e-Refinery lunch lecture | Theoretical studies of the mechanism of C1 and C2 product formation in CO2 electrochemical reduction

Results of atomic scale calculations are presented on the electrochemical reduction of CO2 (CO2RR) and the competing hydrogen evolution reaction (HER) at various metal surfaces, in particular copper, silver and platinum. The calculations include evaluation of the activation energy of the various elementary steps as a function of applied voltage based on methods for finding saddle points on the energy surface to identify transition states of the possible elementary steps. The energy and atomic forces are calculated using density functional theory (DFT). At copper and silver electrodes, CO2RR becomes more facile than HER within a certain window of applied voltage, while HER is always preferred at platinum electrodes [1,2]. While formate is thermodynamically preferred, silver electrodes have high selectivity for CO, and copper electrodes produce both with almost the same onset potential [3]. CO can be further reduced at copper electrodes, and C-C bond formation can occur on Cu(100) without overcoming the rate limiting step for methane formation. Efforts have been made to enhance the reactivity and/or selectivity of copper electrodes by adding more reactive transition metal impurities. Calculations of CO adsorption on such surfaces reveal the adsorption of multiple CO molecules on a single impurity atom [4]. The recent CO2RR and HER calculations have mostly been carried out by explicitly including only a few water molecules around the reacting surface species while the rest of the liquid phase has been described using an implicit solvent approach. Proper inclusion of a liquid electrolyte at the surface of the electrode at a given applied voltage is a significant challenge as it makes the DFT computational effort large. A brief presentation will be given of ongoing efforts to develop a hybrid simulation approach where the liquid electrolyte is fully represented.

[1] M. Re Fiorentin, F. Risplendi, C. Salvini, J. Zeng, G. Cicero and H. Jónsson, J. Phys. Chem. Letters 15, 11538 (2024).
[2] J. Husssain, H. Jónsson and E. Skúlason, ACS Catalysis 8, 5240 (2018).
[3] M. Van den Bossche, C. Rose-Petruck, and H. Jónsson, J. Phys. Chem. C 125, 13802 (2021).