PNNL

Abstract

Membrane-based electrochemical processes such as electrodialysis play an important role in desalination and industrial separation processes. Electrodialysis uses alternating anion- and cation-exchange membranes with a perpendicular electric field to generate concentrate and diluate streams from a feed solution. It is known that under the overlimiting current conditions, reduced charge and mass transfer at the membrane interface leads to regions of high ion-depletion generating instability and vortices referred to as electroconvection. While electroconvective mixing is known to directly impact the separation efficiency of electrodialysis, the influence of ion concentration gradients across the membrane experienced in a functional electrodialysis system is not known. Here, we report the influence of ion concentration gradients across a cation exchange membrane (Nafion) both aligned and opposed to the applied electric field. Experiments were conducted by co-flowing NaCl solutions of different concentrations (0.1 to 100 mM) on either side of the membrane and electroconvection was visualized with a fluorescence dye (Rhodamine 6G). We obtained concentration profiles from fluorescence image data and systematically measured the thickness of the depletion boundary layer dBL under different conditions. We found smaller dBL values at a higher flow rate both with and without concentration gradients. Our results show enhanced electroconvection when the electric field is aligned with the direction of the concentration gradient.