A Prussian blue anode for high performance electrochemical deionization promoted by the faradaic mechanism

Electrochemically selective ammonium recovery from wastewater via coupling hydrogen bonding and charge storage

Electrochemical ammonium (NH4+) storage (EAS) has been established as an efficient technology for NH4+ recovery from wastewater. However, there are scientific difficulties unsolved regarding low storage capacity and selectivity, restricting its extensive engineering applications. In this work, electrochemically selective NH4+ recovery from wastewater was achieved by coupling hydrogen bonding and charge storage with self-assembled bi-layer composite electrode (GO/V2O5). The NH4+ storage was as high as 234.7 mg N g-1 (> 102 times higher than conventional activated carbon). Three chains of proof were furnished to elucidate the intrinsic mechanisms for such superior performance. Density functional theory (DFT) showed that an excellent electron-donating ability for NH4+ (0.08) and decrease of diffusion barrier (22.3 %) facilitated NH4+ diffusion onto electrode interface. Physio- and electro-chemical results indicated that an increase of interlamellar spacing (14.3 %) and electrochemical active surface area (ECSA, 388.9 %) after the introduction of GO were responsible for providing greater channels and sites toward NH4+ insertion. Both non-ionic chemical-bonding (V5+=O‧‧‧H, hydrogen-bonding) and charge storage were contributed to the higher capacity and selectivity for NH4+. This work offers underlying guideline for exploitation a storage manner for NH4+ recovery from wastewater.

Enhanced Desalination Performance Using Phosphate Buffer-Mediated Redox Reactions of Manganese Oxide Electrodes in a Multichannel System

Water desalination mediated by electrochemical reactions to directly capture and release salt at electrode materials offers a low-voltage method for producing freshwater. Developing new system designs has allowed electrode materials to maximize their capacity for salt separation, especially when a multichannel system is used to introduce a separate electrode rinse solution. Here, we show that the use of an additive can provide a new strategy for improving electrode capacity and, hence desalination performance, which so far has been limited to increasing the electrolyte concentration. A custom-built, 2/2-channel flow cell divided by two cation exchange membranes and an anion exchange membrane was fed with 50 mM NaCl as the feed (two inner channels) and 0.5 M NaCl containing up to 0.1 M phosphate as the electrode rinse (two outer channels). Using manganese oxide electrodes with phosphate buffer-mediated redox reactions exhibited an improved desalination capacity of 68.0 ± 5.2 mg g-1 (0.55 mA cm-2) and a rate of 5.6 ± 1.3 mg g-1 min-1 (0.96 mA cm-2). The improvement was attributed to the buffer that served as a proton donor for promoting the H+ insertion reaction of amorphous or poorly crystalline MnO2. Additionally, the buffering capacity against acidification and the creation of insoluble manganese phosphate on the electrode surface prevented the dissolution of Mn2+, which could otherwise occur at the anode due to a decrease in the local pH upon H+ deinsertion. Thus, the use of manganese oxide electrodes coupled with phosphate provides a new strategy of increasing electrode capacity for water desalination.

Ternary-metal Prussian blue analogues as high-quality sodium ion capturing electrodes for rocking-chair capacitive deionization

Prussian blue analogs (PBAs) have gained much attention in the capacitive deionization (CDI) field because of their rigid open structure and good energy storage capacity. However, their desalination performance is still to be improved for practical application. Herein, we reported the NiCoFe ternary-metal PBAs materials and explored their application as Na⁺ capturing electrode in rocking-chair capacitive deionization (RCDI) system. On the one hand, the introduction of Ni²⁺ into CoFe PBA can effectively reduce the lattice changes in the (dis)charging process.On the other hand, the RCDI system with symmetrical structure could avoid the performance deficiency caused by the unbalanced capacity of common HCDI system. Due to the rationalized RCDI cell configuration and ternary-metal PBAs with improved stability, the NiCoFe-PBAs-based RCDI exhibits amazing desalination performance with maximum capacity of 131.4 mg·g^-1 and rate of 0.46 mg·g^-1·s^-1 as well as optimum stability with 90.7 % capacity retention over 300 cycles, surpassing those of PBAs based CDI system reported previously. The special strategy in this work offers inspiration via optimizing the cell structure and electrode materials for the promising development of CDI systems.

Vertically Aligned Bismuthene Nanosheets on MXene for High-Performance Capacitive Deionization

Capacitive deionization has been considered as a promising solution to the challenge of freshwater shortage due to its high efficiency, low environmental footprint, and low energy consumption. However, developing advanced electrode materials to improve capacitive deionization performance remains a challenge. Herein, the hierarchical bismuthene nanosheets (Bi-ene NSs)@MXene heterostructure was successfully prepared by combining the Lewis acidic molten salt etching and the galvanic replacement reaction, which achieves the effective utilization of the molten salt etching byproducts (residual copper). The vertically aligned bismuthene nanosheets array evenly in situ grown on the surface of MXene, which not only facilitate ion and electron transport as well as offer abundant active sites but also provide strong interfacial interaction between bismuthene and MXene. Benefiting from the above advantages, the Bi-ene NSs@MXene heterostructure as a promising capacitive deionization electrode material exhibits high desalination capacity (88.2 mg/g at 1.2 V), fast desalination rate, and good long-term cycling performance. Moreover, the mechanisms involved were elaborated by systematical characterizations and density functional theory calculations. This work provides inspirations for the preparation of MXene-based heterostructures and their application for capacitive deionization.

Synthesis of Prussian Blue Analogue and Its Catalytic Activity toward Reduction of Environmentally Toxic Nitroaromatic Pollutants

Nitroanilines are environmentally toxic pollutants which are released into aquatic systems due to uncontrolled industrialization. Therefore, it is crucial to convert these hazardous nitroanilines into a harmless or beneficial counterpart. In this context, we present the chemical reduction of 4-nitroaniline (4-NA) by NaBH₄ utilizing Prussian blue analogue (PBA) as nanocatalyst. PBAs can serve as inexpensive, eco-friendly, and easily fabricated nanocatalysts. PBA cobalt tetracyanonickelate hexacyanochromate (CoTCNi/HCCr) was stoichiometrically prepared by a facile chemical coprecipitation. Chemical, phase, composition, and molecular interactions were investigated by XRD, EDX, XPS, and Raman spectroscopy. Additionally, SEM and TEM micrographs were utilized to visualize the microstructure of the nanomaterial. The findings revealed the synthesized PBA of the cubic phase and their particles in nanosheets. The band gap was estimated from the optical absorption within the UV-vis region to be 3.70 and 4.05 eV. The catalytic performance of PBA for the reduction of 4-NA was monitored by UV-vis spectroscopy. The total reduction time of 4-NA by PBA was achieved within 270 s, and the computed rate constant (k) was 0.0103 s^-1. The synthesized PBA nanoparticles have the potential to be used as efficient nanocatalysts for the reduction of different hazardous nitroaromatics.

Energy Consumption in Capacitive Deionization for Desalination: A Review

Capacitive deionization (CDI) is an emerging eco-friendly desalination technology with mild operation conditions. However, the energy consumption of CDI has not yet been comprehensively summarized, which is closely related to the economic cost. Hence, this study aims to review the energy consumption performances and mechanisms in the literature of CDI, and to reveal a future direction for optimizing the consumed energy. The energy consumption of CDI could be influenced by a variety of internal and external factors. Ion-exchange membrane incorporation, flow-by configuration, constant current charging mode, lower electric field intensity and flowrate, electrode material with a semi-selective surface or high wettability, and redox electrolyte are the preferred elements for low energy consumption. In addition, the consumed energy in CDI could be reduced to be even lower by energy regeneration. By combining the favorable factors, the optimization of energy consumption (down to 0.0089 Wh·g_NaCl^-1) could be achieved. As redox flow desalination has the benefits of a high energy efficiency and long lifespan (~20,000 cycles), together with the incorporation of energy recovery (over 80%), a robust future tendency of energy-efficient CDI desalination is expected.

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.

Highly Efficient Capacitive Deionization Enabled by NiCo4MnO8.5 Electrodes

The shortage of fresh water resources is one of the major challenges facing this planet. Capacitive deionization (CDI) techniques that are deemed to be highly efficient and require low capital cost have attracted widespread attention in the last few decades. In this work, the cubic ternary metal oxides NiCo4MnO8.5 (Ni-Co-Mn-O) are synthesized by facile hydrothermal method for enhanced symmetrical CDI. Electrochemical measurements illustrate that the Ni-Co-Mn-O possesses low internal resistance and ion diffusion impedance. As a result, the salt removal capacity of the Ni-Co-Mn-O electrode increases from 26.84 to 65.61 mg g-1 by varying the voltage from 0.8 to 1.4 V in 1.0 × 10-2 m NaCl solution, while the charge efficiency stabilizes at ≈80%. After 20 cycles, the capacitance retained is 64.27%, which is due to the irreversibility of Co2+/Co3+ and Mn2+/Mn3+ and the release of Ni3+ from the Ni-Co-Mn-O electrode after long desalination/salination cycles.

Ladder Mechanisms of Ion Transport in Prussian Blue Analogues

Prussian blue (PB) and its analogues (PBAs) are drawing attention as promising materials for sodium-ion batteries and other applications, such as desalination of water. Because of the possibilities to explore many analogous materials with engineered, defect-rich environments, computational optimization of ion-transport mechanisms that are key to the device performance could facilitate real-world applications. In this work, we have applied a multiscale approach involving quantum chemistry, self-consistent mean-field theory, and finite-element modeling to investigate ion transport in PBAs. We identify a cyanide-mediated ladder mechanism as the primary process of ion transport. Defects are found to be impermissible to diffusion, and a random distribution model accurately predicts the impact of defect concentrations. Notably, the inclusion of intermediary local minima in the models is key for predicting a realistic diffusion constant. Furthermore, the intermediary landscape is found to be an essential difference between both the intercalating species and the type of cation doping in PBAs. We also show that the ladder mechanism, when employed in multiscale computations, properly predicts the macroscopic charging performance based on atomistic results. In conclusion, the findings in this work may suggest the guiding principles for the design of new and effective PBAs for different applications.

Microscopic study of ion transport in the porous electrode of a desalination battery based on the lattice Boltzmann method

Cation Intercalation Desalination (CID).

Two-Dimensional MXene-Polymer Heterostructure with Ordered In-Plane Mesochannels for High-Performance Capacitive Deionization

The application of traditional electrode materials for high-performance capacitive deionization (CDI) has been persistently limited by their low charge-storage capacities, excessive co-ion expulsion and slow salt removal rates. Here we report a bottom-up approach to the preparation of a two-dimensional (2D) Ti₃ C₂ T_x MXene-polydopamine heterostructure having ordered in-plane mesochannels (denoted as mPDA/MXene). Interfacial self-assembly of mesoporous polydopamine (mPDA) monolayers on MXene nanosheets leads to the mPDA/MXene heterostructure, which exhibits several unique features: (1) MXene undergoes reversible ion intercalation/deintercalation and possesses high conductivity; (2) mPDA layers establish redox capacitive characteristics and Na⁺ selectivity, and also help to prevent self-stacking and oxidation of MXene; (3) in-plane mesochannels enable the smooth transport of ions at the internal spaces of this stacked 2D material. When applied as an electrode material for CDI, mPDA/MXene nanosheets exhibit top-level CDI performance and cycling stability compared to those of the so far reported 2D materials. Our study opens an avenue for the rational construction of MXene-organic hybrid heterostructures, and further motivates the development of high-performance CDI electrode materials.

Bismuth oxychloride nanostructure coated carbon sponge as flow-through electrode for highly efficient rocking-chair capacitive deionization

Rocking-chair capacitive deionization (RCDI), as the next generation technique of capacitive deionization, has thrived to be one of the most promising strategies in the desalination community, yet was hindered mostly by its relatively low desalination rate and stability. Motivated by the goal of simultaneously enhancing the desalination rate and structural stability of the electrode, this paper reports an anion-driven flow-through RCDI (AFT-RCDI) system equipped with BiOCl nanostructure coated carbon sponge (CS@BiOCl for short; its backbone is derived from commercially available melamine foam with minimum capital cost) as the flow-through electrode. Owning to the rational design of the composite electrode material with minimum charge transfer resistance and ultrahigh structure stability as well as the superior flow-through cell architecture, the AFT-RCDI displays excellent desalination performance (desalination capacity up to 107.33 mg g^-1; desalination rate up to 0.53 mg g^-1s^-1) with superior long-term stability (91.75% desalination capacity remained after 30 cycles). This work provides a new thought of coupling anion capturing electrode with flow-through cell architecture and employing a low-cost CS@BiOCl electrode with commercially available backbone material, which could shed light on the further development of low-cost electrochemical desalination systems.

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.

Achieving Enhanced Capacitive Deionization by Interfacial Coupling in PEDOT Reinforced Cobalt Hexacyanoferrate Nanoflake Arrays

Capacitive deionization (CDI) as a novel energy and cost-efficient water treatment technology has attracted increasing attention. The recent development of various faradaic electrode materials has greatly enhanced the performance of CDI as compared with traditional carbon electrodes. Prussian blue (PB) has emerged as a promising CDI electrode material due to its open framework for the rapid intercalation/de-intercalation of sodium ions. However, the desalination efficiency, and durability of previously reported PB-based materials are still unsatisfactory. Herein, a self-template strategy is employed to prepare a Poly(3,4-ethylenedioxythiophene) (PEDOT) reinforced cobalt hexacyanoferrate nanoflakes anchored on carbon cloth (denoted as CoHCF@PEDOT). With the high conductivity and structural stability achieved by coupling with a thin PEDOT layer, the as-prepared CoHCF@PEDOT electrode exhibits a high capacity of 126.7 mAh g-1 at 125 mA g-1. The fabricated hybrid CDI cell delivers a high desalination capacity of 146.2 mg g-1 at 100 mA g-1, and good cycling stability. This strategy provides an efficient method for the design of high-performance faradaic electrode materials in CDI applications.

Electrocatalytic desalination with CO2 reduction and O2 evolution

Multifunctional electrocatalytic desalination is a promising method to increase the production of additional valuable chemicals during the desalination process. In this work, a multifunctional desalination device was demonstrated to effectively desalinate brackish water (15 000 ppm) to 9 ppm while generating formate from captured CO₂ at the Bi nanoparticle cathode and releasing oxygen at the Ir/C anode. The salt feed channel is sandwiched between two electrode chambers and separated by ion-exchange membranes. The electrocatalytic process accelerates the transportation of sodium ions and chloride ions in the brine to the cathode and anode chamber, respectively. The fastest salt removal rate to date was obtained, reaching up to 228.41 μg cm^-2 min^-1 with a removal efficiency of 99.94%. The influences of applied potential and the concentrations of salt feed and electrolyte were investigated in detail. The current research provides a new route towards an electrochemical desalination system.

Stepwise hollow Prussian blue/carbon nanotubes composite as a novel electrode material for high-performance desalination

As a promising intercalation material for capacitive deionization (CDI), Prussian blue (PB) and its analogues (PBAs) have the superiority of high theoretical capacity and easy synthesis. But they often suffer from low conductivity and severe crystal phase transition, resulting in inferior desalination capacity and poor cycling stability. Herein, the dual strategy of structural optimization and carbon-based materials introduction is proposed to enhance the desalination performance of PBAs. Stepwise hollow structure formed by surface etching has been proved to be more outstanding than cubic structure. Enlarged the specific surface area, the contact area with the electrolyte increases, therefore, more active sites are exposed. Besides, the etching of external surfaces provides more buffer space, improves the tolerance to crystal phase transition, and enhances the cycling stability. The introduction of carbon nanotubes brings high conductivity. Specifically, the desalination test shows that stepwise hollow Prussian blue/carbon nanotubes composite delivers a high desalination capacity of 103.4 mg g^-1 with outstanding cycling stability. Moreover, the low energy consumption of 0.23 Wh g^-1 is also suitable for practical application. The dual strategy opens a window to design advanced electrode materials for CDI.

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.

Metal-Ion Depletion Impacts the Stability and Performance of Battery Electrode Deionization over Multiple Cycles

Prussian blue hexacyanoferrate (HCF) materials, such as copper hexacyanoferrate (CuHCF) and nickel hexacyanoferrate (NiHCF), can produce higher salt removal capacities than purely capacitive materials when used as electrode materials during electrochemical water deionization due to cation intercalation into the HCF structure. One factor limiting the application of HCF materials is their decay in deionization performance over multiple cycles. By examining the performance of CuHCF and NiHCF electrodes at three different pH values (2.5, 6.3, and 10.2) in multiple-cycle deionization tests, losses in capacity (up to 73% for CuHCF and 39% for NiHCF) were shown to be tied to different redox-active centers through analysis of dissolution of electrode metals. Both copper and iron functioned as active centers for Na⁺ removal in CuHCF, while iron was mainly the active center in NiHCF. This interaction of Na⁺ and active centers was demonstrated by correlating the decrease in performance to the concentration of these metal ions in the effluent solutions collected over multiple cycles at different pHs (up to 0.86 ± 0.14 mg/L for iron and 0.42 ± 0.17 mg/L for copper in CuHCF and 0.38 ± 0.05 mg/L for iron in NiHCF). Both materials were more stable (<11% decay for CuHCF and no decay for NiHCF) when the appropriate metal salt (copper or nickel) was added to the feed solutions to inhibit electrode dissolution. At a pH of 2.5, there was an increased competition between protons and Na⁺ ions, which decreased the Na⁺ removal amount and lowered the thermodynamic energy efficiency for deionization for both electrode materials. Therefore, while an acidic pH provided the most stable performance, a circumneutral pH would be useful to produce a better balance between performance and longevity.

Na3(VO)2(PO4)2F nanocuboids/graphene hybrid materials as faradic electrode for extra-high desalination capacity

Capacitive deionization (CDI) is considered as a promising desalination technology due to its low energy consumption and no two-second pollution. But the development of traditional CDI is limited by its two drawbacks, which are low deionization capacity and unavoidable parasitic reactions. Hybrid capacitive deionization (HCDI), which is composed of a faradic electrode and an electrical-double-layer electrode, effectively solves the above problem. Herein, we report a typical NASICON material Na₃(VO)₂(PO₄)₂F and modify it with rGO, then apply it in HCDI firstly and receive a superior desalination performance. Five samples are prepared by adding different contents GO solution and we choose the best one (NVOPF-4) with the lowest resistance for the desalination tests according to electrochemical performance. The result of desalination shows a high desalination capacity of 175.94 mg·g^-1, low energy consumption of 0.35 kWh·kg-NaCl^-1, and the energy recovery is 20% at a current density of 25 mg·g^-1. NVOPF@rGO displays a promising ability for desalination in capacitive deionization, further confirming NASICON be a suitable material type for HCDI electrode materials.

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.

All-in-One, Solid-State, Solar-Powered Electrochemical Cell

Solar-powered electrochemical cells (SPECs) have been perceived as a potential strategy for coping with the intermittent nature of solar power. Most of the SPECs reported so far use corrosive/toxic liquid electrolyte and/or need very careful packaging, which is restricted by the scenario of implementation and arises the fabrication cost. Here, we demonstrate an all-in-one, solid-state SPEC with solar-to-output energy conversion efficiency of ca. 2.8% under AM 1.5 G irradiation. In this SPEC, a LiBr/polyacrylamide (PAM) hydrogel serves both as electrolyte, cathode-active mediator, and separator, which is sandwiched between an FTO/BiVO₄ photoanode and an FTO/Prussian blue (PB) anode. The use of solid-state PAM hydrogel promotes the charge-transfer dynamics at the interface of the photoanode and suppressed the undesired side reactions of electrolyte decomposition, representing an effective strategy by interfacial engineering toward the development of high-performance SPECs.

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.

A Review of Battery Materials as CDI Electrodes for Desalination

The world is suffering from chronic water shortage due to the increasing population, water pollution and industrialization. Desalinating saline water offers a rational choice to produce fresh water thus resolving the crisis. Among various kinds of desalination technologies, capacitive deionization (CDI) is of significant potential owing to the facile process, low energy consumption, mild working conditions, easy regeneration, low cost and the absence of secondary pollution. The electrode material is an essential component for desalination performance. The most used electrode material is carbon-based material, which suffers from low desalination capacity (under 15 mg·g−1). However, the desalination of saline water with the CDI method is usually the charging process of a battery or supercapacitor. The electrochemical capacity of battery electrode material is relatively high because of the larger scale of charge transfer due to the redox reaction, thus leading to a larger desalination capacity in the CDI system. A variety of battery materials have been developed due to the urgent demand for energy storage, which increases the choices of CDI electrode materials largely. Sodium-ion battery materials, lithium-ion battery materials, chloride-ion battery materials, conducting polymers, radical polymers, and flow battery electrode materials have appeared in the literature of CDI research, many of which enhanced the deionization performances of CDI, revealing a bright future of integrating battery materials with CDI technology.

Elucidating Structure-Composition-Property Relationships of Ni-Based Prussian Blue Analogues for Electrochemical Seawater Desalination

Nickel hexacyanoferrate (NiHCF), a type of Prussian blue analogue (PBA), has recently emerged as one of the most promising Na-storage electrodes for use in electrochemical desalination. Previous studies have revealed that NiHCF can be prepared with both cubic and rhombohedral symmetries depending on the oxidation state of Fe (Fe^II vs Fe^III) and the related A-site occupancy. However, our understanding of the effects of the lattice-type of the as-prepared samples on their electrochemical performances, structural transitions that occur during sodiation/desodiation, cyclability, and rate capabilities is presently lacking. Additionally, the optimum structural and compositional features required to prepare high-performing NiHCF electrodes have not yet been clearly established. In this work, we report the synthesis of two sets of cubic and rhombohedral NiHCF samples with different particle sizes, crystallinities, and compositions. Using these samples, we systematically elucidated the structure-composition-property relationships of NiHCF to develop rational design principles to prepare high-performing PBAs. Our results show that high crystallinity, a low number of Fe(CN)₆ vacancies, and a large unit cell size to allow for consistent structural changes during cycling are critical factors to produce NiHCF with a high capacity, good cycling stability, and good rate capabilities, and these factors are considerably affected by the synthesis conditions. One of the samples prepared in this study with optimum structural features demonstrates the best performance and stability among any PBA electrode tested in neutral saline solutions to date.

Electrochemical Ion Pumping Device for Blue Energy Recovery: Mixing Entropy Battery

In the process of finding new forms of energy extraction or recovery, the use of various natural systems as potential clean and renewable energy sources has been examined. Blue energy is an interesting energy alternative based on chemical energy that is spontaneously released when mixing water solutions with different salt concentrations. This occurs naturally in the discharge of rivers into ocean basins on such a scale that it justifies efforts for detailed research. This article collects the most relevant information from the latest publications on the topic, focusing on the use of the mixing entropy battery (MEB) as an electrochemical ion pumping device and the different technological means that have been developed for the conditions of this process. In addition, it describes various practices and advances achieved by various researchers in the optimization of this device, in relation to the most important redox reactions and the cathode and anodic materials used for the recovery of blue energy or salinity gradient energy.

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.

More Efficient Prussian Blue Nanoparticles for an Improved Caesium Decontamination from Aqueous Solutions and Biological Fluids

Any release of radioactive cesium-137, due to unintentional accidents in nuclear plants, represents a dangerous threat for human health and the environment. Prussian blue has been widely studied and used as an antidote for humans exposed to acute internal contamination by Cs-137, due to its ability to act as a selective adsorption agent and to its negligible toxicity. In the present work, the synthesis protocol has been revisited avoiding the use of organic solvents to obtain Prussian blue nanoparticles with morphological and textural properties, which positively influence its Cs⁺ binding capacity compared to a commercially available Prussian blue sample. The reduction of the particle size and the increase in the specific surface area and pore volume values compared to the commercial Prussian blue reference led to a more rapid uptake of caesium in simulated enteric fluid solution (+35% after 1 h of contact). Then, after 24 h of contact, both solids were able to remove >98% of the initial Cs⁺ content. The Prussian blue nanoparticles showed a weak inhibition of the bacterial luminescence in the aqueous phase and no chronic detrimental toxic effects.