Comparative insight into the capacitive deionization behavior of the activated carbon electrodes by two electrochemical techniques

In capacitive deionization (CDI) process, the electrosorption capacity of the electrode is related to the electrochemical characteristics of electrode material and the electrolyte solution concentration. In this study, two electrochemical techniques, including the cyclic voltammetry (CV) and the electrochemical impedance spectrometry (EIS) are used to characterize the electrochemical properties of four activated carbon (AC) electrodes at different NaCl concentrations. The results indicate that the EIS analysis can provide clearer and whole electrochemical pictures of the AC electrodes. From the EIS analysis, the detailed electrochemical information of the ions in solution and their transport behavior can be well clarified. In practical application, EIS analysis can provide a valuable direction to select the appropriate type of AC as electrode material for CDI application. At lower NaCl concentration, the ions are highly hydrated and the solution conductivity is low. The EIS results indicate that the diffusion accessibility of the hydrated ions into the micropores of AC is impeded. It is beneficial to select the AC with high content of external mesopore ratio. At higher NaCl concentration, the hydration degree of ion is less and the ion size is relatively small. The AC with high total surface area becomes more favorable.