## 3 Results and Discussion
### CDI Performance of the Electrodes  
Figure 5 shows the flow rate dependence on the SAC and the removal efficiency for the AC and AC-CPL. The SAC value showed a tendency to decrease for the AC as the flow rate increased but showed a progressively increased value for the AC-CPL electrode at the same flow rate. The SAC values were higher for the AC-CPL than for the AC. In the AC electrode, the SAC value showed a tendency to decrease with a flow rate increase because the electrosorption process requires high contact time between the electrode surface and the ionic solution. The highest removal efficiency of Cs\({}^{+}\) was 5 mL min\({}^{-1}\) for the AC and 10 mL min\({}^{-1}\) for the AC-CPL. Then the efficiency gradually decreased as the flow rate increased because a low flow rate provided sufficient contact time for reactants with the electrode. Figure 4: Nyquist plots of AC, and AC-CPL in 0.5 M CsNO\({}_{3}\) solution. (Color of navy: R\({}_{8}\) and R\({}_{\text{ct}}\) of the AC, color of olive: R\({}_{8}\) and R\({}_{\text{ct}}\) of the AC-CPL). Figure 5: Comparison of the AC, and the AC-CPL for (**a**) specific adsorption capacity of Cs\({}^{+}\) ions and (**b**) removal efficiency according to the flow rate (Experimental conditions: Influent of 200 mg L\({}^{-1}\) Cs\({}^{+}\) ions at pH 3 solution was fed at 1.2 V for 10 min). Figure 6a shows the conductivity of the effluent for the AC-CPL according to the potential variation from 0.8 V to 1.4 V. The tests were conducted by maintaining a constant flow rate at 20 mL min\({}^{-1}\) for 10 min. As soon as an electric potential was applied, the conductivity decreased sharply. The reason for the decreased electrical conductivity at the initial stage was due to the accumulation of charged ions by the electrodes. Following this, the electrical conductivity of the effluent water increased. The higher applied potential induced a stronger electrostatic force, which had a positive effect on the electrosorption performance. [17] A desorption potential of 0 V was applied immediately after adsorption. The conductivity increased rapidly, and the value close to the concentration of the injected water was measured. The results revealed that desorption proceeded rapidly, and the regeneration of the electrode in the CDI system was facile. The specific adsorption capacity and charge efficiency of Cs\({}^{+}\) according to the applied potential are depicted in Figure 6b. The SAC value increased with increasing applied potential, indicating that the electrosorption performance was enhanced. The charging efficiency showed the highest value at 1.2 V. Charge efficiency can be defined as the ratio of the adsorbed amount of salt over the charge, and is an important parameter in evaluating electrosorption performance in CDI cells. The energy requirement per ion removed can be explained in terms of the value of the charge efficiency, and the higher the charge efficiency, the lower the energy consumption. In general, the charge efficiency depends on the cell potential, which increases as the charging potential increases [30]. The applied potential at 1.4 V reduced the charge efficiency, which is reportedly due to the possibility of side reactions (e.g., water electrolysis, oxidation of carbon electrode) [31; 32; 33]. Thus, the applied potential of 1.2 V was used in the follow-up test. Figure 7 shows the maximum adsorption capacity of the AC-CPL depending on the concentration of Cs\({}^{+}\) and the flow rate of the influent water. As shown in Figure 7, the maximum adsorption capacity increased according to the Cs\({}^{+}\) concentration in the influent water regardless of the flow rate. These results can be explained by Gouy-Chapman-Stern (GCS) theory and electric double layer expansion [34]. An electric double layer can be more effectively formed on the porous structure of the carbon electrode, along with an increase in the ion concentration of the solution, thereby providing more advantageous conditions for the electrosorption process.