Sulfur‐Bridge Engineering Enables Reverse Hydrogen Spillover to Atomic Cu for Nitrate‐to‐Ammonia Electrocatalysis

ABSTRACT Copper‐based catalysts are widely regarded as promising candidates for electrocatalytic nitrate reduction (NO 3 RR), an environmentally benign route to ammonia synthesis, yet their efficiency is often constrained by nitrite accumulation and insufficient active hydrogen (*H) supply at high current densities. Here, isolated Cu atoms were anchored into a hollow Co 3 S 4 polyhedral framework (Cu‐Co 3 S 4 ), generating a sulfur bridged asymmetric active center. Electrochemical and computational studies reveal that Co 3 S 4 functions as an efficient *H donor, transferring hydrogen species to Cu sites through a sulfur bridge mediated reverse hydrogen spillover process, thereby accelerating the hydrogenation of nitrogen intermediates. By precisely tuning the Cu site density to balance nitrogen intermediate adsorption with *H supply, the optimized Cu 1.01wt% ‐Co 3 S 4 catalyst delivers an exceptional NH 3 yield rate of 94.52 mg h −1 mg cat. −1 (18.90 mg h −1 cm −2 ) and a Faradaic efficiency (FE) of 95.18% at −0.8 V vs reversible hydrogen electrode. The catalyst also exhibits remarkable durability over 300 h at −200 mA cm −2 and performs effectively in zinc‐nitrate batteries. These findings highlight the importance of coupling intermediate activation with hydrogenation kinetics and provide guiding principles for the rational design of high efficiency NO 3 RR electrocatalysts.