Importance of catalyst stability vis-à-vis hydrogen peroxide formation rates in PEM fuel cell electrodes

The role of catalyst stability on the adverse effects of hydrogen peroxide (H₂O₂) formation rates in a proton exchange membrane fuel cell (PEMFC) is investigated for Pt, Pt binary (PtX, X = Co, Ru, Rh, V, Ni) and ternary (PtCoX, X = Ir, Rh) catalysts supported on ketjen black (KB) carbon. The selectivity of these catalysts towards H₂O₂ formation in the oxygen reduction reaction (ORR) was measured on a rotating ring disc electrode. These measured values were used in conjunction with local oxygen and proton concentrations to estimate local H₂O₂ formation rates in a PEMFC anode and cathode. The effect of H₂O₂ formation rates on the most active and durable of these catalysts (PtCo and PtIrCo) on Nafion membrane durability was studied using a single-sided membrane electrode assembly (MEA) with a built-in reference electrode. Fluoride ion concentration in the effluent water was used as an indicator of the membrane degradation rate. PtIrCo had the least fluorine emission rate (FER) followed by PtCo/KB and Pt/KB. Though PtCo and PtIrCo show higher selectivity for H₂O₂ formation than unalloyed Pt, they did not contribute to membrane degradation. This result is explained in terms of catalyst stability as measured in potential cycling tests in liquid electrolyte as well as in a functional PEM fuel cell.