Enhanced electrocatalytic nitrate reduction to ammonia on cobalt oxide nanosheets via multiscale defect modulation

[Display omitted] •Regulation of multiscale defects on Co3O4 can improve electrochemical activity and mass transfer.•Oxygen vacancy can enhance nitrate adsorption, suppress HER and weaken the rate-determining step.•The binder-free, interlaced, ultrathin nanosheet structure is crucial in sustaining NO3RR stability.•A flow electrolyzer has been employed to continuously and effectively synthesize ammonia. The traditional Haber-Bosch process synthesizes ammonia subjecting to harsh process conditions and high energy consumption, and the nitrate pollution is a common environmental issue due to the excessive use of fertilizers. A sustainable and moderate nitrate electrocatalytic reduction to ammonia (NO3RR) was employed to achieve N-cycle, turning waste into treasure. Pre-density functional theory calculations forecast that Co3O4 will display optimal NO3RR performance among all the spinel cobalt-based catalysts. In this work, Co3O4/Co with unique interlaced nanosheets was fabricated through electrodeposition and calcination. Benefitting from the open-pore, defective nanosheet structure and modulated oxygen vacancy, the Co3O4/Co catalyst can accumulate more nitrate on activated sites and display excellent NO3RR properties in the neutral electrolyte (ammonia yield rate: 4.43 mg h−1 cm−2, Faradaic efficiency: 88.7 %). Theoretical calculations reveal that oxygen vacancy could enhance nitrate adsorption energy, suppress hydrogen evolution reaction, and weaken the rate-determining step of *NO → *HNO. Furthermore, the continuous NO3RR flow electrolyzer has been customized with Co3O4/Co electrocatalyst and it highly enlarges ammonia yield over one day, which could accelerate the NO3RR to commercial application.