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.