Stable, High-Rate CO2 Electroreduction to Multi-Carbon Products in a Membrane Electrode Assembly System
Authors: Gabardo Christine M. , O'Brien Colin P. , Edwards Jonathan P. , Dinh Cao-Thang , Xu Yi , Sargent Edward H. , Sinton David 
Published: February 2020

Rising atmospheric carbon dioxide (CO2) levels have motivated the development of sustainable solutions to reduce emissions and close the carbon cycle. Several promising strategies have been proposed for the capture and utilization of CO2, including the electrochemical CO2 reduction reaction (CO2RR). Through CO2RR, CO2 can be converted to storable fuels and other valuable chemical feedstocks using intermittent renewable energy as input. Of the 16 possible CO2RR products, multi-carbon hydrocarbons and oxygenates, such as ethylene and ethanol, have received a great deal of attention as promising commercial targets due to their high energy densities and market sizes. Copper is the only transition metal capable of catalyzing the reduction of CO2 to multi-carbon products and, as such, the optimization of this electrocatalyst has been the focus of much recent work.

However, to scale CO2RR technology for industrial implementation not just the catalyst, but the entire electrolyzer must be considered. Flow cell electrolyzers, which utilize gas diffusion electrodes to continuously deliver reactant CO2 to the catalyst sites, are capable of operating at the current densities necessary for industrial-scale (>100 mA/cm2). Tuning of the copper catalyst materials and structures, as well as the reaction environment through catholyte optimization, has allowed researchers to achieve high Faradaic efficiencies towards specific multi-carbon products, with low overpotentials. In addition to high reaction rates and efficiencies, both Faradaic and energetic, industrial systems will have to operate over long timescales, on the order of tens of thousands of hours. This extent of continuous operation will require stability from all components of the electrolyzer, including the gas diffusion electrode, the cathode catalyst and the anode catalyst, the electrolytes, and the ion exchange membrane. Although high Faradaic and energetic efficiencies have been achieved using alkaline electrolyzer systems, they suffer from instabilities in several system components due to the use of liquid catholyte. Catholyte-free membrane electrode assembly (MEA) systems, similar to those used in proton exchange membrane (PEM) water electrolyzers, have the potential to overcome these instability issues to produce multi-carbon products continuously over prolonged periods of operation.

Here, we present our MEA electrolyzer system to convert CO2 to multi-carbon products. We compare its performance to an alkaline electrolyzer system with catholyte, to demonstrate advantages in energy efficiency, operating costs, and stability. Through the tuning of the system’s operating parameters and copper cathode catalyst, we achieve over 50% Faradaic efficiency for ethylene at current densities greater than 200 mA/cm2. Moreover, we demonstrate the stability of the catholyte-free system by operating the two-electrode configuration under potentiostatic mode at these industrially-relevant current densities continuously for over 100 hours without a decrease in ethylene Faradaic efficiency or current density.