Chemical Engineering Journal 204–206 (2012) 87–97

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Chemical Engineering Journal

journal homepage: www.elsevier .com/locate /cej

Review

Recent advances in proton exchange membranes for fuel cell applications

Liwei Zhang a, So-Ryong Chae b, Zachary Hendren c,d, Jin-Soo Park e, Mark R. Wiesner c,⇑a Department of Civil and Environmental Engineering, Carnegie Institute of Technology, Carnegie Mellon University, Pittsburgh, PA 15213, USAb School of Chemical and Biomolecular Engineering, The University of Sydney, New South Wales 2006, Australiac Department of Civil and Environmental Engineering, Pratt School of Engineering, Duke University, Durham, NC 27708, USAd RTI International, Global Climate Change and Environmental Sciences Division, Research Triangle Park, NC 27709, USAe Department of Environmental Engineering, College of Engineering, Sangmyung University, Cheonan, Chungnam Province 330-720, Republic of Korea

h i g h l i g h t s

" We summarize recent developments of PEMs that maintain performance at high temperature and low relative humidity." Three types of PEMs are evaluated: polymeric, ceramic, and inorganic–organic composite." The advantages and limitations of three types of PEMs under different operation conditions are discussed.

a r t i c l e i n f o

Article history:Received 29 February 2012Received in revised form 19 July 2012Accepted 19 July 2012Available online 28 July 2012

Keywords:Fuel cellProton exchange membraneTemperatureRelative humidityPolymerCeramicsInorganic–organic composites

1385-8947/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.cej.2012.07.103

⇑ Corresponding author. Tel.: +1 919 660 5292; faxE-mail address: wiesner@duke.edu (M.R. Wiesner)

a b s t r a c t

Fuel cell technology has drawn much attention from scientists and engineers due to its high energy con-version efficiency, quick start-up in the beginning of operation, portability, and minimal pollution. Protonexchange membranes (PEMs) play an important role in the fuel cell systems. A good PEM must meet aseries of requirements such as high proton conductivity, excellent mechanical strength and stability,chemical and electrochemical stability, low fuel or oxidant crossover, and be amenable for fabricationinto membrane electrode assemblies. In this paper, we focus on the recent advances in developmentof novel PEMs that work well under elevated temperature and/or low relative humidity. All PEMsdescribed in this paper are divided into three categories: polymeric, ceramic, and inorganic–organic com-posite membranes. The Nafion membrane with structural and compositional modifications is the mostpromising candidate that can adapt to high temperature and low relative humidity operating conditions.Ceramic and inorganic–organic composite membranes display some desired properties, but the develop-ment of these membranes is still in early stages and further in-depth studies are needed. Possibleapproaches focusing on (1) improvement of proton conductivity and (2) enhancement of mechanicaland thermal stabilities to improve the performance of novel PEMs are also discussed.

� 2012 Elsevier B.V. All rights reserved.

Contents

1. Introduction to fuel cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 882. Novel proton exchange membranes for fuel cell applications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

2.1. Fundamentals of proton exchange membranes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 892.2. Classification of novel proton exchange membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 892.3. Polymeric PEM. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 892.4. Ceramic PEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 912.5. Inorganic–organic composite PEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 932.6. Durability, cost and compatibility of PEMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94

3. Conclusions and future prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

ll rights reserved.

: +1 919 660 5219..

88 L. Zhang et al. / Chemical Engineering Journal 204–206 (2012) 87–97

1. Introduction to fuel cells

Energy shortages and environmental pollution are priority glo-bal concerns. Fuel cell (FC) technology is a promising solution tothese challenges, because fuel cells can generate energy efficientlywithout pollutant release (the final product is water if hydrogen isapplied as the fuel) [1–3]. A FC can be defined as an electrochemicaldevice that continuously converts chemical energy into electric en-ergy (and some heat) as long as fuel and oxidant are supplied [4].The development history of fuel cells can be traced back to the19th century, when William Grove developed the so-called ‘‘gas-eous voltaic battery’’, which used hydrogen and oxygen as reactantsand platinum as electrodes [4]. In 1937, Baur and Preis [4] devel-oped the solid oxide fuel cell, which was operated at high temper-ature (1050 �C) using a mixture of ZrO2 and Y2O3 as the electrolyte.In 1959, GE Corporation made the first attempt to produce polymerelectrolyte membrane fuel cells. Phenolic polymer membraneswere made by polymerization of phenol–sulfonic acid with formal-dehyde, and tested the performance of these membranes in fuel cellsystems. Unfortunately, these membranes had low mechanicalstrength and a short lifetime of 300–1000 h and showed a relativelylow power density [5]. In the 1970s, DuPont made a breakthroughto develop a perfluorosulfonic acid–based membrane called Nafion,which doubled the maximum proton conductivity and extendedthe operational lifetime to 104–105 h [1]. The Nafion membranewas quickly applied in fuel cell systems and helped fuel cells toestablish a competitive role in the energy market. Fuel cells are cur-rently being tested in power stations, portable power generators,and a variety of vehicles such as Chevrolet Equinox (General MotorsCo.) and the fuel-cell electric vehicles (Ford Motor Co.) [3].

In general, fuel cells can be classified based on the differences inions traveling through the electrolyte. Table 1 shows the five maincategories of fuel cells. A solid oxide fuel cell (SOFC) is composed oftwo porous electrodes and a solid oxide ceramic electrolyte issandwiched between the electrodes [3]. SOFCs work at very highoperation temperatures, thus no catalyst layers are needed onthe electrode surfaces [3]. A molten-carbonate fuel cell (MCFC) em-ploys an electrolyte that is composed of a mixture of alkali metalcarbonates. The fuel cell is operated well above the melting pointof the carbonates, and the carbonate ion (CO2�

3 ) was found to bethe charge transport mediators within the molten electrolyte [6].A phosphoric acid fuel cell (PAFC) applies liquid-form phosphoricacid as the electrolyte to transfer charge mediators (H+). This sys-tem can only work at relatively high temperatures (around200 �C), because phosphoric acid solidifies at lower temperatures.Furthermore, the dissolution of air–cathode catalysts in hot

Table 1Fuel cell classifications (modified from [8]).

phosphoric acid under PAFC operating conditions is a significantproblem that limits its applicability [7].

An alkaline fuel cell (AFC) uses potassium hydroxide in aqueoussolution as the electrolyte. The AFC is the first fuel cell type appliedin practical services and it has been proven to possess high powerdensities and long life in space shuttles [9]. However, the AFC sys-tem is easily poisoned because the carbon dioxide can react withthe hydroxide ion present in the electrolyte to form a carbonate(CO2�

3 ), thereby reducing the hydroxide ion concentration in theelectrolyte and subsequently deteriorating the overall efficiencyof the fuel cell [9]. As a result, the AFC is being replaced by polymerelectrolyte membrane fuel cells PEMFCs.

Among fuel cell systems listed in Table 1, PEMFCs are the mostwidely applied fuel cell systems for portable power generation dueto compactness, relatively long operational lifespan, quick start-up,high output power density [2], clean by-products, and quiet oper-ation [10]. A proton exchange membrane (PEM) connecting the an-ode and cathode is a fundamental part of PEMFC systems.Furthermore, the PEM must possess properties that make it com-patible with the environments in both the anode and cathodechambers, which is a relatively difficult requirement.

The current PEMFC system usually works at a temperature low-er than 80 �C [11], as the Nafion membrane would suffer frommechanical stability deterioration and water content reduction athigher temperatures, which can adversely affect the proton-con-ducting performance of Nafion [11]. However, there are someintrinsic problems with operating PEMFCs at these lower temper-atures (around 80 �C). First, careful management of the water bal-ance is required for the PEMFC system because this temperature isnear the boiling point of water. The existence of dual-phase watercan easily result in over humidification inside the fuel cell, whichmeans the water can condense on the electrode surfaces and causeproblems in both proton and electron transfer [12]. Second, lowoperational temperature can reduce the tolerance of the PEMFCsystem to fuel impurities (i.e., carbon monoxide in the hydrogensteam) [12]. At the typical operational temperature (around80 �C) of a FC with Nafion membrane, CO content as low as20 ppm in the fuel steam can result in a significant loss of fuel cellperformance [13]. Third, a PEMFC operated at 80 �C with 40–50%efficiency produces a large amount of heat that has to be com-pletely removed in order to maintain the working temperature of80 �C or less [12]. It is difficult to meet this requirement usingexisting cooling systems. Finally, high-capacity H2 storage tanks(e.g., NaAlH4 tank) cannot be integrated into a low temperaturePEMFC system, as the heat for hydrogen desorption from NaAlH4

must be provided in a temperature range of 100–200 �C [12].

L. Zhang et al. / Chemical Engineering Journal 204–206 (2012) 87–97 89

The advantages to operate the PEMFC system at elevated tem-perature are also obvious. Firstly, water flooding of the catalyst lay-ers can be avoided due to the elimination of liquid water at anoperation temperature above 100 �C; secondly, CO poisoning ofcatalyst can be minimized under elevated operation temperature;thirdly, the kinetics of electrode reactions can be enhanced underelevated operation temperature and the conversion efficiency offuel energy into electricity can be raised as well [14]. In one word,drawbacks of normal low-temperature PEMFC system and advan-tages to operate PEMFC system under high temperature are bothdrivers for the development of novel PEMFC system that can beoperated under elevated temperature.

In summary, the main barrier to the PEMFC system is the Nafionmembrane’s inability to withstand high operation temperatures. Ifthe Nafion membrane used in current PEMFC systems can be re-placed by other membranes that can have robust performance un-der operation temperatures P100 �C, significant performancebenefits can be realized. Therefore, the synthesis and characteriza-tion of a novel PEM are among the most frontier areas of interest infuel cell research. In this review, three types of PEMs that can workunder high operation temperature are summarized and possibleapproaches to further improve the performances of these mem-branes are proposed.

2. Novel proton exchange membranes for fuel cell applications

2.1. Fundamentals of proton exchange membranes

The PEM separates the anode and cathode in a fuel cell and actsboth as a proton-conducting medium and a barrier to avoid directcontact between the fuel and oxidant [2]. A PEM needs to have spe-cific properties that enable it to work well in FCs: (1) high protonconductivity to support high currents with minimal resistive lossesand zero electronic conductivity; (2) adequate mechanical strengthand stability; (3) chemical and electrochemical stability underoperating conditions; (4) extremely low fuel or oxidant by-passto maximize columbic efficiency; (5) low water transport throughdiffusion, and (6) electro-osmosis and capability for fabricationinto membrane electrode assemblies [2,15].

The PEM contains many proton-conductive functional groups,which allows protons to transfer from one group to another.Fig. 1 shows a schematic polymer PEM structure, which displayshow the protons are transferred.

The Nafion membrane was invented by DuPont in the early 1970s.The membrane has a perfluorinated polymeric structure, with at-tached sulfonic acid groups that act as proton-conductive groups.The protonic conductivity of Nafion membranes is strongly depen-dent on water content and temperature. A study shows that for a fullyhydrated membrane (water content = 22, represented by the numberof absorbed water molecules per sulfonic acid group), the protonicconductivity is about 0.1 S cm�1 at room temperature. When watercontent decreases to 14, the protonic conductivity decreases to0.06 S cm�1 [16]. As for temperature, protonic conductivity increases

Fig. 1. Schematic diagram of a polymer PEM. Proton-conductive groups (R) arefixed to polymeric chains [2], but protons are free to migrate from one group toanother. The continuous alignment of proton-conductive groups and vacancies onthese groups provide the pathway for the protons to migrate.

significantly with temperature; at 80 �C, the protonic conductivity ofa fully hydrated Nafion membrane can reach 0.18 S cm�1 [3]. Nafionmembranes possess many of the desired properties that enable it tobe a good candidate for FC systems. The properties include high pro-ton conductivity at normal conditions, excellent chemical andmechanical stability and low permeability of both fuel and oxidant[2,17–19]. However, the upper operation temperature limit of Nafionis low (only 80–100 �C). Normal Nafion will dehydrate (thus lose pro-ton conductivity) when the temperature exceeds 80 �C and a signifi-cant drop of proton conductivity is observed at 120 �C [20]. Theproton-conductive performance of Nafion also becomes poor at lowrelative humidity (RH) [21]. Moreover, Nafion has relatively highmethanol permeability [22–24], which drastically reduces the per-formance of direct methanol fuel cells.

2.2. Classification of novel proton exchange membranes

The development of new PEMs for PEMFC systems is one of themost active areas of fuel cell research. To overcome the drawbacksof the Nafion membrane, many types of novel PEMs have beendeveloped, and some have shown the potential to substituteNafion, especially under high operational temperature. In general,novel PEMs developed in recent years can be classified into threemain categories: (1) polymeric, (2) ceramic, and (3) inorganic–or-ganic composite membranes (Fig. 2). The operation conditions ofthese PEMs (temperature and relative humidity) are summarizedin Table 2 and protonic conductivities of representative PEMs aresummarized in Table 3.

2.3. Polymeric PEM

Polymeric PEM is the most widely used membrane in fuel cells.Generally, proton-conductive groups like sulfonic acid are intro-duced to the main chain or side chains of the polymer. The mainchain of the polymer is often fluorinated to obtain thermal andchemical stability. As discussed in Section 2.1, Nafion is still themost widely used PEM in fuel cells and there were enormous stud-ies on the feasibility to modify the properties of Nafion membrane,so as to make the properties of Nafion more desirable for protonconduction under elevated temperature and low humidity. In thepast five years, researchers have made solid progress in this field,thanks to Neutron and X-ray scattering, nuclear magnetic reso-nance (NMR) and infrared (IR) spectroscopy studies on protonand small molecule transport processes of Nafion (recent develop-ments in proton exchange membranes for fuel cells). To addressthe issue of high methanol permeability, Tricoli doped Nafion withCs+ cations to decrease the size of hydrated micellar domains,which resulted in significantly decreased the methanol permeabil-ity [45]. Other researchers blended Nafion with Teflon-fluorinatedethylene propylene (FEP) or Teflon-perfluoroalkoxy polymer resin(PFA) to reduce methanol permeability, but this also decreasedthe protonic conductivity [2]. To increase thermal stability of Naf-ion, researchers modified the side chains of Nafion and produced anew perfluorinated membrane named Hyflon (Fig. 3). This mem-brane possesses a higher ionic glass-transition temperature thanNafion, which enables operation at a higher temperature withoutdamaging the membrane [26,41]. Moreover, researchers found thatthe number of –CF2-groups in the backbone of Hyflon influencesthe hydrogen bonding network of the water-sulfonic acid groupsand leads to an increase in the number of water molecules requiredfor effective proton transport [46].

Recently, researchers have developed novel hydrophilic–hydro-phobic perfluorinated block copolymers that provide enhancedproton transport, especially at low relative humidity [27]. The syn-thesis involves two main steps: First, oligomers of biphenol-basedpartially disulfonated poly(arylene ether sulfone) (BPSH) are

Fig. 2. Classification of novel proton exchange membranes developed recently.

Table 2Normal operation conditions of various PEMs.

Material Item Normal operation conditions

Polymeric Overall description Most newly developed polymeric PEMs can work at an elevated temperature (>100 �C). Manypolymeric PEMs can be operated under very low relative humidity (<40%). High cost associated withthe synthetic processes is the biggest problem

Modified Nafion (Hyflon) Usually works well at a temperature less than 100 �C, but good performance at an operationaltemperature as high as 120 �C was also reported [25]. The membrane has remarkable durability(5000 h) [25]. Can work over a broad relative humidity range [26]

Hydrophilic–hydrophobic copolymer Normal working temperature ranges from 30 �C to 80 �C [27,28], but the copolymer is expected towork well under a temperature higher than 100 �C [29]. The protonic conductivity of the copolymerdrops at a rate similar to that of Nafion when the relative humidity decreases, thus the copolymer islimited to high relative humidity environments (>80%) [27]

Side-modified hydrocarbons Can work under elevated temperatures as high as 250 �C [30]. Maintains high conductivity at lowrelative humidity (<50%) [31]

Acid–base blends Can be operated at an elevated temperatures as high as 280–325 �C [32]. However, the water uptakecapacity of acid–base blends is lower than that of Nafion [33], which limits the use of acid–baseblends at low relative humidity conditions

Ceramic Overall description Most ceramic PEMs work well under very high operation temperatures. They are also very goodcandidates for PEMFC systems running under extremely low relative humidity. However, the fragilenature of ceramic PEMs makes them difficult to be installed in commercial PEMFC systems

Porous silica-based Have stable performance under operation temperatures as high as 130 �C [34]; Able to maintain aproton conductivity of 1.5 � 10�2 S cm�1 at very low relative humidity (0.7%) [34]

TiO2-based Can only work under an operation temperature of 150 �C [35]; under normal ambient humidityenvironment (�70%), the proton conductivity of TiO2-based ceramic PEM is very low(5.5 � 10�6 S cm�1) [35]

FeOOH-based Works well in wide temperature range (from ambient temperature to 300 �C) [21]. It has goodperformance under relative humidity as low as 30% [21]

Inorganic–organiccomposite

Overall description Most inorganic–organic composite PEMs can work well at operation temperature higher than 100 �C.However, compared with Nafion, the composite PEMs do not show significant performanceimprovement under low relative humidity environment

Nafion-inorganic solid acid composite The Nafion –ZrO2 sol–gel composite PEM works well under an operation temperature of 120 �C and arelative humidity of 40% [36]

Sulfonated poly(arylene ether sulfone) –PTA (H3PW12O40�nH2O) composite

The composite PEMs display good proton conductivity especially at elevated temperatures (e.g.130 �C) [37]. The proton-conducting performance of this membrane is only tested at 100% relativehumidity [37]

Ferroxane (carboxylate-FeOOH)-PVAcomposite

Operation temperature cannot exceed 150 �C (PVA will begin to degrade under a temperature higherthan 150 �C) [38]. It does not work well under low relative humidity (658%) [39]

90 L. Zhang et al. / Chemical Engineering Journal 204–206 (2012) 87–97

synthesized with the use of 2 monomers (4,4-dichlorodiphenylsulfone (DCDPS) and Biphenol (BP)) and fuming sulfuric acid asthe introducer of sulfonic acid groups [47]. Second, hydrophilicphenoxide-terminated BPSH and hydrophobic decafluorobiphenyl

(DFBP) react with each other at 100 �C to form multiblock copoly-mers [27].

BPSH + DFBP multiblock copolymers are reported to have muchbetter proton-conductive performance than BPSH copolymers at

Table 3Protonic conductivities of various PEMs.

PEM Protonic conductivity

Nafion 0.13 s/cm at 75 �C and 100% relative humidity [40]; less than 0.01 s/cm at 20 �C [41]Modified Nafion (Hyflon) 0.013 s/cm at 20 �C [41]Hydrophilic–hydrophobic copolymer 0.09 s/cm at 30 �C and 100% relative humidity [42]Side-modified hydrocarbons (SPEEK-based PEMs) Around 0.1 s/cm at 100 �C [2]Acid–base blends 0.046 s/cm at 165 �C [33]Porous silica-based PEM 1.5 � 10�2 s/cm at very low relative humidity (0.7%) [43]TiO2-based PEM 5.5 � 10�6 s/cm under normal ambient humidity environment (�70%) [44]FeOOH-based PEM Around 0.015 s/cm at room temperature and 33% RH; around 0.025 s/cm at room temperature and

100% RH [21]Nafion-inorganic solid acid composite Maximum conductivity can reach 0.1 s/cm at 90 �C and 0.02 s/cm at 120 �C [36]Sulfonated poly(arylene ether sulfone) – PTA (H3PW12O40�nH2O)

composite0.08 s/cm at room temperature and 0.15 s/cm at 130 �C (100% RH) [37]

Ferroxane (carboxylate-FeOOH)-PVA composite 0.0025 s/cm at 100% RH and room temperature; lower than 10�5 s/cm at 33% RH and roomtemperature [39]

-CF2-CF2CF- CF2-CF2-CF2-

OCF2CF2SO3H

Fig. 3. Chemical structure of Hyflon.

L. Zhang et al. / Chemical Engineering Journal 204–206 (2012) 87–97 91

low relative humidities [42,48], in part because the presence of co-continuous hydrophobic–hydrophilic morphological network canresult in a lower morphological barrier for proton transport [23].Furthermore, the introduction of perfluorinated groups can in-crease the thermal and chemical stabilities of the membrane. Theprotonic conductivity of the BPSH + DFBP multiblock copolymermembrane is comparable to that of Nafion 112 membrane at awide range of RHs [23].

Side-modification techniques enhance the proton-conductiveproperties of polymers. Most side-modification processes involvethe grafting of sulfonic acid groups to polymer side chains, whichcan be achieved either by radiation-induced grafting or by directgrafting. The radiation-induced grafting involves the grafting ofstyrene or its substituents onto polymer films followed by sulfona-tion reaction [49]. Fig. 4 describes the radiation-induced graft pro-cedures with PTFE film (Teflon) as an example. The grafting ofstyrene to polymers is usually done prior to the introduction of sul-fonic groups to styrene, since this yields higher reaction efficiencythan the grafting of sulfonated styrene to polymers [50]. The radi-ation-induced graft of styrene to various polymer films has beenproven to be an effective method to produce alternative PEMs thatare suitable for fuel cell applications [51].

In contrast to the radiation-induced graft that uses monomersas an agent, certain polymers allow sulfonic groups to be grafteddirectly to their chains. Polyether ether ketone (PEEK) is an exam-ple of these types of polymers. The product with sulfonic groupsgrafted to the PEEK main chain is called sulfonated polyether etherketone (SPEEK) (Fig. 5). Compared with Nafion, it has a lowermethanol permeability, better mechanical stability and flexibility,which allow it to be fabricated thin enough to reduce the resis-tance due to membrane thickness [2,30,52,53]. Moreover, theSPEEK membrane can withstand heat up to 250 �C [31] and has ahigh protonic conductivity at high temperature (10�1 S cm�1 ataround 100 �C [2]), which is clearly superior to Nafion for hightemperature fuel cell applications. However, SPEEK needs higherwater uptake to get close protonic conductivity to Nafion’s pro-tonic conductivity, thus SPEEK tends to have poorer conductivitythan Nafion at the same hydration level and temperature.

Previous studies have shown that the presence of pendant sidegroups on the PEEK chain enables post-sulfonation to proceed un-der mild reaction conditions and the post-sulfonation can be com-pleted in much shorter time than required for the sulfonation of

commercial PEEKs [54]. A comparison of the sulfonation on mainPEEK chain and the sulfonation on the pendant side groups onthe PEEK chain is shown in Fig. 6. These sulfonated polymers withpendant side groups have excellent mechanical properties, goodthermal stability and chemical stability under normal fuel celloperation conditions. Moreover, the methanol permeability ofthese sulfonated polymers is extremely low, which are severaltimes lower than that of Nafion 117. The highest proton conductiv-ity of these sulfonated polymers can reach 0.15 S cm�1 at 100 �C,which is higher than reported value of Nafion 117 [54].

The acid–base blend PEMs for fuel cell applications involve theincorporation of an acid component into an alkaline polymer, so asto promote protonic conductivity [1]. These membranes are verystable in oxidizing or reducing environments and maintain rela-tively high conductivity at very high temperatures without signif-icant dehydration effects [1]. Poly(2,21-(m-phenylene)-5,51-bibenzimidazole)/phosphoric acid (PBI/H3PO4) complex isthe most widely studied material that belongs to acid–base blendsand possesses a protonic conductivity of 4.6 � 10�2 S cm�1 at anoperation temperature of 165 �C, which is much higher than thenormal operation temperature of Nafion [33]. However, the PBI/H3PO4 complex has lower water uptake capacity than that of Naf-ion (�20% vs.�33%, respectively, after immersion in distilled waterfor several days) [33]. Recently, researchers developed new acid–base blends that have a better performance than PBI/H3PO4. Guoet al. [55] synthesized a composite membrane composed of a sul-fonated poly(aryl ether ketone) (6FSPEEK) as an acidic componentand an aminated poly(aryl ether ketone) with a naphthyl group(AmPEEKK-NA) as a basic component and this membrane attaineda maximum protonic conductivity of 8.7 � 10�2 S cm�1 at 80 �C,and is expected to increase at higher temperatures. Moreover, thismembrane has high water uptake capacity and has an oxidativestability even higher than that of SPEEK membranes [55], whichmakes it a promising candidate for use in extreme oxidative envi-ronments. Li et al. [56] synthesized another type of acid–baseblend PEM with SPEEK as an acid polymer and various amountsof polysulfone tethered with 5-aminobenzotriazole as a basic poly-mer for direct methanol fuel cell applications. Their test resultsshow that the maximum power density of the acid–base mem-brane used is two times higher than that of Nafion 115 membraneat 80 �C with 1 M methanol feed [56].

2.4. Ceramic PEM

Polymeric membranes play the dominant role in the PEM mar-ket. However, over recent decades, researchers have a growinginterest in testing the performance of ceramic materials as possibleproton exchange membranes for fuel cell applications [21,57,58].

Fig. 4. Preparation of PTFE-based proton exchange membrane with the use of radiation-induced graft method (modified from [49]). Gamma rays or electron radiations hit thesurface of PTFE film and PTFE radicals with lone pair electrons will be generated. The radicals then react with styrene to bond styrene with the PTFE main chain. Finally, thesulfonation process introduces sulfonic groups to the side chains.

Fig. 5. Sulfonation of PEEK to SPEEK (modified from [52]).

Fig. 6. Comparison of the sulfonation on main PEEK chain and the sulfonation onthe pendant side groups on the PEEK chain.

Fig. 7. Grotthuss chain reaction (modified from [16]).

92 L. Zhang et al. / Chemical Engineering Journal 204–206 (2012) 87–97

The ceramic PEM can be classified into two main categories: (1)non-metal ceramic PEM and (2) hydrated metal oxide/oxyhydrox-ide ceramic PEM. The non-metal ceramic PEM, such as porous silicaglass, has good chemical and mechanical stability, low material costand endurance to high temperature. However, the protonic conduc-tivity of non-metal ceramic PEM tends to be much lower than

Nafion (10�6–10�3 S cm�1 at 400–800 �C) [59]. A possible approachto improving the performance of silica glasses is to mix the silicamaterial with acids that contain metals, such as dodecamolybdo-phosphoric acid (MPA, H3W12PO40�29H2O) and undecatungstocob-altoaluminic acid (H7[Al(H2O)Co W11O39]�14H2O) [34,60,61].

Additionally, metal oxide ceramic membranes are mainly usedin solid oxide fuel cells (SOFCs) and O2� ions are the main transferions in the membranes. However, if a metal oxide can be fabricatedhigh water absorption capacity on its surface, it will become a goodcandidate for PEMFCs. Various metal oxides and metal oxyhydrox-ides, including TiO2, Al2O3, BaZrO3and FeOOH, have shown thecapacity to conduct protons at different humidities [62] [21,57].Similar to silica glass, the protonic conductivities of both TiO2

and Al2O3 are quite low. The maximum protonic conductivities ofTiO2 and Al2O3 are 1.1 � 10�3 S cm�1 at 97% RH and5.5 � 10�4 S cm�1 at 81% RH, respectively, which are about one or-der of magnitude less than that of Nafion [57]. Compared with TiO2

and Al2O3, the protonic conductivity of FeOOH is much higher,which is even higher than that of Nafion [63]. Next we discussedthe proton conduction mechanism of FeOOH, the preparation ofnano-scale FeOOH and the protonic conductivity of FeOOH indetail.

There are two main proton conduction pathways in the FeOOHceramic membrane: (i) conduction by hydroxyl groups on the sur-face of the material; (ii) conduction by adsorbed water on the sur-face of the material. At low humidity, there is little or nophysisorbed water on the surface and protons are mainly

Acetic acid

lepidocrocite

(low surface area)

ferroxane

(high surface area, acetic

acid groups present)

ferroxane after sintering

(high surface area, no acetic acid groups,

more physisorbed water available)

Sinter at 300

Fig. 8. Ferroxane membrane production processes (modified from [21]).

L. Zhang et al. / Chemical Engineering Journal 204–206 (2012) 87–97 93

conducted by the chemisorbed hydroxyl layer (pathway i). To bespecific, protons will ‘‘hop’’ onto the adjacent oxide group by hy-droxyl dissociation [64,65]. The chemisorbed hydroxyl group layeris unaffected by subsequent changes in relative humidity, but itcan be removed thermally at high temperatures [21]. At highhumidity, a complete physisorbed water layer will cover the sur-face of the material and protons will be conducted mainly by thephysisorbed water layer (pathway ii). Specifically, a free protoncombines with a water molecule first to form a H3O+. The H3O+

then releases the proton to a nearby H2O molecule at the physi-sorbed water layer and ionizes it to produce another H3O+ mole-cule in a process referred as a Grotthuss chain reaction [21,66–68] (Fig. 7).

The sol–gel process is the traditional approach to prepare cera-mic thin films with nano-sized pores [21]. However, for the FeOOHceramics, the most widely-used preparation method is to makecoarse FeOOH particles (usually lepidocrocite) react with carbox-ylic acid to form nano-sized ferroxane (carboxylate-FeOOH)ceramics. One drawback of the ferroxane ceramic membrane isthat the proton conductivity becomes low at low RHs, due to theexistence of acetic acid groups. Though the particle size of lepido-crocite is greatly reduced in the preparation process, the use of ace-tic acid will introduce acetic acid groups to the surface of thematerial. When the acetic acid groups are present, the transfer ofprotons among physisorbed water molecules will be hinderedand proton conductivity is restricted. To remove acetic acid groupsfrom the surface of the ceramic membrane, a sintering process isneeded. After sintering the membrane at a moderate temperature(300 �C) for several hours, the proton conductivity of the

Fig. 9. Proton conductivities of ferroxane green body, ferroxane sintered at 300 �Cand Nafion at various humidities. The result of Nafion is from [63]; Results offerroxane green body and ferroxane sintered at 300 �C are from [21]. All tests areconducted at room temperature.

membrane increases due to the removal of acetic acid [21]. A briefsummary of the membrane-producing processes are shown inFig. 8.

The advantages of ceramic membranes include: excellent ther-mal and chemical stability, which enables the membrane to beused at high temperature, low material cost, and high proton con-ductivity [21]. Previous works show that ferroxane green body(without sintering) has a proton conductivity of 6 � 10�5 S cm�1

at 58% RH and a proton conductivity of 4.51 � 10�3 S cm�1 at100% RH; Ferroxane sintered at 300 �C has a proton conductivityof 1.29 � 10�2 S cm�1 at 33% RH and a proton conductivity of2.65 � 10�2 S cm�1 at 100% RH at room temperature, which ismuch higher than that of Nafion, especially when the RH is low(Fig. 9). Interestingly, the proton conductivity of lepidocrocite (b-phase FeOOH)-derived ferroxane is much higher than that of goe-thite (a-phase FeOOH)-derived ferroxane, which is due to a higherspecific surface area (davg-lep = 60.24 nm and davg-goe = 234.01 nm)and a higher density of surface hydroxyl (–OH) groups of lepido-crocite-derived ferroxane [69–71]. Moreover, the ferroxane cera-mic membrane is a good candidate for applications to directmethanol fuel cells, due to the extremely low methanol permeabil-ity of the ceramics (only 16% of that of Nafion) [21].

However, the ferroxane ceramic membranes have both poorductility and compression resistance. They are quite brittle andcan be easily broken to very small pieces, because the precursorof those membranes, lepidocrocite, has very poor mechanical prop-erties. Compared with other common materials, the hardness oflepidocrocite is very low (5 GPa), while Al2O3 is 20.6 GPa and a-SiO2 is 30.6 GPa [72,73]. Therefore, it is difficult to fabricate cera-mic membranes from ferroxane nanoparticles that have large areasand apply them in PEMFCs. Moreover, ferroxane-based membranesmay not be compatible with electrodes that are commonly used infuel cells. Therefore, more investigations on the compatibility be-tween ferroxane-based membranes and electrode materials areneeded (see Fig. 9).

2.5. Inorganic–organic composite PEM

A composite (or hybrid) material can be defined as a materialthat includes two or more blended compounds on the molecularscale. In most cases, at least one of those compounds is inorganicand at least one is organic [72]. Although combining inorganicand organic materials is nothing new, in-depth studies on compos-ite materials began at the end of the 20th century, with the help ofnovel physico-chemical characterization methods [72]. The studieson composite materials have generated great attention recently,because inorganic–organic components can combine dissimilarproperties of inorganic and organic materials, and the componentscan exhibit the desired properties of both material types [72]. Theincorporation of inorganic nanoparticles with special properties inorganic polymers to create multifunctional groups is one example[72], while another is the surface modification of plastics with hard

Fig. 10. SEM images of ferroxane-PVA composite and pure PVA membranes (a: surface of ferroxane-PVA composite membrane, b: surface of pure PVA, c: cross-sections offerroxane-PVA composite membrane, and d: cross-sections of pure PVA).

94 L. Zhang et al. / Chemical Engineering Journal 204–206 (2012) 87–97

poly(methylsiloxane)s to increase the abrasion resistance of plas-tics [74].

Inorganic–organic composite membranes can be classified intotwo main categories: (1) membranes composed of proton conduc-tive polymers and less-proton conductive inorganic particles, and(2) membranes composed of proton conductive particles andless-proton conductive organic polymers. The most widely-appliedtechnique for composite membrane preparation is to combine Naf-ion with inorganic solid acids, silica materials or metal oxides, be-cause this process can enhance the proton conductivity as well asmaintain the chemical and mechanical stabilities of Nafion at ele-vated temperatures and low RH [2]. Malhotra and Datta [75] incor-porated Nafion with heteropolyacids (HPAs), and the compositemembrane showed good performance in a PEMFC operated at1 bar and 110–115 �C. Jalani and Thampan [36,76] described thepreparation of Nafion-MO2 (M = Zr or Ti) nanocomposite mem-branes for the application of high-temperature PEMFCs and theirresults showed that both Nafion-ZrO2 and Nafion-TiO2 compositespossess proton conductivities higher than Nafion at 120 �C. Shaoet al. [77] combined silicon oxide (SiO2) powders with 5 wt.% Naf-ion solution and phosphotungstic acid to synthesize a compositemembrane with very promising proton conductivities at both highand low RH (similar to Nafion 115 when RH is high, much higherthan Nafion 115 when RH is low).

As the intrinsic problems of Nafion, including high permeabilityof methanol and loss of mechanical stability at high temperaturescannot be completely overcome by the introduction of inorganicmaterials, alternative approaches to develop new composite mem-branes are still necessary. A few novel composite membranes com-posed from polymers with no or low proton conductivities havebeen developed by adding inorganic materials with very high pro-ton conductivities. These membranes have chemical and mechan-ical stabilities are similar or better than those of Nafion.

Kim et al. [37] incorporated sulfonated poly(arylene ether sul-fone) with PTA (H3PW12O40�nH2O) and the prepared compositemembrane showed a proton conductivity up to 0.15 S cm�1 at atemperature range of 100–130 �C, which is higher than that of pureNafion. Zhang et al. [39] successfully prepared proton-conductive

composite membranes derived from the combination of ferroxanenanoparticles and polyvinyl alcohol (PVA). The motivation to com-bine these two materials is to combine the high protonic conduc-tivity of ferroxane nanoparticles and good mechanical propertiesof PVA to make a composite membrane with the potential to be ap-plied in PEMFC systems. PVA polymer has several desirable advan-tages: it is highly resistant against acid, alkali and organic reagents;the production cost of PVA is relatively low; PVA polymer has verygood mechanical properties and; it has been shown to be a goodhost for various nanoparticle fillers [78–83].

Fig. 10 shows the SEM images at different sections of ferroxane-PVA composite membranes. The ferroxane-PVA membrane has aPVA skeleton with ferroxane nanoparticles distributed on it, notonly on the top surface, but also within the cross-section. The scat-tered ferroxane nanoparticles act as channels for proton conduc-tion through the membrane. This membrane has protonconductivity comparable to Nafion at moderate RH and high RHand performs better than Nafion in the tensile force test (maxi-mum breaking strength (rB) of the ferroxane-PVA composite is60.56 MPa but rB of Nafion only 18.02 MPa) [39]. However, theproton conductivity of this composite membrane remains low atlow RHs (658%) and more studies on the improvement of perfor-mance of this membrane at low RHs are needed.

2.6. Durability, cost and compatibility of PEMs

When evaluating the overall performance of a PEM in a fuel cell,one cannot consider protonic conductivity alone. The durability,cost, and compatibility with catalyst and electrode materials arealso very important issues that need to be considered.

Durability of a membrane is closely associated with the chemi-cal stability of the membrane. For polymeric PEMs, the most com-mon cause for the membrane to lose chemical stability is the socalled ‘‘unzipping reaction’’ originated by peroxide radical attack[84]. The source of the peroxide radical (�OH) species is the decom-position of H2O2. The H2O2 is usually present on the anode side,where the potential is low enough to favor H2O2 formation, andis formed as a consequence of oxygen crossover through the

Polymeric PEM with high sulfonic acid group density

Polymeric PEM with new proton-conductive groups

Sputter ferroxane or other nanoparticles directly

on electrode surface

Composite membrane with CoOOH fillers

Composite membrane with NiOOH fillers

Composite membrane that can be sintered

at high temperature

Composite membrane with smaller filler size

Fig. 11. Possible approaches to improve proton-conductive performances of three main categories of PEMs in the future.

L. Zhang et al. / Chemical Engineering Journal 204–206 (2012) 87–97 95

membrane [41]. SPEEK and PTFE-based proton exchange mem-branes have very good resistance to �OH attack, while PSFA-basedmembranes have relatively lower resistance to �OH attack. As toceramic PEMs, chemical stability is usually not a concern, as mostceramic PEMs do not react with peroxide radicals and commonpollutants, such as CO in methanol fuel cells. Moreover, due tothe decomposition of the organic backbone and the decompositionof the sulfonic groups at around 250 �C in polymeric PEMs [85],ceramic PEMs are the only choices for fuel cells operated under ex-tremely high temperatures. However, it is important to note thatcertain ceramics are very hydrophilic and the integration of certainceramic membranes may be weakened in long-term operation, dueto adsorption of water at their surfaces. The durability of inor-ganic–organic composite PEMs is not well studied, but the durabil-ity is expected to be correlated with both the properties of polymerbackbone and inorganic fillers.

As to the cost of PEMs, the cost of polymeric PEMs is usuallyhigher than the cost of ceramic PEMs, because the preparation pro-cesses of polymeric PEMs are complicated and expensive catalystsare commonly involved during synthesis. A good approach to re-duce the cost of polymeric PEMs is to directly introduce sulfonicgroups to commercially available polymers, thus the cost to syn-thesize polymer backbone can be minimized [54].

Compatibility with catalyst and electrode materials is anotherimportant criterion to evaluate the feasibility to apply a PEM in afuel cell system. It has been reported that if the catalyst layer isNafion-based layer and the PEM is non-Nafion membrane, theinterfacial resistance between the two is larger than the interfacialresistance between Nafion-based catalyst layer and Nafion-basedPEM [86]. Non-Nafion PEM associated with Nafion-based catalystlayer also has larger degradation rate, which is probably due tothe different rates of swelling/contraction of different polymers.The rate difference can then cause the delamination of membraneelectrode assembly in the cycle of hydration–dehydration [87]. Asmost PEMFCs are still using Nafion-based catalyst on surfaces ofelectrodes, the compatibility between non-Nafion-based PEMsand Nafion-based catalyst layers needs to be investigated whenapplying such PEMs in PEMFCs.

3. Conclusions and future prospects

Based on a literature survey, this paper reviews the definition,principles and classification of fuel cells and proton exchange

membranes. The recent progress in development of protonexchange membranes that can work at high temperature and lowrelative humidity conditions is the main focus of this paper, andthree categories of newly developed PEMs: polymeric PEM, cera-mic PEM and inorganic–organic composite PEM are discussed indetail.

Nafion is still the most widely used PEM in commercial applica-tions. However, many novel PEMs have been synthesized with im-proved properties and have shown the potential to substitute forNafion. The drawbacks of Nafion include a significant drop of pro-ton conductivity at high temperatures, poor proton-conductiveperformance at low relative humidities and high methanol cross-over if used in direct methanol fuel cells. Therefore, high protonicconductivity at high temperature and low relative humidity andlow methanol permeability are particularly desired properties asa novel PEM. Moreover, novel PEMs need to possess high mechan-ical and chemical stabilities at high temperature and can be syn-thesized with low cost. Fig. 11 illustrates some suggestions forfuture development of three main categories of PEMs.

For polymeric PEMs, the modification of Nafion is regarded asthe most promising approach to prepare high temp/low RH PEMsthat can be produced at the commercial scale, due to the maturesynthesis technology of Nafion and high level of compatibility be-tween PEMs modified from Nafion and catalyst layers on electrodesurfaces. To improve protonic conductivity at low RHs, it is neces-sary to increase the sulfonic group density on polymer surfaces andadd reagents that can promote the mobility of protons on sulfonicgroups. Radiation-induced grafting of sulfonic groups to perfluori-nated or partially-fluorinated polymers is also a workable ap-proach to produce high temp/low RH PEMs, but the processes arecomplicated and the yield efficiency of products is low. Moreover,the development of new chemical moieties that have better protonconductivities than sulfonic groups and the discovery and applica-tion of these new groups to polymeric PEMs is a challenging andinnovative endeavor. SPEEK and acid–base blends show a great po-tential to substitute Nafion in high-temperature operating condi-tions, and although the synthesis processes are simple, thematerial costs must to be further reduced. Last but not least, thereis a drive to modify available polymeric PEMs, so as to make thesePEMs compatible with unique operation conditions of new fuel cellconfigurations, such as redox flow batteries.

For ceramic PEMs, poor ductility and compression resistanceare the main obstacles for their application in commercial fuel cell

96 L. Zhang et al. / Chemical Engineering Journal 204–206 (2012) 87–97

systems. Sputtering ceramic nanoparticles directly on the fuel cellelectrode surfaces may be a good solution to these mechanicalproblems, but this process is energy consuming and the adhesive-ness of the ceramics to electrode surfaces is uncertain. The durabil-ity of ceramic PEMs under prolonged fuel cell operation needs to beinvestigated as well, as the ceramic materials may disintegrate ordissolve under high moisture operation conditions.

For the composite membranes with proton-conductive inor-ganic nanoparticles and organic polymers, Nafion-based compositePEMs have shown great potential to replace Nafion in high temper-ature and low RH operation conditions. Proton-conductive inor-ganic nanoparticle-organic polymer composites also display somedesired properties. However, relatively low protonic conductivityremains a major problem. The following approaches may addressthis issue: (1) identify new inorganic nanoparticles that have high-er proton conductivities. Cobalt and nickel congeners of iron oxy-hydroxides (i.e., CoOOH and NiOOH) may work better thanferroxane when mixed with PVA or other polymers, (2) use organicpolymers that can withstand the sintering process. As mentionedearlier, ferroxane sintered at 300 �C has a much higher protonicconductivity than the ferroxane green body. However, ferroxane-PVA composites cannot withstand high sintering temperatures.Therefore, polymers that can resist high temperatures, such asTeflon and PFA, may be used to prepare ferroxane-based compos-ites, (3) reduce the particle size of nanoparticles fillers. Generally,a smaller particle size yields a larger specific surface area, whichmeans a larger surface available for water adsorption and protontransport. Various size-reduction methods can be tested, such assonication and chemical reaction. Again, the durability of thesecomposite PEMs needs to be tested to ensure long-term goodperformance.

Acknowledgements

This work was supported by the NSF PIRE program, PERMEANT(OISE-0530174). The authors thank Mr. Yao Xiao and Mr. ShihongLin at Department of Civil and Environmental Engineering, DukeUniversity for providing useful citation information and valuablerecommendations to the authors.

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