Investigation of radiation-grafted PVDF-g-polystyrene-sulfonic-acid ion exchange membranes for use in hydrogen oxygen fuel cells

Solid State Ionics 97 (1997) 299–307

Investigation of radiation-grafted PVDF-g-polystyrene-sulfonic-acid ion exchange membranes for use in hydrogen oxygen fuel cells Sara D. Flint, Robert C.T. Slade* Department of Chemistry, University of Exeter, Exeter EX4 4 QD, UK

Abstract Proton-exchange membranes were synthesised by electron beam irradiation of poly(vinylidenefluoride) (PVDF) films (80 mm) followed by styrene graft polymerisation and sulfonation. Physical and electrochemical properties of the membranes were investigated. The membranes were tested as electrolytes in fuel cell conditions (humidified H 2 / O 2 , T 5 20–608C, ambient pressure); emfs of ca. 0.85 V were observed during operation for . 150 h. The conductivity of PVDF-g-pssa (30% graft) electrolyte membranes at 238C is 0.03 S cm 21 with an overall cell resistance of 6.5 ohm cm 2 under fuel cell conditions. Keywords: PVDF; Styrene; Radiation grafting; Fuel cell; Conductivity; Solid polymer electrolyte Materials: PVDF; PVDF-g-polystyrene sulfonic acid; Styrene; Methyl benzene; Sulfuric acid

1. Introduction Proton exchange membrane fuel cells (PEMFCs) are highly efficient, non-polluting, low mass and volume power generators. In a PEMFC the proton exchange membrane (PEM) is an electronic insulator but an excellent conductor of hydrogen ions. The PEM is used in the form of a thin film (typically 50–100 mm) and acts as an electrolyte in the fuel cell and also as a gas separator. The electrolyte is sandwiched between the anode and the cathode, which are in the form of platinised porous thin sheets. The electrolyte and electrodes are pressed together to produce a single membrane-electrodeassembly (MEA). *Corresponding author. Fax: 144 1392 263434; e-mail: [email protected] 0167-2738 / 97 / $17.00  1997 Elsevier Science B.V. All rights reserved PII S0167-2738( 97 )00037-4

The electrolyte is a vital part of the cell and needs to have (i) high specific conductivity ( . 10 mS cm 21 at 258C), (ii) good mechanical, chemical and thermal stability, and (iii) low cost ( , $200 m 22 ). The most widely used and successful membranes have been DuPont’s Nafion  117 and the Dow Developmental membrane [1]. Both these membranes consist of perfluorinated copolymers with sulfonic-acid-functionalised side chains. These materials are, however, expensive ( . $800 m 22 ) and have complicated synthetic procedures [2]. Alternative PEMs and / or synthetic procedures are being sought. One such method is the radiation grafting of base polymers to produce tailored copolymers which may then be functionalised. The irradiation of polymers creates active radical centres which can be used to initiate the polymerisation of monomers leading to the formation of graft polymers

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¨ [3]. Gupta et al. [4,5] and Buchi et al. [6] have investigated the radiation grafting (using a g radiation source) of tetrafluoroethylene-co-hexafluoropropylene (FEP) with polystyrene sulfonic acid. ¨ Buchi et al. [6] found that FEP-grafted-polystyrene sulfonic acid (FEP-g-SSA) systems have physical and electrochemical properties superior to Nafion  117, but an inferior fuel cell performance attributed to the gas permeability of the membrane. In this work we investigated the possibility of radiation grafting polystyrene into PVDF films, using an electron rather than g radiation source. PVDF was chosen as a base polymer as it is a partially fluorinated polymer, melt processable, mechanically strong, readily available, low in cost and may undergo cross-linking when irradiated [7]. PVDF based PEMs were made by pre-irradiation followed by grafting in styrene / methyl benzene solutions and then sulfonation in sulfuric acid; the resulting membranes are denoted as PVDF-g-pssa. The membranes were characterised by physical, chemical and electrochemical methods. In situ single fuel cell testing was carried out to determine the performance of MEAs. For comparison, Nafion  117 was tested under similar experimental conditions.

2. Experimental

2.1. Synthesis and physical characterisation of radiation grafted membranes PVDF films from Goodfellows Cambridge Ltd. (8065 mm, 5 3 5 cm 2 ) were used throughout this work. Prior to use, the films were washed with ethanol and dried for 24 h at 708C. Drying was repeated until a constant dry film mass was obtained. PVDF films were irradiated in an electron beam at a dose of 15 Mrads and then immediately placed in 50% (by volume) styrene (Aldrich) in methyl benzene (Fisons) solutions at room temperature. The films were then refluxed in the monomer solutions at 80, 90 and 1008C for 15 h. After the grafting reactions the films were washed in methyl benzene, to remove any non-grafted polystyrene homopolymer, then washed in ethanol and dried in a 708C oven until a constant mass was recorded.

2.1.1. Mass gain The degree of grafting was determined by the mass increase of the grafted membrane compared with the ungrafted membrane: [(Mg 2 Mi ) /Mi ] 3 100%, where Mi and Mg are mass of base polymer film and grafted film, respectively. 2.1.2. Infra-red FT–IR spectra, in the range 225–3500 cm 21 , were recorded for base polymer and grafted polymer films using a Nicolet Magna-IR 550 Spectrometer. 2.1.3. Sulfonation Sulfonation was effected by immersing the grafted PVDF films (PVDF-g-polystyrene) in concentrated sulfuric acid (98%, Merck) and refluxing under nitrogen for 3 h at 958C. After sulfonation the membranes were washed free of acid with deionised water until a constant pH, equivalent to that of fresh deionised water, of the washings was measured (pH meter, WPA Linton Cambridge CD720). 2.1.4. Ion-exchange capacities Sections of the PVDF-g-pssa membranes (2 3 2 cm 2 ) in proton form were stirred in KCl (aq.) (0.5 mol dm 23 ) at 208C for 15 h to liberate sulfonic acid protons by exchange with K 1 ions. The resulting solutions were titrated with KOH (0.05 mol dm 23 ) to pH 7. The membrane was stirred overnight in HCl (aq.) (0.5 mol dm 23 ), to convert back to proton form, washed and dried in a 708C oven until a constant mass was observed. The ion exchange capacity was evaluated from the dry mass of the membrane and the proton density in the membrane. 2.1.5. Water Uptake Membrane sections (2 3 2 cm 2 ) were stirred in deionised water at 958C for 5 h, dried with blotting paper, to remove surface water, and weighed. The membranes were then dried in a 708C oven to a constant mass. The amount of water uptake / degree of swelling, was determined using [(Ms 2 Md ) / Md ] 3 100%, where Ms and Md are the masses of the swollen and dried membrane, respectively.

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2.1.6. Thickness Thicknesses of swollen membranes were determined with an electrical transducer gauge (Mercer Parnum, B203) accurate to 62 mm. 2.2. Membrane-electrode-assembly ( MEA) preparation Two types of PVDF-g-pssa MEAs were prepared: 1. Carbon-cloth gas diffusion electrodes (E-Tek) with a platinum loading of 0.35 mg cm 22 were painted with a 2 mass% solution of polystyrenesulfonic acid, sodium salt (Aldrich) and dried in a 708C oven. This procedure was repeated until an impregnation of 0.7 mg cm 22 of sulfonic acid on the electrode was achieved. The PVDF-g-pssa membranes were then hot-pressed between two polystyrene-sulfonic acid and platinum impregnated electrodes for 5 min at 1008C and 1 tonne cm 22 . 2. PVDF-g-polystyrene-sulfonic acid membranes were pressed between plain gas diffusion electrodes in the same manner as described above, but without polystyrene sulfonic acid impregnation of electrodes. For comparison in our test fixture, a MEA with Nafion  117 (DuPont) as the electrolyte was also prepared. The MEA preparation followed the procedure reported by Dhar [8].

2.3. Electrochemical Measurements

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lyzer with SI 1287 electrochemical interface. AC impedance experiments, in the frequency range 0.01 Hz–10 MHz with an oscillating voltage of 100 mV, were controlled and analyzed using ZPlot and ZView for Windows software (v1.2, Scribner Associates, Virginia 1994). Data was presented as complex impedance plane plots with Z9 and Z0 as the X- and Y-axis respectively. Data were recorded at varying cell temperatures (T 5 20, 40 and 608C), after varying operating times and in two gas atmospheres: (1) wet nitrogen and (2) wet hydrogen passed to both sides of the cell. The membrane was kept hydrated by bubbling the gases through water (held at 758C to maximise membrane electrolyte hydration) via a glass sinter. Some condensation was observed in the fuel and oxidant feed-pipes.

2.3.2. DC Measurements DC polarising potentials (10–300 mV) were applied to the cell described above and resulting currents measured, using a Solartron SI 1287 electrochemical interface with CorrWare and CorrView for Windows software (v1.2, Scribner Associates). After application of a voltage, the cell was allowed to equilibrate until a steady state current (after 2 min) was obtained; the cell was isolated between each polarising potential. The measurements were taken under the same temperature and gas conditions as for the AC impedance studies.

The MEAs were mounted in a single fuel cell test fixture (GlobeTech) which consisted of two graphite plates, with a square design gas channel active area (5 cm 2 ), sandwiched between two copper end plates. The fuel cell assembly was heated by two 125 W heating cartridges inserted in the end plates. The output to the cartridges was controlled by a Eurotherm 808 temperature controller with a type ‘K’ thermocouple. Gas flows, typically in the range 5.0–10.060.1 cm 3 min 21 , were controlled by Hi-tec mass flow meters.

2.3.3. Fuel cell conditions Fuel and oxidant gases (H 2 and O 2 , BOC Ltd.) were passed to opposite sides of the cell. Humidification of fuel and oxidant gases was carried out by bubbling the gases through water, via a glass sinter, held at 158C and 58C above the cell operating temperature for H 2 and O 2 respectively. After establishing fuel cell conditions the cell was maintained under open circuit voltage for at least 5 h before electrochemical measurements were taken. Four types of experiment, at constant cell temperatures T 5 20, 40 and 608C, were carried out:

2.3.1. Impedance measurements Conductivities of the membranes were determined by evaluation of two probe AC impedance spectra recorded with a Solartron SI 1260 impedance ana-

1. Monitoring of open circuit voltage (cell emf) with time. 2. Measurement of AC impedances evident in fuel cell conditions.

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3. Long-term (24 h) tests under load, i.e. cell delivering DC into an external resistor of typically 0.7 ohm. 4. Determination of polarisation characteristics of cell, via i /E-curves and cell resistances, R cell (H 2 / O 2 ).

3. Results and discussion

3.1. Physical and chemical properties A schematic representation of the PVDF-g-pssa membranes is given in Fig. 1. Membranes with percentage polystyrene grafts of 18, 23 and 30% were synthesised by carrying out the grafting reaction at temperatures T 5 80, 90 and 1008C respectively. Fig. 2 shows the infra red spectra of the pregrafted and grafted membrane for a 30% graft PVDF-g-polystyrene membrane. It is clear that polystyrene has been introduced into the base polymer. The characteristic peaks in the PVDF base polymer are those near 3000 cm 21 representing C–H stretching vibrations. The grafted polymer mimics the base polymer with the addition of peaks in the region 3080–3010 cm 21 (due to aromatic C–H stretching vibrations), 2975–2840 cm 21 (due to aliphatic C–H stretching vibrations) and 1601, 1500 cm 21 (due to aromatic C=C stretching vibrations). The introduction of acid groups into the grafted membranes creates a pseudo two-phase, hydropho-

Fig. 2. Infra red spectra of (a) PVDF (80 mm) and (b) PVDF-gpolystyrene (30% graft).

bic / hydrophillic, polymer. Ion-exchange, water uptake and protonic conductivity properties are all due to the aqueous phase in the polymer. The physical properties of the PVDF-g-pssa membranes, with Nafion  117 and FEP-g-SSA [6] for comparison, are given in Table 1. The ion-exchange capacities of the PVDF-g-pssa membranes increase with increasing degree of grafting. This observed trend is expected and due to increased amounts of benzene rings available for sulfonation in the more highly grafted polymers. The thickness of the swollen membranes surprisingly decreases slightly with increased degree of grafting. At this stage of our investigations we are unclear on the reason for this, a possible explanation is the incomplete rehydration of some of the membranes; further results will be reported elsewhere.

3.2. Electrochemical measurements

Fig. 1. Structure of polystyrene sulfonic acid grafted onto a PVDF base polymer.

The AC impedance spectra at room temperature, in nitrogen and hydrogen atmospheres, of a MEA consisting of membrane 1 (Table 1) as the electrolyte (denoted MEA 1) are shown in Fig. 3. The spectra are typical of all the PVDF-g-pssa mem-

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303

Table 1 Synthetic conditions and physical properties of PVDF-g-polystyrene sulfonic acid, Nafion  117 and FEP-g PSA [5] Membrane

1

2

3

FEP-g-PSA

Nafion  117

Radiation source and dose (Mrads)

e-beam 15

e-beam 15

e-beam 15

gamma-ray 6

–

Graft reaction conditions

1008C 50% monomer solution 15 h

908C 50% monomer solution 15 h

808C 5-% monomer solution 15 h

608C 40% monomer solution 15 h

–

Degree of grafting (%)

30

23

18

19

–

Sulfonation conditions

conc. (98%) H 2 SO 4 3 h, 958C

conc. (98%) H 2 SO 4 3 h, 958C

conc. (98%) H 2 SO 4 3 h, 958C

CSAa 5 h, 958C

–

Ion exchange capacity (meq mol 21 )

1.7

1.03

0.68

1.39

0.91

60

52

37

68

37

100

107

111

78

209

Water uptake (%) Swollen film thickness (mm) a

30 parts chlorosulfonic acid and 70 parts 1,1,2,2-tetrachloroethane.

branes synthesised in this work. Results for a MEA with Nafion  117 as the electrolyte, measured in the same test fixture, are given for comparison.

ance, R b . The forms of impedance spectra are similar to those found for Nafion  117 by other workers [9,10] in static inert gas atmospheres.

3.2.1. Wet nitrogen atmospheres The impedance spectra consist of a linear impedance variation at low frequencies, representing the predominantly blocking behaviour of the electrode. The electrode is predominantly blocking to protons because of the high charge transfer resistance, R ct . The high R ct values in wet nitrogen conditions may be explained by low charge (H 1 ) density at the three-phase boundary as protons can only be supplied via the energetically unfavourable reaction of water electrolysis (Eq. (1)):

3.2.2. Wet hydrogen atmospheres The AC impedance spectra (Fig. 3) consist of a high frequency arc and a low frequency arc. The high frequency arc represents the parallel component R b Cg (where Cg is the geometric capacitance). The arc is displaced from the origin due to unavoidable cell impedances which affect the AC response. (The high frequency arc is not visible in nitrogen atmospheres because of the dominance of the blocking electrode behaviour.) At low frequencies the linear impedance variation is smaller than that in nitrogen atmospheres; this suggests that in hydrogen atmospheres the electrodes are effectively non-blocking to protons because R ct is small and R ct ,R b (hence the resistance of the electrolyte is the larger factor in determining the MEA resistance). It is therefore

1 ] 2

H 2 O → H 1 1 ]41 O 2 1 e 2 .

(1)

As frequency increases, Z0 tends to zero and the intercept on the Z9-axis gives the membrane resist-

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S.D. Flint, R.C.T. Slade / Solid State Ionics 97 (1997) 299 – 307 Table 2 Electrochemical properties and fuel cell performance of PVDF-gpssa membranes (Table 1) at ambient temperature and pressure with Nafion  117 for comparison MEA

1

2

2–2

Nafion  117

Rb (ohm cm 2 )

0.5

2.7

2.7

1.5

sb a (mS cm 21 )

20

4

4

14

R dc b (ohm cm 2 )

–

–

3.5

1.5

R cell c (ohm cm 2 )

6.5(7)

15.0(9)

9.0(2)

3.5(1)

Open circuit voltage (V)

0.85

0.86

0.84

0.94

Current density at 0.2 V d (mA cm 22 )

41

22

32

112

a

Fig. 3. AC impedance spectra of (a) PVDF-g-pssa (membrane 2, Table 1) and (b) Nafion  117 in hydrogen and nitrogen atmospheres at T5238C.

possible to conclude that the catalytic oxidation of hydrogen (Eq. (2)) is energetically favourable at room temperature and provides sufficient charge density at the three-phase boundary to result in smaller R ct values. 1 ] 2

H 2 → H 1 1 e 2.

(2)

As the frequency decreases, Z0 tends to zero once more, with the intercept on the Z9-axis representing the sum of R b and R ct . The electrolyte conductivities of membranes with different amount of grafting were evaluated from R b and are given in Table 2. The highest conductivities were those of membrane 1 (30% graft) with sb (248C)50.02 S cm 21 . At T5408C the membranes exhibited conductivities which were similar to the room temperature values. At T5608C the conductivities of the membranes were initially found to be similar to the values at T5408C but decreased

Evaluated from AC impedance spectra measured after 24 h in flowing wet nitrogen. Swollen membrane thickness was used to calculate cell constant. b Evaluated from DC measurements after 24 h in flowing wet hydrogen. c Evaluated from linear region of polarisation curves in fuel cell conditions. d After 24 h.

dramatically with time. The degradation of conductivity can be explained by membrane dehydration at elevated temperatures. The PVDF-g-pssa membranes have similar conductivities to Nafion  117 at room temperature, but at elevated temperature, T . 408C, the conductivity decreases. The propensity of the membranes to swell in water (Table 2) indicates that perhaps the conductivities of these membranes are more water dependent, and the membranes more prone to dehydration, than Nafion  117 and Nafionrelated electrolytes.

3.2.3. DC measurements DC studies were made with hydrogen passing through both compartments of the cell. Plots of applied voltage, Eapp (mV) versus resultant current density, i d (mA cm 22 ) for MEA 2–2 are shown in Fig. 4. The relationships between Eapp and i d are

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305

also that there is no electrical conduction in these systems.

3.2.4. Fuel cell conditions Stable emfs of ¯0.85 V, under fuel cell conditions with operating times of over 150 h, were measured for all the membranes investigated. The emfs did not alter significantly over the temperature range T520– 608C. The stability of the emfs for long periods of time suggest that there is little physical membrane degradation and that these membranes show satisfactory impermeability to hydrogen and oxygen at temperatures up to 608C and at ambient pressure. AC impedance spectra in fuel cell conditions of MEA 1 (30% graft) are shown in Fig. 5a. The spectra are difficult to interpret as the measurements are taken under a DC bias and two electrode reactions are involved. Comparison with AC spectra taken in nitrogen and hydrogen alone reveals that the

Fig. 4. Current density, i d , versus applied voltage, Eapp , for MEA 2–2 (electrolyte impregnated, hot-pressed electrodes) at T523 and 408C in wet hydrogen.

ohmic at low polarising potentials (Eapp ,100 mV). (The graph deviates from ohmic behaviour at high Eapp owing to polarisation of the electrodes and increased charge transfer resistance.) The cell resistance, R dc , was calculated from the current–voltage relationship in the linear region of the graph and values are given in Table 2. R dc can be represented as: R dc 5 R b 1 R ct (H 2 ).

(3)

At T5408C, R dc decreases because of decreases in R ct . At T5608C R b increases and so R dc increases. R dc is only slightly higher than R b at the same temperatures (Table 2). This illustrates that R ct is negligible and the electrodes are largely non-blocking in hydrogen atmospheres i.e. fully reversible when subjected to applied potentials of Eapp ,100 mV. In moist nitrogen atmospheres the cell was unable to sustain a DC load, with an exponential current decay resulting after the application of polarising potentials to the cell; it is evident, therefore, that in nitrogen atmospheres R ct (N 2 ) values are large and the electrodes are effectively blocking and

Fig. 5. AC impedance spectra in fuel cell conditions of (a) MEA 1 at T523 and 408C and (b) MEA 2 (hot-pressed) and MEA 2–2 (electrolyte impregnated, hot-pressed electrodes).

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306

high frequency response is similar to those shown in Fig. 2. The low frequency arc under fuel cell conditions is larger than that observed in H 2 and smaller than that in N 2 . The arc may therefore be attributed to the processes occurring at both the electrodes with the low-frequency intercept representing the sum of R b , R ct (H 2 ) and the charge transfer resistance at the oxygen electrode, R ct (O 2 ). The reactions at the oxygen electrode (Eqs. (4) and (5)) are more complex and kinetically slower than the hydrogen electrode reaction. The low frequency arc is, therefore, dominated by the oxygen electrode response with R ct (H 2 )
O

22

1 H electrolyte → ]12 H 2 O. 1

(4) (5)

Fig. 5b shows two AC impedance spectra of MEA 2 recorded at room temperature in fuel cell conditions. Spectrum 2 represents a MEA which was assembled by hot-pressing carbon-cloth electrodes onto the electrolyte membrane. Spectrum 2–2 represents a MEA which was assembled by painting a 2 mass% solution of polystyrene sulfonic acid (sodium salt) onto the electrodes before the hot-pressing step. The low frequency arc of the painted hot-pressed MEA is much smaller than the low frequency arc of the non-painted pressed MEA. This spectral difference is evidence that the low frequency arc represents electrode rather than electrolyte behaviour and shows the significance of the electrolyte / electrode interfaces on the behaviour of the cell. Typical plots of the fuel cell performance (cell potential versus current density) of MEAs 1, 2, 2–2 and Nafion  117 at T5248C are shown in Fig. 6. The overall cell resistances, evaluated from the linear region of the polarisation curve, and the cell current density at 0.2 V after 24 h are given in Table 2. Overall cell resistances for PVDF-g-pssa membranes are higher than those measured for Nafion  117. Increased degree of grafting yields increased fuel cell performance (as expected from the ion-exchange capacities). The lowest cell resistance measured was that for MEA 1 (30% graft) where R cell 56.5(1) ohm cm 2 . At T5408C the cell performance is superior to that at room temperature; this is owing to a decrease in the charge transfer resistances rather than

Fig. 6. Polarisation characteristics of H 2 / O 2 fuel cells at ambient temperature and pressure (H 2 , humidified at 808C) with Nafion  117 and PVDF-g-pssa MEAs 1, 2 and 2–2 (electrolyte impregnated, hot-pressed electrodes).

significant increase in electrolyte conductivity. At T5608C fuel cell performance deteriorates with time owing to the dehydration of the electrolyte. There is a marked difference in performance between samples with electrolyte impregnated electrodes (MEA 2–2) and those with untreated electrodes (MEA 2). The MEAs with electrolyte-impregnated electrodes show a lower resistance. It is evident, therefore, that optimised MEA fabrication is essential in producing fuel cells with useful power densities. Comparison of AC and DC measurements for MEA 2–2 reveals that R dc ¯R b indicating that the charge transfer resistances at the hydrogen electrode are negligible. Furthermore, comparison of measurements in fuel cell conditions with DC measurements in hydrogen alone reveals that R cell .R dc (Table 2) as R cell includes charge transfer and mass transport resistances of the oxygen electrode reaction. Charge transfer resistances at the oxygen electrode can therefore be approximately calculated using Eq. (6): R ct (O 2 ) 5 R cell 2 R dc .

(6)

From the results for MEA 2–2 and Nafion  117 we

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have found that at room temperature the charge transfer resistances at the oxygen electrode account for 61% and 57%, respectively, of the overall cell resistances.

4. Conclusions Electron beam radiation is a satisfactory route to proton-exchange membranes. The ion exchange capacities of the membranes were higher than for Nafion  117, and hence their equivalent weights were lower. The membranes swelled in water with the amount of water uptake increasing with increased degree of grafting. The fuel cell performances of the membranes show a 50% decrease (by comparison of iE curves) in performance compared with Nafion  117, despite having similar conductivities. An explanation for this is the poor electrode contact between the PVDF-gpssa membrane and the carbon-cloth electrode. Improved performances were observed with polystyrene sulfonic acid-sodium salt impregnated electrodes. The membranes dehydrate at T .408C to the detriment of the overall fuel cell performance. An advantage of the these PVDF-g-polystyrene membranes is the relatively mild sulfonation conditions required (compare conc. H 2 SO 4 with chlorosulfonic acid in 1,1,2,2-tetrachloroethane) [11]. Future work will involve making improvements to the MEA (i.e. electrolyte / electrode double layer) and also investigating the effects of changing the synthetic procedure (i.e. varying the radiation dose, grafting reaction conditions and thickness of the base polymer).

307

Acknowledgments We thank the Engineering and Physical Sciences Research Council for funding this work in part (under grant GR / K58722). We thank Raychem Limited for access to electron beam irradiation equipment and Dr. I.D.H. Towle (Raychem Ltd.) for discussion and support.

References [1] M. Wakizoe, O.A. Velev, S. Srinivasan, Electrochim. Acta 40 (1995) 335. [2] A.J. Appleby, F.R. Foulkes, Fuel Cell Handbook, Van Nostrand Reinhold, New York, 1989. ´ Radiation Chemistry of Polymeric Systems, [3] A. Chapiro, Wiley, New York, 1962. ¨ ´ Solid State [4] B. Gupta, F.N. Buchi, G.G. Scherer, A. Chapiro, Ionics 61 (1993) 213. ¨ ´ Polym. [5] B. Gupta, F.N. Buchi, G.G. Scherer, A. Chapiro, Adv. Tech. 5 (1994) 493. ¨ [6] F.N. Buchi, B. Gupta, O. Haas, G.G. Scherer, Electrochimica Acta 40 (1995) 345. [7] J.A. Brydson, Plastics Materials, 6th edn., ButterworthHeinemann, Oxford, 1995. [8] P. Dhar, J. Electroanal. Chem. 357 (1993) 237. [9] M. Cappadonia, J.W. Erning, U. Stimming, J. Electroanal. Chem. 376 (1994) 189. [10] K. Uosaki, K. Okazaki, H. Kita, J. Electroanal. Chem. 287 (1990) 163. ¨ O. Haas, G.G. Scherer, A. Chapiro, ´ [11] M.V. Rouilly, E.R. Kotz, J. Membr. Sci. 81 (1993) 89.