Comparison of faradaic reactions in flow-through and flow-by capacitive deionization (CDI) systems

Abstract

Abstract Use of flow-through electrodes, in which water flows through rather than past porous electrodes, has been shown to be a promising approach to overcoming mass transfer limitations in various electrochemical technologies (including microbial fuel cells, electrochemical disinfection, electrochemical advanced oxidation processes and capacitive deionization). In this work, we fabricated a flow-through CDI cell and investigated the saline water desalination performance and the possible Faradaic reactions occurring in the systems under different flow modes, i.e., forward mode (water flows from anode to cathode) and reverse flow mode (water flows from cathode to anode). Upon applying a charging voltage of 1.2\u202fV, use of flow-through CDI operating under both flow conditions exhibited better desalination performance compared to that of conventional flow-by CDI. Larger pH fluctuations were found in flow-through CDI systems with effluent pH increasing to about 9.3 in forward flow mode and decreasing to 3.8 under reverse flow mode while in flow-by CDI, the treated water pH exhibited only a slight increase to 8.0. In addition, much higher H2O2 production and dissolved oxygen (DO) decay rates were observed in flow-through CDI operated in forward flow mode compared to other scenarios, possibly due to the different ion adsorption and reaction sequences. These findings provide insight into both the desalination capacity of flow-through CDI and the Faradaic reactions occurring in such a system and should be of value in developing a treatment technology capable of simultaneous water desalination, disinfection and/or micro-pollutant removal.