Electrocatalytic synthesis of ammonia using transition metal-based catalysts under ambient conditions: a review

Ammonia is a major chemical that plays vital roles in food supply and energy storage. The electrochemical synthesis of ammonia induces less greenhouse gas emissions and fossil fuel dependence than the traditional Haber–Bosch process. Here we review the electrocatalytic synthesis of ammonia with focus on mechanisms, transition metal catalysts, and economic aspects. Ammonia is synthesized by reduction of dinitrogen, nitrate, or nitric oxide. Catalysts mainly comprise copper-, iron-, and cobalt-based compounds, with recent research focusing on bimetallic and trimetallic catalysts, single-atom catalysts, three-dimensional nanostructures, and sulfides/phosphides. Copper-based catalysts appear as the most active due to their unique electronic configuration. Catalyst design is optimized by calculation of the Gibbs free energy and the adsorption energy. The common mechanisms involved in electrocatalytic ammonia (NH₃) synthesis are dissociative and associative pathways. Strategies for enhancing the Faraday efficiency and ammonia yield include structural optimization, facet engineering, vacancy engineering, and single-atom construction. The cost of electrocatalytic ammonia synthesis becomes competitive with the Haber–Bosch processes at an electricity price below $0.024 per KW and a Faraday efficiency higher than 80%.