Impact of loading in RRDE experiments on Fe-N-C catalysts: Two- or four-electron oxygen reduction?

We have investigated the impact of electrocatalyst loading on rotating ring-disk electrode (RRDE) experiments for the oxygen reduction reaction on Fe-N-C catalysts (ORR) in acid medium. In particular, the fraction of H2O2 produced as a function of catalyst loading was studied. A dramatic increase in H2O2 release was observed as the catalyst loading was decreased. For the same non-noble metal catalyst (NNMC), the fraction of produced H2O2 varied between less than 5% and greater than 95%, depending on the catalyst loading. These observations suggest that oxygen reduction occurs stepwise, via H2O2, and if the catalyst is sparsely loaded, the produced H2O2 cannot be efficiently reduced to H2O before it escapes. These studies have important implications for fundamental studies of ORR on NNMCs.