Dynamic Cobalt Phase Transition Enables Self-Adaptive Electrocatalytic Nitrate-to-Ammonia Conversion in Neutral Media

Electrocatalytic nitrate (NO₃ ^-) reduction to ammonia (NH₃) represents a sustainable pathway for resource recovery and wastewater treatment. However, its application in neutral media typical of real wastewater sources is constrained by weak NO₃ ^- adsorption and proton scarcity, which necessitate high overpotentials that promote the competing hydrogen evolution reaction (HER) and lower selectivity. Here, we overcome this dilemma through a dynamic phase-transition strategy using a defect-engineered Co₃O_4-x catalyst, which achieves ∼100% Faradaic efficiency with a high NH₃ yield rate of 11.6 mg h^-1 cm^-2 at -0.5 V versus RHE in neutral electrolyte. Operando spectroscopy and theoretical calculations reveal an electrochemically reversible phase transition, wherein cathodic potential reduces Co³⁺ to Co²⁺, forming a Co(OH)₂ intermediate that spontaneously reverts upon potential removal. This dynamic restructuring spatiotemporally decouples NO₃ ^- adsorption and hydrogenation: the Co³⁺-rich phase captures NO₃ ^-, while the transient Co(OH)₂ activates water to supply active hydrogen for hydrogenation steps. This self-adaptive process suppresses HER and ensures remarkable stability during 300 h of operation. The catalyst further demonstrates robust performance across diverse real wastewaters without supporting electrolytes and enables efficient NH₃ recovery. This work establishes dynamic phase engineering as a transformative design paradigm for adaptive electrocatalysts, paving the way for practical sustainable nitrogen management.