Generation of H+ and OH− ions in anion-exchange membrane/ampholyte-containing solution systems: A study using electrochemical impedance spectroscopy

Abstract

Generation of H+ and OH− ions at anion-exchange membranes is a phenomenon, which substantially reduces the performance of a number of electromembrane processes, such as the wastewater treatment, fuel cell energy production and others. We show that the mechanisms of this phenomenon are different, when a Neosepta AMX anion-exchange membrane is bathed in a strong electrolyte solution (NaCl and KCl) or in an ampholyte solution (NaH2PO4, KH2PO4, KHT and KH2Cit). The H+ and OH− ions' generation in the case of strong electrolytes requires a voltage threshold (about 0.3 V of the corrected voltage not including ohmic contribution), which relates to a current close to the limiting current. The process occurs mainly through proton transfer reactions involving the membrane functional groups. In the case of ampholytes, this generation takes place without a threshold; this process is separated in space: the generation of H+ occurs at the depleted solution/membrane interface, that of OH− ions, at the membrane boundary with the enriched solution. Electrochemical impedance spectra (EIS) are used for the determination of the effective rate constants for the H+ and OH− ions’ generation. Their values are compared with the rate constants calculated from the acid dissociation constants (Ka).