Nickel-metal-orgsanic framework coupled with Cu sub-nanometer clusters enables reverse hydrogen spillover for efficient electrocatalytic nitrate reduction to ammonia.

Electrocatalytic nitrate reduction (NO3-RR) to ammonia offers a sustainable strategy for wastewater treatment and the balance of the nitrogen cycle. However, this process has long faced challenges from competing side reactions, low selectivity, and sluggish hydrogenation kinetics. Herein, we present a Cu@Ni-MOF catalyst, fabricated by anchoring Cu sub-nanoclusters onto a nickel-metal-organic framework (Ni-MOF) support, which demonstrates exceptional performance for NO3-RR. The catalyst achieves a peak NH4+ Faradaic efficiency (FE) of 96.8% with a production rate of 0.44 mmol h-1 cm-2 and minimal NO2- accumulation, and near-total FE (∼100%), which indicates high selectivity and minimal side products. Integrating electrochemical analysis with theoretical calculations establishes a cooperative tandem mechanism: the Cu sites promote NO3- adsorption and conversion to NO2-, while the Ni-MOF efficiently dissociates water to generate hydrogen species (*H), which migrate via reverse hydrogen spillover through Ni - O - Cu bridges to facilitate further hydrogenation of NO2- intermediates on Cu sites. This synergistic process enhances intermediate conversion and overall catalytic efficiency, providing a promising strategy for sustainable electrocatalytic ammonia synthesis.