The electrochemical reduction of CO₂ is a promising route for converting intermittent renewable energy into storable fuels and useful chemical products. A theoretical investigation of the reaction mechanism and kinetics is beneficial for understanding the electrocatalytic activity and selectivity. In this report, a kinetic model based on Marcus theory is developed to compute the potential-dependent reaction barrier of the elementary concerted proton–electron transfer steps of electrochemical CO₂ reduction reactions, different from the previous hydrogen atom transfer model. It is found that the onset potentials and rate-determining steps for CO and CH₄ formation are determined by the first and third concerted proton–electron transfer steps C1 and C3. The influence of binding energy, electrode potential, and reorganization energy on the computed reaction barriers of the C1 and C3 reactions is discussed. In general, the calculated reaction barrier shows a quadratic relationship with the applied electrode potential. Specifically, the reaction barrier is merely determined by the reorganization energy at equilibrium potential. The present kinetic model is applied to compare the electrocatalytic activities in the electrochemical reduction of CO₂ on various copper crystal surfaces. Among the four studied copper single-crystal surfaces, Cu(211) exhibits the best electrocatalytic activity for CO formation and CH₄ formation due to its low onset potential and overpotential.