K⁺ Transport in Perfluorosulfonic Acid Membranes and Its Influence on Membrane Resistance in CO₂ Electrolysis

In CO₂ electroreduction it is common to use cation exchange membranes in combination with high-molar electrolytes. In a model polymer electrolyte membrane (PEM) water electrolysis setup, which mimics CO₂ electrolysis in a mixed (mode_mix) and in a separate electrolyte mode (mode_sep), this study investigates how K⁺-sulfonate interactions increase membrane resistance dependent on the electrolyte concentration. K⁺-based electrolytes (KHCO₃, K₂SO₄) are used instead of ultrapure water in the PEM-model electrolyzer. At 1.0 M KHCO₃, the membrane resistance is increased by 1.7 Ω cm² (cathode side only) to 4.2 Ω cm² (mode_mix), causing a significant voltage increase that needs to be invested for K⁺ transport over a PFSA membrane. We quantify the underlying ionic interactions to 527–545 mV and observed a further effect, namely a space-charge limitation expressed by a strongly increased voltage, occurring in the case of K⁺ overload when lacking hopping centers for cation transport. Beginning at ca. 300 mA/cm², the current density gets high enough to drive K⁺ back to the cathode side and low enough to prevent large resistive contributions and K⁺ overload. Along with thermodynamic considerations and pH-induced intrinsic operational contributions, the membrane resistance was found to have a significant impact contributing to the total cell voltage V_total and proved that current research towards green and scalable CO₂ electrolysis is on a promising way towards broad application.