Solvent Effects for Methanol Electrooxidation on Gold

A detailed understanding of the methanol electrooxidation reaction mechanism is important for the further development of methanol fuel cells. By modeling the reaction on Au(111) using density functional calculations, we investigate the impact of solvent models, focusing on the potential-determining step and the theoretical limiting potential. Both implicit solvent effects, in the form of VASPsol, and explicit solvation by water molecules are investigated. The use of explicit water molecules changes the energetics of the reaction intermediates, and it requires the addition of six water molecules to reach converged results. An important observation is that the configuration space of the explicit water molecules needs to be treated carefully. Upon comparison of the most simple vacuum model with a more advanced combined solvent model, it is clear that there are some pronounced differences; for instance, both implicit solvent effects and explicit solvation stabilize HCOOH and destabilize CO₂. There are, however, qualitative agreements between the models; for instance, the first deprotonation step of methanol is found to be the potential-determining step, although the more accurate model put forth aldehyde and formate formation as possible competitive steps. The results are experimentally validated by using cyclic voltammetry.