Potassium-Concentration-Gated C-N Coupling Enables a Volcano-Shaped Selectivity Window for Electrocatalytic Urea...

Alkali metal cations influence electrochemical interfaces, but their role in regulating urea electrosynthesis remains incompletely understood. Here, we combine constant-potential ab initio molecular dynamics (AIMD), classical molecular dynamics (cMD), flow-cell electrolysis, and in situ infrared spectroscopy to investigate how cations affect urea formation from CO₂ and nitrate on Cu. Among Li⁺, Na⁺, K⁺, and Cs⁺, K⁺ affords the lowest calculated barrier for direct *CO-*NO coupling and the highest measured urea Faradaic efficiency. Varying the K⁺ concentration reveals a volcano-shaped dependence of urea activity and selectivity, with a maximum at a partial current density of 13.62 mA cm^-2 and Faradaic efficiency of 24.50% near 1.20 M K⁺. AIMD calculations identify a minimum coupling barrier at an intermediate nominal K⁺ concentration and elucidate how K⁺ concentration regulates precursor geometry and charge redistribution. Complementary cMD simulations reveal concentration-dependent changes in interfacial reactant distributions and water structure, with infrared spectral features providing experimental evidence of changes in interfacial reaction chemistry. Together, these results establish an optimal K⁺ concentration window governed by the coupled roles of elementary-step kinetics and electrolyte organization in urea electrosynthesis.