Electrochemical Reduction of CO 2 using Copper Oxide Nanoparticles supported on Glassy Carbon Electrodes
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Electrochemical Reduction of CO2 using Copper Oxide Nanoparticles supported on Glassy Carbon Electrodes Gregory L. Griffin and Joel Bugayong Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803, U.S.A.
ABSTRACT We have studied the electrochemical reduction of CO2 using Cu2O nanoparticles deposited on planar electrodes. Nanoparticles are prepared in aqueous solution by chemical reduction of CuCl2 using ascorbic acid with polyethylene glycol surfactant. The particles are then re-suspended in ethanol with added Nafion binder and brush-coated onto glassy carbon substrates. The CO2 electroreduction activity is measured in KHCO3 electrolyte under flowing CO2 using a two-compartment electrochemical cell. Product formation rates are determined using gas chromatography; major gas phase products include CO, H2, C2H4, and CH4, while liquid phase products include C2H5OH and 1-C3H5OH. The observed product distribution agrees with results obtained previously using similar Cu2O particles deposited on carbon fiber paper supports, as well as Cu2O catalysts prepared by electrodeposition or thermal oxidation. In particular, the catalysts produce a much higher ratio of C2H4 to CH4 than observed using polycrystalline Cu foil. The potential dependence of the formation rates for hydrocarbon and alcohol products is roughly two times greater than for H2 and CO formation. Both XRD and SEM measurements confirm the Cu2O nanoparticles undergo at least partial reduction to Cu metal under CO2 reduction conditions, accompanied by significant surface morphological changes. Thus the kinetic results are consistent with current models that the increased C2H4/CH4 ratio is due to the presence of a more open atomic structure on the freshly reduced Cu surfaces. INTRODUCTION The oxidation state of Cu electrodes is reported to have a significant effect on product selectivity during CO2 electroreduction. For metallic Cu, it is widely accepted that CO and HCOOH are the main products at low overpotentials, while hydrocarbons (notably CH4 and C2H4) become the dominant products at higher overpotentials (1,2). However, a few groups have reported CH3OH as a major product using pre-oxidized Cu electrodes or supported Cu2O nanoparticles (3,4). Others have reported that supported Cu2O layers prepared by thermal oxidation, while being reduced to Cu metal under CO2 reduction conditions, nevertheless exhibit a decreased overpotential for CO formation and increased selectivity for C2H4 formation, relative to Cu metal (5). We have previously reported on the electrochemical reduction of CO2 using Cu2O nanoparticles prepared using the method of Chang et al. (4) and deposited onto carbon fiber paper support using a Nafion binder (6). During CO2 electroreduction at −1.5 V(NHE), the Cu2O particles were reduced to metallic Cu, but the hydrocarbon product distribution remained
different from that reported for conventional metallic Cu electrodes. Ethylene was the major hydrocarbon produced, with a Faradaic efficiency around 25%, while the CH4 e
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