Title: Ni3+-Induced Hole States Enhance the Oxygen Evolution Reaction Activity of NixCo3–xO4 Electrocatalysts

Authors: Meiyan Cui,Xingyu Ding,Xiaochun Huang,Zechao Shen,Tien-Lin Lee,Freddy E. Oropeza*,Jan P. Hofmann,Emiel J. M. Hensen,Kelvin H. L. Zhang*

Abstract: 

This work reports a systematical study on the relationship of electronic structure to oxygen evolution reaction (OER) activity of Ni_xCo_3–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 (E_F), and creates a new hole (unoccupied) state located ∼1 eV above the E_F. Such electronic features favor the adsorption of OH surface intermediates on Ni_xCo_3–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 Ni_xCo_3–xO₄ can be extended to other transition metal oxides for rational design of highly active catalysts.

Full-Link: https://pubs.acs.org/doi/10.1021/acs.chemmater.9b02453