Electrochemical N2 Conversion: Reduction and Oxidation Pathways under Mild Conditions

Electrochemical nitrogen fixation (EN2F) is emerging as a promising technology. This review summarizes recent advances in both nitrogen reduction reaction (N2RR) and nitrogen oxidation reaction (N2OR) pathways for ammonia synthesis under mild conditions. We begin by contrasting the fundamental reactions and mechanisms of N2RR and N2OR, emphasizing the impact of competing hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) on EN2F selectivity. A detailed analysis of various N2RR mechanisms, including dissociative, associative, Mars-van Krevelen, surface-hydrogenation, and dual-site pathways, is presented, highlighting their respective advantages and limitations. For N2OR, we focus on O-preferred and N-preferred reaction routes, emphasizing the role of hydroxyl species in N2 activation. Progress in catalyst development for both N2RR and N2OR is summarized, with particular attention to surface-hydrogenation or dual-site mechanisms, as well as strong/weak N-bonding bimetallic catalysts. Strategies for promoting N2OR by modulating the OER are also discussed. Furthermore, we analyze the kinetic steps of EN2F, identifying nitrogen dissolution and mass transfer as potential rate-limiting factors, and propose strategies for enhancing EN2F selectivity through electrolyte optimization, electrolyzer design, and catalyst modification. Finally, we offer a perspective on the future directions of EN2F, acknowledging the current challenges of active nitrogen pollution and suggesting avenues for technological advancement through refined experimental protocols, innovative catalyst design, and thermochemical analysis.