Current–voltage behaviour of bipolar membranes in concentrated salt solutions investigated with chronopotentiometry

Abstract

Chronopotentiometry is used as a tool to obtain detailed information on the transport behaviour of the bipolar membrane BP-1 in solutions of high sodium chloride concentration above the limiting current density. We discuss critically the interpretation of the observed transition times. The occurrence of two such polarization times for low to moderate current densities is explained by the membrane asymmetry: the two membrane layers of opposite charge in general have different transport properties such as co-ion concentration and diffusion coefficient. The reversible and irreversible contributions to the transmembrane potential can be distinguished which allows the bipolar membrane energy requirements to be addressed. The experiments indicate that the increased voltage drop across bipolar membranes observed with higher solution concentrations can be explained on the basis of stronger concentration gradients in the membrane layers. The gradients become stronger with increased current density, but here the ohmic resistance under steady state transport conditions (the transport resistance) contributes to the increasing electrical potential. The transport resistance decreases with increasing current density due to the ion-exchange of the salt counter ions with the water splitting products. The experiments show that bipolar membranes should be operated at low current densities and low concentrations to minimize energy requirements. These findings are in contrast to the high current densities required to reduce impurities in the produced acid and base.