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Capacitive deionization (CDI) is a promising technology to satisfy the global need for fresh water, since it can be both economical and sustainable. While two-dimensional transition metal carbides/nitrides (MXenes) exhibit great characteristics for use as CDI electrode materials, their tightly spaced layered structure renders intercalation inefficiency. In this study, the interlayer distance of MXenes is precisely modulated by inserting different quantity of one-dimensional bacterial fibers (BC), forming freestanding MXene/BC composite electrodes. Among the studied samples, MXene/BC-33% electrode with the interlayer spacing of 15.2 Å can achieve an optimized tradeoff among various desalination performance metrics and indicators. The salt adsorption capacity (SAC), the average salt adsorption rate (ASAR), the energy normalized adsorbed salt (ENAS), and the thermodynamic energy efficiency (TEE) of the MXene/BC-33% electrode are improved by 24%, 46%, 13%, and 66% respectively compared with those of pure MXene electrode. While the insertion of BC improves the ion diffusion pathways and facilitates the intercalation kinetics, the desalination performance decreases when the insertion amount of BC exceeds 40%. This is attributed to the overlarge resistance of the composite and the resulting increased energy consumption. This study reveals the desalination performance tradeoffs of MXene-based electrodes with different interlayer distances and also sheds light on the fundamental ion storage mechanisms of intercalation materials in a CDI desalination system.

Another technique for desalination, known as membrane capacitive deionization (MCDI), has been investigated as an alternative. This approach has the potential to lower the voltage that is required, in addition to improving the ability to renew the electrodes. In this study, the desalination effectiveness of capacitive deionization (CDI) was compared to that of MCDI, employing newly produced cellulose acetate ion exchange membranes (IEMs), which were utilized for the very first time in MCDI. As expected, the salt adsorption and charge efficiency of MCDI were shown to be higher than those of CDI. Despite this, the unique electrosorption behavior of the former reveals that ion transport via the IEMs is a crucial rate-controlling step in the desalination process. We monitored the concentration of salt in the CDI and MCDI effluent streams, but we also evaluated the pH of the effluent stream in each of these systems and investigated the factors that may have caused these shifts. The significant change in pH that takes place during one adsorption and desorption cycle in CDI (pH range: 2.3–11.6) may cause problems in feed water that already contains components that are prone to scaling. In the case of MCDI, the fall in pH was only slightly more noticeable. Based on these findings, it appears that CDI and MCDI are promising new desalination techniques that has the potential to be more ecologically friendly and efficient than conventional methods of desalination. MCDI has some advantages over CDI in its higher salt removal efficiency, faster regeneration, and longer lifetime, but it is also more expensive and complex. The best choice for a particular application will depend on the specific requirements.

The demand for water and energy in today’s developing world is enormous and has become the key to the progress of societies. Many methods have been developed to desalinate water, but energy and environmental constraints have slowed or stopped the growth of many. Capacitive Deionization (CDI) is a very new method that uses porous carbon electrodes with significant potential for low energy desalination. This process is known as deionization by applying a very low voltage of 1.2 volts and removing charged ions and molecules. Using capacitive principles in this method, the absorption phenomenon is facilitated, which is known as capacitive deionization. In the capacitive deionization method, unlike other methods in which water is separated from salt, in this technology, salt, which is a smaller part of this compound, is separated from water and salt solution, which in turn causes less energy consumption. With the advancement of science and the introduction of new porous materials, the use of this method of deionization has increased greatly. Due to the limitations of other methods of desalination, this method has been very popular among researchers and the water desalination industry and needs more scientific research to become more commercial.

Microfluidic techniques have emerged as promising, efficient, cost-effective, and environmentally friendly desalination solutions. By utilizing fluid dynamics at the microscale, these techniques offer precise control over chemical, biological, and physical processes, presenting advantages such as reduced energy consumption, miniaturization, portability, and enhanced process control. A significant challenge in scaling microfluidic desalination for macro applications is the disparity in flow rates. Current devices operate at microliters per minute, while practical applications require liters daily. Solutions involve integrating multiple units on a single chip and developing stackable chip designs. Innovative designs, such as 3D microfluidic chips, have shown promise in enhancing scalability. Fouling, particularly in seawater environments, presents another major challenge. Addressing fouling through advanced materials, including graphene and nanomaterials, is critical to improving the efficiency and longevity of devices. Advances in microfluidic device fabrication, such as photo-patterned hydrogel membranes and 3D printing, have increased device complexity and affordability. Hybrid fabrication approaches could further enhance membrane quality and efficiency. Energy consumption remains a concern, necessitating research into more energy-efficient designs and integration with renewable energy sources. This paper explores various electrochemical-based microfluidic desalination methods, including dialysis/electrodialysis, capacitive deionization (CDI)/electrochemical capacitive deionization (ECDI), ion concentration polarization (ICP), and electrochemical desalination (ECD).

The effective desalination and purification devices for seawater/ brackish water treatment are crucial in sustainable progress. Techniques that render high salt removal efficiency and water purification ability at low applied potentials play a central role in sustainable water supplies. One of them is capacitive deionization (CDI) which has drawn significant consideration as a promising deionization technology since the last decade. Desalination efficiency profoundly depends on the utilized electrode material. The most widely used CDI electrodes are carbons due to their cost effectiveness and good stability. However, to acquire high electrosorption capacity, extensive researches are reported with modified carbon materials. CDI cell architectures are equally important for practical high salt removal performance. This review focuses on carbon materials in CDI along with other emerging trends in diverse carbon types, e.g., carbon nanotubes and their composites. Various architectures reported in the literature to improve desalination efficiency are also included here.

HighlightsA broad overview of MXenes and MXene-based nanomaterials in desalination is presented.Recent advancement in the synthesis of MXenes for applications in desalination is critically evaluated. Salt removal mechanisms and regeneration capability of MXenes are appraised.Current challenges and future prospect of MXenes in desalination are highlighted. Research directions are provided to safeguard the applications of MXenes in future desalination.MXenes, novel 2D transition metal carbides, have emerged as wonderful nanomaterials and a superlative contestant for a host of applications. The tremendous characteristics of MXenes, i.e., high surface area, high metallic conductivity, ease of functionalization, biocompatibility, activated metallic hydroxide sites, and hydrophilicity, make them the best aspirant for applications in energy storage, catalysis, sensors, electronics, and environmental remediation. Due to their exceptional physicochemical properties and multifarious chemical compositions, MXenes have gained considerable attention for applications in water treatment and desalination in recent times. It is vital to understand the current status of MXene applications in desalination in order to define the roadmap for the development of MXene-based materials and endorse their practical applications in the future. This paper critically reviews the recent advancement in the synthesis of MXenes and MXene-based composites for applications in desalination. The desalination potential of MXenes is portrayed in detail with a focus on ion-sieving membranes, capacitive deionization, and solar desalination. The ion removal mechanism and regeneration ability of MXenes are also summarized to get insight into the process. The key challenges and issues associated with the synthesis and applications of MXenes and MXene-based composites in desalination are highlighted. Lastly, research directions are provided to guarantee the synthesis and applications of MXenes in a more effective way. This review may provide an insight into the applications of MXenes for water desalination in the future.

The recent advances in chloride‐ion capturing electrodes for capacitive deionization (CDI) are limited by the capacity, rate, and stability of desalination. This work introduces Ti3C2Tx/Ag synthesized via a facile oxidation‐reduction method and then uses it as an anode for chloride‐ion capture in CDI. Silver nanoparticles are formed successfully and uniformly distributed with the layered‐structure of Ti3C2Tx. All Ti3C2Tx/Ag samples are hydrophilic, which is beneficial for water desalination. Ti3C2Tx/Ag samples with a low charge transfer resistance exhibit both pseudocapacitive and battery behaviors. Herein, the Ti3C2Tx/Ag electrode with a reaction time of 3 h exhibits excellent desalination performance with a capacity of 135 mg Cl− g−1 at 20 mA g−1 in a 10 × 10−3 m NaCl solution. Furthermore, low energy consumption of 0.42 kWh kg−1 Cl− and a desalination rate of 1.5 mg Cl− g−1 min−1 at 50 mA g−1 is achieved. The Ti3C2Tx/Ag system exhibits fast rate capability, high desalination capacity, low energy consumption, and excellent cyclability, which can be ascribed to the synergistic effect between the battery and pseudocapacitive behaviors of the Ti3C2Tx/Ag hybrid material. This work provides fundamental insight into the coupling of battery and pseudocapacitive behaviors during Cl− capture for electrochemical desalination. A Ti3C2Tx/Ag hybrid is prepared and exhibits exceptional electrochemical desalination rate and capacity, which can be ascribed to the synergistic effect between the battery (Ag nanoparticles) and pseudocapacitive behaviors (layer‐structure Ti3C2Tx). MXene serves a dual role as an intercalation electrode and facile, electron‐conductive network to capitalize on the Ag/AgCl conversion reaction.

Several harmful or valuable ionic species present in seawater, brackish water, and wastewater are amphoteric, weak acids or weak bases, and, thus, their properties depend on local water pH. Effective removal of these species can be challenging for conventional membrane technologies, necessitating chemical dosing of the feedwater to adjust pH. A prominent example is boron, which is considered toxic in high concentrations and often requires additional membrane passes to remove during seawater desalination. Capacitive deionization (CDI) is an emerging membraneless technique for water treatment and desalination, based on electrosorption of salt ions into charging microporous electrodes. CDI cells show strong internally generated pH variations during operation, and, thus, CDI can potentially remove pH-dependent species without chemical dosing. However, development of this technique is inhibited by the complexities inherent to the coupling of pH dynamics and ion properties in a charging CDI cell. Here, we present a theoretical framework predicting the electrosorption of pH-dependent species in flow-through electrode CDI cells. We demonstrate that such a model enables insight into factors affecting species electrosorption and conclude that important design rules for such systems are highly counterintuitive. For example, we show both theoretically and experimentally that for boron removal, the anode should be placed upstream and the cathode downstream, an electrode order that runs counter to the accepted wisdom in the CDI field. Overall, we show that to achieve target separations relying on coupled, complex phenomena, such as in the removal of amphoteric species, a theoretical CDI model is essential.

Capacitive deionization (CDI) is an emerging desalination technology for effective removal of ionic species from aqueous solutions. Compared to conventional CDI, which is based on carbon electrodes and struggles with high salinity streams due to a limited salt removal capacity by ion electrosorption and excessive co‐ion expulsion, the emerging Faradaic electrodes provide unique opportunities to upgrade the CDI performance, i.e., achieving much higher salt removal capacities and energy‐efficient desalination for high salinity streams, due to the Faradaic reaction for ion capture. This article presents a comprehensive overview on the current developments of Faradaic electrode materials for CDI. Here, the fundamentals of Faradaic electrode‐based CDI are first introduced in detail, including novel CDI cell architectures, key CDI performance metrics, ion capture mechanisms, and the design principles of Faradaic electrode materials. Three main categories of Faradaic electrode materials are summarized and discussed regarding their crystal structure, physicochemical characteristics, and desalination performance. In particular, the ion capture mechanisms in Faradaic electrode materials are highlighted to obtain a better understanding of the CDI process. Moreover, novel tailored applications, including selective ion removal and contaminant removal, are specifically introduced. Finally, the remaining challenges and research directions are also outlined to provide guidelines for future research. Faradaic electrodes open a new era for capacitive deionization (CDI) by providing enhanced salt removal capacity and energy‐efficient desalination for high salinity streams. Advances in the development of Faradaic electrodes for CDI are comprehensively summarized, spanning from cell architectures, key performance metrics, advanced materials, and related ion capture mechanisms, to tailored applications as well as main challenges and perspectives.

Constructing faradaic electrode with superior desalination performance is important for expanding the applications of capacitive deionization (CDI). Herein, a simple one‐step alkalized treatment for in situ synthesis of 1D TiO2 nanowires on the surface of 2D Ti3C2 nanosheets, forming a Ti3C2‐MXene partially derived hierarchical 1D/2D TiO2/Ti3C2 heterostructure as the cathode electrode is reported. Cross‐linked TiO2 nanowires on the surface help avoid layer stacking while acting as the protective layer against contact of internal Ti3C2 with dissolved oxygen in water. The inner Ti3C2 MXene nanosheets cross over the TiO2 nanowires can provide abundant active adsorption sites and short ion/electron diffusion pathways. . Density functional theory calculations demonstrated that Ti3C2 can consecutively inject electrons into TiO2, indicating the high electrochemical activity of the TiO2/Ti3C2. Benefiting from the 1D/2D hierarchical structure and synergistic effect of TiO2 and Ti3C2, TiO2/Ti3C2 heterostructure presents a favorable hybrid CDI performance, with a superior desalination capacity (75.62 mg g−1), fast salt adsorption rate (1.3 mg g−1 min−1), and satisfactory cycling stability, which is better than that of most published MXene‐based electrodes. This study provides a feasible partial derivative strategy for construction of a hierarchical 1D/2D heterostructure to overcome the restrictions of 2D MXene nanosheets in CDI. Cross‐linked 1D TiO2 nanowires on the surface of 2D Ti3C2 MXene nanosheets, which contribute to avoiding layer stacking, simultaneously act as the protective layer to prevent access to the dissolved oxygen in water and hinder the oxidation of Ti3C2 in the interior. TiO2/Ti3C2 heterostructure presents a favorable hybrid capacitive deionization performance, with a superior desalination capacity, fast salt adsorption rate, and satisfactory cycling stability.