Synergistic doping and tailoring: realizing in depth modulation on valence state of CoFe spinel oxide for high-efficiency water oxidation

Abstract Modifying the valence of metal active center to the favorable configuration offers a promising strategy to promote the electrocatalytic performance of water splitting. For example, Co2+ is reported to show better performance of water oxidation than Co3+ in the Co-based spinel oxide, but its adjustment degree is small due to the geometrical occupation and the uniform synthetic environment. In this work, on the basis of chemical tailoring for the Ni-doped CoFe Prussia blue precursor, we demonstrate an inverse spinel NiCoFe oxide with dominant Co2+ (NiCoII(5)Fe-ONC). Interestingly, the introduction of Ni can promote the valence modulation in depth, which suggests the superior oxygen evolution reaction (OER) performance of the well-balanced and Co2+-rich CoFe oxide. Electrochemical measurements show that NiCoII(5)Fe-ONC can afford a current density of 10 mA cm-2 at a low overpotential of only 220 mV in alkali, which is much better than that of RuO2. The robust catalytic performance results from the increased active sites, facile electron transfer and favorable adsorption/desorption of intermediate products. The synergistic effect of Ni-doping and chemical tailoring on valence configuration has been studied in detail. This work provides a potential method to develop high-efficiency electrocatalysts and expands researchers’ access to the 3d transition-metal catalysts in favor of valence engineering.