## 2 Electro-Membrane Technologies: Current Status and Challenges  
In this section, the previously mentioned electro-membrane technologies will be presented, and their main characteristics will be illustrated in Figure 1 and Table 2, respectively. As previously mentioned, electro-membrane processes are electrically-driven processes that require IEMs in order to achieve a selective transport of ions between two confronted solutions. Thus, IEMs are semi-permeable membranes that allow the flow of certain ions while blocking others. Cationic membranes (CM) are negatively charged IEMs; therefore, cations are able to flow across them while blocking anions. Otherwise, anionic membranes (AM) are positively charged IEMs that allow anion flow, whereas cations are blocked. Additionally, bipolar membranes, constituted by a cation and an anion exchange layer, generate the dissociation of water when an energy field is applied. Moreover, an electro-membrane stack also requires electrodes which are solid electric conductors that are in charge of carrying the electrical current to the solutions involved in the process. Reduction and oxidation reactions take place at the cathode and anode electrodes, respectively. Whether an electrode operates as a cathode or anode depends on the operation mode of the cell, galvanically (i.e., producing energy) or as an electrolytic cell (i.e., consuming energy), switching from cathode to anode and vice versa if the operation mode is changed. As shown in Figure 1a, in ED, when an electric potential is applied to a saline solution, a selective transport of ions through AM and CM occurs, producing a low salinity solution, or dilute, and a high salinity solution, or concentrate. ED development on an industrial scale started more than 50 years ago. The increase in ED implementation is closely related to the advance in IEM properties, as deeply detailed in several studies [18]. In this way, taking advantage of the reduction in chemical reagents required compared to other processes, a number of works have attempted to optimize ED performance for various applications in the chemical, food, and drug industry as well as wastewater treatment or other environmental applications [19; 20; 21]. When ED is used in the desalination industry for freshwater production, it generates both freshwater (dilute) and brines (concentrate). The ED technology adapts to power fluctuations better than RO; thus, the integration of ED with RE has shown a better performance [15; 22], being a promising alternative if it is integrated with solar PV energy (ED-PV) [13]. Several ED-PV pilot-scale plants have been built [23]. The main current drawbacks and challenges for ED development are related to the intensive use of energy required, the development of antifouling and anti-scaling strategies, and the high costs of IEM [13; 23; 24]. As its name suggests, RED is the opposite process of electrodialysis (ED). So, in RED (Figure 1b), when a low salinity solution, such as river water or wastewater, and a high salinity solution, such as brine, are faced, the difference in concentrations generates ion flux between the different compartments, which generates an electric current between the anode and the cathode [25]. The magnitude of the electric current obtained is dependent on the difference of the concentrations of the input streams and the number of cell pairs (repetitive units) in the stack, among other parameters. Thus, SGE can be harvested by means of RED obtaining a continuous source RE which has great advantages compared to traditional REs that are characterized by their intermittence. Moreover, RED presents insignificant Greenhouse Gases emissions compared to other well-established RE sources such as solar PV, wind, or hydropower [26]. Even though there is financial support for big projects such as the pilot plant in Afshuitdijk site (Netherlands) [26; 27], RED presents many limitations that need to be overcome in order to be marketable.