Ultrahigh-Desalination-Capacity Dual-Ion Electrochemical Deionization Device Based on Na3V2(PO4)3@C-AgCl Electrodes

A review of chloride ions removal from high chloride industrial wastewater: Sources, hazards, and mechanisms

To reduce metal pipe corrosion, improve product quality, and meet zero liquid discharge (ZLD) criteria, managing chloride ion concentrations in industrial wastewaters from metallurgical and chemical sectors has become increasingly important. This review provides detailed information on the sources, concentration levels, and deleterious effects of chloride ions in representative industrial wastewaters, and also summarizes and discusses various chloride ion removal techniques, including precipitation, ion exchange, physical separation, and advanced oxidation (AOPs). Among these, AOPs are particularly promising due to their ability to couple with other technologies and the diversity of their auxiliary technologies. The development of dechlorination electrode materials by electro-adsorption (CDI) can be inspired by the electrode materials used in chloride ion battery (CIB). This review also provides insights into exploring the effective combination of multiple chloride removal mechanisms, as well as the development of environmentally friendly composite materials. This review provides a theoretical basis and development direction for the effective treatment and secondary utilization of chlorine-containing industrial wastewater in the future.

A mini review on metal-organic framework-based electrode materials for capacitive deionization

Capacitive deionization (CDI) is an electrochemical method of extracting ions from solution at potentials below electrolysis. It has various applications ranging from water remediation and desalination to heavy metal removal and selective resource recovery. A CDI device applies an electrical charge across two porous electrodes to attract and remove ions without producing waste products. It is generally considered environmentally friendly and promising for sustainability, yet ion removal efficiency still falls short of more established filtration methods. Commercially available activated carbon is typically used for CDI, and its ion adsorption capacity is low at approximately 20-30 mg g-1. Recently, much interest has been in the highly porous and well-structured family of materials known as metal-organic frameworks (MOFs). Most MOFs are poor conductors of electricity and cannot be directly used to make electrodes. A common workaround is to pyrolyze the MOF to convert its organic components to carbon while maintaining its underlying microstructure. However, most MOF-derived materials only retain partial microstructure after pyrolysis and cannot inherit the robust porosity of the parent MOFs. This review provides a systematic breakdown of structure-performance relationships between a MOF-derived material and its CDI performance based on recent works. This review also serves as a starting point for researchers interested in developing MOF-derived materials for CDI applications.

Anion chemistry in energy storage devices

Anions serve as an essential component of electrolytes, whose effects have long been ignored. However, since the 2010s, we have seen a considerable increase of anion chemistry research in a range of energy storage devices, and it is now understood that anions can be well tuned to effectively improve the electrochemical performance of such devices in many aspects. In this Review, we discuss the roles of anion chemistry across various energy storage devices and clarify the correlations between anion properties and their performance indexes. We highlight the effects of anions on surface and interface chemistry, mass transfer kinetics and solvation sheath structure. Finally, we conclude with a perspective on the challenges and opportunities of anion chemistry for enhancing specific capacity, output voltage, cycling stability and anti-self-discharge ability of energy storage devices.

Soft-hard interface design in super-elastic conductive polymer hydrogel containing Prussian blue analogues to enable highly efficient electrochemical deionization

The poor cycling stability of faradaic materials owing to volume expansion and stress concentration during faradaic processes limits their use in large-scale electrochemical deionization (ECDI) applications. Herein, we developed a "soft-hard" interface by introducing conducting polymer hydrogels (CPHs), that is, polyvinyl alcohol/polypyrrole (PVA/PPy), to support the uniform distribution of Prussian blue analogues (e.g., copper hexacyanoferrate (CuHCF)). In this design, the soft buffer layer of the hydrogel effectively alleviates the stress concentration of CuHCF during the ion-intercalation process, and the conductive skeleton of the hydrogel provides charge-transfer pathways for the electrochemical process. Notably, the engineered CuHCF@PVA/PPy demonstrates an excellent salt-adsorption capacity of 22.7 mg g-1 at 10 mA g-1, fast salt-removal rate of 1.68 mg g-1 min-1 at 100 mA g-1, and low energy consumption of 0.49 kW h kg-1. More importantly, the material could maintain cycling stability with 90% capacity retention after 100 cycles, which is in good agreement with in situ X-ray diffraction tests and finite element simulations. This study provides a simple strategy to construct three-dimensional conductive polymer hydrogel structures to improve the desalination capacity and cycling stability of faradaic materials with universality and scalability, which promotes the development of high-performance electrodes for ECDI.

Local Electric Field Induced by Atomic-Level Donor-Acceptor Couple of O Vacancies and Mn Atoms Enables Efficient Hybrid Capacitive Deionization

Transition metal oxides suffer from slow salt removal rate (SRR) due to inferior ions diffusion ability in hybrid capacitive deionization (HCDI). Local electric field (LEF) can efficiently improve the ions diffusion kinetics in thin electrodes for electrochemical energy storage. Nevertheless, it is still a challenge to facilitate the ions diffusion in bulk electrodes with high loading mass for HCDI. Herein, this work delicately constructs a LEF via engineering atomic-level donor (O vacancies)-acceptor (Mn atoms) couples, which significantly facilitates the ions diffusion and then enables a high-performance HCDI. The LEF boosts an extended accelerated ions diffusion channel at the particle surface and interparticle space, resulting in both remarkably enhanced SRR and salt removal capacity. Convincingly, the theoretical calculations demonstrate that electron-enriched Mn atoms center coupled with an electron-depleted O vacancies center is formed due to the electron back-donation from O vacancies to adjacent Mn centers. The resulted LEF efficiently reduce the ions diffusion energy barrier. This work sheds light on the effect of atomic-level LEF on improving ions diffusion kinetics at high loading mass application and paves the way for the design of transition metal oxides toward high-performance HCDI applications.

A Novel Dual-Ion Capacitive Deionization System Design with Ultrahigh Desalination Performance

Capacitive deionization is an emerging desalination technology with mild operation conditions and high energy efficiency. However, its application is limited due to the low deionization capacity of traditional capacitive electrodes. Herein, we report a novel dual-ion capacitive deionization system with a lithium-ion battery cathode LiMn2O4/C and a sodium-ion battery anode NaTi2(PO4)3/C. Lithium ions could enhance the charge transfer during CDI desalination, while NaTi2(PO4)3/C provided direct intercalation sites for sodium ions. The electrochemical capacities of the battery electrodes fitted well, which was favorable for the optimization of the desalination capacity. The low potential of the redox couple Ti3+/Ti4+ (−0.8 V versus Ag/AgCl) and intercalation/deintercalation behaviors of sodium ions that suppressed hydrogen evolution could enlarge the voltage window of the CDI process to 1.8 V. The novel CDI cell achieved an ultrahigh desalination capacity of 140.03 mg·g−1 at 1.8 V with an initial salinity of 20 mM, revealing a new direction for the CDI performance enhancement.

Structural design enabled a hypotoxic Na3.36FeV(PO4)3 cathode with ultra-fast and ultra-long sodium storage

Among various cathode materials for sodium-ion batteries, Na₃V₂(PO₄)₃ has attracted much attention due to its outstanding electrochemical performance. However, the toxicity and expensive price of vanadium limit its practical application. Therefore, the substitution of vanadium with nontoxic and inexpensive transition metal elements is significant. We select the earth-abundant iron element to partially replace the vanadium element, and successfully synthesize Na_3.36FeV(PO₄)₃ with a Na superionic conductor structure. Furthermore, a Na_3.36FeV(PO₄)₃ cathode with an optimal carbon content can deliver an initial capacity of 97.6 mA h g^-1 at 0.5C with a high capacity retention of 96.4% after 200 cycles. In addition, it also delivers an initial capacity of 90.4 mA h g^-1 at 10C, and a capacity retention of 80% can be obtained after 5000 cycles. We also found that the lack of sodium in the material can be compensated by an electrochemical reaction. Furthermore, the in situ X-ray diffraction analysis reveals that the sodium storage process follows a pseudo-solid-solution reaction mechanism and the volume change ratio is less than 3% during charging/discharging. In order to study the practical application capability of Na_3.36FeV(PO₄)₃, we assemble the pre-activated cathode and a hard carbon anode into a full cell, which exhibits high initial discharge capacities of 103 and 91.3 mA h g^-1 at 0.5C and 10C, respectively. This work will provide new insights into the structural engineering of low-toxicity and ultralong-life NASICON-type cathode materials for SIBs.

Electrochemical Methods for Water Purification, Ion Separations, and Energy Conversion

Agricultural development, extensive industrialization, and rapid growth of the global population have inadvertently been accompanied by environmental pollution. Water pollution is exacerbated by the decreasing ability of traditional treatment methods to comply with tightening environmental standards. This review provides a comprehensive description of the principles and applications of electrochemical methods for water purification, ion separations, and energy conversion. Electrochemical methods have attractive features such as compact size, chemical selectivity, broad applicability, and reduced generation of secondary waste. Perhaps the greatest advantage of electrochemical methods, however, is that they remove contaminants directly from the water, while other technologies extract the water from the contaminants, which enables efficient removal of trace pollutants. The review begins with an overview of conventional electrochemical methods, which drive chemical or physical transformations via Faradaic reactions at electrodes, and proceeds to a detailed examination of the two primary mechanisms by which contaminants are separated in nondestructive electrochemical processes, namely electrokinetics and electrosorption. In these sections, special attention is given to emerging methods, such as shock electrodialysis and Faradaic electrosorption. Given the importance of generating clean, renewable energy, which may sometimes be combined with water purification, the review also discusses inverse methods of electrochemical energy conversion based on reverse electrosorption, electrowetting, and electrokinetic phenomena. The review concludes with a discussion of technology comparisons, remaining challenges, and potential innovations for the field such as process intensification and technoeconomic optimization.

Efficient, Selective Sodium and Lithium Removal by Faradaic Deionization Using Symmetric Sodium Titanium Vanadium Phosphate Intercalation Electrodes

NASICON (sodium superionic conductor) materials are promising host compounds for the reversible capture of Na⁺ ions, finding prior application in batteries as solid-state electrolytes and cathodes/anodes. Given their affinity for Na⁺ ions, these materials can be used in Faradaic deionization (FDI) for the selective removal of sodium over other competing ions. Here, we investigate the selective removal of sodium over other alkali and alkaline-earth metal cations from aqueous electrolytes when using a NASICON-based mixed Ti-V phase as an intercalation electrode, namely, sodium titanium vanadium phosphate (NTVP). Galvanostatic cycling experiments in three-electrode cells with electrolytes containing Na⁺, K⁺, Mg²⁺, Ca²⁺, and Li⁺ reveal that only Na⁺ and Li⁺ can intercalate into the NTVP crystal structure, while other cations show capacitive response, leading to a material-intrinsic selectivity factor of 56 for Na⁺ over K⁺, Mg²⁺, and Ca²⁺. Furthermore, electrochemical titration experiments together with modeling show that an intercalation mechanism with a limited miscibility gap for Na⁺ in NTVP mitigates the state-of-charge gradients to which phase-separating intercalation electrodes are prone when operated under electrolyte flow. NTVP electrodes are then incorporated into an FDI cell with automated fluid recirculation to demonstrate up to 94% removal of sodium in streams with competing alkali/alkaline-earth cations with 10-fold higher concentration, showing process selectivity factors of 3-6 for Na⁺ over cations other than Li⁺. Decreasing the current density can improve selectivity up to 25% and reduce energy consumption by as much as ∼50%, depending on the competing ion. The results also indicate the utility of NTVP for selective lithium recovery.

Asymmetric and Symmetric Redox Flow Batteries for Energy-Efficient, High-Recovery Water Desalination

Electrochemical separation offers an energy-efficient means to desalinate brackish water, a relatively untapped but increasingly utilized water source for freshwater supply. Several electrochemical techniques are being developed to enable low-energy desalination combined with energy storage. We report a new approach that produced a peak power density of 6.0 mW cm^-2 from the energy stored in iron cyanide (Fe-CN) and iron citrate (Fe-Cit) redox couples during water desalination, using asymmetric redox flow batteries (RFBs). Desalination and the charging of the redox couples occurred in a four-channel RFB cell. The stored energy was extracted in a two-channel RFB cell. Desalination of model brackish water (2.9 g L^-1) to freshwater (0.5 g L^-1) was also studied in a symmetric system using the environmentally benign Fe-Cit. The process was characterized by low energy consumption (0.56 kW h m^-3), high productivity (41.1 L freshwater m^-2 area h^-1, representing practical operating conditions for brackish water desalination), and high water recovery (91% product-to-intake water ratio, addressing the environmental and economic challenges of brine disposal). The low cell voltage (<0.5 V) required in the reported system is ideally suited for developing modular desalination systems powered by renewables, including solar energy. Collectively, water-based RFBs for desalination and power production would lead to sustainable water-energy infrastructure.

Recent Advances in Faradic Electrochemical Deionization: System Architectures versus Electrode Materials

Capacitive deionization (CDI) is an energy-efficient desalination technique. However, the maximum desalination capacity of conventional carbon-based CDI systems is approximately 20 mg g^-1, which is too low for practical applications. Therefore, the focus of research on CDI has shifted to the development of faradic electrochemical deionization systems using electrodes based on faradic materials which have a significantly higher ion-storage capacity than carbon-based electrodes. In addition to the common symmetrical CDI system, there has also been extensive research on innovative systems to maximize the performance of faradic electrode materials. Research has focused primarily on faradic reactions and faradic electrode materials. However, the correlation between faradic electrode materials and the various electrochemical deionization system architectures, i.e., hybrid capacitive deionization, rocking-chair capacitive deionization, and dual-ion intercalation electrochemical desalination, remains relatively unexplored. This has inhibited the design of specific faradic electrode materials based on the characteristics of individual faradic electrochemical desalination systems. In this review, we have characterized faradic electrode materials based on both their material category and the electrochemical desalination system in which they were utilized. We expect that the detailed analysis of the properties, advantages, and challenges of the individual systems will establish a fundamental correlation between CDI systems and electrode materials that will facilitate future developments in this field.

Intrinsic Pseudocapacitive Affinity in Manganese Spinel Ferrite Nanospheres for High-Performance Selective Capacitive Removal of Ca2+ and Mg2

Pseudocapacitor-type hybrid capacitive deionization (PHCDI) has been developed extensively for deionization, which enables to address the worldwide freshwater shortage. However, the exploitation of selective hardness ion removal in resourceful hard water via the intrinsic pseudocapacitive effect, rather than the ion-sieving or ion-swapping effect based on the electric double layer (EDL) of porous carbon, is basically blank and urgent. Herein, manganese spinel ferrite (MFO) nanospheres were successfully fabricated by one-step solvothermal synthesis and used as the cathode for PHCDI assembled with commercial activated carbon. The MFO electrode exhibited prominent capacities of 534.6 μmol g^-1 (CaCl₂) and 980.4 μmol g^-1 (MgCl₂), outperforming those of other materials ever reported in the literature. Fascinatingly, systematic investigation of binary and ternary ion solutions showed the high electro-affinity of hardness ions (Ca²⁺ and Mg²⁺) toward Na⁺, especially the leading affinity of Mg²⁺, in which the superhigh hardness selectivity of 34.76 was achieved in the ternary solution with a molar ratio of Na-Ca-Mg as 20:1:1. Unexpectedly, the ion-swapping trace in a multi-ion environment was also first detected in our pseudocapacitive-based electrode. The electrochemical response in unary and multiple electrolytes disclosed that the unique pseudocapacitive affinity based on the cation (de)intercalation-redox mechanism was from the synergistic effect of the relative redox potential, ionic radius, and valence, in which the redox potential was the dominant factor.

Achieving High-Quality Freshwater from a Self-Sustainable Integrated Solar Redox-Flow Desalination Device

Solar-assisted electrochemical desalination has offered a new energy-water nexus technology for sustainable development in recent studies. However, only a few reports have demonstrated insufficient photocurrent, a low salt removal rate, and poor stability. In this study, a high-quality freshwater level of 5-10 ppm (from an initial feed of 10 000 ppm), an enhanced salt removal rate (217.8 µg cm^-2 min^-1 of NaCl), and improved cycling and long-term stability are achieved by integrating dye-sensitized solar cells (DSSCs) and redox-flow desalination (RFD) under light irradiation without additional electrical energy consumption. The DSSC redox electrolyte (I^- /I₃^- ) is circulated between the photoanode (N719/TiO₂ ) and intermediate electrode (graphite paper). Two DSSCs in parallel or series connections are directly coupled to the RFD device. Overall, this hybrid system can be used to boost photo electrochemical desalination technology. The energy-water nexus technology will open a new route for dual-role devices with photodesalination functions without energy consumption and solar-to-electricity generation.

Electrochemical Driven Phase Segregation Enabled Dual-Ion Removal Battery Deionization Electrode

Battery deionization (BDI) offers a powerful platform for integrating water treatment and energy conversion. Exploring novel BDI electrode materials with high energy storage capacity and high efficiency for both cations and anions removal is the key to advancing the BDI technique. Herein, we report the first BDI electrode material capable of simultaneously removing Cl^- (58.4 mg g^-1) and Na⁺ (8.7 mg g^-1) in water with a reversible capacity of 160 mAh g^-1. In situ powder X-ray diffraction (PXRD) unravels that the dual-ion removal capability is attributed to a novel reversible electrochemical driven phase segregation reaction mechanism between NaBi₃O₄Cl₂ and the in situ formed metallic Bi. The unique dual-ion storage capability demonstrated with the NaBi₃O₄Cl₂ electrode indicates that exploring electrochemical reversible phase segregation electrode material holds great promise for advancing the BDI electrode for future desalination techniques and aqueous rechargeable battery systems.

Bismuth Nanoparticle-Embedded Porous Carbon Frameworks as a High-Rate Chloride Storage Electrode for Water Desalination

Capacitive deionization (CDI) is a promising cost-effective and low energy consumption technology for water desalination. However, most of the previous works focus on only one side of the CDI system, i.e., Na⁺ ion capture, while the other side that stores chloride ions, which is equally important, receives very little attention. This is attributed to the limited Cl^- storage materials as well as their sluggish kinetics and poor stability. In this article, we demonstrate that a N-doped porous carbon framework is capable of suppressing the phase-transformation-induced performance decay of bismuth, affording an excellent Cl^- storage and showing potential for water desalination. The obtained Bi-carbon composite (Bi/N-PC) shows a capacity of up to 410.4 mAh g^-1 at 250 mA g^-1 and a high rate performance. As a demonstration for water desalination, a superior desalination capacity of 113.4 mg g^-1 is achieved at 100 mA g^-1 with excellent durability. Impressively, the CDI system exhibits fast ion capturing with a desalination rate as high as 0.392 mg g^-1 s^-1, outperforming most of the recently reported Cl^- capturing electrodes. This strategy is applicable to other Cl^- storage materials for next-generation capacitive deionization.

A Brief Review on High-Performance Capacitive Deionization Enabled by Intercalation Electrodes

Owing to the advantages of cost-effectiveness, environmental-friendliness and high desalination capacity, capacitive deionization (CDI) has emerged as an advanced desalination technique. Recently, the ions intercalation materials inspired by sodium ion batteries have been widely implemented in CDI due to their exceptional salt removal capacity. They are able to extract sodium ions from the brine through intercalation or redox reactions, instead of electrostatic forces associated with the carbonaceous electrode. As a result, the ions intercalation materials have caught the attention of the CDI research community. In this article, the recent progress in various sodium ion intercalation materials as highly-efficient CDI electrodes is summarized and reviewed. Further, an outlook on the future development of ion intercalation electrodes is proposed.

Three-Dimensional Cobalt Hydroxide Hollow Cube/Vertical Nanosheets with High Desalination Capacity and Long-Term Performance Stability

Faradaic electrode materials have significantly improved the performance of membrane capacitive deionization, which offers an opportunity to produce freshwater from seawater or brackish water in an energy-efficient way. However, Faradaic materials hold the drawbacks of slow desalination rate due to the intrinsic low ion diffusion kinetics and inferior stability arising from the volume expansion during ion intercalation, impeding the engineering application of capacitive deionization. Herein, a pseudocapacitive material with hollow architecture was prepared via template-etching method, namely, cuboid cobalt hydroxide, with fast desalination rate (3.3 mg (NaCl)·g^-1 (h-Co(OH)₂)·min^-1 at 100 mA·g^-1) and outstanding stability (90% capacity retention after 100 cycles). The hollow structure enables swift ion transport inside the material and keeps the electrode intact by alleviating the stress induced from volume expansion during the ion capture process, which is corroborated well by in situ electrochemical dilatometry and finite element simulation. Additionally, benefiting from the elimination of unreacted bulk material and vertical cobalt hydroxide nanosheets on the exterior surface, the synthesized material provides a high desalination capacity (117 ± 6 mg (NaCl)·g^-1 (h-Co(OH)₂) at 30 mA·g^-1). This work provides a new strategy, constructing microscale hollow faradic configuration, to further boost the desalination performance of Faradaic materials.

Recent Advances in Desalination Battery: An Initial Review

Desalination is one of the most effective strategies to solve the problem of freshwater shortage, which is one of the most critical challenges facing global development. Recently, the desalination battery has become an emerging desalination technology thanks to its high salt-removal capacity enabled by the high capacity of battery electrodes and low energy consumption mainly rooted from the high energy recovery during the discharge process. To promote the development of the desalination battery, we must understand the recent advances and the remaining issues in the field. Herein, we comprehensively review the development of the concept and the electrode materials for a desalination battery, summarize the performance of a full desalination battery, and propose perspectives and guidelines.

An energy efficient bi-functional electrode for continuous cation-selective capacitive deionization

Effective ion intercalation nanomaterials provide tremendous opportunities to various deionization systems such as capacitive deionization (CDI) to significantly improve the removal capacity of brackish water desalination. However, the asymmetric design of CDI devices causes a low removal rate due to the indispensable regeneration half-cycle. Furthermore, choices of chloride selective electrodes for such devices are limited. This imposes a big challenge on further improvement of CDI systems. Herein, we report a cation-selective CDI system using a single bi-functional Na2VTi(PO4)3@carbon nanomaterial with redox couples of V4+/V3+ and Ti3+/Ti4+ as an advanced symmetric electrode. The as-prepared continuous desalination set-up shows a superior removal rate of 0.022 mg g-1 s-1 (1.32 mg g-1 min-1) with a high half-cycle removal capacity of 35 mg g-1, and extremely low energy consumption of 0.14 W h g-1 (at a current density of 100 mA g-1). In addition, an extremely high cycle-stability of at least 50 cycles is achieved. The bi-functional intercalation mechanism is investigated by in situ XRD and ex situ XPS. The symmetric device yields a simplified and low-cost configuration with improved energy efficiency and high removal capacity. This opens a new horizon towards the commercialization of CDI technologies.

Faradaic Electrodes Open a New Era for Capacitive Deionization

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.

In Situ Formation of Prussian Blue Analogue Nanoparticles Decorated with Three-Dimensional Carbon Nanosheet Networks for Superior Hybrid Capacitive Deionization Performance

Capacitive deionization (CDI) is considered to be an alternative water purification technology because of its low cost and low driven energy. However, the desalination performance of traditional CDI still cannot meet the requirement of actual operations, which is the limited adsorption capacity of carbon electrodes. Here, we report a feasible and simple strategy for the synthesis of a three-dimensional hierarchical composite with homogeneous Prussian blue analogue nanoparticles, decorating hierarchical porous carbon nanosheet networks (NiHCF@3DC-2) as a redox-active intercalation electrode material for hybrid capacitive deionization (HCDI). The interconnected network structure, accompanied by its unique porous characteristic and uniform NiHCF nanoparticles, endows the prepared NiHCF@3DC-2 with enough straining space for alleviating the effect of volume change upon the regeneration process and guarantees fast transmission kinetics for both electrons and salt ions. As a consequence, an HCDI cell with NiHCF@3DC-2 and activated carbon showed superior desalination ability with a high ion removal capacity of 47.8 mg g^-1 (107.5 mg g^-1 NiHCF@3DC-2) and good cyclic regenerative performance. Moreover, the Na⁺ ions storage mechanism and the interfacial synergy of the NiHCF@3DC-2 were also explored by structure and electrochemistry analyses during the CDI process. Our work provides a promising redox-active intercalation electrode material to highly efficient hybrid capacitive deionization for brine.

Combining Battery-Type and Pseudocapacitive Charge Storage in Ag/Ti3 C2 Tx MXene Electrode for Capturing Chloride Ions with High Capacitance and Fast Ion Transport

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 Ti3 C2 Tx /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 Ti3 C2 Tx . All Ti3 C2 Tx /Ag samples are hydrophilic, which is beneficial for water desalination. Ti3 C2 Tx /Ag samples with a low charge transfer resistance exhibit both pseudocapacitive and battery behaviors. Herein, the Ti3 C2 Tx /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 Ti3 C2 Tx /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 Ti3 C2 Tx /Ag hybrid material. This work provides fundamental insight into the coupling of battery and pseudocapacitive behaviors during Cl- capture for electrochemical desalination.

Enabling Superior Sodium Capture for Efficient Water Desalination by a Tubular Polyaniline Decorated with Prussian Blue Nanocrystals

The application of electrochemical energy storage materials to capacitive deionization (CDI), a low-cost and energy-efficient technology for brackish water desalination, has recently been proven effective in solving problems of traditional CDI electrodes, i.e., low desalination capacity and incompatibility in high salinity water. However, Faradaic electrode materials suffer from slow salt removal rate and short lifetime, which restrict their practical usage. Herein, a simple strategy is demonstrated for a novel tubular-structured electrode, i.e., polyaniline (PANI)-tube-decorated with Prussian blue (PB) nanocrystals (PB/PANI composite). This composite successfully combines characteristics of two traditional Faradaic materials, and achieves high performance for CDI. Benefiting from unique structure and rationally designed composition, the obtained PB/PANI exhibits superior performance with a large desalination capacity (133.3 mg g^-1 at 100 mA g^-1 ), and ultrahigh salt-removal rate (0.49 mg g^-1 s^-1 at 2 A g^-1 ). The synergistic effect, interfacial enhancement, and desalination mechanism of PB/PANI are also revealed through in situ characterization and theoretical calculations. Particularly, a concept for recovery of the energy applied to CDI process is demonstrated. This work provides a facile strategy for design of PB-based composites, which motivates the development of advanced materials toward high-performance CDI applications.

Self-Sustained Visible-Light-Driven Electrochemical Redox Desalination

The freshwater scarcity and increasing energy demand are two challenging global issues. Herein, we propose a new route for desalination, self-sustained visible-light-driven electrochemical redox desalination. We propose a novel device architecture involving internal integration of a quasi-solid-state dye-sensitized solar cell and continuous redox-flow desalination units with a bifunctional platinized-graphite-paper electrode. Both the solar cell and redox-flow desalination units are integrated using the bifunctional electrode with one side facing the solar cell operating as a positive electrode and the other side facing the redox-flow desalination unit operating as a negative electrode. The solar cell contains a gel-based tri-iodide/iodide redox couple sandwiched between an N719 dye-modified photoanode and cathode. In contrast, the redox-flow desalination consists of re-circulating ferro/ferricyanide redox couple sandwiched between the anode and cathode with two salt streams located between these electrodes. The performances of bifunctional electrodes in both redox couples were thoroughly investigated by electrochemical characterization. The brackish feed can be continuously desalted to the freshwater level by utilizing visible light illumination. As a device, this architecture combines energy conversion and water desalination. This concept bypasses the need for electrical energy consumption for desalination, which provides a novel structural design using photodesalination to facilitate the development of self-sustained solar desalination technologies.

Symmetric Sodium-Ion Battery Based on Dual-Electron Reactions of NASICON-Structured Na3MnTi(PO4)3 Material

Symmetric sodium-ion batteries possess promising features such as low cost, easy manufacturing process, and facile recycling post-process, which are suitable for the application of large-scale stationary energy storage. Herein, we proposed a symmetric sodium-ion battery based on dual-electron reactions of a NASICON-structured Na₃MnTi(PO₄)₃ material. The Na₃MnTi(PO₄)₃ electrode can deliver a stable capacity of up to 160 mAh g^-1 with a Coulombic efficiency of 97% at 0.1 C by utilizing the redox reactions of Ti^3+/4+, Mn^2+/3+, and Mn^3+/4+. This is the first time to investigate the symmetric sodium-ion full cell using Na₃MnTi(PO₄)₃ as both cathode and anode in the organic electrolyte, demonstrating excellent reversibility and cycling performance with voltage plateaus of about 1.4 and 1.9 V. The full cell exhibits a reversible capacity of 75 mAh g^-1 at 0.1 C and an energy density of 52 Wh kg^-1. In addition, both ex situ X-ray diffraction (XRD) analysis and first-principles calculations are employed to investigate the sodiation mechanism and structural evolution. The current research provides a feasible strategy for the symmetric sodium-ion batteries to achieve high energy density.

Exploration of Energy Storage Materials for Water Desalination via Next-Generation Capacitive Deionization

Clean energy and environmental protection are critical to the sustainable development of human society. The numerous emerged electrode materials for energy storage devices offer opportunities for the development of capacitive deionization (CDI), which is considered as a promising water treatment technology with advantages of low cost, high energy efficiency, and wide application. Conventional CDI based on porous carbon electrode has low salt removal capacity which limits its application in high salinity brine. Recently, the faradaic electrode materials inspired by the researches of sodium-batteries appear to be attractive candidates for next-generation CDI which capture ions by the intercalation or redox reactions in the bulk of electrode. In this mini review, we summarize the recent advances in the development of various faradaic materials as CDI electrodes with the discussion of possible strategies to address the problems present.

Pseudocapacitive Behaviors of Polypyrrole Grafted Activated Carbon and MnO2 Electrodes to Enable Fast and Efficient Membrane-Free Capacitive Deionization

Capacitive deionization (CDI) has emerged as a promising technique for brackish water desalination. Here, composites of polypyrrole grafted activated carbon (Ppy/AC) were prepared via in situ chemical oxidative polymerization of pyrrole on AC particles. The Ppy/AC cathode was then coupled with a MnO₂ anode for desalination in a membrane-free CDI cell. Both the Ppy/AC and MnO₂ electrodes exhibited pseudocapacitive behaviors, which can selectively and reversibly intercalate Cl^- (Ppy/AC) and Na⁺ (MnO₂) ions. Compared to AC electrodes, the specific capacitances of Ppy/AC electrodes increased concurrently with the pyrrole ratios from 0 to 10%, while the charge transfer and ionic diffusion resistances decreased. As a result, the 10%Ppy/AC-MnO₂ cell showed a maximum salt removal capacity of 52.93 mg g^-1 (total mass of active materials) and 34.15 mg g^-1 (total mass of electrodes), which was higher than those of conventional, membrane, and hybrid CDI cells. More notably, the salt removal rate of the 10%Ppy/AC-MnO₂ cell (max 0.46 mg g^-1 s^-1 to the total mass of active materials and 0.30 mg g^-1 s^-1 to the total mass of electrodes) was nearly 1 order of magnitude higher than those in most previous CDI studies, and this fast and efficient desalination performance was stabilized over 50 cycles.

Elucidating the capacitive desalination behavior of NaxCoO2: the significance of electrochemical pre-activation

Hybrid capacitive deionization (HCDI) has emerged as a promising desalination technique due to its ultra-high salt removal capacity in high brine water. However, the mechanism behind HCDI is seldom discussed anywhere. Herein, we perform a comprehensive investigation to have some insight into the HCDI behavior of Na_xCoO₂ by varying x as 0.2, 0.5, 0.7, 1.0 and 1.6. Regardless of x, Na_xCoO₂ are classified as a representative P63/mmc space group with a P2 layered structure. With the increase of the sodium content, the (002) crystal plane of Na_xCoO₂ shifts significantly toward a high angle as the distance between CoO₂ layers decreases. This results from the variation of the Na-O bonding length as well as the bonding energy according to the first-principles simulation. Moreover, it is observed that the Na-O bond broke once the input energy is higher than the Na-O bonding energy, leading to the electrochemical pre-activation of Na_xCoO₂. As a result, Na_0.7CoO₂ exhibits the best HCDI performance, i.e. a salt removal capacity of 63.0 mg g^-1 and a charge efficiency of 97% in NaCl solutions with an initial conductivity of 2000 μS cm^-1. Besides, the intercalation of sodium ions into Na_xCoO₂ has been confirmed by differentiating the respective contributions of pseudo-capacitance together with crystal phase transformation. Our results show that the desalination behavior of Na_xCoO₂ can be mediated by controlling the sodium content and electrochemical pre-activation.

3D printed electrodes for efficient membrane capacitive deionization

There is increasing interests in cost-effective and energy-efficient technologies for the desalination of salt water. However, the challenge in the scalability of the suitable compositions of electrodes has significantly hindered the development of capacitive deionization (CDI) as a promising technology for the desalination of brackish water. Herein, we introduced a 3D printing technology as a new route to fabricate electrodes with adjustable composition, which exhibited large-scale applications as free-standing, binder-free, and robust electrodes. The 3D printed electrodes were designed with ordered macro-channels that facilitated effective ion diffusion. The high salt removal capacity of 75 mg g^-1 was achieved for membrane capacitive deionization (MCDI) using 3D printed nitrogen-doped graphene oxide/carbon nanotube electrodes with the total electrode mass of 20 mg. The improved mechanical stability and strong bonding of the chemical components in the electrodes allowed a long cycle lifetime for the MCDI devices. The adjusted operational mode (current density) enabled a low energy consumption of 0.331 W h g^-1 and high energy recovery of ∼27%. Furthermore, the results obtained from the finite element simulations of the ion diffusion behavior quantified the structure-function relationships of the MCDI electrodes.

Effect of conductive additives on the transport properties of porous flow-through electrodes with insulative particles and their optimization for Faradaic deionization

Deionization devices that use intercalation reactions to reversibly store and release cations from solution show promise for energy-efficient desalination of alternative water resources. Intercalation materials often display low electronic conductivity that results in increased energy consumption during desalination. Accordingly, we performed experiments to quantify the impact of the size and mass fraction of conductive additives and insulative active particles on the effective electronic conductivity, ionic conductivity, and hydraulic permeability of porous electrodes. We find that Ketjen black conductive additives with nodules <50 nm in diameter produce superior electronic conductivity at lower mass fractions than the larger carbon blacks commonly used in capacitive deionization. Hydraulic permeability and effective ionic conductivity depend weakly on carbon black content and size, though smaller active particles decrease hydraulic permeability. Based on these results we analyzed the energy consumption and salt removal rate of different electrode formulations by constructing an electrochemical Ashby plot predicting the variation of desalination performance with electrode transport properties. Optimized electrodes containing insulative Prussian blue analogue (PBA) particles were then fabricated and used in an experimental cation intercalation desalination (CID) cell with symmetric electrodes. For 100 mM NaCl influent energy consumption varied from 7 to 33 kJ/mol when current density increased from 1 to 8 mA/cm², approaching ten-fold increased salt removal rate at similar energy consumption levels to past CID demonstrations. Complementary numerical and analytical modeling indicates that further improvements in energy consumption and salt removal rate are attainable by enhancing transport in solution and within PBA agglomerates.

Dual-Ion Electrochemical Deionization System with Binder-Free Aerogel Electrodes

Desalination devices such as capacitive deionization (CDI) have been developed for many years as an approach to relief freshwater shortage. However, due to the limitation of physical adsorption capacity of CDI, the salt removal capacity is unable to reach high value. To enhance the desalination capacity effectively, battery materials are employed to fabricate a dual-ion electrochemical deionization (DEDI) device. Herein, a binder-free DEDI system with two free-standing aerogel electrodes is reported. A Na₃ V₂ (PO₄ )₃ /graphene hybrid aerogel is used as sodium electrode and a AgCl/graphene hybrid aerogel is used as chloride electrode. With electric current passing through, sodium and chloride ions are released or absorbed by two aerogel electrodes. This system achieves super high desalination capacity, excellent cycling stability, and rapid desalination rate. The desalination capacity is as high as 107.5 mg g^-1 after 50 cycles with the current density of 100 mA g^-1 . The outstanding desalination performance of this system shows a synergistic effect of combining battery materials with graphene for deionization and promises a new potential alternative of future desalination design.