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

Abstract

ABSTRACT Electrocatalytic nitrate (NO 3 − ) reduction to ammonia (NH 3 ) 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 NO 3 − 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 Co 3 O 4‐x catalyst, which achieves ∼100% Faradaic efficiency with a high NH 3 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 Co 3+ to Co 2+ , forming a Co(OH) 2 intermediate that spontaneously reverts upon potential removal. This dynamic restructuring spatiotemporally decouples NO 3 − adsorption and hydrogenation: the Co 3+ ‐rich phase captures NO 3 − , 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 NH 3 recovery. This work establishes dynamic phase engineering as a transformative design paradigm for adaptive electrocatalysts, paving the way for practical sustainable nitrogen management.