nanocomposite membranes for polymer electrolyte fuel cells

Feature Article

Nanocomposite Membranes for PolymerElectrolyte Fuel Cells

Nagappan Nambi Krishnan, Dirk Henkensmeier,* Jong Hyun Jang,Hyoung-Juhn Kim

Nanoparticles are known to play several roles as polymer fillers, among them reducing thebulk material price or adjusting the mechanical properties. In ion conducting polymermembranes, nanoparticles are also used to manipulate the water balance and fuel
permeability of the membrane and to increase theion conductivity. Suitable materials are metal oxides(SiO2, TiO2, etc.), electron deficient boron nitride or ionconductive functionalized metal oxides (S—ZrO2, HPA—SiO2, etc.). This article shows which membrane proper-ties can be addressed by adding nanoparticles anddescribes the underlying mechanisms.
Dr. N. N. Krishnan, Dr. D. Henkensmeier, Dr. J. H. Jang, Dr. H.-J. KimFuel Cell Research Center, Korea Institute of Science andTechnology, Hwarangno 14gil5, Seongbukgu 136-791, Seoul, KoreaE-mail: [email protected]

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ectmethanol fuel cell (DMFC) and the protons

1. Introduction

Fuel cells are devices that electrochemically convert a fuel’s

chemical energy to electrical energywithhigh efficiencyby

prohibiting direct, uncontrolled oxidation.Withno internal

moving parts, fuel cells are similar to batteries in operation

and constituents. The key difference is that while batteries

store energy, fuel cells produce electricity continuously

as long as fuel and oxidant are supplied. The following

introduction aims at recalling the fuel cell basics, which are

necessary to fully understand the elaborations on nano-

composite membranes.

1.1. Polymer Electrolyte-Based Fuel Cells

Depending on the membrane and fuel, polymer electro-

lyte-based fuel cells (PEFCs) are categorized as shown in

Table 1.

In the PEMFC, hydrogen is oxidized at the anode to form

protons, amembrane separates the anode and cathode and

transports the protons to the cathode, where they react

with oxygen (Figure 1). Similarly, methanol, usually in the

form of a 0.5–5M water-based solution, is oxidized at the

anode of a dir

are transported by the membrane to the cathode, where

they react with oxygen.

In the case of solid alkaline fuel cell (SAFC) and alkaline

DMFC, the same reactions proceed under alkaline con-

ditions, and the transported ions thus are not protons but

hydroxide ions and are transported in the opposite

direction, from cathode to anode.

PEMFCs can be further distinguished into low temper-

ature PEMFC (LT-PEMFC, <80 8C) and HT-PEMFC (120–

200 8C). The LT-PEMFC, which is typically used in

automobile applications, is based on a water swollen

acidic polymer membrane (e.g., Nafion, a sulfonated

perfluorinated polymer). HT-PEMFCs are mainly based

on phosphoric acid swollen basic polymer membranes,

e.g., phosphoric acid doped poly(benzimidazole).

1.2. The Catalyst Layer of a PEMFC

As shown in Figure 1, the central components of a fuel

cell are the membrane and the catalyst electrode, which

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N. Nambi Krishnan received his M. Sc. (AppliedChemistry – 2000) fromAnnaUniversity (Chennai)and obtained his M. Tech. (Polymer Technology –2003) from CUSAT (Cochin). He worked as JRF inSPIC Science Foundation (Chennai, 2000–2001),andPA II inNCL, Pune (2003–2004).HeobtainedhisPh. D. in Energy and Power Conversion Engineering(2009) from Korea Institute of Science andTechnology (KIST-UST) under the supervision ofDr. Hyoung-Juhn Kim. He served as visitingscientist and STAR postdoctoral researcher inKIST’s Fuel Cell Research Center with Dr. DirkHenkensmeier (2009–2013). Currently, his researchinterests include development of ion conductingpolymer materials for energy applications.

Dirk Henkensmeier is a principal researcher atKorea Institute of Science and Technology (KIST).He studied chemistry at University of Hamburg,fromwhichhe receivedhis doctoral degree in 2003.After positions at LG Chem/Research Park (Korea),Sartorius (Germany) and Paul Scherrer Institute(Switzerland) he joined KIST’s Fuel Cell ResearchCenter in 2009.Hismain interest is onmembranes,ion conducting polymers, and their applications.

Jong Hyun Jang received his Ph. D. degree inChemical Technology from Seoul National Univer-sity, Korea, in 2004. He is currently a principal

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N. N. Krishnan, D. Henkensmeier, J. H. Jang, H.-J. Kim

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together form themembrane electrode assembly. For stable

performance, a good lamination between catalyst layer

and membrane is necessary. This can be achieved by hot-

pressing or direct formation of the catalyst layer on the

membrane by spray coating, and by choosing similar

chemical structures of the membrane and the polymeric

binder in the porous catalyst layer, which consists of

(supported) catalyst particles and ion conducting polymer

binder (often thesamematerial as themembrane is chosen),

enabling the reaction of (e.g., at the cathode) oxygen (from

the gas phase) with electrons (through carbon/metal

particles) and protons (from the ionomer binder) to water,

which needs to be transported away or else will block the

pores. Thus, the electrodes composition allows for electron

and ion conduction and gas diffusion at the same time.

There exists an optimum amount of proton conducting

polymer in the catalyst layer. Low loading will result in

insufficient proton conduction to the catalyst surface and

therefore increase the interfacial resistance. On the other

hand, very high loading of the polymer may lower the

porosity of the catalyst layer and limit themass transporta-

tion, leading to high overpotentials. Also, the stability of

proton conducting materials in the catalyst layer is an

important factor for stable performance of a PEMFC.
researcher at theFuelCell ResearchCenter ofKoreaInstitute of Science and Technology (KIST). Hiscurrent research focuses on catalyst/MEA develop-ment and electrochemical analysis of variouspolymer electrolyte membrane (PEM)-based elec-trochemical devices: PEMFC, water electrolyzer,electrochemicalhydrogenpump,andelectrochem-ical CO2 conversion.
1.3. Fuel Cell Membranes

Themembrane fulfills several functions in the fuel cell and

thus has to fulfill several requirements: the membrane

should be an electric insulator, but show proton conductiv-

ity in the range of 0.01–0.2 mS cm�1. It should seal the cell

against the outside, and should effectively separate the fuel

and oxidant, measured as low gas (hydrogen, oxygen) or

methanol permeation. However, when the same polymer

is used as an ionomer binder, high gas permeability is

required. The membrane should be thermally stable up to

temperatures exceeding the operating temperature, and

also the glass transition temperature should be well above

the operating temperature to prevent creep. Furthermore,

the membrane should show chemical stability in acidic

or alkaline environment, and against water, H2O2, and

hydroxyl radicals. Mechanical stability (e.g., high Young

modulus) is required to resist compressive forces and stress

due to shrinking and swelling uponhumidification. Ideally,

Table 1. Categories of polymer electrolyte fuel cells (PEFCs).

PEFC type Co

Proton exchange membrane fuel cell (PEMFC)

Direct methanol fuel cell (DMFC)

Solid alkaline fuel cell (SAFC)

Alkaline DMFC

Macromol. Mater. Eng. 2014, DOI

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the mechanical properties of dry and wet membranes are

similar, the membranes show a high dimensional stability

(low shrinking and swelling upon humidification), to

minimize membrane-electrode delamination and the risk

of crack or pin-hole formation.

The typical thickness of a PEMFC membrane is between

25 and 50mm, for DMFC up to 127mm to reduce methanol

crossover.

A major breakthrough in PEMFC technology came

with the advent of perfluorosulfonic acid membranes.

These membranes consist of three regions: a Teflon-

like fluorocarbon backbone (—CF2—CF2—), side chains

nducting species in the membrane Fuel

Hþ H2

Hþ Methanol

OH� H2

OH� Methanol

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Figure 1. Scheme and electrode reactions of PEMFC and DMFC.

Nanocomposite Membranes for Polymer Electrolyte Fuel Cells

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(—O—CF2—CFR—O—CF2—CF2—) and super-acidic sulfonic

acid groups (SO�3 Hþ).

The perfluorinated backbone determines high chemical

and thermal stability,while side chains have the properties

of strong, highly dissociated acids. The negative SO�3 ions

are fixed to the side chain, and only the hydrogen ions

contribute to the conductivity. Thus, fuel cell membranes

usually are single ion conductors. The general formula of

commercially available perfluorinated electrolyte mem-

branes is shown in Figure 2.

Currently, Nafion membranes (Dupont de Nemours and

Co.,USA)arealmostexclusivelyusedasaPEMFCmembrane

due to their excellent stability and relatively high proton

conductivity.[1,2] Furthermore, a lifetime of over 50 000h at

80 8C in a PEMFC was reported.[3]

Other commonly investigated membrane materials are

based on sulfonated aromatic polymers (e.g., polysulfone

or PEEK).

Several models have been proposed to describe the

morphology of Nafion. The most common is the Gierke

modelorclusternetworkmodel,which isgenerallyaccepted

to explain Nafion’s microstructure well.[4,5] According to

Figure 2. Simplified chemical structure of commercial perfluoro-sulfonic acid membranes.

Macromol. Mater. Eng. 2014, DO

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this model, the carbon–fluorine backbone forms a semi-

crystalline hydrophobic region, the sulfonic acid groups and

absorbed water form an amorphous hydrophilic region,

and the side chains form a connecting region. Clusters of

around4nmdiametersarearrangedperiodicallyamongthe

hydrophobic region, the distance between clusters (Bragg

distance) is around 5nm, and the clusters are connected by

channels (1nm in diameter) as shown in Figure 3. This

model satisfies the desire for forming hydronium ions from

sulfonic acidgroupsandavoids strong interactionsbetween

water molecules and the hydrophobic backbone. Cluster

structures have been observed in many ionomers. Many

physical techniques such as small angle X-ray diffraction

(SAXS),[6] infrared (IR) spectroscopy,[7] nuclear magnetic

resonance (NMR) spectroscopy,[4] and transmissionelectron

microscopy (TEM)[8] support the cluster model of perfluori-

nated PEMs. Nevertheless, the exact microstructure of

Nafion is still under debate.[9–12]

1.4. Proton Conduction Mechanism

Generally, there are two mechanisms concerned in proton

conduction in proton exchange membranes,[13] the vehicle

mechanism and the Grotthuss mechanism.

In the vehicle mechanism, the protons migrate through

the medium, carrying a solvation shell, e.g., as H3Oþ or

H5O2þ. Theoverall proton conductivity is fullydependant on

the vehicle diffusion rate. This mechanism explains the

observedelectro-osmotic drag, describing the fact thatwater

is transported from the anode to the cathode site during fuel

cell operation. While this effect dehydrates the membrane,

back-diffusion of product water rehydrates the membrane.

In the Grotthuss mechanism, protons are transferred

fromanode to cathode through rearrangement ofhydrogen

bonds. Therefore, an uninterrupted path for proton migra-

tion is needed. The overall proton conductivity is very high

and determined by the proton transfer rate and also

reorganization rate of its environment. In general, the

Grotthuss mechanism is favored under fully humidified

conditions.

These two proton conduction mechanisms are correlat-

ed. In sulfonated or phosphonated polymer electrolyte

Figure 3. Cluster network model for Nafion membranes.[5]

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membranes, the vehicle mechanism is predominant but

the Grotthuss-typemechanism is also present. In phospho-

ric acid doped poly(benzimidazole) systems, the Grotthuss

mechanism isdominant andvehicle-type is presentaswell.

Generally, both mechanisms are present and partially

contribute to the overall proton conduction. Under very dry

conditions, proton hopping from proton binding sites is

anticipated to occur.

Understanding these processes and mechanisms is

necessary in order to understand the potential effects of

nanoparticles on the membranes functionality.

1.5. Challenges in Membrane R&D

A number of reviews concerning the development of

proton-conducting membranes are available. They

reveal information on materials, their electrochemical

properties, water uptake, and thermal stabilities. The

most promising alternative membranes are high perfor-

mance polymers such as polyimides, poly(ether ketone)s,

poly(arylene ether sulfone)s, poly(benzimidazole)s,

etc.[3,14–20]

Most major car makers have targeted 2015 for initial

commercial sales. Hyundai Motor Company plans the

first full-scale commercial production run for 2015, with

a target price of 50 000 $ per car. Nevertheless, a major

challengearises fromthecar industries’wishto increase the

operating temperature (up to 90–120 8C) and to lower the

relative humidity of automotive fuel cell stacks (<50% RH).

Expected benefits are smaller and thus cheaper cooling

systems, lower adsorption of catalyst poisons (especially

carbonmonoxide, which is a trace component of industrial

hydrogen, of which about 95% is produced by reforming of

natural gas), faster electrode kinetics, and lower costs for

humidifying systems.

A common strategy that has been adopted to improve

the membrane’s water retention capacity is to integrate

hygroscopicmaterials, formingnanocomposites. Especially

organic–inorganic hybrid materials are discussed as a

possible solution toenhance thehydrationproperties of the

polymer electrolytes at high temperatures.

In the DMFC, nanocomposites are expected to decrease

the methanol crossover, which decreases both the fuel

efficiency and the fuel cell power.

2. Functions of Nanoparticles in Fuel CellMembranes

In the last decade, nanocomposite membranes, consisting

of polymericmaterials andnanofillers, haveheld scientific,

industrial, and academic significance due to their improved

properties. The inorganic nanomaterial selection mainly

depends on the hygroscopicity, surface area, and porosity.




Ideally, theadditivecanbe immobilizedwithin thepolymer

matrix. They are compatible with the electro-catalyst (no

leaching of catalyst poisons) and they can maintain or

increase the thermo-mechanical properties of the polymer

at high temperature.

Nanocompositemembranes aremainly prepared by two

methods.[21,22] They are

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(a) Mixing of polymer solution and inorganic particles

followed by membrane casting.

(b) In situ nanoparticle synthesis via a sol–gel process.

Particles can be either physically incorporated into

the polymer matrix, or they can be bound covalently.[23]

Furthermore, nanocomposites can be obtained by

in situ impregnation processes[24] or by grafting from

nanoparticles.[25]

The major reason for the development of nanoparticle-

based membranes is to combine both components’ best

properties and to overcome the drawbacks of an individual

component by synergetic interaction. For example, hygro-

scopic nanomaterials adsorb water molecules on their

surface or in their pores. This effect can help to balance the

water uptake of the membrane under dry conditions. Ionic

conductivity of the membranes highly depends on the cell

temperature and relative humidity conditions. When the

operating temperatureexceeds100 8C, a conductivity loss isobserved for the sulfonated fluorocarbon and sulfonated

hydrocarbon membranes due to evaporation of water

molecules.

As stated in the introduction, the fuel cell operation at

high temperature with low relative humidity is a highly

desirable goal, since the operation of fuel cells at high

temperature increases the reaction kinetics, improves

CO tolerance, and reduces the problems associated

with water management. There have been extensive

research efforts to find alternative membranes, which

are stable at high temperature. In fact, a desirable high

temperature PEM must have high proton conductivity

under hot and dry conditions. It should be thin for low

resistance, show a good contact with catalyst electrodes,

thermal and dimensional stability and low gas and

fuel permeability.

In general, nanoparticles can help to mitigate peroxide

related radicals which attack the polymer, increase the

available number of protons, offer sites for proton

hopping, and increase the water retention (the presence

of a hygroscopic additive binds a larger amount of

water in the membrane, increasing the membrane water

content at a given RH). Furthermore, nanoparticles can

change the fuel and oxidant permeability, increase the

thermal and mechanical stability (e.g., improvement of

Tg and Young’s modulus), change and/or stabilize the

morphology, and lead to higher electrode performance

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(higher membrane humidification can help to humidify

the catalyst layer).

2.1. Mitigation of Reactive Oxygen Species

Chemicalmembranedegradation is amajor cause of proton

exchange membrane fuel cells failure. Basically, degrada-

tion of themembranes is classified as chemical and chemo-

mechanical degradation. Chemical degradation of the

PEM occurs via a two-step process: a) formation of

reactive oxygen species such as hydroxyl radicals (OH�),superoxide radicals (O��

2 ) and hydroperoxyl/peroxyl radi-

cals (HOO�/ROO�) within the MEA and b) reaction of

reactiveoxygenspecieswiththePEM, leading tomembrane

decay. Chemo-mechanical degradation occurs, when

membranes shrink and swell. During these processes,

membranes are subjected to huge stress, and polymer

chains, which are highly entangled can only yield to

the stress by chain scission. The radical yield is high

enough to be measured by ESR spectroscopy and to form

measurable amounts of hydrogen peroxide, e.g., when a

water filled silicone tube is squeezed repeatedly.[26] For

sulfonated PEEK and PPEK-based fuel cell membranes, this

effectwas shownbyRoduner et al.,who identified sulfonyl,

phenyl, and phenoxy radicals by ESR spectroscopy of

membranes subjected to wet/dry cycles directly inside

of a spectrometer.[27] A similar experiment for Nafion

is missing, but Nafion radical chemistry is well known

through work done by Conti et al.[28,29]

Reactive oxygen species can be mitigated by the use

of free radical scavengers or peroxide decomposition

catalysts like MnO2, which do not form OH� but O2.

While decomposition agents and regenerative scavengers

continuously protect the membrane, the use of simple

radical scavengers can only delay the onset of degradation

for a definite period due to steady consumption of

the scavenger molecules. Since, peroxide formation is

expected to occurmainly at the electrodes as a consequence

of small but unavoidable gas crossover, especially of

oxygen to the anode, inclusion of these agents in the

electrodes is mainly investigated. But also inclusion of

agents in the membrane showed impressive results. Good

examples for a regenerative scavenger are cerium-based

compounds. Mechanistically, cerium can easily change

the redox state from þ3 to þ4 and back again. In detail,

Ce(III) can be oxidized by OH, forming harmless hydroxide

ions. Regeneration of the Ce(III) ions occurs by reaction

with H2O2 or HOO radicals. The efficiency of CeO2 nano-

particles in mitigating free radical induced PEM degrada-

tion was investigated by Ramani and coworkers.[30]

Commercially obtained CeO2 and synthesized nanopar-

ticles were incorporated within recast Nafion membranes

with 0.5, 1, and 3wt% CeO2. The composite membranes

exhibited similar proton conductivities and hydrogen




crossover currents (�1mAcm�2) as Nafion when tested

in a H2/air fuel cell at 80 8C and 75% RH. Accelerated

tests showed that the fluoride emission rate (concentration

of HF, the final degradation product in the cathode off-

gas) was lowered by more than 1 order of magnitude

upon addition of CeO2 into a Nafionmembrane, suggesting

that CeO2 nanoparticles can enhance the membrane

durability.

A typical decomposition catalyst is MnO2. It was used

as a model hydrogen peroxide decomposition catalyst

by Trogadas and Ramani.[31] A Pt/C-MnO2 electrocatalyst

demonstrated a 50% reduction in H2O2 generation. A

three- to fourfold reduction of the F� ion concentration

was observed in the anode condensate whereas the

cathode results were not reproducible, indicating that

MnO2 was not stable in acidic media at high potentials. To

overcome the oxide stability, the same group developed

carbon–tungsten oxide catalysts (C-WO3) and platinum/

carbon–tungsten oxide (Pt/C-WO3) electro catalysts. They

found that a 15wt% WO3-C catalyst produced approxi-

mately 60% less H2O2 whereas Pt/C-WO3 produced

approximately 30% less hydrogen peroxide than Pt/C.

The fluoride emission rate was found to be reduced by

�70% at the anode and �60% at the cathode condensate

when Pt/C-WO3 replaced Pt/C in the electrode. So, the

addition of metal nanoparticles and metal oxides with

radical scavengingabilities isapromising route tomitigate

PEM degradation. Recently, Ramani and coworkers[32]

reported also the use of metal nanoparticles. Freestanding

and silica supported platinum, palladium, silver, and gold

nanoparticles, cerium oxide and manganese oxide sup-

ports, and ceria supported platinum nanoparticles were

prepared and their radical scavenging ability investigated.

Composite membranes were prepared by adding 3wt% of

the nanoparticles to Nafion, followed by solvent casting.

The fluoride emission rate (FER) was ascertained for

each membrane from accelerated tests. The addition of

Au, Pd, Pt, and Ag nanoparticles reduced the FER by an

order ofmagnitude, 75, 60, and35%, respectively,while the

addition of all MnO2, CeO2, and Pt on CeO2 nanoparticles

reduced the FER by an order of magnitude.

2.2. Additional Proton Sources

The conductivity of membranes is strongly correlated with

the ion exchange capacity, expressed asmmol Hþ per 1 g of

dry polymer (weight-based IEC). For most membranes,

increasing IEC increases theconductivity.However, also the

water uptake increases with the IEC, and in some cases, a

veryhigh IEC candecrease the conductivity by lowering the

proton concentration in the water swollen, hydrophilic

channels of themembrane. This effect is taken into account

by using the volume based IEC, expressed as mmol Hþ

per 1ml wet polymer.

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One strategy to lower the membrane resistance is the

addition of proton donating (acidic) nanoparticles. Obvi-

ously, theweight-based IEC can only increase,when the IEC

of the nanoparticles is higher than that of the membrane

matrix. This can bemore easily achieved for perfluorinated

membranes (density of Nafion is 1.97 gml�1) than for

hydrocarbon-based membranes, which usually show a

density in the range of 1.2–1.3 gml�1. An interesting aspect

is that inorganic nanoparticles might swell less in water

than the membrane polymer and thus might increase the

conductivity even at very high IEC values.

Sulfated zirconia, zirconium phosphate (ZrP), and heter-

opolyacids are inorganic additives to enhance the proton

concentration during fuel cell operation. Though this

approach seemspromising, the success so far seems limited

and the interaction between the inorganic phase and

proton conductor is not sufficiently understood. For some

materials, also thestability in thepolymerhost isuncertain.

For example, heteropolyacids dissolve in the water

produced during fuel cell operation and finally leach out.

There have been numerous candidates developed for

high temperature fuel cell operation, both by modifying

Nafion membranes and also by developing completely

new membrane systems.[15,21,22]

Malhotra and Datta[33] first proposed the incorporation

of inorganic solid acids in the conventional polymeric ion-

exchangemembranes suchasNafion. Themainobjective of

thiswork is improvingwater retention aswell as providing

additional acidic sites. Thus, theydopedNafionmembranes

with a heteropolyacid (phosphotungstic acid (PTA)). The

composite membranes showed improved fuel cell perfor-

mance at high temperature and low relative humidity,

indicating a high proton concentration and improved

water retention. However, PTA leaches out from the

PEM due to its high water solubility. Fenton et al. showed

that the leaching of PTA from Nafion-PTA membranes

can be reduced by heat treatment.[34,35] Zaidi et al.[36]

embedded heteropolyacids to different extents in sulfonat-

ed poly(ether ether ketone) (SPEEK). The best performing

nanocomposite was a tungstophosphoric acid doped,

80% sulfonated PEEK based PEM, which showed similar

conductivity to that of Nafion. Colicchio et al. reported

the synthesis of SPEEK-silica membranes doped with

phosphotungstic acid (PWA). The silica is generated in situ

via sol–gel process of poly(ethoxysiloxane) (PEOS), a liquid

hyperbranched inorganic polymer of low viscosity. At

100 8Cand90%RH themembranepreparedwith PEOS (20%

silica content) exhibits two times higher proton conductiv-

ity than the pristine SPEEK. The addition of 2wt% PWA

during the membrane preparation further increases the

conductivity.[37]

Kim et al. evaluated the feasibility of a HPA/BPSH

composite membrane for use in PEM fuel cells. Hydrated

membranes consisting of 30wt% HPA and 70wt% BPSH




had a conductivity of 150 mS cm�1 at 130 8C. In contrast,

the pure copolymer had a proton conductivity of just

70 mS cm�1 at room temperature and 90 mS cm�1 at

elevated temperatures.[38]

Gomes et al. synthesized novel nanocomposite mem-

branes using sulfonated polyoxadiazole and sulfonated

dense and mesoporous (MCM-41) silica particles. A proton

conductivity of 34 mS cm�1 at 120 8C was obtained,

which was approximately twice the value of the pure

membrane.[39]

The protonic conductivity of zirconium hydrogen phos-

phate/disulfonated poly(arylene ether sulfone) as a func-

tion of temperature was demonstrated by Hill et al.

The study dealt with the in situ blending of zirconium

hydrogen phosphate with disulfonated poly(arylene

ether sulfone) copolymers to form transparent organic/

inorganic hybrid PEMs. Introduction of the inorganic

components into the membranes decreased the water

uptake, but increased the protonic conductivity, modulus,

and mechanical stability.[40]

Sulfonic acid-functionalized heteropolyacid-SiO2 nano-

particles were synthesized by grafting and oxidizing a

thiol-silane compound onto the heteropolyacid-SiO2 nano-

particle surface. The composite membrane containing

the sulfonic acid-functionalized heteropolyacid-SiO2 nano-

particles was prepared by blending with Nafion ionomer.

These composite membranes were thermally stable up to

290 8C, and the DMFC operation could be increased from

80 to 200 8C.[41]

Sulfonic acid functionalized silica was synthesized by

condensation of 3-mercaptopropyltrimethoxy silane in a

sol–gel approach, followed by oxidation. SPEEK composites

with these nanoparticles were prepared by casting.[42]

At 80 8C and 75% RH the measured conductivity was

50 mS cm�1 for SPEEK containing 10% sulfonic acid

functionalized silica and 20mS cm�1 for the pristine SPEEK

membrane.At 80 8Cand50%RHthemeasured conductivity

was 18 mS cm�1 for SPEEK containing 10% sulfonic acid

functionalized silica and 4 mS cm�1 for the unmodified

SPEEK membrane.

Nanocomposite PEMswere also prepared from sulfonat-

ed poly(phtalazinone ether ketone) (SPPEK) and various

amounts of sulfonated silica nanoparticles. The use of

sulfonated silica compensates for the decrease in ion

exchange capacity of membranes observed when non-

sulfonated nano-fillers are used. The strong interaction

between sulfonated polymer and sulfonated silica particles

leads to ionic crosslinking in the membrane structure,

which increases both the thermal stability and methanol

resistance of the membranes. A membrane with 7.5 g

sulfonated silica/100 g of SPPEK showed low methanol

permeability, high bound water content, and a 3.6-fold

increased proton conductivity compared to that of the

pristine SPPEK membrane.[43]

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Nanocomposite membranes based on recast Nafion

filled with SiO2 supported sulfonated zirconia particles

(Si-SZ, hygroscopic and high proton conductivity) were

fabricated by Bi et al.[44] The proton conductivity of three

membranes (Nafion/Si-SZ, Nafion/Si, and recast Nafion)

was studied and compared under dry and wet H2/O2

conditions at 60 8C. The order of proton conductivity at

0% RH was Nafion/Si-SZ>Nafion/Si> recast Nafion. The

Nafion/Si-SZ composite membrane showed also the

highest conductivity at 100% RH.

Also inorganic–organic hybrid nano-materials were

investigated. Nanoparticles possessing a core–shell struc-

ture consisting of a silica core (<10nm) and a densely

grafted oligomeric ionomer layer were dispersed in Nafion

solutionandcast intoamembrane.Theprotonconductivity

of a Nafion membrane containing 4wt% of the nano-

materials were significantly higher than that of an

unmodified recast Nafion membrane and the composite

membrane showed the expected superior fuel cell

performance.[45]

Hou et al. prepared layered ZrP and phosphonates-based

composite Nafion 115 membranes by ion exchange and

testedthemforDMFCapplication.When23wt%ofZrPwere

incorporated, the IEC value of the resulting membrane

increased significantly from 0.91mmol g�1 (Nafion 115)

to 1.93mmol g�1. A DMFC test with highly concentrated

(10 M) methanol feed at 75 8C showed that the composite

membrane performed better than the pristine Nafion

115 membrane, with peak power densities of 76 and

42mWcm�2, respectively.[46]

These examples show that nanoparticles can be used to

increase the ion exchange capacity.

2.3. Additional Sites for Proton Hopping

Proton hopping describes a process in which a proton is

first bound to a hydrogen bond acceptor, loosens this bond,

and finally forms a new bond with another acceptor. The

Grotthuss mechanism in bulk water is one example for

protonhopping. However, protonhopping can also occur at

waterbound toa surfaceor evenothergroups, like silanolor

sulfonate, carboxylate, orphosphonategroups, etc.[47] Thus,

nanoparticles can form a surface onwhich proton hopping

can occur, possibly even at low relative humidity if the

surface provides enough bound water or other suitable

functional groups.

The incorporation of hygroscopic inorganic fillers may

result in an increased water uptake at lower relative

humidity. Hence, the transport of protons through the

membrane becomes easier. Composite membranes based

on Nafion and triazole functionalized mesoporous silica

(SBA-15) had aproton conductivityof8.52� 10�4 S cm�1 at

a low humidity of 10% RH and 140 8C. At higher temper-

atures, the triazole groups, attached to the nanopores of




SBA-15, played a key role in aligning the SBA-15 channels

through which proton transfer might be facilitated.

SBA-15 is well known to have higher water retention

capability in its nano-sized pores at high temperatures

than Nafion.[48]

2.4. Increase the Conductive Strength of Sulfonic

Acid Groups

Nanoparticlesmightalsoplayan important role to enhance

the conductive strength of sulfonic acid groups. Hu et al.

reported the incorporation of inorganic electron-deficient

compounds (i.e., B2O3, H3BO3, and BN nanoparticles) into a

polybenzimidazole (PBI-OO)/Nafion (proton donor/accep-

tor) blend. Theproton conductivity of aNafion-1.7wt%PBI–

OO–0.5wt% nano BN composite membrane was higher

than that of the pure PFSA by 3 orders of magnitude at

ambient temperature andmore than 1 order of magnitude

at 140 8C. In this case, the interaction of the electron-

deficient boron nitride particles could be shown nicely by a

shift in the FT-IR bands of the sulfonic acid group.[49]

2.5. Increase the Water Uptake

The proton conductivity of a membrane is strongly

correlated with its water uptake, and effective fuel cell

operation at low relative humidity is difficult to achieve.

Water can be incorporated into membranes as ‘‘normal’’

bulk water, boundwater and strongly bound, non-freezing

water, depending on its interactions with the membrane

material.[50] Besides direct chemical interaction, e.g., by

hydrogen bonding, capillary condensation can play a

significant role. As a consequence, the water uptake of

polymer electrolyte membranes is affected by their hygro-

thermal history, type of counter ions, elasticity of the

polymer matrix, hydrophobicity of the polymer surface,

and the dissociation constant of the ionic groups. Obvious-

ly, additionofnanoparticleswill haveaneffecton thewater

uptake, especially on the equilibrium water uptake at low

relative humidity. This can be surface effects, but also

changes in the polymer morphology.

Water sorption properties depend on the surface

characteristics of metal oxides, surface functional groups

acting as hydrophilic centers available for hydrogen

bonding and dipole–dipole interactions. An approach to

quantification of these characteristics can be made via

surface acidity properties and the particle size and surface

area. For Aerosil (Sigma S5130, 7 nm, 400 m2 g�1), the

product specification states four silanol groups per nm2,

which is equivalent to2.6mmol SiOH/g.Assuming that one

water molecule is interacting with two silanol groups at

100 8C,[51] it can be calculated that 1 g Aerosil (7 nm) can

bind 23mg water. This is too low to have an effect on the

membrane humidity, and very probably other effects than

I: 10.1002/mame.201300378



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surface bound water are stronger. In the fuel cell also non-

equilibrium effects have to be taken into account, like the

ability to assist in the back diffusion of product water, or

by lowering the electro-osmotic drag by reducing the

average number of water molecules solvating protons.

The first Nafion-metal oxide composite membranes

were reported by Watanabe et al.[52,53] Based on the

above mentioned research effort, considerable develop-

ment of organic–inorganic nanocomposite membranes

were reported. Various studies have compared the efficien-

cy of Al2O3, TiO2, ZrO2, and SiO2 as additives in composite

membranes prepared with Nafion or other sulfonated

polymers. The presence of silica or titania improves the

measured water retention properties of membranes, and

the composite membranes show higher current density in

PEMFC operation than similar thickness-based pristine

Nafion membranes.

An alternative approach, first proposed byMalhotra and

Datta,[33] is to incorporate inorganic acidicmaterialswithin

the conventional polymer in order to improve water

retention while simultaneously increasing the number of

available acid sites. This approach shows promise for

developing PEMs that function adequately at high temper-

ature under low relative humidity conditions and has

consequently become a very active area of research. Ren

et al. prepared sulfated zirconia/Nafion115nanocomposite

membranes by ion exchange with zirconium ions into

Nafion, followed by precipitation of sulfated ZrO2 by

treatment in sulfuric acid. Sulfated ZrO2 containing Nafion

showed nearly twice the water uptake than the pristine

Nafionmembrane at 120 8C. The proton conductivity of the

nanocomposite membrane was three times higher than

that of Nafion 115 at 25 8C (15 mS cm�1 vs. 5 mS cm�1). At

110 8C and above, the proton conductivity of S-ZrO2/Nafion

115 membrane is still twice as high.[54]

The water retention properties of Nafion/phosphor-

silicate hybridmembranes are reported byKannan et al.[55]

For the hybrid membrane and pure Nafion, the water

retentionwas 15.8 and 4.5wt%, respectively. Thismight be

due to the strong hydrophilic nature of phospho-silicate

gels, allowing for formation of hydrogen bonds. A similar

trend was also observed for phospho-silicate incorporated

sulfonated poly(2,6-dimethyl-1,4-phenylene oxide).[56,57]

Nafion-metal oxide (ZrO2, SiO2, and TiO2) nanocomposite

membranes were also synthesized and examined by Jalani

et al.[58] At 120 8C, all nanocompositemembranes exhibited

higher water uptake than pristine Nafion, Nafion-ZrO2

showed 45% higher water uptake, and Nafion-SiO2 showed

15% higher water uptake.

Amjadi et al.[59] reported Nafion/TiO2 membranes for

high temperature PEMFC application. Water uptake of the

nanocomposite membrane with 3wt% doping level was

found to be 51% higher than that of a pristine Nafion

membrane.




Besides passive humidification, fuel cells allow also for

active humidification (self-humidification). This concept

was first proposed by Watanabe et al., and is based on

membranes containing catalyst particles, on which

permeating oxygen and hydrogen (which inevitably exist

in the membrane in a low concentration) react to water.

Ideally, membranes based on this concept should not

require external humidification and should suppress

the crossover of the reactant gases. Water from this

reaction directly humidifies the membrane.[52,53,60]

Expected drawbacks are the high price of these mem-

branes, thepossibilityofperoxide formationdirectly in the

membrane, increased electric conductivity and reduced

ionic conductivity.

Even though higher water uptakes can be observed for

nanocompositemembranes, the literature usually does not

convey if the data are really equilibrium values under

fuel cell operation. In several cases, the reported highwater

uptakesmightnotbeequilibriumvalues, but relative stable

values due to initial water retention. In those cases, the

observed water uptake and the correlated conductivity

will decrease with time, e.g., when water molecules are

removed from the membrane by electroosmotic drag.

2.6. Membrane Morphology

In a recent review, Park et al.[61] pointed out that Aerosil

380’s silanol groups are protonated by sulfonic acids, when

the pH is lower than ca. 2, which is the iso-electric point

of the silanol groups. Hence, a strong ionic interaction

between protonated silica nanoparticles and the sulfonate

groups of the polymer matrix can be anticipated. Another

important factor is the size of Aerosil’s primary particles

and aggregates. As can be seen in Figure 4, the primary

particles (7 nm)are larger thanNafion’shydrophilic clusters

(4 nm). This raises the question, how the introduction of

nanoparticles influences, stabilizes and even shapes the

membrane morphology. Naturally, the hydrophilic chan-

nels in a membrane form a tortuous pathway. It could

be possible that introduction of nanoparticles leads to

shortcuts, connecting channels. Even the formation of a

‘‘proton highway,’’ connecting anode and cathode, could be

possible, if the interaction between nanoparticles and

polymer matrix leads to a continuous pathway. However,

direct analysis is not easily possible. Besides AFM and

TEM, which show the channel structure in well phase

separated polymer samples, SAXS is the standard method.

As all X-ray diffraction methods, SAXS delivers the

regular patterning of structural units. In the case of Nafion

and other phase separated polyelectrolytes, the so called

ionomer peak in the SAXS spectrum gives the average

distance between two hydrophilic domains.

For a good overview, the reader is referred to a paper

by Jones and Rozi�ere,[62] who compared the structural

: 10.1002/mame.201300378



Figure 4. TEM images, comparison of Aerosil 380 nanoparticles (silica, ca. 390 m2 g�1,7 nm primary particle size) and the size of Nafion’s hydrophilic clusters according to theGierke model (red structures).


www.mme-journal.de

morphology of sulfonated fluorocarbon, sulfonated hydro-

carbon, and composite membranes. The morphology of

hybrid Nafion-ZrP membrane was reported by Yang et al.

The SAXS result implies that ionic clustering persists also

in the hybrid system. Interestingly, there is no difference

between the spacing in fully hydrated Nafion 115 and

fully hydrated Nafion115-ZrP.[63]

We developed a sulfonated poly(ether sulfone) contain-

ing 5–20% silica nanoparticles of 7 nm diameter and

390� 40 m2 g�1 surface area. TEM analysis proved that

thenanoparticlesarewell embedded inthepolymermatrix.

The separation length between the ion-rich domains was

determined by SAXS to be 2.8, 2.9, and 3.0 nm for pure

polymer, nanocomposite membrane, and Nafion NRE 212,

respectively.[64]

In summary, it seems tobeworthy to investigate further

into the morphological changes induced into membranes

upon addition of nanoparticles. An open question is the

size of the channels formed around the nanoparticles.

Since this is not a repetitive structural detail, it cannot be

seen well by X-ray methods. Molecular modeling might

fill this gap, but to our knowledge, there is no literature

available.

2.7. Enhance Mechanical Stability

It is generallyaccepted that introductionofnanoparticles to

polymers can mimic covalent crosslinking. It can be

expected that introduction of nanoparticles to polyelec-

trolytes like Nafion will increase the modulus and tensile

strength. Thus, incorporation of metal oxides like silica or

zirconia should increase the water retention, mechanical

stability, and working temperature of the composite

membranes. Shao et al.[65] reported that Nafion supported

Macromol. Mater. Eng. 2014, DOI: 10.1002/mame.2013

� 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhwww.MaterialsViews.com

Early View Publication; these are NOT the final pag

by inorganic oxides resulted in the

formation of membranes, which showed

improvement in tensile properties at

elevated temperature, indicating that

inorganic particles have a significant

degree of interconnection with the

membrane at elevated temperature.

However, care needs to be taken to find

the right balance between flexibility

and high tensile strength, to prevent

that the membrane is not too brittle for

use in the fuel cell.

Generally, tensile strength of SiO2

containing nanocomposite membranes

is expected to increase with increasing

SiO2 loading, but the literature also

knows exceptions. The tensile strength

of a pristine sulfonated poly(fluorinated

aromatic ether) (SDF-F) was reported as

33MPa, which is higher than that of Nafion (21MPa).

Nevertheless, Kim et al.[66] found that the tensile strength

decreased with the silica load. Presumably the addition of

excess SiO2 phase prevented the formation of a well-

interpenetrated network during the sol-gel process. This

example shows that introduction of nanoparticles is not a

trivial process and needs to be controlled well.

ForNafionbasedmembranesfilledwithMxOy (M¼ Ti, Zr,

Hf, Ta, andW),DiNotoetal.[67] proposeddynamic crosslinks

between sulfonic acid groups and nanoparticles, based on

dynamic mechanical analysis (DMA) results.

Tripathi et al.[68] reported sulfonated poly(ether ether

ketone) (SPEEK)–zeolite–ZrPnanocompositePEMs, inwhich

infiltration of zeolite and surface modification with ZrP

improves the thermal, mechanical, oxidative, and dimen-

sional stabilityalongwith thewater retentioncapacity. The

mechanical properties of sulfonated poly(arylene ether

sulfone) (SPAES) containing different types of SiO2 (particle

size 7, 14, and 24nm) nanocomposite membranes were

reported by Lee et al.[69] Pure SPAES showed a tensile

strength of 57.6MPa, and variation of the nanoparticle load

showed amaximal tensile strength of 65.4MPa at a load of

1%, from which on the tensile strength decreased to just

24MPa at 5% nanoparticle load. At the same time, the

tensile strength slightly decreased with increasing particle

size at constant particle load, down to 58.4MPa for 14nm

sized aerosol, which is sill slightly higher than the value of

the pure material. One key point in this paper was the use

of a non-ionic surfactant, pluronics L64, which helped to

disperse and distribute the nanoparticles evenly over the

membrane volume.

Sulfonated montmorillonite (SMMT) filled SPEEK was

prepared and studied by Gosalawit et al.[70] The study

revealed that the inorganic aggregation in SPEEK increased

with respect to SMMT loading. The stability inwater and in

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aqueous methanol solution as well as the mechanical

stabilitywereenhancedwith theSMMT loading. TheMeOH

crossover was reduced when the SMMT loading increased.

The proton conductivity was also improved with the

incorporation of SMMT. As a result, SMMT/SPEEK nano-

composite membranes exhibited a significantly improved

DMFCperformancecompared topureSPEEKandNafion117

membranes.

The strong interaction between nanoparticles and

membranepolymer not only influences the tensile strength

and modulus, but also is expected to increase the glass

transition temperature. This is important when Nafion

shouldbeusedat temperaturesover 100 8C,which is close to

the glass transition temperature of Nafion (110 8C). Adje-mian et al.[71,72] introduced nanosized SiO2 into several

perfluorinated membranes (e.g., Aciplex 1004, Nafion 112,

andNafion115)byasol–gelprocess.At130 8Candapressureof 3 atm to allow for full humidification and a sufficient

oxygen partial pressure, a Nafion 115 based hydrogen/

oxygen fuel cell failed within the first hour (at 650mV, the

current density dropped from 200mAcm�2 to zero). On

the contrary, all composite membranes showed a stable

current density at 650mV over the tested time of 50h.

It can be concluded that addition of nanoparticles

indeed increases the tensile strength, modulus, and Tg of

nanocomposite membranes. This is especially important

for Nafion, which has a Tg of around 110 8C and will

start to creep from this temperature on. However, a very

fine particle dispersion seems to be vital. Furthermore,

there exists an optimum particle load, from which on the

material properties again start to decrease.

2.8. Reduction of Fuel and Oxidant Crossover

Fuel permeation is a major problem for the fuel cell’s

electrochemical performance (permeated methanol can

poison the cathode catalyst in the DMFC), fuel efficiency

and chemical stability (because oxygen and hydrogen can

react to peroxide and related radicals). Especially in the

DMFC,ahighprotonconductivity-fuelpermeability ratio (so

called ‘‘selectivity’’) isanimportantproperty,whichstrongly

affects the fuel cell power. A higher fuel cell permeation

decreases the selectivity ratio. Many researchers expect

that the incorporation of nanoparticles creates tortuous

pathways (also the opposite might be true as well, as

discussed in Section 2.6) and offers resistance to fuel

crossover. Hence, the transport of protons through the

nanocomposite membrane becomes easier or slightly

hindered,butthemethanolpermeabilitystronglydecreases.

Ahmad et al. reported that the crossover current density

measuredwithhybridmembranes isanorderofmagnitude

lower than that of the unmodified polymermembranes.[73]

Song et al. observed that the methanol permeability of

pure recast Nafion was 2.3� 10�6 cm2 s�1, but decreased




to 1.6� 10�7 cm2 s�1 for Nafion/MMT nanocomposite

membranes by addition of just 1wt% nanoparticle

loading, which is more than 90% reduction.[74] Chen and

Kuo[75] found that the robust hybrid membrane filled

with covalently bound silica showed both good proton

conductivity (s¼ 3.4� 10�2 S cm�1) and low methanol

permeability (P¼ 1.1� 10�8 cm2 s�1), comparing to

s¼ 4.5� 10�2 S cm�1 and P¼ 2.2� 10�8 cm2 s�1 for

Nafion 117 under similar operating conditions. Lin et al.[76]

studied clay composite membranes developed from mont-

morillonite and Nafion. They reported that the composite

membraneexhibitedaslightdecreaseinprotonconductivity

but about 40% decrease in methanol permeability for a 5%

MMT-POP-400-Nafion composite membrane.

3. Conclusions

Incorporation of small amounts of nanoparticles into the

polymericmatrix of fuel cellmembranes leads to enhanced

thermal and mechanical stability, ionic conductivity, and

water retention in comparison to that of pristine polymeric

material. Especially, the fuel crossover can be reduced.

However, the fuel cell relevant membrane properties

cannot be adjusted separately, and adjustment of one

parameter in most cases leads also to changes in other

parameters. Fromascientificpointofview, it is stillnot fully

understood what is the exact reason for the reported

property changes. In the case of DMFC membranes, the

increased tortuosity is supposed to decrease the fuel

crossover, while the same materials show a strong

interaction between nanoparticles and polymer matrix

and thus rather might show well-connected hydrophilic

channels. It is hoped that future research can focus on

these contradictions and simple, but valid, models can be

established, leading to further advances in this field.

Acknowledgements: This work was partially supported by KIST’sK-GRL program and the Joint Research Project funded by the KoreaResearch Council of Fundamental Science & Technology (KRCF),Republic of Korea (Seed-10-2).

Received: September 27, 2013; Revised: December 15, 2013;Published online: DOI: 10.1002/mame.201300378

Keywords: fuel cells; ionomers; membranes; nanocomposites;nanoparticles

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