## 3 Results
### Effects of Electrode Hydrophilicity on MCDI Performance  
Figure 2 shows the MCDI performance using P60 and YS-2 electrodes in terms of (a) normalized conductivity profile; (b) specific current profile; (c) cumulative SAC; and (d) ASAR. The black solid line and blue dashed line represent the data obtained with the P60 electrode and YS-2 electrode, respectively. As seen in Figure 2a, the normalized conductivity displayed a lower and broader drop in the case of the YS-2 electrode during the charging step in comparison to the P60 electrode. Specific currents shown in Figure 2b also appear higher for the YS-2 electrode at the beginning of the charging step. Figure 2a,b in the MCDI indicates that more ions were adsorbed onto the surface of the YS-2 electrode in the initial few minutes of the charging step. Figure 2c,d shows the cumulative SAC and ASAR, calculated by Equations (1) and (2), respectively. In both the figures, the blue dashed line (YS-2 electrode) was positioned higher than the black solid line (P60 electrode), which means that faster and higher salt adsorption was achieved on the YS-2 electrode. It could be inferred that the accessible surface area for ions on the hydrophilic YS-2 electrode was larger than on the hydrophobic P60 electrode due to an easier approach for the water molecules into the micropores.  
Figure 1: (a) Contact angle changes of two electrodes fabricated with P60 and YS-2 as function of time and (b) FT-IR spectra of P60 and YS-2.  
Table 2 displays the SACs of the P60 electrode and the YS-2 electrode in the MCDI calculated from the normalized conductivity changes. The SAC of the YS-2 electrode was larger (\(13.0\pm 0.9\) mg g\({}^{-1}\)) than that of the P60 electrode (\(11.1\pm 1.2\) mg g\({}^{-1}\)). Note that the p-value for those SACs was approximately 0.01, indicating that those SACs are statistically different [39]. As the physical properties of both the electrodes were similar, the larger SAC of the YS-2 electrode reflects the effect of hydrophilicity on the desalination performance. Notably, similar behavior was observed in the CDI; the SAC of the YS-2 electrode was \(10.1\pm 0.7\) mg g\({}^{-1}\) and that of the P60 electrode was \(9.5\pm 0.8\) mg g\({}^{-1}\), as shown in Supplementary Figure S1. Previous studies on the improved CDI performance using hydrophilic electrodes have demonstrated that a hydrophilic surface resulted in enhanced ion transportation [30, 40] and ion accessibility to the pores [41], and better interaction between the carbon surface and the electrolytes [42, 43, 44]. The results from Figure 2 and Table 2 demonstrate the effect of hydrophilicity on MCDI when the physical properties of the electrodes were similar.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Activated Carbon Electrode** & **Salt Adsorption Capacity (mg g\({}^{-1}\))** \\ \hline P60 & \(11.1\pm 1.2\) \\ YS-2 & \(13.0\pm 0.9\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Salt adsorption capacity (SAC) of P60 electrode and YS-2 electrode in MCDI.  
Figure 2: Membrane capacitive deionization (MCDI) performances using P60 and YS-2 electrodes; (**a**) normalized conductivity profiles; (**b**) specific current (mA g\({}^{-1}\)); (**c**) cumulative salt adsorption capacity (SAC, mg g\({}^{-1}\)); and (**d**) average salt adsorption rate (ASAR, mg g\({}^{-1}\) s\({}^{-1}\)) with time of P60 electrode (black) and YS-2 electrode (blue). All graphs were the result of the nearest set to the average value.