Promotion of oxygen evolution through the modification of Co-O bond in spinel NiCo2O4

Abstract

The coordination environment of magnetic ions in transition metal oxides has a significant impact on the oxygen evolution reaction (OER) activity. Herein, we synthesized inverse spinel oxide NiCo2-xFexO4 (x = 0–1.5) on the nickel foam (NiCo2-xFexO4@NF) to explore the impact of Fe3+ ions doping in Co site on OER activity. The electrochemical results indicate that the introduction of Fe3+ ions drastically improved the OER performance for x ≤ 1.0. While the OER activity would decrease with further doping of Fe3+. The overpotential of 10 mA cm−2 decreased from 331 mV (NiCo2O4@NF) to 260 mV versus RHE (NiCoFeO4@NF) and lower Tafel slope (41.46 mV/dec) for OER in 1.0 M KOH electrolyte. Furthermore, there is a slightly decrease in overpotential after the stability test and the difference is not more than 2%. By comparing the structure of NiCoFeO4@NF and pristine NiCo2O4@NF, it was found that the doping of moderate amounts Fe3+ (x ≤ 1.0) would help to form the structure which can expose more active sites. Furthermore, XPS results indicate that transition from Co2+ to Co3+, the higher Co3+/Co2+ ratio contributes to good reducibility and oxygen mobility, and 3d state of Co3+ would generate strong hybridization with O 2p state. The EIS results imply that the incorporation of Fe can effectively improve the conductivity of materials and reduce the charge transfer resistance. Together with the magnetic properties, we found that Fe3+ ions replaced Co site in octahedral site with low spin state for x ≤ 1.0 implies beneficial effect for OER activity. While For x > 1.0, part of the Fe3+ ions locate in tetrahedron site with high spin state which show harmful effect for the OER activity.