Salt precipitation and water flooding intrinsic to electrocatalytic CO2 reduction in acidic membrane electrode assemblies: fundamentals and remedies

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 low CO₂ single-pass conversion (SPC_CO2) 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 overcoming the SPC_CO2 limits in their AEM counterparts. Alkali 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⁻ concentration, and water management in gas diffusion electrodes (GDEs). We hope that this mini-review will spur more insightful solutions to address the salt precipitation and water flooding issues and push acidic eCO₂R MEAs toward industrial implementations.