(66c) Electrochemical CO2 Reduction over Cuag Bimetallic Electrodes and Well-Defined Surface Alloys with Enhanced Oxygenate Selectivity

Atmospheric CO₂ is a potential source of renewable carbon for the production of fuels and chemicals. However, for this process to be sustainable the hydrogen required must be derived from water and the necessary energy must be supplied by the sun. One approach to this goal is the utilization of renewable electricity to drive the electrochemical CO₂ reduction reaction (CO₂RR). Previous research has shown that the overall efficiency of CO₂RR and the distribution of generated products depend primarily on the electrocatalyst used as the cathode. Cu is the only monometallic electrocatalyst capable of reducing CO₂ into potential fuels or multi-carbon chemicals with a total Faradaic efficiency (FE) in excess of 1%. The principal products formed over polycrystalline Cu are H₂, CH₄, C₂H₄, and C₂H₅OH. However, at potentials cathodic of -1 V vs RHE excessive H₂ evolution leads almost exclusively to the methanation of CO₂. Since multi-carbon products are more valuable precursors to fuels and chemicals than CH₄, recent studies have focused on identifying factors governing the multi-carbon product selectivity. These studies have revealed that the multi-carbon product selectivity is sensitive to the surface morphology, electrolyte composition, and the applied potential. However, these studies have not managed to substantially reduce the fraction of current lost to the parasitic evolution of H₂.

Herein we report our investigations of electrochemical CO₂ reduction over CuAg bimetallic electrodes and well-defined surface alloys, which we have found to exhibit an enhanced activity for the evolution of products derived from CO relative to H₂ compared to pure Cu. Furthermore, we have found that these electrocatalysts display an unusually high selectivity for the formation of carbonyl-containing compounds, such as acetaldehyde. While this bimetallic system was investigated on the hypothesis that CO spillover from phase-segregated domains of Ag to Cu would increase the rate of C-C coupling, further characterization by photoelectron spectroscopy and electrochemical cycling led to a very different conclusion. The revised hypothesis was confirmed by synthesizing well-defined Cu+Ag surface alloys, which reproduced the activity and selectivity trends observed over the bimetallic electrodes. The results are explained in terms of an observable shift in the electronic properties of Cu and the resulting impact that these modifications have on the adsorption energies of key reaction intermediates.