Competing HCOOH and CO Pathways in CO2 Electroreduction at Copper Electrodes: Calculations of Voltage-Dependent Activation Energy

In CO2 electrochemical reduction on copper electrodes, the formation of HCOOH and CO has been observed at a similar onset potential even though the former is thermodynamically more stable. Results of theoretical calculations of the formation of these products are presented here where the transition state energy for various mechanisms is evaluated as a function of applied voltage. The calculations are based on electron density functional theory combined with searches of saddle points on the energy surface describing the system at a given voltage, and the simulated system includes a few water molecules explicitly while the rest of the dielectric is represented as a continuum. The mechanism for HCOO- formation is found to involve concerted electron and hydrogen atom transfer to CO2 while CO is formed in two concerted electron and proton transfer steps with adsorbed COOH as an intermediate. Conversion of COOH to HCOOH is not a viable process as it involves a transition state of high energy so the formation of HCOOH and CO products does not have a common intermediate but rather represents competing reaction paths. The energies of the transition states of the rate-limiting steps for these two paths are almost equal, consistent with the onset potential being similar.