Influence of ion exchange membrane arrangement on dual-channel flow electrode capacitive deionization: Theoretical analysis and experimentations

Flow electrode capacitive deionization (FCDI) has emerged as a viable technology for brackish water desalination. However, the mutual restriction between desalination rate and energy consumption is the current bottleneck hindering the widescale and practical application of FCDI. At this point, the relationship between the complex electrical double layer and electrodialysis mechanism is still unclear. Herein, we have constructed two kinds of dual-channel FCDI systems to investigate the influence of ion exchange membranes (IEMs) arrangement on desalination performance. The diffusion coefficient of chlorine is higher than that of sodium, resulting in the improved salt removal rate and current efficiency of dual-channel FCDI based on the NaCl electrolyte. In contrast, both systems performed identically using KCl as the majority salt in the feed stream due to the similar diffusion coefficients of both the cation and anion. The higher salt removal capability of dual-channel FCDI was achieved by (i) the thicker ion depletion layer of cation exchange membrane (CEM) enabling higher salt removal efficiency and (ii) the higher concentration of the ion enrichment layer accelerating the electro-adsorption of flow-electrodes. Both the numerical simulation and experimental results indicate that the composite mechanisms of dual-channel FCDI are beneficial to the excellent desalination performance and great energy efficiency.