Theoretical atomic scale calculations of the electrochemical reduction of CO2 and the competing hydrogen evolution reaction are presented. The calculations are based on efficient methods for finding saddle points on the energy surface, representing transition states for the reactions, so as to evaluate the activation energy of the various elementary steps as a function of applied voltage. The energy and atomic forces are calculated using density functional theory (DFT). Copper is found to be special among the transition metals in that the activation energy for CO2 reduction becomes lower than that of hydrogen evolution reaction (HER) within a certain window of applied voltage. The fact that the onset potential of formate and CO formation is similar can be explained by the fact that the energy barrier for these two competing processes turns out to be similar. The rate limiting step for further reduction beyond CO is a Heyrovsky step to form COH rather than formation of CHO. The rate of C-C bond formation is strongly dependent on the surface structure, Cu(100) being the most active facet, and it can be affected by H-adatom coverage as well as addition of a small alloying component. The optimal mechanism for C-C bond formation is found to involve simultaneous electron-proton transfer to form *OCCOH in a concerted manner. So far, the calculations have been carried out by explicitly including a few water molecules around the reacting surface species while the rest of the electrolyte is described with an implicit solvent approach. Proper inclusion of a liquid electrolyte at the surface of the electrode is a challenge as it makes the DFT calculations too heavy. Ongoing methodology development based on a hybrid simulation approach where the liquid electrolyte is fully represented will be introduced. A mechanism is also proposed to explain recent experimental results where absorption of the CO2 in clathrates is found to increase the CO2 reduction rate strongly with respect to HER at small applied voltage, thereby improving the energy efficiency significantly.

Mechanism of CO2 electrochemical reduction to form hydrocarbons and alcohols, C1 and C2 products

ACS Spring 2022

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