Nitrogen Defective Engineering of a Metal-Free Carbon Catalyst for Ammonia Electrosynthesis from Nitrate

Electrocatalytic nitrate reduction reaction (ENO3RR) to NH3 provides an appealing route to valorize pollutants needed to close the nitrogen cycle. The development of metal-free carbon catalysts with high stability and well-developed active sites for ENO3RR is highly desirable, while the role of structural defects (such as vacancies or functional groups) on NH3 electrosynthesis is not fully understood. Herein, we developed a group of carbon-based catalysts with regulated quaternary-N and N vacancies, and the effect of dual defect sites on the ENO3RR to NH3 process was systematically investigated. The as-prepared NHC-1000 catalyst with atomic-level engineered active sites exhibited a NH3 Faradaic efficiency of 91.2% associated with a NH3 yield rate of 2.6 mmol h–1 g–1 at –0.5 V (vs RHE), better than most of the reported metal-free carbon electrocatalysts. According to the structure characterization and theoretical calculations, the yielded NH3 was dependent on the nitrogen defective involved catalytic sites. The quaternary-N moiety facilitated the potential-determining step of *NO protonation to *NHO and further contributed to the formation of *NH2 intermediates by the synergistic action of N-vacancies, which enhanced the NO3– to NH3 activity effectively. This work provides a fundamental principle and deeper understanding for designing advanced carbon-based catalysts by defect engineering applied in the ENO3RR process effectively.