Experimental observation of phenomena developing on ion-exchange systems during current-voltage curve measurement

Abstract

Ion-exchange systems represented, for example, by ion-exchange membranes or ion-exchange resin particles exhibit nonlinear current-voltage curves on which one finds three distinct regions. These regions are referred to as underlimiting, limiting and overlimiting ones. Each of these regions reflects proceeding phenomena responsible for the ion transport. It is known that all important transport processes take place on the depletion side of the ion-exchange systems. Here, we capture the situation at the interface between a cation-exchange system and an electrolyte on the depletion side as it develops during the measurement of a current-voltage curve by using fluorescent and optical observations. Our observations allow to describe qualitatively transport phenomena occurring in ion-exchange systems and to assign their onset to the particular points on the current-voltage curve. We show that current-voltage curves for the studied systems, namely a single cation-exchange particle and a heterogeneous cation-exchange membrane, have a shape typical for ion-exchange systems, i.e. we can clearly recognize underlimiting, limiting and overlimiting region. Interestingly, the overlimiting region of both systems can be divided into two parts based on the slope of the current-voltage curve in this region. We show that there is a qualitative and quantitative change in the developed electroconvection as the major mechanism governing the overlimiting current when the system transitions from the first part of the overlimiting region to the second one. This transition of electroconvection causes its mixing effect to change from local to global.