Heterogeneity of heterogeneous ion-exchange membranes investigated by chronopotentiometry and X-ray computed microtomography

Chronopotentiometry is a powerful experimental technique for characterization of ion-exchange membranes, especially phenomena associated with ion transport and the occurrence of overlimiting current. One also uses this technique to estimate heterogeneity of ion-exchange membranes, or more precisely the fractions of conductive and nonconductive regions in the membrane, by measuring so called transition times and their substitution into Sand equation. Here, we test this approach on small pieces of heterogeneous cation-and anion-exchange membranes by combining two experimental techniques, namely chronopotentiometry and X-ray computed microtomography. While chronopotentiometry provides data for the theoretical analysis based on Sand equation, microtomography provides a detailed analysis of the membrane structure both on its surface and in its volume. Such a structural analysis allows for evaluation of volumetric and surface fractions of ion-exchange resin and also quantification of the surface area of the active ion-exchange material on the depletion side of the membrane. We compare the experimental data from both techniques to see how they correlate and discuss the obtained results. We conduct the same experimental investigation on single ion-exchange resin particles which are a component of the aforementioned heterogeneous membranes. The ion-exchange particles can be viewed as homogeneous ion-exchange systems. Our structural analysis by means of the newly developed technique based on micro-computed tomography showed that the volume and the surface compositions of the heterogeneous membranes differ. The surface fraction of the ion-exchange resin is two to three times smaller than the corresponding volumetric fraction. The experimental transition times showed very good agreement with the predictions of the Sand equation in case of the ion-exchange particles. Unlike that the transition times were not well predicted by the Sand equation in case of the heterogeneous ion-exchange membranes. This fact along with observed differences in the structure of these membranes on their surface and in their volume limit the use of the Sand equation to estimate the content of the ion-exchange resin in these membranes.