Reversing Electrochemical Spongification by Asymmetric Breathing for 380-hour Hydroxylamine Electrosynthesis

Electrocatalytic nitrate-to-hydroxylamine (NH₂OH) conversion offers a sustainable route for reactive nitrogen upcycling. Metallic bismuth (Bi) is intrinsically active for this reaction, yet undergoes severe structural degradation under continuous electrolysis-a failure mode we identify as electrochemical spongification, wherein the compact Bi framework cathodically relaxes into an electrically disconnected porous network. Herein, we report an "asymmetric electrochemical breathing" strategy that effectively decouples long-term catalytic durability from structural failure. Using pulsed potentials between -1.36 and +1.24 V versus the reversible hydrogen electrode (RHE), BiOCl nanosheets serve as a dynamic template, delivering a high Faradaic efficiency of 71.7% and a yield rate of 4.29 mmol h^-1 cm^-2, among the highest reported for NH₂OH electrosynthesis. Experimental and theoretical insights reveal that BiOCl rapidly reduces in situ to dense metallic Bi to drive efficient nitrate reduction. Crucially, a brief anodic pulse reconstructs the framework before irreversible degradation occurs, enabling continuous structural rejuvenation for over 380 h. This dynamic reconstruction strategy successfully suppresses the Faradaic efficiency decay rate by two orders of magnitude, establishing a robust paradigm for stabilizing highly active yet metastable electrocatalysts.