Efficient Oxygen Evolution Activity of Nickel-Based Compounds and Their Structural and Electronic Effects

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Present global energy issues have prompted demand for the actualization of carbon neutral by the establishment of hydrogen society. Green hydrogen production through anion-exchange membrane water electrolysis using renewable energy-derived electricity is a promising process because inexpensive nonprecious metal-based electrocatalysts can be used in alkaline media. However, oxygen evolution reaction (OER) at the anode side in the water electrolysis is sluggish; thus, the development of highly active electrocatalysts bearing nonprecious metals for OER is considerably desirable.Recently, we have intensively researched the OER activity of iron-based oxides and developed outstanding iron-based multimetal oxides using our proposed descriptors in terms of atomic configurations.[1–3] Furthermore, we have expanded our research on OER electrocatalysts to iron-based phosphates and reported that electrodes made by iron-based phosphates overcame those of the most active iron-based multimetal oxides and IrO2.[4] To further enhance the OER efficiency, we focused on nickel-based compounds. Nickel intrinsically catalyzes OER efficiently compared to other nonprecious transition metals.[5] In this study, we investigate the OER activity trend in Ni-based compounds and clarify the main factor to determine their OER activity. Remarkably, the OER activity exhibited a clear correlation with metal−oxygen bond length in their crystals, i.e., shorter metal−oxygen bond lengths are more beneficial for the OER, which shows a good agreement to the findings in the case of iron-based oxides, as displayed in Figure 1a.[1,2] Additionally, the plots for nickel-based compounds in Figure 1a are located at higher OER activities than those for the iron-based oxides. This trend demonstrated an improvement of OER efficiency on nickel compared to iron. Furthermore, to get insights into electronic effects on the activity, we analyzed electronic states of the nickel-based oxides by density functional theory (DFT) calculations. The DFT calculations revealed that the OER activities were linearly correlated with the level of their occupied Ni 3d orbitals, as shown in Figure 1b; i.e., the upshifted Ni 3d orbital strengthened interaction with nickel active site and Oxygen intermediates and boosted their OER activities. Thus, the electronic states of nickel-based compounds were reflected in metal−oxygen bond length and intrinsically determined the OER activity. This study provided a useful design guideline to improve nickel-based OER electrocatalysts and contributed to the innovation of green hydrogen production.[6] The part of this paper is based on results obtained from a project, JPNP14021, JPNP20003, commissioned by the New Energy and Industrial Technology Development Organization (NEDO).