Catalysts for the Electrochemical Conversion of Carbon Dioxide

Abstract

Currently, there are no large, commercial processes that convert carbon dioxide back to useful chemicals, save for the water-gas shift that leads to the Fischer-Tropsch process, but both of these reactions require a large amount of energy input. Simpler methods exist, such as the electrochemical conversion of carbon dioxide, which can be performed in aqueous solution at room temperature1. This carbon dioxide utilization pathway is not yet fully developed for several reasons: lack of a selective and robust catalyst, large energy input required for desirable products, and low current densities (low yields)2. All of these issues are being addressed by researchers in the field. Copper metal has been found to produce the most desirable products, hydrocarbons, which can be used as fuel3. High surface area nanoporous copper catalysts were prepared using an in-house method. Galvanic displacement reactions using aqueous transition metal salt solutions were used to create nanoporous copper catalysts with a coating of a second transition metal. Electrolysis was conducted using a custom built electrochemical cell and a Princeton Applied Research potentiostat. CO2 reduction products were analyzed in-situ using a gas chromatograph with a mass spectrometer and thermal conductivity detectors. Liquid products were analyzed using nuclear magnetic resonance. Results show unique selectivity for all variations of nanoporous bimetallic copper catalyst tested. The pure nanporous Cu preferentially producing ethylene and ethane with very minimal methane. On a typical polycrystalline copper catalyst, methane and ethylene are the majority hydrocarbon gases while ethane is rarely observed. A significant amount of formate was produced on nanoporous Cu, reaching 10% faradaic efficiency at potentials as low as -0.55 V vs RHE. Selectivity of the catalysts can be significantly altered on bimetallic Cu surfaces. Substantial increase in ethylene and ethane is observed and greater amounts of ethanol and propanol (~5% FE each) were produced. Preliminary experiments using 13CO2 saturated solution have been performed and may provide insight into CO2 reduction pathways. Using mass spectroscopy and NMR, reaction products were monitored. Product species containing the 13C isotope were observed for all CO2 reduction products and a comparison to 12CO2 experiments was made. Differences in the 13C/12C ratio may provide clues as to the carbon species responsible for the measured products.