Modulating surface oxygen species via facet engineering for efficient conversion of nitrate to ammonia

Electrochemical reduction of nitrate, a common pollutant in aquatic environment, to valuable ammonia (NO3−RR) using renewably-sourced electricity has attracted widespread interests, with past efforts mainly focused on designing electrocatalysts with high activity and selectivity. The detailed correlation between catalyst properties and NO3−RR kinetics, nevertheless, is still not fully understood. In this work, we modulate the surface oxygen species of Cu2O via facet engineering, and systematically study the impact of these oxygen species on the NO3−RR activity. Combining advanced spectroscopic techniques, density functional theory calculations and molecular dynamics simulations, we find that while oxygen vacancies on Cu2O (111) surface promote the adsorption of reactants and reaction intermediates, hydroxyl groups effectively inhibit the side reaction of hydrogen evolution and facilitate the hydrogenation process of NO3−RR. These two effects work in concert to render Cu2O (111) facet the highest NO3−RR activity relative to those from other facets. Our study provides critical insights into the synergistic effect of exposed facets and surface oxygen species on heterogeneous catalysis, and offers a generalizable, facet engineering-based strategy for improving the performance of a variety of electrocatalysts important for renewable energy conversion.