Tailoring Multivalent Nickel Sites via Mild Surface Reconstruction for Enhanced Nitrate Reduction.

Electrocatalytic nitrate reduction to ammonia (NO3RR) represents a sustainable route for green ammonia synthesis. However, its practical deployment is fundamentally compromised by a severe kinetic mismatch between the proton-supplying water dissociation step and the subsequent hydrogenation of nitrogenous intermediates, inevitably triggering intermediate accumulation and parasitic reactions. Herein, utilizing Ni(OH)2 as a model precatalyst, we demonstrate that operando electrochemical reconstruction tailored within a high-concentration nitrate environment (Ni(OH)2-N) effectively overcomes this critical bottleneck. Within this reconstructed catalyst, emergent high-valence Ni3+ sites efficiently cleave water to supply active protons (*H), while adjacent medium-valence Ni2+ sites concurrently anchor and activate nitrogenous intermediates. This spatially and electronically synergistic ensemble orchestrates kinetic matching across the tandem reaction pathways. Consequently, the optimal Ni(OH)2-N catalyst delivers an exceptional ammonia Faradaic efficiency of 95.5% and a yield rate of 0.82 mmol h-1 cm-2 at -0.2 V vs reversible hydrogen electrode, maintaining robust performance even at ampere-level current densities. Furthermore, a prototype aqueous Zn-NO3- battery assembled with this engineered cathode achieves a peak power density of 6.78 mW cm-2 alongside continuous and highly selective ammonia electrosynthesis. This work provides profound mechanistic insights into dynamic catalyst evolution, establishing a rational interfacial engineering paradigm for advanced tandem electrocatalysis.