Capacitive deionization of aqueous solutions: modeling and experiments

Capacitive deionization of NaHCO3, KCl, CaCl2, MgSO4 solutions is studied under dynamic conditions. An electrochemical cell containing activated carbon electrodes is used. A two-dimensional model of this cell based on the model of a static cell is developed. The model takes into consideration ion adsorption-desorption and transport, surface conductivity of electrodes, electric double layer capacitance, as well as characteristics of the porous structure of the electrodes and separator obtained using the method of standard contact porosimetry. Characteristics of the deionization processes are shown to depend on the structure of the electric double layer. The time evolutions of fields of solution concentration were calculated from process beginning. The results are in agreement with experimental data which supports the model. This allows determining the optimal conditions for deionization processes. It is shown that both deionization and concentration are enhanced at an increase in voltage and at a decrease in the solution flow rate. The possibility of obtaining very pure water is determined by surface conductivity of the electrodes. The maximal deionization degree of 82% has been achieved for the 0.005 M KCl solution. The influence of the porous structure and hydrophilic-hydrophobic properties of the electrodes on characteristics of the deionization processes has been established.