Electrocatalytic nitrogen reduction reaction (eNRR) offers a sustainable pathway for ammonia (NH3) production. Defect engineering has emerged as a promising strategy to enhance eNRR activity
however, it can concurrently amplify the competing hydrogen evolution reaction (HER), posing challenges for achieving high selectivity. Herein, VOx with systematically tuned defect sizes was engineered to establish a structure-activity relationship between defect size and eNRR performance. In-situ spectroscopy and theoretical calculations reveal that medium-sized defects (VOx-MD, 1-2 nm) provide an optimal electronic environment for enhanced N2 adsorption and activation while maintaining sufficient spatial flexibility to facilitate efficient hydrogenation and suppress HER. Consequently, VOx-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 and selectivity, offering a scalable strategy for the rational design of advanced electrocatalysts.