Analysis of Faradaic Reaction and Its Effect on Desalination Performance in Capacitive Deionization

Abstract

Capacitive deionization (CDI) is a desalination technique that removes salt from the saline water, applying a potential difference between two porous activated carbon electrodes. Membrane assisted CDI (MCDI) is a commonly used system that improves performance by installing an ion exchange membrane in the CDI system. CDI is mainly known to be deionized by Non Faradaic reaction, but recently there have been limited reports on H2O2 generation, pH change and oxidation of carbon electrode in relation to Faradaic reaction. Therefore, the detailed study of Faradaic reaction characteristics in CDI and MCDI systems is required. The purpose of this study is to investigate the characteristics of the Faradaic reaction and the quantitative effect on the desalination performance in flow-mode CDI and MCDI systems. As major results, firstly, the H2O2 formation and effluent pH change in case of MCDI ([H2O2] = 5.5 μM) were much smaller than that of CDI (37.0 μM), indicating the less occurrence of Faradaic reactions for MCDI. However, the surface pH of carbon electrode in the MCDI was much more acidic (pH ~1.5) at the anode and more basic at the cathode (pH ~ 11.7), indicating the deterred transfer of byproducts (OH- and H+) generated in the compartment inside of ion change membrane. In addition, the quantitative analysis regarding the extent of Faradaic reaction between these two systems were provided based on measured byproducts. Secondly, the long-term study indicated that the performance reduction of deionization in MCDI (17%) was much smaller than that of CDI (85%), which can be supported by their asymmetrically oxidized anode analyzed by FT-IR, CV, and XPS. In addition, the strategies to overcome the performance reduction by coating the carbon electrode with ion exchange polymer or applying alternate reverse potential were suggested. Finally, the deep understanding about Faradaic reactions affecting short-term or long-term CDI performance can ultimately contribute to the development of CDI performance and long-term stability.