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

Authors: 
Clark, E. L., Department of Chemical and Biomolecular Engineering, University of California - Berkeley
Bell, A. T., University of California - Berkeley
Hahn, C., Stanford University
Jaramillo, T. F., Stanford University
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.

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