The principle of ED technology is to use an electric field to drive the migration of ions; cations move to the cathode through the cation exchange membrane, anions move to the anode through the anion exchange membrane, and finally get fresh water in the fresh water chamber [28] . ED technology has the advantages of low energy consumption and large treatment capacity, but at the same time there, are shortcomings in the treatment of low concentrations of water with low efficiency [29] .

EDI technology is a combination of electrodialysis and ion exchange, in which the pale chamber of the electrodialysis unit is filled with a mixture of anion and cation exchange resins, and deionization is achieved under the action of a direct current electric field.

In the process, the fresh chamber is filled with conductive ion exchange resin, which plays a role in improving the ion transport and, at the same time, achieves the effect of deep deionization. EDI technology overcomes the shortcomings of electrodialysis technology, which is prone to concentration polarization and unsuitable for the treatment of dilute solutions, and at the same time retains the advantages of the depth treatment of ion exchange technology and the continuous operation of electrodialysis technology, but there are high requirements for the quality of feed water, and the membrane stack is prone to scaling.

Capacitive deionization technology relies on the double-layer capacitance effect of electrode materials and a reversible electrochemical adsorption process, applicable to low salinity water desalination and specific ion resource recovery, with low energy consumption, no chemical by-products, and other advantages [30] .

In the 1990s, Farmer et al. [31] discovered the potential of CDI technology for practical applications by using high specific surface area carbon aerogel electrodes in a CDI device, which led to an increase in the adsorption capacity of the CDI device. As the demand for fresh water increases, ordinary carbon-based material electrodes cannot meet the development needs. Most of these materials are microporous materials; this structure will limit the rate of ion diffusion and thus lead to a decline in the performance of the device desalination [32] . To overcome this shortcoming, researchers have developed electrodes that store ions through a Faraday reaction, an electrode known as a Faraday electrode. In 2012, Pasta et al. [33] applied a Faraday material electrode in a capacitive deionization device for the first time, which used Na_2−xMn₅O₁₀ as well as Ag as the electrode for the desalination test. The negative electrode is Na_2−xMn₅O₁₀ to store Na⁺, and the positive electrode is the Ag electrode for storing Cl⁻, but due to the high cost of Ag, it is not suitable for large-scale industrialized application.

In 2014, Lee et al. [34] investigated a hybrid capacitor deionization device, which realized the embedding and detachment of anions and cations through the Faraday reaction principle and the theory of double electric layer, which not only improved the desalination performance but also reduced the cost of the device. In general, CDI technology has the advantages of high efficiency, environmental protection, simple device, and high efficiency of water production, which is the hot spot of electrochemical desalination technology in recent years.

Compared with ED and EDI technology, CDI technology also has the advantages of low energy consumption, a wide range of concentrations of treatment solution, etc. At the same time, its equipment structure is simple and low cost, and with the development of electrode materials, this technology has broader and broader prospects for development.

Improvements on the basis of traditional capacitive deionization technology can be divided into Flow-Through Capacitive Deionization (FT-CDI) [35] , Membrane Capacitive Deionization (MCDI) [36] , Flow Capacitive Deionization (FCDI) [36] , Inverted Capacitive Deionization (I-CDI) [36] , and Hybrid Capacitive Deionization (HCDI) [36] .

1) Flow-Through Capacitive Deionization

Flow-Through Capacitive Deionization was first proposed by Johnson et al. [37] The core design employs a porous structure in which the electrolyte solution flows directly through the interior of the electrode, rather than the traditional CDI structure in which the solution flows on the surface of the electrode. As shown in Figure 1(a) , this design significantly improves ion transfer efficiency and deionization performance. FT-CDI has the advantages of high efficiency, low energy consumption and compact structure. However, in long-term operation, the internal pores of the electrode may be contaminated by organic matter or particles, which may easily lead to electrode clogging and contamination, and the cost of the equipment is high and is not applicable to the treatment of high-concentration salt water [38] .

2) Membrane Capacitive Deionization

As shown in Figure 1(b) , the membrane capacitor deionization technology is based on the traditional capacitor deionization device by adding an ion-exchange membrane between the electrode and the compartment [39] . The ion exchange membrane can selectively carry out charge transfer in the adsorption stage, which improves the charge utilization efficiency and enhances the regeneration efficiency of the electrode. In the regeneration stage, the ion exchange membrane inhibits the re-adsorption of desorbed ions, which guarantees the regeneration of the electrode and provides certain protection for the electrode [40] .

3) Flow-Electrode Capacitive Deionization

Flow Capacitor Deionization, the core feature of which is the use of Flow Electrode instead of the fixed electrode in conventional CDI [41] . As shown in Figure 1(c) , the flow electrode consists of a suspension of electrically conductive particles, such as activated carbon, and an electrolyte solution capable of circulating through the system. This design significantly improves ion adsorption capacity and system continuity for high salinity water treatment and large-scale applications [42] .

4) Inverted Capacitive Deionization

Inverted capacitive deionization is an improved technique for capacitive deionization to avoid anodic carbon oxidation proposed by Gao et al. in 2015 [43] . As shown in Figure 1(d) , this technology reduces the risk of oxidation of the electrodes and improves the regeneration efficiency of the electrodes by periodically reversing the polarity of the electrodes, which enhances the stability of long-term operation of the device. The problem with I-CDI is that its desalination efficiency is limited by the electrode working voltage, and the desalination efficiency is generally [44] .

5) Hybrid Capacitive Deionization

Heterogeneous capacitance deionization technology combines new material electrodes on the basis of traditional CDI device, which mainly provides large adsorption capacity through the double storage mechanism of new material electrodes, mainly including two storage modes of double layer capacitance and pseudo-capacitance. As shown in Figure 1(e) , A Hybrid Capacitive Deionization (HCDI) cell is fundamentally composed of a capacitive electrode, typically made of porous carbon like activated carbon for electrostatic ion adsorption, paired with a Faradaic (battery) electrode, made of materials such as sodium manganese oxide or silver chloride that store ions through reversible electrochemical reactions; these electrodes are often separated by ion-exchange membranes (cation-exchange and/or anion-exchange) to enhance selectivity and prevent crossover, with current collectors (e.g., titanium or graphite foil) attached to each electrode to facilitate electrical connection, all housed within a structure featuring flow channels for saline water passage and providing necessary electrical connections and structural support, thereby combining capacitive and Faradaic processes for improved desalination performance. HCDI has the advantages of large adsorption capacity, high adsorption efficiency, and the ability to operate continuously and stably. However, it has a certain limitation in the range of ions handled by the electrode material. However, this limitation is also an advantage in that the target ions can be selectively removed ( Table 1 ).

Table 1. Comparison of advantages and disadvantages of different types of CDI systems.

1) Transition metal oxide

Transition metal oxides are pseudocapacitive transition metal compounds that remove sodium ions by forming a pseudocapacitance through a Faraday reaction. This material has the advantage of high desalination capacity and good cycling stability. Depending on the morphology of the prepared titanium dioxide, it can be classified into nanoparticles, nanorods/nanofibers and nanotubes, with nanoparticles being the most widely used [45] . Laxman et al. prepared granular TiO₂ by depositing TiO₂ onto a carbon surface, and showed that the structure of this material can improve the specific capacitance and desalination capacity of ordinary TiO₂ [46] . Reduced graphene-coated nanorod-type TiO₂ was prepared by El-Deen et al. The results showed that the incorporation of reduced graphene oxide increased the electrical conductivity of the material and the structure of the coated material increased the specific surface area. The specific capacity was up to 443 F∙g⁻¹ and the desalination capacity was up to 16.4 mg∙g⁻¹ at 10 mV∙s⁻¹, 1 mol·L⁻¹ NaCl [47] .

2) Transition metal sulfide

The most widely studied transition metal sulfide is molybdenum disulfide (MoS₂), a layered material with a graphene-like structure in which the layers are bonded by van der Waals forces and the molybdenum (Mo) and sulfur (S) atoms within the layers are connected by strong covalent bonds [48] . Xing et al. prepared MoS₂ with sizes and lattice spacings of 200 - 600 and 0.61 nm, respectively, by chemical stripping, and used it as a CDI electrode material with a desalination amount of 8.81 mg/g [49] . Wang et al. successfully prepared MoS₂/PDA complexes with flower-like structure by introducing hydrophilic polydopamine (PDA) into the MoS₂ system, which significantly enhanced the surface wettability of the material, and the results showed that its desalting capacity could reach 16.94 mg/g [50] .

3) Prussian Blue Analogs

Prussian blue (PB) is a material with a unique framework structure characterized by advantages including highly reversible redox reactions, excellent electrochemical stability, non-toxicity and low cost. These properties give it potential for a wide range of applications in electrochemical energy storage, catalysis, and sensing.

Guo et al. synthesized single-crystal FeFe (CN) 6 with a cubic structure and a size of about 200 - 300 nm, and further prepared CDI electrode materials by compositing FeFe (CN) 6 with GE, which had a desalination capacity of up to 120.0 mg/g and excellent cycling stability [51] . In addition, PBAs, as an important class of cation-embedded materials, have been widely used in the fields of batteries and supercapacitors due to their advantages such as low preparation cost and low toxicity [52] .

The above is an introduction to common electrode materials for heterocapacitor deionization systems.