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 omega cm(2) (cathode side only) to 4.2 omega cm(2) (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/cm(2), 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.