Induced-charge membrane capacitive deionization enables high-efficient desalination with polarized porous electrodes

Exposure of a conducting porous material to an electric field in electrolytes induces an electric dipole, which results in capacitive charging of cations and anions at opposite poles. In this letter, we investigate a novel desalination method using this induced-charge capacitive deionization (IC-CDI). To do this, we devise a microscale IC-CDI platform that can visualize in situ ion concentrations, pH shifts, and fluid flows, and study ion transport dynamics and desalination performances compared to conventional CDI with unipolar / bipolar connections. Similar ion concentration and fluid flow characteristics were observed in Ohmic, limiting, and overlimiting regimes, but variations in desalination performance trends were noted based on the number of stacks. In a single cell, IC-CDI generates a higher electric field at the opposite poles of porous electrodes than simple conducted electrodes in CDIs with unipolar/bipolar connections, leading to superior salt removal and/or lower ionic current at a given applied voltage. This marks a clear contrast from CDI with bipolar connection, which lacks any advantage over CDI with unipolar connection in a single cell. These metrics of IC-CDI however deteriorated as the stack number increased, likely due to short-circuiting between the dipoles. As a result, IC-CDI in current form shows higher desalination efficient than conventional CDIs with low stack numbers (< 6), so we offer the scale-up module by repeating 4-stack IC-CDI units. Our study enhances comprehension of ion transport dynamics and desalination performance in IC-CDI, and the results could aid in the development of IC-CDI for energy/cost-efficient desalination.