Hydrogel-Empowered Interfacial Reactive Hydrogen on Copper Enables Selective Nitrate-To-Ammonia Electrocatalysis

Precise control of interfacial reactive hydrogen is crucial for improving the selectivity of proton-coupled electron transfer reactions in electrocatalysis. In the nitrate electroreduction to ammonia reaction, a fundamental trade-off is identified for copper (Cu)-based catalysts, where slow water dissociation at low potentials causes reactive hydrogen scarcity, whereas at high potentials, elevated reactive hydrogen flux is diverted into hydrogen evolution. To overcome this mismatch, we coated a poly(sodium acrylate) hydrogel on a Cu foam-supported Cu nanoarray, empowering the Cu catalyst surface with a hydrophilic, hydrogen-bonded network. The resulting Cu@PANa/CF catalyst achieved a Faradaic efficiency for NH3 exceeding 90% over a wide potential window and reached a peak of 98% at -0.3 V vs RHE, while delivering a 2-fold higher NH3 yield rate at low overpotentials compared to bare Cu/CF. In situ characterizations and mechanistic analyses corroborated that the PANa hydrogel enriched water on the interface and facilitated the generation and stabilization of reactive hydrogen, thereby sustaining an effective hydrogen supply. In parallel, hydrogen-bonding interactions between the PANa hydrogel network and the nitrogenous intermediates stabilized key reaction species (*NO2H, *NH2O, and *NH2OH) and lowered the free energy difference of the rate-determining step (*NO2 to *NO2H). Together, these effects suppressed competing reactions and directed reactive hydrogen toward sequential hydrogenation, enabling efficient NO3--to-NH3 conversion. This work demonstrates that designing microenvironments of Cu catalyst surface through a hydrogen-bonded network can effectively complement electronic tuning and provides a broadly applicable framework for selectivity control in electrocatalysis.