Size Effect of Surface Defects Dictates Reactivity for Nitrogen Electrofixation

Abstract Electrocatalytic nitrogen reduction reaction (eNRR) offers a sustainable pathway for ammonia (NH 3 ) production. Defect engineering enhances eNRR activity but can concurrently amplify the competing hydrogen evolution reaction (HER), posing challenges for achieving high selectivity. Herein, VO x with systematically tuned defect sizes is engineered to establish a structure–activity relationship between defect size and eNRR performance. In situ spectroscopy and theoretical calculations reveal that medium‐sized defects (VO x ‐MD, 1–2 nm) provide an optimal electronic environment for enhanced N 2 adsorption and activation while maintaining spatial flexibility to facilitate efficient hydrogenation. Consequently, VO x ‐MD exhibits outstanding eNRR performance, achieving an NH 3 yield rate of 81.94 ± 1.45 µg h −1 mg −1 and a Faradaic efficiency of 31.97 ± 0.75 % at −0.5 V (vs RHE). These findings highlight the critical role of defect size in governing eNRR activity, offering a scalable strategy for designing advanced catalysts for competitve electrocatalytic reactions.