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• K 2 FeO 4 is first utilized for the solid-phase oxidation of pitch by a high-energy ball mill. • The solid-phase reaction mechanism involving in oxidation and condensation was revealed. • Advanced pitch-based porous carbon (H-CBPF) was prepared by a solid–liquid synergistic strategy. • The synergistic effect improves both the porous geometric configuration and O content of H-CBPF. • H-CBPF delivers an excellent salt adsorption capacity and a superior charge efficiency. To address limited desalination capacity and insufficient charge efficiency for carbon materials in capacitive deionization (CDI), a solid–liquid synergistic oxidation strategy of potassium pertechnetate (VI) for pitch was proposed to construct porous carbon with a reasonable pore structure and high reactive surface. On the one hand, K 2 FeO 4 is first utilized for the solid-phase oxidation (SPO) of pitch by a high-energy ball mill, introducing rich oxygen-containing groups such as ether bonds. Meanwhile, dehydro-condensation reactions occurred during SPO, resulting in the formation of the oxidized pitch with a high degree of condensation. Importantly, the pyrolysis yield of the oxidized pitch is as high as 46.9 % at 800 °C, almost twice that of the pristine pitch, meaning that the oxidized pitch with high polymer degree and rich C-O bridge bonds facilitates the carbon fixation and emission reduction during carbonization. On the other hand, the self-oxidation of K 2 FeO 4 in aqueous solution forms bifunctional activators (KOH and Fe(OH) 3) for the preparation of pitch-based porous carbon. Benefiting from the crosslinking structure of oxidized pitch and the multi-effective activation, the prepared porous carbon (H-CBPF) exhibits a robust layered stacking topology with a high surface area and oxygen content. Thereby, it can deliver an excellent salt adsorption capacity (SAC) of 22.1 mg g−1 with an average desalination rate of 1.89 mg g−1 min−1 and a high charge efficiency of 85 % at 1.2 V in 500 mg L-1 NaCl solution. This work opens up a green and economical route for the preparation of pitch-derived porous carbon for high-performance CDI. [ABSTRACT FROM AUTHOR]

[Display omitted] • A comprehensive review to highlight the application of 3D faradic materials in CDI field is presented. • The desalination performances of non-3D faradic electrodes and 3D faradic electrodes are systematically compared. • The perspective of 3D faradic materials is discussed to provide guidelines for future CDI development. To address the water scarcity issue, capacitive deionization (CDI) as a burgeoning desalination technology for removing different ions from sea and wastewater have been intensively explored. However, the low salt adsorption capacity and poor cycling stability of widely used carbon materials are difficult to fulfill the demand of practical CDI application, while the non-three-dimensional (non-3D) faradic electrode materials manifest the drawbacks of low specific capacitance and dissolution loss of metal ions, which severely limit their CDI application. Notably, three-dimensional (3D) faradic materials usually possess higher desalination capacity and better cycling stability than carbon materials and non-3D faradic materials, due to their wide ion diffusion channels and versatile structure, which are highly promising for achieving practical CDI application. However, few review papers have specifically focused on the limitations and advantages of 3D faradic materials in CDI field. In this review, a comprehensive insight into 3D faradic materials in terms of the solutions of limitations and corresponding desalination performance (via comparing with the desalination performance of non-3D faradic materials) is provided and the development prospect of 3D faradic materials is also expounded. [ABSTRACT FROM AUTHOR]

[Display omitted] • Electrode material with efficient and selective phosphate adsorption was developed. • ZnFe-PANI/CNT electrode exhibited remarkable capacitive behavior and charge storage capacity. • ZnFe-PANI/CNT had excellent phosphate adsorption capacity over a wide parameters range. • The high phosphate removal rate was maintained after 10th regeneration processes. Capacitive deionization technology possesses puissant competitiveness in the field of water treatment due to its low cost and high efficiency. In this study, polyaniline (PANI) doped and carboxyl intercalated metal hydroxide composite (ZnFe-PANI/CNT) was developed by a simple co-precipitation method, and used as anode electrode for phosphate removal in sewage. The results show that ZnFe-PANI/CNT electrode can efficiently remove phosphate at concentration of 2–10 mg/L at a wide range of pH (3–10) by electric double layer capacitive adsorption and pseudocapacitive adsorption. The concentration of phosphate in the effluent was as low as 0.095 mg/L, and the coexisting anions (Cl−, NO 3 −, SO 4 2− and HCO 3 −) hardly inhibited the phosphate adsorption by ZnFe-PANI/CNT composite electrode. In addition, under the optimal phosphate adsorption conditions (pH = 7 and applied voltage of 1.2 V), the energy consumption of CDI system was calculated as low as 0.00274 kWh/g P and 0.017 kWh/m3 water, realizing the expectation of low cost and high efficiency operation. [ABSTRACT FROM AUTHOR]

• A novel capacitive deionization with a three-phase architecture (T-CDI) was proposed. • Cathodic oxygen reduction was enhanced in T-CDI without affecting desalination. • SAC value and H 2 O 2 generation were 14.91 mg/g and 25.55 mg/L, respectively. • Simultaneous desalination and organics control was achieved by T-CDI/UV. • Applicability of T-CDI in long-term operation was investigated. Cathodic oxygen reduction, as a Faradaic side reaction, has been proven to be thermodynamically favorable for H 2 O 2 generation in capacitive deionization (CDI), which may provide a greener manner for organics control alongside desalination. However, the low levels of dissolved oxygen (DO) in saline water and its potential Faradaic degradation of carbon-based electrodes limit the H 2 O 2 generation in the conventional CDI (C-CDI). Herein, a novel three-phase CDI architecture (T-CDI) was proposed to simultaneously achieve stable desalination and supplementary H 2 O 2 generation. Unlike C-CDI architecture, T-CDI was open-ended at the cathode side, where porous nickel foam was used as current collector and air diffusion layer. Accordingly, a stable gas–liquid-solid three-phase interface was created inside the cathode, thus enhancing the diffusion and utilization of atmospheric air and preventing the damaging effects of excessive DO on the electrode. A stable electrosorption and electrocatalysis performance for T-CDI was obtained by regulating electrode hydrophilic-hydrophobic properties, comparing with other CDI configurations, and altering operating conditions. The salt adsorption capacity and H 2 O 2 accumulative concentration were 14.91 mg/g and 25.55 mg/L, respectively. Furthermore, the simultaneous desalination and organics control in T-CDI were investigated with the assistance of UV irradiation (T-CDI/UV). When treating NaCl and sulfamethazine (SMR) mixture solution, T-CDI/UV significantly reduced the irreversible electrode organic fouling and improved the desalination stability. Additionally, the potentially hazardous SMR was effectively removed alongside desalination. Finally, the long-term operation of T-CDI demonstrated that the generated H 2 O 2 cannot induce an additional substantial deterioration in desalination stability. The techno-economic assessment showed T-CDI was economically competitive compared with C-CDI. This study provides insights into the synergy of non-Faradaic and Faradaic processes and further diversifies the application of CDI in water treatment. [ABSTRACT FROM AUTHOR]

[Display omitted] • Dual cation titanosilicate(DTS) was utilized in CDI to capture Cs+ and Sr2+. • By applying −1.2 V, the DTS electrode exhibits a high adsorption capacity. • DTS electrode showed rapid kinetics over 95 % of adsorption capacity within 30 min. • It provides a high selectivity of 5x103 mL/g in 10000-fold Na concentrated solution. • It was regenerated by applying + 2.0 V and its structural integrity remains intact. The utilization of ion intercalate capacitive deionization technology is garnering attention as an electrified water treatment method. This study proposes a capacitive deionization technology incorporating Dual-cation TitanoSilicate (DTS) to enable efficient electro-sorption/desorption of radioactive ions Cs and Sr. The synthesized DTS possesses a robust structure, high selectivity for Cs and Sr, and substantial adsorption capacity, incorporating redox-active titanium. Employing DTS as an active material in the electrode demonstrated a high adsorption capacity (186 mg Cs/g, 177 mg Sr/g) and rapid adsorption kinetics, reaching maximum capacity within 30 min, even in environments with a tenfold excess of competing ions. Furthermore, our system exhibited stable performance over 5 cycles of adsorption/desorption, and post-desorption analyses confirmed the durable structure stability without significant alterations. This study highlights the feasibility of CDI technology utilizing DTS for cesium and strontium removal. [ABSTRACT FROM AUTHOR]

Capacitive deionization (CDI) holds great promise for phosphorus adsorption and recovery from wastewater, while current electrodes exhibit poor salt adsorption capacity (SAC) and selectivity due to co-ion effect and interference of co-existed ions with high concentration. Herein, to overcome these issues, a facile route was proposed to prepare a covalent organic framework-membrane coating electrode (COF-MCE) by depositing polyvinyl alcohol/polyethyleneimine membrane onto activated carbon fiber (ACF) followed by the in-situ growth of COF-LZU1. Compared to ACF and MCE, COF-MCE, with abundant functional groups and specific sites, facilitated electrical double layer (EDL)-based adsorption and induced hydrogen-bond interaction with phosphorus, contributing to a higher SAC of phosphorus (10.5 mg/g) in treating 1 mM NaH 2 PO 4. When treating different mixtures of NaH 2 PO 4 /NaCl, NaH 2 PO 4 /NaNO 3 , and NaH 2 PO 4 /Na 2 SO 4 with molar ratio of 1:5, COF-MCE exhibited the high phosphorus selectivity of 3.62, 5.98 and 7.01, respectively, which was attributed to the synergistic effects of EDL-based adsorption, hydrogen-bond and pseudocapacitance. Desorption experiments revealed that high reversal voltage and alkaline condition can weaken the hydrogen-bond interaction and strengthen the electrostatic repulsion between COF-MCE and phosphorus, thus improving the desorption and recovery of phosphorus. These findings confirmed the high potential of COF-MCE in the effective electrosorption and recovery of phosphorus from wastewater. • COF-MCE was prepared by casting followed by in-situ growth of COF powders. • Inhibited co-ion effect and enhanced EDL formation led to superb P adsorption. • Hydrogen-bond and redox-reaction interaction contributed to high selectivity for P. • A reversal voltage and high pH weakened the interaction and favored P recovery. [ABSTRACT FROM AUTHOR]

Capacitive deionization (CDI) technology has shown practicality and cost-effectiveness for water desalination. However, electrode development remains a major challenge. Carbon electrodes, commonly used in CDI, have limited adsorption capacity, necessitating the improvement of their physicochemical and electrochemical properties. To address these limitations, Faradaic electrodes based on the redox process have emerged as an alternative. Faradaic reactions in CDI systems offer benefits such as pseudocapacitive/intercalation effects and the development of charged species. Different types of Faradaic reactions include anodic oxidation reactions (to be avoided in CDI), cathodic oxygen reduction reactions, and Faradaic ion storage processes involving intercalation effects. Faradaic electrodes exhibit higher ion storage capacity due to their redox reactions and have been produced in various forms, expanding the range of available CDI electrodes. This indicates a promising future for Faradaic materials in CDI technology. The review highlights recent strategies in the development of capacitive and Faradaic electrode materials for CDI, along with progress in material synthesis and its impact on desalination. The review also introduces approaches to address complexities in defining optimal electrode parameters and achieve material optimization in electrochemical desalination. Overall, advancements in material development for electrochemical desalination processes are overviewed. [Display omitted] • Electrochemical desalination of seawater/brine was reviewed. • A wide range of electrode material design has been analyzed. • Faradaic redox reactions in CDI were represented. • The performances of various electrode materials have been summarized and compared. • Future challenges in the field of CDI have been discussed. [ABSTRACT FROM AUTHOR]

Cathode materials with high capacity and cyclic durability are urgently needed for capacitive deionization. Na 3 V 2 (PO 4) 3 is an emerging pseudocapacitive candidate for this process on account of its high capacity and Na+ reversible insertion, while it suffers from the intrinsic low electrical conductivity and small electrolyte-accessible surface area. Herein, a sea urchin-like Na 3 V 2 (PO 4) 3 @carbon (NVP@C) compound material was synthesized from MXene-V 2 C via hydrothermal method followed by annealing treatment, and then assembled as capacitive deionization (CDI) electrode material. Benefiting from three-dimensional sea urchin-like structure, where the nanoneedles of NVP@C serve as fast transport channel for Na+, NVP@C performed high specific capacity (437 F g−1 at 5 mV s−1) and fast ionic diffusion kinetics. Moreover, the NVP@C showed attractive desalination performance as cathode material for CDI, achieving a remarkable adsorption capacity of 74.0 mg g−1 in a NaCl salt solution (1000 mg L−1) and long life cycle stability. The present work provides a novel perspective to construct practical redox-active and stable CDI electrode with 3D structure from MXene for highly capacitive and durable desalination application. [Display omitted] • An urchin-like NVP@C derived from V 2 C MXene was used for capacitive deionization. • The NVP@C showed superior electrochemical performance with high capacity of 437 F g−1. • The NVP@C exhibited an outstanding salt (Na+) adsorption capacity of 74.0 mg g−1. • The unique structure provides fast ion transfer channels and ensures the stability. [ABSTRACT FROM AUTHOR]

Heteroatom doping, involving the introduction of atoms with distinct electronegativity into carbon materials, has emerged as an effective approach to optimize their charge distribution. In this study, we designed a strategy to synthesize in-situ Mn, N co-doped carbon nanospheres (Mn-NC) through the polycondensation of 2,6-diaminopyridine and formaldehyde in synchronization with Mn2+ chelation to form Mn-polytriazine precursor, followed by calcination to form carbonaceous solid. Then Mn-NC was fabricated into a capacitive deionization (CDI) electrode for the selective removal of uranium ions (U (VI)), which is commonly found in radioactive water. Interestingly, Mn-NC exhibited good selectivity for UO 2 2+ capture with a demonstrated adsorption capacity of approximately 194 mg/g @1.8 V. The systematic analysis of the adsorption mechanism of UO 2 2+ revealed that N dopants within Mn-NC can coordinate with the U (VI) ions, thereby facilitating the removal process. Our study presents a straightforward and convenient strategy for removing UO 2 2+ ions by harnessing the coordination effect, eliminating the requirement for pore size control. [Display omitted] • Trace Mn in N doped carbon can boost charge density and adsorption energy. • Mn–N–C adsorbs UO 2 2+ ions by coordination interaction with high selectivity. • This material has a good adsorption capacity of 194 mg/g @1.8 V. [ABSTRACT FROM AUTHOR]

[Display omitted] • Dopamine-functionalized carbon interface enhances interfacial activity. • Boron and oxygen are codoped to optimize the capacitive behavior of carbon materials. • The inside-out pore creating method of body phase improves the activation efficiency. As a typical double-layer capacitive electrode material, carbon nanomaterials show desirable application potential in the field of capacitive deionization (CDI) and supercapacitors (SCs). In this work, interfacial functional groups and B\O co-doping defects are simultaneously introduced into multi-chamber carbon with confinement effect, and their practical applications in CDI and flexible capacitors are also studied. Specifically, the oxygen-rich carbon source and boron source, dissolved and mixed completely via co-crystallization method, released amount of gas during the pyrolysis process to produce multi-chamber honeycomb-like carbon with ultrathin walls, which were further introduced into abundant interfacial functional groups by dopamine to optimize the carbon surface. At the same time, part of boron atoms entered the carbon lattice to form boron doping at high temperature, followed by the integration with the oxygen heteroatoms in carbon source to generate abundant boron and oxygen codoping defects. The as-prepared diatomic defects by the synergistic effect promote the wettability and pseudocapacitance of the material. In addition, the derived carbon materials are endowed with good hydrophilic ability and surface activity under the modification of dopamine functional groups. The ideal hydrophilic ability ensures that the porous structure of material can be infiltrated by electrolyte to provide large effective specific surface area, which enhances the electrical double-layered capacity. The interfacial functional groups also offer additional pseudocapacitance, so as to improve the ion adsorption capacity of PDA-BOC. [ABSTRACT FROM AUTHOR]

• A system based on membrane rejection and electrochemical technique was developed. • MIL-101 endowed the mixed matrix membranes (MMMs) with phosphate adsorption capacity. • MMMs of capacitive deionization capacity was prepared with porous carbon cloth. • The conductive MMM performed both high selectivity and permeability. The great potential of ultrafiltration (UF) membranes for phosphate rejection has been challenged by the selectivity-permeability trade-off. Here, we propose a filtration system based on a novel dual-layer UF mixed matrix membrane (MMM) with a capacity for capacitive deionization (CDI), which was fabricated by incorporating NH 2 -MIL-101 into a polyethersulfone (PES) polymer matrix and the in-situ combination of carbon cloth (CC). The MMM exhibited an improved porosity of 85.52%, a declined water contact angle of 53.25°, and a 5.7% increase in water permeability in comparison to those of the PES membrane due to the hydrophilic and mesoporous NH 2 -MIL-101 particles. While incorporating the CC provided the non-conductive PES polymer with high electroconductivity. As an electrode, the CC/MMM exhibited a relatively high permeance of 424.63 L/h MPa m2, good antifouling performance, and remarkable phosphate rejection that reached 100% during the 2-h filtration period in the electrically-enhanced dead-end filtration cell. The removal capacity of the cell exceeded 22.51% and 74.0% for the energized CC/PES membrane and uncharged CC/MMM, respectively, due to the superior phosphate adsorption ability of NH 2 -MIL-101 and enhanced deionization capacity due to the electric field. UF MMM separation coupled with the electrochemical technique exhibited great potential to enhance the selective rejection of phosphate ions at low levels, while maintaining high membrane permeability and shortening the treatment time required by the CDI technique. [ABSTRACT FROM AUTHOR]

Novel two-dimensional carbon nitride hexagonal-like sheets were fabricated from thermal condensation, which uses hexaazatriphenylene-based precursor prepared microporous and oxidation resistant carbons materials (HAT-CN). The results show that the as-prepared HAT-CN is the first electrode utilized in capacitive deionization, due to the rich heteroatom and border-group content, as well as the potential collaborative effect of C and N atoms, displaying the quick ion diffusion and strong charge transfer performance. Additionally, the capacitive deionization performance of HAT-CN is improved by the synergistic contribution of a large accessible surface area, large pore volume, and high graphitization. The electrode of HAT-CN was carbonized at 550 °C which display an excellent specific capacitance of 179.2 F g−1 in 1-M NaCl solution at a scan rate of 5 mV s−1. Subsequently, a high salt adsorption capacity of 24.66 mg g−1 was attained for an applied voltage of 1.2 V in 500 mg L−1 NaCl solution, showing good cyclic stability at 30 cycles. Considering those excellent properties of the prepared HAT-CN, the capacitive deionization electrode should be an ideal candidate for high-performance deionization application. [ABSTRACT FROM AUTHOR]

Abstract Capacitive deionization (CDI) has emerged as an attractive approach for desalination and water treatment for its high energy efficiency, low cost, and easy regeneration. Here, we report the preparation and application of villiform carbon fiber paper (v-CFP) as the current collector for CDI device with high areal electrosorption capacity. The v-CFP was prepared by thermal annealing and subsequent O 2 plasma etching of the commercial pristine carbon fiber paper (p-CFP). With greatly improved surface area and hydrophilicity, the v-CFP itself showed an areal capacitance as high as 0.70 F·cm−2 in 0.5 M NaCl solution, and delivered an areal electrosorption capacity up to 122 and 400 mg·m−2 when feeding a NaCl solution with initial concentration of 50 and 3500 mg·L−1, respectively. Upon loading with additional electrode materials (EMs), e.g., graphene hydrogel, activated carbon, carbon nanotubes, and porous carbon spheres, the v-CFP/EM electrodes exhibited further improved areal electrosorption capacities, generally outperforming the corresponding p-CFP/EM electrodes by >2 times. Our studies demonstrate that proper surface treatment can largely improve the capacitance of the commonly used p-CFP current collector, making it greatly attractive in CDI devices. Graphical abstract Unlabelled Image Highlights • Annealing and O 2 plasma can improve the surface area and hydrophilicity of p-CFP. • The resultant v-CFP itself exhibits a high areal electroadsorption capacity. • The electrosorption capacity of v-CFP/AEM is higher 2 times that of p-CFP/AEM. • v-CFP has great potential as a new current collector in CDI device. [ABSTRACT FROM AUTHOR]

Abstract Capacitive deionization (CDI) performance, as measured by salt adsorption capacity (SAC) and energy normalized adsorption of salt (ENAS), is frequently limited by anion repulsion at the positive electrode. In this work, we investigate the ability to prevent co-ion repulsion by increasing complementary fixed charged within the electrode macropores by binding composite CDI electrodes with the ionically charged structural polysaccharides chitosan and carboxymethyl cellulose. When employing asymmetrically charged electrode binders, co-ion repulsion was prevented, resulting in SAC and ENAS values that were three times greater than composite electrodes bound with polyvinylidene fluoride (PVDF) and similar to CDI electrodes composed of chemically modified carbon. Polysaccharide binders did not modify the charge balance in the carbon micropores but did shift the discharge voltage of maximum adsorption, enabling a shift in operating voltage that prolonged cycle lifetime without a significant loss in performance. The mechanism of improved salt accumulation with polysaccharide binders was explored with a one-dimensional model that integrated CDI and ion-exchange membrane covered (MCDI) sub-units. Model simulations indicate that carbon macropores covered with thin layers of charged polysaccharides increase adsorption by a sequential accumulation and release of salt to depleted uncovered pores. Graphical abstract Image 1 Highlights • Charged polysaccharide binders form stable capacitive deionization electrodes. • Binding anode with deacetylated chitosan prevented co-ion repulsion. • Model simulations indicate covered macropores accumulate and release ions. • Ionic flux between covered and uncovered macropores enhanced salt adsorption. [ABSTRACT FROM AUTHOR]

Transition metal oxides (TMOs) have great application potential in capacitive deionization (CDI), which can selectively extract ions from water with high charge efficiency and low contamination. Previous studies have found that cationic segregation of TMOs has adverse effects on their electrochemical performance, but its effect on CDI remains unclear. Herein, we demonstrate for the first time that Ca2+ segregation provides more sites for F− adsorption by synthesizing Ca 2 Mn 3 O 8 for CDI selective defluorination. The Ca 2 Mn 3 O 8 material has a graded porous structure, low charge transfer resistance, high specific capacitance (255.64 F/g, 1 mV/s, 0.5 M NaF solution) and good charge/discharge reversibility (about 100 % charge specific capacity retention after 50 cycles). Moreover, Ca 2 Mn 3 O 8 has a high specific adsorption capacity (SAC) for NaF (1419.07 μmol/g) and excellent cyclic stability (capacity retention rate is 96.69 % after 100 cycles). The Ca2+ segregation from lattice to surface provides more sites for F− adsorption and greatly shortens ion transport paths. Density functional theory (DFT) calculation further suggests that the adsorption energy of Ca 2 Mn 3 O 8 for F− is much greater than that of Cl−. As a result, the removal rate of F− by Ca 2 Mn 3 O 8 in mixed solutions is up to 20 times higher than that of Cl−. [Display omitted] • It is proved that cationic segregation on the surface provides additional sites for ion adsorption. • Ca 2 Mn 3 O 8 electrode delivers outstanding fluoride removal performance and cycle stability. • Ca 2 Mn 3 O 8 electrode is much more selective for F− than for Cl−. [ABSTRACT FROM AUTHOR]

[Display omitted] • Divergent/convergent CDI cells are established and compared with straight ones. • The spacer geometry shifts the location of the initial adsorption and the depletion zone. • The channel slope mainly affects the desalination output rate. • Convergent channels show a faster ASAR than the regular straight geometry. • Steeper slopes give larger benefits. This research aims to explore the impact of tilted channel configurations of CDI cells on desalination performance. The results reveal that the titled convergent channels have a faster average salt adsorption rate (ASAR) than the regular straight geometry. For desalination operations that end at a quarter of the equilibrium salt adsorption capacity (SAC), the convergent spacer with a slight slope of 1.5 degrees has a 20 % higher ASAR than the typical straight geometry (0.15 mg/g/min for convergent and 0.12 mg/g/min for straight). This gain increases to about 24, 29.5, and 33%, respectively, for slopes of 3.5, 5.5, and 7 degrees, compared to the straight geometry with the same spacer thickness. By looking at the underlying mechanisms, the spacer geometry is found to shift the location of the initial adsorption. This affects how quickly the device outputs the cleaned water. Interestingly, the geometry angle can also affect the location of the depletion zone, so tilted spacers can also affect the behavior during electrode starvation. Specifically, the convergent geometry has the depletion zone in the middle of the electrode instead of the corner near the outlet, as seen for straight and divergent channels. Together, these findings indicate how to construct tilted spacers to enhance CDI performance. [ABSTRACT FROM AUTHOR]

[Display omitted] • Charging time plays a decisive role in CDI performance improvement. • UQ analysis quantitively provides guidance for CDI optimization. • The pore distribution and connectivity are highlighted. Capacitive deionization (CDI) is considered to be one of the most promising technologies to deionize water with low/medium concentration. Great attention has been paid to improve its performance by optimizing electrode architectures and operation conditions. However, no guidelines are built for developing such strategies due to the nonlinear impact of the parameters concerning electrodes and operations. Therefore, our work quantitatively reveals the influence of some key parameters involving electrode characteristics, operation conditions and ion properties using the statistical analysis based on uncertainty quantification. We find the porosity solely has a small influence on all the performance metrics. But the pore might play a significant role by the pore size distribution and connectivity that greatly determine the effective diffusion coefficient of salt ions. The high diffusion coefficient obviously raises the salt adsorption by facilitating ion transport, which also reduces the energy consumption while most of the consumed energy could even be recovered. The current exerts an opposite influence by shortening the charging period in the constant current (CC) mode. The salt concentration in the inflow water increases the adsorption capacity but decreases the removal efficiency. The decreased removal efficiency requires less external energy, enabling the usage of CDI as an economic pre-treatment unit. We highlight the ion transport resistance and its influence on CDI performance by acting upon charging time in the CC mode. We also suggest the application of the constant voltage mode to the low-salinity wastewater instead of the CC mode due to the high resistance. Our work addressed the importance of proper electrode and operation condition for high desalination performance and energy efficiency. [ABSTRACT FROM AUTHOR]

The resin-activated carbon composite (RAC) electrodes were fabricated and applied in capacitive deionization for recovery of V(V) from complex vanadium solution. The adsorption capacity of the RAC electrode for V(V) is extremely low and the reduction of V(V) is significant in low pH solution, but the adsorbed V(V) on the electrode increases obviously and the reduction of V(V) gradually diminishes with the rise of pH. However, as the pH is increased to 10, the adsorbed V(V) on the RAC electrode declines. The higher applied potential is beneficial to the adsorption of V(V) and 1.0 V is appropriate for the adsorption. The impurities ions (Al, P and Si) are mainly adsorbed in the electric double layers on the RAC electrode and V(V) is dominantly adsorbed by the resins in the electrode. The adsorbed impurity ions can be easily removed by diluted H 2 SO 4 and V(V) can be effectively eluted by 10% NaOH solution. The vanadium-bearing eluent can be recycled to recover and enrich vanadium from the complex solution. The performance of the RAC electrode keeps stable during the cyclic operation. This study may provide a promising and novel method for the recovery and separation of metals from aqueous solution. [ABSTRACT FROM AUTHOR]

Desalination technologies can increase potable water supplies worldwide. One possible approach, capacitive deionization (CDI), could prove optimal for brackish water treatment. At present, CDI development has largely focused on increasing salt adsorption capacity and desalination rates of materials and devices. In addition to these metrics, ultimately, optimizing salt rejection and throughput will be essential. In this work, we present a framework for the design of flow-between CDI cells for maximum salt rejection. We developed a model that is dependent on device specific parameters (system volume, flow rate, inlet and outlet water quality), to generalize the design of a cell for any given requirement. We showed that decreasing the advection-diffusion Péclet number and increasing the aspect ratio of the electrode compared to the channel space yield the highest salt rejection. In addition, tuning the cycle frequency time for salt rejection instead of complete electrode charging can yield faster water production rates and optimal salt rejection. These modeling results were validated through experimental prototypes that made use of vertically-aligned carbon nanotube (VA-CNT) electrodes. This framework to maximize salt rejection can be extended to a multitude of porous electrodes used in flow-between CDI devices. [ABSTRACT FROM AUTHOR]

[Display omitted] • EDTA grafted MXene was designed and fabricated as CDI electrode for treating simulant nuclear wastewater. • EDTA grafted MXene demonstrates high selectivity of Cs+ from concentrated HNO 3 solutions. • Cs+ can be concentrated to form an enriched waste stream that can be further utilized or disposed as a small waste package. • Stripping of adsorbed Cs+ by simple reverse of applied voltage led to easy regeneration with outstanding performance over 320 cycles. A capacitive deionization (CDI) electrode comprising ethylenediamine triacetic acid (EDTA) and 2D MXene (EDTA-MXene) is fabricated to separate Cs+ from strongly acidic process water. The method provides new direction for an advanced aqueous recycle process to separate fission products from spent fuel liquor. Grafting EDTA on MXene has no detrimental effect on its structure but does diversify the modes of ion interaction and increase the number of binding sites. The composite CDI electrode has a Cs+ adsorption capacity of 2.07 mmol g−1 at 1.2 V with 97.3% removal efficiency within 15 min. Step-wise adsorption and desorption cycling of the electrode highlights the chemisorption effect of EDTA which immobilizes the ions as the applied voltage is lowered, although almost all ions can be stripped by reversing the voltage to −1.4 V and without the need for chemical treatment. The EDTA-MXene electrode demonstrates outstanding cyclic performance with a capacity retention of >80% after 320 cycles. Furthermore, the electrode maintains its performance in strongly acidic solution, removing 0.66 mmol g−1 Cs+ at 1.2 V, as well as being stable after immersing in 3 mol L−1 HNO 3 for 7 days. Through continuous cycling it is possible to enrich the Cs+ into a highly concentrated solution for element recovery or safe disposal. The EDTA-MXene material is robust and maintains good performance in harsh chemical environments, levering its multiple binding sites to successfully isolate Cs+ from strongly acidic solutions and in the presence of competing ions, Sr2+ and Ce(IV). [ABSTRACT FROM AUTHOR]

• Novel MoSe 2 /MCHS composite with unique 2D/3D interconnected structure was synthesized. • The MoSe 2 /MCHS electrode exhibited excellent HCDI water desalination performance. • Excellent performance came from synergy of intercalation pseudocapacitance and EDLC. • The MoSe 2 /MCHS electrode exhibited great recyclability and low energy consumption. Transition-metal dichalcogenides can be used for capacitive deionization (CDI) via pseudocapacitive ion intercalation/de-intercalation due to their unique two-dimensional (2D) laminar structure. MoS 2 has been extensively studied in the hybrid capacitive deionization (HCDI), but the desalination performance of MoS 2 -based electrodes remains only 20–35 mg g−1 on average. Benefiting from the higher conductivity and larger layer spacing of MoSe 2 than MoS 2 , it is expected that MoSe 2 would exhibit a superior HCDI desalination performance. Herein, for the first time, we explored the use of MoSe 2 in HCDI and synthesized a novel MoSe 2 /MCHS composite material by utilizing mesoporous carbon hollow spheres (MCHS) as the growth substrate to inhibit the aggregation and improve the conductivity of MoSe 2. The as-obtained MoSe 2 /MCHS presented unique 2D/3D interconnected architectures, allowing for synergistic effects of intercalation pseudocapacitance and electrical double layer capacitance (EDLC). An excellent salt adsorption capacity of 45.25 mg g−1 and a high salt removal rate of 7.75 mg g−1 min−1 were achieved in 500 mg L−1 NaCl feed solution at an applied voltage of 1.2 V in batch-mode tests. Moreover, the MoSe 2 /MCHS electrode exhibited outstanding cycling performance and low energy consumption, making it suitable for practical applications. This work demonstrates the promising application of selenides in CDI and provides new insights for ration design of high-performance composite electrode materials. [Display omitted] [ABSTRACT FROM AUTHOR]

In this work, active carbon nanofibers (ACFs) and reduced graphite oxidate (RGO) nanocomposites were facilely synthesized and proposed as capacitive deionization (CDI) electrodes for removal of different salty ions. According to this study, the dosage of ACFs has a great effect on the CDI performance of these electrode materials. Cyclic voltammetry results illustrated that sample RGO-ACF 20 (RGO with 20 wt% ACFs) showed the best CDI performance among the all RGO-ACF x samples. The electro-adsorptive capacity of RGO-ACF 20 electrode was 2.99 mg/g with electrical voltage of 2.0 V in about 20 min. Furthermore, the electrosorption of salty ions onto RGO-ACF x follows the Langmuir isotherm, implying the monolayer adsorption. [ABSTRACT FROM AUTHOR]

Capacitive deionization (CDI) is a promising technology for the desalination of brackish water due to its potentially high energy efficiency and its relatively low costs. One of the most challenging issues limiting current CDI cell performance is poor cycling stability. CDI can show highly reproducible salt adsorption capacities (SACs) for hundreds of cycles in oxygen-free electrolyte, but by contrast poor stability when oxygen is present due to a gradual oxidation of the carbon anode. This oxidation leads to increased concentration of oxygen-containing surface functional groups within the micropores of the carbon anode, increasing parasitic co-ion current and decreasing SAC. In this work, activated carbon (AC) was chemically modified with titania to achieve additional catalytic activity for oxygen-reduction reactions on the electrodes, preventing oxygen from participating in carbon oxidation. Using this approach, we show that the SAC can be increased and the cycling stability prolonged in electrochemically highly demanding oxygen-saturated saline media (5 mM NaCl). The electrochemical oxygen reduction reaction (ORR) occurring in our CDI cell was evaluated by the number of electron transfers during charging and discharging. It was found that, depending on the amount of titania, different ORR pathways take place. A loading of 15 mass% titania presents the best CDI performance and also demonstrates a favorable three-electron transfer ORR. [ABSTRACT FROM AUTHOR]

As an indispensable element in the field of nuclear energy, uranium (U (VI)) is widely concerned because it is abundant and easy to obtain from seawater. Herein, we report a promising dual-template strategy to prepare heteroatom-doped carbon materials with hierarchical micro−/ meso −/macroporous from low-cost biomass precursors. The carbon materials with hierarchical porosity nitrogen/oxygen are synthesized through freeze-drying, high-temperature thermal treatment and subsequent HF etching treatment by in-situ growth of SiO 2 on the precursor with ethyl silicate as silicon source and introduction of ZnCl 2 as hard template and starch/ammonia as carbon and nitrogen source. Based on the advantages of three-dimensional interconnected hierarchical porous heteroatom doping, the electrosorption capacity reaches 67.25 mg/g, and the removal efficiency is as high as 98.45%, when the initial UO 2 (NO 3) 2 concentration is 80 mg/L in capacitive deionization. The isothermal adsorption of nitrogen doped hierarchical porous carbon (NHPC) is consistent with the Langmuir model, with a maximum electrosorption capacity of 152.43 mg/g, good cycle stability and high charge efficiency (energy consumption is only 0.77 kg-U kWh−1). Fascinatingly, the initial UO 2 (NO 3) 2 concentration is 80 mg/L and it also has a high adsorption capacity in 3.5 wt% NaCl solution, and the U (VI) removal efficiency could reach 42.5% after 8 h of continuous adsorption, with high selectivity. The prepared adsorbent NHPC shows good selectivity for U (VI) in the solution of 11 metal ions co-existing, and the effective selectivity K d value of U (VI) is 1620.84 mL/g. Finally, X-ray photoelectron spectroscopy analysis reveals that in addition to Coulomb interaction and chelation during the adsorption process, there is also a synergistic effect of the conversion of U (VI) to UO 2 under electric drive to improve the adsorption capacity of U (VI). • Dual-template synthesis of biomass-derived hierarchical porous heteroatom-doped carbon materials. • The removal efficiency of the NHPC electrode for U (VI) in 3.5 wt% NaCl solution is 42.5%. • The synergistic effect between UO 2 2+ and NHPC through coulomb interaction and reduction reaction makes it selective for UO 2 2+. [ABSTRACT FROM AUTHOR]

Prussian blue analogue (PBA) as a typical Faraday electrode material has important application value in capacitive deionization (CDI) for its rigid open framework and unique ion storage properties. But poor electrical conductivity and deformation of crystal structure during the desalination process restrict its development. To enhance the stability of PBA and provide rapid charge transfer, a new composite hollow core-shell polyaniline-encapsuled Ni-Prussian blue analogue (H-NP@PANI) is constructed. Experiments illustrate that PANI formed an omnidirectional uniform and highly conductive layer on the crystal surface, which tremendously improved the electrochemical capacity of H-NP@PANI (242.24 F·g−1). In constant voltage mode, the adsorption capacity of AC║H-NP@PANI cell is 35.86 mg·g−1, which was the highest to H-NP (20.82 mg·g−1) and pure Ni-PBA (16.08 mg·g−1). The retention rate remains 96.1 % after 50 cycles. XRD tests show that superior cycle stability is due to the hollow structure and buffer space provided by PANI shell, which can effectively inhibit the degree of crystal transformation. Furthermore, DFT calculation confirms the existence of a highly stable tight electron coupling between H-NP and PANI, and the electron aggregation center around Fe atom facilitates the Fe3+/Fe2+ redox cycle. Hence, this work can provide a feasible strategy for designing stable and efficient electrodes. [Display omitted] • Hollow core-shell PANI-encapsuled Ni-PBA (H-NP@PANI) with omni-directional conductive layer was constructed. • H-NP@PANI electrode showed superior electrochemical performance with high specific capacity of 242.24 F·g-1. • H-NP@PANI electrode had remarkable desalination performance of 35.86 mg·g-1 with superior desalination cycle stability. • Both DFT calculation and experimental analysis revealed the synergistic effect between PANI shell and H-NP in CDI process. [ABSTRACT FROM AUTHOR]

[Display omitted] • A low-carbon strategy was proposed for discarded mask recycling. • High Cr(VI) removal (358.68 mg g−1) was achieved using low-cost CPM/PDA/PPy. • Excellent desalination performance was obtained via capacitive deionization. • It has shown excellent reusability in Cr(VI) adsorption and desalination. The global mask consumption has been exacerbated because of the coronavirus disease 2019 (COVID-19) pandemic. Simultaneously, the traditional mask disposal methods (incineration and landfill) have caused serious environmental pollution and waste of resources. Herein, a simple and green mass-production method has been proposed to recycle carbon protective mask (CPM) into the carbon protective mask/polydopamine/polypyrrole (CPM/PDA/PPy) composite by in situ polymerization of PPy. The CPM/PDA/PPy composite was used for the removal of Cr(VI) and salt ions to produce clean water. The synergistic effect of PPy and the CPM improved the removal capability of Cr(VI). The CPM/PDA/PPy composite provided high adsorption capacity (358.68 mg g−1) and economic value (811.42 mg $−1). Consequently, the CPM/PDA/PPy (cathode) was combined with MnO 2 (anode) for desalination in CDI cells, demonstrated excellent desalination capacity (26.65 mg g−1) and ultrafast salt adsorption rate (6.96 mg g−1 min−1), which was higher than conventional CDI cells. Our work proposes a new low-carbon strategy to recycle discarded masks and demonstrates their utilization in Cr(VI) removal and seawater desalination. [ABSTRACT FROM AUTHOR]

Fluorine pollution has been recognized as one of the major problems of global concern. Metal-organic frameworks (MOFs), as potential fluoride adsorbents, possess unique chemical reactivity and broad functionalities. However, their application is restricted by instability in fluoride or acid/base solution and difficult separation. Here, lotus flower-like Al/N-doped porous carbon composite was prepared by a simple pyrolysis method using Al-based MOF as a precursor and used for capacitive deionization (CDI) defluorination. Through pyrolysis, MOF-derived carbon not only obtained a larger specific surface area and N-rich structure, but also possessed multiple F− capture sites. These carbon substrate enabled the metal centers to be uniformly dispersed and contributed to enhanced electrical conductivity and chemical stability of electrode material. Through the CDI defluorination system, F− migrate rapidly with the aid of the electric field and was captured under the synergistic effect of metal sites and carbon substrate. According to the Langmuir isotherm, the maximum F− removal capacity was 76.28 mg g−1 at 1.2 V, and the adsorption kinetics followed pseudo-second-order model. The electroenhanced adsorption method exhibits high adsorption capacity and selectivity for F− due to combining the advantages of adsorbent adsorption and electrosorption, which already exceeds the capacities of most electrode material. [Display omitted] • MOF-derived Al/N-doped porous carbon composite (AlNC) is firstly proposed in Capacitive deionization defluorination. • The F− removal capacity reached 76.28 mg g-1 at 1.2 V. • Presented a possible mechanism for the elctrosorption of F- on AlNC. [ABSTRACT FROM AUTHOR]

Capacitive deionization (CDI) has been considered as the most promising and environmentally friendly electrical desalination technology owing to its low energy consumption and no secondary pollution. CDI is based on the principle of electric double layer for salt ion adsorption, but the existence of co-ions repulsion reduce the charge efficiency, leading to the low salt adsorption capacity. To prevent the intrinsic "co-ion effect" inside the porous carbon electrodes, membrane capacitive deionization (MCDI) by applying an ion-exchange membrane (IEM) to the surface of electrode is of increasing interest. However, MCDI brings various resistances, such as the internal and interface resistances of membrane, as well as the contact resistance between membrane and carbon electrode. More recently, by integrating "membrane" with carbon electrode without the introduction of free-standing IEM, integrated-MCDI has shown great merits to enhance counter-ion selectivity of electrode and decrease resistance, resulting in improving adsorption rate and reducing energy consumption. In this review, the preparation methods of innovative electrodes, the working mechanisms and performances of integrated-MCDIs, and the advanced advantages are summarized. It is hoped that this work will provide insight into integrated-MCDI and shed light on the future direction of water desalination based on CDI technology. [Display omitted] • Integrated-MCDI has been developed without using free-standing IEM on electrode. • Integrated-MCDI outperforms MCDI on energy consumption, cost, and kinetic rate. • Integrated-MCDI has the potential of anti-organic pollution and ion-selective removal. • Review summarized current status of integrated-MCDI for water desalination. • The mechanisms of integrated-MCDI with different fabrication methods are analyzed. [ABSTRACT FROM AUTHOR]

[Display omitted] • The universal low carbon mass loading was used in capacitive deionization (CDI) for efficient deionized (DI) water production. • Partial desalination performance weakened with increasing carbon mass loading. • CDI cells using YEC-8–5.81 as electrodes produced DI water with a low conductivity of 1.89 μS/cm. • Carbons with a suitable micro- meso pore distribution and high specific capacitance may be a better choice. • 82.2% DI water recovery was achieved under stopped flow (SF) discharge mode. High-purity deionized (DI) water can be widely used in high-end manufacturing fields, but its efficient and energy-saving production is a huge challenge. Here, simple carbon mass loading control was developed for efficient DI water production via advanced capacitive deionization (CDI). Supercapacitor carbon (YEC-8) electrodes with diverse mass loading of 2.50, 5.81, 12.63 and 24.72 mg/cm2 were prepared for desalination. As expected, YEC-8–2.50 acquired the optimal CDI performance. High mass loading of 12.63 and 24.72 mg/cm2 not only caused an overt property drop but also serious wasted materials. Competitively, YEC-8–5.81 achieved a similar volumetric adsorption capacity (VAC) to YEC-8–2.50 as well as made up for the lack of total mass of YEC-8–2.50 and the resultant solution conductivity was as low as 1.89 μS/cm (grade III DI water). This strategy has been successfully extended to other carbon materials and carbons with suitable micro- meso pore distribution and high specific capacitance may be a better choice. Besides, the reverse effect of acetylene black (AB) mass on desalination was verified. To gain a high water recovery (W R), stopped flow (SF) discharge was introduced and realized a high W R of 82.2 %. Meanwhile, cycling tests indicated its splendid cycling stability. This study provides a universal direction for using low mass loading strategy for efficient DI water production. [ABSTRACT FROM AUTHOR]

Capacitive deionization (CDI) is attracting increasing attention as an emerging technology for the facile removal of ionic species from water. In this work, the feasibility of fluoride removal from low-salinity groundwaters by single-pass constant-voltage CDI was investigated and a model developed to describe the dynamic fluoride electrosorption behavior. Effects of operating parameters including charging voltage and pump flow rate as well as impact of fluoride and chloride feed concentrations on the effluent fluoride concentration and equilibrium fluoride adsorption capacity were studied and the obtained data used to validate the model. Using the validated model, the effects of various design parameters, including arrangement of multiple CDI cells, on fluoride removal were assessed. Single-pass constant-voltage CDI was found to be effective in removing fluoride from low-salinity groundwaters but, as expected, removal efficiency was compromised in waters of high salinity. The relatively simple electrosorption model developed here provided a satisfactory description of both fluoride removal and current evolution and would appear to be a useful tool for prediction of CDI performance over a range of operating conditions, cell arrangements and feed water compositions though scope for model improvement exists. [ABSTRACT FROM AUTHOR]

[Display omitted] • Nitrogen-doped porous carbon is synthesized via a dual-salt template approach. • The carbon exhibits high ion adsorption capacity and ultrafast adsorption rate. • Ion adsorption proceeds via electrocapacitive and pseudocapacitive mechanisms. • The electrocapacitive mechanism plays a dominant role in the ion adsorption. Porous carbon materials hold tremendous potential for capacitive deionization (CDI). However, realizing high specific surface area (SSA), suitable pore size distribution, and hierarchical porosity in carbons with facile and sustainable techniques is still challenging. In this work, we prepare hierarchical porous carbons (HPCs) with high SSA, abundant pore structures, and N-self doping via a versatile and sustainable modified salt-template approach. The optimized electrode gives a high SSA of 1639.9 m2/g, a large pore volume (2.7 cm3/g), and hierarchical porous structures (micro-meso-macro pores). For applications, the symmetric CDI electrode assembly delivers a salt adsorption capacity (SAC) of up to 17.67 mg/g in a 500 mg/L NaCl at 1.2 V together with ultrafast salt adsorption kinetics. We systematically study their associated desalination mechanism by combining several electrochemical experiments with the in-situ Raman spectroscopy technique, which revealed that the total salt storage is from the synergistic effect of both electrical double-layer (EDL) storage and pseudocapacitive mechanisms, with the EDL storage being dominant. This work paves the way to design economical and eco-friendly porous carbon materials for high-performance CDI desalination. [ABSTRACT FROM AUTHOR]

Capacitive deionization (CDI) is an emerging technology in brackish water desalination. Several types of CDI electrodes have been fabricated and tested throughout the years and most, if not all, require hours of CDI performance testing to assess their capability. In this study, a model was created to predict the salt adsorption capacity of different carbon-based CDI electrodes. Random forest, a statistical classification method, was utilized in the making of the model. Based on the results, the generated models had high accuracy and precision in estimating the salt adsorption capacity, representing R2 values mostly over 0.9. Furthermore, feature importance analysis showed that specific capacitance at higher scan rates can reduce the performance of the model. Therefore, it is recommended that low scan rate specific capacitance be used in creating any kind of CDI model. This study also illustrated that a universal model is possible for CDI with a minor sacrifice in precision. Depending on the goal of the end user, individual and universal models can be used in the prediction of salt adsorption capacity of various CDI electrodes. • The SAC of different CDI electrodes was predicted by the RF model. • Carbon electrodes doped with heteroatom and metal/metal-heteroatom were examined. • Variable importance of operational condition and electrode property was explored. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ppy grafted CMC carbon aerogels are applied in CDI for the first time. • Ppy/CA-2 shows excellent electrochemical and electrosorption performance. • The salt adsorption capacity of Ppy/CA-2 reaches 34.03 mg g−1. • Ppy could capture Cl− via surface redox reaction during CDI. Capacitive deionization (CDI) has been identified as a prospective desalination technology. There has been increasing interest in the exploration of electrodes with high electrochemical properties and salt adsorption capacity. Here, polypyrrole (Ppy)/CA electrodes were prepared by grafting Ppy on carbon aerogel (CA) via in-situ polymerization and applied in CDI desalination. Compared with CA, the Ppy/CA samples display higher specific capacitance, lower charge transfer resistance as well as enhanced ion diffusion rate. Particularly, Ppy/CA shows enhanced pseudocapacitance generation and the synergistic effect of the diffusion-controlled dominating process and capacitive process by Ppy introduction, conducive to the desalination performance. The optimal Ppy/CA-2 CDI system exhibits high adsorption capacity (34.03 mg g−1 with 500 mg L−1 solution), fast salt adsorption rate (0.053 mg g−1 s−1 with 500 mg L−1 solution) together with reasonable long-term stability. The outstanding desalination performance is principally on account of the positively charged nitrogen atoms of Ppy could capture Cl− well through the surface redox reaction in CDI and the most positively charged nitrogen in Ppy/CA-2 contribute its best adsorption capacity. The simple preparation method, excellent electrochemical properties, outstanding adsorption performance, and good cycling stability render Ppy/CA-2 a potential CDI electrode material. [ABSTRACT FROM AUTHOR]

Given that carbon-based electrodes suffer from poor long-term stabilities and limited desalination capacity, the optimized charge storage mechanism used for improving desalination performance remains requisite. In this work, the KC-pre-intercalated carbon nanosheets (PCNs) with abundant conjugated carbonyl groups were designed via a one-step smelting method for the first time as a novel intercalation electrode in the symmetric capacitive deionization. Interestingly, below 500 mg/L, the physical electrical double-layer mechanism is mainly influential. Benefitting from the proposed dynamic intercalation mechanism, a remarkable ion removal capacity of 1.65 g g−1 and impressive cyclability of 300 cycles (97.96%) were observed in the high salt concentrations (>1000 mg/L) and simulated seawater system rather than pure NaCl solution. The ultrahigh adsorption capacity and prominent cycling stability make the PCNs-based device rank top three among these of all the symmetric CDI systems reported in the literatures so far. The Na+ ions are intercalated in the enlarged interlayer spacing without a host structure deformation, thus offering a high ion removal rate (2.04 mg g−1 min−1) and a low energy consumption (0.28 Wh g−1). Our work exemplifies the importance of engineering conjugated carbonyl groups and pre-intercalated structure to capacitive deionization for brine. [Display omitted] • A novel dynamic intercalation mechanism leads to a remarkable SAC of 1.65 g g−1 in the simulated seawater system. • Pre-intercalation carbon nanosheets deliver superior SAC (40.72 mg g−1) as well as life-span (300 cycles) in 500 mg/L NaCl. • Engineering conjugated carbonyl groups and pre-intercalated structure to capacitive deionization. [ABSTRACT FROM AUTHOR]

Cadmium (Cd2+) is the main toxic heavy metal in desulphurisation wastewater for coal-fired power plants. Here, a novel carbon skeleton/molybdenum disulphide composite material (MoS 2 /AC) with expanded layer spacing was synthetized in one step for Cd2+ removal through the capacitive deionization method. As the results showed, the Cd2+ removal capacity of the MoS 2 /AC cell was 22.15 mg g−1, and the concentration of Cd2+ in the simulated wastewater was effectively reduced from 10 ppm to <0.1 ppm. According to the characterisation, the layer spacing of MoS 2 nanosheets in MoS 2 /AC increased by 58.06% compared with the original commercial MoS 2 , which could provide more active sites for Cd2+ adsorption. Moreover, the XPS results showed that electrosorption and complexation on the surface of MoS 2 /AC were the main routes for Cd2+ removal, especially the complexation between Cd2+ and metal-sulphur. In addition, the selective adsorption efficiency of MoS 2 /AC was more than 98% under the coexistence of other ions, such as Na+, Mg2+, and Ca2+. The regenerated MoS 2 /AC also had a high adsorption capacity for Cd2+ even after 5 cycles. The MoS 2 /AC material also exhibited excellent removal capability for heavy metal ions in the actual desulphurisation wastewater from coal-fired power plants (<1 ppb). The results indicated that MoS 2 /AC may be a prospective material for heavy metal ion removal from wastewater. [Display omitted] • MoS 2 /AC was developed as an effective CDI electrode material. • High Cd2+ absorption capacity of 22.15 mg g−1 and excellent recyclability. • The obtained MoS 2 /AC possess enlarged interlayer spacing to expose S adsorption sites. • The exposed sulphur atoms complexed with cadmium ions to remove Cd2+. [ABSTRACT FROM AUTHOR]

Capacitive deionization (CDI) technology is a high-efficiency, energy-saving, green and environmentally friendly new water treatment technology, which will have potential in the field of low-salinity seawater desalination. Current carbon-based electrode materials have limited desalination capacity and rate due to insufficient ion-accessible specific surface area and slow ion transmission rate. Here, we demonstrate layered porous carbon (CHPC) derived from Chitosan, colloidal SiO 2 and polytetrafluoroethylene (PTFE) mixed carbonization for efficient CDI desalination. CHPC has abundant macropores that are connected with mesopores. This layered porous structure enables CHPC to have a faster ion migration rate. The salt removal capacity of CHPC reaches 26.5 ± 1.5 mg/g, and the average adsorption rate is 8.8 ± 0.5 mg/g/min, which are about 1.6 and 10 times than that of commercial activated carbon, respectively. In addition, CHPC has extremely strong anti-interference performance against organic macromolecules, which could maintain more than 95% adsorption capacity after 30 cycles. Our work helps to promote the practical application of hierarchical porous structures based on typical biomass in large-capacity and fast-rate CDI desalination technology. [Display omitted] • CHPC forms a macroporous-mesoporous hierarchical carbon network framework through a multi-step etching reaction. • CHPC achieves an ultra-fast adsorption efficiency of 8.8 ± 0.5 mg/g/min due to fast ion/electron transport. • CHPC has excellent cycle stability and resistance to interference from macromolecular substances. [ABSTRACT FROM AUTHOR]

Activated-carbon-electrodes are widely used in capacitive deionization (CDI). However, the desalination capacity of most activated carbons remains to be improved, which makes it necessary to explore activated carbons with special structure and improved desalination capacity. In this context, we studied the desalination performance of super activated carbons with ultra-high total pore volume (V Tot). This carbon showed a salt adsorption capacity of 27.2 mg/g in 500 mg/L NaCl solution at 1.2 V, which ranked among the highest value ever reported for activated carbons. The high adsorption capacity was due to the large V Tot caused by the abundant presence of both micropores and mesopores in the material. The adsorption capacity of the carbon after 10 adsorption-desorption cycles (saturated adsorption capacity) was as high as 67.6 mg/g, which was dominated by V Tot and water contact angle. The sample also showed excellent desalination rate and superior charge efficiency (76.5%). This study indicated that superior desalination performance could be obtained by using super activated carbon with high pore volume as a CDI electrode material. [Display omitted] • SAC with high pore volume (AC-P-CO 2) showed excellent desalination performance. • The salt adsorption capacity of AC-P-CO 2 was 27.2 mg/g. • The salt adsorption capacity ranked among the highest values for activated carbons. • The salt adsorption capacity after 10 cycles of AC-P-CO 2 was 67.6 mg/g. • AC-P-CO 2 showed superior desalination rate and charge efficiency (76.5%). [ABSTRACT FROM AUTHOR]

[Display omitted] • Developed a carbonized MIP-202/MXene (CMM) for selective electrosorption of U(VI). • MXene provides pseudocapacitance/intercalation effect and conductive base. • Two composite structures were observed during preparation of CMMs. • The CMM electrode exhibited excellent adsorption capacity and selectivity for U(Ⅵ). As the key element in the nuclear industry, uranium (U) has been widely concerned for its stable acquisition and efficient recycling. Capacitive deionization (CDI), which known as a seawater desalination technology, has shown great potential for uranium-adsorption. However, the poor selectivity restricts the application of CDI in uranium extraction. In this work, a carbonized MIP-202/MXene (CMM) composite was developed for highly efficient selective electrosorption of U(VI) from multi-ionic water. The uniformly decorated MIP-202 can prevent MXene from aggregation and provide rich pore structures, large specific surface area and additional nitrogen source for the efficient electrosorption of U(VI). With the addition of MXene nanosheets, a conductive network was introduced into the CMM and improved mechanical robustness of material to facilitate rapid charge transfer at the interface. As a result, the CMM electrode exhibited a maximum adsorption capacity of 582.46 mg g−1 calculated from Langmuir model and a removal ratio of 89.1% at 1.2 V in UO 2 2+ solution. The selectivity adsorption of CMM for UO 2 2+ can reach 92.3% when treated with multi-ionic solution. Besides, there is an interesting fact that the decreased ratio of added MXene can got a higher adsorption capacity, which may cause by the two composite structures formed when the mixing ratio changed. Overall, the excellent conductivity, high adsorption capacity and selectivity make CMM a promising electrode in CDI for efficient extraction of uranium from water. [ABSTRACT FROM AUTHOR]

• The first high entropy oxides based multifunctional electrode material is synthesized by simple grinding technique. • Nano-sized (< 10 nm) high entropy oxides are incorporated in surface functionalized biowaste derived activated carbon. • High salt adsorption capacity of 241 mg g−1 and cycle repeatability upto 20 cycles is demonstrated. • The excellent desalination capacity is attributed to the combined electric-double layer and pseudocapacitive mechanism. Nano-sized high entropy oxides (particle size < 10 nm) are expected to possess enhanced stability, electrochemical properties and increased potential in versatile applications than bulk metal oxides. In this study, nano-sized high entropy oxides incorporated surface functionalized coconut shell derived activated carbon (HEO/f-CSAC) is synthesized, characterised and used as an advanced multifunctional material for electrochemical energy storage and desalination. Attributed to its unique microstructure and extraordinary properties, HEO/f-CSAC shows an excellent salt adsorption capacity of 241 mg g−1 at 1.2 V in 500 mg L−1 concentration of NaCl solution, which is very high as compared to previous reports. The reason behind high salt adsorption capacity is its outstanding specific capacitance of 147.5 F g−1 at current density 1 A g−1. Moreover, the assembled lab-scale CDI unit demonstrates extremely good performance upto 20 adsorption-desorption cycles due to the long cycle life with good capacitance retention of fabricated HEO/f-CSAC based two-electrode supercapacitor. Additionally, a red light-emitting diode illumination by connecting three supercapacitors in series, indicates the high efficiency and potential of HEO/f-CSAC for real life energy-related application. These results prove that HEO/f-CSAC can be implemented for water desalination with enhanced supercapacitive performance manifesting its multifunctionality for energy and environmental applications. [Display omitted] [ABSTRACT FROM AUTHOR]

A self-supported MoS 2 /CoS 2 @TiO 2 nanotube (TNT) hybrid electrode was fabricated by an in-situ hydrothermal process-assisted growth strategy, which achieved a significantly high gravimetric Na+ adsorption capacity and excellent regeneration performance. It also demonstrated amazing removal efficiencies close to 90% for various heavy metal ions (i.e., Fe3+, Cr3+, Cd2+, Pb2+, Cu2+ and Ni2+), revealing its great potential for wastewater purification. [Display omitted] • A self-supported MoS 2 /CoS 2 @TNT hybrid electrode was constructed. • The electrode achieved a gravimetric Na+ adsorption capacity of 44.22 mg g−1. • The electrode exhibited excellent regeneration performance and removal rates. • The electrode also realized removal efficiencies up to 90% for heavy metal ions. Recently, the rational design and construction of graphene-like layered transition-metal disulfides (TMDs) with chemical and physical superiorities to act as capacitive desalination (CDI) electrodes for water purification have attracted increasing interest worldwide. Herein, an advanced self-supported MoS 2 /CoS 2 @TiO 2 nanotube (TNT) hybrid electrode was grown on a TNT scaffold by a hydrothermal process-assisted in-situ growth strategy, and an asymmetric CDI cell was constructed with the as-fabricated MoS 2 /CoS 2 @TNT and commercial AC electrodes as cathode and anode, respectively. Benefiting from the effective structural feature (i.e., fully open nanosheet-assembled flowers and hierarchical pore alignment) and synergy effect of metal-like CoS 2 , state-of-the-art MoS 2 and TNTs with an efficient charge "superhighway", the MoS 2 /CoS 2 @TNT electrode achieved a significantly high gravimetric Na+ adsorption capacity of 44.22 mg g−1 in a 600 mg L−1 NaCl solution at 1.2 V. Moreover, the hybrid electrode possessed excellent regeneration performance with a capacity retention of nearly 100% even after 20 cycles. Most notably, such electrode also demonstrated amazing removal efficiencies close to 90% for various heavy metal ions (i.e., Fe3+, Cr3+, Cd2+, Pb2+, Cu2+ and Ni2+), revealing its great potential for wastewater purification. [ABSTRACT FROM AUTHOR]

Capacitive deionization (CDI) has aroused significant concern for its application in treating low-concentration brackish water. However, some carbonaceous materials suffer from the limited salt adsorption capacity (SAC), sluggish adsorption kinetics, and inferior selectivity to particular ions. Hence, we develop the well-dispersed few-layered two-dimensional pseudocapacitive material MoS 2 connected with a robust 3D conductive architecture constructed by carbon nanotubes (CNT) and carbon spheres (CS) (MoS 2 @CNT-CS). The enhanced dispersibility and expanded interlayer of MoS 2 nanosheets provide sufficient faradic effective sites for ion storage and transfer, while the interlaced carbonaceous network affords improved hydrophilicity, convenient electron transport path, and low charge transfer resistance for fast charge transportation and permeation. Therefore, the asymmetric hybrid CDI cell (CNT-CS for anode and MoS 2 @CNT-CS for cathode) delivers a prominent SAC (25.35 mg g−1), fast SAR (3.9 mg g−1 min−1), enhanced desalination kinetics, and favorable cycling stability. In addition, the specific ion selectivity of MoS 2 @CNT-CS electrode was evaluated systematically in the equimolar multi-salt solutions, and a selectivity order of Ca2+ > Na+ > Mg2+ > K+ was obtained. The molecular dynamic (MD) calculation provides the lowest intercalation energy for Ca2+ (−2.55 eV), further confirming the strong interaction and preferable pseudocapacitive deionization of MoS 2 @CNT-CS electrode to Ca2+ over other cations. [Display omitted] • Few-layered MoS 2 is dispersed in a robust 3D carbonaceous CNT-CS architecture. • MoS 2 @CNT-CS has improved active sites, conductivity, and hydrophilicity. • MoS 2 @CNT-CS electrode shows a prominent salt adsorption capacity and rate. • MoS 2 @CNT-CS has an ion selectivity of Ca2+ > Na+ > Mg2+ > K+. [ABSTRACT FROM AUTHOR]

[Display omitted] • HBC was synthesized via a "sacrifice template" method and applied in CDI. • The salt adsorption capacity was 28.06 mg g−1, the ASAR was 1.56 mg g−1 min−1. • Hollow bowl-like structure and N-doped 3D carbon reached high ion removal capacity. As a bright technology for desalinating brackish water, the desalination ability of capacitive deionization (CDI) has been limited by the electrode materials. However, it was still a challenge to adjust the construction of CDI electrode material to improve the salt removal ability. Here, nitrogen-doped hollow bowl-like carbon (HBC) has been successfully constructed in ZIF-8 by facile templating approaches. Physical measurements displayed that HBC had a hollow bowl-like structure, a certain number of nitrogen-containing groups and high specific surface area (1244 m2 g−1). These unique features made HBC have high capacitance (210.7F g−1), remarkable cycle stability and low charge transfer resistance in electrochemical double layer capacitors, indicating its promising application in CDI desalination. Due to the abundant ion accumulation active sites and rapid ion diffusion of HBC materials, provided by unique characteristics of regular hollow bowl-like structure, microporous dominated structure, interconnected channels and nano-scale wall thickness, HBC showed high salt removal ability of 28.06 mg g−1 with 1.2 V in 0.5 g L-1 NaCl solution. Besides, HBC exhibited excellent average salt ion adsorption rate (1.56 mg g−1 min−1) and initial salt adsorption capacity of 90% after 20 cycles. [ABSTRACT FROM AUTHOR]

Rocking-chair capacitive deionization (RCDI) is a semi-continuous desalination technology, however, suffering from issues pertaining to effluent salinity and pH. We thoroughly investigated the composition evolution and pH-related behavior in RCDI and disclosed the underlying causes for quality decay. RCDI concomitantly produces two streams: the desalted cathodic effluent becomes basic, whereas the brine anodic stream becomes acidic. These pH-related species deteriorate the outflow quality and distort conductivity profiles. Under variation in dissolved oxygen and potential distribution, we verified the carbon oxidation and oxygen evolution reactions as causes for the acidic anode and oxygen reduction as the cause for the basic cathode. Faradaic reactions and resultant pH fluctuation were exacerbated with increasing charging voltages, even at 0.9 V, whereas they were negligibly affected by the resembling potential distribution at different flowrates. To improve pH stability and water recovery, we integrated the ion exchange and flowrate adjustment procedures; consequently, water recovery increased to 71.43% and pH was stabilized at 5–6.5 for over 80 cycles, with significant adsorption capacity enhancement owing to the ion exchange with pH-related species. Thus, with a better understanding of pH-related behaviors, RCDI can be optimized and extended more specifically for demands such as acid/base production and resource enrichment. • RCDI with semi-continuity and high capacity was thoroughly investigated. • Evolution of pH and mechanism for performance variation was disclosed fundamentally. • The effect of voltage and flowrates on pH and RCDI behavior was expounded. • Facile modification for stable outflow composition and high efficiency were proposed. [ABSTRACT FROM AUTHOR]

• Porous carbon nanoflakes were prepared by carbonizing xylose with KHCO 3. • Porous carbon nanoflakes exhibited a satisfactory salt adsorption capacity. • Porous carbon nanoflakes have great potential for large-scale application. Capacitive deionization (CDI) works by using the electrical double layer on various materials including nanoporous carbons to separate ions from saline water. To help realize industrial application, there has been an increasing interest in the exploration of carbon materials from low cost, eco-friendly and abundant biomass for CDI to align with the demands of sustainable development strategies. Herein we report pyrolysis of xylose with KHCO 3 to prepare hierarchically porous carbon nanoflakes which display a satisfactory salt adsorption capacity of 16.29 mg g−1. This novel strategy can design highly efficient carbon materials from naturally-developed biomass materials with its low preparation cost, environmentally friendliness and superior desalination performance. Our xylose-derived hierarchically porous carbon nanoflakes are promising for potential industrial application for CDI. [ABSTRACT FROM AUTHOR]

Electrosorptive desalination is a very simple and appealing approach to satisfy the increasing demand for drinking water. The large-scale application of this technology calls for the development of easy-to-produce, cheap and highly performing electrode materials and for the identification and tailoring of their most influential properties, as well. Here, biosynthesised bacterial cellulose is used as a carbon precursor for the production of three-dimensional nanostructures endowed with hierarchically porous architecture and different density and type of intrinsic and hetero-atom induced lattice defects. The produced materials exhibit unprecedented desalination capacities for carbon-based electrodes. At an initial concentration of 585 mg L−1 (10 mmol L−1), they are able to remove from 55 to 79 mg g−1 of salt; as the initial concentration rises to 11.7 g L−1 (200 mmol L−1), their salt adsorption capacity reaches values ranging between 1.03 and 1.35 g g−1. The results of the thorough material characterisation by complementary techniques evidence that the relative amount of oxygenated surface functional species enhancing the electrode wettability play a crucial role at lower NaCl concentrations, whereas the availability of active non- sp 2 defect sites for adsorption is mainly influential at higher salt concentrations. • Bacterial-cellulose (BC) is biosynthesised by using Gluconacetobacter medellinensis as a strain. • By carbonising BC, microporous nanocarbons with different amount and type of defects are produced. • BC-derived nanocarbons are tested as electrodes for the capacitive deionization of water. • The BC-based electrodes remove 55–79 mg g−1 of NaCl from a solution with a 585 mg L−1 initial concentration. • The C network defectiveness plays a key role in the electrode capacitive and electrosorptive behaviour. [ABSTRACT FROM AUTHOR]

Capacitive deionization (CDI) is an electrochemical method of removing salt ions from brackish water. A common assumption in CDI is that monovalent ions (e.g., Na+, Cl−) are removed in a 1:1 symmetry on the electrodes. Validation of this assumption with techniques such as ion chromatography is not commonly performed, but is important to better understand how parasitic process, such as faradaic reactions, affect ion removals. In this study, we quantified the removals of Na+ and Cl− as a function of electrode orientation in flow-through CDI. When the cathode was positioned upstream, Na+ and Cl− removals approached a 1:1 symmetry, but when the anode was located upstream, we observed a significant drop in Na+, but not Cl−, removals. We attributed this drop to oxygen reduction reactions at the cathode that competed with Na+ adsorption. Oxidation of carbon in the upstream anode yielded H+ that enhanced the reduction of oxygen to H 2 O 2 at the downstream cathode, which in turn diverted electrons from Na+ adsorption. In the absence of oxygen, Na+ removals increased in the upstream anode orientation and were comparable to Cl− removals, confirming that competition with oxygen reduction reactions was the primary reason for decreased Na+ removal. In the upstream cathode orientation, we show that H 2 O 2 generated at the cathode can be oxidized at the downstream anode, possibly enhancing Na+ removals via internal electron recycling. Salt adsorption capacities calculated using actual ion removals did not always agree with those estimated using changes in solution conductivity, with the largest disagreement observed when conductivity data were corrected for pH changes. Our results highlight that faradaic reactions, particularly oxygen reduction reactions, can contribute to asymmetrical removals of monovalent ions in flow-through CDI. Image 1 • Removal of Na+ and Cl− in flow-through capacitive deionization was studied as a function of electrode orientation. • Positioning the anode upstream led to a signficant decrease in Na+, but not Cl−, removal. • Oxygen reduction reactions at the cathode caused the drop in Na+ removals. • Salt adsorption capacities adjusted for actual ion removals did not always agree with estimates based on changes in solution conductivity. [ABSTRACT FROM AUTHOR]

Capacitive deionization (CDI) is an emerging technology for energy-efficient water desalination, and attracts more and more attention in recent years. It has been concluded that CDI technology shows competitiveness and perspectives on seawater desalination and wastewater treatment. The ionic adsorption mechanism can be clarified by electric double-layer capacitive adsorption and pseudocapacitive adsorption. The performance of CDI depends on both device and materials. The adsorption capacity and energy efficiency was improved significantly with fast growth of researches on material and novel energy storage techniques. This review summarizes researches on CDI technologies with an emphasis on electrode material design and improved adsorption performance. This review outlines the ion storage mechanisms and electrode materials of capacitive deionization. Unlabelled Image • Electrochemical extraction of ions from seawater/brine was reviewed systematically. • An emphasis on electrode material design thought has been analyzed. • The performance of various extraction devices has been summarized and compared. • Future challenge on this technology has been clarified. [ABSTRACT FROM AUTHOR]

• Freundlich model can well describe the phosphate electroadsorption behavior. • CDI electroadsorption is an endothermic process according to thermodynamics. • pH was determined by water electrolysis and forms transformation of phosphate. • Surface charge can reflect the amount of ions adsorbed by GCS model. The advanced treatment for phosphate (P) has been focused on in the wastewater treatment field in recent years. Developing an innovative P removal technology with high efficiency, low cost, and low energy consumption has become an imperative requirement. Capacitive deionization (CDI) is an emerging environmentally friendly technology for removing charged ions from aqueous solutions. In this study, we investigated the performances of P removal by CDI with homemade activated carbon electrodes. Furthermore, kinetics, thermodynamics and the equilibrium Gouy-Chapman-Stern (GCS) double-layer model were studied to reveal the mechanism of P adsorption. It was observed that the adsorption capacity was positively correlated with the voltage. The pH first increased and then decreased during the adsorption because of water electrolysis and forms transformation of phosphate. The experimental results were validated by pseudo-first-order adsorption kinetics and Freundlich isotherms. The adsorption rate increased with increasing temperature, voltage, and concentration. The adsorption capacity decreased with increasing temperature, and the maximum adsorption capacity was 8.53 mg/g. Thermodynamic analysis confirmed that the electrosorption of P on the activated carbon electrode was an endothermic process, and the high temperature was conducive to reducing the adsorption potential of P on the activated carbon electrode. The theoretical results of the double layer model were in agreement with the experimental data, which showed that the ion adsorption can be reflected by surface charge. This study can help to understand the adsorption mechanism of P on activated carbon electrodes and provide a theoretical basis for the application of P removal by CDI. [ABSTRACT FROM AUTHOR]

• A biocharcoal-based electrode was used to improve Cl− and Cr3+ removal. • The adsorption and regeneration performance of the KOBC15 were analyzed and evaluated. • Optimization of operation parameters for the CDI system was performed. Red oak biocharcoal, a novel electrode material that is inexpensive and highly conductive, was used for capacitive deionization (CDI) to simultaneously remove Cl− and Cr3+ from leather processing wastewater. The results showed that both the carbonization temperature and time played important roles in reducing the resistivity (ρ) of biocharcoal. Additionally, at 1000 °C and 3 h, the ρ value was reduced from 4.68 to 1.19 Ω·cm after the addition of Fe 2 O 3. KOH impregnation enhanced the desalination capacity of the biocharcoal electrodes by increasing the specific surface area and hydrophilicity. Under the optimal operating parameters (voltage of 1.8 V, flow rate of 10 ml/min and electrode spacing of 2 mm), the KOBC15 electrode exhibited a maximum adsorption capacity of 13.79 mg/g. After 10 cycles of CDI system operation, the removal rates of Cl− and Cr3+ reached 86.7% and 100%, respectively, in simulated leather processing wastewater. This work provides new ideas and methods for the treatment of high-salinity wastewater. [ABSTRACT FROM AUTHOR]