Inversion phenomenon and effective charging quantity in capacitive deionization device

The inversion desalination in capacitive deionization (CDI) due to overcharging leads to low salt adsorption and high energy consumption. Carefully controlling the charging process can effectively avoid the energy consumption from the inversion desalination and improve the desalination performance. In this study, by using the activated carbon (AC) electrode as a representative, the CDI charging process was analyzed in detail by adjusting the charging current, voltage, and the concentration of salt solution. The effective charging quantity of the electrode, defined by the inflection point of the inversion phenomenon, was found to be the inherent property of porous carbon material, which is independent of the test conditions and only depends on the surface characteristics and pore structure of AC material. The effective charging quantity of AC was increased only through varying the surface charge state by surface modification or applying an ion exchange membrane. The improvement of the pore structure of AC by KOH treatment led to the increase of effective charging quantity by retarding the appearance of inversion phenomenon. The present study provides a vital guidance for better understanding the inversion deionization phenomenon. The discovery of the intrinsic charging quantity is very helpful to avoid overcharging in practical CDI engineering application.