Tuning the Electronic Structure of NiO via Li Doping for the Fast Oxygen Evolution Reaction

Transition metal oxides are being actively pursued as low-cost electrocatalysts for the oxygen evolution reaction (OER) in many electrochemical energy devices. A fundamental understanding of the oxide electronic structures, along with the ability to rationally tune them, is a key step toward designing of highly active catalysts. Here, we report the tuning of the electronic structure of NiO via Li doping (Li x Ni 1– x O) to enhance the OER activities. We identified that Li 0.5 Ni 0.5 O (LiNiO 2 ) has the highest OER activity, comparable to or exceeding that of the benchmark perovskite Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3−δ and LaNiO 3 . More importantly, a synergistic combination of synchrotron-based photoemission spectroscopy, X-ray absorption spectroscopy, and density functional theory was used to unravel the electronic structure of Li x Ni 1– x O with unprecedented accuracy, thus providing deep insight into the origin of the enhanced catalytic activity. The results unambiguously reveal the creation of a new hole state at 1.1 eV above the Fermi level and an enhanced degree of O 2p–Ni 3d hybridization induced by Li doping optimize the adsorption energetics of OH intermediates and thereby facilitate the fast kinetics for the OER. The Li x Ni 1– x O would serve as a new platform to study the relationship of composition–electronic structure–activity for OER electrocatalysts, beyond the extensively studied Co-based perovskites.