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• 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]

Capacitive deionization (CDI) has been attracted great interest as an emerging desalination technology and the electrode materials play a critical role on improving CDI performance. In this work, nitrogen-enriched activated carbon fibers (AN-CFs) were prepared from the natural based silk cocoon through simple carbonization and CO 2 activation. Their electrochemical and electrosorption behaviors were studied in NaCl solution. The results show that AN-CFs exhibit a fibri form porous structure with rich nitrogen element and CO 2 activation enhances their specific surface area. Compared with unactivated nitrogen-enriched carbon fibers (196.05 F g − 1 at 1 mV s − 1 , 12.02 mg g − 1 in 1000 mg l − 1 NaCl solution), AN-CFs exhibit more excellent capacitive (236.03 F g − 1 ) and CDI (16.56 mg g − 1 ) performances, showing that CO 2 activation is proved to be an effective method to improve electrochemical and electrosorption performances and the AN-CFs should be a promising electrode material for CDI application. [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]

Nitrogen-doped porous carbon spheres (NPCSs) were prepared through a fast microwave-assisted approach using sucrose as the precursor in a microwave system and subsequent thermal treatment in ammonia atmosphere at different temperatures. The morphology, structure and electrochemical performance of the NPCSs were characterized by scanning electron microscopy, nitrogen adsorption-desorption, X-ray photoelectron spectroscopy, cyclic voltammetry and electrochemical impedance spectroscopy, and their electrosorption performance in NaCl solution was studied. The results show that NPCSs treated at 1000 °C exhibit an extremely high electrosorption capacity of 14.91 m g g −1 when the initial NaCl concentration is 1000 mg l −1 , which shows great improvement compared with their undoped counterpart. The nitrogen doping is suggested to be a very effective method to improve the electrosorption performance, and the NPCSs should be a very promising candidate as electrode material for CDI application. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a comparative study on electrosorptive behavior of commercial single walled and double walled carbon nanotubes (CNTs) and as-prepared graphene as potential electrode materials for capacitive deionization (CDI). Their electrosorptive behaviors are evaluated under the same experimental conditions and described by various adsorption isotherms and kinetics models. It was found that the electrosorptive capacity of CNTs (single walled or double walled)-based CDI is higher than that of graphene-based CDI, which is related to their specific surface area, pore structure and hydrophilic functional groups. Electrosorptive kinetics analysis further confirms that CNTs are more suitable as electrode materials than graphene. [Copyright &y& Elsevier]

Metal oxide nanoparticles are one kind of important electrode materials for capacitive deionization (CDI) application. Unfortunately, most metal oxide materials usually exhibit poor electrical conductivity and often experience slow dissolution of metallic species, thereby resulting in limited desalination performance. Overcoming these obstacles still remains challenging due to the lack of synthetic approaches. Taking it into consideration, herein we proposed a dual confinement strategy to promote the CDI application of metal oxide nanoparticles. Nb 2 O 5 was synthesized as a typical example for designing metal oxide/MXene-reduced graphene oxide (rGO) derived from Nb 2 CT x MXene/graphene oxide (GO) under hydrothermal condition. In the hybrid, Nb 2 CT x and rGO serve as both the confinement agent and conductive support for Nb 2 O 5 nanoparticles, endowing Nb 2 O 5 /Nb 2 CT x -rGO with improved electronic conductivity and structural stability. As a result, the hybrid shows an outstanding desalination performance, including a NaCl adsorption capacity of 41.07 mg g−1, an ultra-fast average NaCl adsorption rate of 4.14 mg g−1 min−1 and excellent long-term cycling stability of 50 cycles (81 % desalination capacity retention rate). This work is expected to bring new insights in the synthesis of metal oxide materials for electrochemical desalination. [Display omitted] • A "dual-confinement" system of Nb 2 O 5 /MXene-rGO was constructed from Nb 2 CT x /GO. • Nb 2 O 5 /MXene-rGO was applied as electrode for hybrid capacitive deionization. • Nb 2 O 5 /MXene-rGO exhibited high desalination capacity/rate and good cycling stability. [ABSTRACT FROM AUTHOR]

To improve the long-term stability of capacitive deionization (CDI), one effective strategy is to introduce ion-exchange membranes into CDI (called as MCDI) owing to their ability to separate electrodes from oxidants. However, detailed explorations on the electrode corrosion in MCDI are seldom reported up to now. In this work, we studied in details the cathode oxidation in an anion-exchange membrane CDI (AEM-CDI) by employing activated carbon (AC) anode and different cathodes, AC and carbon sphere (CS). The results show that the anodes display a slight change during long cycling due to the isolation effect of AEM. However, the electrosorption capacity of AC cathode is slowly reduced, while CS cathode exhibits an improved electrosorption ability during the long-term operation. By analyzing the structure, morphology and element variations of original and cycled cathodes, it is found that both of specific surface area (SSA) and zero charge potential ( E pzc ) of AC cathode are reduced, while CS cathode exhibits an enhanced SSA and E pzc , indicating that the desalination performance of AEM-CDI cell, even with cathode materials with low E pzc value and SAA, can be enhanced via effectively utilizing cathode oxidization. [ABSTRACT FROM AUTHOR]

• HATN with rich electroactive sites and MXene jointly build a unique 3D structure. • HATN/MXene hybrid was applied as capacitive deionization electrode. • HATN/MXene exhibits a good performance with high desalination capacity and rate. In this work, we report a hybrid material of pyrazine based π -conjugated organic molecule, hexaazatrinaphthalene (HATN), and MXene. In this hybrid, MXene serves as a conductive substrate and HATN provides multiple electroactive sites, which are jointly combined to build a unique 3D structure with more ion transport channels for accelerating the charge transfer and ion diffusion rates. The as-obtained HATN/MXene composite displays a superior desalination capacity of 57.5 mg g−1 with an excellent desalination rate of 13.2 mg g−1 min−1 and good long-term stability. This work provides a meaningful guidance for designing and applying organic molecules in capacitive deionization field. [ABSTRACT FROM AUTHOR]

A novel system combining photocatalysis and capacitive deionization (CDI) was proposed and used to efficiently remove the total Cr from the aqueous solution for the first time. MIL-53(Fe) with wide visible-light absorption was successfully prepared by a simple solvothermal method, and applied as positive photoelectrode material of the photocatalysis-CDI system (PCS) to convert Cr(VI) to Cr(III). Active carbon was used as negative electrode material to absorb Cr(III). An enhanced removal of Cr(VI) can be obtained by applying visible light irradiation and 1.3 V direct current (DC) voltage simultaneously and the Cr(VI) removal can reach a maximum value of 81.6%, much higher than those under individual 1.3 V DC voltage (39.4%) or visible light irradiation (54.2%), demonstrating the synergistic effect from CDI and photocatalysis. More importantly, effective removal of the total Cr with high removal ratio (72.2%) can be achieved, which is difficult to be realized using current other technologies. The strategy in this work provides a promising method for the complete removal of high-valence heavy metal ions. [ABSTRACT FROM AUTHOR]

• NiHCF/MXene was prepared via in situ close packing of NiHCF on MXene nano-stacking. • NiHCF/MXene exhibits 3D charge transfer pathway with unique vertical charge transfer. • NiHCF/MXene exhibits both excellent desalination capacity and high desalination rate. Prussian blue analogues (PBAs), as a kind of metal–organic framework–like materials, has attracted great attention in capacitive deionization (CDI) field due to excellent redox activity, but its desalination performance, especially its desalination rate, is greatly limited due to its poor electrical conductivity. Hybridization of PBAs with MXene can potentially solve this problem, but still remains intrinsic limitation, such as poor vertical charge transfer between nano-sheets. Herein, we engineered a three-dimensional (3D) charge transfer pathway in nickel hexacyanoferrate (NiHCF)/MXene through in situ close packing of NiHCF nanoparticles on MXene nano-stacking by electrostatic attraction. Compared to charge transfer mode in two-dimensional (2D) nano-sheets, 3D MXene nano-stacking can not only have excellent conductivity like MXene to provide horizonal charge transfer pathway alongside nano-sheets, but also possess unique vertical charge transfer pathway between nano-sheets. As a result, the NiHCF/MXene exhibits a superior desalination performance with a high desalination capacity of 30 mg g−1, ultrahigh desalination rate of 9.5 mg g−1 min−1 and good cycling stability over 30 cycles. This work demonstrates an effective way to address the poor CDI performance of nickel hexacyanoferrate nanoparticles by employing 3D MXene nano-stacking as charge transfer support, and is of significance to be applied for other nanoparticle materials. [ABSTRACT FROM AUTHOR]

Capacitive neutralization deionization with flow electrodes (FCND) was proposed by combining neutralization dialysis (ND) and flow-electrode capacitive deionization (FCDI). Its key property is that by employing flow electrodes, capacitive adsorption of salt ions occurs simultaneously during the neutralization dialysis process. To compare the desalination performance between FCND and FCDI, a series of experiments were conducted by changing salt concentration and applied electrical voltage. The hybrid exhibits a promising average salt adsorption rate (ASAR) and salt removal efficiency (SRE) compared to either ND or FCDI process. The ASAR and SRE of FCND are 203.27 mmol m −2 min −1 and 72.20% in 0.1 mM NaCl solution at 1.2 V after 120 min, whereas the values of FCDI are 120.86 mmol m −2 min −1 and 42.92%, respectively. In order to facilitate the understanding of the ion transportation in this complex system, a theory was also set up based on simplified Nernst-Planck equation and Donnan equilibrium, which helps to understand the experimental data. [ABSTRACT FROM AUTHOR]

One of the most challenging issues in developing supercapacitor and capacitive deionization (CDI) technologies is the rational design and synthesis of active electrode materials, with favorable morphologies, reasonable porous structure and high surface area. Recently, three-dimensional (3D) graphene frameworks (3DGF)-based hybrids have been considered as one of the most promising candidates for supercapacitor and CDI applications. By incorporating functional nanomaterials into 3DGF, the obtained 3DGF-based hybrids exhibited enhanced properties or unique character compared to 3DGF. Here we report novel hierarchical hybrids with microporous carbon spheres (MCS) decorated 3DGF (3DGF-MCS), which not only possesses a hierarchical porous structure, but also maintains favorable features for supercapacitor and CDI applications, such as a large surface area and good electrical conductivity. The results indicate that 3DGF-MCS with 10 wt% MCS exhibits excellent electrochemical performances, including a superior specific capacitance of 288.77 F g −1 , and an ultrahigh electrosorption capacity of 19.8 mg g −1 . The 3DGF-MCS should be promising electrode materials for highly efficient supercapacitor and CDI applications. [ABSTRACT FROM AUTHOR]

Here a facile strategy was proposed to prepare graphene (GE) sheets with unique mesoporous morphology (denoted as mGE) via a simple carbothermal reaction by using KMnO 4 as the etching agent. The morphology, structure and electrochemical properties of mGE were investigated by scanning electron microscopy, transmission electron microscopy, Raman spectra, nitrogen adsorption-desorption, cyclic voltammetry and electrochemical impedance spectroscopy. The electrosorption performance of mGE in NaCl solution was studied and compared with GE. The results show that mGE electrode exhibits a high electrosorption capacity of 6.38 mg g −1 when the initial NaCl concentration is ∼75 mg L −1 and applied voltage is 1.2 V, which is about 2.3 and 1.5 times of those of GE (2.76 mg g −1 ) and recently reported microporous GE (∼4 mg g −1 ), respectively, and also higher than those of other carbon materials in the literatures, due to short ion diffusion path, large accessible surface area and low charge transfer resistance offered by its novel mesoporous structure. Therefore, it is believed that mGE is a promising candidate for practical capacitive deionization applications. [ABSTRACT FROM AUTHOR]

Porous carbon spheres (PCSs) were fabricated through a fast microwave-assisted approach using sucrose as the precursor in a microwave system and subsequent thermal treatment at 600, 800 and 1000 °C. The morphology, structure and electrochemical performance of the PCSs were characterized by scanning electron microscopy, Raman spectroscopy, nitrogen adsorption-desorption, cyclic voltammetry and electrochemical impedance spectroscopy. Their electrosorption performance in NaCl solution was studied and compared with activated carbon, carbon nanotubes, reduced graphene and carbon aerogels. The results show that due to their high specific surface area and low charge transfer resistance, PCSs treated at 1000 °C exhibit high electrosorption capacity of 5.81 m g g −1 when the initial solution concentration is 500 mg l −1 , which is higher than those of other carbon materials. [ABSTRACT FROM AUTHOR]

MXene-derived nitrogen-doped 3D nanofibrous architecture enables excellent fast deionization rate by breaking through the intrinsic structural limits of common 2D nanosheets. [Display omitted] • Nitrogen-doped 3D nanofibrous architecture was successfully fabricated from MXene. • This architecture breaks through the intrinsic structural limits of common 2D nanosheets. • Superior deionization rate and remarkable cycling stability are achieved. Two-dimensional (2D) nanosheets are promising electrode materials for electrochemical desalination by capacitive deionization (CDI). Like most 2D nanosheets, the delicate design of MXene-based materials can achieve state-of-the-art desalination capacities, but the intrinsic low ion diffusion characteristic of 2D nanosheets limits the desalination rate. To address this problem, we synthesize a new-family of nitrogen-doped three-dimensional (3D) nanofibrous architectures from MXenes (denoted as N-TNF) via direct alkalization and subsequent nitrogenization of common MXene stacks. N-TNF has a unique nanofiber structure and plentiful nitrogen dopants, resulting in expanded interlayer spacing, high specific surface area and excellent electrochemical activity. As a result, the N-TNF shows an ultrahigh mean desalination rate of 5.6 mg g−1 min−1, along with superior desalination capacity of 44.8 mg g−1, as well as good long-term cycling stability, which is comparable to state-of-the-art MXene electrodes and better than most 2D materials. This work demonstrates the fabrication of MXene-derived 3D materials, and provides a new approach to overcome the limits of 2D nanosheets for efficient CDI. [ABSTRACT FROM AUTHOR]

Highlights: [•] A modified membrane capacitive deionization device was proposed. [•] The electrodes consisted of ion exchange polymers and carbon nanotubes. [•] The polymers improved the contact adhesion and reduced the co-ion expulsion effect. [•] The device exhibited a high NaCl removal with a good regeneration ability. [ABSTRACT FROM AUTHOR]

Abstract: Carbon nanotubes (CNTs)–polyacrylic acid (PAA) composite film has been successfully fabricated by electrophoretic deposition method in PAA aqueous solution containing CNTs and used as electrodes for capacitive deionization (CDI). The PAA serves as the cation-exchange polymer and the matrix to incorporate the CNTs. The CDI unit cell based on CNTs–PAA composite film electrode exhibits a high NaCl removal of 83%, 51% higher than that of the cell based on pure CNTs electrode, with a good regeneration ability. [Copyright &y& Elsevier]

Abstract: Multi-walled carbon nanotubes (CNTs) films have been successfully fabricated by electrophoretic deposition technique and used as electrodes for capacitive deionization (CDI). The as-deposited CNTs electrode exhibits a comparable electrosorption capacity in NaCl solution to other conventional carbon electrodes. The results suggest that electrophoretic deposition has much potential for the facile and controllable fabrication of CDI electrodes. [Copyright &y& Elsevier]

Capacitive deionization (CDI) is an emerging water desalination technique that holds great potential in solving the global water shortage issue. It is known that, H 2 O 2 formation at the cathode interface causing the performance degradation of activated carbons (ACs) electrode has limited the large-scale implementation of CDI, but up to date there has seldom any effective solution being reported. In this work, we proposed the use of nitrogen-doped carbon spheres (NCSs)-based CDI cathode instead of that made up of activated carbons (ACs). Benefited from the enhanced oxygen reduction ability offering by the abundant active nitrogen dopants, the asymmetrical apparatus assembled by NCSs cathode and ACs anode exhibits an enhanced cycling stability with a retention ratio of 79.6% even after 50 cycles, which surpasses that of symmetrical apparatus (only 18%). This work provides a simple and effective approach to improve the cycling stability of CDI, highlighting the significance of surface nitrogen-doping modification and the importance of interfacial electrochemistry at the nanoscale. [ABSTRACT FROM AUTHOR]