## 3 Results
### Enhancement of Hydrophilicity by Polydopamine Modification  
Polydopamine surface modification was performed to further evaluate the effects of hydrophilicity on the electrode. Figure 3 shows the changes in surface hydrophilicity and elemental composition of PDA-modified electrodes. After PDA modification, the initial contact angles of both electrodes were significantly reduced (i.e., from 130\({}^{\circ}\) to 50\({}^{\circ}\) for the P60 electrode and from 118\({}^{\circ}\) to 41\({}^{\circ}\) for the YS-2 electrode) and the water droplets were almost completely absorbed into both the electrodes after 3 s Figure 3a,b. These changes in contact angles indicate that both electrodes had higher hydrophilicity compared to pristine electrodes (i.e., 130\({}^{\circ}\) for P60 electrode and 118\({}^{\circ}\) for YS-2 electrode). Notably, the changes in the hydrophilicity of IEMs after PDA modification were negligible (Supplementary Figure S2). The results of IEM modifications are discussed later. Figure 3c,d represent the XPS spectra of the P60 and YS-2 electrodes in pristine (black line) and PDA-modified (red line) forms. Three major peaks--C1s, O1s, and F1s--were commonly observed in the spectra of the pristine electrode, indicating carbons, oxygenated functional groups, and fluorine from a PTFE binder, respectively. After the PDA modification, an additional N1s peak, an increase in the O1s peak, and a decrease in the C1s peak were observed. These changes were commonly attributed to the presence of polydopamine on the electrode surface because dopamine includes both hydroxyl and amine groups [30]. Figure 4 shows the relative SAC in MCDI for three combinations of the electrode and IEM: PDA-modified electrode and pristine IEM (E\({}_{\text{PDA}}\) + IEM\({}_{\text{P}}\)), pristine electrode and PDA-modified IEM (E\({}_{\text{P}}\) + IEM\({}_{\text{PDA}}\)), and PDA-modified electrode and IEM (E\({}_{\text{PDA}}\) + IEM\({}_{\text{PDA}}\)). The SACs were summarized in Table 3. Relative SAC was calculated from these SACs by subtracting from and dividing by the  
Figure 3: Contact angle images of (**a**) P60 electrode and (**b**) YS-2 electrode after polydopamine (PDA) modification (upper-right images were taken after 3 s) and XPS spectra of (**c**) P60 and (**d**) YS-2 as pristine (black) and PDA-modified electrodes (red). SAC of the MCDI (refer to Table 2) with a pristine electrode and IEM. As shown in Figure 4, the PDA-modified P60 electrode significantly increased the SAC by approximately 20%, while the changes in SAC using the YS-2 electrode before/after PDA modification were insignificant. The probable explanation is that the PDA modification of the hydrophobic P60 electrode had a large effect on the desalination performance, while the YS-2 electrode with already good wettability did not get affected by additional hydrophilic modification. A similar large change in the SAC in case of the P60 electrode compared to the YS-2 electrode was observed in the CDI configuration without IEM, as shown in Supplementary Figure S3. In CDI, the observed SACs were 10.2 \(\pm\) 1.0 and 10.2 \(\pm\) 1.5 mg g\({}^{-1}\) for the PDA-modified P60 and YS-2 electrodes, respectively, with 0.7 and 0.1 mg g\({}^{-1}\) improvement. This improvement is consistent with a previous study showing a higher desalination performance achieved by decreasing the contact angle of the modified electrodes [35; 36]. As can be seen in Figure 4 and Supplementary Figures S1 and S3, PDA modification improved the SAC of P60 electrodes to a level almost similar to the SAC of PDA-modified YS-2 electrodes. This result with the PDA-modified electrodes in both the MCDI and CDI suggests that the SAC of approximately 13-14 mg g\({}^{-1}\) for MCDI (10 mg g\({}^{-1}\) for CDI) may be the maximum achievable value because both the electrodes became sufficiently hydrophilic with a good wettability.