Determination of Ionic Diffusion Coefficients in Ion-Exchange Membranes : Experiments and Simulations

Ionic diffusion coefficients in the membrane are needed for the modelling of ion transport in ion-exchange membranes with the Nernst-Planck equation. We give estimates of the ionic diffusion coefficients of Na+, OH−, H+, Cl−, SO42− and HSO4− from the diffusion experiments of dilute NaCl, NaOH, HCl, Na2SO4, and H2SO4 solutions through ion exchange membranes (IEMs) and the conductivity data obtained with impedance measurements in these solutions. The diffusion rate across the anion exchange membrane (AEM) is found to be as H2SO4 > HCl > NaCl > Na2SO4 > NaOH, whereas for cation exchange membrane (CEM) it was NaOH > NaCl > Na2SO4 ≥ H2SO4. Special attention is given to sulfuric acid because of the partial dissociation of bisulfate to sulfate and proton, which makes use of the Nernst-Hartley equation, i.e., splitting the electrolyte diffusion coefficient into its ionic contributions, impossible. The diffusion behavior of bisulfate ions inside CEM and AEM is determined in three different concentrations of sulfuric acid. This work offers a convenient approach to finding diffusion coefficients of various ions inside ion-exchange membranes