Co-Electrolysis As an Approach for Efficient Electroreduction of CO2 to Fuels or Chemicals

Abstract

Using CO2 as a feedstock for the production of intermediates for fuels/chemicals such as formic acid, CO, ethylene, and ethanol is one of several approaches being explored to help reduce anthropogenic CO2 emissions, while also reducing society’s dependence on diminishing fossil fuel reserves [1]. A range of active electrocatalysts for the selective reduction of CO2 have been reported. For CO production selectivity easily exceeds 95%, and current densities approaching 500 mA/cm2 can be achieved, while overall energy efficiencies are 50-60% [2,3]. Also, ever more selective catalysts for ethylene / ethanol production are being developed. Recently we reported an electrodeposited copper-silver alloy catalyst able to produce ethylene and ethanol at a combined selectivity exceeding 80% (3:1 ethylene to ethanol) at a rate of 170 mA/cm2 [4]. Techno-economic analyses of these processes indicate that the availability and cost of renewable energy is the most important factor in determining economic feasibility [5]. After a brief summary of state-of-the-art electrocatalysts for the reduction of CO2 to CO and to ethylene / ethanol this presentation will focus first on a number of factors, such as electrolyte composition and pH, that help to optimize the electrocatalytic reduction of CO2 to CO and C2 hydrocarbons. The second and main part of the presentation will explore anode chemistries to help improve the economics of CO2 conversion at scale. An analysis of Gibbs free energies indicates that about 90% of the total energy required for CO2 electrolysis is consumed at the anode when the oxygen evolution reaction (OER) is taking place there. In other words, 90% of the energy is used to produce oxygen for which no large market exists. Organic chemicals often can be oxidized at significantly lower potentials than the OER. We identified glycerol as an easily oxidizable and abundantly available option, being a large volume by-product of industrial biodiesel and soap production. Using a 2M glycerol solution as the anolyte lowers the overall cell potential by approximately 0.8 V, regardless of the CO2 electroreduction chemistry on the cathode (CO, formic acid, or ethylene/ethanol formation). The 0.8 V lower cell potential translates to a 45-53% reduction in overall energy requirement and improves the energetic efficiency by about 15%. The presentation will conclude with a summary of the improvements in techno-economic feasibility and in life-cycle analysis upon co-electrolysis of CO2 (cathode) and glycerol (anode).