Ni³⁺-Induced Hole States Enhance the Oxygen Evolution Reaction Activity of Ni_<i>x</i>Co_3–<i>x</i>O₄ Electrocatalysts

Abstract

This work reports a systematical study on the relationship of electronic structure to oxygen evolution reaction (OER) activity of NixCo3-xO4 (x = 0-1) mixed oxides. The specific OER activity is substantially increased by 16 times from 0.02 mA cm-2 BET for pure Co3O4 to 0.32 mA cm-2 BET for x = 1 at an overpotential of 0.4 V and exhibits a strong correlation with the amount of Ni ions in the +3 oxidation state. X-ray spectroscopic study reveals that inclusion of Ni3+ ions upshifts the occupied valence band maximum (VBM) by 0.27 eV toward the Fermi level (EF), and creates a new hole (unoccupied) state located ∼1 eV above the EF. Such electronic features favor the adsorption of OH surface intermediates on NixCo3-xO4, resulting in enhanced OER. Furthermore, the emerging hole state effectively reduces the energy barrier for electron transfer from 1.19 to 0.39 eV, and thereby improves the kinetics for OER. The electronic structure features that lead to a higher OER in NixCo3-xO4 can be extended to other transition metal oxides for rational design of highly active catalysts.