In electrocatalysis, there are two mechanisms for hydrogenation reactions: Heyrovsky mechanism via the proton-electron transfer and Tafel mechanism via surface coupling with adsorbed hydrogen. In CO/CO2 electroreduction over copper catalysts, CH4 is a final product from the subsequent hydrogenation of carbon. In this work, by performing the constrained ab initio molecular dynamics simulations, we firstly identify that CH4 production on Cu(100) via,the proton-electron transfer is more kinetically favored than via surface coupling, due to the negatively charged C atoms in CHX*. Our findings highlight the significance of proton-electron transfer in CO2 electroreduction, and help understand the essential difference between electrocatalysis and heterogeneous catalysis. (C) 2016 Elsevier B.V. All rights reserved.