Potential Sweep Method to Evaluate Rate Capability in Capacitive Deionization

Capacitive deionization (CDI) is a desalination technology attracting growing attention nowadays. To understand desalination performance in CDI, the capacitive behavior of the electrode is significantly important because desalination is accomplished through capacitive charging and the subsequent double layer formation. So far, a variety of studies have reported on desalination performance along with the electrochemical properties. However, the rate capability of desalination has been poorly defined, though it is an important indicator showing the ability of an electrode to exhibit rapid desalination. In this study, the rate capability of CDI was evaluated by introducing a potential sweep method, using electrodes with different thicknesses. The deionization capacity was obtained at various scan rates, and its retention ratio upon increasing the scan rate was used as an indicator of the rate capability. The results showed that the thinner electrode retained more deionization capacity compared to the thicker electrodes as the scan rate was increased, which indicates that the electrode thickness is one of parameters affecting the rate capability in CDI. In addition, a higher mean deionization rate was found in constant current operations for the thinner electrode. To support the desalination performance, the rate-dependent capacitive behavior was analyzed by cyclic voltammetry and electrochemical impedance spectroscopy, which also resulted in a similar trend; the rate capability of the thinner electrode was better than that of the thicker electrodes. In particular, the ion penetrability analyzed by equivalent circuit analysis was suggested as one of the major parameters determining the rate capability. In this regard, careful consideration is required in determining the electrode thickness when developing a versatile CDI system producing various effluent concentrations with an outstanding rate capability.