Low-Temperature Electrocatalytic Ammonia Production: Study on the Nitrogen-Reducing Activity of Vanadium Sulfide Catalysts

Abstract

Several transition metal dichalcogenides (TMDs) that are active for electrochemical nitrogen reduction (e-NRR) remain underexplored, particularly regarding the correlation between their surface electronic states and e-NRR activity. For example, π-backdonation supposedly occurs in the associative e-NRR at the V2+ active sites in vanadium sulfides (VSx); but the hypothesis was not experimentally validated with the reaction-oriented surfaces remained under debate (V2+ vs V4+). To resolve these issues, we synthesized several metal sulfides, including VSx specimens with controlled oxidation states and studied the effects of electronic structure modulation on the e-NRR activity. The incorporation of Cu into VSx resulted in enhanced e-NRR activity (131.24 μg mgcat–1 h–1 or 0.62 mA cmgeo–2 in 0.5 M Na2SO4, pH7), primarily owing to the increased V2+ content (51%) and improved binding of e-NRR intermediates through π-backdonation. This π-backdonation-induced enhancement of e-NRR activity was further observed in other transition metal sulfides, including Cu, Ni, Co, and Mn sulfides. Our findings underscore the significance of enabling electric charge control of catalysts and establish a foundation for further enhancing e-NRR activity.