## 1 Introduction  
The availability of freshwater resources for the survival of humanity on the planet has become a critical global issue with the increase of world population and rapid industrial development, as well as the increasingly serious water pollution and climate changes. At present, nearly 750 million people worldwide have limited access to clean drinking water, and this number of people is forecasted to rise in the upcoming years. The United Nations World Water Development report (2021) stated that the world would tackle a 40% shortage of freshwater resources by 2030 unless the water resource management is improved [1]. In this scenario, considering that seawater represents approximately 97% of the total available water resources on the Earth, the desalination process becomes one of the most promising ways for many societies to secure fresh water and to face global water scarcity [2]. Presently, reverse osmosis (RO) is the most widely adopted water desalination technique, with the largest capacity installed and operating in the world [3]. However, since it operates under high pressures (50-70 bar), generates large amounts of brine containing chemicals, and greatly consumes electricity, it has a significant impact on the environment and is not suitable for use in remote areas [3]. Several potential alternative desalination techniqueshave been developed to address these issues. An in-depth discussion on the benefits they offer over conventional RO and the challenges faced by these emerging techniques to become competitive with it can be found in ref. [3]. Among them, capacitive deionization (CDI) appears to be one of the most appealing because of the easy regeneration of the electrodes, operation under ambient temperatures, ambient pressures, and low voltage (0.8-2.0 V), and suitability for remote areas [4; 5; 6]. CDI is based on a complementary approach with respect to RO: it removes salt from water rather than water from salt [3]. Salt ions are electro-adsorbed inside porous electrodes under the action of the small applied voltage, which generates an electrical double-layer (EDL) on their surface [7; 8]. The electrodes are easily regenerated by reversing, reducing, or short-circuiting the voltage of the cell [9]. Moreover, during the discharge, up to 83% of energy stored in the EDL can be recovered [3] and reused, with a reduction of the energy consumption of the process [5; 6]. However, at the current stage of development, CDI is competitive with respect to RO only in the treatment of brackish water (salinity \(<\) 10 g L\({}^{-1}\)), with an electrical energy consumption below 1 versus 3-4 kWh m\({}^{-3}\) potable water [10; 11]. Conversely, in the desalination of seawater having a higher salinity (37 g L\({}^{-1}\)), CDI is not yet a mature technology for commercialization, as its energy consumption is by far higher than RO [3; 11; 12]. For a comparison based on a theoretical study between the energetic performance of CDI and RO as a function of feed salinity, water recovery, salt rejection, and average water flux, see ref. [11]. Another relevant aspect is the economic feasibility of CDI. It depends critically on the reduction of material costs through the increase in their life [12]. Possessing suitable properties for electro-sorption [13; 14; 15; 16], porous carbons (such as activated carbons [17], graphene [18], carbon aerogels [19], carbon nanotubes [20], carbon nanofibers [21], and combinations of them [22; 23]) are to date the most used electrode materials. Nevertheless, modifying or replacing them could be the key to improving CDI performance [3]. In fact, there are many studies in which their surface is functionalized to increase their wettability and provide additional adsorption sites [24]. Moreover, the addition of metal sulfide or oxides is another strategy to enhance the desalination performance of these carbon-based CDI electrodes by a charge transfer mechanism [3; 9; 25].