Electrochimica Acta 164 (2015) 235–242

Enhanced Methanol Tolerance of Highly Pd rich Pd-Pt CathodeElectrocatalysts in Direct Methanol Fuel Cells

Baeck Choi a,1, Woo-Hyun Namb,2, Dong Young Chung a,d, In-Su Park a, Sung Jong Yoo a,c,Jae Chun Song e, Yung-Eun Sung a,d,*a School of Chemical & Biological Engineering, Seoul National University, Seoul 151-742, South KoreabDepartment of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju 500-712, South Koreac Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, South KoreadCenter for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 151-742, South KoreaeHanwha Chemical Corporation, Seoul 100-797, South Korea

A R T I C L E I N F O

Article history:Received 2 January 2015Received in revised form 23 February 2015Accepted 24 February 2015Available online 26 February 2015

Keywords:ElectrocatalystAlloyMethanol toleranceDirect methanol fuel cellsOxygen reduction reaction

A B S T R A C T

Methanol crossover critically restricts the practical application of direct methanol fuel cells (DMFCs). Toresolve this crucial difficulty from the standpoint of electrocatalysis, an electrode material having highactivity for the oxygen reduction reaction and low activity for the methanol oxidation reaction comparedto widely used Pt-based electrodes is needed for DMFC cathodes. In this research carbon-supportedPd-rich Pd–Pt bimetallic nanoparticle electrocatalysts with 60 wt.% metal content were prepared for thispurpose by sodium borohydride reduction of metal chlorides. The physical features of the preparednanoparticles were investigated by transmission electron microscopy, energy dispersive X-rayspectroscopy, atomic absorption spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy,and X-ray absorption near edge spectroscopy. Methanol tolerance was tested by means of rotating diskelectrode (RDE) voltammetry using oxygen-saturated methanol-containing electrolyte solutions as theanode fuel for DMFC unit cell performance tests. In the RDE measurements, Pd-rich electrocatalysts(carbon-supported Pd19Pt1 nanoparticles) showed excellent methanol tolerance compared with Pd-freePt electrocatalyst. When Pd19Pt1 nanoparticles were used as a DMFC cathode catalyst, unit cellperformance tests showed that the i-V curves of the Pd19Pt1 electrocatalyst decreased slightly withincreasing methanol concentration, while that of the Pt electrocatalyst decreased rapidly. The results in aliquid-feed DMFC unit cell test were in good agreement with the methanol tolerant characteristicsidentified in the RDE measurements.

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1. Introduction

Direct methanol fuel cells (DMFCs), which are being consideredas portable energy conversion devices, produce electrical energyfrom the catalyzed electrochemical reactions of methanoloxidation and oxygen reduction [1–5]. The oxygen reductionreaction (ORR) occurs at the cathode using 4 electrons, 4 protons,and an oxygen molecule as reactants. Considerable attention hasbeen directed toward electrocatalytic enhancement of this

* Corresponding author. Tel.: +82 2 880 1889; fax: +82 2 888 1604.E-mail address: ysung@snu.ac.kr (Y.-E. Sung).

1 Present address: Department of Chemical Engineering, University of Florida,1006 Center Drive, Gainesville, FL 32603.

2 Present address: Hyundai Next Generation Vehicle Technology (NGV), Seoul151-742, S. Korea.

http://dx.doi.org/10.1016/j.electacta.2015.02.2030013-4686/ã 2015 Elsevier Ltd. All rights reserved.

reaction over the last several decades. However, the level ofelectrode efficiency achieved to date is still insufficient forsuccessful application and marketing of DMFCs [6,7].

Although liquid methanol is considered to be a suitable fuelfor portable fuel cells because of its easy handling andsubstantial energy density upon direct electrocatalyticoxidation, DMFC cathodes are subjected to unique conditionsthat originate from the so-called “methanol crossover” [8–10].The decrease in the DMFC cathode performance is mainlyowing to methanol crossover, in which methanol is transportedfrom the anode to cathode through a polymer electrolytemembrane (PEM) [11,12]. This transport occurs owing todiffusion caused by methanol concentration gradient andelectroosmotic drag accompanying the proton transfer [13,14].Pt-based nanoparticles are well known for their effectiveelectrocatalytic activity in cathodic reactions as well as anodic

236 B. Choi et al. / Electrochimica Acta 164 (2015) 235–242

reactions such as methanol oxidation [15–18]. Consequently,transported methanol molecules can be oxidized on thecatalytic Pt-rich DMFC cathodes. This phenomenon decreasesDMFC performance, restricts the use of highly concentrated

Fig. 1. TEM images of carbon-supported (a) Pt, (b) Pd1Pt1, (c) Pd3Pt1 (d) Pd19Pt1, and (e) Pnanoparticles.).

methanol as an anode fuel, and makes the balance of plant(BOP) systems complex [19–21].

Methanol-impermeable PEMs can be introduced into themembrane electrode assembly (MEA) to reduce methanol crossover

d nanoparticles containing 60 wt.% metal. (Insets are the EDS spectra of each set of

Table 1Summary of physical characterizations by XRD, AAS, and EDS.

Electro-catalysts Crystallite size(XRD, nm)

Metal composition(AAS, wt.%)

Atomic composition (%)

AAS EDS

Pt Pd Pt Pd

Pt 4.71 61 100 0 100 0Pd1Pt1 3.81 64 54.74 45.26 56.52 43.48Pd3Pt1 3.39 – – – 73.21 26.79Pd19Pt1 2.82 54 4.26 95.74 4.42 95.58Pd 3.23 57 0 100 0 100

Fig. 2. (a) Wide-range and (b) (111) crystallographic plane XRD patterns ofcarbon-supported Pt, Pd1Pt1, Pd3Pt1, Pd19Pt1, and Pd nanoparticles.

B. Choi et al. / Electrochimica Acta 164 (2015) 235–242 237

[22–24]. Cathode electrocatalysts must show selective oxygenreduction activity compared to methanol oxidation when Pt-basednanoparticlesareusedasDMFCcathodeelectrocatalysts[4,8,25–29].Non-Pt-basednanoparticleshavebeenproposedascathodecatalysts[9,30–34]. Meanwhile, sufficient oxygen reduction activity has to beensured andthe electrocatalystmust possess considerablemethanoltolerance [35–37]. Pd-based electrocatalysts with high oxygenreduction activity and low methanol oxidation activity have beensuggested as possible DMFC cathode catalysts [38–45].

Herein, we report the DMFC performance of Pd-rich Pd–Ptbimetallic nanoparticles (containing less than 5 at% Pt) to establishthe possible use of materials with extremely low Pt contentas DMFC cathode catalysts. The results show that Pd-richelectrocatalysts exhibit better i-V performance curves in highlyconcentrated methanol feed solution, and help minimize the use ofexpensive Pt in Pd–Pt bimetallic catalysts.

2. Experimental

2.1. Fabrication of carbon-supported Pd, Pd–Pt, and Pt nanoparticles

Activated carbon-supported (Vulcan XC-72R, Cabot) Pd, Pd–Pt,and Pt nanoparticles were synthesized by the conventional sodiumborohydride (NaBH4, Aldrich) method combined with freezedrying. It was intended to produce material with 60 wt.% metal.First, carbon powder was dispersed in deionized water (18.2 mVcm) and ultrasonicated for one hour. Then, metal salts palladiumchloride (PdCl2, Aldrich) and chloroplatinic acid hexahydrate(H2PtCl6�6H2O, Aldrich) were added to the carbon powderdispersion and stirred. The metal salts were then reduced by afivefold excess of borohydride. The resulting particles were washedseveral times with deionized water to remove residual chloride ion(Cl�) and were dried by a freeze drying process.

2.2. Physical characterizations

Nanoparticle composition was examined by an energydispersive X-ray spectroscopy (EDS) facility equipped with aTEM microscope and an atomic absorption spectrometer (AAS).The X-ray diffraction (XRD) patterns and photoelectron spectra(XPS) were measured by using a D/MAX 2500 PC powder X-raydiffractometer (Rigaku) and an ESCALAB 250 XPS spectrometer(VG Scientifics). Transmission electron microscopy (TEM) imageswere obtained with a JEOL EM-2000 EXII microscope. X-rayabsorption near edge spectra (XANES) were measured withLab-EXAFS (R-XAS, Rigaku).

2.3. Electrochemical characterizations

Electrochemical measurements were performed using a3-electrode cell with an AUTOLAB (Eco Chemie) potentiostat at

room temperature. A Pt wire and a Ag/AgCl (saturated KCl)electrode were used as the counter and reference electrodes,respectively, and electrode potentials were reported with respectto the normal hydrogen electrode (NHE). A glassy carbon rotatingdisk electrode (RDE, Eco Chemie) was used as the workingelectrode. It was polished successively with 1-, 0.3-, and 0.05-mmalumina (Al2O3) paste and washed ultrasonically with Milliporewater. The catalyst ink was pipetted onto the mirror-finish glassycarbon substrate. The metal catalyst content on the RDE electrode(0.000196 m2) was 0.039 gmetal/m2. Voltammetry experimentswere conducted at a scan rate of 5 mV/s in 0.5 M H2SO4, 0.5 M

Fig. 3. (a) Pd 3d spectra of carbon-supported Pd, Pd19Pt1, Pd3Pt1, and Pd1Pt1 nanoparticles. (b) Pt 4f spectra of carbon-supported Pd19Pt1, Pd3Pt1, Pd1Pt1, and Pt nanoparticles.(c) Change in the reduced metallic state of Pd and Pt as a function of Pd content in the nanoparticles.

238 B. Choi et al. / Electrochimica Acta 164 (2015) 235–242

H2SO4 + 0.1 M CH3OH, and 0.5 M H2SO4 + 0.5 M CH3OH supportingelectrolytes purged with Ar gas and saturated with oxygen gas.For CO stripping voltammetry, CO was adsorbed on the catalystsurface at 0.1 V vs. NHE using 99.5% CO gas in 0.5 M H2SO4

electrolyte for 2 min. Then, Ar gas was purged into the electrolyteto remove the dissolved CO gas, and cyclic voltammetry wasconducted at a scan rate of 5 mV/s.

2.4. DMFC unit cell performance test

The catalyst slurry was prepared by dispersing an appropriateamount of the catalyst in deionized water and by adding 15% and7% NafionTM solution (Aldrich) to the anode and cathodecompartments, respectively, and 2-propanol (Aldrich). Unsup-ported Pt-Ru 1:1 black (Johnson Matthey) was used as the anodecatalyst. The prepared catalyst inks were brushed onto a carbonpaper (Toray), which was wet-proofed by treating it with 20 wt.%TeflonTM (PTFE). NafionTM 117 was used as a PEM after beingpretreated in a hydrogen peroxide (H2O2, Fisher) and sulfuric acid(H2SO4, Aldrich) solution. The catalyst loadings of the anode andcathode were 5 mgmetal /cm2 and 2.8 mgmetal /cm2, respectively.The MEA was prepared by hot pressing (110 �C, 800 psi) for 3 min.The unit cells, which had an active area of 2 cm2, were measuredat 30 and 70 �C by flowing 2.0, 4.0, and 6.0 M methanol at a rate of1 mL/min to the anode and dry air at a rate of 500 mL/min to thecathode without any back pressure, using a computer-controlledelectric load.

3. Results and discussion

3.1. TEM, EDX, and AAS analyses

In the TEM images (Fig. 1), it can be seen that small-sized Pt,Pd–Pt, and Pd nanoparticles were well-dispersed on the activatedcarbon surface. For the Pd19Pt1 nanoparticles, the bulk atomiccompositions of Pd and Pt obtained from EDS were 4.42% and95.58%, respectively. Atomic absorption spectroscopy (AAS) wasused to establish the total metal content of the nanoparticles aswell as the compositions of Pd and Pt. The atomic compositions ofPd and Pt obtained by AAS were 4.26% and 95.74%, respectively.(Table 1) These compositions were in good agreement with thosefound in the EDS spectra. In addition, it was confirmed by AASanalysis that the total metal contents of carbon-supported Pd,Pd19Pt1, and Pt were 61, 64, and 57 wt.%, respectively.

3.2. XRD

Wide range X-ray diffraction (XRD) patterns showed facecentered cubic (fcc) structured Pt, Pd–Pt, and Pd nanoparticles onthe carbon support surface (Fig. 2a). The calculated crystal sizebased on the Debye-Scherrer equation with (2 2 0) crystalline planefor Pd, Pd19Pt1, Pd3Pt1, Pd1Pt1, and Pt were 3.23, 2.82, 3.39, 3.81, and4.71 nm, respectively (Table 1). The gradual shift in the diffractionpeak position to lower 2u values with increasing Pd content(Fig. 2b) revealed the successively formed Pd–Pt alloy nano-particles. Here, the diffraction peak positions of the (111) plane are

Fig. 4. (a) Pd K-edge XANES spectra of carbon-supported Pd, Pd19Pt1, Pd3Pt1, andPd1Pt1 nanoparticles. (b) Pt L3-edge XANES spectra of carbon-supported Pd19Pt1,Pd3Pt1, Pd1Pt1, and Pt nanoparticle catalysts.

B. Choi et al. / Electrochimica Acta 164 (2015) 235–242 239

39.12�, 39.27�, 39.68�, 39.75�, and 39.92� for Pd, Pd19Pt1, Pd3Pt1,Pd1Pt1, and Pt nanoparticles. The TEM images and FWHM(calculated by the XRD (111)) shows that the nanoparticle sizedecreases as the content of the Pd precursor increased in the Pd–Ptalloy. Standard reduction potential of PdCl2/Pd is 1.88 eV, whilethat of PtCl62�/Pt is 1.44 eV. The high standard reduction potentialinduced increasing the amounts of seed and inhibiting the particlegrowth due to the low concentration of precursor after burstnucleation. We expect the standard reduction potential affects theparticle size as varying the Pd:Pt ratio.

3.3. XPS and XANES analyses

We investigated the modification of electronic states in Pd–Ptnanoparticle catalysts by examining the oxidation state of eachelement using XPS and XANES. Fig. 3 shows Pd 3d XPS spectra ofcarbon-supported Pd, Pd19Pt1, Pd3Pt1, and Pd1Pt1 and Pt 4f XPSspectra of Pd19Pt1, Pd3Pt1, Pd1Pt1, and Pt nanoparticles. As the Pdcontent is increased, the intensity of the Pd+2 peaks (Pd–O)increased significantly in both the 3d5/2 (336.00 eV) and 3d3/2(340.80 eV) spectra, and the reduced Pt metallic state alsoincreased. Considering that NaBH4 is a strong reducing agent, Ptand Pd are presumed to be reduced to similar extents. An increasein the Pd content indicates that the surface Pd has probablyincreased and that the surface Pt has probably decreased. Becausesurface Pd and Pt can be easily oxidized, the proportion of reducedmetal states decreases at locations nearer the surface. These results

are summarized in Fig. 3(c). Also, the binding energy (BE) of themetallic Pd0 peak of Pd19Pt1 (335.00 eV) shifted to a slightly lowervalue than that of Pd (335.10 eV) nanoparticles. Normally, thewhite-line intensity of XANES spectra is characteristic of theoxidation state of each element. In Fig. 4, the Pd K-edge spectrashow slightly increased white-line intensity whereas the PtL3-edge spectra show a decreased intensity with increasing Pdcontent. These tendencies may result from the electronicmodification of Pd caused by the small fraction of Pt atoms inPd-rich Pd–Pt nanoparticle catalysts. These modifications mayalter the adsorption properties of small molecules such asmethanol (CH3OH) and carbon monoxide (CO) and their oxidativeactivity at the electrochemical interface of the Pd–Pt nanoparticlecatalysts.

3.4. Oxygen reduction reaction kinetics

In Fig. 5(a), different Pt, Pd–Pt, and Pd electrocatalysts showdifferent cyclic voltammetric wave shapes, especially in thelow-voltage, hydrogen adsorption/desorption region. The kineticsof the ORR was investigated using a rotating disk electrode (RDE) inmethanol-free and methanol-containing electrolyte solutions. InFig. 5(b), the Pt electrocatalyst shows better activity than thePd-based electrocatalysts in aqueous 0.5 M H2SO4 solution. Theonset potential of oxygen reduction gradually reduced from 0.98 V(Pt) to 0.91 V (Pd) as the Pd content increased, and Pt exhibited thehighest cathodic current density in the limiting current region. Inthis regard, Luo et al. reported low MOR activity for a Pd-modifiedPt electrode in 0.1 M H2SO4 + 0.5 M CH3OH electrolyte [46] andArenz et al. reported with the help of situ FTIR experiments that theMOR is highly inhibited on a Pd-modified Pt electrode [47].Further RDE experiments were performed in oxygen-saturatedmethanol-containing electrolyte solutions (Fig. 5(c, d)). Based onthe results, it can be confirmed that the enhanced ORR of Pd–Pt canbe attributed to the selective oxygen reductive activity of Pd–Pt inthe presence of methanol.

3.5. CO stripping voltammograms

Fig. 6 shows CO stripping voltammograms of Pt, Pd1Pt1,Pd19Pt1, and Pd electrocatalysts. As an intermediate of methanoloxidation, CO is strongly adsorbed on electrocatalyst surfacesduring the methanol oxidation reaction (MOR). In the COstripping voltammograms, Pd-based electrocatalysts exhibiteda more positive CO stripping peak potential than the Ptelectrocatalyst. Papageorgopoulos et al. reported that Pt–Pdbimetallic carbon-supported electrocatalysts are CO-tolerant interms of both their more positive stripping peak potential andreduced CO coverage [48]. They also observed that the peakpotential shifted in the negative direction as the Pt contentincreased in Pd–Pt bimetallic systems. The decrease in the Pdcontent implies the presence of more weakly adsorbed COmolecules on the surface of Pd–Pt electrocatalysts because of theless positive peak potential. Alvarez et al. also observed a slightdecrease in CO coverage on Pd-deposited Pt (111) electrodescompared to bare Pt (111) [49]. Low CO coverage on Pd–Ptbimetallic electrocatalysts could provide fresh active sites for theORR, because CO is strongly adsorbed on Pt-based electro-catalysts. Most of the methanol tolerance originated not fromweak CO binding, but from decrease in CO generation during theMOR. Consideration of the more positive oxidation potential withincreasing Pd content suggests that the improved methanoltolerance has originated from the weak CO binding energy.Decreased CO generation arises from the atomic ensemble effect,wherein methanol oxidation occurs by a direct pathway that does

Fig. 5. (a) Cyclic voltammetry curves for carbon-supported Pt, Pd1Pt1, Pd19Pt1, and Pd nanoparticle catalysts in Ar-purged 0.5 M H2SO4 electrolyte at a scan rate 50 mV s�1.Linear sweep voltammetry curves of the same nanoparticle catalysts in O2-saturated (b) 0.5 M H2SO4, (c) 0.5 M H2SO4 + 0.1 M CH3OH, and (d) 0.5 M H2SO4 + 0.5 M CH3OHsolutions at 5 mV/s.

240 B. Choi et al. / Electrochimica Acta 164 (2015) 235–242

not generate adsorbed CO as an intermediate. A second reason fordecreased CO generation during the reaction is the low methanoloxidation current shown in Fig. 5(c) and 5(d).

3.6. DMFC unit cell performance

Using Pt and Pd19Pt1 electrocatalysts, DMFC unit cell testswere performed at 30 and 70 �C by feeding 2, 4, and 6 M methanolto the anode and air to the cathode. Fig. 7(a, b) shows the

Fig. 6. CO stripping voltammetry of carbon-supported Pt , Pd1Pt1, Pd19Pt1, and Pdnanoparticle catalysts.

methanol concentration dependence of Pt electrocatalysts at70 and 30 �C, and Fig. 7(c, d) show the results of the performanceof Pd19Pt1 electrocatalysts. In spite of the decrease in the Ptcontent in the cathode catalysts, the more preferable perfor-mance was obtained using the Pd-rich Pd19Pt1 electrocatalyst. Theresults of the DMFC unit cell test shown in Fig. 7(a) and (c)indicates that the cell voltage of the Pd19Pt1 electrocatalystincreased slightly with decreasing methanol concentration, whilethat of Pt electrocatalyst increased rapidly at 70 �C. Thus, theseresults could be attributed to the fact that the Pd19Pt1 electro-catalyst has more unsusceptible to methanolthan Pt to the MOR,as suggested by the results of the RDE measurements inmethanol-containing oxygen-saturated sulfuric acid electrolyte.These discrepancies make it clear that highly Pd-rich PdPt/Ccatalysts have a significant methanol tolerance compared to Pt/C.In addition, the cell voltages of Pd19Pt1 electrocatalysts under aload of 100 mA/cm2 were 351, 314, and 307 mV at 2.0, 4.0, and6.0 M methanol feed, while those of Pt were 334, 260, and 155 mV,respectively. The decrease in the cell voltage with increasingmethanol concentration was much slower at 30 than 70 �C. Aconsiderable methanol tolerance was observed at 30 �C, evenwhen 6.0 M methanol is used with the Pd19Pt1 electrocatalyst.This cell voltage behavior caused by the increase in the celltemperature can be explained by the methanol permeability ofNafionTM, which increases with temperature, and which results ina rapid drop of cell voltage as the temperature increases. Fromthese results, it can be inferred that the Pd19Pt1 electrocatalystsprovide excellent DMFC performance because of their methanoltolerant property, in spite of their minimized Pt composition. ThePd19Pt1 electrocatalysts show such remarkable performancebecause that are MOR inactive.

Fig. 7. DMFC unit cell performances of carbon-supported Pt at (a) 70 �C, (b) 30 �C and Pd19Pt1 at (c) 70 �C, (d) 30� electrocatalysts. Anode: Pt1Ru1 black (5 mgmetal/cm2), 2.0, 4.0,and 6.0 M CH3OH solutions fed at 1 mL/min; Cathode: Pt/C, Pd19Pt1/C (2.8 mgmetal/cm2) and 500 cc/min dry air without back pressure.

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4. Conclusions

In this study, we have attempted to produce methanol-tolerantPd-rich Pd–Pt electrocatalysts with minimal Pt content for use as aDMFC cathode. By comparing the performance of a carbon-supported Pd19Pt1 electrocatalyst with Pt, we could conclude thathighly Pd-rich Pd–Pt bimetallic catalysts show methanol tolerancewith enhanced DMFC performance. Even more remarkable resultswere obtained for the Pd19Pt1 electrocatalyst compared to thoseobtained for Pt when a highly concentrated methanol solution wasfed into the anode. Therefore, Pd-rich Pd–Pt electrocatalysts couldbe a viable candidate for DMFC cathodes, especially for highlyconcentrated methanol feed solutions.

Acknowledgement

This work was supported by IBS-R006-G1.

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