Potassium‐Concentration‐Gated C–N Coupling Enables a Volcano‐Shaped Selectivity Window for Electrocatalytic Urea Synthesis

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 CO 2 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.