Structural Reconstruction of Ternary CuCo2S4 for Boosting Electrocatalytic Nitrate Reduction to Ammonia

Electrocatalytic nitrate reduction to ammonia (NO3RR) represents a promising strategy for sustainable ammonia synthesis while simultaneously addressing nitrate pollution in wastewater. However, the irreversible structural reconstruction commonly observed in catalysts during NO3RR obscures the true active sites and complicates mechanistic understanding. In this study, flower-like CuCo2S4 nanocrystals synthesized via a hot-injection method serve as a model ternary sulfide system to systematically probe reconstruction dynamics and identify active phases under operating conditions. Experimental characterizations reveal that CuCo2S4 gradually evolves into Cu-Co(OH)2/Co9S8 through electrochemical reconstruction. This reconstructed catalyst exhibits excellent NO3RR performance, achieving an average Faradaic efficiency of 97.3%, an ammonia production rate of 363.1 μmol h-1 mg-1, and remarkable cycling stability. In situ FTIR spectra indicate that Cu incorporation promotes the conversion of *NO2 and accelerates subsequent hydrogenation steps. Density functional theory calculations further demonstrate that Cu doping facilitates *NH3 desorption and optimizes the balance between NO3- adsorption and *NH3 release. This work systematically deciphers the reconstruction pathway of sulfide catalysts during NO3RR, offering fundamental insights into electrocatalyst reconstruction mechanisms and providing a design strategy for high-performance ammonia-production electrocatalysts.