Calculations of the Electrochemical Reduction of CO2 and the Competing Hydrogen Evolution Reaction

Results of theoretical calculations of electrochemical CO2 reduction to formate, hydrocarbons and alcohols will be presented. The mechanism for the formation of various products is established and the rate evaluated and compared with experimental measurements. The rate of the main side reaction, the hydrogen evolution reaction, is also estimated. The calculations are based on a detailed atomistic model of the electric double layer (metal slab and water layer) and density functional theory calculations to evaluate not only the free energy of intermediates as a function of applied voltage but also the activation energy for each elementary step, both Heyrovsky and Tafel reactions [1]. Comparison is also made with calculations using an implicit solvation model [2]. A range of close packed metal surfaces are compared, including Cu, Ag, Au, Ni, Fe, Rh, Ir and Pt. The results are in remarkably good agreement with the available measurements. A two parameters descriptor is established that can help identify improved catalysts for this important reaction. The elucidation of the reaction mechanism of CO2 electroreduction to hydrocarbons and alcohols as well as the competing hydrogen evolution reaction is an important step towards the design of a selective and energy efficient catalyst for small scale, decentralized fuel production using renewable energy sources and CO2 as reactant.