Engineering Nitrogen Vacancy in Polymeric Carbon Nitride for Nitrate Electroreduction to Ammonia.

Electrocatalytic nitrate reduction to ammonia is of great interest in terms of energy conservation and environmental protection. However, the development of abundant metal-free electrocatalysts with high activity, selectivity, and stability is still a big challenge. Herein, polymeric graphitic carbon nitride (g-C3N4) with controllable numbers of nitrogen vacancies is reported to exhibit high Faradaic efficiency (89.96%), selectivity (69.78%), and stability toward nitrate-to-ammonia conversion. 15N isotope labeling experiments prove the produced ammonia originating from nitrate reduction. The combined results of ex situ and in situ characterizations unveil the reaction pathway based on the captured critical intermediates. Density functional theory calculations reveal that nitrogen vacancies could introduce a new electron state at the Fermi level and promote the adsorption, activation, and dissociation of nitrate. An appropriate content of nitrogen vacancies is beneficial for modulating the adsorption energies of reaction intermediates (*NO, *NOH, *NH2, etc.), facilitating the enhancement in ammonia selectivity and Faradaic efficiency.