The electrochemical CO2
reduction reaction has the potential to produce industrial chemicals and to mitigate the increasing CO2
concentrations in the atmosphere. Polycrystalline copper, the only transition metal catalyst to produce hydrocarbons at reasonable Faradaic efficiency, produces a reasonable 1mA/cm2
current density at ~1V overpotential.1
Polycrystalline Cu has shown similar onset potentials for C1
hydrocarbons at -0.75V vs. RHE2
, suggesting both C1
pathways to be limited by the initial hydrogenation of *CO to form *CHO. However, the (100) facet and nanostructured Cu have been shown to have an earlier onset for C2
products in alkaline conditions.4-7
These results suggest a C-C coupling pathway that proceeds prior to *CO hydrogenation. Recent calculations have found CO-CO coupling to be feasible in the presence of a solvent and cation, which induces a field in the Helmholtz plane.8
We present a DFT study on the effect of coverage, strain, and electric field on CO-CO coupling energetics on Cu (100), (111), and (211). Our calculations indicate that CO-CO coupling is facile on all three facets in the presence of a cation-induced electric field in the Helmholtz plane, with the lowest barrier on Cu(100). The CO dimerization pathway is therefore expected to play a role in C2
formation at potentials negative of the Cu potential of zero charge, corresponding to CO2
/CO reduction conditions at high pH. Both increased *CO coverage and tensile strain further improve C-C coupling energetics on Cu (111) and (211). Since CO dimerization is facile on all 3 Cu facets, subsequent surface hydrogenation steps may also play an important role in determining the overall activity towards C2
products. Adsorption of *CO, *H, and *OH on the 3 facets were investigated with a Pourbaix analysis. The (211) facet has the largest propensity to co-adsorb *CO and *H, which would favor surface hydrogenation following CO dimerization. These results suggest that the (100) and (211) facets should be the most active due to field and coverage effects.
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