Comprehensive Electrochemical Impedance Spectroscopy Study of Flow-Electrode Capacitive Deionization Cells

Flow-electrode capacitive deionization (FCDI) offers infinite ion adsorption for continuous desalination of high-concentration saline water by supplying a flow-electrode to the cell. Although extensive efforts have been made to maximize the desalination rate and efficiency of FCDI cells, the electrochemical properties of these cells are not fully understood. This study investigated the factors affecting the electrochemical properties of FCDI cells containing activated carbon (AC; 1–20 wt %) and various flow rates (6–24 mL/min) for the flow-electrode using electrochemical impedance spectroscopy before and after desalination. Examination of the impedance spectra using the distribution of relaxation time and equivalent circuit fitting analysis revealed three distinctive resistances such as internal, charge transfer, and ion adsorption resistances. The overall impedance decreased significantly after the desalination experiment due to increased ion concentrations in the flow-electrode. The three resistances decreased with increasing concentrations of AC in the flow-electrode due to the extension of electrically connected AC particles that participated in the electrochemical desalination reaction. The ion adsorption resistance decreased significantly due to the flow rate dependence of the impedance spectra. In contrast, the internal and charge transfer resistances were invariant.