Oxygen Reduction Reaction Kinetics on Silver- and Platinum Thin-Layer Electrodes in AEMFC

The kinetics of oxygen reduction reaction (ORR) on Ag and Pt thin-layer electrodes was studied in an anion exchange membrane fuel cell (AEMFC). The two-dimensional nature of these layers minimizes the effects of current distribution and mass transport. The ORR activities were evaluated at 80 °C and 100% RH via polarization curves. Compared to Pt, Ag displays a lower open circuit potential and a lower performance at high voltages. For Ag an anodic peak at 0.82 V was obtained by cyclic voltammetry. This peak is related to the formation of Ag-oxides which were also observed in scanning electron microscopy images. At 100% O 2 , the Tafel slope for Ag was 160 mV dec −1 . For Pt above 0.8 V the slope was 75 mV dec −1 . By decoupling the first proton- and electron-transfer step of an associative ORR mechanism, a theoretical model captures the Tafel-slope response of Pt when the first proton transfer is the rate-determining step (rds). If the electron transfer is the rds, the theoretical slope fits well with the Tafel behavior of Ag. In fuel cell conditions, Ag performs better than Pt below 0.5 V, but the stability of Ag is compromised above 0.8 V.