NO3– and *H Regulation on CoFe/FeN via Coordination Engineering for Efficient Nitrate Removal and Electroreduction to Ammonia

Abstract The electrochemical nitrate reduction reaction (NO3–RR) to ammonia shows high potential for balancing the global nitrogen cycle. Owing to facile *NO3 adsorption, nontoxic and inexpensive Fe-based materials are particularly suitable for nitrate reduction. However, generating active hydrogen (*H) on the Fe surface is challenging and inefficient, leading to low Faradaic efficiency (FE) and limited ammonia yield. This study reports that effective modulation of metal–metal and metal–nitrogen coordination in CoFe/FeN-2 by optimizing specific Co/Fe ratios and gas compositions in a two-step approach, which significantly promote the reduction and hydrogenation processes of NO3–RR. In situ characterization and theoretical calculations show that coordination-engineered Co–Fe and Fe–N bonds regulate NO3– and *H adsorption, improve the intrinsic activity of the CoFe/FeN-2 catalysts, and reduce the occurrence of competitive hydrogenation side reactions. Consequently, CoFe/FeN-2, with an optimal FE of 99.05% and ammonia selectivity of almost 100% at −0.68 V vs RHE, has significantly better NO3–RR activity than CoFe/FeN-1 and CoFe. This well-matched two-step method effectively modulates the coordination of iron-based electrocatalysts and provides a new strategy for achieving the design and synthesis of high-performance nitrate reduction electrocatalysts.