Maximizing Volumetric Removal Capacity in Capacitive Deionization by Adjusting Electrode Thickness and Charging Mode

Abstract

Capacitive deionization (CDI) is an emerging desalination technology based on the same charge storage principles as in electrical double-layer supercapacitors (EDLC's). In this study, electrodes of differing thicknesses were tested using constant current (CC-CDI) and constant voltage (CV-CDI) operational modes in order to study the best way of sequestering the highest amount of ions under different salt concentration scenarios. CV-CDI was used to calculate electrode time constants (RC) and thereby determine a suitable current density for desalinating the solution. Results showed that the voltage pulse produced a fast but heterogeneous layer of ion adsorption, presumably, on the most accessible part of the electrode surface. Thus, volumetric specific capacitance under CV-CDI mode might vary from 17–24 F cm−3 and 14–20 F cm−3 for 50 μm and 180 μm electrodes, respectively. Nevertheless, results demonstrated that, under the constant current mode, it is possible to increase charge storage by 30% for a CDI cell consisting of thin electrodes and as much as 80% for thicker electrodes simply by controlling current density and, therefore, the rate capability. Moreover, the analysis of the charge efficiency indicated that a proper selection of the current density can result in efficiencies above 80% regardless the salt concentration scenario.