Electrocatalytic C−N coupling process is indeed a sustainable alternative for direct urea synthesis and co-upgrading of carbon dioxide and nitrate wastes. However, the main challenge lies in the unactivated C−N coupling process. Here, we proposed a strategy of intermediate assembly with alkali metal cations to activate C−N coupling at the electrode/electrolyte interface. Urea synthesis activity follows the trend of Li⁺< Na⁺ < Cs⁺ < K⁺. In the presence of K⁺, a world-record performance was achieved with a urea yield rate of 212.8±10.6 mmol h⁻¹g⁻¹ on a single-atom Co supported TiO₂ catalyst at −0.80 V versus reversible hydrogen electrode. Theoretical calculations and operando synchrotron-radiation Fourier transform infrared measurements revealed that the energy barriers of C−N coupling were significantly decreased via K⁺ mediated intermediate assembly. By applying this strategy to various catalysts, we demonstrate that intermediate assembly at the electrode/electrolyte interface is a universal approach to boost sustainable urea synthesis.