## 2 Capacitive Deionization (CDI)
### Advancement of Capacitive Deionization (CDI) and Its Limitations  
CDI is developed in various types of electrodes and architectures, as depicted in Figure 3. First, electrodes can be categorized by two ion removal mechanisms: The capacitive and faradaic electrodes. In the category of capacitive electrodes, the EDL is formed on the surface of the electrode by applying electrical energy. Therefore, without charge transfer reactions, the ions are preferentially ad/desorbed into the EDL formed at the interface of the electrode surface and the water stream [4]. Porous carbonaceous materials like activated carbon, activated carbon fiber, carbon aerogels, carbon nanotube, and graphene have been utilized as capacitive electrodes, and they are widely used as electrodes in conventional CDI [11]. In the category of faradaic electrodes, the ions favorably interacted with the surface and lattice of the electrodes, resulting in the insertion of the ions into the lattice structures by the changing oxidation state or redox reactions. Accordingly, their ion removal capacity can be much larger than that of capacitive electrodes [43]. Battery and battery-like materials such as sodium manganese oxide, Prussian blue analogs, 2D materials (e.g., Mxene, titanium disulfide, and molybdenum disulfide), and silver/silver chloride have been utilized as faradaic electrodes in CDI [4; 18; 20]. Second, the classification of electrodes originates from the mobility of the electrodes. Instead of static-electrodes, flow-able conductive particles have been employed in CDI cell architectures, known as flow-electrode CDI (FCDI) [1; 27]. This concept is similar to the slurry-based energy storage systems, such as electrochemical flow capacitors and semi-solid lithium-ion batteries. The FCDI exhibits two major unique features and functionalities compared with the static-electrode CDI cell architecture: (1) FCDI is able to desalinate feed water continuously because the charged activated carbon particles can be sustainably discharged downstream of the cell, and (2) a continuous supply of discharged carbon particles leads to a higher ion adsorption capacitance than static-electrode CDI. Therefore, FCDI can desalinate highly concentrated feed water. The introduction of ion exchange membranes not only allows a significant enhancement of desalination performance in conventional CDI but also leads to various types of cell architectures such as hybrid CDI (HCDI) [16], cation intercalation desalination (CID) [25], multichannel flow stream membrane CDI (MC-MCDI) [15], and battery desalination combined with the oxidation processes [44]. Due to the permselective characteristics of ion exchange membranes, membrane-involved CDI reveals high charge efficiency and enhanced salt removal capacity. By reducing co-ion repulsion and ion swapping (Figure 4), additional ions could be captured to balance the charge neutrality in the macropores of electrodes or at the interface of membranes and electrodes [30]. Figure 2: Conditions for flowing the feed solutions: Semi-batch, single-pass, and batch modes. Despite of numerous efforts on the advancement of electrode materials and architectures, CDI has crucial limitations for its industrial applications as listed below:  
* **Discontinuous desalination process** Conventional static-electrode CDI is operated in a sequential charging and discharging steps, resulting in the production of fresh and concentrated water, respectively. This intermittent operation causes discontinuous production of clean water [45]. * **Low concentration of the feed water** For most of static-electrode CDI, the energy consumption strongly depends on the salt concentration of the feed water because CDI desalinates ions from the feed water, while other desalination technologies  
Figure 4: Ion removal mechanism of CDI with charge compensation on pores in the porous carbon electrode: (**A**) Ideal ion adsorption, (**B**) presence of the ion swapping, and (**C**) presence of the co-ion repulsion.