Single-atom catalysts toward electrocatalytic urea synthesis via C-N coupling reactions

Urea, with a global annual production that exceeds 200 million tons, occupies an irreplaceable position in agriculture, pharmaceuticals, and materials science. The conventional Haber-Bosch process and its derivatives are constrained by high energy consumption and considerable carbon emissions. Global urea production, for instance, utilizes approximately 1.4-2% of total energy, accompanied by 1.5-2.0 tons of CO₂ emissions per ton of product. Electrocatalytic technology utilizes a simultaneous reduction of CO₂ and nitrogen-containing compounds to achieve urea synthesis, offering advantages such as ambient temperature and pressure operation, renewable energy driving, and the potential for carbon neutrality. Life cycle assessments have indicated the potential for a 75% reduction in carbon footprint. Single-atom catalysts (SACs), distinguished by their atomically dispersed active sites, the ability to precisely adjust their coordination environments, and an extremely high metal atom utilization, have demonstrated remarkable efficacy in electrocatalytic urea synthesis. Following the initial report of Co-N-C SAC catalyzed urea synthesis in 2020, the field has witnessed a rapid expansion in related research, with a notable increase in urea faradaic efficiency (FE) from approximately 2% to 60.11% and substantial improvements in production rates, reaching 212.8 ± 10.6 mmol h^-1 g^-1. This review systematically summarizes the advancements in SACs based on carbon-based, two-dimensional materials, metal-organic frameworks, and metal oxide supports. It delves into the regulatory mechanisms of supports on the electronic structure and coordination environment of active centers, while emphasizing the C-N bond formation mechanisms under diverse nitrogen sources. It also discusses the main challenges and future development directions in this field, providing theoretical and experimental guidance for the design of efficient electrocatalytic urea synthesis catalysts.