This mesostructured

thin film was 20 times more active than Pt/C (Figure 6D) andshowed better durability. The improved activity and durabilitywere assigned to the high percentage of (111) domains andstable Pt-skin surface.Despite their unique advantages, the NSTF thin film

structures have severe issues such as flooding and poor protonconductivity in the electrode due to low Pt surface area andpore volume, they are extremely thin, and they have an ionomerfree electrode.283−285 Several approaches including coating anionomer and silica nanoparticles on the NSTF surface, andintroducing an additional carbon or Pt/C layer adjacent to theNSTF layer, have been proposed to improve the protonconductivity, water removal, and storage capabilities.286−288

4.5. Porous Pt Alloys

As compared to the solid particles, catalysts with nanoporousstructures offer more reaction sites and enhance activity viathe so-called nanoconfinement effect.289 One of the mostefficient ways to prepare porous Pt alloys is dealloying. In thisprocess, the less noble metals are selectively dissolved from analloy by either a chemical or an electrochemical method.290−294

The dissolution of the less noble atoms on the surface of thealloy generates defects and vacancies, which result in reductionof coordinated numbers of noble atoms. As a result, themobility of the noble atoms increases. The morphology of thedealloyed material depends on the competition between thedissolution rate of less noble metals and the surface diffusion ofnoble metals. In bulk alloys, porous structures are generallyformed due to the slow diffusion rate of noble atoms on thesurface. For Pt-based alloy nanoparticles, the morphology of thefinal dealloying products strongly correlates with the particlesize. For instance, Strasser et al. found a critical particle size of15 nm for PtNi3, below which the particles tend to form anonporous (core−shell) structure, while particles larger than15 nm showed a porous structure.132,295,296 A similar observation

was reported by Oezaslan et al., although the critical particle sizewas slightly different due to the differences in composition andsynthesis method.297

Synder et al. found that the ORR activity of nanoporousPt−Ni particles (15 nm) formed by dealloying was slightlyhigher than that of solid ones.145 When these porous particleswere encapsulated with a hydrophobic protic ionic liquid, theirORR activities were further enhanced by 30 mV as measured inboth aqueous HClO4 solutions (Figure 7) and fuel cells.298,299

The high solubility of O2 in the ionic liquid and the confinedenvironment of the porous structures were proposed to be themain reasons for the activity enhancement. Chen et al.300

created Pt3Ni nanoframes by dispersing solid PtNi3 polyhedralparticles with an average size of 20 nm in hexane or chloroformunder ambient conditions for 2 weeks. The transformationprocess was illustrated in Figure 8. The free-standing nanoframeswere then supported on carbon black and further heat-treatedat 400 °C to obtain a Pt-skin surface. The hollow structureconsisting of 24 edges with width of ∼2 nm was maintained afterthe heat treatment. The nanoframes showed 22- and 16-foldenhancement over 5 nm Pt/C on mass and specific activities,respectively. The positive effect of ionic liquid in ORR on porouscatalysts was also confirmed in this work. After ionic liquidtreatment, the enhancement factors increased to 36 and 22 formass and specific activities, respectively.Dealloying Pt alloy thin films could result in hierarchical

porous structures that also showed high ORR activity.301−304

Galvanic displacement of non-noble metal particles (Co, Ni,etc.) with Pt has also been explored to fabricate porous Ptalloys.305−307 The best porous Pt−Ni/C showed a 6-foldenhancement in Pt mass activity over Pt/C and excellentdurability in potential cycling.307

4.6. Shape-Controlled Pt Alloys

4.6.1. Synthesis. It has been known that the activityenhancement on Pt alloys depends on their crystallineorientations. For example, the Pt3Ni(111) is much more activethan Pt3Ni(100) and (110).35 The surfaces of conventional Ptalloy particles consist of mixed facets, edges, corners, and otherdefects that result in low activities. To maximize the structuraleffect of the Pt alloys, one may want to synthesize shape-controlled

Figure 6. Schematic illustration and corresponding HRTEM images ofthe mesoscale ordering during annealing and formation of themesostructured thin film started from the as-deposited Pt−Ni onwhiskers (A), annealed at 300 °C (B), and 400 °C (C). Specificactivities of Pt−Ni NSTF as compared to polycrystalline Pt and Pt-NSTF at 0.9 V (D).279 Reprinted with permission from ref 279.Copyright 2012 Nature Publishing Group.

Figure 7. Comparison of oxygen reduction curves of Pt/C, dealloyedPt−Ni (np-NiPt/C), and ionic liquid encapsulated dealloyed Pt−Ni(nm-NiPt/C + IL) in O2-saturated 0.1 M HClO4 solution at 60 °C,scanning rate = 20 mV s−1, rotation speed = 1600 rpm. The inset is thecartoon illustrating the ionic liquid encapsulated porous nano-particles.298 Reprinted with permission from ref 298. Copyright2013 Wiley-VCH.



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Pt alloy nanocrystals (NCs) with only {111} facets exposed.The Sun group at Brown University first reported the synthesisof cubic PtFe NCs in 2006.308 Following this pioneering work,other Pt−M (M = Mn, Co, Ni, Pd) bimetallic alloy NCs weresynthesized.141,146,190,309−319 The synthesis of octahedral Pt−MNCs with particle size in the range of 4−15 nm has beenreported by several groups with or without the assistance of COcontaining chemicals. In synthesis protocols without COcontaining chemicals, Pt(acac)2 and Ni(acac)2 mixed withdimethylfomamide (DMF) in a sealed autoclave were heated toa certain temperature (typically between 120 and 200 °C) toproduce sub-10 nm octahedral Pt−Ni.141,314,315 It was foundthat the precursor ligands played a critical role in controllingthe size and shape of Pt−Ni NCs.141 The formation of Pt−Nioctahedra was not observed by mixing K2PtCl6 with Ni acetate,suggesting that acetyl acetonate may modify the nucleationand particle growth kinetics by specific interactions with thenucleation seeds and facets. The detailed mechanisms of theformation of octahedral Pt−Ni NCs, however, have not beenstudied. Wu et al.314 improved the shape selectivity of octahe-dral Pt−Ni NCs by adding capping agents poly(vinylpyrrolidone)(PVP) and benzoic acid in the reaction solution, and replacingDMF with benzyl alcohol as the solvent. The complete removalof remaining PVP adsorbed on the surface might be difficult.Huang et al. recently reported a synthesis of ∼4 nm Pt−Nioctahedra using only Pt(acac)2, Ni(acac)2, DMF, and benzoicacid.320

In the synthesis involving CO containing chemicals, themixture of Pt(acac)2, Ni(acac)2, oleylamine (OAm), and oleicacid (OA) in the presence of W(CO)6 was heated to 230 °C,resulting in 12 nm octahedral Pt−Ni NCs.309 The residualOAm and OA that served as capping agents had to be removedby Ar plasma or acid washing treatments. By introducing aproper solvent (benzyl ether) in the reaction mixture, Choiet al.317 achieved a high yield of octahedral Pt−Ni NCs withnegligible capping agents on the surface. The importance of thesynergistic combination effects of OAm, OA, and W(CO)6 hasbeen recognized by different groups, but their individual role inthe shape selective synthesis of Pt bimetallic NCs is still underdebate. The OAm may act not only as a surface stabilizerslowing the NCs growth and preventing them fromagglomeration, but also as a coordination ligand with Pt ions

resulting in a reduced nucleation rate of Pt at the early stage ofthe synthesis. Fang and co-workers proposed that the yieldedW0 from the thermal-decomposition of W(CO)6 could helpreduce Pt ions, resulting in a rapid nucleation of Pt clusters inthe early stage of the synthesis.309 On the other hand, the workfrom the Yang group suggested that the CO gas released fromthe W(CO)6 may play a significant role in shape control of NCsby preferentially binding to certain Pt facets.146 New synthesisprotocols based on the use of CO gas were subsequentlydeveloped to make shape controlled Pt alloy NCs includingcubes, (truncated-)octahedral, and icosahedra.146 The effect ofCO gas was further confirmed by a recent study by Choi et al.,who demonstrated that only an irregular shape of Pt−Ni nano-particles was formed when CO gas generated from W(CO)6was diluted with Ar.317 It was thought that CO preferentiallyadsorbed on the {100} facets of Pt, resulting in the formation ofcubic Pt NCs. The introduction of Ni(acac)2 may alter theadsorption preference of CO from {100} facets of Pt to {111}facets of Pt−Ni. Zhang et al.318 developed a simple impreg-nation method to obtain octahedral Pt−Ni alloys by heating thedried metal precursors and carbon black mixture in a CO/H2environment at 200 °C. The critical role of CO in determiningthe shape of Pt alloys was again demonstrated. Further studiesare still necessary to clarify the mechanism of CO adsorption inthe development and growth of a particular shape. One ofthe common properties shared by octahedral Pt−M NCssynthesized by different methods is their large crystallite sizewith edge length ranging from 9 to 15 nm depending on themethods, precursors, and additives, which is 2−3 times largerthan the commercially available conventional Pt alloy catalysts(4−5 nm). As a result, more than 80% of Pt atoms are wastedinside of the particles. One strategy to further improve the Ptmass activity is to synthesize octahedral Pt−M NCs withsmaller crystallite sizes by modifying the synthesis protocols.Recent studies have reported Pt−Ni octahedra with 4−6 nmedge length.318,320,321

4.6.2. ORR Activity. The cubic Pt−M NCs only showedlimited ORR activity. As compared to conventional Pt−Mnnanoparticles, cubic Pt−Mn NCs were much less active in theHClO4 solutions.310 The cubic Pt3Ni and Pt3Co/C NCsshowed slightly higher activities than conventional Pt/C.309

These results are not surprising and consistent with work on

Figure 8. Schematic illustrations and corresponding TEM images of the samples obtained at four representative stages during the evolution processfrom polyhedra tonanoframes. (A) Initial solid PtNi3 polyhedra. (B) PtNi intermediates. (C) Final hollow Pt3Ni nanoframes. (D) Annealed Pt3Ninanoframes with Pt(111)-skin-like surfaces dispersed on high-surface area carbon.300 Reprinted with permission from ref 300. Copyright 2014American Association for the Advancement of Science.



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the bulk Pt alloy single crystals.35 Because of much weakeradsorptions of SO4

2− and HSO4− on the (100) than on (111)

and (110) planes, the cubic Pt−M NCs showed significantlyhigher activity than conventional Pt−M and Pt nanoparticles inH2SO4 solutions.309,310 A wide range of Pt mass activities ofoctahedral shape Pt−M (mainly Pt−Ni) NCs in HClO4

solutions were reported.322 The absolute value ranges from aslow as 0.3 A mg−1 to as high as 6.98 A mg−1 at 0.9 V, as shownin Table 2. The low mass activities can origin from a fewsources, imperfection of the structure, measurement condition,unoptimized composition, and particle size. Choi et al. foundthat the ORR activity of octahedral Pt−Ni NCs was highlydependent on the Pt:Ni ratio.321 For Pt−Ni NCs with an edgelength of 9 nm, Pt2.5Ni had higher activity than Pt1.4Ni, Pt2Ni,Pt3.2Ni, and Pt3.7Ni, as shown in Figure 9. If the Ni content wastoo low, the oxygen binding energy would be rather high,resulting in a slow step of further reduction of adsorbed oxygenadsorbates. On the other hand, if the Ni content was too high,the binding energy would be too weak to facilitate the breakingof O−O bond and charge transfer. The oxygen binding energyon Pt2.5Ni might be not too weak nor too strong, resulting in amaximum mass activity. A similar composition dependence wasreported by Chou et al.323 and Zhang et al.,318 although theoptimized composition was PtNi and Pt1.5Ni in their respectivestudies. The PtNi was also reported to have the highest activityby Cui et al.190 The discrepancy may come from the differencesin synthesis and post-treatment methods, final structure, andcomposition of the catalysts. Cui et al.190 found that the Pt and

Ni distribution was nonuniform in Pt−Ni octahedra. Ptlocalized at the corners and edges, while Ni enriched in thefacets. During initial electrochemical treatment, Ni was easilydissolved resulting in a structure evolution. The sample with ahigh Ni content (PtNi1.5) formed a concave structure and lostmost of its active (111) facets. On the other hand, the samplewith a low Ni content (Pt1.5Ni) maintained its octahedralstructure but with a thick Pt shell (5−12 atomic layers). Thesample with a balanced Pt and Ni composition (PtNi) retainedmost of the (111) active sites and formed a relatively thinPt shell (1−4 atomic layers). As expected, the activity ofoctahedral Pt−Ni NCs is also dependent on the crystallite size.Choi et al. found that the 9 nm Pt2.5Ni NCs showed higher Ptmass activity than the 6 and 12 nm ones.321

One interesting finding reported by Huang et al.320 is thatthe ORR activity and stability can be enhanced by doping thePt3Ni octahedra surface with a tiny amount of other transitionmetals, which were introduced by adding corresponding metalcarbonyls in the synthesis solutions. For instance, the Pt massactivity at 0.9 V increased from 1.80 to 6.98 A mg−1 by doping1.6 molar % of Mo in the octahedral Pt3Ni. Theoretical studyrevealed that Mo atoms preferred to sit near the edges andvertices of the particles. They stabilized both Ni and Pt atomsagainst dissolution by forming relatively stronger Mo−Pt andMo−Ni bonds. The activity enhancement was also observed bydoping other transition metals (Cr, Co, Fe, V, Mn, W, et al.).This study implied that previous synthesis of Pt−Ni octahedrainvolving W(CO)6 might dope a small amount of W in the

Table 2. Synthesis Conditions of Pt−Ni Octahedra and Their Corresponding Physical and Electrochemical Properties

methods solventcapping/shapedirecting agents

temp(°C)

shapeselectivity

edgelength(nm)

Pt/Nimolar ratio

specific activity(mA cm−2)

Pt mass activity(A mg−1) ref

without assistance of COcontaining chemicals

DMF none 200 fair 8−9 1 3a 0.68 315DMF none 120 good 9 1 3.14b 1.45 141benzylalcohol

PVP, benzoic acid 150 excellent 12 0.5 314

DMF benzoic acid 160 good 4 3 2.2b 1.80 320none none 200 fair 5.8 1.5 3.99a 1.96 318

with assistance of COcontaining chemicals

none OAm, OA,W(CO)6

230 excellent 11 3 1.2a 0.3 309

benzylether

Oam, OA,W(CO)6

230 excellent 9 2.5 10.1b 3.3 317

diphenylether

OAm, CO 210 fair 10 3 1.26 0.44 146

DMF benzoic acid,Mo(CO)6

170 good 4 3 8.2b 6.98c 320

aNormalized to the surface areas derived from the H adsorption charges. bNormalized to the surface areas derived from the CO stripping charges.cMo-doped Pt3Ni octahedra.

Figure 9. Comparison of ORR polarization curves of the octahedral Pt−Ni/C catalysts with different compositions in O2-saturated 0.1 M HClO4solutions. Scanning rate = 10 mV s−1. Rotation speed = 1600 rpm (A). The specific activity (SA) and mass activity (MA) at 0.9 V as a function of Niatom % for the octahedral Pt−Ni/C catalysts (B).321 Reprinted with permission from ref 321. Copyright 2014 Wiley-VCH on behalf ofChemPubSoc Europe.



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surface of Pt−Ni, resulting in an enhanced activity. This hypo-thesis needs further confirmation.

4.7. Nanowires, Nanorods, and Nanotubes

One-dimensional (1-D) nanostructures such as nanowires(NWs), nanorods (NRs), and nanotubes (NTs) have attractedgreat attention due to their unique anisotropic structure bene-ficial to many catalytic reactions.324 The exterior surfaces of 1-Dnanostructures consist of smooth and low energy facets, onwhich the oxygen binding energy is weaker than that on conven-tional nanoparticles resulting in higher ORR activities.325−327

The 1-D nanowires can form a 3-D interconnected porousaerogel structure. Liu et al.328,329 found that Pt80Pd20 bimetallicaerogel had 5 times higher Pt mass activity than commercialPt/C.One concern of NWs in fuel cell applications is their low

surface area/volume ratio. Great efforts have been taken tosynthesize thin and ultrathin Pt alloy NWs. Pt−M (M = Fe, Co,Ni, Pd, etc.) NWs with small diameters have been synthesizedby electrospinning, microwave irradiation, and polyol methods,and their ORR performances were evaluated.330−333 Forinstance, Pt−Fe and Pt−Co NWs with diameter in the rangeof 2.5−6.3 nm were prepared by thermal decomposition ofFe(CO)5 or Co2(CO)8, and reduction of Pt(acac)2 in anorganic solvent in the presence of capping agents, as shown inFigure 10A. The Pt mass and specific activities of 2.5 nmPt80Fe20 NWs were 0.84 A mg−1 and 1.53 mA cm−2, respect-ively, at 0.9 V.331 The specific activity could be further increasedto 3.9 mA cm−2 by increasing the diameter to 6.3 nm.331 Zhuet al. found that the dissolution of transition metals wasdramatically reduced by adding a third metal, for example, Cu,in Pt−Fe to form NRs. The morphology of the Pt−Fe−CuNRs is shown in Figure 10B.334 Other NRs with variouscompositions including Pt−Ni,335 Pt−Cu, Pt−Ni−Fe,214 andPt−Ni−Cu336 have been studied as well. The surface areas ofNTs are typically high because both inner and outer walls inNTs are available for reactions.155,337−341 For instance, quater-nary Pt5Cu76Co11Ni8 NTs (Figure 10C) prepared by a one-stepdirect electrodeposition approach using a porous anodicaluminum oxide (AAO) membrane as the template showedan ECA of 104 m2 g−1.338 Its Pt mass activity was about 5-foldthat of Pt/C. The nanoporous NWs and NTs fabricated bydealloying (see section 4.5) or sacrificial displacement of a CuNW template were also evaluated for ORR.342−345

5. CORE−SHELL STRUCTURESThe Pt dispersion (utilization) in conventional nanoelectroca-talysts is low due to the fact that only the surface atoms exposeto the electrolytes and participate in the electrocatalytic reac-tions. For example, for a 3 nm Pt nanoparticle, only about 30%

of atoms are on the surface with the rest of the 70% wastedinside of the particles. The idea of core−shell structure isto improve the utilization of Pt atoms by depositing a thinPt-based shell around a less expensive core, such as Pd-, Ru-,and Re-based nanoparticles. There are various approaches tosynthesize core−shell catalysts including but not limited toCu-mediated Pt deposition,346−348 chemical reduction, sponta-neous deposition,349 dealloying,316,350 electrochemical deposi-tion, and atomic layer deposition.351,352

5.1. Cu-Mediated Deposition

In the Cu-mediated deposition method, a Cu monolayer iselectrochemically deposited on a noble metal core at potentialslower than its bulk deposition (underpotential deposition,UPD). The Cu monolayer then is displaced by Pt atoms viaa spontaneous reaction Cu + Pt2+ → Pt + Cu2+. Ideally, a Ptmonolayer (ML) is coated on the core. The Cu UPD on a Pdcore and Pt displacement reactions are illustrated in Figure 11.

The Adzic group at Brookhaven National Laboratory pioneeredthis approach and has conducted an extensive study.346−348,353,354

The advantages of this Pt ML structure include not only high Ptutilization (100% in theory), but also possible activity enhance-ment due to property changes caused by the structural andelectronic effects from the core materials.

5.1.1. Core Material Effect. Because of the mismatch ofthe lattice constant, the Pt ML undergoes either a compressiveor a tensile strain when it is deposited on a foreign substrate.The presence of the strain affects the d-band center of the PtML.354 For instance, a large tensile strain exists in the Pt MLsupported on Au(111), while a compressive strain is expectedwhen it is deposited on Ru(0001). The tensile and compressivestrains result in upshifting and downshifting of the d-bandcenter of Pt ML, respectively.355 The binding energies ofadsorbates have a strong correlation with the position of d-bandcenters. In addition, the electronic coupling between the Pt MLand the substrate causes additional electronic (ligand) effect.Both strain and ligand effects have been confirmed by thedensity functional theory (DFT), and their roles in controllingthe electrocatalytic activities of Pt MLs have been demon-strated.354,356,357 Recently, Stolbov et al. proposed that thecore−shell interlayer hybridization was the main factor

Figure 10. TEM images of 2.5 nm wide Fe56Pt44 NWs (A),331 2 nm wide Fe29Pt41Cu30 NRs (B),334 and Pt5Cu76Co11Ni8 NTs (C).

338 Reprinted withpermission from ref 331. Copyright 2013 Wiley-VCH. Reprinted with permission from ref 334. Copyright 2013 American Chemical Society.Reprinted with permission from ref 338. Copyright 2011 Wiley-VCH.

Figure 11. Illustration of Pt monolayer deposition on a foreign metalcore (Pd as an example) involving the Cu UPD and subsequent Ptdisplacement.



L


controlling the surface reactivity, whereas the contribution fromthe strain effect was not significant.358 Zhang et al. comparedthe ORR activities of Pt MLs supported on different substratesincluding Ir, Ru, Rh, Pd, and Au, and found a strong correlationbetween the activities and the d-band centers of Pt MLs, whichwere dependent on the type of the substrates.354 FollowingSabatier’s principle, a good ORR catalyst should exhibit amoderate metal−O interaction.359 If the oxygen binding energyis too weak, the O−O bond cleavage and electron transfer willbe difficult. On the other hand, the subsequent reduction ofadsorbed oxygen containing species will be slow if the oxygenbinding energy is too strong. Furthermore, the tardiness inremoving oxygen containing species results in blocking theadsorption and dissociation sites for oxygen molecules. It hasbeen known that the Pt(111) surface binds oxygen too stronglyand the ORR kinetics is limited by the rate of removal ofstrongly adsorbed oxygen containing species. A slightly loweroxygen binding energy, as in the case of Pt ML on Pd(111),achieved by lowering the d-band center of Pt, is expected toenhance the activity. The electronic properties of Pt ML canbe further tuned resulting in even higher ORR activities byalloying the Pd core with other metals such as Co,348 Ni,360

Fe,361 Cu,362,363 Ir,363,364 Au,365 etc. Shao et al. achieved thehighest ORR activity (2.8 A mg−1Pt) of core−shell catalysts bydepositing the Pt shell on nanoporous Pd−Cu nanoparticlesobtained by electrochemical dealloying.362 Later, it was foundthat Cu leached from the dealloyed core during the fuel celloperation poisoning the ionomer and membrane by occupyingthe SO3

− group, even affecting the anode performance. Thepoisoning issue was largely mitigated by using a chemicallydealloyed Pd−Ni core due to the fact that Ni2+ does notredeposit on the anode.360

According to Zhang et al., Au(111) is not a good support forPt ML due to the larger lattice constant of Au than that ofPt.354 However, contrary to this observation, a recent study ofPt overlayers supported on Au single crystals demonstrated thatAu(111) is indeed a good support.366 An atomic layer thick Ptshell on Au(111) was 2 times more active than bulk Pt(111)surface. The ORR activities of Pt overlayers on Au(100) andAu(110) were much lower due to easier alloying and recon-struction of these surfaces. The significant compressive strain inthe surface of small Au nanoparticles results in a 1.6-foldincrease of ORR activity on Pt ML supported on 3 nm Aunanoparticles over that supported on Pd particles with the samesize.367 The activity enhancement from small Au core has alsobeen confirmed by other groups.40,349,368−371 The structuraland electronic properties of the Au-based core can be furthertuned by alloying with transition metals. For example, Gonget al. synthesized core−shell AuNi0.5Fe nanoparticles consistingof 3−5 atomic layers of Au on the multimetallic alloy core.372

The combination of lattice contraction of Au and electrontransfer from Ni and Fe atoms to Au might weaken the oxygenbinding strength on the Pt ML, resulting in a high ORR activity.Another improvement on core−shell catalysts is to introduce

an interlayer between the Pt shell and the core. The interlayersconsisting of Pd, Au, or their alloys were deposited via a Cu-mediated process before the Pt ML was deposited. Xing et al.found that the ORR activity of Pt ML on Pd core enhanced by3 times by introducing a Pd9Au interlayer (Table 3).373 Asimilar enhancement (2-fold) was observed with an incompleteAu layer.367,374 Pt monolayers on Ir−Co-based alloy cores didnot show a high ORR activity due to strong oxygen bindingenergies. For example, the Pt mass activity of Pt ML on IrCo

was only 0.15 A mg−1. In comparison, with a Pd interlayer, theactivity of 1.18 A mg−1 was considerably higher.375 The Pdinterlayer plays an essential role in achieving high catalyticactivity by adjusting the electronic interaction of the Pt MLwith the IrCo core, resulting in a weaker oxygen binding energy.Zhang et al. demonstrated that PtML on Ru(0001) bulk

single crystal had a significantly lower ORR activity than that onPd(111) due to a too weak oxygen binding energy.354 If twoto three Pt MLs instead of only one were deposited on Runanoparticles, the Pt mass activity of core−shell catalyst wascomparable to that of [email protected] The activity enhancement wasattributed to the proper oxygen binding energy on a thicker Ptshell based on DFT calculation results.

5.1.2. Core Structure Effect. The structure of the corematerials, including the shape, particle size, porosity, andsurface morphology, has a significant effect on the ORR activityof the core−shell catalysts.346,377−380 The activity of Pt shell onPd octahedra was 3.5 times higher than that on Pd cubes,suggesting the importance of maintaining high coverage of(111)-oriented facets.378 Cai et al. were able to smooth theconventional Pd nanoparticle surfaces and increase the fractionof {111} facets by using Br− adsorption/desorption approach.381

The low coordinated Pd atoms formed a Pd−Br2 pair in solutionthat was redeposited onto the Pd surface in the following anodicscan, accompanied by the oxidative adsorption of bromide. As aresult, the activity increased by 25−50% after the Pd core wastreated by Br−.381 Pd NW has less defects than nanoparticle andis expected to be a better choice as a core. Koenigsmann et al.confirmed the high activity of Pt ML supported on Pd NWs(1.83 A mg−1Pt).

382 The degree of enhancement resulting fromthe Au interlayer also depends on the structure of the Pd core.The Au interlayer improved the activity of Pt MLs on Pd cubesand octahedra by 3 and 1.2 times, respectively; that is, its effecton ORR activity was much smaller on (111) surface than that on(100).374 The larger enhancement factor at the (100) sites maybe due to the larger decrease of oxygen binding energy caused byAu interlayer (0.275 eV) than that at (111) sites (0.075 eV).In the systems of Pt MLs on dealloyed nanoporous Pd−Cu

and Pd−Ni nanoparticles, in addition to the strain and ligandeffects from the transition metals, the unique porous structurewas believed to change the electronic properties of the Pt shell,and consequently enhance the activity.360,362 This argumentwas supported by the work of Zhang et al.,383 who found thatthe Pt MLs on hollow Pd and Pd−Au nanoparticles formed bygalvanically displacing Ni nanoparticles were 2 times moreactive than that on corresponding solid particles. Instead of atensile strain (0.46%) in the Pt ML on a solid Pd9Au core, a0.46% compressive strain was found on a hollow core. Thehollow-induced lattice contraction, together with the smoothsurface morphology and the mass-saving geometry of thehollow particles, improved the total PGM activity by 2−3 times.

5.1.3. Scale-Up and MEA Testing. Most of the core−shellcatalysts made with the Cu-mediated method were synthesizedon a microgram scale, that is, with the Cu UPD and Pt displace-ment reactions occurring on a RDE tip. When this process wastransferred to a gram scale, the high activity of core−shellcatalyst was not fully realized.384 One of the main reasons isthat the quality of the Pt shell is more sensitive to the reactionconditions in a large reactor such as concentration and injectionspeed of Pt solutions and rotation speed of solution (transportspeed of Pt cations). It is possible that Pt cluster rather than auniform Pt shell was deposited on the core in large batches, asdemonstrated by in situ XRD results in an H2 environment.

385



M


The problem associated with the Pt cluster formation is that thePt−Cu displacement involves the electron transfer from thecore to Pt cations, rather than direct electron exchange fromCu, as recently proposed by Thambidurai et al.386 That meanselectrons generated anywhere on the surface can move freelythrough Pd substrate, reducing Pt cations wherever theiractivity and surface energy are the greatest. In other words, thePt atom may not deposit on the same site left by the Cudissolution, but rather on Pt deposited previously leading to theformation of Pt clusters. The mass transport of Pt cations inlarge batches exacerbates this problem. As a result, there areonly a limited number of core−shell materials that have beensynthesized at a gram scale and tested in MEAs. Naohara et al.first reported the synthesis of Pd@Pt on a gram scale withreproducible Pt mass activity around 0.6 A mg−1.387 Zhou et al.synthesized Pd2Co@Pt with the same protocol resulting in aslightly higher activity (0.72 A mg−1).348 By adding specialadditives in the Pt−Cu displacement solutions, the coverageof the Pt shell could be improved significantly due to selectiveadsorption of additives on Cu and Pt surfaces but noton Pd surfaces; that is, the Pt atoms were forced to depositon the Pd core rather than Pt atoms already deposited.385

With this improvement, the Pt mass activities of Pd@Pt anddealloyed Pd−Ni@Pt could reach 0.95 and 1.40 A mg−1,respectively.360

The optimization of MEA performance of core−shellcatalysts has not been seen in public domain. The existingdata showed that there were significant activity gaps betweenMEA and liquid cell measurements for core−shell catalystssynthesized in gram batches.384 For example, the activitiesdropped from 0.6 (liquid cell) to 0.3 A mg−1 (MEA) and from0.7 (liquid cell) to 0.15 A mg−1 (MEA) for Pd@Pt andPd−Co@Pt, respectively.348 The dealloyed Pd−Ni@Pt outper-formed Pt/C by 40 mV (i.e., 4-fold enhancement over Pt/C)through the whole potential range in the MEA testing.360 Theenhancement was also smaller than the expected 7-fold. Thelow performance of core−shell catalysts may simply be due tounoptimized MEA fabrication parameters. To optimize theperformance of core−shell catalysts in fuel cells, a significantamount of work is required to determine the optimal ink andcatalyst layer composition, and preparation techniques.The durability of core−shell catalysts during fuel cell opera-

tion is a big concern in the fuel cell community. Sasaki et al.demonstrated that the activity decay of Pd@Pt indeed wasmuch slower than that of the benchmark Pt/C.59,388 After100 000 potential cycles (0.7−0.9 V, with 30 s dwell time at80 °C), the Pt mass activity of Pd@Pt decreased by 37%. Forcomparison, Pt/C lost its initial activity by 70% after only60 000 cycles. During potential cycling, a large amount of Pddissolved from the core and migrated to the membrane and the

Table 3. Summary of ORR Activities of Core−Shell Catalysts Synthesized by the Cu-Mediated Method (Measured at 0.9 V)

core materialsPt mass activity RDE/MEA

(A mg−1)PGM mass activity

(A mg−1)PGM mass activity (cost adjusted)

(A mg−1)aspecific activity(mA cm−2)

batchsize ref

Pt benchmark 48% Pt/CTKK

0.2 0.2 0.2 0.24 317

Pd NPs 0.75 0.18 0.38 0.31 RDE 385Pd NWs 1.9 0.2 0.6 0.8 RDE 382Pd NRs 1.7 0.4 0.9 0.75 RDE 389Pd tetrahedra RDE 380Pd octahedra 2.2 0.93 RDE 378Pd cubes 0.64 0.27 RDE 378Pd NPs 0.95 0.6 gram 360Pd NPs 0.60/0.30 gram 387Pd9Ru NPs 0.38 0.26 RDE 390IrCo NPs 0.15 0.036 0.069 RDE 375IrCo NPs with Pdinterlayer

1.18 0.16 0.45 RDE 375

PdIr alloy NPs 2.17 0.13 0.43 0.94 RDE 391Ir2Re NPs with Pdinterlayer

0.60 0.18 0.25 RDE 364

Au NPs 1.06 0.42 RDE 368Au NPs 1.2 0.51 RDE 367Ru NPs (2 ML) 0.95 0.65 RDE 376Os NPs (2 ML) 0.70 0.45 1.33 RDE 392dealloyed PdCu NPs 2.8 0.46 1.2 1.18 RDE 362dealloyed PdNi NPs 1.40 0.53 0.99 0.93 gram 360dealloyed IrCu 1.35 0.57 0.71 RDE 393AuNi0.5Fe NPs 1.38 0.18 1.12 RDE 372Ir2Re NPs 0.38 0.12 0.16 RDE 364PdNi NPs 1.1 0.43 0.6 RDE 394PdIrNi2 NPs 0.9 0.79 RDE 395AuPdNi NPs 1.35 0.53 0.7 RDE 394Pd9Au NPs 0.9/0.36 0.32 0.47/0.38 gram 388Pd2Co NPs 0.72 0.50 gram 348hollow Pd NPs 1.50 0.45 0.90 RDE 383hollow Pd20Au NPs 1.62 0.57 0.85 RDE 383aAssuming the cost of Pd is 1/3 of that of Pt.



N


anode. The formed Pd2+ was reduced by H2 diffusing fromthe anode forming a Pd band in the membrane. The rest ofthe Pd2+ diffused further to the anode and was reduced in theanode catalyst layer. With continuous removal of Pd in thecore, the particle gradually shrinks and the Pt atoms onthe surface were able to form bi-/multilayers. With a thickerPt shell, the stability of the core−shell structure was furtherimproved. The Pd dissolution prevented Pt shell via a cathodicprotection mechanism. The stability of the core−shell structurewas further improved by alloying Pd core with a small amountof Au (Pd9Au).

388 As shown in Figure 12D, after 200 000 cycles(0.6−1.0 V), the Pt mass activity only decreased by ∼30%.Even under a much harsher cycling condition (0.6−1.4 V),about 30% of activity was maintained after 20 000 cycles. Inboth cases, the core−shell structure was maintained (Figure 12Band C). The only difference is that the Pt shell of the latterbecame thicker. DFT calculations suggested that Au atomssegregated preferentially at defect sites in Pt shell suppressing Pddissolution from the core. In addition, the Au clusters might alsodelay the Pt oxidation/dissolution and thereby stabilize the Ptsurfaces.388

5.2. Chemical Reduction

In the chemical reduction method, the Pt salts are reduced ineither aqueous or organic solvents by various reduction agentsand deposited on the cores, which serve as the seeds for theshell growth. Different types of cores, solvent, capping, andreduction agents have been explored.264,370,396−399 Forinstance, Pd@Pt core−shell nanoparticles were synthesizedby coating the Pd nanoparticles in an ethylene glycol and/ordiethylene glycol solution with PVP as the capping agent.172,400

Thin and uniform Pt shells were formed by this approach asshown in Figure 13A and B, and their thickness (from as low as0.38 nm to over 1 nm) could be tuned by varying the amountof Pt precursors. The activities of Pd@Pt catalysts, however,were not high. The low ORR activity may be due to the nega-tive effect from the PVP, which is not easy to remove from thePt surfaces. Other groups explored deposition method withoutPVP. Zhang et al.401 synthesized Pd@Pt core−shell catalystswith different Pd:Pt ratios using ascorbic acid as the reducingagent in the presence of PEO106PPO70PEO106 as capping agent.In general, the ORR activity of Pd@Pt was low (up to 2 timesimprovement over Pt/C) in the RDE testing. Interestingly, thefuel cell performance of Pd@Pt improved significantly (70 mVat 600 mA cm−2) after 40 000 potential cycles (0.65−1.05 V,100 mV s−1) instead of decay, and the activity was 4.5 timesover Pt/C after cycling. The same group also used amphiphilictriblock poly(ethylene oxide)−poly(propylene oxide)−poly-(ethylene oxide) copolymer as the reducing and capping agentto synthesize Pd@Pt core−shell nanoparticles with various shellthickness.402 When more Pt atoms were deposited, the layer-by-layer growth mode was changed to island-on-wetting-layergrowth mode. The RDE and fuel cell testing results showed2 and 3 times activity improvement over Pt, respectively. Thecore−shell transferred to a nanocage structure resulting fromPd dissolution during electrochemical cycling. The activity ofthe nanocage was found to be higher than that of the [email protected] The core−shell structure was also synthesized inethanol solvent, which reduced the Pt cations at 70 °C. DFTcalculations showed that the 2-D rather than 3-D growth wasenergetically favorable for Pt on Pd nanoparticles. The uniform

Figure 12. Distribution of Pd, Pt, and Au in the core−shell nanoparticles as synthesized (A), after 100 000 potential cycles between 0.6 and 1.0 V(B), and after 20 000 potential cycles between 0.6 and 1.4 V (C) obtained by the line-scan analysis using EDS/STEM. (D) The Pt mass activity ofthe Pt/Pd9Au/C catalysts at 0.9 V as a function of number of potential cycles during fuel cell testing. The result of Pt/C is included for comparison.Absolute pressure for H2/O2 = 150 kPa. Relative humidity = 100%. Pt loadings in the cathodes: 0.105 mg cm−2 for Pt/Pd9Au/C, 0.102 mg cm

−2 forPt/C.388 Reprinted with permission from ref 388. Copyright 2012 Nature Publishing Group.



O


coating of 1 or 2 Pt MLs on Pd was promoted by the slowerkinetics of ethanol oxidation.404 The Pt mass activity of Pd@Ptsynthesized in ethanol was 0.64 A mg−1, which was higher thanthose discussed above and comparable to that synthesized viathe Cu-mediated method. The high activity could be attributedto the thin, uniform, and clean Pt shell. Using the proprietarymethod, Ball et al. deposited Pt shell on Pd−Co alloy nano-particles and achieved a Pt mass activity of 0.7 A mg−1.405,406

The same material, however, only showed a Pt mass activity of0.14 A mg−1 in the fuel cell testing. Alia et al. coated Ptoverlayers with various thicknesses (1.1, 1.7, and 2.2 Pt MLs)on Pd nanotube by reducing Pt salts with PVP at 108.7 °C. ThePt mass activity was as high as 1.8 A mg−1 for the thinnestcore−shell sample.407A few works also synthesized core−shellstructures consisting of Pt nanobranches (dendrites, nano-particles, etc.), which showed moderate ORR activities.408−410

Another promising synthesis protocol is epitaxial overgrowth ofPt on Pd nanoparticles.411−415 A smooth and thin Pt shell onthe scale of one monolayer to 2 nm could be deposited on Pdcores. Recent reports of 1−3 monolayers of Pt shells on 20 nmPd cubes415 (as shown in Figure 13C) and 2−5 monolayers onPd octahedra416 showed 3- and 5-fold enhancement over Pt/Cin mass activities, respectively. The same group further reducednoble metal loading by etching away most of the Pd atoms inthe core in a FeCl3/HCl mixed solution, resulting in a cubicnanocage structure (Figure 13D).417 The mass activity enhance-ment factor was slightly increased from 2 to 3. For the octahedralnanocage, the mass activity was higher with an enhancementfactor of 5 over Pt/C.By utilizing the ability of Pd to absorb hydrogen to form a

Pd hydride, which is a strong reduction agent, Wang et al.

developed a simple approach to coat Pt shell on Pd NPs. Aftertreatment with H2, the Pd NPs suspension was mixed with asolution containing Pt2+ cations, which were spontaneouslyreduced by Pd hydride forming a uniform thin Pt shell. In analternative process (as shown in Figure 14),418,419a sacrificial

Cu thin layer was deposited first deposited on Pd NPs with theassistance of Pd hydride. The Cu layer was then displaced by Ptto form a 0.3 nm thick Pt shell. The specific activity and Ptmass activity were double those of Pt/C. Liu et al. synthesized asandwich structure with a Y layer between Pd core and Pt layerutilizing the same mechanism.420

The Sun group developed a new strategy by epitaxial over-growth of Pt-based bimetallic shell on Pd and Au NPscore.421−424 Pd, Au, or even Ni NPs seeds were first synthesizedwith the polyol methods and mixed with Pt(acac)2 solvent at110−120 °C. Fe(CO)5 was injected into the mixture, whichwas further heated to 200 °C. The composition of the Pt−Feshell could be controlled by adjusting the molar ratio of Fe:Ptprecursors. The thickness of the bimetallic shell varied from1 to 3 nm depending on the amount of meal precursors added.Figure 15 showed the synthesis process, HAADF-STEM, andelement mapping of Au@Pt3Fe NPs consisting of 7 nm Aucores and 1.5 nm Pt3Ni shells.

423 The initial Pt mass andspecific activities were 5- and 3-fold higher than those of Pt/C.In addition to the higher activities, the core−shell catalysts wasalso more durable. After 60 000 potential cycles between0.6 and 1.1 V, the activity improvement factors increased to17 and 7 for mass and specific activities, respectively.423 Thestable icosahedral Au core was hypothesized to be responsiblefor the excellent durability of Au@Pt3Fe. The Au−Cu alloyNPs were also explored as the core to introduce compressivestrain.425 Using a similar approach, Pt−Fe binary shell withtunable thickness ranging from 0.3 to 1.3 nm was successfullycoated on FePtPd(Au) NWs.426 The core−shell structure withthe shell thickness of 0.8 nm showed the highest activityand 2−3 times more activity than the core (Pt−Fe NWs).As compared to Pt/C catalyst, the Pt mass and specific activitieswere 14.5 and 12.5 times higher, respectively.426

The Pt-based bimetallic shells discussed above are mixedfacets. As discussed in section 3.2, the {111} facet has thehighest activity. If one can control the final structure with only{111} facets exposed, the ORR activity of the core−shellcatalysts will be maximized by the combination of the followingthree aspects: a high Pt utilization, electronic effect from thealloying elements, and desired structure (Table 4). Choiet al.427 deposited a ∼1 nm Pt2.5Ni shell on a 5 nm Pd seedusing a similar method in the synthesis of PtNi octahedron,317

forming an octahedral core−shell nanoparticle. The Pt massand specific activities were enhanced by 12.5- and 14-fold,respectively. A similar approach has been conducted by Zhaoet al. to synthesize [email protected] octahedra with a somewhatlower activity.428

Figure 13. HAADF-STEM image (A) and HAADF-STEM-EDSmapping images of Pd@Pt nanoparticles (B). Scale bars in (A) and(B) are 2 and 5 nm, respectively.400 HAADF-STEM image of Pd@Pt(Pd and Pt are the core and shell, respectively) cubes with a Pt shellthickness of 2−3 monolayers (C),415 and after chemically removingthe Pd core (D).417 Reprinted with permission from ref 400.Copyright 2013 Wiley-VCH on behalf of ChemPubSoc Europe.Reprinted with permission from ref 415. Copyright 2014 AmericanChemical Society. Reprinted with permission from ref 417. Copyright2015 American Association for the Advancement of Science.

Figure 14. Illustration of the synthesis of Pd@Pt NPs based on a Cuthin layer formed by chemical reduction of Cu2+ by absorbed H2 in Pd(Pd hydride).419 Reprinted with permission from ref 419. Copyright2011 American Chemical Society.



P


5.3. Dealloying

As discussed in section 4.5, dealloying can not only formporous structures, but also solid core−shell structures. Theunique core−shell structure consisting of a Pt alloy core and apure Pt shell showed higher ORR activity than conventional Ptalloy catalysts.128,134,222,431−433 In addition, a solid core−shellstructure is preferred from the durability point of view becausethe nanoporosity may result in a faster activity decay duringORR.132 The compressive strain formed in the Pt shell wasfound to play a critical role in the activity improvement.Strasser et al. extensively studied the dealloying of Pt−Cunanoparticles.434−437 A few atomic layers (0.6 nm) of Pt-richshell was formed on a PtCu3 core after 200 potential cycles inacid media. The measured lattice distance in the Pt shell wassmaller than that of pure Pt due to the lattice mismatchbetween the Pt shell and the Cu-rich alloy core.350 Themagnitude of the compressive strain was determined by thePt:Cu ratio and annealing temperature of the alloys. Thehighest ORR activity was achieved on a dealloyed PtCu3 with aPt mass activity of 0.56 A mg−1. The activity enhancementshould mainly come from the surface strain effect because theligand effect from the core was negligible due to the thick Ptshell. The same group studied the dealloying of Pt−Ni alloynanoparticles with different Ni contents.438 The dealloyedPtNi3 showed the highest Pt mass activity (0.81 A mg−1) andspecific activity (2.27 mA cm−2). Unlike a conventional core−shell structure observed in the dealloyed PtNi, a Ni-enrichedlayer between the Pt shell and the Pt alloy was formed near the

surface at a depth of 2−3 nm, as shown in Figure 16. Thecompetition between the Ni segregation to the surface inducedby the adsorption of oxygen species and the formation of aprotective Pt shell might be the reason for the accumulation ofNi atoms below the Pt shell. The presence of the Ni-enrichedlayer resulted in a larger compressive strain in the Pt shell andhence a higher ORR activity.438

The dealloying condition also plays a critical role in deter-mining the structure, activity, and durability of the catalysts.142,439

Take PtNi3 as an example,439 dealloying in a hot diluted H2SO4solution resulted in a thicker Pt shell and a higher Ni content inthe core as compared to that in a more oxidative HNO3 solution.The denser core−shell structure obtained in the H2SO4 solutionled to a higher ORR activity and durability revealed in MEAtesting. When the particle size is large enough, a porous structureinstead of a core−shell structure is easy to form.Wang et al. also studied the core−shell structure as a

function of initial compositions of the Pt−Ni alloys.440 After100 potential cycles (0.06−1.1 V), the Pt shell of dealloyedPtNi3 (1 nm) was thicker than that of dealloyed PtNi (0.5 nm)due to easier removal of Ni in the former. The thicker Pt shell,however, was not beneficial for oxygen reduction due tosignificant relaxation of the compressive strain in it. Thisargument is consistent with the fact that dealloyed PtNi wasmore active than PtNi3.

440 They also found that the ORRactivity of as-dealloyed Pt−Ni nanoparticles could be enhanced2-fold by heat treatment at 400 °C.114 The heat treatment wasbelieved to smooth the surface and reduce the low coordination

Figure 15. Schematic illustration of Au@Pt3Fe nanoparticle synthesis (A), HAADF-STEM characterization (B), and elemental distribution analysisof Au@Pt3Fe (red, Pt; green, Au) (C), and the elemental distribution analysis of a single core−shell particle (D).423 Reprinted with permission fromref 423. Copyright 2010 American Chemical Society.



Q


Table

4.Summaryof

ORRActivitiesof

Core−

ShellCatalysts

Synthesizedby

Chemical

Metho

ds(M

easuredat

0.9V)

core

materials

Ptmassactivity

RDE/MEA

(Amg−

1 )

PGM

mass

activity

(Amg−

1 )PG

Mmassactivity

(cost

adjusted)a

(Amg−

1 )specificactivity

(mAcm

−2 )

method

refs

Ptbenchm

ark48%

Pt/C

TKK

0.2

0.2

0.2

0.24

317

PdNPs

0.64

0.26

0.58

ethanolat

70°C

404

PdNPs

0.08−0.21

0.4−

0.5

ethylene

glycol

anddiethylene

glycol

with

PVPat

200°C

400

PdNPs

0.16−0.37

0.25−0.55

amphiphilic

triblock

poly(ethyleneoxide)−poly(propylene

oxide)−

poly(ethyleneoxide)

copolymer

at80

°C402

PdNPs

0.2

0.5

ethylene

glycol

with

PVPat

140°C

172

PdNTs

1.80

0.18

0.47

2.20

aqueoussolutio

nin

thepresence

ofPV

Pat

108.7°C

407

Pd3CoNPs

0.70/0.14

0.25

0.49

proprietary

methods

406

Nib

2−3tim

esover

Pt/C

3−4tim

esover

Pt/C

1,2-propanediol,oleicacid,K

BH

4reductionat

138°C

397

Pd−Co

0.15−0.22

0.17−0.31

ultrasound-assistedpolyol

ethylene

glycol

reduction

429

Au−

Cu

0.57

oleylamineat

240°C

425

Pd−Au

0.94

aqueoussolutio

nin

thepresence

ofL-ascorbicacid

underam

bientconditions

430

Pdcube

0.35

0.85

ethylene

glycol

inthepresence

ofascorbicacid,K

BrandPV

Pat

200°C

415

Pdoctahedron

0.49

0.15

0.9

aqueoussolutio

nin

thepresence

ofcitricacid

andPV

Pat

95°C

/orethylene

glycol

inthepresence

ofascorbicacid,K

BrandPV

Pat

200°C

416

AuNPs

(Pt 3Ni

shell)

0.55

1.5

oleicacid

andoleylamineat

200°C

423

PdNPs

(octahedral

Pt2.5N

ishell)

2.5

1.6

2.7c

oleylamine,oleicacid,and

W(C

O) 6

at200°C

427

PdNPs

(octahedral

Pt1.8N

ishell)

0.79

0.48

hydrazinehydrate,citricacid,and

PVPat

65°C

428

aAssum

ingthecostof

Pdis1/3of

that

ofPt.bMeasuredat

0.85

V.cNormalized

tothesurfacearea

derived

from

thecharge

associated

with

CuUPD

.



R


surface atoms, as shown in Figure 17. The activity and durabilityenhancement by postannealing was also observed by Han et al.439

5.4. Other Methods

Other methods to deposit Pt-based shell on foreign metalcores have been explored, including the spontaneous galvanicdisplacement,441−446 electrochemical deposition,447−450 ther-mal annealing,451,452 evaporation,453 atomic layer deposition(ALD),454 microwave-,443 and ultrasound-assisted reduc-tion.429,455 Because the equilibrium electrode potential ofPtCl4

2−/Pt couple (0.775 V vs SHE) is higher than that ofmost other couples, such as PdCl4

2−/Pd (0.591 V), Cu2+/Cu(0.24 V), and Co2+/Co (−0.28 V), the galvanic displacementreaction of the core consisting of these metals by Pt can occurspontaneously.441−443,456 For instance, the Pd−Cu@Pt core−shell catalysts fabricated by this method showed a 3-foldenhancement in Pt mass activity over Pt/C.441 Liu et al. devel-oped a self-terminating process, in which the Pt deposition onAu(111) substrate was terminated with the assistance of Hupdresulting a 2D Pt monolayer without formation of 3D Ptclusters.448 It will be of interest to check whether the samemechanism also works on nanostructure substrates.

6. Pd-BASED ELECTROCATALYSTS

Pd and Pt are in the same group and share similar electronicproperties. Indeed, Pd shows a reasonable good activity towardORR, which is about 5 times lower than that of Pt.457 This,combined with the fact that the historical cost of Pd is only1/2−1/4 of Pt, makes it an attractive alternative to Pt.457

6.1. Structure Dependence

Pd is a more reactive metal than Pt, and binds oxygen morestrongly. It oxidizes at more negative potentials than Pt and isexpected to be less active for ORR.458 The study of ORR on Pdbulk electrodes in acidic media has received less attention thanthat on Pt due to the lower activity of the former, difficulty ofpreparing Pd single crystals, and their poor stability. Kondoet al.459 reported the activity trend of ORR on low index planesof Pd single crystals recently. They found that the ORR activitystrongly depended on the structure of the Pd surface. Incontrast to Pt(hkl) surfaces, where Pt(110) and Pt(111) showmuch higher activity than Pt(100) in a HClO4 solution, theactivity of the Pd(100) surface is the highest in the samesolution. The comparison of the kinetic current densities ondifferent Pd and Pt crystal orientations at 0.9 V is shown inFigure 18A. Surprisingly, Pd(100) is 14 and over 2 times more

active than Pd(111) and Pt(111), respectively. Their resultssuggest that (100) plane is the most active site for ORR on Pdcrystals, which has been confirmed by several studies report-ing Pd nanocrystals enriched with {100} facets with exceptionalORR activities in acidic solutions.460,461 Erikson et al. demon-strated that cubic Pd particles with an average size of ∼27 nmhad a higher ORR activity than spherical Pd particles

Figure 16. HAADF images (insets) and EELS compositional lineprofiles of individual dealloyed PtNi (A) and PtNi3 (B) nanoparticles.Red, Pt; green, Ni. A Ni-enriched shell is observed between the Ptshell and Pt−Ni alloy core in (B).438 Reprinted with permission fromref 438. Copyright 2012 American Chemical Society.

Figure 17. Illustration of the formation of a smooth Pt shellsurrounding a Pt−Ni shell by dealloying and subsequent thermalannealing.114 Reprinted with permission from ref 114. Copyright 2011American Chemical Society.

Figure 18. Comparison of specific activity of oxygen reduction on lowindex facets of Pd and Pt single crystals (A), and Pd and Pt nano-structures (B) at 0.9 V.457,461 Reprinted with permission from ref 457.Copyright 2011 Elsevier. Reprinted with permission from ref 461.Copyright 2011 Royal Society of Chemistry.



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(2.8 nm).460 A systematic study on the structural dependenceof electrochemical behavior of Pd nanocrystals with a muchsmaller particle size (5−6 nm) was conducted by Shao et al.461

The specific activities of cubic, octahedral, and conventionalPd/C at 0.9 V in a 0.1 M HClO4 solution were compared inFigure 18B. The activity enhancement of Pd/C cubes wasabout 10 times over octahedra, consistent with the bulk surfacestudy. The high ORR activity on Pd cubes can be contributedto its lower oxide coverage and consequently more availablereaction sites than octahedra.461 Pd-based nanorods are alsomore active than conventional Pd nanoparticles. For instance,Pd nanorods prepared by electrodepostion methods showed a10-fold activity enhancement over Pd nanoparticles.462,463 Theproblem with Pd nanocubes is their extremely low stability.Upon potential cycling, the well-defined structure converts toan irregular shape quickly. By alloying with a small amount ofRh, Yan et al.464 discovered that the activity of Pd octahedronwas improved significantly. The addition of Rh also enhancedthe stability of the octahedral structure.A recent DFT study revealed a slightly weaker oxygen

binding energy (0.04 eV) on Pd(100) than that on Pd(111).378

A weaker oxygen binding energy helps improve the ORRkinetics on Pd surfaces by decreasing the coverage of oxygencontaining species and increasing their removal rates. However,the difference in binding energy between Pd(100) and Pd(111)surfaces is rather too small and may not be the only reason forthe activity enhancement. More experimental and theoreticalstudies are needed to further understand the mechanisms ofstructural dependence of ORR activity on Pd and Pd alloyfacets.378,465,466

6.2. Pd Alloys

Similar to Pt, the activity of Pd can be enhanced by alloyingwith many other metals. Pd−M (M = Co, Fe, Ni, Cr, Mn, Ti, V,Sn, Cu, Ir, Ag, Rh, Au, Pt) alloys showed much higher ORRactivities than pure Pd, and some were comparable to Pt.467−476

As expected, the compositions of Pd alloys play an importantrole in ORR activity. The optimum Pd:M ratios for ORRwere strongly dependent on the alloying elements, synthesismethods, and annealing temperatures. Shao et al.477 found thatPd−Co/C nanoparticles synthesized by the impregnationmethod at 900 °C exhibited the highest ORR activity whenPd:Co = 2:1. The same optimum ratio was reported by Wanget al.478 for Pd−Co/C synthesized by the coreduction methodin ethylene glycol followed by thermal annealing at 500 °C. Onthe other hand, different optimum rations (30−40 at. %479 or10−20 at. %480 Co) were reported for Pd−Co/C synthesizedby coreduction method in aqueous solutions. In the studies ofPd−Fe/C systems, the highest activity was commonly observedat Pd:Fe = 3:1.481−485 The high activity of Pd3Fe was confirmedby Zhou et al., who demonstrated that a well-preparedPd3Fe(111) surface had an ORR activity comparable to thatof Pt(111).361 Similar to Pt-based alloys, it is expected that purePd skin- and skeleton-like surfaces are formed during annealingand electrochemical measurements. The systematic study ofPd-based alloy single crystals on ORR with various morphologyand compositions has yet to be reported. A small amount of Ircan further improve the activity of Pd3Fe.

482 For Pd−W alloysannealed at 800 °C, the maximum activity was observed whenthe alloy consisted of only 5% W.486 Some Pd−nonmetallicelement alloys487,488 were also studied with limited activityimprovement.

Dealloying has been also applied to the Pd-based alloys tocreate solid core−shell structure. Yang et al. tried to preparecore−shell type Pd−Cu structure by electrochemical dealloyingPdCu3 alloy films. A thick pure Pd overlayer (∼2 nm) wasformed on the PdCu3 substrate after Cu atoms were removedfrom the near-top surface. The activity enhancement, however,was only 2-fold due to the thick Pd overlayer, which limitedthe strain and ligand effects from the PdCu3 substrate.489

Nanoporous Pd-based structures can also be fabricated by(electrochemical or chemical) dealloying methods startingwith transition metal enriched Pd alloys nanoparticles, such asPdNi6 and PdCu6. Even though the electronic properties of thesurfaces of these dealloyed nanoparticles were significantlydifferent from those of pure Pd nanoparticles, there ORRactivities have not been measured. Xiong et al.490 prepared a3D-porous Pd−Cu bimetallic film by partial displacing Cu foamdeposited on Au substrate with Pd salts. However, its ORRactivity could not be accurately measured by RDE due to thethick (20 μm) and rough porous structure.Other nanostructures and compositions including sponge-

like PdAu,491 Rh−Pd alloy nanodendrites,522 nanocubes andoctahedra,464 Pd−Co−Ni, and carbon nitride composite472

have been explored.The ORR activity of Pd alloys can be enhanced by proper

supports like metal carbides and oxides. The Shen groupexplored the WC prepared by the intermittent microwaveheating method on carbon black as the support for Pd alloysnanoparticles.493 Pd−Au and Pd−Fe nanoparticles supportedon WC/C showed significant improvement over Pd/Cevidenced by the synergy effect from the support. By addinga small amount of Ce in the synthesis of Pd3Co alloy, theactivity of catalyst was enhanced by 100 mV.494 The promotioneffect of ceria (CeO2) contributed to the electronic interactionbetween PdCo and CeO2 particles, and Pd enrichment on thealloy surface caused by Ce modification. Kwon et al. alsoobserved the synergy effect from ceria on PdCo alloy with apositive shift of the ORR onset potential by 50 mV.495

The stability of most of the binary alloys is poor. By incorpo-rating a more corrosion resistant metal, like Au, Mo, and Mn,the stability of the alloy could be enhanced.496−499 Anotherapproach to improve the durability of Pd-based nanomaterialsis to fabricate nanocomposites with exfoliated montmorillonite(ex-MMT). Wei et al.500 found that the Pd/ex-MMT catalystswere considerably more stable in an acidic solution than Pd/C.The Pd-d states and O(AlO6)-p states of MMT had similarenergy levels. As a result, electrons were easy to transferbetween these levels to form a strong Pd−O(AlO6) bond.During the attack by the Oads, the Pd atoms remain linkedtogether through a Pd−Pd bond and anchored tightly on theex-MMT.The origin of the activity enhancement of Pd alloys has been

studied by several groups. Bard et al.501,502 suggested that forPd−M alloys the reactive metal M constitutes the site forbreaking the O−O bonds, forming Oads that would migrate tothe hollow sites dominated by Pd atoms, where it would bereadily reduced to water. On the basis of this mechanism, thealloy surface should consist of a relatively reactive metal such asCo, and the atomic ratio of this metal should be 10−20% sothat there are sufficient sites for reactions of O−O bondbreaking on M and Oads reduction at hollow sites formed by Pdatoms. Similar thermodynamic guidelines were proposed byBalbuena et al.503 and Savadogo et al.504 However, the reactivemetals are unstable and easily leach from the alloy surface



T


during electrochemical measurements. It will be of interest toconfirm the existence of transition metals on the surface inacidic media. Theoretical calculations and experimental datasuggested that Pd−M alloys underwent Pd segregation, inwhich Pd atoms migrated to the surface to form a pure Pd skinon the bulk alloys upon annealing at elevated temperatures.The Pd lattice contracted upon alloying with 3d metals,generating compressive strain in the Pd skin. For example, thecompressive strain alone weakened the Pd−O binding by0.1 eV for Pd3Fe(111).

505 The electronic structure of the Pdskin is further modified by transition metals via electron transfer(ligand effect). In Pd3Fe(111), the ligand effect further weakenedthe Pd−O binding by 0.25 eV. The combined strain and ligandeffects were the main reasons for the ORR activity enhancementof Pd alloys. Similar results were obtained for the Pd−Co andPd−Ir−Co alloys.506,507

The durability of Pd-based ORR catalysts is their biggestconcern. One way to introduce them into the electrodes ofpractical fuel cells is to modify their surfaces with Pt-basedoverlayers. This approach was discussed in detail in section 5.

7. METAL OXIDES, NITRIDES, OXYNITRIDES, ANDCARBONITRIDES

7.1. Metal Oxides

Many metal oxides, particularly group IV and V metal oxides,are chemically stable in acidic electrolytes and proposed forcatalyst supports to replace carbon. The problems associatedwith them are low electrical conductivity and lack of adsorptionsites for oxygen species on metal oxides’ surfaces resulting inextremely low ORR activity in their bulk form. Extensive effortshave been made to solve these issues by surface modifica-tion, doping, alloying, and forming highly dispersed nano-particles.508−511

The Ota group found that many metal oxides,512−514 includ-ing ZrO2−x, Co3O4−x, TiO2−x, SnO2−x, and Nb2O5−x, preparedby sputtering with respective metal oxide targets in the Aratmosphere showed clear ORR activities in H2SO4 solutions.The introduction of surface defects such as O vacancies insputtering was believed to be the main reason for the enhancedcatalytic activities. The ORR activity on metal oxides may alsodepend on the surface structure. For instance, it was foundthat the ORR activity on Ti oxide catalysts increased with theincrease of the percentage of TiO2 (rutile) (110) plane.515

Takasu et al. prepared TiOx, ZrOx, and TaOx, with corre-sponding binary oxide films on a Ti substrate by a dip-coatingmethod and annealing at temperatures between 400 and500 °C in air.516 The ORR activity of the pure TiOx wasimproved by adding certain amounts of Zr and Ta into TiOx toform binary oxides such as Ti0.7Zr0.3Ox and Ti0.5Ta0.5Ox. Thesame group also tested RuOx, IrOx, RuM (M = La, Mo, V), andIrM (M = La, Ru, Mo, W, V)Ox films prepared by the samemethod.517−519 The dramatic improvement of ORR activity inRu−LaOx and Ir−VOx binary oxides as compared to RuOxand IrOx films in 0.5 M H2SO4 was observed, as shown inFigure 19.517 Note that the similar current density of Ir−VOxfilm and a Pt plate does not mean that these two materials havecomparable activities because the former has a much highersurface area due to high porosity of the coated film. The specificadsorption of (bi)sulfates on Pt surface but not on metal oxidesmight be another reason. The mechanisms underlying theenhanced activity in binary oxides are not clear.

Another approach to improve the catalytic activities of metaloxides is to reduce their crystalline sizes,520 which can increasethe available reaction sites, surface defects, and electricalconductivity. TaOx powders (particle size not characterized)prepared by heat treating the Ta precursors at 450 °C in airshowed high ORR onset potential (0.9 V vs RHE) and currentdensity.516 As compared to bulk TaOx oxides, the Ta 4f7/2 peakof the powder was 0.5 eV lower (26.0 eV vs 26.5 eV),suggesting a lower valence of Ta ions in the powder form.Recently, Seo et al.521,522 were able to deposit highly dispersedfine metal oxides nanoparticles including NbOx, ZrOx, andTaOx on carbon black using a potentiostatic electrodepostionmethod. Because group IV and V metal precursors are insolublein aqueous solutions, the electrodeposition was conducted inethoxide ethanol solutions. These oxide nanoparticles showedmuch higher ORR activities than their bulky particles/filmswith high onset potentials of 0.96 VRHE (NbOx), 1.02 VRHE(ZrOx), and 0.93 VRHE (TaOx), respectively. By adjusting thedeposition conditions, such as deposition potential and time,the particle sizes of the metal oxides could be well controlledranging from 1 to 14 nm. As shown in Figure 20, the massspecific activity increased with decreasing of particle sizes in

Figure 19. ORR−current curves of various RuO2- and IrO2-basedelectrodes supported on Ti substrate: (a) RuO2, (b) IrO2, (c) RuLaO2,(d) IrVO2, and (e) a Pt electrode. Electrolyte is 0.5 H2SO4, sweep rateis 5 mV s−1, and temperature is 60 °C.517 Reprinted with permissionfrom ref 517. Copyright 2009 Elsevier.

Figure 20. Linear sweep voltammograms normalized to the mass ofTaOx nanoparticles deposited on carbon black electrodes with anaverage particle size of 1.0 (a), 2.6 (b), 4.6 (c), and 13.5 (d) nm.522

Reprinted with permission from ref 522. Copyright 2014 Royal Societyof Chemistry.



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TaOx. The enhanced ORR activities with smaller particles aremainly due to increased surface areas, densities of surfacedefects, and electrical conductivities. The changes of the surfacestructures and electronic properties such as oxygen vacanciesand work functions as particles become smaller need morestudies to confirm. The Ota group managed to deposit smallTa2O5 particles (<10 nm) on multiwalled carbon nanotubesusing organometallic complex oxy-tantalum phthalocyanine(TaOC32H16N8) as the precursor.523 The carbon film on themetal oxide particles formed during heat treatment acted as anelectric conducting medium improving the activity of Ta2O5,but also a barrier covering the active sites. Thus, the amountand structure of the carbon film played a significant role indetermining the activity of the metal oxide particles. Similarresults were also found on ZrO2-based particles.524

7.2. Metal Nitrides and Oxynitrides

Metal nitrides nanoparticles have also been investigated asORR electrocatalysts in acidic solutions.525−536 In general, theiractivities are lower than most of the metal oxides discussed insection 7.1. For instance, MoN nanoparticles with an averageparticle size of 4 nm supported on carbon powder showedan onset potential at 0.58 V in 0.5 M H2SO4 at roomtemperature.525 For comparison, the onset potentials of metaloxide particles are higher than 0.9 V. The highest activity so farwas achieved by Isogai et al. with a composite consisting of TiNnanoparticles (6 nm) and carbon nanotubes.537 The TiN/CNTcomposite was synthesized by using a mesoporous graphiticC3N4 hard template, which was a nitrogen source as well. Itshowed a high ORR activity with an onset potential of ∼0.85 V.Doping N in metal oxides forming metal oxynitride may

result in narrowing band gap and hence increasing electricalconductivity due to the hybridization between O and N.538,539

Ta-, Nb-, and Zr-based oxynitride thin films prepared by a radiofrequency magnetron sputtering method in the Ar + O2 + N2atmosphere showed some ORR activity.540 The ionizationpotentials of transition metals in the sputtered oxynitrides werelower than the corresponding bulk ones, suggesting that therewere surface defects in the former. Ota et al. proposed that notonly substitutional doping of N but also creating surface defectswere essential for the enhanced activity. Oxynitrides can alsobe synthesized by nitridation of metal oxides in the flow ofammonia at elevated temperatures.541,542 For example,tantalum oxynitride (TaO0.92N1.05) powders were synthesizedby heating the Ta2O5 powders at 850 °C in ammonia andshowed much higher ORR activity (onset potential = 0.8 V)than that of Ta3N5 (onset potential = 0.4 V).541 ZrOxNy,

543

TiOxNy,544−547 CoxMo1−xOyNz,

548 and HfOxNy549−552 nano-

particles were also synthesized and showed moderate ORRactivities with onset potentials in the range of 0.7− 0.8 V.Domen et al. found that the ORR onset potential of carbonsupported NbOxNy nanoparticles shifted positively by 80 mVfrom 0.78 V for that of NbOx, indicating the role of nitrogen inthe activity improvement.553 The same group also preparedbinary (Ba−Nb−O−N)554,555 and ternary (Ba−Nb−Zr−O−N) oxynitrides555 supported on carbon. The addition ofBa and Zr was found to suppress the formation of Nb4+ andincrease the content of Nb5+ on the surface. The Ba−Nb−Zr−O−N/CB had a very high ORR onset potential of 0.93 V, andthe reaction proceeded primarily via a four-electron transferreaction. Further studies are required to understand the struc-ture and electronic property changes caused by the incorpora-tion of Ba and Zr into NbOx matrix.

7.3. Metal Carbonitrides

Inspired by the high ORR activities of macrocyclic complexesand transition metal−N modified carbons, several groups haveexplored the ORR of transition metal carbonitrides,556

including Fe−C−N,557 Co−C−N,558 Cr−C−N,559 and Ta−C−N560 thin films prepared by sputtering with metal andcarbon targets under N2 and subsequent annealing. It wasfound that the ORR onset potential highly depended on thecontent of N. For instance, the onset potential increaseddramatically from 0.6 to 0.82 V when N content increased from0% to 3% (atomic). The further increase of N content did notchange the ORR activity due to the saturation of active sites onthe surface.509 Even though the ORR activities of carbonitridesare enhanced from their metal nitrides counterparts, they arestill not superior. Partial oxidation may create an interestingcore−shell structure consisting of a metal carbonitride core andan oxide shell.561−566 The Ota group did a systematic study onthe dependence of ORR activities on the degree of oxidation(DOO) of TaCxNy, which was heat-treated with a trace amountof O2 at 1000 °C. The onset potential of ORR increasedsharply with the increase of DOO from 0 (0.55 V) to 0.2 andkept relatively constant at 0.9 V until it was completely oxidizedto Ta2O5.

565 The sharp increase of ORR activity indicated thatthe active sites immediately formed by a slight partial oxidation,and they did not change during further oxidation until thecomplete transformation of TaCxNy to Ta2O5. The results ofthe surface-sensitive conversion-electron-yield X-ray absorptionspectroscopy (CEY-XAS) revealed that the TaCxNy phaseexisted within 30 nm from the surfaces for the DOO below0.15, and the structure in the Ta2O5 phase for the DOO in therange of 0.15−0.96 was very similar.566 The subtle difference inthe coordination number for the shortest Ta−O bond betweenthe partially oxidized TaCxNy (2.4) and the bulk Ta2O5 (2.8)suggested the existence of oxygen vacancies in the former.These oxygen vacancies may act as active sites for oxygenadsorption and electron transfer. A similar activity improve-ment was observed on partially oxidized ZrCxNy with amaximum onset potential of 0.97 V.567−569

The general trend of ORR activities of different metal comp-ounds discussed in section 7 is summarized in Figure 21. Theonset potential of ORR follows a general trend: metal nitrides <metal carbonitrides < metal oxynitrides < metal oxides <partially oxidized metal carbonitrides.

8. METAL CHALCOGENIDESMetal chalcogenides catalysts, mainly Se- and S-based, haveattracted significant attention since Alonso-Vante andTributsch570 discovered that Ru2Mo4Se8 had a ORR activitycomparable to that of Pt in H2SO4.

571−599 Many metals canform chalcogenides with S, Se, and Te, and showed good ORRactivity.

8.1. Noble Metal-Based Chalcogenides

Ru-based chalcogenides are the most extensively investigated inthis category. They were initially synthesized at high temper-ature (1000−1700 °C) and high pressure by using pure Ru andchalcogene elements to achieve a Chevrel phase.600 Later, amore environmental-friendly method involving the chemicalcoreduction of RuCl3·xH2O and SeO2 by NaBH4 at lowtemperature (lower than 80 °C) significantly simplified thesynthesis procedure.601 Impregnation method has also beendeveloped to modify the surfaces of carbon supported Runanoparticles (Ru/C) with Se by mixing Ru/C with H2SeO3



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and subsequent heating at elevated temperatures. Recently,carbon supported highly dispersed RuxSey chalcogenide nano-particles (1.7 nm) were synthesized using RuCl3·xH2O andNa2SeO3 as the Ru and Se precursors in a simple microwave-assisted polyol process.595,602

Bulk Se is a semiconductor. Interestingly, Se atoms becomemetallic when they are coordinated with Ru atoms on thesurface of RuxSey chalcogenide due to the electron transfer fromRu.603 In situ EXAFS experiments have suggested that Ruatoms on the RuSe surfaces act as the catalytically active centersfor oxygen reduction.589,604 Fiechter et al.605 proposed analternative reaction pathway, in which incoming O2 moleculesreact with Se atoms to form −(Ru−SeO3

2−)− units instead ofdirectly adsorbing on the Ru atoms. The −(Ru−SeO3

2−)−units will be further reduced via electrons transferred from Rucores. In either case, the change of d-band center of Ru atomsplayed a sufficient role in the activity enhancement of RuxSey.The ORR activity on a bulk Ru electrode is very low due tothe ready formation of oxygen-containing species at lowpotentials.606 With Se modification, the charge transfer fromRu to Se reduces the oxygen binding energy on Ru and makesit less susceptible to oxidation. Figure 22 shows the ORR

activity as a function of OH binding energy on various surfacesincluding Ru, Co, Rh, and Pd chalcogenides and puremetals.584,607 It suggests that all selenides and sulfides exceptPd0.5Se0.5 (surface composition) are more active than their puremetals due to lowered oxygen binding energies caused by S andSe modifications. The oxygen binding energy on Pd0.5Se0.5 maybe too weak and prevent the adsorption of oxygen specieson Pd surfaces.607 Figure 22 also predicts that Rh-based

chalcogenides, in particular Rh0.5Se0.5, are more active thanRu-based ones, which is in contrast to previous experimentalresults.608,609 Further investigation is needed to clarify thisdiscrepancy.The electrochemical behaviors of RuxSey chalcogenides with

various compositions and synthesized by different methodshave been evaluated. The typical ORR onset potential of RuxSeychalcogenides is around 0.85 V in the RDE measurements inH2SO4 solution at 80 °C. Depending on the catalyst loading,the half-wave potential is around 0.65 V. The highest half-wavepotential at 0.77 V was observed on RuxSey prepared by core-duction of RuCl3 and SeO2 with a Ru loading of 56 μg cm

−2.601

Even though the ORR activity is improved by ∼200 mV ascompared to pristine Ru nanoparticles, it is still at least100−150 mV lower than Pt/C, as shown in Figure. 23. The

activation energies of 18−24 kJ mol−1 at 0.7 V for theRuxSey catalysts were found to be comparable to that of Pt/C(18 kJ mol−1).610 The strong metal−support interaction(SMSI) effect was also observed when RuxSey nanoparticleswere loaded on metal oxides, such as TiO2. The half-wavepotential can be shifted positively by as much as 20 mV in thepresence of TiO2. The mechanisms of how the metal oxideschange the electronic properties of RuxSey and consequentlyenhance the ORR activity deserve further study. Kulesza et al.611

also found that the H2O2 decomposition rate was accelerated bymodifying the RuxSey with WO3. Thus, novel nanocompositesconsisting of additives that can further tuning the electronicproperties of Ru atoms are promising.Se is essential to modify the electronic properties of

Ru atoms. The inherent activity of Ru atoms continuouslyincreases with increasing Se content due to stabilization of theRu surface toward oxidation by Se modification. The possibilityof forming new active phases with higher Se contents can notbe excluded. However, the ORR activity of RuxSey is not simplyproportional to the Se/Ru ratio. It is easy to understandbecause Se atoms are not directly involved in the oxygenadsorption and not the active sites for ORR. If the Se coverageis too high on the surface, there are not many Ru active sitesavailable for reaction. The overall activity is the result ofbalancing between the specific activity of Ru active sites and theavailability of these sites. A volcano-type dependence of the

Figure 22. ORR activity on Ru, Rh, and Co modified by selenium andsulfur (square symbols). Pure metals are indicated as circles.607

Reprinted with permission from ref 607. Copyright 2011 Elsevier.

Figure 23. ORR current−potential curves obtained on RDE in 0.1H2SO4 solutions at 1600 rpm. (a) CoSe2/C (50 wt %; 28 mg cm−2;0.5 M H2SO4), (b) Ru/C (20 wt %; 56 mg cm−2), (c) RuxSey/C(20 wt %; 56 mg cm−2), and (d) Pt/C (10 wt %; 56 mg cm−2). Scanrate = 5 mV s−1.583 Reprinted with permission from ref 583. Copyright2011 Springer.

Figure 21. Comparisons of onset potentials of oxides, nitrides, oxynitrides,carbonitrides, and partially oxidized carbonitrides in acid solutions.



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ORR activity on the Se/Ru ratio has been reported by severalgroups.517,601,610 The maximum mass activity was observedwhen the Se/Ru ratio was in the region of 15−37%.To reduce the material cost, more abundant transition metals

have been introduced into the Ru-based chalcogenides to formRuxMySez (M = Cr, W, Mo, Fe, Ni).575,585,586,596,612−614 TheORR activity was improved by 50 mV by adding a very smallamount (2−3 at. %) of Mo in the Ru−Se/C catalyst in RDE.The H2/O2 fuel cell performance of Ru−Mo−Se/C is about3-fold higher than that of Ru−Se/C with a maximum powerdensity of 300 mW cm−2 at 80 °C and 400 kPa (0.5 mgRucm−2).612 In addition, a much higher stability was achievedfor Ru−Mo−Se/C. Its half-wave potential did not shift after1000 cycles between 0.7 and 0.9 V in 0.1 M H2SO4 solutions,while the binary potential decreased by 70 mV in RDEmeasurements. These results suggested that the presence of Mocould help stabilize Se atoms in the catalyst possibly due to astronger Mo−Se bond than Ru−Se. Other ternary composi-tions including RuxCrySez and RuxWySez had lower perform-ances than the typical RuxSey catalyst and followed the trend ofRuxMoySez > RuxCrySez > RuxWySez.

613 When the Ru−Se/Ccatalyst was diluted with Fe, the activity was unchanged orslightly higher than the origin with Fe content as high as50%.585 So far, the maximum power density for Ru−Fe−Se/Cwas 140 mW cm−2 at 80 °C and 250 KPa (0.4 mgRu cm

−2). Thestability of Ru−Fe−Se/C during potential cycling is a bigconcern due to significant Fe dissolution leading to an activitydrop together with an increased H2O2 yield.

585

Even though RuxSey catalysts showed good ORR activity, thetoxicity of Se remains one of the concerns for this type ofmaterials. The feasibility of the RuxSy as an alternative to RuxSeyhas been studied. It was concluded that the ORR activityimprovement of the former over bare Ru is very limited(∼20 mV).589 The synthesis and evaluation of other noblemetal-based chalcogenides have also been carried out, eventhough they were not as intensive as Ru-based ones. Figure 22predicts that Rh-based selenides and sulfides were more activethan pure Rh and comparable to Pt. However, Cao et al.608

demonstrated that Rh particles modified with Se and S showedmuch worse activity as compared to the unmodified particles.The Se and S atoms most likely poisoned the catalysts ratherthan enhanced the activity by covering the active Rh sites.Ir-based selenides and sulfides have also been synthesized bydifferent methods. The modification of Ir particles with Se andS did not change the activity dramatically (within 10−20 mV)in RDE measurements.615 In another study, Ir85Se15/C with anaverage particle size less than 2 nm synthesized via amicrowave-assisted polyol process showed the best fuel cellperformance among the chalcogenide catalysts, with amaximum power density of 500 mW cm−2 at 80 °C and200 kPa (0.4 mgIr cm−2).616 The maximum power densityof the Ir85Se15/C catalysts was further improved by 80% byadding Nafion in the synthesis, possibly due to a higher activesurface area through better particle size control and catalystutilization.617

8.2. Non-noble Metal-Based Chalcogenides

The family of NPM (Co, Ni, Fe)-based chalcogenides has beenstudied as ORR catalysts for more than four decades.618−627 Ascompared to Ru and Rh, these transition metals have a lowercost and higher abundance on earth. A general conclusioncan be drawn on the basis of the previous studies that theORR activity follows the trend MxSy > MxSey > MxTey.

628

In addition, the activity also depends on the catalytic centerswith Co, and its alloys are superior to others. Both theoreticaland experimental results confirmed that Co selenides were lessactive than its sulfides by ∼0.2 V.629 Two kinds of crystalstructures having different activities were discovered on CoSe2by Alonso-Vante and co-workers. An orthorhombic structurewas observed at a heat treatment temperature of 250−300 °C,while a cubic structure was present at high temperatures (400−430 °C). The latter had a higher ORR activity by 30 mV inH2SO4.

620 Despite intensive development and recent advances,the activities of NPM-based chalcogenides are still significantlylower than that of RuxSey. As shown in Figure 23, the half-wavepotential of CoSe2 is about 200 mV lower than that of RuxSey.

9. CARBON-BASED NON-NOBLE METAL ANDMETAL-FREE CATALYSTS

Another possible replacement for Pt at the cathode of PEM fuelcells with Fe-based catalysts has indeed shown interestingproperties for the electroreduction of O2 in acid medium. Asseen in Figure 1, Fe is a cheap metal, and its production iscurrently the most abundant of all metals.1 The Fe-basedcatalysts are said to be of the type Fe/N/C because they areproduced by the pyrolysis of an iron precursor, a nitrogenprecursor, and a carbon support at high temperature. The lattermay eventually be replaced by a carbon precursor as well.Co-based catalysts of the type Co/N/C and catalysts based onCo and Fe (Co−Fe/N/C) have also shown interesting proper-ties, even if, according to Figure 1, Co is a much less commonmetal than Fe. Several book chapters and review articles havealready been written about Fe- and/or Co-based catalystsmade to perform the oxygen reduction reaction (ORR) in fuelcells.11,630−638 In this literature, it is also reported that, besidesthe non-noble metal-based catalysts, some metal-free catalystsmade of carbon doped with several heteroatoms (the mostcommon one being nitrogen) may also reduce O2 electro-chemically in acid medium.639−644

To replace Pt with a non-noble catalyst at the cathode of aPEM fuel cell, it is necessary that the catalyst fulfills thefollowing three requirements: (i) be able to provide a powerequivalent to that provided by Pt in the low power regime ofthe fuel cell; this requirement essentially depends on the ORRkinetics at the surface of the non-noble catalyst; (ii) be able toprovide a power equivalent to that provided by Pt when the fuelcell delivers the work for which it has been designed (forinstance, replace the internal combustion engine in an auto-mobile by providing the necessary power to an electricalmotor); and (iii) demonstrate a stability of at least 1000 h whenthe fuel cell is running at useful power. In this Review, we willnot discuss the kinetics of oxygen reduction at the surface ofnon-noble catalysts, nor the structure of the active center(s) asthese topics have already been covered in many of the previousreviews. Instead, we will focus on PEM fuel cell results only andwill (i) review what has been published recently on the use ofnon-noble (metallic or metal-free) catalysts at the cathode ofPEM fuel cells; (ii) document their performance and stability infuel cells; and (iii) compare them with those of Pt.A series of papers published after 2006, all presenting the

synthesis of non-noble catalysts, their characterization, and theiruse in PEM fuel cells, at the beginning of life of the catalysts,were selected.639,645−691 The stability in fuel cells has also beenstudied for some of these catalysts.682,692−737 The variables ofsuch studies are multiple. Most of the catalysts are synthesizedaccording to novel procedures. The inks used to prepare the



X


membrane-electrode-assemblies (MEAs) are made according tovarious recipes, especially with regards to the weight ratio of theionomer to the catalyst loaded at the cathode. Furthermore, thethickness of the ionomer membrane, the fuel cell temperature,the nature of the gases (H2/O2 or H2/Air), their humidity, andpressure are also variables. When the non-noble catalysts aretested for their stability in fuel cell, other variables are added tothose already enumerated above for the measurement of theirinitial fuel cell performance. For instance, durability experi-ments are not necessarily performed with the same gases (andthe same experimental conditions) as those used to determinethe catalyst’s performance. Furthermore, stability tests may berun for different time lengths, either at constant potential or atconstant current density, and the values of the potential or thecurrent density chosen may vary from one catalyst to another.It is therefore difficult to compare published performance anddurability results among non-noble metal catalysts.9.1. Initial Performance in H2/O2 PEM Fuel Cells

The emergence of several general trends in all reported resultsis seen by first ordering all of these catalysts according to theirinitial maximum performance (expressed in W cm−2) in H2/O2PEM fuel cells (a value very often available from the publishedresults). This is illustrated in Figure 24, where the abscissa,

divided into 0.05 W cm−2 peak power increments, reportsinitial peak powers of 0.05−0.10 W cm−2 to 0.95−1.00 W cm−2

for all catalysts used at the cathode of H2/O2 fuel cells. Eachindividual rectangle in Figure 24 represents the initial maximumperformance for one catalyst. A column made with severalrectangles is obtained when several catalysts are characterizedwith an initial maximum performance falling into the samespecific power increment. For instance, 12 catalysts were reportedto have an initial peak power between 0.10 and 0.15 W cm−2, butonly one was reported to have an initial peak power between0.70 and 0.75 W cm−2. A list of non-noble cathode catalysts forH2/O2 fuel cells, with their associated review code, their initialpeak power, and their reference, is provided in Tables 5 and 6.The distribution of initial peak powers in Figure 24 seems to

follow a Poisson distribution with a maximum at rather lowinitial peak power and a decreasing probability for obtaininghigher initial peak powers. All blank rectangles and blank

rectangles marked (MOF) in Figure 24 represent non-noblecathode catalysts that were made with a nitrogen precursor anda Co and/or Fe precursor. Their ORR properties are mainlyattributed to metal-containing active sites (usually labeled FeNxor even FeN4 or their Co equivalent). These active site struc-tures are derived from experimental evidence and supportedby theoretical considerations.640,738,739 A typical example ofsuch catalyst is obtained as follows:676 Fe-8CBDZ-DHT-NH3was prepared by a sacrificial support method. Carbendazin(a nitrogen precursor) and iron nitrate (the iron precursor) in a8:1 ratio and for a nominal iron loading of 15 wt % were addedto a dispersion of fumed silica in water. After evaporation ofthe solvent, the obtained solid was then subjected to a firstheat-treatment for 1.5 h at 800 °C under N2 atmosphere. Theresulting material was leached by 25 wt % HF overnight toremove the silica sacrificial support and the leachable metal.After being rinsed with DI water, the material was thensubjected to a second 30 min heat-treatment, at 950 °C underNH3 atmosphere, to obtain Fe-8CBDZ-DHT-NH3. The catalysthas a Fe content of 0.1−0.3 at. % (determined by XPS) and atotal N content up to 7.7 at. % (also determined by XPS). TheORR catalytic activity of Fe-8CBDZ-DHT-NH3 is attributed toFeNx centers formed during the heat-treatments. This catalysthas an initial peak power of 0.56 W cm−2 in H2/O2 fuel celland appears therefore in Figure 24 as a blank rectangle between0.55 and 0.60 W cm−2.All light-blue rectangles in Figure 24 represent catalysts that

were made with a nitrogen precursor and with either Co and/orFe precursors, but for which, according to their authors, theORR properties do not derive from metal-containing sites, butare the result of the nitrogen-doped carbon obtained duringtheir synthesis. In this case, all of the ORR active sites in thesecatalysts, according to the authors, are of the CNx type, and themetals are only intermediates to initiate, during the pyrolysisstep, larger CNx concentrations in the catalyst. The metals usedduring the synthesis of the catalysts and identified as Co or/andFe in the rectangle that represent each of these catalysts inFigure 24 are, according to their authors, completely leached atthe end of the synthesis or are inaccessible to O2 behind aprotecting carbon layer. A typical example of such catalyst isobtained as follows:702 NMCC-SiO2-800-3 was prepared by asacrificial support method. Ethylenediamine (EDA; the nitro-gen precursor) was added to a solution of cobalt nitrate andiron nitrate (the metal precursors) in isopropyl alcohol tocreate Co−Fe−EDA complexes. Next, silica was added to themixture. After evaporation of the solvent, the resulting powderwas pyrolyzed for 1 h at 800 °C in Ar atmosphere. Thepyrolyzed sample was washed with NaOH to remove silica andwith H2SO4 to remove surface metal. The resulting materialwas pyrolyzed a second time for 3 h at 800 °C in Ar to obtainNMCC-SiO2-800-3. A similar catalyst, NMCC-C-800-3, wassynthesized by using carbon black (Ketjenblack EC-300)instead of silica. In this case, the carbon support cannot beremoved. Both NMCC-SiO2-800-3 and NMCC-C-800-3 haveidentical nitrogen-containing groups identified by XPS. Theonly difference between these catalysts is the total nitrogencontent (5.93 and 1.64 at. %, respectively). Metal particlesformed during the synthesis are encapsulated within the carbonsubstrates. The authors conclude that the metal (Fe or Co) inthe final NMCC catalysts is likely not part of the active sites.Hence, the nitrogen doped in the carbon matrix is presumably atthe origin of the main active site for ORR. NMCC-SiO2-800-3 ischaracterized by an initial peak power of 0.45 W cm−2, while the

Figure 24. Initial peak power for non-noble catalysts tested in H2/O2PEM fuel cells. The nature of the main active site at work in thesecatalysts has been proposed by their authors.



Y


initial peak power of NMCC-C-800-3 is 0.35 W cm−2. Only thebest catalyst was used to build Figure 24, where it appearsas a light-blue rectangle (marked Co Fe) between 0.45 and0.50 W cm−2.

All dark-blue rectangles in Figure 24 represent catalysts thatare made without any Co or/and Fe precursors, but only withnitrogen precursor and carbon (or carbon precursor). Here,according to the authors, the only sites able to reduce oxygen inthese catalysts are CNx sites. A typical example of such catalystis obtained as follows:698Ketjenblack (EC-300) was prewashedwith HCl to remove metallic impurities and then chemicallyoxidized with HNO3. Urea formaldehyde (UF) and melamine(two nitrogen precursors) were condensed onto the function-alized Ketjenblack. The resulting material was then heat-treatedfor 1.5 h at 800 °C under N2 atmosphere to obtain UF-C.The amount of N detected by XPS in UF-C was 2.2 wt %.No Fe nor Co was detected in the catalyst by ICP-MS. Theinitial maximum peak power of UF-C in H2/O2 fuel cell is0.18 W cm−2, which appears in Figure 24 as a dark-bluerectangle between 0.15 and 0.20 W cm−2.It is obvious from Figure 24 that the average initial peak

power developed by the catalysts, represented by dark-bluerectangles, is lower than the average peak power of the catalysts,represented by light-blue rectangles. The fact that, accordingto the authors, only CNx sites are ORR active in the lattercatalysts has been challenged in a recent review640 where it issaid that, even if catalysts represented by light-blue rectangleshave more CNx sites to reduce O2 than catalysts represented bydark-blue rectangles, the ORR activity of CNx sites is rather low(more than 1 order of magnitude in solution for CNx sites ascompared to FeNx or CoNx sites). Therefore, CNx sitesalone cannot account for the power difference seen in Figure 24between light- and dark-blue rectangles and their associatedcatalysts. Instead, it is proposed that the extra performance ofcatalysts represented by light-blue rectangles has its origin in alow concentration of FeNx or CoNx sites that appear in thesecatalysts as the result of the use of these metal precursorsduring their synthesis. Indeed, acid leaching of the reducedmetallic species obtained during the high temperature heat-treatment is never fully complete as some metallic ions adsorbon the carbon support during the leaching step instead of beingrinsed away from these catalysts, hence becoming equivalent to

Table 5. List of Non-noble Cathode Catalysts for H2/O2 FuelCells, Giving Their Code Used in This Review, Their InitialPeak Power, and Their Reference

polarization curve only polarization curve and durability

codeinitial max power

(W cm−2) ref codeinitial max power

(W cm−2) ref

08#22 0.12 645 07#08 0.06 69208#37a 0.15 646 07#32 0.12 69308#37b 0.11 646 08#03e 0.12 69408#54c 0.33 647 08#03f 0.23 69408#54d 0.51 647 08#15 0.53 69608#61 0.26 649 08#34 0.09 69709#03 0.16 651 09#06 0.18 69809#22 0.05 652 09#34 0.36 69909#36 0.36 653 09#74 0.44 70010#10 0.21 654 10#01 0.14 70110#28 0.13 655 10#11 0.45 70210#29 0.38 656 10#29 0.44 70310#76 0.20 657 11#39 0.30 70510#98 0.18 658 11#73 0.35 70611#64 0.36 660 11#79 0.42 70711#69 0.16 661 11#87 0.33 70811#127 0.15 662 11#97 0.34 70912#70 0.35 663 11#109 0.12 71012#100 0.26 664 11#137 0.55 71112#147 0.48 665 11#138 0.91 71212#156 0.30 666 12#15 0.37 71312#160 0.42 667 12#44 0.28 71412#180 0.30 668 12#79 0.27 71513#96 0.08 670 12#104 0.29 71613#113 0.32 671 13#02 0.05 71713#152 0.33 672 13#02 0.10 71713#206 0.77 673 13#23 0.32 72014#01 0.64 674 13#30g 72114#12 0.15 675 13#124 0.73 72214#31 0.56 676 14#04 0.62 68214#56 0.10 677 14#38 0.20 72314#157 0.35 679 14#45 0.20 72414#184 0.46 681 14#84 0.36 72514#217 0.62 682 14#97 0.17 72615#07 0.30 683 14#114 0.60 72715#28 0.12 684 14#133 0.24 72815#30 0.46 685 14#151 0.14 72915#51 0.60 686 14#nic 0.47 73015#58 0.17 687 14#214 0.98 73115#59 0.07 688 15#01 0.17h 73215#61 0.49 689 15#01 0.23i 73215#62 0.46 690 15#11 0.18 733

15#54 0.90 73415#60 0.94 73515#63 0.11 73615#66 0.23 737

a08#37: Backpressure = 200 kPag. b08#37: Backpressure = 0 kPag.c08#54: Nafion 117 membrane. d08#54: Nafion 112 membrane.e08#03: Backpressure = 0 kPag. f08#03: Backpressure = 200 kPag.g13#30: No polarization curve was provided in this paper, only adurability curve. h15#01: For N-G-CNT, 0.5 mg cm−2 + KB 2 mgcm−2. i15#01: For N-G-CNT, 2 mg cm−2 + KB 2 mg cm−2.

Table 6. List of Non-noble Cathode Catalysts for H2/AirFuel Cells, Giving Their Code Used in This Review, TheirInitial Peak Power, and Their Reference

polarization curve only polarization curve and durability

codeinitial max power

(W cm−2) ref codeinitial max power

(W cm−2) ref

08#37a 0.11 646 08#03 0.05 69408#37b 0.08 646 08#13 0.10 69508#56 0.14 648 10#84c 0.14 70408#64 0.07 650 10#84d 0.18 70411#42 0.25 659 11#109 0.06 71011#69 0.07 661 11#137 0.14 71113#206 0.30 673 13#11 0.33 71814#112 0.30 678 13#18 0.16 71914#162 0.26 680 14#84 0.18 72515#52 0.38 691 14#114 0.32 727

14#151 0.07 72914#nic 0.20 73014#214 0.41 73115#11 0.07 733

a08#37: Backpressure = 200 kPag. b08#37: Backpressure = 0 kPag.c10#84: In Figure 2b of the original article. d10#84: In Figure 4a of theoriginal article.



Z


load a metallic precursor on the already N-doped carbon struc-ture. Furthermore, in many cases for the catalysts representedby light-blue rectangles, there is even a last pyrolysis stepperformed after acid leaching during their synthesis. This isideal to activate FeNx or CoNx sites from the adsorbed iron orcobalt ions and the nitrogen functionalities of the N-dopedcarbon structure, adding therefore an additional activity to thatof the CNx sites already present at the surface of these catalystsrepresented by light-blue rectangles. Even for catalysts thatare represented by dark-blue rectangles and are supposedlymade with nitrogen and carbon precursors devoid of Fe, it isimportant to stress that iron is a very common impurity presentat the ppm level in many chemicals, and it may be un-intentionally added during the catalyst synthesis. This is not thecase for the example of synthesis reported above, because zerometal content was found by ICP-MS, but it could be the casefor the most performing dark-blue catalysts. Only very powerfulanalytical techniques like neutron activation analysis (NAA) orinduced coupled plasma (ICP) analyses are able to detect thesetraces of iron in the otherwise reputed metal-free CNx-basedcatalyst.740

Finally, there are four yellow rectangles in Figure 24 thathave not yet been identified. They are characterized byencapsulated metal particles that are, according to their authors,at the origin of the ORR activity of these catalysts. The firstexample is a catalyst containing pea-pod-like carbon nanotubeswith enclosed metal particles and N-doped carbon walls. Theywere synthesized according to the following procedure:717

Ferrocene (the iron precursor) and sodium azide (the nitrogenprecursor) were reacted at 350 °C in N2 atmosphere in astainless-steel autoclave. The resulting sample (FeOx/Pod-Fe)was leached with HCl to remove FeOx particles on the outsidewall of the carbon nanotubes and yield the Pod-Fe catalyst. Itcontains 12.8 wt % Fe detected by ICP and 0.8 wt % Ndetected by XPS. Its initial peak power is 0.05 W cm−2. Thiscatalyst is represented by a first yellow rectangle surrounded bya thick black line in Figure 24. A second version of this type ofcatalyst, but this time containing more N than Pod-Fe, wasobtained by heating Co2Fe(CN)6 for 2 h at 600 °C in Ar. Theresulting sample (Pod(N)-FeCo) was also leached with HCl toproduce Pod(N)-FeCo, which is also a pea-pod-like carbonnanotube containing 7.9 wt % Fe, 8.2 wt % Co (both measuredby ICP), and 3.3 wt % N (measured by XPS). The initial peakpower of Pod(N)-FeCo is 0.10 W cm−2. It is represented by thesecond yellow rectangle surrounded by a thick black line inFigure 24. According to the authors and their DFT calculations,the ORR activity of these catalysts arises from electron transferfrom Fe (or Co) encapsulated particles to the carbon nanotubeleading to a decrease of the local work function on the carbonsurface and resulting in some interaction with O2. The latter isactivated and reduced on the carbon nanotube outer wall.N-doping of the carbon nanotube walls would enhance thiseffect. So far, however, powers obtained using this procedureare quite low as seen in Figure 24.Several catalysts at the high end of the initial peak power

have been identified as MOF (for metal−organic framework).MOF is a material made by repetition in three dimensions of(−metal−organic−metal−) chains. The use of MOFs in thesynthesis of cathode catalysts for ORR will be discussed later.The initial peak power representation of Figure 24 is now

used to determine the influence of (i) the catalyst loading at thecathode, (ii) the fuel cell temperature, (iii) the backpressure ofO2 at the cathode, and (iv) the membrane thickness, all of them

being important variables identified for the fuel cell tests. Figure 25Apresents the influence of the cathode non-noble catalyst loadingon the initial peak power for H2/O2 fuel cell tests. The colorintensity of each individual rectangle increases incrementallywith the catalyst loading. Note that, for each power incrementin Figure 25A, there is no relation between the order of thecatalysts in each column in Figure 25A and the catalyst locatedat the same place in the equivalent column in Figure 24. Forinstance, the catalyst identified by a light-blue rectangle at thebottom of the fifth column (from 0.25 to 0.30 W cm−2) inFigure 24 is not necessary the same as that loaded between8 and 10 mg cm−2 at the bottom of the fifth column inFigure 25A. The only common point between both catalysts isthat their initial peak power is in the range of 0.25−0.30 W cm−2.This remark will also apply to the other panels of Figure 25.From Figure 25A, one sees that the most common catalyst

loading at the cathode of the MEAs using non-noble catalystsranges between 3.8 and 4.1 mg cm−2 (a blue rectangle with awhite spot). As this loading is usually obtained by buildingseveral layers of catalyst on top of each other, the thicknessof the cathode may become important. When each layer ofcatalyst containing 1 mg cm−2 of catalyst has a thickness ofabout 20−25 μm, the cathode becomes 80−100 μm thick. Thisis much larger than cathode layers using high Pt loadings oncarbon, which enable one to get about 10 μm (and even lower)thick cathodes,2,28,741 but this higher loading is necessary fornon-noble catalysts to compensate for their ORR activity lowerthan that of Pt. Doing so, it has been shown that, at low power,a non-noble catalyst is able to deliver as much current as Pt/Cin H2/O2 PEM fuel cell.700 Of course, such a thick electrodenecessarily involves mass transport problems at the cathodewhen the cathode will deliver currents larger than 0.1 A cm−2.It is therefore important to develop non-noble catalysts thatwould roughly be 10 times more active to be able to reduce thecathode thickness.Figure 25B presents the temperature at which fuel cell mea-

surements were recorded for all of the catalysts of Figure 24.Evidently, 80 °C is the most popular temperature. It is also thetemperature suggested by the DOE for fuel cell experiments.742

As expected, lower temperatures yield lower initial peak powers.Figure 25C presents the back-pressures used at the cathode ofH2/O2 PEM fuel cells. The most popular back-pressures arecomprised between 150 and 200 kPa (1.5−2.0 atm). Here,lower back-pressures (100−138 kPa) do not necessarily meanlower initial peak performance as evidenced by the six catalystsproviding the highest peak powers. However, no or very lowback pressures (0−30 kPa) seem to lead to low peak powers.The last panel of Figure 25 presents the effect of membranethickness used with each catalyst on the peak power initiallydeveloped in H2/O2 fuel cell by this catalyst. It is obvious fromFigure 25D that higher peak powers are obtained with thinnermembranes. The most popular membrane seems, however, tobe about 50 μm thick. Another parameter that would be veryinteresting to plot versus maximum peak power would be thecatalyst to ionomer ratio of the inks used to obtain the MEAcathode layers. There was, however, not enough data reportedin the literature about this parameter to obtain valuablestatistics.

9.2. Synthesis of Non-noble Metal Catalysts

We previously said that, to obtain a Me/N/C catalyst that willhave some ORR properties, we need to start from a metalprecursor (either Fe or/and Co), a nitrogen precursor, and a



AA


carbon support (which can also be replaced by a carbonprecursor). A high temperature pyrolysis of the mix of all ofthese precursors is, however, necessary to obtain the catalyticsites that will be able to perform the oxygen reduction reactionin the acid medium of PEM fuel cells. Noticeably, after lookingto Figure 24, one may conclude that some precursors andsynthesis procedures are able to yield catalysts in a large rangeof initial peak powers. So the question is: How can we prepare acatalyst that is highly performing at the beginning of its life?In Figure 24, several high peak power catalysts were made

with MOFs, which are essentially used as highly porous carbon(and nitrogen) precursors. It means that no carbon black wasinitially involved in the synthesis of these catalysts. To find outif there are other hints leading to highly performing catalysts,we will herein describe the synthesis procedure used for12 selected catalysts appearing in Figure 24, starting from themost performing one and decreasing in performance until a lastcatalyst yielding an initial peak power of 0.35 W cm−2 isreported. The elemental analysis of each catalyst, its porosity,and its content in ink used to prepare the cathode aresummarized in Table 7 (when available) to find out whetherthere are common points and practices that are preferred toobtain performing catalysts.(1) The highest performing catalyst is PAN-Fe03-

1000NH3.731 It was made by dispersing polyacrylonitrile

(PAN) in a solution of FeCl2·4H2O in tetrahydrofuran. Theinitial Fe loading was 0.3 wt % to obtain PAN-Fe03. Theresulting material was pyrolyzed at 600 °C for carbonization.It then was ball-milled for dispersion. The dispersed sample was

first activated at 800 °C, then at 1000 °C in NH3 to become acatalyst. The latter was characterized by an initial peak power of0.98 W cm−2.(2) The second best performing catalyst is Fe/N/C-SCN.735

It was prepared on carbon black (KJ600) that was firstfunctionalized with sulfophenyl groups. Polymethylphenylene-diamine (PmPDA) was coated on the functionalized carbonblack through oxidative polymerization of mPDA monomer by(NH4)2S2O8. After Fe(SCN)3 was added, the resulting materialwas subjected to the sequence of a first pyrolysis step at 950 °Cunder Ar, acid leaching, and a secondary pyrolysis step again at950 °C under Ar to obtain the catalyst. Its initial peak powerwas 0.94 W cm−2.(3) The third best performing catalyst is NC Ar + NH3.

712

It was obtained by mixing ZIF-8 (a commercial ZnII zeoliticimidazolate framework of formula ZnN4 C8 H12)

743 with 1,10-phenathroline and ironII acetate in methanol and water. Thedried slurry was then ball-milled (planetary) in N2 in a steel vialwith chromium-steel balls. The resulting material was first heat-treated at 1050 °C in Ar, then a second time at 950 °C in NH3

to give NC Ar + NH3 characterized by an initial peak power of0.91 W cm−2.(4) The fourth best performing catalyst is Fe/N/CF.734 It

was prepared by electrospinning a mixture in dimethylforma-mide (DMF) of PAN, polymethylmethacrylate, and ZIF-8 ball-milled with tris-1,10-phenanthroline iron(II) perchlorate. Thisprecursor mixture was electrospun to form polymeric nano-fibers. The latter were cured in air at 200 °C before being firstheated in flowing Ar at 1000 °C, then submitted to a flow of

Figure 25. Influences of various parameters on the initial peak power for non-noble catalysts tested in H2O2 PEM fuel cells: the cathode loading (A),the fuel cell temperature (B), the back-pressure (C), and the membrane thickness (D).



AB


Table

7.Characterizationof

Perform

ingNon

-nob

leCatho

deCatalysts

Usedin

H2/O

2PEM

Fuel

Cells

elem

entalanalysiswt%

(orat.%

)porosity

(m2g−

1 )

code/initialpeak

power

(Wcm

2 )catalyst’sname

onlyCprecursor(s)

insynthesis

Cblack(orsimilar)

insynthesis

NFe

Co

other

BET

micropores

mesopores

inkcompositio

n(w

t%

cat.−

wt%

Nafion)

114#214

0.98

PAN-Fe03-1000NH3

yes(PAN)

no(3.22)

3.7

(0.24)

1.11

1096

??

?

215#600.94

Fe/N

/C-SCN

noyesKJ600

4.4

1.4

S2.1

751

414

337

50−50

311#138

0.91

NC

Ar+NH

3yes(M

OF)

no(5.3)6.0

(0.78)

3.51

Zn(0.01)

0.05

964

814

184

40−60

415#540.90

Fe/N

/CF

yes(PAN

+PM

MA)

no9.03

0.56

Zn?

809

780

840−60

513#124

0.73

PFeT

PP-1000

yes(POP)

no1.92

4.95

S1.14

758

mostly

?50−50

614#040.62

Zn(mIm

) 2TPIP

yes(M

OF)

no2.22

1.07

Zn1.86

1277

??

?

714#114

≥0.60

Fe(PI-1000-III-N

H3)

yes(PI-composite)

no3.1

1.1

1050

844

206

63−37

814#31≥0.56

Fe-8

CBDZ-DHT-

NH

3

yesa

+sacrificialSiO

2no

(7.7)8.9

(0.3)1.4

400−

600

??

55−45

911#137

0.55

PANIFeCo-C

noyesKetjenblack

??

??

??

65−35

1008#150.53

C-com

posite-

Co 3Fe

1Nx

noyes

3.9

1.4

4.6

??

?75−25

1108#540.51

catalyst-900

noyesKetjenblack

(0.70)

0.80

(0.66)

3.00

??

?77−23

1210#110.45

NMCC-SiO

2-800-3

yesa

+sacrificialSiO

2no

(5.93)

6.85

??

??

?75−25

0.35

NMCC

C-800−3

noKetjenblack

(1.64)

1.91

??

??

?75−25

aSeedetails

aboutthenature

ofthecarbon

precursorin

thesynthesisdescrib

edin

thetext.



AC


NH3 at 900 °C to produce a material that was acid washed.The dried sample was heat-treated again at 700 °C in NH3to produce a final catalyst with an initial peak power of0.90 W cm−2.(5) The next best performing catalyst is PFeTPP-1000 with

an initial peak power of 0.73 W cm−2.722 It was obtained fromFeTPP POP, where POP is a porous organic polymer madefrom the 3D polymerization of repetitive units containing afunctionalized iron tetraphenylporphyrin. The catalyst pre-cursor was heat-treated at 1000 °C in N2.(6) The following catalyst is Zn (mIm)2TPIP with an initial

peak power of 0.62 W cm−2.682 Here, ZIF-8 was synthesizedfrom ZnO and 2-methyl imidazole in an autoclave at 180 °C for18 h. The iron precursor was (1,10-phenanthroline)3 ironperchlorate. The resulting material was pyrolyzed at 1050 °C inAr. It then was acid leached before being pyrolyzed a secondtime at 950 °C in NH3.(7) The next catalyst is Fe/PI-1000-III-NH3 with an initial

peak power of ≥0.60 W cm−2.727 Here, the symbol ≥ is used asthe reported polarization curve of that the catalyst stops at0.6 V and 1.0 A cm−2. A polyimide (PI) was first obtained byreacting pyromellitic acid dianhydride with 4,4′-oxidianiline.A composite material then was made using −PI−/Fe(acac)3,where acac is acethylacetonate. The resulting material was firstpyrolyzed at 600 °C in N2, then at 800 °C in 50% NH3, thenagain at 1000 °C in 50% NH3.(8) The following catalyst is Fe-8 CBDZ-DHT-NH3 with an

initial peak power of ≥0.56 W cm−2.676 Again, the polarizationcurve stopped at 0.4 V and 1.4 A cm−2. The synthesis of thiscatalyst was already described as a typical example of catalystwhose ORR activity resulted from the presence of FeNx sites.(9) The next catalyst is PANI FeCo-C with an initial peak

power of 0.55 W cm−2.711 This catalyst used Ketjenblack[EC-300] as carbon support. The carbon black was first treatedwith HCl to remove its metal impurities. It then was mixed withaniline oligomers, cobalt nitrate, and iron chloride, as well aswith ammonium persulfate. The latter chemical was used topolymerize aniline and obtain PANI onto the carbon support.The resulting material was first pyrolyzed at 900 °C in N2,leached with H2SO4, then pyrolyzed a second time in N2 toobtain PANI FeCo-C.(10) This catalyst labeled C-composite-Co3Fe1Nx is

characterized by an initial peak power of 0.53 W cm−2.696

Carbon black was first treated with HCl to remove metalimpurities before being refluxed with HNO3 to introduceO- and N-bearing functional groups on the carbon. Theoxidized carbon surface was first modified by reaction withnitrogen-containing organic precursors followed by thedeposition of Co and Fe complexes with ethylene diamine.The resulting material was first heat-treated in Ar, then leachedin H2SO4.(11) The following catalyst, labeled catalyst-900, is

characterized by an initial peak power of 0.51 W cm−2.647 Toprepare this catalyst, carbon black (Ketjenblack EC-300) wasfirst functionalized with N- and O-functionalities by refluxingit in HNO3. It then was mixed with cyanamide (CN−NH2)and iron sulfate. The resulting material was first heat-treatedat 900 °C in N2, leached in H2SO4, then re-heat-treated at900 °C in N2.(12) The last catalysts are NMCC SiO2-800-3 and NMCC

C-800-3, which are displaying initial peak powers of 0.45 and0.35 W cm−2, respectively.702 Their synthesis has already been

described as typical examples of catalysts for which, accordingto the authors, the main active sites are CNx.It is apparent from the description of the synthesis and from

Table 7 that the most common factor to obtain high perform-ing catalysts is the use of a carbon precursor like a MOF (hereZIF-8; a Zn imidazolate framework), a POP (a porous organicpolymer made of functionalized tetraphenyl porphyrin units),or N-bearing polymers like polyacrylonitrile, poly m-phenyl-enediamine, or a polyimide composite. All of these carbon (andnitrogen) precursors yield catalysts with very high BET (total)surface area (between 758 and 1277 m2 g−1). These catalystsalso contain a large fraction of micropores, whose surface arearanges between 780 and 844 m2 g−1 (according to the valuesreported in Table 7). In these catalysts, the active sites are madeat the same time as the carbon support and are therefore wellintegrated in the catalyst structure. These active sites are, atleast initially, in contact with O2 gas diffusing through thecathode. Furthermore, the high microporous surface area ofthese catalysts is also an important factor in their performancebecause it is known that highly active catalytic FeN4 sites arelocated in micropores.739,744

The use of a sacrificial support like silica (for the catalystsFe-8 CBDZ-DHT-NH3 and NMCC-SiO2-800-3 in Table 7),which is also a way to produce catalysts from nitrogen andcarbon precursors, does not seem, however, to be as efficient tosynthesize high performing catalysts as is the use of nitrogenand carbon precursors like ZIF-8, POP, or N-bearing polymers.So far, when a silica sacrificial support has been used to prepareFe-8 CBDZ-DHT-NH3, the BET did not increase to the levelof that measured for catalysts 1−7 in Table 7. However, whenboth types of catalysts 12 are compared, one may conclude thatreplacing SiO2 with Ketjenblack does involve a decrease of theinitial peak power of NMCC C-800-3 as compared to that ofthe same catalyst (NMCC SiO2-800-3) made with a sacrificialsupport, all other parameters of the synthesis being the same.From Table 7, it seems also that, as long as there is enough

nitrogen and metal in the catalyst, neither nitrogen nor metalcontent seems to be a factor limiting the peak powerperformance of these catalysts. Finally, a last conclusion fromTable 7 is that higher ORR performance in fuel cells is obtainedwith a higher percentage of Nafion in the ink used to preparethe cathode, as this percentage decreases similarly to the peakpower from the top to the bottom of Table 7. It is expected thata large Nafion content in the ink will result in a better coverageof the non-noble catalyst in the cathode, and this shouldfacilitate the access of protons to active sites located at thesurface and in the micropores of these catalysts.

9.3. Initial Performance in H2/Air PEM Fuel Cells

Some of the catalysts that were tested for their beginning of lifeperformance in H2/O2 PEM fuel cells (Figure 24) were alsotested in H2/air PEM fuel cells. Their initial peak power dis-tribution is shown in Figure 26. The same figure also includesthe initial peak power of the catalysts that were only tested inH2/air fuel cells. It seems evident from Figure 26 that reportingperformance of non-noble catalysts in H2/air is not as commonas reporting their performance in H2/O2 fuel cells. Note thateach power increment in Figure 26 is only one-half that of eachpower increment in Figure 24. This indicates that switchingfrom O2 to air reduces the initial peak power by more than afactor of 2. This reduction factor is larger than for Pt/C forwhich switching from O2 to air only decreases the peak powerof the Pt-based fuel cell from 1.36 to 0.96 W cm−2 (as it will be



AD


shown later on). This is noticeably an effect of mass transportproblems that are more acute in the thick cathode of non-noblecatalysts than in the thin cathode made with Pt/C.A list of non-noble cathode catalysts for H2/air fuel cells,

their review code, their initial peak power, and their reference isprovided in Table 6. On one hand, there are not enough resultsreported for H2/air fuel cell tests to obtain statistics similar tothose discussed in Figure 25 for H2/O2 fuel cell tests. However,we may assume that the conclusions reached about catalystloading, fuel cell temperature, back pressure, and membranethickness will be the same for H2/air as for H2/O2 PEM fuelcells. On the other hand, we may now enlarge the previousdiscussion about how to prepare an initially performing catalystbecause the important characteristics of the catalysts providingan initial peak power ≥0.30 W cm−2 in H2/air fuel cells maynow be added to those already found from H2/O2 fuel cellexperiments.(1) The highest performing catalyst is PAN-Fe03-

1000NH3.731 Its synthesis has already been described as it is

also the highest performing catalyst in H2/O2 in Table 7. InH2/air, the initial peak power of PAN-Fe03-1000NH3 is0.41 W cm−2.

(2) The second best performing catalyst is Fe Phen@MOF-ArNH3. It was made by mixing a solution A of 2-methyl-imidazole in methanol with a solution B of Zn(II) nitrate and1,10-phenanthroline in water. Iron(II) acetate was then addedto the reaction. Zn(II) nitrate and 2-methylimidazole are thenecessary ingredients to obtain ZIF-8. Doing so, it is claimedthat the complex Fe phenanthroline is trapped in themicropores of ZIF-8. The latter material is then heat-treatedfirst in Ar at 1050 °C, then in NH3 at 1050 °C. The resultingcatalyst is Fe Phen@MOF-ArNH3. It is characterized by aninitial peak power of 0.38 W cm−2 in H2/air.

691

(3) The synthesis and characterization of catalyst 3 inTable 8 have already been described. It is catalyst 7 (14#114)727 inTable 7. When measured in H2/air, the initial peak power ofcatalyst 3 in Table 8 is ≥0.32 W cm−2 (the polarization curve inH2/air stops at 0.8 A cm−2 at 0.4 V).(4) Catalyst 4 in Table 8 is Fe-PANI/C-Mela with an initial

peak power of 0.33 W cm−2.718 This catalyst was made bydispersing melamine, aniline, and iron chloride in an HClsolution. Ammonium peroxidisulfate was added to induceaniline polymerization. After drying, the resulting mixture wasfirst pyrolyzed at 900 °C in Ar. The sample then was leached inH2SO4 before being pyrolyzed a second time at 900 °C in Ar toobtain Fe-PANI/C-Mela.(5) Catalyst 5 in Table 8 is C700/950 with an initial peak

power of 0.30 W cm−2.678 This catalyst was made from aFe-based MOF obtained from the reaction of anhydrousferrous chloride and 1,3,5 tris (2H-tetrazol-5-yl) benzenehydrochloride in a mixture of dimethylformamide and dimethylsulfoxide. The dried Fe-based MOF was heat-treated at 700 °Cin Ar, then leached in H2SO4, then heat-treated a second timeat 950 °C in NH3.The conclusions that were reached for catalysts 1−12 tested

in H2/O2 fuel cells, and for which the synthesis was previouslydescribed and the characterizations were summarized in Table 7,are the same as the conclusions that may be reached now for thethree catalysts of Table 8 that were tested in H2/air. For the latterfuel cell tests, one sees that (i) the highest performances are againobtained for catalysts that are made by using one or several carbonprecursors in their synthesis; (ii) the porosity of these catalysts ishigh with a major contribution of the micropore surface area(when available) to the total BET surface area; and (iii) nitrogenand Fe contents are in the same range as all other catalysts alreadyreported in Table 7. Finally, and contrary to what was observed inFigure 24 for other catalysts whose ORR activity is claimed to be

Figure 26. Initial peak power for non-noble catalysts tested in H2/AirPEM fuel cells. The nature of the main active site at work in thesecatalysts has been proposed by their authors.

Table 8. Characterization of Performing Non-noble Cathode Catalysts Used in H2/Air PEM Fuel Cells

elemental analysis wt % (or at. %) porosity (m2 g−1)

code/initialpeak power(W cm2)

catalyst’sname

only Cprecursor(s)in synthesis

C black (orsimilar) insynthesis N Fe Co other BET micropores mesopores

ink composition(wt % cat.−wt % Nafion)

1 14#214 0.41 PAN-Fe03-1000NH3

yes (PAN) no (3.22) 3.7 (0.24) 1.11 1096 ? ? ?

2 15#52 0.38 Fe Phen @MOF-ArNH3

yesa no ? 3.1 Zn0.13

1200 ? ? 40−60

3 14#114≥0.32

Fe(PI-1000-III-NH3)

yes (PI-composite)

no 3.1 1.1 1050 844 206 63−37

4 13#11 0.33 Fe-PANI/C-Mela

yesa no ? ? 702 ? ? 75−25

5 14#112 0.30 C700-950 yesa (MOF) no (2.2) 2.5 5.3 687 516 171 40−60

aSee details about the nature of the carbon precursor in the synthesis described in the text.



AE


based on encapsulated metal, catalyst 15#52 in Table 8 displays anexceptionally high ORR performance with an initial peak power of0.38 W cm−2 in H2/air, as seen in Figure 26, where it is rep-resented by a yellow rectangle.

9.4. Durability of Non-noble Metal Catalysts

The first part of this Review about non-noble catalysts wasfocusing on the initial performance of these catalysts. Ifperformance is important for non-noble catalysts to becomeserious contenders to Pt, their durability is important as well.The latter will now be analyzed. The 2017 DOE durabilitytarget for Pt catalysts to be used in automotive application is5000 h.742 However, to accelerate the durability tests, Pt-basedcatalysts are frequently probed in cycling conditions, either inMEA or in half cell (RDE). Two potential-cycle durability testshave been designed: (i) the start/stop cycle, and (ii) the loadcycle.745 On the one hand, for the start/stop cycle, the protocolconsists of applying a 2 s per cycle triangular wave from1.0 to 1.5 V vs RHE with a sweep rate of 0.5 V s−1. The reasonfor these conditions is that during the shut-down of a fuel cell,the gas in the anode flow field is replaced with air. Thus, at thenext start-up, the potential at the cathode is increased to about1.5 V vs RHE.746 Under these conditions, the electrochemicaloxidation of the carbon support is believed to play an importantrole in the degradation of Pt-based catalysts. On the otherhand, for the load cycle, it is known that Pt dissolution occursduring potential change.745 In an actual PEM fuel cell vehicle,the potential range of each cell in the stack will approximatelychange between 0.6 and 1.0 V vs RHE. The former potentialcorresponds to the maximum load, and the latter to the opencircuit potential during idle stop operation. The load cycleprotocol consists of applying rectangular waves from0.6 to 1.0 V vs RHE (3 s at 0.6 V and 3s at 1.0 V) after aninitial potential hold at 0.6 V for 30 s.It has already been shown that applying potentials of ≥1.2 V

to MEAs containing non-noble catalysts at the cathode resultsin serious degradation of the non-noble catalyst.728,747

Therefore, start/stop cycles up to 1.5 V applied to non-noblecatalysts will inevitably degrade them. The question now isknowing that load cycles, occurring at potentials between0.6 and 1.0 V vs RHE, are able to replace much longerdurability tests for Pt-based catalysts, are similar load cyclesperformed in acid solution able to replace fuel cell durabilitytests for non-noble catalysts? As will be seen in Figure 27 anddiscussed below, the answer to this question is no!Figure 27A and B show a series of RDE curves recorded at

25 °C, in Ar (a) and in (b) O2-saturated 0.1 M HClO4, formultiple cycles followed by polarization curve measurements ofFeIM/ZIF-8. This catalyst was made from a mixture of an ironimidazolate framework and ZIF-8, which was first pyrolyzed at1050 °C in Ar, followed by a second pyrolysis at 950 °C inNH3. The initial peak performance of FeIM/ZIF-8 was0.29 W cm−2 measured in H2/air fuel cell.716 It is obviousfrom Figure 27A and B that 10 500 cycles from 0.0 to 1.1 V at50 mV s−1 in Ar or 2000 cycles in O2 at 10 mV s−1 have nearlyno influence on the RDE properties of the catalyst in acidsolution. However, Figure 27C shows that the catalyst isunstable at 0.5 V and 80 °C in a 100 h durability test in fuel cell.Similar conclusions were also reached in another paper fromthe same group722 and in a paper from a different group718 thatused Fe-PANI/C-Mela, whose synthesis and properties werealready reported as catalyst 4 in Table 8. Here, cycling wasperformed up to 10 000 cycles and at different potentials

(from 0.55 to 0.95 V) in air-saturated 0.1 M HClO4. AlthoughFe-PANI/C-Mela is a rather stable catalyst at all potentials insolution, a drastic decrease of current density, measured at0.6 V in H2/air fuel cell, is however observed. Another group

748

also reports similar results with a catalyst prepared by thesacrificial SiO2 support method, where a mesoporous orderedsilica was mixed with two prophyrins (ClFeTMPP andCoTMPP). The resulting material was heat-treated at 800 °Cin N2, and then the silica was leached out with HF to increasethe total surface area of the catalyst to 930 m2 g−1. Althoughthis is also an interesting catalyst, as far as its initial peakpower is concerned, it was not included in Figure 24 becauseits polarization curve was reported as iR-free potential and thevalue of R was not provided in the paper. The uncorrectedcell potential could therefore not be determined and com-pared to the other catalysts reported in Figure 24. It is,however, shown in the article that 10 000 cycles in O2-saturated0.1 M HClO4, recorded between 0.6 and 1.0 V vs RHE at50 mV s−1, leave this catalyst practically unchanged in solution,while it is unstable over 100 h at 0.5 V in the H2/O2fuel cell.

Figure 27. ORR polarization curves of FeIM/ZIF-8 measuredduring RDE stability test. The test conditions include cycling from0.0 to 1.1 V at 50 mV s−1 in (A) 0.1 M Ar-purged HClO4 or (B) 0.1 MO2-purged HClO4. At 25 °C for multiple cycles, followed bypolarization curve measurements in O2-purged HClO4 at the scanrate of 10 mV s−1. (C) 100-h stability test by measuring the currentdensity at 0.5 V of a single cell with FeIM/ZIF-8 as the cathodecatalyst (Nafion 117 membrane) operated with H2/air. For allmeasurements, the temperature of the cell was kept at 80 °C.



AF


Now that it was demonstrated that the durability of non-noble catalysts in MEAs in fuel cell cannot be extrapolated fromRDE measurements in acid solution, it is therefore imperativeto carry out durability experiments in fuel cell to obtain the truedurability behavior of non-noble catalysts. Durability tests canbe performed either in H2/O2 or in H2/air fuel cells. Othervariables like constant potential or constant current have also tobe considered in the durability tests. Only a few catalysts weremeasured for their durability at constant current, while most ofthem were reported at constant potential, especially at 0.4 V.In the following, we will first report durability tests in H2/O2

followed by tests in H2/air.Figure 28A and B presents the time evolution, for a

maximum of 200 h and at 0.5 V in H2/O2 fuel cells, of catalysts1−6 and catalysts 7−11 of Table 9, respectively. Figure 28Cpresents the time evolution for a maximum of 200 h of all ofthe catalysts that were measured at 0.4 V in H2/O2 fuel cell,while the time evolution for a maximum of 1100 h at 0.4 V inH2/O2 fuel cell is shown in Figure 28D for the same catalysts.A maximum of 200 h was chosen for all of the catalysts ofFigure 28A−C, to compare them on the same time scale. Forthe sake of comparison, all catalysts of Figure 28 are also shownon the same power scale, from 0 to 0.5 W cm−2. In Table 9,the catalysts measured either at 0.5 V or at 0.4 V appear indecreasing order according to the initial power they display inFigure 28. Besides reporting the catalyst’s name given by theirauthors, Table 9 also reports (when available) important data

about the synthesis procedure of the catalysts, their elementalcomposition, their main catalytic site according to their authors,their porosity, the composition of the ink used to produce thecathodes, and also if any carbon etching, either with NH3 orwith CO2, was used during their synthesis.Figure 29A−C presents the time evolution for a maximum of

200 h of all catalysts of Table 10, but this time at 0.6, 0.5, and0.4 V in H2/air fuel cell, respectively, while the time evolutionfor a maximum of 850 h is only shown in Figure 29D for thecatalysts probed at a constant potential of 0.4 V in H2/air fuelcell. Figure 30A presents the time evolution of all catalysts thatwere measured in H2/O2 fuel cell at a constant current of 0.1 or0.2 A cm−2 and whose properties are gathered in Table 11.Figure 30B is the equivalent figure for all catalysts that werealso measured at a constant current, but this time in H2/air fuelcell. Their properties are detailed in Table 12.It is important to note that, to compare the power obtained

at constant current with those obtained at constant potential,all power axes in Figures 28, 29, and 30 are the same withpowers ranging from 0 to 0.5 W cm−2 for H2/O2 andH2/air fuel cells. Note also that durability results at constantpotential or constant current cannot be deduced from oneanother because the correlation between potential andcurrent in fuel cell (the polarization curve) is not linear.Furthermore, it will also be shown later that the durabilitybehavior of a catalyst in fuel cell is not the same at all potentialsbut is better at lower potentials. When durability is measured

Figure 28. Time evolution in H2/O2 fuel cell for catalysts 1−6 (A), and catalysts 7−11 (B), which were measured at constant potential of 0.5 V.Time evolution in H2/O2 fuel cell for all catalysts (C,D) that were measured at constant potential of 0.4 V. In (D), one of the catalysts was tested for1100 h. The properties of the catalysts are detailed in Table 9. The numbers appearing at the end of each curve in (A) are the microporous surfacearea/total BET surface area of each catalyst and their ratio.



AG


Table


Non

-nob

leCatho

deCatalystsUsedforDurability

Tests

inH

2/O

2PEM

Fuel

Cells:Con

stantPotentials

elem

entalanalysiswt%

(orat.%

)porosity

(m2g−

1 )

potential

(V)

catalyst’snameref

onlyCprecursor(s)

insynthesis

Cblack(orsimilar)in

synthesis

NFe

Co

maincatalytic

site

BET

micropores

mesopores

inkcatalyst−

Nafion

(wt%)

useof

NH

3or

CO

2

10.5

Fe/N

/C-SCN

15#60

noyesKetjenblack

(KJ600)

4.4

1.4

FeNx

751

414

337

50−50

no

20.5

Fe/N

/CF15#54

yes(PAN

+PM

MA)

no9.0

0.56

FeNx

809

780

840−60

NH

3

30.5

PFeT

PP-100013#124

yes(POP)

no1.92

4.95

FeNx

758

mostly

?50−50

no4

0.5

NC

Ar+NH

311#138

yes(M

OF)

no(5.3)6.0

(0.78)

3.51

FeNx

964

814

184

40−60

NH

3

50.5

Zn(mIm

) 2TPIP14#04

yes(M

OF)

no2.22

1.07

FeNx

1277

??

?NH

3

60.5

SCIAr+NH

309#74

noyesblackpearls

(2.4)2.7

(0.44)

2.01

FeNx

767

605

162

40−60

NH

3

70.5

NC

Ar11#138

yes(M

OF)

no(3.7)4.2

(0.65)

2.92

FeNx

550

504

4640−60

no8

0.5

Fe/m

elam

ine/KB11#39

noyesKetjenblack

4.1

?FeNx

??

?67−33

no9

0.5

N-G-CNT:2/0.5/0.15

+KB15#01

noyesNCNT

(4.1)4.7

0aCNx

422

??

50−50b

NH

3

10Fe/Phen/N-C

flakes

13#30

noyescarbon

nanoflakes

(1.88)

2.14

(0.28)

1.28

FeNx

??

??

NH

3

110.5

CHb35090008#03

yes

no+vulcan

after

synth

?0.47

FeNx

453

??

29−71

CO

2

10.4

Co-ED

/PPy-CNF11#97

noyescarbon

nanofiber

4.5

2.7

CNx(C

o)?

??

?no

20.4

P-CNF11#87

noyesplatelet

Cnanofiber

3.8

?CNx(C

o)?

??

?no

30.4

Co-ED

/Ppy-CNF-10

12#44

noyescarbon

nanofiber

4.5

2.7

CNx(C

o)?

??

?no

40.4

Fe-N

SG15#66

noyesfunctio

nal

graphite

oxide

(3.7)3.9

(1.1)4.8a

(FeN

x)?

(EFeNc )?

??

??

no

50.4

bNGr14#97

noyesball-milled

graphite

oxide

7.3

1.7

0.3

CNx(C

o/Fe)

??

?33−67

no

60.4

FeCo-ED

A-600

10#29

noyesKetjenblack

(2.18)

2.51

4.14

4.31

Fe/C

o-Nx

483

136

347

50−50

no7

0.4

UF-C09#-06

noyesKetjenblack

2.2

0.0

0.0

CNx(0

Me)

321

??

?no

aDeduced

from

TGA(nometalresiduein

N-G-CNT).b(C

atalyst+Ketjenblack)/Nafion

=1/1.c EncapsulatedFe-containingnanoparticles.



AH


galvanostatically in fuel cell for an unstable catalyst, a decreasein current is compensated by a decrease in potential, whichmeans a change in potential toward a potential of better stabi-lity. Therefore, a durability test at constant current (starting atan initial potential Vstart) will give the impression that thecatalyst is more stable than if the durability test of the samecatalyst was measured using Vstart as constant potential.Let us discuss now the durability results illustrated in Figures 28

and 29. These are results obtained at constant potential either inH2/O2 or in H2/air fuel cells. Focusing first on catalysts tested inH2/O2 fuel cells that deliver initial powers ≥0.10 W cm−2, wenotice essentially three behaviors: (i) behavior 1 illustrated inFigure 28A, where all catalysts are initially very performing (withan initial power around 0.5 W cm−2 at 0.5 V) but are also veryquickly losing their activity; (ii) behavior 2 illustrated by 11#138Ar and 11#39 in Figure 28B, where the catalysts’ performancefirst increases, goes through a maximum (between 0.20 and0.30 W cm−2 at 0.5 V), then decreases; to a certain extent, it isalso the durability behavior of the three first catalysts listed inFigure 28C (it is easier to see the negative slopes of the same threecurves on Figure 28D, drawn on an extended scale); and (iii)behavior 3 illustrated in Figure 28 D by catalyst 14#97 that is ableto deliver a rather stable but low power of 0.13 W cm−2 for 1100 hat 0.4 V. Looking to Table 9 for properties of catalysts displayingbehavior 1, one can see that four out of six of these catalystswere made without any addition of carbon black during theirsynthesis, and, when carbon black was used in the synthesis, eitherKetjenblack or black pearls were chosen as both carbon supports

are highly porous. Furthermore, all of these catalysts arecharacterized by a very high porosity with a total surface area(BET) comprised between 751 and 1277 m2 g−1. Most of thepores in these catalysts are micropores, which are known to hosthighly active FeNx sites.744 The numbers appearing at the end ofeach curve in Figure 28A are the microporous surface area/totalBET surface area of each catalyst and their ratio. It seems that thehigher is this ratio, the lower will be the power at which the catalystwill reach some stability. All durability tests stop at 100 h. It seems,however, that the curve of catalyst 15#54 with a microporous to totalsurface area ratio of 0.96 will cross for t > 100 h the durability curveof the catalyst 11#38_Ar + NH3. From the behavior of all durabilitycurves in Figure 28A, one may conclude that, if a high microporoussurface area is beneficial to promote the initial peak performance of acatalyst, the same high microporous surface area will be detrimentalfor its stability!Catalyst 11#138 Ar in Figure 28B is typical of behavior 2.

Here, the performance of the catalyst goes through a maximum,then decreases. The synthesis of catalyst 11#138 Ar + NH3,which is a highly performing catalyst in this Review with aninitial peak power of 0.91 W cm−2, has been described undercatalyst 3 in section 9.2. The synthesis of 11#138 Ar is exactlythe same as that of 11#138 Ar + NH3, except for the lastpyrolysis step in NH3 that is missing in 11#138 Ar. A 57FeMossbauer study of these two catalysts revealed that bothcatalysts contained about the same number of Fe-based cata-lytic sites, but that many of these sites were initially secluded in11#138 Ar. This was not the case for 11#138 Ar + NH3 for

Figure 29. Time evolution in H2/air fuel cell for all catalysts that were measured at constant potential of either 0.6 V (A) or 0.5 V (B). Timeevolution in H2/air fuel cell for all catalysts that were measured at constant potential of 0.4 V for 200 h (C), and up to 900 h (D). The properties ofthe catalysts are detailed in Table 10.



AI


Table


Non

-nob

leCatho

deCatalystsUsedforDurability

Tests

inH

2/AirPEM

Fuel

Cells:Con

stantPotentials

elem

entalanalysiswt%

(orat.%

)porosity

(m2g−

1 )

potential

(V)

catalyst’snameref

onlyC

precursor(s)

insynthesis

Cblack(or

similar)

insynthesis

NFe

Co

maincatalytic

site

BET

micropores

mesopores

inkcatalyst−

Nafion

(wt%)

useofNH

3or

CO

2

10.8

PAN-Fe03-

1000NH314#214

yes(PAN)

no(3.22)

3.7

(0.24)

1.11

FeNx

1096

??

?NH

3

10.6

Fe-PANI-Mela

13#11

yes

no?

?FeNx

702

??

?no

20.6

NC

ArF(90)

14#N

ICyes75

wt%

(MOF)

yes25

wt%

Cfibers

(3.90)

4.4

2.75

FeNx

338

310

2840−60

no

30.6

PANI-Fe-C

13#18

noyesKetjenblack

?7.4(t=0h)

1.6(t=200h)

FeNx

??

?70−30

no

10.5

NC

Ar+NH

311#138

yes(M

OF)

no(5.3)6.0

(0.78)

3.51

FeNx

964

814

184

40−60

NH

3

20.5

NC

Ar11#138

yes(M

OF)

no(3.7)4.2

(0.65)

2.92

FeNx

550

504

4640−60

no

30.5

15#54

40.5

FeIM

/ZIF-8

12#104

yes(M

OF)

no4.50

5.29

FeNx

572

mostly

?50−50

NH

3

50.5

CHbM

g-1000-5

min

14#151

yeshemoglobin

no+vulcan

after

synth

2.35

0.32

FeNx

1562

?mainly

39−61

NH

3

60.5

CHbMg3s

400900

11#109

yeshemoglobin

no+vulcan

after

synth

?0.25

FeNx

1110

?mainly

?no

70.5

CHb200

90008#03

yeshemoglobin

no?

0.41

FeNx

831

??

29−71

CO

2

10.4

Py-B12/C

12#15

noyesvulcan

?0.95

CoN

x?

??

67−33

no

20.4

SCIAr+NH

309#74

noyesblackpearls

(2.4)2.7

(0.44)

2.01

FeNx

767

605

162

40−60

NH

3

30.4

PyP-CoT

MPP

-600

10#84

yes+sacrificial

SiO

2

no?

?CoN

x?

??

?no

40.4

NMCC-SiO

2-800-3

10#11-800C

yes+sacrificial

SiO

2

no(5.93)

6.85

??

CNx(C

o/Fe)

??

?75−25

no

50.4

PANIFeCo-C

11#137

noyesKetjenblack

??

?Fe-CoN

x?

??

65−35

no

60.4

NMCC-SiO

2-1000-3

10#11-1000C

yes+sacrificial

SiO

2

no(2.38)

2.76

??

CNx(C

o/Fe)

??

?75−25

no

70.4

CoF

e(1:3)N-C

11#79

noyesKetjenblack

(6.0)6.7

(0.6)2.7

(0.0)0.0

FeNx/CNx(C

o)370

??

65−35

no

80.4

PANI-Fe-C

13#18

noyesKetjenblack

?7.4(t=0h)

1.6(t=200h)

FeNx

??

?70−30

no

90.4

PyP-CoT

MPP

-700

08#13

yes+sacrificial

SiO

2

noN/C

o=(2.6)

?CoN

x?

??

?no

100.4

Py-Co-corrole/

C-700

12#79

noyesvulcan

?1.50

CoN

x?

??

33−67

no

110.4

Fe/C

15#11

yes

no∼4.0

∼8.0

EFeN

375

??

40−60

no



AJ


which most of the sites were initially accessible to performORR.749 Looking to Table 9, one sees that the BET surfacearea of 11#138 Ar is only 550 m2 g−1 (with a microporoussurface area of 504 m2 g−1), as compared to the BET surfacearea of 11#138 Ar + NH3 (964 m2 g−1) with a microporoussurface area of 814 m2 g−1. The effect of NH3 is therefore toetch 11#138 Ar to practically double its surface area in 11#138

Ar + NH3 and also to increase somewhat its total N content(from 3.7 at. % for 11#138 Ar to 5.3 at. % for 11#138 Ar +NH3). It was proposed that the activation of 11#138 Ar duringits durability test was the result of the dissolution of someinactive material, giving access to O2 and to the electrolyteand to increasingly more catalytic sites that were oncesecluded in the material. However, once activated, these sitesbegin to perform ORR and show a rapid decay as they do for11#138 Ar + NH3.

749 To obtain behavior 2, it is, however,necessary that the rate at which sites are activated is faster thanthat of their decay. We believe that this explanation may alsohold for the three best catalysts of Figure 28C (or D), a fit tothe experimental curve being obtained by balancing activationand decay rates for these catalysts.According to their authors, all of the catalysts of Figure 28C

and D (except for 10#29 and 15#66) are supposed to reduceoxygen on CNx sites, but they are all made (except for 09#06)with Co or/and Fe precursors. The only catalyst also believedto function with CNx catalytic sites but made without Co or Feprecursors is 09#06. It is, however, also a poor performing,yet rather stable catalyst. The catalyst that displays a stable0.13 W cm−2 for 1100 h (14#97) was made by first ball-millinggraphite oxide, then mixing this material with dicyanamide(a nitrogen precursor) with CoCl2 and FeCl2. The mixture waspyrolyzed at 900 °C in Ar, then leached in H2SO4 at 80 °C,then pyrolyzed again at 900 °C in Ar. According to theirauthors, the main catalytic sites of catalyst 14#97 are CNx (withsome minor contribution of MeNx sites).726 Catalyst 10#29 issupposed to reduce oxygen on Fe/CoNx sites. However, it doesnot show the typical decay observed for the catalysts reputed toreduce oxygen on FeNx or CoNx active sites. This is becausetheir authors waited until the catalyst displayed some stabilitybefore recording its durability behavior.703 As far as catalyst15#66 is concerned, it is not obvious to know what type ofcatalytic site is responsible for the ORR activity andperformance attributed to this catalyst.737

Looking now at the durability tests recorded for H2/air, wefirst note in Figure 29A that catalyst 13#11 displays a strongtypical behavior 1 at a constant potential of 0.6 V. Again, accordingto Table 10, this catalyst was made from carbon precursors only,like most of the catalysts presented in Figure 28A. Catalyst 13#11is characterized by a high total BET surface area of 702 m2 g−1, avalue similar to the BET surface area of many catalysts of Table 9,also made without any carbon black in their synthesis. Catalyst13#18 displays a mild behavior 1, while catalyst 14#Nic730 displays

Figure 30. Time evolution for all catalysts that were measured atconstant current in H2/O2 (A) and H2/air (B) fuel cells. Theproperties of the catalysts are detailed in Tables 11 and 12.

Table 11. Characterization of Non-noble Cathode Catalysts Used for Durability Tests in H2/O2 PEM Fuel Cells: ConstantCurrents

elemental analysiswt % (or at. %) porosity (m2 g−1)

current(A cm−2)

catalyst’s nameref

only Cprecursor(s) in

synthesis

C black (orsimilar) insynthesis N Fe Co

main catalyticsite BET micropores mesopores

ink catalyst−Nafion(wt %)

use ofNH3 orCO2

1 0.2 NMCC-80009#34

no yesKetjenblack

? ? ? CNx (Co/Fe) ? ? ? 75−25 no

2 0.2 CoTETA/C10#01

no yesKetjenblack

? ? α-Co? ? ? ? ? no

3 0.2 NMCC-110009#34

no yesKetjenblack

? ? ? CNx (Co/Fe) ? ? ? 75−25 no

4 0.2 C-composite-Co3Fe1Nx08#15

no yesKetjenblack

3.9 1.4 4.6 CNx (Co/Fe) ? ? ? 75−25 no

5 0.1 MNC-3 15#63 no yesKetjenblack

3.7 FeNx 873 ? ? 70−30 no



AK


a behavior 2 durability curve. In Figure 29B, which presents thedurability behavior at a constant potential of 0.5 V, we alsorecognize a behavior 1 in the durability curves of 11#138_Ar +NH3, while catalyst 11#138 Ar displays a behavior 2 durabilitycurve. Both catalysts are the same as those already described inFigure 28A. However, after 100 h, both 11#138 + NH3 Ar and11#138 Ar catalysts deliver the same power (∼0.14 W cm−2) at0.5 V in Figure 29B, catalyst 11#138 Ar + NH3 starting at0.3 W cm−2 at t = 0 and 11#138 Ar starting at 0.15 W cm−2.This is quite different from the behavior of the same catalystsmeasured in H2/O2 durability tests, where catalyst 11#138Ar + NH3 started at 0.45 W cm−2 and dropped to 0.08 W cm−2

after 100 h of test, while catalyst 11#138 Ar started at0.23 W cm−2 and dropped to about 0.16 W cm−2 after 100 h(see Figure 28A). Therefore, the use of pure O2 gas at thecathode exacerbates the power decay during the durability testsof the catalysts.The last figures to discuss in the series of experiments

performed at constant potential are Figure 29D and the detailof its first 200 h test presented in Figure 29C. Here, thepotential of the catalysts has been maintained constant at 0.4 V.In these figures, catalysts 11#137 and 13#18 display ratherstable durability behaviors (during 200 h for catalyst 13#18 andabout 700 h for catalyst 11#137). The synthesis of catalyst11#137 was already described as catalyst 9 in the previoussection reporting on the initial performance of the catalysts.Alike 11#137, catalyst 11#18 is also made with PANI asnitrogen precursor. Both catalysts use Ketjenblack as carbonblack in their synthesis. Here, it is important to note that ifcatalyst 13#18 displays a stable behavior for 200 h at about0.11 W cm−2 when it is measured in H2/air fuel cell at aconstant potential of 0.4 V, the same catalyst is not stable inH2/air fuel cell when it is measured at a constant potentialof 0.6 V. In that case, the power delivered at t = 0 is about0.12 W cm−2. It drops to about 0.07 W cm−2 (a 40% powerdrop), after 200 h after a durability test at 0.6 V. No wonderdurability experiments are most popular in H2/air fuel cell testsat a constant potential of 0.4 V! In these conditions, the authorsare able to report larger and more stable powers than at higherpotential or in H2/O2 fuel cells. It is particularly important toreport here on the durability of catalyst 14#214, which was theonly catalyst measured at 0.8 V and should therefore decay veryquickly at that high potential. However, this catalyst only loses10% of its initial performance after 7 h of test in H2/air fuel cell.This is a power loss comparable to that of catalyst 13#18presented in Figure 29A, but at 0.6 V instead of 0.8 V. It is apity that the durability test performed on catalyst 14#214 wasnot longer as this catalyst seems quite promising. Indeed, inH2/O2 fuel cell, the same catalyst was characterized by the

highest initial peak power (0.98 W cm−2) recorded in thisReview (see catalyst 1 in Table 7).Another point that is important to note in Figure 29C is the

behavior of catalyst 10#11 pyrolyzed at two temperatures,either 800 or 1000 °C. For this catalyst, 10#11_800Cpractically displays a behavior 1-like type curve, while thedecay curve for 10#11_1000C looks more like a behavior2 type of curve. The catalyst was made using sacrificial SiO2support on which complexes of ethylene diamine with cobaltnitrate and iron sulfate were impregnated. After a first pyrolysisat 800 °C (or 1000 °C) in Ar, the SiO2 sacrificial supportwas removed using NaOH. The resulting material was acidleached, then re-heat-treated at 800 (or 1000 °C) in Ar.According to the authors, CNx are the main active sites inthese two catalysts, despite the use of Co and Fe precursorsin their synthesis. Therefore, the pyrolysis temperaturegreatly influences the decay rate of a catalyst, slowing it forhigher pyrolysis temperatures. This positive effect is, however,counterbalanced by some loss of power for the resultingcatalyst pyrolyzed at the highest of both temperatures (800 or1000 °C).The last durability (Figure 30A and B) has been measured

under constant current either in H2/O2 or in H2/air fuel cell,respectively. As it was already mentioned, it is not possible tocompare durability results obtained at constant current withthose obtained at constant potential. Furthermore, except forthe two catalysts tested under H2/air, the initial power of allcatalysts tested under H2/O2 is quite low (≤0.10 W cm−2) andtherefore of little interest. Both 14#84 and 14#114 catalystswere made with a carbon precursor (instead of a carbon black),and they have again a high BET surface area with a largefraction of micropores. It is expected that, if the same catalystswere tested at constant potential, they would display a fast initialdecay starting at 0.15 W cm−2 for 14#114 or 0.10 W cm−2 for14#84. According to their authors, CNx is the main ORR site inthese catalysts, even if an iron precursor was used in theirsynthesis.

9.5. Origin of Activity Loss in Fuel Cells

Durability tests for ORR catalysts measured in fuel cell are veryoften reported without any comments. When some commentsare provided, a list of possible hypotheses to explain the lackof durability is also provided, most frequently without anyelaborate explanation. These hypotheses are, for instance: (i)the electrooxidation of the catalyst carbonaceous supportcausing the loss of ORR catalytic sites, which are known tobe integrated for Me/N/C and for CNx in the carbonaceoussupport of the catalysts; (ii) the loss of the metallic ion in theMe/N/C sites, leaving behind free metal ions, like Fe or Coions, and also pyridinic type CNx sites; (iii) the generation ofH2O2 by less performing ORR sites that may further oxidize the

Table 12. Characterization of Non-noble Cathode Catalysts Used for Durability Tests in H2/Air PEM Fuel Cells: ConstantCurrents

elemental analysiswt % (or at. %)porosity (m2 g−1)

current(A cm−2)

catalyst’s nameref

only Cprecursor(s) in

synthesis

C black (orsimilar) insynthesis N Fe Co

maincatalyticsite BET micropores mesopores

ink catalyst−Nafion(wt %)

use ofNH3 orCO2

1 0.2 1000-III-NH3-g14#84

yes no 1.9 1.6 CNx(Fe)

918 698 220 ? NH3

2 0.25 Fe(PI-1000-III-NH3)14#114

yes no 3.1 1.1 CNx(Fe)

1050 844 206 63−37 NH3



AL


ionomer and/or the catalyst, especially if metallic ions like Fe orCo ions are around, because the latter are able to generateFenton’s type reagents that are highly oxidizing species;750,751

(iv) Fe or Co ions released from catalytic Me/N/C sites mayalso exchange for protons in the ionomer, increasing thereforeits resistance; (v) water flooding of the catalyst pores impedingthe transport of oxygen in the cathode; and (vi) reaction ofprotons with pyridinic CNx sites poisoning the activity of thesesites, etc. An excellent review about the lack of durability ofnon-noble metal and metal-free catalysts and its potentialcauses has recently been published.752 Note that it is possiblethat the lack of durability of most Me/N/C catalysts may havemore than one cause, complicating therefore the resolution ofthis problem. Let us now focus on some of the hypotheses forwhich the authors have provided elaborate information.For Popov and collaborators who defend the idea that CNx

are ORR catalytic sites, even if Co or Fe precursors are used asmetal precursors during the preparation of the catalysts, themetal is only present during the synthesis to favor N-doping ofthe carbon support.639 There are, however, several kinds ofCNx sites depending on where N substitutes for C. A pyridinic-type N substitutes for a carbon atom at the edge of grapheneplatelets (or carbon sheets), while a graphitic N substitutes forcarbon on the base planes of the carbon support. Both pyridinicand graphitic type CNx sites are providing some ORR activity.However, pyridinic-type N is a basic functionality that will reactwith protons in the acid medium of the fuel cell to form NH+

species, which then become ORR inactive. Durability experi-ments showing a fast decrease of ORR with time are thereforeinterpreted as the result of the protonation of pyridinic-typeCNx sites. Furthermore, Popov and collaborators also advocatethat preparing catalysts at 800 or 1000 °C, as it was done forthe two types of 10#11 catalysts in Figure 29C, results in theloss of more of pyridinic N-type sites, which are mainly foundat 800 °C than of graphitic N-type sites, explaining thereforewhy the catalyst prepared at 1000 °C is initially less performingbut more durable than that prepared at 800 °C.For Ozkan and collaborators, who recognize the existence of

CNx sites and that of FeNx sites in Fe/N/C catalysts, the lossof durability of Fe/N/C catalysts results from the loss of Feions from the FeNx catalytic sites. Because FeNx sites areresulting from the coordination of a Fe ion between pyridinictype nitrogen atoms,640,738,739a demetalation of the FeNx siteswill leave behind pyridinic type N, increasing therefore the totaldensity of CNx sites in the catalyst.643,753,754 Nothing is said,however, in Ozkan’s group publications about the reasonwhy FeNx sites in Fe/N/C catalysts are demetalated whenthese catalysts are in contact with the acidic medium of a PEMfuel cell.Our explanation for behavior 1 durability curves mainly

illustrated by all of the curves in Figures 28A, for Me/N/Ccatalysts, is that these materials are highly microporous andtherefore also highly prone to water flooding. As the maincatalytic sites in our catalysts are hosted in micropores,744

flooding them with water will oblige O2 to change its mode oftransport to the sites from a gaseous to a dissolved mode in thenow water-filled micropores. This would explain the drop withelapsed time (behavior 1) in the power delivered by thesecatalysts, being more drastic when the microporous surface areabecomes a larger fraction of the total BET surface area. It wouldalso explain why using O2 at the cathode is much more efficientto flood the micropores of the catalysts than using air, becausemore water is provided for the same volume of gas at the fuel

cell cathode when it is fed with O2 instead of air. Moreover, thehypothesis of flooding the micropores would also explain thedifference in stability behavior (presented in Figure 27) forthe same catalyst between RDE and fuel cell tests. For RDEtests, where the electrolyte is an aqueous acid solution, themicropores of the catalyst are instantly filled with water fromthe acid solution; it is therefore difficult to measure a decay inperformance similar to behavior 1 in that case. On the contrary,in fuel cell tests, where the electrolyte is a protonic ionomer,water flooding of micropores is the result of the ORR; in thiscase, the performance decay becomes measurable. Thishypothesis of flooding of micropores has been put to test ina recent paper where the hydrophobicity of the carbon supportwas modified by changing the synthesis procedure to obtainsuch catalysts.755 In our recent attempt, we produced catalystswith various hydrophilicity by ball-milling ZIF-8 with an ironporphyrin: ClFeTMPP. The mixture was then heat-treated athigh temperature in Ar. The temperature of this first pyrolysisvaried from 850 to 1150 °C in steps of 100 °C to provide fourdifferent catalysts labeled: NC ClFeTMPP-T Ar, where T waseither 850, 950, 1050, or 1150 °C. All four catalysts then weresubmitted to a second pyrolysis, this time identical for allcatalysts and involving a heat-treatment at 950 °C in NH3. Thefinal catalysts were labeled: NC ClFeTMPP-T Ar + NH3 withT representing the temperature of the first pyrolysis in Ar. Thenominal Fe loading of all of these catalysts was always 0.8 wt %as Fe in ClFeTMPP.Figure 31 illustrates the power durability behavior of the four

catalysts. It is clear from this figure that the catalyst delivering

the largest initial power is NC ClFeTMPP-1050 Ar + NH3, butthe catalyst showing the best durability is that produced at thehighest first pyrolysis temperature (1150 °C) in Ar. These fourNC ClFeTMPP-T Ar + NH3 catalysts have been characterizedfor the change in their graphitization when the pyrolysis tem-perature in Ar was modified from 850 to 1150 °C. Changes intheir total nitrogen and oxygen contents, in the type of nitrogen

Figure 31. Chronoamperometry curves at 0.6 V in H2/O2 fuel cell forNC ClFeTMPP-T Ar + NH3 catalysts, where T is the temperature(between 850 and 1150 °C) of the first pyrolysis in Ar. A secondpyrolysis is then performed in NH3 at 950 °C. The open symbols arethe current density values read at 0.6 V on the polarization curvesmeasured either initially or between two segments of chronoamper-ometry curves.



AM


functionalities doping their carbon support, and in theirporosity (total BET and microporosity) were also determined.All of this information is summarized in the three panels ofFigure 32, where it can be seen that NC ClFeTMPP-1150Ar + NH3 is the most graphitic catalyst among the four ones.It is also characterized by the lowest heteroatom (N and O)

content at the surface of its carbon support, as well as by theirlowest surface concentration of pyridinic and pyrrolic nitrogenatoms. The same catalysts displays a high total surface area,mainly composed of micropores. It has been shown that thebehavior of the four catalysts in Figure 31 cannot be explainedneither by using the hypothesis proposed by Popov and hiscollaborators nor by that provided by Ozkan and hercollaborators.755 On the contrary, the variable lack of durabilityof the four catalysts illustrated in Figure 31 can be rationalizedin terms of water flooding of the catalyst’s micropores: NCClFeTMPP-1150 Ar + NH3 being the least hydrophilic and themost graphitic of all four catalysts. It remains now to bechecked if the same behavior cannot be explained as well bysome general electrooxidation of the catalyst’s support or by anelectrooxidation limited only to their active sites.As far as an increase of the MEA resistance with time is

concerned, we checked this hypothesis to explain behavior1 decay, while recording the data necessary for Figure 31, as acomplete polarization curve and an impedance measurementwere recorded at specific times marked by symbols on eachcurve in Figure 31. The impedance measurements for allcatalysts indicated that the resistance of the MEA (mostlyrelated to Nafion ionomer and Nafion membrane) in fuel cellremained practically constant and even slightly decreasedwith time during the durability test. For instance, for NCClFeTMPP-1150 Ar + NH3, the initial resistance was0.070 Ω cm2, while it was 0.064 Ω cm2 after 88 h at the endof the durability test. This observation suggests that if iron waseventually released from the catalyst into the MEA, it should benegligible as otherwise it would have increased the resistance ofthe MEA by exchange with protons. The most difficult causefor a lack of durability to be investigated in fuel cell is certainlythe possible release of peroxide in an MEA. However, if someamount of peroxide is indeed released, it should be low becausethe presence of any free iron ions in the MEA would result in astrong chemical oxidation of the catalyst’s carbon support bythe Fenton’s reagent produced by the chemical reaction of freeiron ions with hydrogen peroxide. It remains to be checked infuel cell whether the electronic conductivity around catalyticsites is not drastically affected by chemical corrosion of thecarbon structure of the catalysts.

9.6. Comparison with Pt

It is important to compare performance and durability mea-sured for non-noble catalysts with those of Pt-based catalysts.As far as performance is concerned, we have seen in Figure 24that the catalyst displaying the largest initial peak power inH2/O2 fuel cell was 14#214 with a maximum initial peak powerof 0.98 W cm−2.731 The durability in H2/O2 and H2/air fuelcells was not reported in the same experimental conditions forat least 100 h at any potential for the same catalyst, but this wasdone for catalyst 11#38_Ar + NH3, the third catalyst on the listin Table 7 at a constant potential of 0.5 V (Figure 28A, H2/O2;and Figure 30, H2/air). The original publication about catalyst11#38_Ar + NH3 was also reporting on the activity of thatcatalyst measured in A cm−3 at 800 mViR‑free in H2/O2 fuel celland in experimental conditions recommended by the DOEfor non-Pt catalysts activity per volume of supported catalyst(for transportation applications).742 The volumetric activity of11#138 Ar + NH3 was 39 A cm−3 (measured at 800 mViR‑free)and 230 A cm−3 (extrapolated at 800 mViR‑free from a linearTafel slope between 1 and 0.9 V (both iR-free)). The 2020volumetric activity target set by the DOE is 300 A cm−3.742

Figure 32. (A) Changes with the pyrolysis temperature in Ar, in fwhmof the D band and D/G surface area ratio, for all of the NCClFeTMPP-T_Ar + NH3 catalysts first pyrolyzed in Ar then in NH3.(B) Changes with the first pyrolysis temperature in Ar, in the totalnitrogen and oxygen contents of NC ClFeTMPP-T_Ar + NH3catalysts. (C) Left axis: Changes with the first pyrolysis temperaturein Ar, in the individual types of nitrogen contents for NC Por_0.8-TAr + NH3 catalysts. Right axis: Changes with the first pyrolysistemperature in Ar, in the microporous surface area (lines and points)and total surface area (points only) of NC ClFeTMPP-T_Ar + NH3catalysts.



AN


Note that the highest volumetric activity (450 A cm−3), mea-sured at 800 mViR‑free, has been reported for catalyst 15#54.734

The latter is the fourth catalyst on the list in Table 7.Coming back to the performance and the durability of non-

noble catalysts, it is certain that the first applications of thesecatalysts will not be in transportation but rather in lower powerapplications like in portable PEM fuel cells for which thePt-group metal loading in the fuel cell MEAs will not be asrestrictive as those recommended by the DOE for trans-portation applications. In the latter case, the 2020 DOE targetfor Pt group metal-based catalysts is 0.125 mg Pt group metalper cm2 of electrode area (total loading). In this Review, weused a cathode loading of 0.3 mg Pt cm−2 to make our com-parison. The polarization curves and power curves for a H2/O2and a H2/air fuel cell using that Pt loading are given in Figure 33,

where it is seen that, with a loading of 0.3 mg Pt cm−2 at thecathode, a peak power of 1.36 W cm−2 is obtained at 0.41 V and3.4 A cm2 in a H2/O2 fuel cell, and 0.96 W cm−2 is obtained at0.45 V and 2.13 A cm−2 in a H2/air fuel cell. The maximuminitial peak power of 0.98 W cm−2, measured in H2/O2 fuel cellfor a cathode using the non-noble catalyst 14#214 PAN-Fe03-1000NH3,

731 represents therefore 72% of the maximum peakpower generated by the Pt/C electrode of Figure 33. As far asH2/air is concerned, the maximum initial peak power of the bestcatalyst in Figure 26 is 0.41 W cm−2. It is also reported forcatalyst 14#214.731 This is only 43% of the maximum peak powergenerated in H2/air by the Pt/C cathode catalyst in Figure 33.We have seen that the best durable non-noble catalyst in

H2/O2 and H2/air fuel cells is 14#97 that delivers in H2/O2 andat 0.4 V a constant power of 0.13 W cm−2 over 1100 h,726 while11#137 delivers about 0.14 W cm−2 over 700 h at 0.4 V inH2/air.

711 At 0.4 V in H2/O2 fuel cell, the Pt/C cathode ofFigure 33 yields 3.37 A cm−2 or 1.35 W cm−2 of power. At0.4 V in H2/air fuel cell, the Pt/C cathode of Figure 33 yields2.31 A cm−2 or 0.92 W cm−2 of power. The 2015 DOE targetfor durability of portable fuel cell systems is defined as amaximum of 20% of power loss over 5000 h. If we assume (i)that the entire loss of power may be attributed to loss of thePt/C catalyst performance, and (ii) that the decline of powerloss is linear with elapsed time, then at 0.4 V in H2/O2 fuel cell,

the power loss of the Pt/C catalyst in Figure 33 should be0.0297 W cm−2 per 100 h and 0.0259 W cm−2 in H2/air fuelcell. Therefore, after 1100 h in H2/O2 fuel cell, the powerdelivered by Pt/C of Figure 33 should be 1.318 W cm−2.In these conditions, the stable behavior of catalyst 14#97 at0.13 W cm−2 represents 9.8% of the power delivered by Pt/Cafter 1100 h. Accordingly, after 700 h in H2/air fuel cell, thepower delivered by Pt/C of Figure 33 should be 0.898 W cm−2,and the stable behavior of catalyst 11#137 at 0.14 W cm−2

represents 15.6% of the power delivered by Pt/C after 700 h.At the end of this Review of the carbon-based non-noble

metal and metal-free catalysts, one may wonder how muchfurther these catalysts need to go to become serious contendersas Pt-based catalysts in automotive application. According tothe manufacturing cost analysis of Fuel Cell Systems publishedin 2011 by the DOE for an estimated annual production rateof 500 000 units for automotive application, a 0.125 mg cm−2

Pt-based cathode catalyst would represent ca. 30% of thetotal cost of the stack (25$/kWnet) or 15% of the system cost(51$/kWnet).

756 The calculation of this study was based onMEAs of 0.125 mg Pt cm−2 of a nanostructured thin film plati-num catalyst producing 833 mW cm−2 at 55% cell efficiency(0.676 V) under H2/air. In this Review, the most performingnon-noble metal catalyst produces 230 mW cm−2 at 55% cellefficiency (0.676 V) under the same conditions.731 However, ina PEM fuel cell, the catalyst is not the only element composingthe stack. A decrease in performance from a non-noble metalcatalyst as compared to a Pt cathode will inevitably imply anincrease in the number of bipolar plates, membranes, gaskets,and current collectors to keep the same power output. There-fore, by using the cathode of 500 000 units of Fuel Cell Systems,the most performing non-noble metal catalyst, and assuming thatthis catalyst will be as durable as Pt and that its production costwill be 10 times less expensive than that of a Pt cathode, thestack cost will nevertheless jump to 51$/kWnet or twice the costof a Pt stack! From this analysis, it is then obvious that the futureresearch axes for non-noble (or metal free) catalysts for ORR willbe (i) continue to improve their initial peak power performance;(ii) improve their durability to at least that of Pt; and (iii)decrease their production cost to the smallest possible fraction ofthe cost of Pt-based catalysts.

10. CONCLUSIONSThis comprehensive Review covers almost all of the state-of-the-art developments of low cost and high performance electro-catalysts for oxygen reduction reactions in acidic media. For thepast eight years, great progress has been made in developingmore active electrocatalysts based on both Pt-based nanoma-terials and non-noble metal compounds/composites. For theformer, the size, composition, morphology, porosity, surfacestructure, synthesis method, and post-treatment play significantroles in determining their activity and stability. In general, theactivity of Pt-based electrocatalysts can be improved by: (1)Incorporating proper transition metals to increase the disper-sion of Pt atoms and specific activity. The Pt mass activityimprovement in conventional Pt alloys is typically limited up to4-fold due to large particle size and imperfect structure. (2)Forming a core−shell structure to improve the utilization of Ptatoms and modify the electronic properties by strain and ligandeffects from the core. On the microgram scale synthesis, thehighest Pt mass activity improvement factor can reach as highas 14-fold (2.8 A mg−1).362 On the gram scale synthesis, it islimited to 7-fold.360 (3) Forming and maintaining structures

Figure 33. Polarization and power curves obtained at 80 °C in H2/O2and H2/air PEM fuel cells using a Pt-based commercial catalyst with acathode loading of 0.3 mgPt cm

−2. The membrane was Nafion NR211.Backpressures of 15 psig and 0.3 lpm for either H2, O2, or air wereused in these experiments.



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with only the highest active facets exposed to electrolytes.This strategy so far showed the highest activity of 6.98 A mg−1,that is, 35-fold enhancement with a Mo-doped Pt3Ni octahedralstructure.320 (4) Creating a porous structure by dealloying toincrease the surface area and strain. A 20-fold activity improve-ment has been observed with a Pt−Ni nanoframe structure.300

The activity can be further improved by post surface modifi-cation via annealing or acid treatment. In summary, the highestactivities according to RDE evaluations have been observed onthe shape-controlled Pt alloys and core−shell structures so far.Whether these highly active ORR electrocatalysts can exhibitthe same enhancement factor in a real fuel cell is still an openquestion. Great efforts on demonstrating their feasibility in fuelcell applications with desired performance and durability arerequired.Even though significant progress has been made on metal

oxides, nitrides, oxynitrides, carbonitrides, and chalcogenides,their activities are still not comparable to those of Pt-basedmaterials. In the frame of this Review, we also reported recentresults for the behavior of non-noble catalysts of the typeFe/N/C and Co/N/C at the cathode of PEM fuel cells. Moreparticularly, we focused on the performance of these catalysts inH2/O2 and H2/air fuel cells as well on their durability at thecathode of the same cells. Our observations and comparisonsare made in terms of electrical power delivered by the PEM fuelcells. For the performance, the initial peak power of the non-noble catalysts was listed, discussed, and compared to Pt/C.It was found that in H2/O2 fuel cells, the initial peak power fornon-noble catalysts is comprised between 0.05 and 0.98 W cm−2,depending on their mode of synthesis. The most performingcatalysts are those made using precursors for the metal and thenitrogen atoms and either very porous carbon supports orcarbon precursors that will also yield very porous carbonsupports upon pyrolysis. Doing so, the density of sites able toreduce oxygen is maximized as those sites are disseminated onthe entire surface area of the catalyst material. An intermediatepossibility with intermediate initial peak power performance isobtained when a carbon precursor is used with a sacrificialsupport. The latter has to be removed chemically, once thecatalyst is obtained, after precursors and sacrificial support havebeen heat-treated at high temperature.For all catalysts, this Review also lists the main catalytic

site(s) able to perform ORR as reported by the authors ofthese catalysts. These sites are of three types: MeNx, CNx, andencapsulated metal. Results reporting the initial peak powerperformance of non-noble catalysts in H2/air fuel cells aremuch less numerous than those obtained in H2/O2 fuel cells.However, the conclusions drawn from H2/air experiments alsoconfirm what was reported about the initial peak powerperformance of non-noble catalysts in H2/O2 PEM fuel cells. Ascompared to a Pt/C cathode loaded at 0.3 mg Pt per cm2, theinitial peak power of the most performing catalyst in H2/O2(14#214) is 72% of that of Pt/C, while in H2/air, the initialpeak power of the most performing catalyst (14#214) is only43% of that of Pt/C. This is the sign of an important masstransport problem of non-noble catalysts used at the cathode ofH2/air fuel cells.As far as durability is concerned, it was seen that it is

absolutely necessary to test non-noble catalysts in fuel cells asresults of RDE cycling may indicate a durability, which is notconfirmed in fuel cell. The most important result about the lackof durability of most of the non-noble catalysts is that the mostperforming ones are also the least durable ones. The latter are

also the catalysts that are characterized by the largest micro-porous surface area being practically equivalent to the totalBET surface area of the material. This may have importantconsequences because the most active catalytic sites (the MeNxsites) are known to be hosted in the micropores of thecarbonaceous support. A large microporous surface area willtherefore lead to a large number of highly active sites, but thosemay easily be flooded if the carbonaceous catalyst supportbecomes too hydrophilic in a running fuel cell. Rendering thiscarbonaceous support less hydrophilic by improving itsgraphitization inevitably leads to a decrease of the numberof heteroatoms (like N and O) on the carbon support andtherefore to a lower site density and catalyst performance. Anelectrooxidation of the carbonaceous support of the catalystand the accumulation of some H2O2 from an incompleteoxygen reduction in the cathode may also be potential causes ofthe lack of durability of non-noble catalysts. The latter potentialcauses need to be studied in detail (in fuel cell absolutely, not incycling tests in solution) to determine if they have an importantinfluence on the durability of non-noble catalysts in actualfuel cells.So far, the most stable catalysts are those made with a pre-

existing carbon support to host the catalytic sites. For 14#97,which has been tested during about 1100 h at 0.4 V and inH2/O2 fuel cell, the stable power delivered represents 9.8% ofthe power that should be delivered (according to DOE) byPt/C after the same test duration, while the power of the moststable catalyst (11#137) that has been tested during about700 h at 0.4 V in H2/air fuel cell represents 15.6% of the powerthat should be delivered by Pt/C after the same test duration.In 14#97, the authors believe that the main catalytic sites areCNx, while in 11#137, the ORR activity has been mainlyattributed to MeNx sites. As 9.8% or 15.6% represents only asmall fraction of the 2015 targeted performance for Pt/C inportable applications, it is important, as a first step, to be able tostabilize the best non-noble catalysts at their initial peak power.However, this will not be enough for these catalysts to becomeserious contenders to Pt/C at the cathode of PEM fuel cells.Their performance still needs to be improved to reach that ofPt catalysts, and their production cost needs to be drasticallydecreased to a very small fraction of the Pt catalyst productioncost.

AUTHOR INFORMATIONCorresponding Author

*Tel.: +852-34692269. E-mail: [email protected]

The authors declare no competing financial interest.

Biographies

Minhua Shao earned his B.S. (1999) and M.S. (2002) degrees inchemistry from Xiamen University, and a Ph.D. degree in materialsscience and engineering from the State University of New York atStony Brook (2006). He joined UTC Power in 2007 to lead thedevelopment of advanced catalysts and supports for PEMFC andPAFC. He was promoted to UTC Technical Fellow and ProjectManager in 2012. In 2013, he joined Ford Motor Company to conductresearch on lithium-ion batteries for electrified vehicles. He thenjoined the Hong Kong University of Science and Technology in theDepartment of Chemical and Biomolecular Engineering as anAssociate Professor in 2014. He received the Supramaniam SrinivasanYoung Investigator Award from the ECS Energy Technology Division



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(2014) and the Student Achievement Award from the ECS IndustrialElectrochemistry and Electrochemical Engineering Division (2007).His research mainly focuses on electrocatalysis and advanced batteries.

Qiaowan Chang received a B.S. in chemical engineering from TianjinUniversity and B.S. (Double Degree) in finance from NankaiUniversity in 2014. She is now undertaking a MPhil degree at theHong Kong University of Science and Technology under thesupervision of Professor Minhua Shao. Her main research interestsare advanced materials for fuel cells.

Jean-Pol Dodelet got his Ph.D. in 1969 in Physical-Chemistry from“L’Universite Catholique de Louvain”, Belgium. The same year, he leftfor Canada where he became Postdoctoral Fellow (from 1969 to1971), then Research Associate (from 1971 to 1976) in RadiationChemistry at “The University of Alberta”, Edmonton, Alberta, Canada.In 1976, he became Professor of Physical-Chemistry at “L’Universite du Quebec a Trois Rivieres” in Quebec, Canada, where he worked onthe properties of molecular photoconductors. In 1981 he becameProfessor at INRS, which stands for “Institut National de la RechercheScientifique”, in Quebec, Canada, where he was still working until hisretirement in 2015. At INRS, he became interested in electrocatalysis,especially in non-noble electrocatalysts for the reduction of oxygen inPEM fuel cells, a research topic that he has now pursued since 1990. Inthe last several years, he collaborated with General Motors in theframe of an NSERC Industrial Research Chair in electrocatalysis todevelop non-noble electrocatalysts. The chair ended in December2009. Since that time, he continued to be involved in research anddevelopment on the same topic with a focus on the durability of thesenon-noble electrocatalysts at the cathode of H2/air PEM fuel cells.

Regis Chenitz was born in 1980 in Paris, France. He received his“diplome d’ingenieur” in Physics and Chemistry from ENSCBP andhis M.Sc. in Chemistry and Materials from “L’Universite de BordeauxI”, France, in 2006. He then moved to Canada and obtained his Ph.D.in Energy and Materials Science from “L’INRS-EMT”, Canada, in2012 under the supervision of Professor Jean-Pol Dodelet. Presently,he is a Research Associate at “L’INRS-EMT”, Canada. His researchinterests involve catalysis and polymer membranes for PEMFC, directmethanol and formic acid fuel cells, non-noble metal catalyst for ORRin acid and alkaline media, and nanomaterial synthesis andcharacterization.

ACKNOWLEDGMENTS

Work on this Review at the Hong Kong University of Scienceand Technology (HKUST) was supported by the ResearchGrant Council of the Hong Kong Special AdministrativeRegion (IGN13EG05 and 26206115) and a startup fund fromthe HKUST. Work on this Review at INRS Energie, Materiauxet Telecommunications, was supported by funds provided fromMESRST (Gouvernement du Quebec) and from NSERC, theCanadian funding agency.

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