Controlling ion transport with pattern structures on ion exchange membranes in electrodialysis

Abstract

In electrodialysis (ED), ion transport through ion exchange membranes governs its desalination performance. In this paper, we decipher the effects of non-conductive pattern structures on the membrane in ED and identify two factors to control conductive/convective ion transports: i) electric "view factor" (deduced from the analogy of view factor of radiative heat flux) representing the possibility of ion conduction, and ii) vortex intensity representing the strength of ion convection. To do this, we build a two-dimensional ED platform with six different pattern structures on the membrane, and then visualize in situ ion concentration and fluid flow over these patterns in Ohmic, limiting, and overlimiting current regimes. In the Ohmic-limiting regime (V 2 V), the electric view factor determines current, Ohmic conductance, and salt removal. In the overlimiting regime (V 2 V), electroconvection (EC) occurs on the membranes, so the vortex intensity of EC determines current and salt removal. Moreover, overlimiting conductance can be predicted by the density of this vortex intensity. Lastly, taken together, we develop new scaling law for the electric Nusselt number, which presents the degree of ion convection vs. ion conduction, as a function of the pattern shape and EC strength.