Quantitative Decoupling of Diffusion and Electromigration of Ions across Membranes in Flow Capacitive Deionization Device

In the flow electrode capacitive deionization (FCDI) device, ion transfer across the membranes under an electrical field was controlled by both concentration-gradient-induced diffusion and electromigration. Decoupling diffusion and electromigration during deionization operation are critically important to rationally optimize the deionization process, while it remains a challenge so far. In this work, a modified diffusion-electromigration ratio (DMR) model was proposed by considering diffusion. With modified DMR, the contribution of diffusion and electromigration can be differentiated in both continuous operation and deionization/regeneration modes. In both modes, electromigration has been demonstrated to be the dominating working mechanism. The quantification of the electromigration portion also enables a deeper understanding of the ion adsorption kinetics, which reveals a pseudo-first-order kinetics of the deionization/regeneration process and pseudo-second-order kinetics of the continuous desalination process. DMR provides a deeper understanding of the role of electromigration in the FCDI process and offers theoretical guidance for the selective separation of complex ionic systems.