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 produced as a function of catalyst loading was studied. A dramatic increase in release was observed as the catalyst loading was decreased. For the same non-noble metal catalyst (NNMC), the fraction of produced varied between less than and greater than , depending on the catalyst loading. These observations suggest that oxygen reduction occurs stepwise, via , and if the catalyst is sparsely loaded, the produced cannot be efficiently reduced to before it escapes. These studies have important implications for fundamental studies of ORR on NNMCs.