Stoichiometry control and phosphorus doping as strategies for the enhancement of nickel iron spinel oxides as electrocatalysts for water oxidation

Energy storage and conversion may depend on the development of (photo)electrochemical devices relying on water oxidation. Finding electrode materials based on earth abundant elements is critical in this respect. Multinary transition metal oxides, such as the iron group spinels, can be efficient water oxidation electrocatalysts in alkaline media. Both Ni-rich and Fe-rich nickel ferrites have been synthesized through a chemical bath method by selecting specific bath temperature and composition. These electrodes, constituted by nanosheets, have been modified by the incorporation of phosphorus (as phosphate) in the structure, leading to either a substantial decrease (of around 0.1 V) in the overvoltage for oxygen evolution or a drastic decrease in the Tafel slope. Phosphate seems to have a dual role: it stabilizes the high oxidation states of surface nickel and probably provides sites for fast proton transfer. Both factors are needed for an efficient oxygen evolution reaction.