Metal-free urea synthesis on nitrogen-deficient carbon nitride via direct carboxyl-nitrite coupling.

Electrocatalytic urea synthesis offers advantages such as low energy consumption, mild conditions, and environmental friendliness, making it a promising alternative to the energy-intensive Haber-Bosch process. However, the kinetic mismatch between CO2 reduction reaction and NO3- reduction reaction, coupled with the high thermodynamic barrier of CN coupling, often leads to low faradaic efficiency for urea production. This work reports a strategy of introducing nitrogen vacancies into graphitic carbon nitride to create localized electron-rich sites for driving efficient urea electrosynthesis. Results show that the electron-enriched structure enhances CO2 adsorption and activation, promoting the pathway (CO2 → *CO2 → *COOH) and thereby balancing the kinetic mismatch between nitrogen and carbon reduction. This effectively improves the faradaic efficiency for urea synthesis. Furthermore, a thermodynamically more favorable CN coupling route is established on the catalyst surface: *COOH + *NO2 → *COOHNO2. The strong electron-donating effect of the active sites stabilizes the post-coupling intermediates and renders the CN coupling step thermodynamically more favorable. Experimentally, at a low applied potential of -0.6 V vs reversible hydrogen electrode (RHE), a urea faradaic efficiency of 26.8 ± 2.5% and a urea formation rate of 1345 ± 76 μg h-1 mgcat-1 are achieved, rivaling the performance of many metal-based catalysts. This study provides a new perspective for designing stable and efficient metal-free catalysts for urea synthesis.