An aromatic ionomer in the anode catalyst layer improves the start-up durability of polymer electrolyte fuel cells

Abstract

A sulfonated polyphenylene ionomer (SPP-QP) was used as a catalyst layer binder in polymer electrolyte fuel cells. SPP-QP functioned well in the proton-conducting thin layers to show high electrochemically active surface area (ECSA) for the Pt catalysts. When used as the cathode binder, however, specific adsorption of SPP-QP on the Pt catalyst lowered the oxygen reduction reaction (ORR) activity, resulting in lower fuel cell performance compared to that using Nafion binder. In contrast, SPP-QP supported the hydrogen oxidation reaction (HOR) in the anode, with a negligibly small overpotential, similar to that for Nafion. Furthermore, the fuel cell with SPP-QP as the anode binder (SPP-QP(a)-cell) exhibited improved durability in a gas exchange cycle test simulating start-up conditions (according to the protocol suggested by the Fuel Cell Commercialization Conference of Japan). After 1000 cycles, the remaining ECSA was 37% for the SPP-QP(a)-cell, compared to 19% for the Nafion(a)-cell. The better durability was further demonstrated in the I–V curves, where the cell voltage remaining at a current density of 0.8 A cm⁻² was 80% of the pre-test value for the SPP-QP(a)-cell compared to 47% for the Nafion(a)-cell. The specific adsorption of SPP-QP on the Pt catalyst suppressed the unfavorable ORR in the anode and accordingly the so-called reverse current reaction under start-up conditions, mitigating the degradation of the cathode catalyst layer.