Size Effect of Surface Defects Dictates Reactivity for Nitrogen Electrofixation

Electrocatalytic nitrogen reduction reaction (eNRR) offers a sustainable pathway for ammonia (NH₃) 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 N₂ adsorption and activation while maintaining spatial flexibility to facilitate efficient hydrogenation. Consequently, VO_x-MD exhibits outstanding eNRR performance, achieving an NH₃ 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.