Evaluation of long-term stability in capacitive deionization using activated carbon electrodes coated with ion exchange polymers

Although capacitive deionization (CDI) is an energy-efficient and environment-friendly desalination technique, the severe performance decrease during long-term operation has been a critical obstacle to its practical application. Compared to various other approaches for stability improvement, the ion-exchange polymer (IEP) coating on the electrode seems to be both efficient and economically feasible. Nevertheless, there have only been limited studies aimed at understanding the role of IEP on stabilizing CDI operations. In this study, we investigated the effect of IEP on CDI performance by varying the amount of IEP coated on the electrodes. The polymer layer thickness was varied across the three IEP-coated electrodes used in this study (0, 30, and 100 mu m). By monitoring the salt adsorption capacity (SAC) during the 50-h operation, it was found that the long-term stability of the system was dramatically improved upon using the IEP-coated electrodes. Additionally, the SAC retention was further improved with increasing IEP layer thickness. Based on the experimental analysis, we could conclude that the activated carbon particles' coating layer acted as a barrier to block the water molecules from the electrode surface, hence impeding carbon oxidation. The outer polymer layer formed on the electrode could additionally block the diffusion of oxygen sources from the bulk solution to the electrode, which further reduced the possibility of carbon oxidation. The results suggest that the IEP coating is effective towards maintaining the performance of the electrodes, and thicker IEP layers increased the electrode stability.