Beginning-of-life MEA performance - efficiency loss contributions

Beginning-of-life H2-fed fuel cell performance of membrane electrode assemblies (MEA) made using state-of-the-art subcomponents (25–50 µm membranes, 47 wt% Pt on carbon, 0.15–0.4 mg cm−2 Pt cathode loading, ≤1100 equivalent weight (EW) ionomer/binder) was studied. Performance data were taken using small-scale (50 cm2) cells and a large-scale (500 cm2, 22 cell) stack, and voltages were corrected for membrane resistance in order to isolate cathode polarization losses. Based on catalyst and MEA cyclic voltammetry and in-situ testing with pure O2 feed, we observed loading independent O2 kinetic control up to 1.8 A cm−2 at 80 °C under fully humidified conditions; this indicates negligible effects of cathode layer proton transport limitations with pure O2. With air as the oxidant, we observed O2 kinetic control below 0.2 A cm−2 and the onset of O2 transport resistances at higher current densities. Results with helox (21% O2 in helium) as the cathode feed revealed that the O2 transport resistances were roughly evenly distributed between gas and solid phases. With incomplete reactant humidification, additional membrane resistance was observed and proton conductivity losses in the cathode layer became significant. Moreover, thin (25 µm) low-equivalent weight (EW) MEAs outperformed thicker (50 µm) high EW MEAs even after correcting for membrane ohmic losses. This is due to faster water transport of the thinner low EW MEAs resulting in improved retention of cathode layer proton conductivity under dry conditions. Keywords: catalyst loading; exchange current density; membrane electrode assembly (MEA); membrane thickness; oxygen reduction reaction; polymer electrolyte fuel cell (PEFC); Tafel slope