Anion conductivity of cation exchange membranes in aqueous supporting electrolytes

Cation exchange membranes are used for electrochemical reduction of CO₂ in alkaline media, where the anion conductivity of hydroxide (OH⁻) and bicarbonate (HCO₃⁻) are essential for the process. Any partial conductivity of anionic species containing CO₂ (i.e. bicarbonate (HCO₃⁻) or carbonate (CO₃²⁻) ions) causes parasitic transport of CO₂, a critical issue for the technology. In this work, the anion conductivity of cation exchange membranes contacted with alkaline support electrolytes is experimentally studied in a hydrogen pumping electrolysis cell. The total conductivity of the membranes is calculated from the resistance measured by electrochemical impedance spectroscopy, while the partial anion conductivity is obtained from the steady state I-V polarization for the hydrogen oxidation and evolution process. For membranes in the acid form, the two measurements give identical values of the conductivity in pure water. For membranes contacted with aqueous solutions of KHCO₃ or KOH of concentrations up to 2.5 M the anion (OH⁻ or HCO₃⁻) partial conductivity is estimated to be about 1% of the total conductivity.