Professor Hannes Jónsson, School of Engineering and Natural Sciences, University of Iceland

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

Theoretical atomic scale calculations of the electrochemical reduction of CO2 and the competing hydrogen evolution reaction are presented. The calculations include evaluation of the activation energy of the various elementary steps as a function of applied voltage based on efficient methods for finding saddle points on the energy surface that represent transition states for the reactions. 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 ^[1]. 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 ^[2]. The most likely step for reduction of CO, which also turns out to be the rate limiting step for methane formation, involves a Heyrovsky mechanism 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. The optimal mechanism for C-C bond formation is found to involve a nearly simultaneous electron-proton transfer to form *OCCOH ^[3]. The calculations have mostly been carried out by explicitly including a few (4 or 5) 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 ^[4]. 

References
^[1] J. Husssain, H. Jónsson and E. Skúlason, ACS Catalysis 8, 5240 (2018).
^[2] M. Van den Bossche, C. Rose-Petruck, and H. Jónsson, J. Phys. Chem. C 125, 13802 (2021).
^[3] A. Pena-Torres, R. Brevik and H. Jónsson (preprint).
^[4] M. Lyu, Z. Li, M. Van den Bossche, H. Jónsson and C. Rose-Petruck, Chemical Physics 568, 111839 (2023).