Carbon Dioxide Electroreduction to Multi-Carbon Products Using a Large-Scale Membrane Electrode Assembly

Abstract

Rising atmospheric carbon dioxide (CO2) concentrations has led to an increased focus on electrochemical CO2 reduction using low-cost renewable electricity to produce low-carbon-intensity fuels and chemical feedstocks. The electrosynthesis of multicarbon (C2+) hydrocarbons and oxygenates, such as ethylene and ethanol, from CO2 feedstocks is appealing due to the large global market demand of these high-value products. Industrial-scale implementation of this technology requires operation at reaction rates above 100 mA/cm2 to minimize electrolyzer capital costs. Gas diffusion electrodes (GDEs) feed CO2 directly to the electrocatalyst surface, alleviating mass transport limitations faced in traditional aqueous environments and enabling operation at industrially relevant reaction rates. However, when implemented in conventional liquid flow cells with alkaline or neutral electrolytes, major challenges emerge in maintaining electrolyte stability with detrimental carbonate and bicarbonate formation, consequently having adverse effects on energy efficiency. Here we introduce therefore a membrane electrode assembly (MEA) based CO2 electrolyzer for C2+ production. We investigate the extent to which it can overcome the electrolyte stability limitations by eliminating alkaline electrolyte degradation and neutral electrolyte salt precipitation typically observed in the conventional approach. The MEA design maintains its performance while operating at elevated temperatures and low partial pressures of CO2, important considerations for industrial reactors. We further design a copper catalyst that takes advantage of the high CO2 concentrations afforded by the MEA. This catalyst design strategy achieves more than 50% Faradaic efficiency towards ethylene and more than 80% Faradaic efficiency towards C2+ products while operating at more than 300 mA/cm2. We demonstrate the stability of the system design through the continuous generation of ethylene while operating at industrially-relevant currents for more than 100 hours. The outcome is an engineered system that provides the longest stable ethylene production amongst all CO2 electrolyzers at reaction rates greater than 100 mA/cm2. To reduce costs associated with gas product separation from dilute streams, maximizing CO2 utilization is important. We investigate alternative flow field designs for the MEA reactor to achieve uniform reactant distribution, leading to improved current densities and reduced hydrogen evolution by at least 25% each. Finally, we demonstrate the process and system scalability through C2+ production in a large-scale MEA electrolyzer as part of the NRG COSIA Carbon XPRIZE.