Mechanistic insights into potential dependence of CO electrochemical reduction into C1 products based on a H coverage-dependent Cu(111)/H2O interface model

Abstract

A H coverage-dependent Cu(111)/H₂O interface model incorporated with electronic structure analysis is employed to investigate the potential dependence of CO electroreduction into C₁ products with the aim of solving the long dispute over CO₂ electrocatalytic reduction mechanisms. The results indicate that CH₄ formation mainly proceeds through CO, CHO, CH₂O, CH₂OH and CH_x (x = 2 and 3) species at various applied potentials. CH₃OH may be formed via a CH₃O intermediate at high overpotential and the present study can confirm that CH₃OH is only produced in a trace amount as detected in experiments. The high overpotential results in the formation of CH₄, explaining the experimentally required high overpotential on Cu. The calculated energetics concludes that CO electroreduction into CHO may be a potential-limiting step, being regarded as the origin of the required high overpotentials for CO₂ electroreduction in this paper. The electronic structure calculations show that more electronic transfer to the adsorbed H atoms occurs with increasing H coverage, which can be considered as the origin of the more negative electrode potentials. Interestingly, it is observed that the s orbital of the C atom in the valence shell of the adsorbed CO molecule gains more and more electrons, whereas the s orbital of the O atom gains less and less electrons, and even loses electrons with increasing H coverage, implying easier and easier proton transfer towards the C-center site. Thus, the easier occurrence of CO electroreduction may be ascribed to the more electron transfer into the s orbital of the C atom at high overpotential. We believe that the present study represents theoretical progress to systematically study potential-dependent CO₂ electroreduction mechanisms on Cu electrodes.