Directing the C-N Coupling Pathway Enables Efficient Urea Electrosynthesis

Electrocatalytic synthesis of urea presents a promising approach to closing the artificial nitrogen cycle. However, a major challenge arises from the sluggish C-N coupling step in urea formation compared to other competing electrochemical steps involving nitrate and CO₂ reduction, ultimately limiting the urea selectivity and energy efficiency during the coelectroreduction of nitrate and CO₂. In this study, we identify *NO₂ as a key intermediate that governs urea activity and selectivity as it can either undergo hydrogenation or couple with CO₂ to form the desired C-N bond. Our theoretical investigations reveal that a decrease in the adsorption energy of *NO₂ on Cu can suppress *NO₂ hydrogenation while enhancing its coupling with CO₂ to favor C-N bond formation. We further discover that doping of Cu with boron increases the energy requirement for *NO₂ hydrogenation and lowers the energy barrier for C-N coupling involved in the formation of *NO₂CO₂. Encouragingly, with the synthesized boron-doped Cu catalysts, we achieve a high urea Faradaic efficiency exceeding 80% at a low overpotential of -0.22 V vs the reversible hydrogen electrode, along with a substantial urea production rate of 101.2 μmol h^-1 cm^-2. In contrast, the pristine Cu only shows a low urea selectivity (19%) and production rate (<20 μmol h^-1 cm^-2) under identical conditions. Furthermore, a comprehensive life-cycle assessment underscores the significance of abundant nitrate sources for urea electrosynthesis. This work introduces an effective approach for selective and efficient urea synthesis, offering valuable insights into C-N coupling and guiding the design of energy-efficient electrocatalysts for urea production.