Understanding Cation and pH Effects for Oxygen Reduction Reaction Activity and Selectivity

The optimization of electrocatalytic water splitting reactions has the potential to enhance the storage of intermittent renewable electricity. Traditionally, the structure and composition of catalyst active sites are engineered to optimize catalyst activity. However, recent work indicates that interactions between supporting electrolyte ions and the catalyst surface can have a significant impact on water splitting rates. Here, we study the role of electrolyte pH and alkali metal cations on oxygen reduction reaction (ORR) activity and selectivity using rotating ring-disk electrode and Koutecký-Levich experiments. Over oxophilic metals (Pt, Ru, and Ir), ORR rates are not strongly influenced by electrolyte pH. On the other hand, ORR rates significantly increase with electrolyte pH over noble metals (Ag and Au). In alkaline electrolytes, ORR rates increase with alkali metal cation size over all metals except Ag. Interestingly, both electrolyte pH and cations do not significantly influence ORR selectivity except over Au. This study demonstrates how electrocatalyst activity can be engineered beyond the catalyst active site.