Chlorination‐Induced Symmetry Breaking in Cu Single‐Atom Sites for Boosting Electrocatalytic Nitrate Reduction to Ammonia

Electrochemical nitrate reduction to ammonia is attractive for green fertilizer synthesis and pollution remediation, yet its performance is normally hindered by the mismatched generation and consumption of active hydrogen (*H). Here, we engineer the planar chloride coordination to break the local symmetry of Cu single atoms over oxygen‐deficient WO 3‐x , forming Cu 1 Cl–WO 3‐ x with Cu–O/Cl and Cu–Cl–W motifs to enable dual interfacial relay of *H and *NO 2 derived intermediates. The Cu 1 Cl–WO 3‐ x catalyst achieves an NH 3 Faradaic efficiency of 99.7% at −0.8 V versus the reversible hydrogen electrode and delivers an NH 3 yield rate up to 63.6 mg h −1 cm −2 at −0.9 V with stable operation. In situ electrochemical characterization and theoretical calculations reveal that the asymmetric Cu–Cl–W interface promotes water dissociation and *H relay, while facilitating thermodynamically favorable *NO 2 relocation and subsequent hydrogenation, thereby shifting the potential‐determining step and lowering the overall energy barrier. This dual‐relay strategy might offer a generic route toward efficient NO 3 − ‐to‐NH 3 electrosynthesis.