The water splitting reaction provides a promising method to store intermittent renewable energy resources in the form of hydrogen fuel. Currently the bottleneck of water splitting is its oxidative half reaction, the oxygen evolution reaction (OER), which has a high thermodynamic potential and is kinetically sluggish due to the involvement of four consecutive electrons and protons transfer steps. Considerable efforts have been devoted to develop heterogeneous OER catalysts composed of earth-abundant metals. However, the heterogeneous nature of these catalysts makes it difficult to probe the nature of their active sites.