CO2 reduction at transition metal electrodes

A detailed understanding of the mechanism of electrochemical reduction of CO2 to form hydrocarbons can help design improved catalysts for this important reaction. Density functional theory calculations were used here to model the various elementary steps in this reaction on transition metal surfaces, in particular Cu(111) and Pt(111). The minimum energy paths for sequential protonation by either Tafel or Heyrovsky mechanism were calculated using the nudged elastic band method for applied potentials comparable to those used in experimental studies, ranging from -0.7 V to -1.7 V. A detailed mechanism for CO2 reduction on Cu(111) has been identified where the highest activation energy is 0.5 eV at -1.3 V vs. RHE. On Pt(111), a different mechanism is found to be optimal but it involves a higher barrier, 0.7 eV at -1.0 V vs. RHE. Hydrogen production is then a faster reaction with activation energy of only 0.3 eV on Pt(111) at the same potential, while on Cu(111) hydrogen production has an activation energy of 0.9 eV at -1.3 V. These results are consistent with experimental findings where copper electrodes are found to lead to relatively high yield of CH4 while H2 forms almost exclusively at platinum electrodes.