## 3 Multi-Channel Membrane CDI (MC-MCDI)
### Operational Studies and Principle of Mechanism  
With its key advantages, MC-MCDI has successfully been demonstrated as next-generation cell architecture by various operational studies. In general, MC-MCDI provides a high saline environment to electrodes in the side channels, while the low saline water is fed into the middle channel as treating water. Both channels are independently controlled and separated by ion exchange membranes [32]. The salinity gradient between channels enhances desalination performance with an unprecedented improvement in SAC, high charge efficiency, and low energy consumption (Figure 6) [15]. Particularly, under reverse-voltage operation of +1.2 V for charging and -1.2 V for discharging, MC-MCDI can be operated as a semi-continuous desalination process for the production of clean water with even greater enhanced desalination performance. As a result, under reverse-voltage operation, MC-MCDI resulted in approximately four times greater desalination performance than the system under operational condition for conventional CDI (see Figure 6A,D). The enhancement of desalination performance in MC-MCDI is mainly attributed to the enhanced intrinsic capacitance of the porous carbonaceous electrodes in the high saline environment. Additionally, considering the mechanism of the reverse-voltage operation shown in Figure 7, further enhancement can be interpreted by the block of co-ion repulsion, additional ion flux for charge neutrality, and the double-ion ad/desorption processes. During the first half-cycle (i.e., charging step), co-ions are released from the pores of carbonaceous electrodes in the side channels, while the ion exchange  
Figure 6: Desalination performance of multichannel membrane CDI (MC-MCDI) with a concentration-gradient operation between the middle and the side channels under zero voltage operation (ZV, +1.2 V vs. 0 V) (**A,B**) and reverse voltage operation (RV, +1.2 V vs. -1.2 V) (**C,D**). The M\({}_{5}\)S\({}_{5}\) and M\({}_{5}\)S\({}_{1,000}\) indicate the concentration of the middle (5 mM NaCl) and side channel (5 mM NaCl or 1000 mM NaCl) (Reprinted with permission from Reference [15]). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Mode**} & \multirow{2}{*}{**Material**} & **Cell Voltage** & **Feed Concentration (mM)** & \begin{tabular}{c} SAC (mg/g)/Energy \\ Consumption (kT) \\ \end{tabular} & \begin{tabular}{c} Charge \\ Efficiency \\ \end{tabular} & **Ref.** \\ \hline \multirow{4}{*}{**MC-MCDI**} & \multirow{4}{*}{**ACC(Kynol)**} & +1.2/0 & **M: 5 (S: 5)** & **14.1**/-20 & \multirow{4}{*}{\textgreater{90}} & \multirow{4}{*}{[15]} \\ \cline{3-4} \cline{6-6}  & & +1.2/-1.2 & & 22.6/-20 & & \\ \cline{2-6} \cline{5-6}  & & +1.2/0 & 29.8/-20 & & \\ \cline{2-6} \cline{5-6}  & & +1.2/-1.2 & & 56.8/-20 & & \\ \cline{2-6} \cline{5-6}  & \multirow{2}{*}{**ACC(Kuraray)**} & +1.2/0 & \
\begin{tabular}{c} **M: 10 (S: 100 NaCl,** \\ 100 NaFe(CN)\({}_{6}\))** \\ \end{tabular} & 67.8/-27 & \textgreater{90} & [34] \\ \cline{2-6} \cline{5-6}  & & **Prussian blue** & **+1.0**/-1.0 & **M: 10 (S: 1000)** & 52.9/-38 & \textgreater{98} & [35] \\ \cline{2-6} \cline{5-6}  & \multirow{2}{*}{**ACC(Kynol)**} & +2.4/0.0 &
\begin{tabular}{c} M: 5 (S: 1 M NaCl in H\({}_{2}\)O, \\ **1 M NaCl\({}_{4}\) in PC)** \\ \end{tabular} & 63.5/27 & \textgreater{90} & [33] \\ \hline \hline \end{tabular}
\end{table} 
Table 3: _Cont._membrane blocks those ions due to its permselectivity property. This characteristic phenomenon allows additional migration of counter-ions from the middle channel to balance the charge neutrality in the side channels. Consequently, the feed stream in the middle channel is desalinated, while the side channel is concentrated. During the second half-cycle (i.e., discharging step), co-ions can migrate from the side channels to the middle channel through the ion exchange membrane, producing concentrated effluent in the middle channel and desalinated solution in the side channels.