Water oxidation is a thermodynamically and kinetically demanding reaction and has been identified as a key step in future solar photocatalytic light harvesting technologies. Developing earth-abundant, chemically stable and low cost electro-catalysts to facilitate water oxidation has been an on-going challenge.
Since the water oxidation reaction releases protons, it is anticipated that its activation energy will be influenced by pH. Towards this goal, we have studied the pH dependence of two promising catalysts. Thin films of manganese oxide (MnOx) and iron oxide (Fe2O3) were prepared by a facile technique, photochemical metal-organic deposition (PMOD). They were tested for electrocatalytic activity in water oxidation over a range of neutral (pH=6) to alkaline (pH=13) solutions both with and without buffering. Both electro-catalysts are stable throughout the whole pH range of the experiments. In the unbuffered solution and at pH>10.5, the onset potential of the oxygen evolution reaction for MnOx is 0.1 V lower than that of Fe2O3. However, at pH<10.5 their onset potential is almost the same and invariable with respect to change in pH. This is in contrast with the buffered solution where the onset potential changes with a Nernstian slope almost over the entire pH range. This observation suggests a change in the mechanism of water oxidation around pH=10.5, which we plan to identify and investigate by future spectro-electrochemical studies. Our results are relevant for better understanding the influence of protons on water oxidation at the metal oxide catalyst surfaces.