The radiation grafting method is of interest for the preparation of ion-exchange membranes for electrochemical and other applications. Typically styrene is used in this method because the grafted polystyrene can be readily modi®ed to introduce a variety of functionalities. The grafting of poly(ethylene-alt-tetra¯uoroethylene), or ETFE, ®lms with styrene by the pre-irradiation method has been investigated and compared to that of poly(tetra¯uorethylene-co-hexa¯uoropropylene), or FEP, and poly(vinylidene ¯uoride), or PVDF. The in¯uence of base polymer ®lm properties such as ®lm thickness, extent of orientation, and molar mass on the grafting behavior of ETFE ®lms is reported. Film orientation was found to often have a dominant in¯uence either directly, as in the case of monoaxially oriented ®lms, or indirectly, as a result of the ®lm extrusion process. In addition, the effects of the irradiation type and atmosphere and grafting temperature on the grafting behavior of one ETFE ®lm type were examined in more detail.

Radiation grafted proton conducting membranes are developed with the prospect of low cost, while aiming at maintaining or improving the functionalities of PFSA ionomer membranes. We show key polymer design aspects (base film, grafting monomer, processing) to prepare membranes that show favorable performance and durability attributes in the fuel cell compared to commercial Nafion 212 and Nafion XL-100 membranes.

Anion exchange membrane fuel cells (AEMFCs) are considered as potential future alternative for proton exchange membrane fuel cells (PEMFCs) due to their potential to not require platinum. However, many properties of alkaline ionomers/membranes are not yet well-characterized. The goal of this study is to evaluate the suitability of current AEMs for application in a wide range of operating conditions, especially at temperatures below the freezing point of water. For this, a method was developed to reversibly convert the counter ion of the cationic group in the membrane electrode assembly (MEA) from (bi-)carbonate to hydroxide and vice versa. Subsequently, the through-plane membrane conductivity in an AEMFC was evaluated by electrochemical impedance spectroscopy at different temperatures (−20°C to 50°C) and water contents, whereby the electrical resistance contribution (contact and through-plane) to the high frequency resistance of the cell was determined in an ex-situ experiment. The results obtained in this study were compared to a standard PEM (Nafion 212) and to a sulfonic acid based membrane with a hydrocarbon backbone. The here acquired conductivity data suggest that the conductivity of the evaluated anion exchange membrane, particularly in its (bi-)carbonate form, would be too low at sub-zero temperature to meet automotive freeze start requirements.

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The influence of temperature and moisture activity on the viscoelastic behavior of fluorinated membranes for fuel cell applications was investigated. Uncrosslinked and crosslinked ethylene tetrafluoroethylene (ETFE)-based proton-conducting membranes were prepared by radiation grafting and subsequent sulfonation and their behavior was compared with ETFE base film and commercial Nafion V R NR212 membrane. Uniaxial tensile tests and stress relaxation tests at controlled temperature and relative humidity (RH) were carried out at 30 and 50 C for 10% < RH < 90%. Grafted films were stiffer and exhibited stronger strain hardening when compared with ETFE. Similarly, both uncrosslinked and crosslinked membranes were stiffer and stronger than Nafion V R . Yield stress was found to decrease and moisture sensitivity to increase on sulfonation. The viscoelastic relaxation of the grafted films was found to obey a power-law behavior with exponent equal to 20.04 6 0.01, a factor of almost 2 lower than ETFE, weakly influenced by moisture and temperature. Moreover, the grafted films presented a higher hygrothermal stability when compared with their membranes counterparts. In the case of membranes, a power-law behavior at RH < 60% was also observed. However, a markedly different behavior was evident at RH > 60%, with an almost single relaxation time exponential. An exponential decrease of relaxation time with RH from 60 s to 10 s was obtained at RH 70% and 30 C. The general behavior of grafted films observed at 30 C was also obtained at 50 C. However, an anomalous result was noticed for the membranes, with a higher modulus at 50 C when compared with 30 C. This behavior was explained by solvation of the sulfonic acid groups by water absorption creating hydrogen bonding within the clusters. A viscoelastic phase diagram was elaborated to map critical conditions (temperature and RH) for transitions in time-dependent behavior, from power-law scaling to exponential scaling.