Effect of nanostructured carbon support on copper electrocatalytic activity toward CO2 electroreduction to hydrocarbon fuels

The effect of support on electrocatalytic activity of Cu nanoparticles (NPs) towards CO2 electroreduction to hydrocarbon fuels (CH4 and C2H4) is investigated for three types of nanostructured carbons: single wall carbon nanotubes (SWNT), reduced graphene oxide (RGO) and onion-like carbon (OLC). Cu/SWNT, Cu/RGO and Cu/OLC composite catalysts are synthesized and characterized by X-ray diffraction analysis, transmission electron microscopy and electrochemical surface area measurements. Electrocatalytic activities of the synthesized materials, as measured in an electrochemical cell connected to a gas chromatograph, are compared to that of Cu NPs supported on Vulcan carbon. All four catalysts demonstrate higher activity towards C2H4 generation vs CH4, with production of the latter mostly suppressed on Cu NPs supported on nanostructured substrates. Onset potentials for C2H4 vs CH4 generation demonstrate 200 mV positive shifts for Cu/SWNT, Cu/RGO, and Cu/OLC catalysts. The Cu/OLC catalyst is found to be superior to the other two nanostructured catalysts in terms of stability, activity and selectivity towards C2H4 generation. Its faradaic efficiency reached 60% at -1.8 V vs Ag/AgCl. The enhanced stability and activity of the Cu/OLC catalyst can be attributed to the unique catalyst design, wherein a shell of OLC surrounds the Cu NPs. Such a configuration enables the outer layer to act as a filter that protects the Cu surface from adsorption of undesirable species, enhances its electrocatalytic performance, and improves its viability in CO2 electroreduction reaction. The enhanced selectivity of the Cu/OLC catalyst towards the C2H4 production is most likely related to the enhanced electrocatalytic activity of the OLC support towards the CO2 electroreduction to CO. Higher CO surface concentration from CO2 electroreduction on the OLC support, rather than a greater number of available sites for CO dimerization, likely originated this behavior. CO molecules generate on surfaces of OLCs, dimerize on Cu NP (100) planes, and, subsequently, yield additional C2H4 molecules. Both the reduction of the CO2 to CO on the surface of OLCs and reduction of "extra" CO molecules to C2H4 on the surface of Cu NPs are expected to lead to an increase in the local pH, which is also beneficial for the C2H4 production. Published by Elsevier B.V.