The “Best Catalyst” for Water Oxidation Depends on the Oxidation Method Employed: A Case Study of Manganese Oxides

Manganese oxides are a highly promising class of water-oxidation catalysts (WOCs), but the optimal MnO_<i>x</i> formulation or polymorph is not clear from previous reports in the literature. A complication not limited to MnO_<i>x</i>-based WOCs is that such catalysts are routinely evaluated by different methods, ranging from the use of a chemical oxidant such as Ce⁴⁺, photoactive mediators such as [Ru­(bpy)₃]²⁺, or electrochemical techniques. Here, we report a systematic study of nine crystalline MnO_<i>x</i> materials as WOCs and show that the identity of the “best” catalyst changes, depending on the oxidation method used to probe the catalytic activity