Porous Metallic Membranes As High-Performance Gas Diffusion Electrodes for CO2 Reduction in a MEA-Type Electrolyzer

Abstract

Combining the electrochemical reduction of CO2 (eCO2RR) with carbon-neutral energy sources creates groundbreaking possibilities for using CO2 as a raw material for the sustainable production of fuels and chemicals. Recent technoeconomic analyses have emphasized the importance of achieving high current densities (i.e., hundreds of mA cm‒2) for the eCO2RR to be practically viable,[1] leading to a growing awareness of the need to evaluate catalysts under industrially-relevant operating conditions.[2] Cell designs that integrate selective eCO2RR catalysts into a membrane electrode assembly (MEA) are a promising approach to reduce ohmic losses and achieve high energy efficiency at elevated current densities.[2,3] In this context, we recently demonstrated that unmodified silver membranes—commonly used as porous filtration media in biotechnological applications—sustain high rates of CO2 reduction to CO (> 200 mA cm‒2) when used as gas diffusion electrodes (GDEs) in a zero-gap configuration.[4] These metallic electrodes are simple, highly stable, and cost-competitive vis-à-vis carbon-based GDEs. However, CO2 crossover across the anion-exchange membrane (AEM)—required to suppress the hydrogen evolution reaction—results in poor reactant utilization and in the early onset of mass transfer limitations. These findings highlight the importance of carrying out a carbon balance, in addition to traditional measurements of activity and selectivity, to adequately assess the performance and to identify the operational limitations of realistic devices for CO2 reduction.We will also discuss how these porous silver membranes can be employed as templates for the synthesis of freestanding copper GDEs and, particularly, how different electrode morphologies and compositions can be targeted via the synthesis procedure to tune the resulting selectivity toward multi-carbon products. In addition, we will show how the homogeneous structure and uniform pore network of these porous metallic membranes provides insights into the influence of the GDE’s mass transport properties on CO2 reduction in a zero-gap configuration.[1] M. Jouny, W. Luc, F. Jiao, Ind. Eng. Chem. Res. 2018, 57, 2165-2177[2] T. Burdyny, W.A. Smith, Energy Environ Sci. 2019, 12, 1442-1453[3] D. Higgins, C. Hahn, C. Xiang, T.F. Jaramillo, A.Z. Weber, ACS Energy Lett. 2019, 4, 317-324[4] G.O. Larrazábal, P. Strøm-Hansen, J.P. Heli, K. Zeiter, K.T. Therkildsen, I. Chorkendorff, B. Seger, ACS Appl. Mater. Interfaces 2019, 11, 41281-41288