Effect of surface hydrophobization on chronopotentiometric behavior of an AMX anion-exchange membrane at overlimiting currents

Abstract

Surface modification of ion-exchange membranes is already proved to be an effective and low-cost way to improve membrane properties. Usually, the object of modification are cation-exchange membranes. We study commercial anion-exchange Neosepta AMX membrane and two its modifications obtained by electrospray deposition method. Hydrophobic non-conducting spots of a fluoropolymer are deposited with different density; they screen 8 or 12% of the membrane surface. When a constant overlimiting current density is applied to an electrodialysis cell with a membrane sample, in all studied cases there were voltage oscillations superimposed on the general upward trend in voltage–time dependence. For the first time, we observed on the chronopotentiograms of ion-exchange membranes the oscillations arisen essentially earlier than the transition time, where the potential drop (PD) across the membrane (reduced by the initial ohmic drop) was 20–40mV. The modified membranes showed higher amplitude of oscillations, lower voltage growth and lower water splitting rate. The limiting and overlimiting steady-state current was up to 1.5 times higher over modified membranes. The early oscillations are explained by equilibrium electroconvective instability recently theoretically studied by Rubinstein and Zaltzman [24]. Thе occurrence of this mode of electroconvection at low PD is explained by undulated surface (commercial AMX) and hydrophobic spots (modified membranes) favouring higher tangential electric force applied to equilibrium space charge region. At higher voltages occurring in quasi-steady state, there are stronger and more frequent oscillations governed by non-equilibrium electroconvection. It is shown that electroconvection can not only enhance mass transfer, but essentially reduce water splitting.