## 1 Introduction  
Capacitive deionization (CDI) has drawn increasing interest in water desalination, wastewater treatment, and resource recovery [1; 2; 3; 4]. CDI is considered more versatile than other water treatment technologies because it is an energy-efficient and eco-friendly process that does not require extreme temperatures or pressures or use of chemicals [5; 6; 7; 8]. Along with the simplicity of the process, the relatively small size of the module allows for simple adaptation for decentralized water treatment and in combination with other desalination technologies, including multistage flash distillation and reverse osmosis (RO) [9; 10]. Over the last few decades, CDI has become remarkably advanced with various types of electrodes including capacitive electrodes with carbonaceous materials (i.e., activated carbon, carbon aerogel, carbon nanotubes, and graphene) [11; 12; 13; 14; 15] and faradaic electrodes with battery and battery-like materials (i.e., sodium manganese oxide, molybdenum disulfide, Mexene, silver/silver chloride, and Prussian blue analogs) [16; 17; 18; 19; 20]. These electrode materials have led to novel systematic developments. For instance, with the modification of carbonaceous electrodes, flow-by CDI [21], flow-through CDI [22], and inverted CDI (i-CDI) [23] were introduced. With the development of faradaic electrodes, hybrid CDI (HCDI) [16; 24], cation intercalation desalination (CID) [25], rocking-chair CDI (RCDI) [17], and desalination battery (DB) [26] have been widely researched. Instead of a static electrode, the flowable electrode was employed in CDI, resulting in a continuous desalting system such as flow-electrode CDI (FCDI) [27] and fluidized bed CDI [28]. In addition, the introduction of ion exchange membranes in CDI (MCDI) allowed not only enhanced desalination capacity and charge efficiency by blocking the co-ion repulsion but also a significantly improved stability by reducing the side reactions such as oxygen reduction and carbon oxidation reactions [1; 29]. In particular, the systematic development in CDI has been accelerated by the theoretical approaches to understand the electrochemical behavior of ions on the porous carbonaceous electrodes. For instance, the modified Donnan model is considered as fundamental knowledge of the ion sorption mechanism on porous electrodes, which explains the desalination performance and the relationship among charges, feed concentrations, and operation voltages [30]. Several theoretical studies discovered that the desalination performance of CDI was directly related to a correlation of the pore size distribution and the ion removal capacity [31]. All of these theoretical investigations provide one of the most important criteria for choosing an appropriate material and for considering the synthetic method of electrodes in CDI. Nevertheless, CDI suffers from the following fundamental limitations: (1) CDI is plausible for treating low concentrated water (ex. Brackish water) compared with other water treating technologies such as the RO. (2) However, such low feed salinity causes the limited ion removal capacity because the electrical double layer (EDL) could not be sufficiently formed in the nanopores of the porous electrodes. (3) CDI in aqueous media is operable only in a narrow range of operational cell voltage (E\({}_{\text{cell}}\) < 1.23 V vs. normal hydrogen electrode, NHE), and thus the choice of electrodes as well as ion removal capacity are severely restricted [1; 2]. Recently, in order to overcome these limitations, multichannel flow stream membrane CDI (MC-MCDI) was proposed as an innovative concept of cell design in CDI. MC-MCDI utilizes two independent flow streams for electrodes and treating water, resulting in a favorable environment for the formation of EDL [15; 32; 33; 34; 35]. Through several systematic studies, MC-MCDI has not only successfully been demonstrated as an alternative of conventional CDI systems, but also utilized in various applications.