3d transition metal (Fe, Co, Ni, Cu) compounds for electrocatalytic nitrate reduction to ammonia: fundamentals, strategies, and progress.

Under ambient conditions, electrocatalytic nitrate reduction (eNO3RR) converts nitrate pollutants into ammonia, enabling sustainable ammonia production and wastewater treatment. However, practical applications are limited by the lack of efficient, selective, and stable electrocatalysts. 3d transition metal compounds are low-cost, tunable alternatives to noble metals. This review summarizes recent progress on these materials for eNO3RR, covering reaction mechanisms, synthesis methods with strategic selection criteria for structural engineering targets, and four performance-enhancing strategies including vacancy engineering, doping engineering, interface engineering, and multi-strategy synergy, elucidating their structural and electronic modulation mechanisms on nitrate adsorption, intermediate conversion, active hydrogen (*H) utilization, and side reaction suppression. Current challenges and future directions are also discussed, including dynamic mechanism analysis, industrial-scale performance, and real wastewater testing. This review aims to guide the rational design of high-performance eNO3RR catalysts and advance practical green ammonia synthesis.