Hole for faster OER: The hole state induced by Fe4+ promotes the OER process. It reduces the energy barrier for electron transfer at the interface and facilitates a faster electron transfer from reaction intermediates to the catalyst.

This work reports a systematical study on the relationship of electronic structure to oxygen evolution reaction (OER) activity of Ni x Co3–x O4 (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 x Co3–x O4, 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 x Co3–x O4 can be extended to other transition metal oxides for rational design of highly active catalysts.

Double perovskite oxides are one of the most promising bifunctional electrocatalysts for efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) due to their adjustable electronic structures via doping with different metal cations or engineering of defects. Herein, we report a systematic study on the tuning of the electronic structure of La 2−x Sr x NiMnO 6 with 0 ≤ x ≤ 1.0 to promote the bifunctional OER/ORR activity. The bifunctional index (ΔE) is substantially reduced with increasing of Sr contents and achieves an optimal value of 0.85 V for La 1.4 Sr 0.6 NiMnO 6 , exceeding that of widely studied LaNiO 3 . Our study on electronic structures reveals that the enhancement of the ORR and OER activities strongly correlates with the appearance of Ni 3+ oxidation states and the upshift of the O 2p-band center promoted by Sr doping. Furthermore, an increase of hole states, derived from Ni 3+ states, reduces the energy barrier for the electron transfer from 0.44 to 0.12 eV and hence improves the intrinsic OER activities. The tuning of the electronic structure that leads to higher OER and ORR activities in La 2−x Sr x NiMnO 6 can be extended to other materials for the design of active bifunctional electrocatalysts.

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