Electrochemically reconstructed defect-rich Cu/Co nanoreactor for ultrafast ammonia electrosynthesis from low-concentration nitrate.

Electrocatalytic nitrate reduction enables ammonia synthesis and wastewater treatment, but at low nitrate levels, slow enrichment and mismatched deoxygenation‑hydrogenation kinetics limit efficiency. Here we report an in-situ electrochemically reconstructed Cu/Co yolk-shell nanoreactor (Cu/CoYSNC-0.5 h) that balances a two-step process: the reduction of NO3- to NO2- and the hydrogenation of the resulting NO2-, enabling rapid NH3 production from dilute nitrate. Electrochemistry and in-situ characterization emphasize the critical role of Cu in modulating the phase transition of Co and inhibiting HER. DFT calculations reveal that the introduction of Cu induces additional active electronic states in the 3d orbitals of Co active sites, strengthening d-π* coupling and lowering the energy barrier for the transformation of *NO to *NOH, thereby accelerating NO2- hydrogenation. Finally, the coupling device driven by renewable energy provides an environmentally sustainable ammonia synthesis pathway, with carbon dioxide emissions almost negligible. This study highlights the essential role of CuCo bimetallic synergy in matching the two-step reaction to enhance catalytic activity, and elucidates the connection between the intermediate phases of the reconstruction process and the critical steps of NO3- RR, offering a promising strategy for rational catalyst design.