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.