Effect of adsorption of sulfate anions on the activities for oxygen reduction reaction on Nafion®-coated Pt/carbon black catalysts at practical temperatures

Abstract

The influence of the concentration of H2SO4, a typical decomposition product from polymer electrolyte membranes, on the cathode catalyst performance was investigated in the practical temperature range (30–70°C) for fuel cells. Changes in activities for the oxygen reduction reaction (ORR) and H2O2 yield at Nafion®-coated film electrode of Pt nanoparticles dispersed on high-surface-area carbon black (Nafion®-Pt/CB) were examined as a function of H2SO4 concentration [H2SO4] from 1μM to 50mM added in 0.1M HClO4 supporting electrolyte solution by a channel flow double electrode method. It was found that the apparent rate constant kapp (per real Pt active surface area) for the ORR decreased approximately linearly with the logarithm of [H2SO4], accompanied with an increase in H2O2 yield. The apparent activation energy εa for the ORR of ca. 41kJmol−1 was nearly independent of [H2SO4], with only a slight increase from that obtained in H2SO4-free 0.1M HClO4 (38kJmol−1). These results indicate that the decrease in the ORR activity at Pt/CB by the addition of H2SO4 can be ascribed predominantly to the pre-exponential factor in the rate constant, i.e., decrease in the number of active sites due to the specific adsorption of SO42−. The decrease in the ORR activity by H2SO4 was found to be mitigated with increasing temperature, probably because the specific adsorption of SO42− could be weakened, e.g., in a solution of [H2SO4]=50mM, the ORR activity at 0.85V vs. RHE decreased by ca. 60% at 30°C, whereas it decreased only by ca. 30% at 70°C.