Radiation-Grafted Membranes for Polymer Electrolyte Fuel Cells

Review pubs.acs.org/CR

Radiation-Grafted Membranes for Polymer Electrolyte Fuel Cells: Current Trends and Future Directions Mohamed Mahmoud Nasef*,†,‡ †

Advanced Materials Research Group, Institute of Hydrogen Economy, and ‡Environmental and Green Technology Department, Malaysia-Japan International Institute of Technology (MJIIT), Universiti Teknologi Malaysia (UTM), International Campus, Jalan Semarak, 54100 Kuala Lumpur, Malaysia 5.2. Radiation-Induced Grafting onto New or Modified Polymer Substrates Followed by Sulfonation 5.2.1. Radiation Cross-Linking of Substrate, Grafting of Styrene or Its Substituent 5.2.2. Grafting of Styrene onto Porous Films/ Pore-Filling 5.2.3. Grafting of Styrene onto Engineering Plastics 5.2.4. Grafting onto Powder Followed by Film Formation Using Thermal Processing or Solution-Casting 5.2.5. Grafting onto Electrospun Nanoweb Polymer Substrates 5.3. Applying New Radiation Grafting Techniques 5.3.1. Direct Sulfonation of Irradiated Polymer Films 5.3.2. Grafting by Mediated Living Radical Graft Polymerization Followed by Functionalization 5.3.3. Single-Step Grafting of Sulfonated Monomers 5.3.4. Patterned Grafting of Monomers on Masked and Irradiated Films 5.3.5. Simultaneous Grafting and In Situ Sol− Gel Reaction Followed by Sulfonation 5.3.6. Modification of Nafion Membranes by Radiation Grafting of Functional Monomers 6. Radiation-Grafted Membranes for High Temperature PEMFC 6.1. Grafting of Basic Monomers Followed by Phosphoric Acid Doping 7. Radiation-Grafted Anion Exchange Membranes for Alkaline Fuel Cells 7.1. Grafting of Styrene Followed by Chloromethylation and Functionalization 7.2. Grafting of Vinyl Benzyltrimethyl Ammonium Chloride 7.3. Grafting of Vinylbenzyl Chloride Followed by Quaternization, Cross-Linking, Alkylation, and a Final Quaternization

CONTENTS 1. Introduction 2. Fuel Cells Based on Polymer Electrolyte Membranes 3. Preparation of Radiation-Grafted Membranes for Fuel Cells 3.1. Principles of Radiation-Induced Grafting 3.2. Monomer/Polymer Combinations for the Preparation of Membranes 3.3. Parameters Affecting the Properties of Radiation-Grafted Membranes 4. Radiation-Grafted Membranes for Low Temperature PEMFC 4.1. Grafting of Styrene onto Hydrocarbon Polymers and Subsequent Sulfonation 4.2. Grafting of Styrene onto Fluorinated Polymers and Subsequent Sulfonation 4.3. Stability of Radiation-Grafted and Sulfonated Membranes 5. Strategies To Enhance Stability of Membranes for Low Temperature PEMFC 5.1. Radiation-Induced Grafting of Monomer with Cross-Linker Followed by Sulfonation 5.1.1. Grafting of Styrene/Cross-Linker Mixtures 5.1.2. Grafting of Substituted Styrene Monomers 5.1.3. Grafting of Mixtures of Styrene or Its Substituents and Comonomer with or without Cross-Linker 5.1.4. Grafting of Functional Monomers 5.1.5. Grafting of Other Monomers

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12297 12298 12300 Received: October 13, 2013 Published: November 24, 2014

© 2014 American Chemical Society

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Chemical Reviews 7.4. Grafting of Vinyl Imidazole or Mixtures with Styrene Followed by Quaternarization and Alkalization 8. Radiation-Grafted Membranes for Other Types of Fuel Cells 9. Challenges and Future Directions 10. Conclusions Author Information Corresponding Author Notes Biography Acknowledgments Abbreviations References

Review

the remaining challenges for the commercialization of PEMFC in various vehicular applications has also appeared.8 Among fuel cell types, those whose operation is based on polymer electrolyte membranes (PEMs) such as the proton exchange membrane fuel cell (PEMFC), the direct methanol fuel cell (DMFC), and the polymer electrolyte alkaline fuel cell (PEAFC) are all promising candidates for low temperature operations. The replacement of a liquid electrolyte by PEM in such systems has eliminated the corrosion problems and conferred on the system additional advantages such as simplicity of construction, compactness, and quick self-starting at ambient temperatures. The successful performance of these kinds of fuel cell systems depends critically on the role played by the PEM. Currently, several commercially available membranes such as Nafion (DuPont), Aciplex (Asahi Chemicals Co.), Flemion (Asahi Glass Co.), Gore-Select (Gore and Associate), Aquivion PFSA (Solavy), Fumapem (Fumatech.), and Dais membranes (Dais Analytical Co.) have all been considered for use in PEMFCs because of their attractive properties. Table 1 shows the currently commercially available PEMs and their properties. Of all of the PEMs, Nafion membranes, which are obtained by the copolymerization of variable amounts of unsaturated perfluoroalkyl sulfonyl fluoride with tetrafluoroethylene, possess long-term stability under both oxidative and reductive environments (more than 60 000 h in stationary application). This stability is prompted by the Teflon-like backbone structure of these membranes.9 Nafion or perfluorosulfonic (PFSA) membranes are available in various versions with different thicknesses such as Nafion 1110, Nafion 1135, Nafion 112, Nafion 115, and Nafion 117. Of all, Nafion 117 is the most established PEM material that has been widely tested, and hence it is not surprising that the majority of the available PEMFC systems are utilizing them. However, it should be noted that Nafion membranes are considered to be expensive, and another disadvantage is that they have a high methanol permeability (in DMFC). In addition, they are prone to viscoelastic relaxation and water loss at high temperatures (low hydrated Tg), which decreases both the mechanical properties and the proton conductivity.10 Therefore, it is not suitable for use at temperatures higher than 100 °C.11 This situation has motivated many researchers worldwide to expand extensive efforts to developing alternative cost-effective and highly conductive membranes. Since then, a range of varieties of alternative PEMs have been developed for application in PEMFCs. Various approaches have been explored for developing new alternative PEMs. They include four main categories. The first category is based on modification of Nafion membranes themselves through several methods including: (i) physical or chemical treatment, (ii) reinforcement by porous support materials, and (iii) addition of organic or inorganic compounds.2c,12 When used as electrolytes for PEMFCs, the modified Nafion membranes were found to exhibit far better performance than the corresponding pristine Nafion membranes.13 More details on the various approaches used in modifying Nafion membranes and the subsequent performance of the PEMs can be found in the literature.2c,12 The second category involves the formation of acid−base complexes that provide a viable alternative for membranes that can maintain high conductivity at elevated temperatures without suffering from dehydration effects. The acid−base complexes considered for use in PEMFC involve the

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1. INTRODUCTION Fuel cell technology is one of the key emerging technologies that is currently attracting tremendous effort with the aim to provide alternative environmentally friendly and efficient power sources. The worldwide move away from conventional fossil fuel combustion power generation technologies is driving much of this important research.1 Unlike batteries, which have outputs limited to their stored chemical energy, fuel cells generate electricity continually as long as fuel is supplied. Furthermore, fuel cells have very high current densities and high energy per weight and per volume as compared to other conventional power sources. Fuel cells also possess high efficiency conversion because there are fewer limitations imposed by the second law of thermodynamics. It is not surprising, therefore, that fuel cell technology provides very promising energy conversion devices that have the ability to power stationary, mobile, and portable applications in the 21st century.2 After some decades of technological evolution, fuel cells developed into a range of categories that can be classified into: (1) solid oxide fuel cells (SOFC) that have a ceramic ion/solid oxide conducting electrolyte; (2) molten carbonate fuel cells (MCFC) with a molten carbonate salt electrolyte; (3) alkaline fuel cells (AFC) with an alkaline solution electrolyte (such as potassium hydroxide KOH); (4) phosphoric acid fuel cells (PAFC) with an acidic solution electrolyte; (5) proton exchange membrane fuel cells (PEMFC); and finally (6) direct methanol fuel cells (DMFC).3 Historically, fuel cells were invented in the middle of the 19th century. However, they only really found relevant application when space exploration started in the 1960s. Of particular note is the fact that PEMFC was first developed for the Gemini space vehicle. Launched by the National Aeronautics and Space Administration (NASA), typically it was required to work around 60−90 °C using pure hydrogen as a fuel. This was followed by a major breakthrough in the production of perfluorosulfonic acid PEM under the Nafion trademark developed by Du Pont de Nermous, Inc.4 A review of this early work, including the preparative chemistry of Nafion membranes, was given in Appleby and Yeager.5 The advances in the applications of Nafion membranes were reviewed in Mauritz and Moor.6 Later, PEMFC was developed for a range of transportation, stationary, and portable applications. The automotive industry’s first attempt at an automobile powered by PEMFC was by GM in their 1966 Electrovan. To date there have been numerous prototype cars, buses, and other vehicles all based on PEMFC developed around the world.7 A review on 12279


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Table 1. Commercially Available PEMs and Their Properties membranes Nafion N-112 N-1135 N-115 N-117 Aciplex Flemion-T Flemion-S Flemion-R Dow Gore Select Fumapem F-930 Fumapem F-1050 Aquivion PFSA R1010 R4010 61AZL386-389 MC 3470-3142 OEM

manufacturer

equivalent weight (g mol−1/−SO3H)

thickness/dry (μm)

conductivity (mS cm−1)

water content (wt %)

DuPont

Asahi Chemicals Asahi Glass

Dow Chemicals Gore & Associate Fumatech GmbH

Solvay IonClad IonClad Ionics Inc., Watertown Ionac Chemical Co. Prochema

1100 1100 1100 1100 1000 1000 1000 1000 800 900−1100 900

50 89 125 175 120 120 80 50 125 5−20 30

165 1100 90 80 110 N/A N/A 140 114 28−96 >100

∼21 ∼21 ∼22 ∼23 43 N/A N/A 38 54 32−43 ∼35

1000 850−890 835 835 400−450 650−950 N/A

50 90−110 40 20 50 60−80 50

>90 N/A 70 80 67−81 75−114 100

∼31 5000 h), (v) substantial morphological and dimensional stability, (vi) outstanding mechanical properties both dry and hydrated, (vii) a thickness 500 h of operation, for non or low DVB cross-linked membranes, to more than 2500 h in the temperature range of 60−80 °C. This is shown in Figure 14.129 In an earlier study, a DVB cross-linked membrane based on 25 μm FEP was tested in PEMFC at a temperature of 75 °C and was found to last for 1130 h without losing performance.112

FEP-based membranes has led to an optimized DVB content. The reaction conditions consisted of a monomer solution (St/ DVB 90:10) and a DG range of 10−15% and yielded PEMs with performance comparable to that of Nafion 117 (FEP-75 based membrane) when tested in a single cell. They were alo found to have a lower methanol permeability.39a,132 DVB-cross-linked PEMs based on other fully fluorinated films have also been reported. Nasef et al.100f prepared crosslinked PEM by the RIG of St with varying DVB content (2% and 4%) onto PFA films followed by sulfonation. Cross-linking was found to reduce the water uptake by 30%. The membranes showed ion conductivities of 15.0 and 6.3 mS cm−1 at DGs of 24% (for 2% DVB) and 17% (for 4% DVB), respectively. The structure of similarly cross-linked PEM was also investigated, and it was found that the grafted polySt cross-sectional distribution depends on the cross-linker content in addition to the irradiation dose and solvent.124 Cross-linked PTFE films were also used to prepare PEMs, which exhibited enhanced stabilities. The membranes were obtained by RIG of a St/cross-linker mixture onto cross-linked PTFE, which in turn were obtained by irradiation at molten temperature leading to double-cross-linked membrane.122b More details of this double-cross-linking approach will be discussed later in this Review in section 5.2 on new substrate polymers to improve the stability of PEMs. DVB-cross-linked membranes based on ETFE films with an optimum DG of 20−30% exhibited a PEMFC stability of about 770 h without significant degradation as shown in Figure 15.62h

Figure 14. Polarization characteristic curves and ohmic cell resistance results for various DVB-cross-linked PEMs based on 25 μm FEPgrafted polystyrene. Single cell durability at a current density 500 mA cm−2 of MEA with ETFE-based radiation-grafted membrane crosslinked from 10% DVB grafting solution. Cell temperature, 60−80 °C; reactants, H2/O2 at a stoichiometry of 1.5/1.5; gas pressure, 1 bar; H2 humidified at 80 °C. Reprinted with permission from ref 129. Copyright 2005 Electrochemical Society.

Figure 15. Single cell durability at a current density 500 mA cm−2 of a MEA with ETFE-based radiation-grafted membrane with DG of 24.2%. Cell temperature, 80 °C; reactants, H2/O2 at a stoichiometry of 1.5/1.5; gas pressure, 1 bar, H2 humidified at 80 °C, O2 dry. Ohmic cell resistance measured using the current-pulse technique. Reprinted with permission from ref 62h. Copyright 2005 Elsevier.

To further improve the stability of cross-linked PEMs based on a 25 μm FEP film, grafting parameters were optimized in a way that led to a membrane with a DG of 18% after crosslinking with 10% DVB, and it had an IEC of 2.0 mmol g−1. The membrane that was obtained using a 2-propanol/water mixture (instead of toluene) at 3 kGy (a dose 10-fold lower than the usual one) recorded 4000 h stability in PEMFC (at a temperature of 80 °C and steady state with a current density of 500 mA cm−2).60,98a This remarkable improvement in the mechanical properties (% of elongation) of the membrane was found to enhance the MEA stability. The performance was further increased to more than 7900 h in a single cell under an operating temperature of 80−85 °C with further cell parameter manipulation.62f DVB-cross-linked PEMs were also found to be promising candidates for DMFC applications. A series of investigations of

The performance of PEMFCs was found to be a function of the ohmic resistance of the membrane, which increases with a high degree of cross-linking. Grafting of St/DVB onto an ETFE film (25 μm) was found to produce membranes with better stability.113 For example, a cross-linked membrane stability (in a single) cell as long as 2180 h with a DG of 25% (cross-linked from 5% DVB) was reported at a current density of 500 mA cm−2 and a cell temperature of 80 °C.131 As reported in an earlier study, such membrane durability was 4-fold higher than that of another cross-linked PEM obtained by grafting a similar combination of St/DVB onto a 50 μm ETFE film followed by sulfonation.133 To improve the cross-linking quality and properties of the cross-linked PEMs further, a range of other cross-linkers and their combinations with DVB were investigated. For example, 12293


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a

a

styrene/10DVB/sulfonic acid

RX-PTFEa/15 EB/preirradiation

EB/preirradiation

EB/preirradiation EB/preirradiation γ-radiation/ preirradiation

γ-radiation/ preirradiation

∼27

γ-radiation/ preirradiation γ-radiation/ preirradiation

36 38.3 38.3

39

25

24

57

up to 88

25

∼45

18

∼20

∼28

11−34

39

γ-radiation/ preirradiation

γ-radiation/ preirradiation γ-radiation/ preirradiation EB/preirradiation

γ-radiation/simultaneous EB/preirradiation

17−24

γ-radiation/simultaneous γ-radiation/ preirradiation γ-radiation/ preirradiation 19

18−73 19−103

DG (%)

EB (175 kV)/ preirradiation

radiation source/method

RX-PTFE = radiation cross-linked PTFE.

styrene/DVB (10%)/sulfonic acid

(styrene + DVB), (styrene + BVPE)/cholorulfonic acid styrene + DVB (2−4%)/ cholorulfonic acid styrene + DVB (12%)/ cholorulfonic acid styrene + DVB (5%) + TAC (1%)/cholorulfonic acid styrene + DVB/cholorulfonic acid styrene + DVB (9%) + TAC (5%) cholorulfonic acid styrene + DVB (10%)/ cholorulfonic acid styrene + DVB (8%)/ cholorulfonic acid styrene + DVB (9%) + TAC (5%)/cholorulfonic acid styrene + DVB (9%) + TAC (5%)/cholorulfonic acid styrene + DVB (5%)/ cholorulfonic acid MeSt, tBuSt + DVB or/ and + BVPE/cholorulfonic acid MeS or tBuSt + 2.5% DVB + 17.5% BVPE/sulfonic acid styrene + DVB (10%)/ cholorulfonic acid styrene + DVB (5%)/ cholorulfonic acid styrene/BVPE/sulfonic acid

monomer/activating agent

RX-PTFEa/15

RX-PTFE /50

ETFE/25

ETFE/25

ETFE/25

ETFE/50

ETFE/50

ETFE/25

ETFE/100

FEP/25

FEP/25

FEP/75

FEP/50

FEP/75

FEP/125

PFA/120

PVDF/80

film type/thickness (μm)

2.1 2.2

2.0

1.35

2.0

up to 2.5

2.4

∼1.4

0.78−2.08

1.92

1.15

1.59−1.62

0.48−2.51 0.19−2.95

ion exchange capacity (mmol g−1)

100

29

60

equivalent to 80

17.0

113 (in situ at 60 °C)

41


23.0−3.0


36.0

6.3−15.9

BVBE > TAC. It was suggested that a mixture of the cross-linkers with lower concentrations of DVB and higher concentrations of BVPE or TAC in the grafting solution is most likely to provide an optimum combination for the preparation of PEMs with high chemical stability and high proton conductivity.137

Despite the fact that the majority of cross-linkers used in the preparation of PEMs are aromatic compounds, apparently for stability reasons, aliphatic cross-linkers such as MBAA have also been proposed.123 In particular, MBAA was used to cross-link PEM obtained by the RIG of St onto both FEP and ETFE films that were subsequently sulfonated. Cross-linking with MBAA was found to reduce the swelling in water and methanol and slightly improved the stability of the membranes in a simulated oxidative environment. This was established by sandwiching the membrane between two Pt electrodes in 0.1 N sulfuric acid for 100 h at room temperature with a current flow of 1 A cm−2. A summary of previous studies on the preparation of cross-linked PEMs by the RIG method is presented in Table 4. On the basis of previous studies pertaining to cross-linked PEMs, it can be concluded that cross-linking is essential for achieving the desired membrane stability/durability under dynamic fuel cell conditions. The effect of cross-linking has a remarkable impact on the physical, chemical, and mechanical properties of radiation-grafted PEMs. Such effects as well as the levels of change in the properties vary depending on the reaction parameters. These include dose, solvent, cross-linker concentration, and the nature of the polymer substrate. Thus, optimization of the reaction parameters including the type of cross-linker, concentration, and combination (if any) has to be judicious to establish a much needed balance in the properties of the PEMs with respect to obtaining the desired stability and durability. 5.1.2. Grafting of Substituted Styrene Monomers. 5.1.2.1. Alkyl-Substituted Monomers. Substituted St monomers such as those shown in Figure 4 are proposed to improve the stability of PEMs. The substitution of tertiary hydrogen at the α-carbon of PS with alkyl groups in such monomers is expected to provide protection against chemical attack (by oxidative species generated during the PEMFC operation), and this leads to better chemical stability. For example, the RIG of α-methylstyrene (MSt) was found to improve the chemical stability of poly-MSt grafted and sulfonated membranes in an oxidative environment.138 However, only a few studies using this approach have been reported. This is most likely due not only to the limited number of substituted-St monomers, but in addition to poor radical polymerization kinetics in some cases, such as when MSt is found. Such poor kinetics is caused by the instability of the free radicals and thus limits its use under experimental conditions because it requires irradiation at dry ice temperature. Chen et al.139 attempted to improve the chemical stability of ETFE-g-PSSA by the RIG of MeSt and tBuSt using the preirradiation method. A combination of DVB and BVPE was used to cross-link the membrane to bring about more stability. The ETFE-g-P(MeSt)SA and ETFE-g-P(tBuSt)SA membranes exhibited better chemical stability and sufficient mechanical strength and thermal properties, making them suitable for DMFCs.143 Particularly, the membrane (DG = 57%) crosslinked from 2.5% DVB and 17.5% BVPE showed an IEC value of about 2.0 mmol g−1 and conductivity of 59 mS cm−1 coupled with 6 times lower methanol crossover (1.16 × 10−6 cm2 s−1) than Nafion 117 (6.63 × 10−6 cm2 s−1).143 Despite the improvement in durability of this membrane as compared to the DVB or BVPE cross-linked membranes, its stability, however, remained inferior to that of Nafion 117. Another membrane obtained by the RIG of vinyltoluene (VT) and a VT/DVB mixture onto ETFE film followed by sulfonation was reported. The membrane with a 35% DG 12295


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showed an IEC and a proton conductivity of 2.0 mmol g−1 and 60 mS cm−1 at 25 °C, respectively. The membrane also exhibited a methanol crossover 5.4 times lower (1.8 × 10−6 cm2 s−1) than Nafion 112 (7.9 × 10−6 cm2 s−1). Such ex situ results appear promising. However, the in situ performance of these membranes in fuel cell systems under dynamic conditions is yet to be revealed.143 In another study, Nho and co-workers 144 prepared sulfonated PEMs with improved chemical stability by using RIG of VBC onto FEP and PFA films using simultaneous grafting followed by sulfonation as shown in Figure 16.144 The

higher chemical stability was attributed to the presence of a methylene spacer group between the benzene ring and the sulfonic acid group in the grafted moiety unlike what is present in PSSA-containing membranes. These membranes displayed an interesting combination of physicochemical properties. Of particular interest are the FEP-based membranes, which attained IECs in the range of 0.8−1.62 mmol g−1 at DGs in the range of 36−102%, respectively. Methanol permeability lower than that of Nafion 212 was also recorded. The membrane also exhibited a 1-fold lower methanol crossover (2.5 × 10−7 cm2 s−1) than Nafion 212 (5.3 × 10−7 cm2 s−1).145 Similar membranes based on PFA films with favorable IEC and proton conductivity have also been reported.146 These membranes showed 1.2 times lower methanol crossover (2.2 × 10−7 cm2 s−1) than Nafion 212 (5.3 × 10−7 cm2 s−1). Another styrene substituent of inorganic nature was used to prepare a new hybrid (organic/inorganic) PEM membrane with SA groups. p-Styryltrimethoxysilane (StTMS) was radiationgrafted onto ETFE film followed by sulfonation, hydrolysis, and condensation to introduce silane-cross-linking between the graft chains in the resulting PEM.147 A plausible mechanism for the preparation of the new hybrid PEM is shown in Figure 17. The IEC, water swelling, and ionic conductivity (at room temperature under wet conditions) of the hybrid membrane were determined and found to be 1.56 mmol g−1, 56.7%, and 15 mS cm−1, respectively. The thermal stability of this membrane was remarkably high as compared to those obtained by St grafting and was comparable to that of Nafion. Further characterization and fuel cell applications are yet to be reported. A similar PEM was obtained by simultaneous RIG of StTMS onto PTFE film followed by sulfonation, hydrolysis, and condensation. The PEM showed an IEC of 1.53 mmol g−1, a proton conductivity of 15 mS cm−1 at 90 °C (DOG 72.5%), and a low methanol permeability (0.82 × 10−6 cm2 s−1 at DG = 50.1%). This is much lower than that found for Nafion (5.3 ×

Figure 16. Mechanism for preparation of sulfonated PEM by radiation-induced grafting of VBC onto fluorinate film followed by sulfonation reactions. Reprinted with permission from ref 144. Copyright 2012 Elsevier.

Figure 17. Mechanism illustrating the preparation of hybrid PEMs by RIG of p-styryltrimethoxysilane onto ETFE film followed by sulfonation, hydrolysis, and condensation. Reprinted with permission from ref 147. Copyright 2007 Elsevier. 12296


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Figure 18. Mechanisms for preparation of PEMs by grafting of St/AN and AMS/AN and St/MAN mixtures onto ETFE films followed by sulfonation.

10−6 cm2 s−1). However, such a high DG usually does not provide a good stability when the membrane undergoes dynamic fuel cell testing.148 It can be concluded that the replacement of St with alkyl-substituted monomers is effective in improving the transport properties and chemical stability of PEMs. However, such monomers are generally expensive and less kinetically well understood as compared to St. This, of course, will have an impact on the economic viability of the membranes. 5.1.2.2. Fluorinated Monomers. The RIG of fluorinesubstituted St monomers onto fluorinated substrates followed by sulfonation provides PEMs with enhanced stability because both the backbone and the grafted moiety are fluorinated. The early work by Momose et al.,149 which reported the RIG of TFSt onto ETFE films and subsequent sulfonation, yielded PEMs with an excellent combination of physicochemical properties and high chemical stability.150 However, such membranes were challenged by the high cost of the TFSt (a fluorinated monomer). In addition, its poor kinetics led to prolonged reaction times. Nevertheless, it is strongly believed that membranes obtained by this method have conceived PEMs for commercial applications. In a recent study, Gürsel et al.151 reported new PEMs obtained by the grafting of a TFSt derivative monomer (pCF2CFC6H4OCF2CF2SO2F) onto preirradiated ETFE films and subsequent hydrolysis. The grafting was attempted both in emulsion and in a solvent. Grafting in alcohol yielded membranes with DGs between 104% and 229%. The fuel cell-relevant properties were investigated, and the membrane performance was tested in a single H2/O2 fuel cell. The membrane with a DG of 229% showed an in situ area resistance lower than that of Nafion 112, and its performance in the single

cell exhibited the highest durability of 524 h (at 60 °C, full humidification and H2/O2 ratio of 1.5/9.5). Another PEM was obtained by the radiation-induced grafting of 2-bromotetrafluoroethyl trifluorovinyl ether (BrTFF) onto ETFE film. Interestingly, no phase separation between the poly(BrTFF) grafts and the ETFE films was observed in such membranes. However, a relatively high dose such as 400 kGy was needed to achieve a reasonable DG of 20%. No further details on the performance of such membranes needed to evaluate their suitability for PEMFC were provided.152 It can be concluded that the use of fluorine-substituted monomers was found to improve the chemical stability of PEMs. However, fluorinated monomers are mostly expensive, and their kinetics is rather slow. The use of such monomers may also require special processing facilities, which will certainly affect the ultimate economic viability of the membranes. 5.1.3. Grafting of Mixtures of Styrene or Its Substituents and Comonomer with or without CrossLinker. The RIG of a comonomer with St is an interesting but less-explored approach for the preparation of PEMs for use in fuel cells. The strategy is to replace the tertiary hydrogen of an α-carbon of the grafted PS with the incoming comonomer molecules to prevent chemical attacks. Various St/comonomer or substituted St/comonomer mixtures were grafted onto films to boost the PEM stability. The RIG of a St and acrylic acid (AAc) mixture was grafted onto preirradiated FEP films under various reaction conditions followed by sulfonation to prepare bifunctional PEMs having carboxylic acid and SA moieties.65b The physicochemical properties of these membranes were found to be dependent on the DG.153 The IEC recorded values in the range of 0.4−1.7 mmol g−1 and area-specific resistance in the range of 0.80−0.42 12297


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Ω cm2 for membranes having DGs in the range of 7−46%. The thermal and mechanical properties were found to be strongly influenced by the DG in the membranes,154 although the performance of these membranes is yet to be reported. Becker et al.155 reported the RIG of St/acrylonitrile (AN) and MSt/AN mixtures onto FEP and ETFE films using a preirradiation method followed by sulfonation. The PEMs were found to have lower water and methanol swelling, which coincided with a better stability than the corresponding Stgrafted PEMs despite both of these systems achieving higher conductivities than Nafion 117. The kinetics of RIG of MSt/ AN onto ETFE at wide concentration ranges in the grafting solutions has been established.94a However, details on the properties of the sulfonated membranes in these studies were not reported, and their stability for PEMFC applications could not be evaluated. An interesting PEM was prepared by the RIG of a novel combination of St/MSt/DVB monomers onto RX-PTFE films followed by sulfonation. The new PEMs showed an improved glass transition temperature and high ex situ chemical stability as compared to the PEMs prepared by the RIG of St/DVB on the same film. The balance of the cost, grafting kinetics, thermal properties, and the properties of the resulted PEM are the key points addressed in this work.156 To enhance the grafting of AMS, known to have poor kinetics, and improve stability of polySt PEMs, Gubler et al.157 reported cografting AN and methacrylonitrile (MAN) as comonomers with St during its RIG onto ETFE films followed by sulfonation. Figure 18 illustrates the mechanism for preparation of the PEMs by grafting combinations of St/AN, MSt/AN, and St/MAN onto ETFE films and subsequent sulfonation. The use of AN and MAN as the comonomers was found to significantly increase the intrinsic oxidative stability of the membranes by conferring them with a protected α-position and a strong dipolar pendant nitrile group.158 The performance of the membrane grafted with St/MAN at 500 mA cm−2 current density showed a longer durability than the membrane grafted with only St and a fuel cell lifetime exceeding 1000 h.159 The fuel cell performance of AMS/MAN grafted membranes was found to be very close to that of Nafion-NR212.160 The effect of the nitrile-containing comonomer on the chemical stability of the membranes was also investigated by comparing membranes based on St/AN and St/MAN comonomer mixtures. Both membranes showed better performance than St-based membranes and showed a similar pattern of degradation. However, the St/AN-based membranes are more prone to hydrolysis of the nitrile (i.e., H2N−CO → HO− CO) group in the fuel cell tests. The α-methyl group in the MAN unit was shown to improve the stability of the nitrile groups against hydrolysis in the fuel cell environment. This membrane exhibited durability and lifetimes exceeding 2000 h as indicated in Figure 19. Here, the polarization curves and high frequency resistance of PEMs grafted with St, St/AN, and St/ MAN and Nafion 212 are also presented.161 Other PEMs were prepared by the RIG of MSt and t-BuSt in the presence of 1−3 vol % DVB. The membranes showed better chemical stability than grafted PS and sulfonated membranes under stimulated fuel cell conditions (191 h with 3% DVB as compared to only 1 h for a membrane obtained by RIG of St). However, the membranes were very brittle after sulfonation even without cross-linker and cannot be used for fuel cell tests.162 Such observations emphasize the significance of the need to optimize the cross-linking.

Figure 19. Polarization curves and high frequency resistance of PEMs grafted with St (DG = 21%), St/AN (DG = 40%), and St/MAN (DG = 42%) and Nafion-NR212 of H2/O2 (fully humidified) fuel cells at 80 °C and 2.5 bar. Reprinted with permission from ref 161. Copyright 2013 Elsevier.

The possibility of developing PEM membranes by the RIG of glycidyl methacrylic acid (GMA) and St onto PVDF followed by sulfonation was recently investigated.163 However, no details on the electrochemical properties of the membranes or their performance in fuel cells were reported.163 Grafting of methyl methacrylic acid (MMA) onto ETFE films and subsequent sulfonation has also been reported.164 In a very recent study, a new and cheap PEM was obtained by the RIG of GMA/St onto ETFE film using a preirradiation method followed by a mild sulfonation reaction with a sodium sulfite and bisulfite mixture.165 The GMA-based membrane obtained (DG = 23%) demonstrated a better initial performance in a single PEMFC as compared to the St-based ones that was almost as good as the Nafion 212. The cell could be operated at a constant current of 0.5 A cm−2 and 80 °C for more than 160 h as compared to only 60 h for the St basedmembrane having a similar conductivity and under the same conditions. Despite the potential of these GMA-based membranes, they are still far from meeting the stability requirements for use in PEMFCs. In conclusion, the approach of introducing a comonomer such as AN or MAN into St or its derivatives during grafting onto polymer films has brought about a remarkable improvement in membrane stability. This paves the way to creating a third generation of fuel cell membranes after regorious investigation to meeting PEMFC requirements. However, more work is needed to establish the optimum preparation procedures and optimize the stability of the membranes under dynamic fuel cell conditions. 5.1.4. Grafting of Functional Monomers. The RIG of SStS in association with AAc as a comonomer onto both low density polyethylene (LDPE) or high density polyethylene (HDPE) films was used for the preparation of a bifunctional PEM containing −SO 3 H and −COOH groups using simultaneous166 and preirradiation166a,167 methods as depicted in the mechanism presented in Figure 20. The reaction parameters were investigated, and the resulting membranes showed high IECs of 5.8 mmol g−1 in addition to good conductivity. Nevertheless, PE and AAc are not desirable in fuel 12298


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Figure 20. Mechanism of preparation of PEMs by radiation-induced grafting of acrylic acid and styrene sodium sulfonate onto PE films using two different methods.

Figure 21. Processes for the preparation of the PEEK-based polymer electrolyte membranes by a two-step grafting method. Reprinted with permission from ref 169. Copyright 2009 Elsevier.

that AAc-grafted PFA membranes have a very poor open circuit voltage of 0.190 V, and they were unable to sustain any current flow.71b The RIG of ethylstyrenesulfonate (ETSS) was used to prepare a new PEM made from sulfonated styrene-PEEK (ssPEEK) as reported by Chen et al.169 The membrane was prepared by thermal grafting of DVB onto PEEK followed by RIG of ETSS and subsequent hydrolysis.169 Figure 21 shows the mechanism for the preparation of ssPEEK membranes by thermal cross-linking and RIG. The membrane was found to

cells because the former can easily undergo chemical degradation, whereas the latter does not undergo complete dissociation. Similar bifunctional PEMs were prepared by Patri et al.136 using different methods in which the RIG of AAc onto FEP films was performed followed by sulfonation reaction. The membrane with a 23% DG was reported to give a steady performance in PEMFC for 100 h at 50 °C under a constant current density of 1 A cm−2.168 Earlier, attempts to prepare PEMs by RIG of AA onto various fluorinated films revealed 12299


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Figure 22. Mechanism of preparation of sulfoalkyl/carboxylic PEM. Reprinted with permission from ref 174a. Copyright 2008 Elsevier.

Figure 23. Mechanism of preparation of the ETFE-g-PVESA membrane by RIG of CEVE onto ETFE film followed by sulfonation. Reprinted with permission from ref 176. Copyright 2012 John Wiley.

Figure 24. Mechanism for preparation of PEMs containing alkylsulfonic acid and hydrophilic hydroxyl group by radiation-induced grafting VAc onto ETFE film followed by saponification and alkylsulfonation. Reprinted with permission from ref 177. Copyright 2012 Elsevier.

(IEC) region. This is in contrast to cross-linked PTFE PEM, which showed a higher conductivity at a high IEC region.173 Despite the potential of grafting functional monomers for obtaining PEMs and the use of PEEK as an alternative hydrocarbon polymer to fluorinated ones, it is too early to judge the viability of such a route based on the few published studies. In addition, the kinetics of these mostly expensive monomers are not well established. More research work is definitely needed to resolve such issues as well as addressing the membrane stability in a meaningful manner before their full potential can be realized. The future success of these membranes could lead to a new generation of fluorine free PEMs. 5.1.5. Grafting of Other Monomers. Radiation-grafted PEMs were also prepared by the RIG of monomers other than St and its derivatives despite the limitations of nonstyrenic monomer grafting by free radical polymerization. In addition, there is the issue of the sluggish sulfonation reaction required to introduce the ionic group into the grafts without creating hydrolyzable bonding such as ester linkages. Recently, PEMs containing alkylsulfonic acid were prepared by RIG of methyl acrylate (MA) onto ETFE films. According to the mechanism shown in Figure 22, this was followed by sulfonation at an αcarbonyl carbon of the MA units in the grafting chain using an equimolar complex of chlorosulfonic acid and dioxane.174 Similar radiation-grafted PEMs with alkylsulfonic acid were prepared by replacing MA with a MA/methyl methacrylate

have better properties (proton conductivity and mechanical properties) than Nafion 212.169 The fuel cell performance with ssPEEK membrane was comparable to that with Nafion at 80 °C and 100 RH%. However, the performance was better when tested at 95 °C and 40 RH% and delivered a stable performance at 0.3 A cm−2 for more than 250 h without any substantial voltage drop unlike that of the Nafion-based cell, which continuously dropped over the same duration. Further testing of the lifetimes of ssPEEK membranes to 1000 h under the same conditions was accompanied by a slow voltage degradation of 18 μV h−1.170 In a relevant study, Chen et al.171 also prepared PEMs for DMFC by the thermal grafting of DVB onto a thin PEEK film (6 μm) followed by RIG of ETSS. The PEM with IEC of 2.71 mmol g−1 was tested in a DMFC at 95 °C. A maximum power density of 110 mW cm−2 was achieved, which is better than Nafion tested under the same conditions. However, a further increase in temperature above 95 °C led to a drastic drop in the cell performance. Structural studies of ssPEEK membranes revealed that proton transfer takes place in ion channel domains located at an average distance of 13 nm, in which there are nanostructures of sulfonic acid groups at an average distance of 1.8 nm.172 Further structural−property relationships were established in ssPEEK as compared to its corresponding PEM-based cross-linked PTFE. The PEEK-based PEMs showed higher conductivity at a low ion exchange capacity 12300


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(MMA) mixture for grafting onto ETFE film, followed by a similar sulfonation procedure.175 However, the yield of sulfonation was below 30% leading to lower IECs and ionic conductivity, which are not sufficient for fuel cells operating at high temperatures and low humidity. More data on the performance of these membranes in PEMFC or DMFC are yet to be reported. Another new PEM for possible use in DMFCs comprised of poly(vinyloxy)ethanesulfonic acid-grafted ETFE (ETFE-gPVESA) was prepared by simultaneous RIG of 2-chloroethyl vinyl ether (CEVE) onto ETFE film followed by sulfonation as shown in Figure 23.176 The preparation involves formation of a thiouronium salt with thiourea, hydrolysis with NaOH to form a thiol, and finally oxidation of the thiol using hydrogen peroxide to achieve the desired sulfonic acid group. Membranes with DGs of 48 and 71, which respectively correspond to IECs of 1.48 and 2.28 mmol g−1, were obtained.176 No details on evaluating the performance of these membranes have been published. In another study, PEM membranes containing alkylsulfonic acid and hydrophilic hydroxyl groups (PEM-OH) were prepared by the RIG of vinyl acetate (VAc) onto ETFE film followed by saponification and alkylsulfonation with 1,3propane sultone. The mechanism is shown in Figure 24.177 Membranes with IECs of ∼1.3 and 1.9 mmol g −1 showed ionic conductivities of 1.0 and 3.8 mS cm−1, respectively (at 30% RH and 80 °C). The conductivities of these membranes were found to be higher than those of PEMs based on sulfonated polyimide and poly(ether sulfone) with similar IEC values.178 However, the predicted inferior performance of such PEMs in fuel cells was not discussed. Despite some potential for exploring monomers other than St, it must be acknowledged that this route is challenged by lengthy preparation procedures and unknown reaction kinetics that undoubtedly will have an impact on the overall economics. More research to shorten preparation procedures and establish the reaction kinetics is required before any certainty regarding credible fuel cell performance and desired stability can be established.

To enhance the stability of PEMs based on RX-PTFE, the concept of double cross-linking with the use of thin films was introduced. Hiraiwa et al.142 prepared a cross-linked PEM by the RIG of St/DVB onto RX-PTFE of different thicknesses followed by sulfonation. Gas permeation experiments showed that thinner membranes (13.6 ± 0.5 μm) have higher gas crossover values than their thicker counterparts (73.8 ± 2.7 μm). A maximum power density of 469 mW cm−2 was obtained for a MEA based on a 18.6 ± 1.3 μm thick PEM with a DG of 38.3% at 60 °C and a relative humidity of 16%.142 In another study, Yamaki et al. used RX-PTFE as a substrate for grafting St with a BVPE cross-linker.122b Their membrane exhibited high chemical durability coupled with reasonable swelling, chemical, and mechanical properties.122b The double cross-linked membrane was prepared using the following conditions: (i) γ-ray preirradiation of a 50 μm thick RXPTFE film at a dose of 15 or 60 kGy under an Ar atmosphere, (ii) the grafting reaction was in St contained 20 mol % BVPE at 60 °C, and (iii) sulfonation was in a chlorosulfonic acid/1,2dichloroethane mixture at 50 °C. It was subsequently tested in a fuel cell and compared to both the corresponding non-crosslinked and the Nafion 112 membranes. The polarization curves of these membranes are shown in Figure 25. The PEMFC

5.2. Radiation-Induced Grafting onto New or Modified Polymer Substrates Followed by Sulfonation

5.2.1. Radiation Cross-Linking of Substrate, Grafting of Styrene or Its Substituent. Recently, interest in using PTFE as a polymer substrate for preparing PEMs has been renewed after it was shown that the radiation resistance could be improved by the radiation cross-linking of the PTFE at molten temperatures. The cross-linked PTFE (RX-PTFE) showed remarkable improvement in radiation resistance, thermal durability, and mechanical properties, as compared to those of non-cross-linked PTFE.179 The use of RX-PTFE with its network structure as the starting polymer allows the use of thin films for making the membrane while reducing the gas crossover when using in PEMFC. Yamaki et al.180 prepared new PEMs by the RIG of St into the RX-PTFE films using a preirradiation method with γradiation and subsequent sulfonation. DGs in the range of 5− 75% were obtained and achieved IECs up to 2.6 mmol g−1.127,181 Surprisingly, these membranes suffered from some deterioration in their mechanical properties, and a higher water uptake as compared to their counterparts based on non-crosslinked PTFE was also observed.

Figure 25. Polarization curves of PEMFC with membranes: (#1) with BVPE cross-links, (#2) without cross-links, and (#3) Nafion 112. Reprinted with permission from ref 182. Copyright 2009 The Electrochemical Society.

performance of the double cross-linked membrane was high enough to match that of Nafion 112 with the cell voltages at 1.0 A cm−2 and OCV 0.603 and 0.947 V, respectively. The double cross-linking system was found to induce not only a decrease in the reactant permeability, but also an improvement in the chemical stability and interfacial properties resulting in higher cell performance and stability for >500 h of the steady-state operation. This suggests that double cross-linking involving BVPE can in fact effectively enhance the durability of radiationgrafted membranes.182 Similar double cross-linking with DVB was reported by Li et al.62g The cross-linked membrane, based on a RX-PTFE film, showed a reduction in the gas crossover and an enhancement in 12301


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the chemical stability.62i The chemical composition and microstructure of doubled cross-linked membrane properties have also been investigated from a materials research point of view.62j The PS grafting mechanism was found to proceed in two steps: (1) first, grafting occurs at an interface of the RXPTFE microcrystals and (2) a thin layer of pure PS chains surrounding the RX-PTFE microcrystals is formed. Next, either grafted PSt chains or a thin layer start to bridge between the crystalline domains, leading to 40% of the total microcrystals being covered by the PSt thin layer as revealed by time-resolved small-angle neutron scattering.54a Other alkyl vinyl ether monomers such as propyl vinyl ether (nPVE) and isopropyl vinyl ether (iPVE) have been used by Chen et al. to replace St to yield PEMs based on RX-PTFE films.183 The grafting reaction was initiated under simultaneous irradiation conditions and in the presence of AlCl3 or at a temperature close to the boiling point of each monomer used. The grafted RX-PTFE films were subsequently sulfonated using chlorosulfonic acid in dichloromethane. The structures and the thermal stabilities of the membranes were established. Interestingly, these membranes recorded higher proton conductivity than Nafion 112 despite their lower ion exchange capacity (0.75 mmol g−1). A new cross-linked PEM for DMFC applications was prepared by the RIG of substituted St monomers such as MeSt and tBuSt, as well as their mixtures, with DVB, BVPE, and a DVB/BVPE mixture onto RX-PTFE films.140 The resulting membranes were found to have high thermal stabilities as well as significantly lower methanol permeability as compared to Nafion membranes. Interestingly, these membranes exhibited better performance in DMFC especially when a high methanol feed concentration was used. Of particular note is the fact that membrane cross-linking from St containing 2.5% DVB + 17.5% BVPE showed the highest chemical stability combined with reasonable water uptake, proton conductivity, and mechanical strength in addition to lower methanol permeability than Nafion 117. The performance of DMFC with this membrane at a high methanol feed concentration of 5 M recorded a maximum power density of 70 mW cm−2. This is more than double that of Nafion 117 under the same conditions. The potential of these membranes for PEMFC applications was also further explored.184 Li et al.62g extended the concept of double cross-linking by preparing new PEMs by RIG of a mixture of MSt/St/DVB onto the cross-linked RX-PTFE films. They were subsequently sulfonated. The new membranes showed an improved glass transition temperature and chemical stability as compared to the membranes prepared by RIG-induced grafting of St/DVB. However, no data on the ionic conductivity of these membranes, nor on their performance in PEM fuel cells, were reported. Lappan et al.166b,172 reported the preparation of new FEP-gPSSA and PFA-g-PSSA membranes by the RIG of St onto radiation-cross-linked FEP and PFA films using the preirradiation method and subsequent sulfonation. The FEP and PFA films were cross-linked by irradiation with EB under a N2 atmosphere at 290 and 350 °C, respectively.181b,185 However, the properties of such membranes have not been been disclosed. The double cross-linking approach appears to be very promising in producing highly conductive thin PEMs while maintaining good barrier properties. However, only membranes based on cross-linked PTFE were subjected to investigation in

PEMFC and DMFC, and, in our opinion, there are insufficient results to draw solid conclusions regarding their durability. Further work is needed especially using other cross-linked fluorinated films (e.g., ETFE, FEP, and PFA), replacing St with various substituted St monomers and using comonomers or cross-linkers to further boost the stability. 5.2.2. Grafting of Styrene onto Porous Films/PoreFilling. Pore-filled membranes can be obtained by pore-filling using the RIG of a monomer into the porous microstructure of selected films. The result is a class of composite membranes that is composed of two components in the form of a polymer electrolyte immobilized in a porous substrate. The former component provides facile ion transport, whereas the latter suppresses swelling and provides mechanical support to the membrane. Recently, PEM pore-filled membranes were proposed as alternatives to PFSA membranes for use in DMFC applications. They offer various advantages, which include nonswelling, low methanol crossover, high proton conductivity, low cost, and a wide range of substrate materials. Early work on pore-filled membranes for the use in DMFCs reported by Yamaguchi and co-workers involved the preparation of a membrane with high conductivity and low methanol permeability based on a poly(vinylsulfonic acid/AAc) crosslinked gel in a PTFE substrate.186 Similar, yet highly durable, pore-filled membranes for DMFC were developed starting from a porous polyimide (PI) film and sulfonated polyarylene ether sulfone. The PI-based pore-filled membrane was compared in several tests with a commercial Nafion 117. The performance of this new membrane (40 μm thick) successfully exhibited a methanol crossover 300 times lower when tested with 30 wt % methanol solution at 25 °C. In addition to showing a lower ohmic loss in MEA, the new membrane showed a power density of ∼550 mW cm−2 at 1500 mA cm−2. It was also tested in a H2/O2 PEMFC at 60 °C.187 Nasef et al.61 also reported the preparation of radiationgrafted pore-filled membranes for use in DMFCs by impregnating the microporous structure of PVDF films with St followed by direct EB irradiation and subsequent sulfonation. The use of direct irradiation with EB was shown to greatly reduce the monomer consumption, shortened reaction times, and improved the overall economics. Membranes with DGs in the 8−46% range were obtained, and those having DGs of 40% and 46% exhibited excellent combinations of physicochemical properties as compared to Nafion 117. For example, under the same conditions, the 46% grafted membrane achieved a 61 mS cm−1 conductivity (as compared to 53 mS cm−1 for Nafion 117 under the same conditions) and a 5-fold lower methanol permeability (0.7 × 10−6 cm2 s−1) than for Nafion 117 (3.5 × 10−6 cm2 s−1).188 The DMFC potential was tested at 70 °C with 5 M methanol using two MEAs made of commercial electrodes (0.4 mg cm−2) and pore-filled PEM with DGs of 40% and 46%. The results showed maximum power densities of 110 and 120 mW cm−2, respectively.48 Although this performance is promising, it does require further stability evidence. To improve the transport in pore-filled membranes, Yamaki et al.189 used PVDF with a nanoporous structure in the form of cylindrical pores with diameters of 100 nm (track-etched films) to prepare PEMs for possible use in DMFCs. The membranes were obtained in four steps, which involved (i) irradiation of the PVDF films with swift heavy ions, (ii) chemical etching, (iii) RIG of St in the nanopores using γ radiation, and (iv) functionalization of the membrane by sulfonation. The 12302


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Figure 26. Schematic representation for the preparation of pore-filled PEMs by swift heavy ion irradiation, track etching, irradiation with γ rays, and sulfonation. Reprinted with permission from ref 191. Copyright 2007 Elsevier.

Figure 27. Mechanism for preparation of PEM by RIG of styrene onto alicyclic polybenzimidazole (ChPBI) followed by sulfonation. Reprinted with permission from ref 196. Copyright 2013 Elsevier.

They were prepared by RIG of St onto porous P(VdF-HFP) cast from a polymer solution containing SnO2 nanoparticles followed by sulfonation and oxide loading. The incorporated SnO2 particles reduced the DG and promoted the thermal stability of the resulting membranes. The composite membranes (9 wt % of SnO2, DG of 40%, and IEC of 0.9 mmol g−1) exhibited lower methanol crossovers with a reasonable ionic conductivity value (98 mS cm−1 at 100 °C). This was at even higher temperatures and lower humidity conditions as compared to similar membranes without SnO2. Testing the performance of this composite membrane in DMFC revealed an improved efficiency. For example, the SnO2-containing membranes were reported to have a power density of 110 mA cm−2 as compared to 80 mA cm−2 for the SnO2-free membrane when E-Tek commercial electrodes with the 1 mg cm−2 (Pt) and 2 mg cm−2 (Pt−Ru) catalyst loading at 60 °C were used.193 In conclusion, preparation of PEMs by pore-filling is found to be an effective strategy for improving the economics of the reaction and for yielding membranes that have low methanol crossovers, thus rendering them suitable for DMFC applications. Nevertheless, the use of the alternative lengthy procedures (ion beam irradiation followed by track etching and RIG followed by functionalization) required to produce such membranes is rather daunting and endures high cost that

membranes were found to have lower water uptake and methanol permeabilities as compared to the Nafion 117 membrane.189 The evolution of the cylindrical nanopores formed in these membranes was monitored by the in situ measurement of the conductance through the membrane. This made it possible to establish the relationship between the etching kinetics under various experimental conditions.190 Similar PEM pore-filled membranes were obtained by the irradiation, with swift heavy ion-beams, of 129Xe23+ followed by γ-ray irradiation, then grafting of St onto the ETFE films followed by sulfonation as shown in Figure 26. A proton conductivity of 110 mS cm−1 was achieved when measured in the thickness direction.191 No additional information was given. Similar pore-filled membranes based on a 25 μm-thick PTFE film were prepared by the RIG of St in latent pores with the γrays. The membrane demonstrated a low DG of 7.5%, an IEC of 0.06 mmol g−1, a proton conductivity of 60 mS cm−1 in the thickness direction, a methanol permeability of 0.35 × 10−6 cm2 s−1, a tensile strength of 19 MPa, and finally an elongation (%) at break of 210%. The methanol crossover was found to be about 3-fold lower than that of Nafion 112.192 No details on any testing in fuel cells were reported. Another range of composite pore-filled PEMs containing PSSA grafted P(VdF-HFP)/tin oxide for DMFC were reported. 12303


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45 mW cm−2 at 60 °C (1 M methanol, O2, 2 mg cm−2 Pt−Ru anode, 1 mg cm−2 Pt cathode).197 This approach is of high interest with respect to migration from fluorinated polymers and merits further investigation using a range of polymer powders with a view to improving stability and the move to more environmentally friendly PEM preparation approaches. In another investigation, solution casting was also used to convert grafted polymer powders into homogeneous PEMs. Li et al.200 prepared PEMs by the RIG of St onto PVDF powders. These were subsequently dissolved in N-methylpyrrolidone and cast into films. This method was found to be highly effective for yielding membranes with a uniform distribution of grafted PSt chains. The proton conductivity of a PEM with a DG of 21.8% was found to be about 10 mS cm−1. This corresponds to an IEC and water uptake of 1.41 mmol g−1 and 40.6%, respectively.200 5.2.5. Grafting onto Electrospun Nanoweb Polymer Substrates. Electrospun polymer fibers with high molecular orientation and large specific surface areas are capable of hosting functional groups. However, such new promising materials have not yet found a wide range of applications as substrates for PEMs. On the other hand, PEMs with SA groups were successfully prepared on the basis of electrospun nanofibers of PVDF by the RIG of SStS in the presence of comonomers such as St, AAc, or NVP. The proton conductivity at 20 °C was in the range 20−53 mS cm−1, and an average DG of 30−32% and IEC of 2.8 mmol g−1 were found.201 Further details on their performance in fuel cells were not reported. However, this approach appears interesting and has the potential to lead to thin membranes suitable for DMFC because they can combine high conductivity and low methanol crossover. Producing more stable polymers is a definite possibility. However, low grafting levels and the lack of mechanical stability of the final membranes especially after sulfonation are challenges facing further use of this technique. Therefore, it is possible that developing this type of membrane would have to involve cross-linking and the use of nanofillers to improve the strength and transport properties of such membranes.

undermines the economics improvement associated with filling of porous matrixes. Cheaper hydrocarbon polymers such as porous PE or PP could provide alternative substrates for further reducing the cost and eliminating the need for fluorinated polymers. 5.2.3. Grafting of Styrene onto Engineering Plastics. Nonfluorinated polymers like engineering plastics including PEEK, PI, and polybenzimidazole (PBI) are alternative substrates that can improve some environmental aspects of radiation-grafted membranes. Of particular note are the PEEKbased PEMs, which were prepared for the first time by the RIG of St onto PEEK films followed by sulfonation. The resulting PEM displayed an IEC and a proton conductivity in the 1.2− 2.9 mmol g−1 and 0.03−180 mS cm−1 range, respectively.194 However, primarily the membrane exhibited better mechanical properties and lower methanol permeability as compared to Nafion membranes. In addition, a promising durability in DMFC that reached a maximum power density as high as 106 mW cm−2 at 95 °C was also observed.195 Alicyclic PBI (ChPBI) is another engineering plastic that was used to prepare radiation-grafted PEMs as reported by Park et al.196 This PEM was prepared by the RIG of St onto alicyclic PBI (ChPBI) followed by sulfonation. Initially, the ChPBI was prepared from trans-1,4-cyclohexanedicarboxylic acid and 3,3′diaminobenzidine using polycondensation as shown in the scheme presented in Figure 27. The resulting membranes had IECs of 2.1 and 2.9 mmol g−1 and exhibited proton conductivity of the order 1−10 mS cm−1 (at 25 °C and RH = 100%). This was found to be higher than the corresponding membranes based on linear PBIs with sulfonic acid. The membrane also showed thermal durability with no significant deterioration in proton conductivity after 600 h at 120 °C in liquid water.196 The use of engineering plastics such PBI and PEEK films as starting substrates is a major step toward eliminating fluorinated polymers and improving the radiation-grafted membranes preparation environmental aspects. Thus, it is highly significant to extend the application of engineering plastics substrate films with various monomers to produce fluorine-free new membrane generations. The performance of these new membranes will need to be established in PEMFC or DMFCs under dynamic conditions. 5.2.4. Grafting onto Powder Followed by Film Formation Using Thermal Processing or SolutionCasting. RIG onto polymer powders and their conversion into films is another approach used to improve the economics of PEMs. Sherazi et al.197 reported this method for the preparation of PEMs based on ultrahigh molecular weight polyethylene (UHMWPE) powders that were converted into films by compression molding and subsequently sulfonated using chlorosulfonic acid. The mechanisms of the grafting reactions were established by studying the effect of various preparation conditions on DG in both simultaneous and preirradiation grafting reactions.198 PEMs with IECs in the range 0.97−2.77 mmol g−1, which corresponds to proton conductivities of 25−290 mS cm−1 at 90 °C, were obtained. Using this approach, they succeeded in achieving a homogeneous distribution of the PSSA grafts in the membranes at low DG. The recorded methanol permeability, which is in the range 4.86 × 10−8 to 1.45 × 10−6 cm2 s−1 for membranes with DGs of 12−44%, is far lower than that of Nafion (1.65 × 10−6 cm2 s−1).199 The power density of a membrane with a DG of 22% in a single cell was found to be about 32 mW cm−2 at 40 °C and

5.3. Applying New Radiation Grafting Techniques

5.3.1. Direct Sulfonation of Irradiated Polymer Films. Sundholm and co-workers reported a new method for the preparation of PEMs by the direct sulfonation of PVF films by irradiation with heavy charged particles prior to sulfonation.68,202 The resulting membranes (PVF-g-SA) displayed ionic conductivities of up to 20 mS cm−1. The hydration properties203 and the structures204 of these membranes were established. The hydration properties of such membranes were found to be lower than those of either Nafion 117 or Nafion 112. Interestingly, PEMFC tests with these membranes showed short-term performances better than Nafion 117 membranes. This trend is reversed by Nafion 112, which performed better than PVF-g-SA membranes as shown by their polarization curves depicted in Figure 28.205 However, these membranes failed after 200 h of testing due to rupture between the membrane active area and the gasket. PTFE films were used to replace PVF in the preparation of another sulfonated PEM, which subsequently exhibited better stability. This was carried out by the direct introduction of SA groups into the PTFE films by EB irradiation. The PTFE films were irradiated in water under atmospheric conditions followed by treatment with fuming sulfuric acid.206 However, no details on the basic properties of these membranes and their fuel cell 12304


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5.3.2. Grafting by Mediated Living Radical Graft Polymerization Followed by Functionalization. Mediated radical polymerization methods have been used on a few occasions to develop PEMs membranes. This method relies on a completely pure reactant to prevent any termination caused by impurities. Polymerization stops only when the monomer is totally consumed and not with termination. Only ATRP and nitroxide-mediated graft polymerizations have been used for the preparation of PEMs. 5.3.2.1. ATRP-Mediated Graft Polymerization. Novel PEMs prepared by the ATRP of St onto PVDF-g-PVBC membranes obtained in turn by the RIG of VBC onto PVDF films have been investigated. The ATRP was initiated with CuBr/2,2dipyridyl as a catalyst. The resulting block copolymer was sulfonated and demonstrated a proton conductivity of 70 mS cm−1, which is considered sufficient for fuel cell applications.209 Figure 29 shows a mechanism for the preparation of sulfonated PVDF-g-PVBC-g-PS proton conducting membranes. Another PEM, with better chemical stability and homogeneous graft distribution, was prepared by RIG followed by ATRP.210 The procedure involved grafting BrTFF onto an ETFE film to form bromine-containing perfluorinated grafts in which the bromine atoms acted as initiators. The films were then treated with a Cu(I)-based catalytic system of a CuBr and 2,2′-bipyridyl (bpy) for the ATRP step. By optimizing the molar ratio of the initiator/CuBr/bpy and the reaction temperature, PSt grafts with a yield of over 100% were formed in the ETFE-g-PBrTFF films. SA groups were further introduced leading to a membrane with a DG of 15% and proton conductivity higher than that of Nafion 117 membranes. Moreover, the membrane has a chemical stability better than the corresponding ETFE-g-PSSA membrane obtained by conventional RIG. The data on fuel cell testing are yet to be reported. 5.3.2.1. Nitroxide-Mediated Graft Polymerization. PEMs have also been prepared by a combination of RIG with nitroxide-mediated living free radical graft polymerization followed by sulfonation as depicted in Figure 30. The procedure involved first reacting 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) with a preirradiated PVDF film leading to the formation of TEMPO-capped macroinitiator sites. Subsequently, these were used in the nitroxide-mediated pseudoliving radical polymerization of St or for the controlled grafting of St and N-phenylmaleimide onto the PVDF precursor. The membranes were either directly sulfonated, or alternatively the alkoxyamine moieties at the polymer chain ends were substituted by a maleimide derivative before sulfonation. The use of such combinational techniques

Figure 28. Polarization characteristic curves of the tested membranes, PVF-SA (25 μm) (□), Nafion 117 (◇), and Nafion 112 (△), at optimal fuel cell temperatures for the PVF-SA membrane. The gas pressure was 2 bar, and the temperatures varied slightly around 50 °C. The mechanical pressure was 10 bar. Reprinted with permission from ref 205. Copyright 2002 Elsevier.

performance were reported. These membranes could be expected to have low acid content and to be prone to fast degradation due to the inherent strong hydrophobicity and the high-radiation sensitivity of PTFE. The direct sulfonation approach was also found to produce another PEM by the direct incorporation of SA groups into the ETFE films by irradiation of the film with a proton beam followed by sulfonation.64a After ex situ characterization, the resulting membrane was found to be promising.207 Of particular note is the fact that the membrane had an exceptional lower water uptake, better dimensional stability, and 10% lower methanol permeability than Nafion 115.208 The performance of these membranes was tested in DMFC at 30−85 °C. The maximum power densities of 40−65% are lower than the corresponding values for Nafion 115. Both the chemical and the mechanical stabilities of new ETFE-based membranes appeared to be promising after testing for over 2000 h in the DMFC without obvious performance loss. Considering the simplicity of the direct sulfonation of polymer films after irradiation, one would expect more research to have been carried out with a view to establishing preparation procedures and membrane properties using a range of polymers. The resulting membranes would need to be tested systematically in fuel cells for durability under dynamic conditions. The economics of these promising membranes would also need to be established.

Figure 29. Mechanism for preparation of sulfonated PVDF-g-PVBC-PS proton-conducting membranes by RIG of VBC onto PVDF films followed by ATRP of St onto PVDF-g-PVBC membranes. Reprinted with permission from ref 209. Copyright 2002 John Wiley. 12305


Chemical Reviews

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Figure 31. Single-step method for the preparation of PEM by grafting of sodium styrenesulfonate. Reprinted with permission from ref 213. Copyright 2009 Elsevier.

Figure 30. Mechanism for preparation of PEM membrane using TEMPO-mediated grafting and sulfonation. Reprinted with permission from ref 211. Copyright 2004 American Chemical Society.

acid into the grafting mixture, and this played a significant role in boosting the DG to levels suitable for fuel cell membranes. The effect of the acid addition followed the sequence: H2SO4 > HCl > CH3COOH > HNO3.214 The membranes exhibited a very good combination of physicochemical properties, which were suitable for DMFC applications.215 The properties were also found to be better than similar PVDF-g-PSSA membranes obtained by the conventional two-step RIG method.216 The single fuel cell test at 60 °C with three membrane-electrodeassemblies (MEAs) made of commercial electrodes (0.4 mg cm−2) and the membranes with a DG of 65% showed a maximum power density of 78 mW cm−2.48 Li et al.217 prepared a hydrocarbon-based PEM for DMFC applications by the RIG of SStS onto aromatic polyamide (PAm) films followed by subsequent hydrolysis. The new membrane exhibited an IEC of up to 1.63 mmol g−1, which is comparable to the proton conductivity of Nafion 117 (83 mS cm−1). However, it has a much lower methanol permeability. Further details are yet to be reported. The fact that so few studies have been published using singlestep preparation route suggests it has not been well explored despite its high potential for improving the overall membrane economics. More work is clearly needed to establish the reaction kinetics and to test other polymer substrates prior to rigorous testing in fuel cell under dynamic conditions. 5.3.4. Patterned Grafting of Monomers on Masked and Irradiated Films. Patterned RIG in which graft copolymerization occurs in defined areas of the base film has been used for the preparation of microstructured PEMs.218 Patterning with lateral dimensions as small as 100 nm was achieved by irradiation with EB or lithographic X-rays through high aspect ratio Ni-masks, which were used to create patterns of free radicals on the irradiated film. This was followed by monomer grafting and subsequent sulfonation.219 In the grafting step, the growth of the polymer occurs only on the irradiated area, and the polymer chains form “brush-like” structures on the polymer substrate surface. The resulting membrane has sulfonated grafted areas, which provide the required ionic conductivity, while the remaining ungrafted areas maintain the mechanical stability. A detailed study on the synthesis, characterization, fuel cell performance, and durability of the microstructured proton-exchange membranes has been reported.220 Patterned and cross-linked PEMs were later prepared by the irradiation of ETFE with hard synchrotron X-rays through high aspect ratio Ni-masks followed by reaction with St/DVB. The patterned ETFE-g-PSSA membranes obtained (shown in Figure 32) exhibited more homogeneous grafting as compared to the unpatterned ones. The performance of the patterned

produced membranes with homogeneous graft distributions at a DG of 14%. This is approximately 2-fold lower than that found in the corresponding PEM obtained by conventional RIG of St onto PVDF film. The preliminary fuel cell tests showed a steady performance for 930 h at 70 °C. In contrast, conventionally prepared PEMs failed within 150−200 h under similar operating conditions.211 Similar PEMs containing SA with higher DGs were prepared using a three-step procedure involving the addition of TEMPO onto preirradiated PVDF and ETFE films, the TEMPOmediated living radical grafting of St into the films at 125 °C, followed by sulfonation.212 The PEMs were found to have homogeneous distribution of SA (at a DG of 38%) with a maximum IEC of 2.2 mmol g−1 and a proton conductivity of 160 mS cm−1. Such properties render this PEM superior to counterparts obtained by the RIG of St and sulfonation. Despite the success in producing membranes with controlled side chain grafts and properties comparable with Nafion 117, it must be noted that the procedure is obviously long and rather expensive as a result of using the mediating agents. In addition, the substrate polymers tested were mainly limited to PVDF and ETFE, and there is a lack of information on the stability of these membranes for long-term performance in fuel cells. One way to to shorten the procedure and improve the stability would be to use graft- functionalized St monomers such as ETSS and SStS as discussed in the next section. 5.3.3. Single-Step Grafting of Sulfonated Monomers. With a view to improving PEM stability, the development of a method to graft monomers containing SA groups such as SStS directly onto polymer films is yet another approach that has been attempted. Potentially, this could be realized by eliminating the sluggish sulfonation reaction that adversely affects the crystalline structure and hence the mechanical strength of the starting polymer films. This would shorten the membrane preparation time and thus promote better overall economics. However, the RIG of SStS is very difficult. This is due to its poor polymerization kinetics caused by the incompatibility between its highly ionized sulfonic acid groups with their hydration spheres and the hydrophobic polymer backbone.213 However, such poor kinetics can be improved by introducing acrylic acid as a comonomer during or prior to SStS grafting as discussed earlier. Alternatively, SStS can be grafted in special acidic solvent mixtures. Nasef et al.213 were the first to report a single-step route to prepare PEM by the RIG of SStS onto EB-irradiated PVDF films without the adition of an acrylic acid comonomer. This single-step approach is shown in Figure 31.213 The grafting reaction was synergized by the addition of 12306


Chemical Reviews

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well established, and the resulting modified Nafion membranes are most likely to be expensive.

6. RADIATION-GRAFTED MEMBRANES FOR HIGH TEMPERATURE PEMFC PEMFCs made with commercial PFSA membranes such as Nafion and its analogues often operate within a temperature range of 60−80 °C and a pressure range of 1−5 bar. In addition, they have low CO and sulfur tolerances. This is designed to suit the water-dependence mechanism of proton conductivity and to tolerate the variations in the viscoelastic properties of the PFSA membranes.223 It is essential, therefore, to prevent the membrane from drying out as well as over wetting. This can be achieved by installing a water humidification management system, which in turn, however, adversely adds to the overall cost and inherent complexity of the PEMFC system. To overcome such challenges, operating the PEMFC above 100 °C becomes highly desirable. This is necessary to enhance its efficiency through better electrode kinetics, elimination of the need for a humidification system, greater enhancement of the tolerance to reformed fuel impurities such as CO, and finally it opens the possible utilization of excess heat for cogeneration.225 Various approaches have already been used to develop PEMs for HT-PEMFC applications. This includes modification of the Nafion membranes using inorganic fillers and acid−base composite membranes obtained by acid doping of fluorinated and engineering plastic films. Phosphoric acid (PA) and its derivatives loaded onto polymeric substrates were recently proposed as alternative PEMs for use in HT-PEMFC applications. The details of the various approaches used to prepare membranes for HT-PEMFCs and recent progress in their use were recently reviewed.12b,14c,46,226 PA doped PBI membranes were proposed as suitable for PEMFCs operating above 100 °C.227 The incorporation of PA into the structure through doping with PBI yields an acid−base complex capable of conducting protons at elevated temperatures. This advance prompted further investigation, which led to the production of PA/PBI membranes that exhibited reasonable performance in PEMFCs at temperatures up to 190 °C. One advantage is that this was achieved without any additional humidification.228 However, there is a challenge in the synthesis of such PA/PBI membranes. They often involve polymerization followed by both doping with PA and subsequent casting of the doped PBI solution or alternatively casting PBI into the film followed by PA doping. Both encounter the difficulty of having defect-free, even, and uniform morphologies in the membranes. Moreover, despite the similarity in acid doping levels, both syntheses routes lead to membranes with different physicochemical properties. In addition, the conductivity of the PA/PBI membranes above 100 °C is lower than the corresponding values of perfluorosulfonic membranes operating at 80 °C. The RIG of basic monomers onto fluorinated polymer films followed by acid doping provides an alternative promising route for obtaining these acid−base complex membranes for use in HT-PEMFCs.

Figure 32. SEM of microstructured PEM membrane obtained by pattern grafting: (A) cross-sectional view and (B) surface view. Grafted parts are revealed in dark, whereas ungrafted areas appear bright. Reprinted with permission from ref 220. Copyright 2008 Elsevier.

membranes in PEMFC was slightly lower than the unpatterned ones due to the fact they have an overall 10% lower active area. However, they were found to have a considerably longer lifetime due to the confined diffusion of oxidative chemical degradation species by ungrafted components in the membrane.220 Despite being effective in the preparation PEMs with controlled grafts, this route appears to be both too complex and costly, neither of which, of course, serves the desired objective for cost reduction in the production of such PEMs. 5.3.5. Simultaneous Grafting and In Situ Sol−Gel Reaction Followed by Sulfonation. A combination of RIG and in situ sol−gel reactions was introduced to prepare a hybrid PEM with conductivity higher than that of Nafion membranes. The membrane was prepared by the simultaneous RIG of St onto a porous PTFE membrane with the in situ sol−gel reaction of tetraethoxysilane (TEOS) followed by sulfonation in a fuming sulfonic acid.221 The hybrid membrane obtained demonstrated a high conductivity of 143 mS cm−1 at a temperature of 80 °C with an acceptable water uptake of 51 wt %.221 The introduction of silicon oxide is expected to maintain the high proton conductivity with a reduced methanol permeability. However, the kinetics of the grafting reaction was not established, and the monomer cost is of some concern. 5.3.6. Modification of Nafion Membranes by Radiation Grafting of Functional Monomers. A composite PEM for DMFC was obtained by the modification of Nafion membranes by the RIG of vinyl phosphonic acid (VPA) and 2acrylamido-2-methyl-1-propanesulfonic acid (AMPSA).222 The modified Nafion membranes were found to have higher acidic group content with lower equivalent weights than the parent Nafion. Membranes with a poly-VPA content of 14.2 wt % made it possible to obtain a higher power density of 282 mW cm−1 in DMFC as compared to that of the parent Nafion membrane of 250 mW cm−1. Grafting of AMPSA did not lead to an improvement in the cell performance in terms of power density. The maximum power density reached 200 mW cm−1.222 A summary of studies on radiation-grafted PEMs for DMFC applications is presented in Table 5. A hybrid PEM, denoted as FN, obtained by mixing a radiation-grafted sulfonated polystyrene FEP-based membrane (s-FEP) with Nafion membrane has also been reported.62n The interfaces of the PEM and electrodes were coated with a Nafion solution to improve the lamination of the MEA and reduce interfacial resistance. The hybrid FN membrane showed a high IEC, water uptake, and ionic conductivity. This coincided with lower charge transfer resistance than for both Nafion 112 and sFEP membranes. A maximum power density of 861 mW cm−2 (at 500 mA and 60 °C cm2), which is approximately twice that of both s-FEP and pure Nafion membranes, was obtained as depicted in Figure 33.62n However, the use of this route is not

6.1. Grafting of Basic Monomers Followed by Phosphoric Acid Doping

Recently, the RIG of nitrogen-containing monomers such as 1VIm, 4-VP, N-vinyl-2-pyrrolidone (NVP), 2-vinylpyridine (2VP), allylamine, and N-vinylformamide (shown in Figure 34) onto fluorinated polymer films (e.g., ETFE and FEP) has been 12307


12308

ETFE/50

ETFE/50

porous PVDFHFP 9 wt % SnO2 loaded porous PVDF-HFP PFA/50

DVB grafted PEEK/6 porous PTFE/60

ETFE/25

PVDF/50 ETFE/50

FEP/75

FEP/25

γ-radiation/simultaneous γ-radiation/simultaneous γ-radiation/preirradiation

styrene/cholorulfonic acid

p-styryltrimethoxysilane/ sulfonation VT + DVB (5%)/cholorulfonic acid

124.9

γ-radiation/preirradiation γ-radiation/simultaneous γ-radiation/simultaneous

57

γ-radiation/preirradiation

35

2.0

1.53

0.90

40

30 50 80 23

1.55

2.71

2.0

1/0.14 1/1.23

1.49 1.88

55

122

36 27

14.6

17.3

23 34 43 31 42

14.2 24.8



EB/preirradiation


VT or tBuSt + 2.5% DVB + 17.5% BVPE/cholorulfonic acid ethylstyrenesulfonate/ cholorulfonic acid styrene/in situ sol−gel reaction with tetraethoxysilane/sulfonation styrene/cholorulfonic acid

styrene + DVB (20%)/ cholorulfonic acid styrene + DVB (10%)/ cholorulfonic acid styrene + DVB/cholorulfonic acid

styrene + DVB (6%)/ cholorulfonic acid

ETFE/100

ETFE/125

vinylphosphonic acid 2-acrylamido-2-methyl-1propanesulfonic acid styrene + DVB/cholorulfonic acid

Nafion 117/170

1.38

1.92

γ-radiation/preirradiation proton beam/ direct sulfonation EB/simultaneous (10 kGy)


ETFE/50

ETFE/50

1.2 1.2


IonClad R1010 (36 μm) IonClad R4010 (63 μm)


PTFE/30 PTFE/50

DG (%)


monomer/activating agent

film/thickness (μm)

60

performance (mA cm−2)

ex situ methanol crossover was 1.6 × 10−6 as compared to

113 123 118 at 0.4 V

80

98 at 100 °C

2.12 2.75 3.36 15 at 90 °C

110

110

53

98 60

385

477

400

307

204 204

600

>160

2000

2000

durability (h)

refs

9 cm2 active area, 60 °C, Pt loading 2 mg cm−1, 1.0 M methanol solution (1.3 cc/min), air (200 cc/min), current density: 150

5 cm2 active area, 2 M methanol flow rate of 4 mL min−1, air flow rate of 200 mL/min with a back pressure of 20 psi, cell temp 60 °C

5 cm2 active area, methanol 1.0 M, flow rates of methanol and oxygen are 50 mL min−1, and cell temp is 95 °C

2 M methanol, solution flow rate of 5.9 cm3 min−1, cell temp of 80 °C, air flow rate of 100 and 570 cm3 min−1 at ambient pressure air. 5 cm2 active area, temp 70 °C, methanol of 1 M (1 mL min−1) or 5 M (0.2 mL min−1) methanol dry air (100 mL min−1) at 1 atm

224

148

102

193

221

171

140

69b

39a,133

62e

temp 110 °C, 0.4 V, 1 M methanol (1 atm)/air (2 atm)

150 h at 90 °C, and 450 h at 110 °C, 400 mA cm−1, 0.5 M methanl (1 atm)/air (2 atm)

95

222

64a

89

107

13 cm2 active area, 97 °C, 1 M methanol (1 atm)/air (3 atm)

9 cm2 active area, 90 °C, 2 M methanol solution (1 atom)/ air (2 atom) 5 cm2 active area, 30 °C, 2 M methanol solution (1 atom)/air (2 atom), methanol flow rate 2.0 mL min−1 and oxygen/air: 270 mL min−1 5 cm2 active area, temp Tfuel cell, 130 °C, Tmethanol 146 °C, Toxygen, 120 °C, methanol flow rates 2.4 cm3 min−1, oxygen flow rate 0.6 dm3, P methanol 2 atom, Poxygen 5 atom

conditions

Chemical Reviews Review


5 cm2 active area, temp 70 °C, methanol of 1 M (1 mL min−1) or 5 M (0.2 mL min−1) (1 atm) dry air (100 mL min−1) at 2 atm

48

Review

9.7 × 10−6 for Nafion 112 membrane 110 120

Figure 33. Polarization characteristic curves of MEAs based on Nafion coated sulfonated FEP-based membrane (FN), sulfonated FEP-based membrane (s-FEP), and Nafion 112. Fuel cell temperature, 60 °C; H2/O2 gas flow, 50 mL min−1; gas pressure, 0.2 MPa; H2 humidified, O2 dry. Reprinted with permission from ref 62n. Copyright 2007 Elsevier.


EB/simultaneous

40 46

2.1 2.3

51 58

used to provide basic membrane precursors that can be doped with acids such as PA. The grafting of N-vinylformamide forms a primary basic amine polymer after hydrolysis. The mechanisms for the preparation of PEM by RIG of poly(1VIm) and poly(4-VP) onto ETFE films followed by PA doping are shown in Figure 35. A number of studies have recently reported the preparation of various radiation-grafted and PA-doped membranes. Schmidt et al.229 investigated PEMs obtained by the RIG of various monomers such as 4-VP, VIm, vinylamine (1-VAm) onto ETFE films followed by PA acid doping at 100 °C for at least 2 h. This led to composite membranes with high doping levels and reasonable thermal (160 °C in air) and mechanical stability. Specific proton conductivities in the range 20−100 mS cm−1 at 120 °C were recorded without the need for humidification. The membrane delivered maximum power densities of up to 150 mW cm−2 (without humidification) when tested in PEMFC at 120 °C. This is comparable to values reported in the literature for PA/PBI membranes.229 Similar membranes prepared by the RIG of 4-VP onto ETFE film followed by PA doping have also prepared by Nasef et al.230 These membranes achieved proton conductivities of 39 and 48 mS cm−1 (at room temperature without humidification) for DGs of 34% and 49%, respectively.230 The performance of these two membranes in a single cell fuel cell using E-Teck electrodes (5 cm 2 and Pt/C, 1.5 g cm−2) under dry feed conditions and a pressure of 3 bar exhibited maximum power densities of 127 and 146 mW cm−2, respectively. Another PEM with high ionic conductivity was obtained by the RIG of 1-VIm onto ETFE film.223 The membrane that was obtained with an acid doping level of 6.54 molecules per repeat unit proved to be less-water dependent and achieved a proton conductivity of 140 mS cm−1 (at 120 °C and ∼20% relative humidity).223 Similar membranes prepared by the RIG of 4-VP onto PVDF film that had reasonable chemical, thermal, and

porous PVDF (70% porosity)/118

monomer/activating agent film/thickness (μm)

Table 5. continued


DG (%)


ionic conductivity (mS cm−1)

performance (mA cm−2)

durability (h)

conditions

refs

Chemical Reviews

12309


Chemical Reviews

Review

Figure 34. Molecular structures of some nitrogen-containing monomers.

Figure 35. Mechanisms of preparation of PEMs by RIG of 1-VIm and 4-VP onto ETFE films followed by PA doping.

Figure 36. Mechanism for preparation of proton-conducting phosphonic acid membranes by direct or preirradiation grafting of VBC onto fluorinated films followed by phosphonation.

onto ETFE films.232 However, the acid doping of these precursors and the properties of the resulting PEMs are not reported. The overall assessment of membrane preparation using the RIG of basic monomers and the acid doping method showed it to be simpler and with fewer problems than synthetic methods

structural properties were also characterized recently.231 A maximum proton conductivity of 62 mS cm−1 was achieved at 100 °C (without humidifcation) in a membrane having a DG of 81%. Other heterocyclic monomers such as NVP and 2-VP were also used for the preparation of PEM precursors by using RIG 12310


Chemical Reviews

Review

chloromethylated and quaternary-aminated to obtain the final AEMs. The membranes exhibited a very good combination of electrochemical properties. For example, the PE-based membrane demonstrated an area resistance as low as 4.4 Ω cm−2 and an IEC of 0.86 mmol g−1. However, no specific reports on the application of such AEMs for PEAFCs have been sighted. Instead, these membranes were used for the production of caustic soda and the electrodialytic treatment of water.236 Recently, El Moussaoui and Martin34a,d developed AEMs containing ammonium sulfonamide groups. They used the RIG of St onto ETFE and PE films followed by functionalization with chlorosulfones and quaternary ammonium salts. PS grafts were also cross-linked with DVB during the grafting step. The membrane having quaternary ammonium groups obtained by treatement with N,N,2,2-tetramethyl-1,3-propanediamine showed a remarkable chemical stability. This was indicated by the absence of change in the IEC after soaking for 6 days in NaOH solution (2m) at 60 °C. The membrane showed interesting properties including its IEC and conductivity of 1.6 mmol g−1 and 44 mS cm−1, respectively. The membrane (

Radiation-Grafted Membranes for Polymer Electrolyte Fuel Cells

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