Dielectric Analysis of Concentration Polarization Structure at Ion-Exchange Membrane/Solution Interface Under DC Bias Voltage Application.

Abstract

Dielectric measurement coupled with measurement of steady voltage-current (V-I) characteristics was carried out on cation-exchange membrane CMV and anion-exchange membrane AMV immersed in aqueous NaCl solution under application of de bias voltage. A marked dielectric relaxation was observed when the bias voltage exceeds the voltage that yields the critical current density (CCD). The dielectric relaxation was analyzed on the basis of the dielectric model which consists of a Nernst diffusion layer, a thin salt depletion layer and a bulk solution phase. The values of the thickness of the diffusion layer were 0.062 cm for the CMV membrane and 0.026 cm for the AMV membrane, being in good agreement with those values, 0.05 cm for the CMV membrane and 0.03 cm for the AMV membrane, calculated from the values of CCD obtained by the V-I characteristics. The thickness of the salt depletion layer is 0.7-1.6 μm for the both ion-exchange membranes and the conductivity of the depletion layer is 0.2-0.25 μSem-1 for the CMV membrane and 0.6-0.8 μScm-1 for the AMV membrane. The conductivity of the depletion layer at the AMV membrane/solution interface is higher than that at the CMV membrane/solution interface, which may suggest that the water splitting takes place at the AMV membrane/solution interface.