Temporal and spatial distribution of pH in flow-mode capacitive deionization and membrane capacitive deionization

Capacitive deionization (CDI) is a desalination technique that removes salt from saline water by applying a potential difference between two porous activated carbon electrodes. Although undesirable Faradaic reactions causing a deterioration in the desalination performance in long-term operation have been reported with pH fluctuations and hydrogen peroxide generation, the effect of the temporal and spatial pH distribution and quantitative analysis of charge consumption have rarely been reported or interpreted, especially in membrane capacitive deionization (MCDI). In this study, more detailed and precise investigations were quantitatively conducted to determine the charge consumption for ion adsorption, Faradaic reaction, the co-ion repulsion by analyzing the pH distribution on the electrode surface, the H2O2 and salt concentration, and the current profiles for short-term and long-term operations of CDI and MCDI. As major results, active Faradaic reactions caused a decrease in the deionization performance in the short-term operation, whereas in the long-term operation of CDI the sustained oxidation of carbon electrode caused its performance to decrease even further. On the other hand, regardless of a short or long term operation, less Faradaic reactions and less oxidation of the carbon electrode were observed with a superior deionization performance in MCDI.