Regulating surface oxygen species on copper (I) oxides via plasma treatment for effective reduction of nitrate to ammonia

Electrochemical nitrate reduction (NO3-RR) to synthesize ammonia is considered to be a promising strategy to enable artificial nitrogen cycle. Great efforts have been devoted to improving the efficiency and selectivity of the electrocatalysts for NO3-RR. Herein, we demonstrate that tuning the oxygen chemical environment via Ar plasma treatment is an effective approach to improve the NO3-RR activity of Cu2O. Combining synchrotron-based X-ray absorption spectroscopy and other advanced spectroscopy techniques, we find that plasma treatment can effectively promote the formation of oxygen vacancies and hydroxyl groups on Cu2O surface. In-situ diffuse reflectance infrared Fourier transform spectroscopy and density functional theory calculation further reveal that oxygen vacancies and hydroxyl groups facilitate the adsorption of nitrate and proton transfer on the Cu2O surface, thus leading to improved ammonia selectivity. Our results clarify the critical role of surface oxygen species for NO3-RR and can guide the design of other electrocatalysts via surface engineering.