In Situ Electrochemical Reconstruction of Cation‐Vacancy‐Enriched Ni@Ni ₂ P Particles in Hollow N‐Doped Carbon Nanofibers for Efficient Nitrate Reduction

Electrochemical nitrate (NO₃ ^-) reduction to ammonia (NH₃) under ambient conditions is promising to promote the artificial nitrogen cycling. Despite the development of transition metal-based catalysts, their incident in situ electrochemical reconstruction always leads to the ambiguity of veritable active sites and reaction mechanisms. In this work, we report an approach to encapsulate Ni@Ni₂P particles with cationic Ni vacancies in hollow N-doped carbon nanofibers (designated Ni@Ni_2- _xP@N-CNFs) for electrocatalytic NO₃ ^- reduction to NH₃ and have investigated their surface reconstruction and reaction mechanisms using various in situ electrochemical characterizations and theoretical calculations. Specially, the regulation of cationic Ni vacancy concentration in the three defective Ni@Ni_2- _xP@N-CNFs catalysts leads to the 3.92-fold NH₃ yield rate difference at -0.2 V versus RHE. During the electrocatalytic reaction process, new amorphous Ni(OH)₂ and NiOOH species form on the surface of Ni@Ni_2- _xP@N-CNFs and the stable amorphous Ni(OH)₂ species benefits the generation of more active hydrogen (*H) for hydrogenation with NO₃ ^-. This is further verified by the different reaction rate-determining steps on the pristine and reconstructed defective catalysts. Integration of the optimized defective catalyst as cathode into a stable aqueous Zn-NO₃-battery provides high power density and Faraday efficiency for NH₃.