Introduction

Polymer electrolyte fuel cells (PEFC) have attracted much attention for stationary and electric vehicle applications. Much progress has been made to improve their performance recently. However there are still several problems to overcome for commercialization. Among them, the cost of polymer electrolyte membranes seems to be rather critical, because a cost estimate of a practical fuel cell stack shows that the membrane cost must be reduced at least by two orders of magnitude based on current perfluorosulfonic acid membranes eg. NafionĀ®. Thus the development of new membrane materials is strongly desired. Styrene grafted tetrafluoroethylene-hexafluoropropylene copolymer (FEP) membranes have been studied for a fuel cell application by G. Scherer et al. (1). These authors showed that membranes obtained by radiation grafting served as an alternative membrane for fuel cells although there were several problems to overcome in the future. These problems include shorter life time which was concluded to result from the decomposition of grafted polystyrene side chains. We report here the performance of our fuel cells which were fabricated from our radiation grafted membranes (IMRA MEMBRANE) and gas diffusion electrodes.

EXPERIMENTAL Membrane synthesis

The following four different types of membranes were prepared by radiation graft polymerization:

sulfonated styrene grafted polytetrafuloroethylene (PTFE)

sulfonated styrene grafted tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA) sulfonated styrene grafted tetrafhioroethylene-hexafluoropropylene copolymer (EEP) sulfonated styrene grafted terafluoroethylene-ethylene copolymer (ETFE).

A matrix film (PTFE, PFA, FEP, or ETFE) of 50 pm in thickness was irradiated by gamma ray (S0Co). The irradiated film was contacted with styrene for graft polymerization. The styrene grafted film was sulfonated using a chlorosulfonic acid solution and then hydrolyzed with a potassium hydroxide solution.

Gas diffusion electrodes and cell assembly

A carbon black/PTFE composite sheet of 100 pm in thickness was used for an electrode substrate. The sheet of 36.5mm in diameter (10cm2) was dipped with a H2PtC16 solution, dried, and then reduced in H2 gas flow to form a platinum catalyst layer. Platinum loading amount was 0.5mg/cm2. A Nafion solution was impregnated onto the catalyst layer. A cell assembly was prepared by hot pressing of a electrolyte membrane, gas diffusion electrodes, and electricity collectors (230pm carbon paper with FIFE).

Fuel cells were operated with either H2/02 gases or H2/Air gases at the pressures of 1.0 to 1.5 atm. The gases were humidified with water bubblers. The cell temperatures ranged between 55 and 75 *C depending on other conditions. Current density and cell voltage characteristics were recorded to evaluate cell performance.

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