Salt Precipitation and Water Flooding Intrinsic to Electrocatalytic CO2 Reduction in Acidic Membrane Electrode Assemblies: Fundamentals and Remedies

Abstract

Renewable electricity powered electrocatalytic CO2 reduction (eCO2R) is an emerging carbon-negative technology that upgrades CO2 into valuable chemicals and simultaneously stores intermittent renewable energy. eCO2R in anion exchange membrane (AEM) based membrane electrode assemblies (MEAs) has witnessed high Faradaic efficiency (FE). But severe CO2 crossover in AEMs results in the low CO2 single-pass conversion (SPCCO2) and burdens the energy-intensive CO2 separation process. Utilizing cation exchange membranes (CEMs) and acidic anolytes, eCO2R in acidic MEAs is capable of addressing the CO2 crossover issue and overcome the SPCCO2 limits in their AEM counterparts. Alkaline metal cations such as K+/Cs+ are always adopted in acidic MEAs to suppress the competing hydrogen evolution reaction (HER) and boost eCO2R kinetics. However, K+/Cs+ accumulates and precipitates in the form of carbonate/bicarbonate salts in the cathode, which accelerates water flooding, deteriorates the gas-electrode-electrolyte interface, and limits the durability of acidic eCO2R MEAs to a few hours. In this mini-review, we discuss the fundamentals of salt precipitation and water flooding and propose potential remedies including inhibiting K+/Cs+ accumulation, decreasing local CO32−/HCO3−, and water management in gas diffusion electrodes (GDEs). We hope this mini-review can spur more insightful solutions to address the salt precipitation and water flooding issues and push acidic eCO2R MEAs toward industrial implementations.