Impact of heterogeneous cation-exchange membrane surface modification on chronopotentiometric and current–voltage characteristics in NaCl, CaCl2 and MgCl2 solutions

Abstract

Chronopotentiograms and current-voltage curves were measured to characterize the transport of sodium, calcium, and magnesium ions through cation-exchange membranes MK-40 and MK-40MOD. A microfluidic electrodialysis cell was used to visualize electroconvective vortices. The MK-40MOD membrane was obtained by covering the heterogeneous surface of a commercial MK-40 membrane with a thin (about 15 μm) homogeneous film of the MF-4SK sulfonated fluoropolymer. It has been shown that the combination of relatively high hydrophobicity of the MK-40MOD membrane surface with a certain electrical and geometrical (surface undulation) heterogeneity facilitate the generation of electroconvective vortices. The effect of vortices is manifested in reduction of the potential drop under a fixed current density and potential oscillations on the chronopotentiogram. It was found that in all studied systems potential drop in stationary state decreases in the order Na+>Ca2+>Mg2+. This correlates with the amplitude of potential drop oscillations (which increases in the range Na+<Ca2+<Mg2+) and with the size of electroconvection vortices observed in the microfluidic cell: its size is higher in the case of MgCl2 compared to the CaCl2 solution. Oscillations of potential drop on chronopotentiogram were registered not only in stationary state, but also at times of the order of several seconds, lower than the transition time. These oscillations occurred in the case of MgCl2 for both membranes, in the case of CaCl2 for the MK-40MOD membrane. No earlier oscillations were observed in the case of NaCl. The appearance of earlier potential drop oscillations is accompanied by an increase in the transition time and a reduction in the potential drop. The earlier oscillations are explained by the onset of unstable equilibrium electroconvection recently described by Rubinstein and Zaltzman. Stronger electroconvection in the case of MgCl2 in comparison to CaCl2 and NaCl is attributed to the impact of the residual space charge of bulk solution, which is the highest for the MgCl2 among the studied electrolytes. The contribution of bulk electroconvection is discussed on the basis of the estimation of the thicknesses of the diffusion layer and its constituting zones as functions of the current density.