CO2 Electroreduction to Hydrocarbons on Carbon-Supported Copper Nanoparticles

Abstract

High-surface area catalysts are crucial for electrochemical devices that rely on kinetically-limited electrochemical reactions. Proton exchange membrane fuel cells (PEMFC) and electrolyzers are examples of such devices. Commercialization of these devices became possible due to the development of nanoscale Pt catalysts on high-surface area carbon supports.Here we discuss the electrocatalytic activities of carbon-supported Cu nanoparticles for CO2 electroreduction, a reaction that is also kinetically limited and have a potential for implemention in CO2 electrolyzers for the production of hydrocarbon fuels such as CH4 and C2H4. Cu nanoparticles supported on Carbon Black (VC), Ketjen Black (KB) and Single Wall Carbon Nanotubes (SWNT) are synthesized and their activities towards CO2 electroreduction to hydrocarbon fuels (CH4, C2H2, C2H4 and C2H6) are evaluated using online gas chromatography (GC). Faradaic efficiencies of three home-made catalysts towards CO2 electroreduction are compared to that of commercial 20 wt% Cu/VC and films of electrodeposited Cu.Supported Cu catalysts were synthesized using three different synthetic routes. X-Ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) were used for nanoparticle ex-situ analysis. The average Cu particle size as determined by Scherer’s equation was 12, 19, 24 and 27 nm for 40 wt% Cu/VC, 20 wt% Cu/SWNT, 50 wt% Cu/KB and commercial 20 wt% Cu/VC, respectively. Pb underpotential deposition was used for in-situ evaluation of the electrochemical surface area (ECSA) of Cu nanoparticles and Cu films [1]. Electrodeposited Cu films had a smooth surface, close to the mirror-polished surface of a RDE electrode.Activity of Cu nanoparticles towards CO2 electroreduction to hydrocarbon fuels was evaluated using a sealed RDE setup connected to GC. Thin films of 40 wt% Cu/VC, commercial 20 wt% Cu/VC, 20 wt% Cu/SWNT and 50 wt% Cu/KB were deposited on the surface of custom-made 7 mm glassy carbon disks (Pine Instruments, Inc), following a well-established protocol in the PEMFC community [2]. GC sampling from the electrochemical cell was performed at 5, 25, 45 and 65 min from the start of experiment, while holding the electrode potential at -1.2, -1.4, -1.6, -1.8, -2 and -2.2V vs. Ag/AgCl reference electrode. CH4 and C2H4 were the only gaseous hydrocarbons detected during CO2 electroreduction on Cu nanoparticles and Cu films. Figure 1 compares faradaic efficiencies vs. potential for CO, H2, CH4 and C2H4 generation on thin films of 40 wt% Cu/VC (12 nm Cu particle size) and electrodeposited Cu. In the potential region between -1.4 and -2.2V, the hydrocarbon product distribution is dominated by C2H4 for Cu nanoparticles (Fig. 1a) and CH4 for the films of electrodeposited Cu (Fig. 1b). In addition, the onset potential for C2H4 generation is shifted by 0.2 V (-1.4 vs. -1.6 V) for 40 wt% Cu/VC vs electrodeposited Cu, indicative of the higher activity of Cu nanoparticles towards C2H4 generation. Higher activity and reverse product distribution for 40 wt% Cu/VC vs electrodeposited Cu films is likely due to a large number of under-coordinated sites, such as corners, edges and defects on the surface of nanoparticles vs. a smooth Cu surface [3].This presentation will discuss the differences in electrocatalytic activities of four supported catalysts towards CO2 electroreduction and the influence of support on product distribution in more detail. Acknowledgements OAB is grateful to the Office of Naval Research for financial support of this project.