Current efficiency enhancement in membranes with macroscopic inhomogeneities in the fixed charge distribution

Abstract

We present a theoretical study of the effects that a macroscopic fixed charge inhomogeneity exerts on ion transport when an electric current is passed through a charged membrane. The results show that the inhomogeneity can modify the ratio of counterion to co-ion fluxes, but not the current efficiency (Ec), in highly charged membranes. However, for weakly charged membranes both the ratio of the fluxes and the current efficiency can experience important changes because of this inhomogeneity. It is also found that the inhomogeneous membranes should exhibit a much more pronounced dependence of Ec on the electric current than homogeneous membranes at high currents. A phenomenological theory relating the particular non-uniform fixed charge distribution to the resulting membrane transport properties is proposed. This theory can predict which distributions will lead to higher Ec values than those for a homogeneous membrane with the same average fixed charge concentration. The transport model employed is based on the Nernst–Planck equations, and the results obtained are of interest for both the study of the physical principles underlying transport in non-uniform charged membranes and the design and application of membranes.