## 4 Results and Discussion  
Figure 4 shows the surface images of the uncoated activated carbon ((a) and (b)) and LDH coated carbon electrode ((c) and (d)) using FESEM. As shown in Figure 4c,d, the LDH structures were aligned vertically on the surface of the activated carbon electrode, resulting in a porous LDH layer. The LDH coating layer was obtained under a hydrothermal reaction with an in situ growth method for better LDH crystal porous structure without blocking the pores on the surface of the activated carbon powder. During this coating process, the LDH nanosheets in the LDH precursor were attached onto the surface of the substrate and quickly formed a thin LDH film. After that, larger LDH particles began to grow and crystalize on top of the primary LDH film under the hydrothermal condition gradually forming a porous and well-oriented LDH layer. It was observed that the one side of the activated carbon electrode surface was well coated with the LDH layer with a depth of approximately 50 \(\upmu\)m (verified by the  
Figure 3: Schematic diagram of the CDI unit cell. cross-sectional SEM images in Figure 4e,\(f\). The detailed mechanism of the porous LDH layer formation from the LDH precursor has been reported in previous [24]. Figure 5a,b shows the contact angle images of the activated carbon and LDH coated activated carbon electrodes using distilled water. As seen in the images of the water droplets, the contact angles of the uncoated and LDH coated activated carbon electrodes were approximately 130\({}^{\circ}\) and 35\({}^{\circ}\), respectively. These results show that the LDH coated activated carbon electrode has a better wettability than the uncoated one. Since the LDH materials have a hydrophilic characteristic because of their hydroxyl groups, the LDH coating layer results in an enhanced wettability of the activated carbon electrode [19; 27]. Figure 4: Field emission scanning electron microscope (FESEM) images of that activated carbon (CEP-21) sheet (**a**,**b**), LDH coated activated carbon sheet (**c**,**d**), and cross-sectional view of the LDH coated activated carbon sheet (**e**,**f**). Figure 6 shows the cyclic voltammetry (CV) curves for the uncoated and LDH coated activated carbon electrodes with a potential range of 0-0.6 V in 1 M NaCl solution. Both CV curves are nearly rectangular in shape, showing that the ions are adsorbed onto the surface of both uncoated and LDH coated electrodes by forming an electrical double layer as ideal capacitors [6, 28]. The specific capacitance of the LDH coated activated carbon electrode calculated by Equation (1) was 124 F/gelectrode, which shows a slightly increased (6%) value compared with that of the uncoated activated carbon electrode (116 F/gelectrode). These results indicate that the LDH coating layer improves the wettability of the electrode without disturbing the pore structure in the activated carbon, and this enhancement of the wettability is attributed to the increase in the capacitance. Figure 5: The contact angle of the activated carbon (**a**) and LDH coated activated carbon electrode (**b**). Figure 6: The cyclic voltammetry (2 mV/s) of the activated carbon electrode and LDH coated activated carbon electrode in 1M NaCl solution. Furthermore, Figure 7a,b shows the conductivity changes during the ion adsorption/desorption tests and the salt adsorption capacity curves during the third charging step. As seen in Figure 7a, the conductivity rapidly decreased as an electrical potential (1.2 V) was applied during the charging step, while, it increased as the adsorbed ions were released during the discharging step. From the conductivity data, it was observed that the LDH coated activated carbon electrode had a better performance than that of the uncoated activated carbon electrode for overall cycles. Figure 7b shows the accumulated salt adsorption capacity during the third cycle of the charging step. Note that the salt adsorption capacity was expressed as the accumulated mass of adsorbed ions per total mass of the electrodes.