Sub-3 nm CoO Nanoparticles with Oxygen Vacancy-Dependent Catalytic Activity for the Oxygen Reduction Reaction

Developing transition metal-based electrocatalysts toward the oxygen reduction reaction (ORR) with high activity has attracted much attention for high-powered electrochemical energy conversion devices. Earth-abundant and low-cost cobalt oxide has attracted ever-growing interest; however, insufficient active sites and poor electrical conductivity hamper the improvement of catalytic activity for the ORR. Herein, the high-dispersed ultra-small CoO nanoparticles on three-dimensional porous carbon are synthesized by a facile wet chemistry and low-temperature calcination strategy. The characterization with multiple techniques shows that the oxygen vacancy defects are in situ formed on sub-3 nm CoO, and oxygen vacancy concentrations can be adjusted to investigate the related ORR performance. The computational and experimental results demonstrate that moderate oxygen vacancy concentration in CoO improves electrical conductivity, reduces the energy barrier in the rate-limiting step, and optimizes the adsorption of *O and *OH intermediates, thus achieving a high half-wave potential of 0.80 V and a limiting current density of 5.26 mA cm–2. This work points out an avenue to the future design of high-efficiency metal oxides for diverse renewable energy applications.