Equivalent Salt Concentration: A Quantitative Criterion for Predicting and Mitigating Salt Precipitation in MEA CO2 Electrolysis

Membrane electrode assembly (MEA)-based CO2 electrolyzers are promising for industrial applications due to high energy efficiency and scalability, yet their long-term stability is severely limited by cathode salt precipitation. This work innovatively uncovers the salt precipitation mechanism from the perspective of water dynamics. This study clarifies that water plays dual key roles in regulating salt precipitation: first, as the essential carrier for cation removal, and second, as a dynamic modulator of transmembrane water and cation fluxes. Based on this insight, the concept of equivalent salt concentration (Ceq) is proposed, defined as the ratio of transmembrane potassium ion flux to liquid water flux. By comparing Ceq with the saturation solubility of KHCO3 (2.24 mol/L at 20 °C), a quantitative model is established to accurately predict the risk of salt precipitation under different anolyte concentrations, which is validated by long-term stability experiments. Systematic investigations show that membranes with high water permeability and low electro-osmotic drag (EOD) coefficients (e.g., X37–50 RT membrane reducing Ceq by 62.8% vs A40), enhanced gas diffusion layer (GDL) hydrophobicity, and optimized CO2 flow rates effectively mitigate salt precipitation. This work not only establishes a rapid prediction method for salt precipitation risk but also provides targeted strategies for the design of salt precipitation-resistant anion exchange membranes and GDLs in MEA CO2 electrolyzers, which is of great significance for advancing the industrial application of this technology.