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[Display omitted] • A novel two-stage FCDI (TS-FCDI) configuration is proposed. • PANI-CC-AC freestanding electrode construction by one-pot ectropolymerization method. • Secondary ion transfer between the two types of electrodes improves the desalination performance of TS-FCDI. • Role of the two types of electrodes in deionization process was comprehensive analyzed. The desalination of seawater/wastewater utilizing flow-electrode capacitive deionization (FCDI) has received significant interest. However, challenges like the low electrical conductivity of flow electrode and excessive energy consumption have prevented the scaling up of desalination operations. To overcome these issues, this study introduces a new FCDI system termed the two-stage FCDI (TS-FCDI) system, in which desalination modules are equipped with freestanding and flow electrode. The TS-FCDI system offers a graded desalination module design that produces an average salt removal rate (ASRR) of 113 µg cm2·min−1. However, the TS-FCDI system requires substantially reduced infrastructure investment, device size, and energy expenses. Notably, the design allows for the simultaneous operation of freestanding and flow electrode and enables the transport of ions and charges in the electrode region. Experimental evidence from multiple device configurations (CDI, FCDI, and TS-FCDI) supports this claim. Notably, the TS-FCDI system shows about twice the desalination performance over FCDI, with approximately 50 h of single-cycle (SC) operation. This enhancement links the freestanding electrode with the flow electrode, enabling the device to be well-suited for practical applications. The continuous flow of electrode particles in contact with the freestanding electrode initiates secondary ion transport, merging two different types of electrodes to enhance the active space of the electrode, effectively improving ion adsorption performance. In summary, the results of this study indicate the potential of the TS-FCDI system as a suitable desalination technology for scaling up applications. [ABSTRACT FROM AUTHOR]

A vertically oriented porous PVA hydrogel and PDA/CB modified 3D raised-fabric are innovatively designed as an solar evaporator to enhance the evaporation rate, conversion efficiency and self-desalting performance. [Display omitted] • A super water transport PVA hydrogel and PDA/CB modified 3D raised-fabric are innovatively designed as solar evaporators. • It exhibits high water evaporability (1.73 kg m-2h−1, 1 sun) and conversion efficiency (91.9 %). • The 3D raised-fabric solar evaporator system can be readily fabricated for scalable production. • The collected water is in full compliance with the drinking requirements of WHO and EPA. A solar evaporator with simple fabrication process, exceptional photothermal conversion efficiency and dependable continuous water evaporation holds significant potential for seawater desalination and wastewater treatment. Despite the substantial research dedicated to developing high-efficiency solar evaporators, there remains a need for additional exploration to tackle concerns such as salt crystallization, low evaporation efficiency, poor durability and stability. Here, an efficient water transport PVA hydrogel (water transport layer) and polydopamine/carbon black modified 3D raised-fabric (photothermal layer) are innovatively designed as solar evaporators. The synergistic effect between the periodic raised structure and polydopamine/carbon black results in outstanding photothermal conversion capabilities of PDA/CB@RF. Meanwhile, the porous structure of PVA hydrogel and its vertically aligned channels facilitate ion exchange and ensure a consistent water supply to the photothermal layer. This phenomenon serves to prevent the accumulation of salt in the photothermal layer, thereby markedly enhance the water evaporation capability of the PDA/CB@RF evaporator. The results demonstrate that the PDA/CB@RF evaporator system exhibits excellent photothermal conversion performance with its surface temperature rapidly ascending to 73.3 ℃ under 1 sun light intensity, and effectively boost the water evaporation rate (1.73 kg m-2h−1, 1 sun). Furthermore, the PDA/CB@RF evaporator exhibits excellent conversion efficiency (91.9 %) and self-desalting capability. Overall, this solar evaporator system with a 3D raised fabric design integrates light and thermal energy, consistent water evaporation and self-desalting functionalities, which makes it hold the potential to solve the pressing challenge of water shortage. [ABSTRACT FROM AUTHOR]

[Display omitted] • Mn dopant-reinforced NaTi 2 (PO 4) 3 possesses superior structural stability and conductivity. • The optimal NT 1.5 M 0.5 P exhibits ultrahigh salt removal capacity of 128.5 mg/g and cycle stability. • The ultrahigh durability is rooted from the solid-solution reaction induced by Mn-ions pillar. • NT 1.5 M 0.5 P||NaFeHCF rocking-chair desalination battery owns stable, rapid and low energy consumption desalination. Battery deionization (BDI) technique has attracted great attention as a potential candidate to tackle the freshwater crisis, but its application is plagued by the lack of electrode materials with large desalination capacity and excellent durability. Herein, Mn dopant-reinforced NaTi 2 (PO 4) 3 (NT 2–x M x P) composites with significantly enhanced structural stability and electronic/ionic conductivity were fabricated toward seawater desalination. Especially, the NT 1.5 M 0.5 P exhibits excellent desalination performance, affording a salt removal capacity (SRC) of 128.5 mg/g with a salt removal rate (SRR) of 18.4 mg g−1 min−1 and almost no capacity attenuation after 500 cycles. Furthermore, the NT 1.5 M 0.5 P further possesses favorable technical feasibility in different aqueous media, including desalination at various NaCl concentrations, temperature, solution pH, and natural seawater. Mechanism studies reveal that the NT 1.5 M 0.5 P undergoes a solid-solution reaction based on the reversible electrochemical reaction of the Ti4+/Ti3+ redox pair during the desalination/salination process. A rocking-chair desalination battery consisting of NT 1.5 M 0.5 P and NaFeHCF demonstrates a favorable desalination ability (average SRC of 49.4 mg/g and SRR of 1.73 mg g−1 min−1), outstanding cycling stability (92 % of capacity retention in 100 cycles), low energy consumption (0.288 kWh/kg-NaCl) and high average charging efficiency (86.9 %). This work sheds light on the rational design of advanced NASICON-type electrodes for seawater desalination with remarkable desalination capacity and long serving life. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Capacitive deionization (CDI) has emerged as a promising way to obtain freshwater. • Nitrogen-doped mesostructured carbon nanocrystals (NMCs) are useful for CDI. • NMCs show a high nitrogen content and large accessible surface area. • NMCs exhibit a high salt adsorption capacity of 20.63 mg g−1. • The salt adsorption capacity is much higher than for the typical AC benchmarks. Abstract Capacitive deionization (CDI) has emerged as a promising way to obtain freshwater from saline water, but its implementation is in its infancy and remains challenging due to the low salt adsorption capacity (SAC) of commonly used activated carbons (ACs). It is thus desirable to develop carbon electrodes that can exceed the performance of ACs benchmarks. In this work, nitrogen-doped mesostructured carbon nanocrystals (NMCs) have been developed by direct carbonization of highly ordered mesostructured polymers. Due to their mesoporous structure, high N content and large surface area, NMCs exhibit a maximum SAC of 20.63 mg g−1. This state-of-the-art carbon electrode largely surpasses common ACs. This work demonstrates the significance of the material synthetic chemistry and the importance of nanostructuring carbon materials for CDI applications. [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]

[Display omitted] • A ternary composite slurry by introducing AC, CB and CNT particles is proposed. • CNT and CB particles serve as bridging and filling effects, respectively. • A unique connection of charge percolation networks in AC/(CB + CNT) slurry was investigated. • Charge transfer enhanced mechanism in AC/(CB + CNT) slurry was explored by CFD simulation. • AC/(CB + CNT) slurry in FCDI exhibits a faster removal salt rate and lower molar energy consumption. Weak contact between activated carbon (AC) particles results in a poor conductivity for the electrode slurry, which limits the improved desalination performance in flow-electrode capacitive deionization (FCDI). Based on the bridging and filling effects of nanoparticles, we report a ternary composite slurry with multi-dimensional sized carbon particles by introducing the carbon nanotubes (CNT) and carbon black (CB) into AC-based slurry, namely AC/(CB + CNT) slurry. Further enhanced electrical conductivity relative to conventional pure AC slurry. By comparison analysis, FCDI system with the ternary carbon composite slurry had a faster average salt removal rate (ASAR) of 1.89 μmol cm−2 min−1 under at 1.2 V in a feed salinity of 2.0 g L−1, which is 3.27 1.92 and 1.46 times that of pure AC, AC/CB and AC/CNT slurry. In long-term operation (1800 min, U = 1.6 V), the AC/(CB + CNT) slurry exhibited a stable ASAR of 4.41 μmol cm−2 min−1 and maintained a charging efficiency (CE) of around 96% and molar energy consumption (E m) of 0.043 kWh mol−1. Electrochemical analysis and computational fluid dynamics (CFD) simulation further confirms that AC/(CB + CNT) slurry enable system with a point-to-line-to-surface connection morphology, which generates a robust charge percolation network with diverse pathway and facilitates the charge transfer. In summary, this study developed a facile strategy for increasing the slurry conductivity to provide a feasible approach in efficient desalination application. [ABSTRACT FROM AUTHOR]

Asymmetrical capacitive deionization (A-CDI) is expected to be a promising desalination technique with high salt removal capacity. In this work, the novel A-CDI are proposed where the negative and positive electrode are Na 4 Ti 9 O 20 (NTO) and activated carbon (AC), respectively. Basically, the NTO nanotubes with the specific surface area of 142.43 m 2 /g have been prepared by hydrothermal reaction. The electrochemical test shows that the specific capacitance of optimum NTO electrode can reach 120.45 F/g in 1 M Na 2 SO 4 solution with the scan rate of 1 mV/s by employing three-electrodes method. Moreover, the electrosorption capacity of AC/NTO asymmetrical electrodes is approach to 23.35 mg/g in NaCl solution with an initial concentration of 250 ppm. To improve the conductivity of NTO, reduced graphene oxide (rGO) is proposed to couple with NTO, named as rGO@NTO, to constitute the AC/rGO@NTO A-CDI. As a result, the AC/rGO@NTO A-CDI exhibited an ultrahigh desalination capacity of 41.8 mg/g in NaCl solution with an initial concentration of 250 ppm under 1.4 V, which is almost twice higher than that of AC/NTO electrodes and the highest value among all reported data anywhere. Besides, the charge efficiency of AC/rGO@NTO approaches to 1, implying the co-ions impaction has been successfully restricted. [ABSTRACT FROM AUTHOR]

[Display omitted] • Capacitive deionization enables membrane- and chemical-free boron removal. • Reversing discharge polarity enhances boron removal by capacitive deionization. • Order-of-magnitude enhancement of boron electrosorption per charge. • Single-pass separation process meeting WHO recommendations is demonstrated. • Low energy consumption of 0.2 kWh per m3. Increasing global water stress motivates a growing interest in seawater desalination by reverse osmosis. Boron is typically weakly removed by reverse osmosis membranes at seawater pH, necessitating expensive post-processes such as caustic agent dosing of the permeate followed by additional filtration steps. It has been previously demonstrated that membraneless capacitive deionization can enable chemical-free boron removal from reverse osmosis permeate, with basic design rules established. However, the level of boron electrosorption per cell charge was limited to ∼0.5 µmol/g, a level too low for practical applications. We here explore, both theoretically and experimentally, methods to enhance boron removal by capacitive deionization. We found that reversing the polarity of the applied voltage during the discharge step resulted in an order of magnitude increase in boron electrosorption to nearly 4 µmol/g with promising energy consumption of 0.2 kW⋅h/m3. The promise of these results is highlighted when compared with recently-developed boron electrosorption cells requiring bipolar membranes, which demonstrate similar boron removal of 4.35 µmol/g but with much higher energy consumption of 18.3 kW⋅h/m3 while incurring significant membrane costs. Overall, we demonstrate for the first time that membrane- and chemical-free electrochemical technologies can remove sufficient boron from RO permeate in a single pass, significantly enhancing its potential to provide energy- and cost-efficient boron removal. [ABSTRACT FROM AUTHOR]

Nitrogen exists in municipal wastewater in the form of ammonia/ammonium which is a recoverable resource. Recovering nitrogen to improve nitrogen-use efficiency should be carefully considered to promote global environmental sustainability. However, ammonia/ammonium is always not well recovered but removed as one of the main pathways for nitrogen release into the environment, which is due to ineffective ammonia capture. In this study, a new method of ion-exchange resin ammonium recovery combined with membrane capacitive deionization (MCDI + IE process) pretreatment is proposed. Selective ion removal by MCDI is demonstrated and ammonium ratio to the total prior ion (K + , Ca 2+ and Mg 2+ ) in outflow of MCDI cell is respectively increased from ∼15% to over 30% and 45% after 3 and 5 MCDI cycles. The MCDI + IE process could deal with 2.48 and 3.89 times of inflow, respectively, compared with the single IE process after 3 and 5 MCDI charge–discharge cycles and recover more than 65% of ammonium in a resin regeneration cycle after 3 MCDI cycles. It ascribes to MCDI desalination eliminating the negative effects of divalent cations, such as Ca 2+ and Mg 2+ , on the IE absorb–desorb process. [ABSTRACT FROM AUTHOR]

• An in-depth overview of MXene applied to capacitive deionisation for desalination. • Performances of MXene are compared to current best performing faradaic electrodes. • MXene salt desalination mechanisms and its stability in water are addressed. • Challenges and prospects of MXene capacitive deionisation are highlighted. Due to the continually growing world population, water desalination has played a vital role for many countries with a shortage of clean water services. A relevant process for brackish water desalination is capacitive deionization (CDI) with the potential of producing clean water via low energy consumption electrochemical interactions. Due to the recent availability of layered and nanostructured materials to fabricate faradaic electrodes acting as pseudocapacitors to increase the salts' storage capacity, CDI-like desalination methods have gained renewed interest. As a way of improving the desalination performance and energy efficiency of CDI, faradaic deionization (FDI) based on faradaic electrodes such as Ti-MXene (Ti 3 C 2 T x) have been proposed for brackish water desalination. This paper provides a review of the theoretical and experimental framework underpinning CDI and FDI using MXene as electrodes, discussing the actual unique role of MXene in CDI-like desalination methods and its prospects. We will examine MXene's current achievements in the broader area of faradaic and hybrid capacitive deionization if compared with other materials and with various architectures and determine the research gaps in the use of MXene for these applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • As a hydrophobic modifier, rGO makes CGA stably float on the water–vapor interface. • The porous structure promotes the supply of bulk water to the water interface. • The effect of ambient factors on water evaporation is dominant in open system. • Clean water recycling from actual seawater and heavy metal wastewater is achieved. • Purification of heavy metal sewage is achieved mainly for interfacial evaporation. Solar-driven water evaporation plays an important role in the recycling of low-energy water resources. Here, we have prepared porous hydrophilic/hydrophobic cellulose graphene aerogel (CGA) to achieve efficient water recycling in solar-driven desalination and purification of heavy metal wastewater. The aerogel can reach water evaporation of 7.2 kg·m−2 for 4 h from simulated or actual seawater under 1 sun irradiation, which meets the daily water demand of two adults. Due to the interfacial evaporation and the formation of hydrogen and chelating bonds between heavy metal ions and oxygen-containing functional groups, CGA can effectively reduce the amount of heavy metal sewage to obtain water without biotoxicity. The efficient water recycling of aerogel is mainly attributed to the photothermal conversion of hydrophobic reductive graphene oxide (rGO), the interconnected porous structure formed by random stacking and cross-linking of hydrophilic cellulose sheets as well as holes in it serve as supply channel of water evaporation. In addition, ambient factors such as wind, humidity and ambient temperature boost the water evaporation of CGA in open system, while in closed system reducing the light reflection of condensed water and the relative humidity of the space is more conducive to improving evaporation efficiency. [ABSTRACT FROM AUTHOR]

[Display omitted] • Superelastic and versatile cellulose aerogel was prepared by Schiff base cross-linking and freeze-drying assembly. • Porous water channels with reverse transportation achieved long-term desalination without salt accumulation. • This solar steam generator realized efficient purification of seawater and wastewater. • As a proof of concept, a simple device that can adsorb the dye pollutant was designed. Solar steam generators (SSGs) that continuously supply clean water resources under solar radiation have been recognized as a sustainable approach to mitigate the global water and energy crisis. However, most SSGs currently face the accumulation of salt on the surface during long-term seawater desalination, which hinders the water supply and vapor escape and significantly reduces the evaporation efficiency. Inspired by the porous water channels of reverse transportation, the SSG (PPy@PEI@A-CNF aerogel) with superelastic and photothermal properties was obtained by the Schiff base reaction of aldehyde-based cellulose nanofibers (A-CNF) and polyethyleneimine (PEI) and freeze assembly, and further in-situ polymerized with polypyrrole (PPy). This aerogel with porous hierarchical structure performed outstanding mechanical robustness (89.9% strain remaining after 100 compress-release cycles), ultra-low density (0.021 g cm−3) and thermal conductivity (0.042 W m−1 K−1), high porosity (97.72%) and full spectrum solar energy absorption (98.4%). Moreover, the aerogel could be used for solar-driven water evaporation, and its evaporation rate and efficiency were as high as 1.66 kg m–2h−1 and 94.62% under 1.0 sun. Owing to the rapid re-dissolution of the salt in the reverse transport of macroporous structure, the aerogel exhibited excellent salt-resistant and self-desalting properties during long-term seawater desalination, and also showed superior purification effects and reusability. These findings provide a new method for designing reusable macroporous SSGs to meet eco-friendly, efficient and sustainable fresh water access. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ion removal performance of FCDI was greatly enhanced via cathodic iodide redox couple. • A similar positive effect noticed during desorption upon polarity reversal. • The system exhibited a stable behavior over multi-cycle consecutive runs. • The approach holds great promise in treatment of iodide-containing saline wastewater. Flow-electrode capacitive deionization (FCDI) is attracting growing attention owing to its advantages such as effective ion removal capacity and easy management of the electrodes. Optimization of the flow-electrode comprising carbon materials and aqueous electrolyte is always a research hotspot aimed at improving FCDI performance. Herein, we innovatively introduced appropriate amount of KI 3 into the cathodic flow-electrode to provide a highly efficient means of enhancing FCDI desalination performance with a stable behavior in salt electrosorption and desorption over multi-cycle consecutive runs. The accelerated salt removal rate originated from the reversible redox reaction of I 3 −/I− and the maintenance of the electro-neutrality of the global electrolyte. Although possible minor release or leakage of I−/I 3 − through the cation-exchange membrane and into the treated water occurred, which makes the proposed approach not quite suitable for potable water production, we demonstrated that this system has great application prospects in treatment of iodide-containing saline wastewater without causing secondary pollution. [ABSTRACT FROM AUTHOR]

[Display omitted] • CDI hybrid systems are more promising than standalone CDI technology. • Current developments and challenges are extensively discussed. • MFC powered CDI systems are a suitable solution for sustainable water treatment. • Technical and economic barriers need to be overcome to make hybrid systems practical. The collective negative impact of both energy depletion and freshwater scarcity has inspired the researchers to develop effective water treatment approaches with lower energy consumption. Capacitive deionization (CDI), an emerging technology for water treatment and desalination is gaining global interest because of its ion selectivity, high water recovery and energy efficiency. Over the last two decades, CDI has gone through several modifications and advancements in terms of module configuration, electrode material and its applicability. For further enhancement in practicability and commercialization of CDI, researchers have shown a broad range of interest in CDI hybrid systems over the past few years. CDI modules have been integrated with photochemical, membrane separation, ion exchange, electrochemical oxidation and bioelectrochemical processes for desalination, wastewater treatment, energy recovery, resource recovery and performance enhancement. These integrated systems are more promising compared with standalone CDI technology. This comprehensive and state of the art review examine different CDI hybrid systems on the basis of working mechanism, module configuration, operating parameters and their effects on system performance. Moreover, synergetic effects, limitations and challenges of these hybrid systems towards their large scale implementation also have been addressed in this review paper. [ABSTRACT FROM AUTHOR]

• MoS 2 /NOMC electrode exhibited an electrosorption capacity of 28.82 mg/g. • Porous architecture of electrodes facilitated the desalination performance. • Increased conductivity and hydrophilicity were beneficial to the CDI process. • Capacitive and diffusion-controlled processes contributed to electrosorption. • A good regeneration property of MoS 2 /NOMC was demonstrated. Capacitive deionization (CDI) is a promising technology for seawater and brackish water desalination but still suffers from several drawbacks, such as low electrosorption capacity and less efficient desalination performance. In this study, MoS 2 /NOMC (nitrogen-doped highly ordered mesoporous carbon) composite with enhanced hydrophilicity, conductivity and capacitance was successfully synthesized by a hydrothermal process. The MoS 2 /NOMC electrodes showed efficient desalination performances with an electrosorption capacity of 28.82 mg/g at 1.6 V in 250 mg/L NaCl solution and a maximum electrosorption capacity of 30.49 mg/g based on Langmuir fitting. The mechanism of the enhanced desalination performance of MoS 2 /NOMC electrodes was explored through the characterization of the contributions of the capacitor-controlled (electrical double layer capacitor without charge transfer) and diffusion-controlled processes (charge transfer with N-doping of NOMC as well as the various oxidation states (from + 2 to + 6) of molybdenum atoms (Mo) in MoS 2). Capacitor-controlled process played a major role (with the contribution of 73.17%) for the improved desalination performances of MoS 2 /NOMC electrodes compared to the diffusion-controlled process (with the contribution of 26.83%). In addition, MoS 2 /NOMC electrodes showed an excellent regeneration performance, indicating the potential of MoS 2 /NOMC composite as a promising CDI electrode material. [ABSTRACT FROM AUTHOR]

Cellulose fibers extracted from facial tissue paper were used as a porous carbon substrate to coat exfoliated two-dimensional Ti 3 C 2 T x MXene nanosheets in order to prepare unique CLF@Ti 3 C 2 T x nanostructures composites. The designed CLF@Ti 3 C 2 T x material was assembled in symmetric capacitive deionization cell, which showed the desalination capacity of 35 mg·g−1, with good cycling stability. Inspired by the conventional chemical gilding process, the proposed low-cost synthesis strategy and innovative design open a new way to develop MXene-carbon based electrodes for CDI water desalination. • 2D Ti 3 C 2 T x nanosheets@CLF core-shell structures are prepared by chemical gilding. • CLF@Ti 3 C 2 T x electrodes are used in symmetric CDI cell for water desalination. • Pseudo-capacitance-effect is induced by ultrathin 2D Ti 3 C 2 T x NSs shell. • Ti 3 C 2 T x NSs coating on CLF significantly improved the overall CDI performance. • CLF@Ti 3 C 2 T x shows salt absorption capacity (SAC) 35 mg·g−1. We introduce a novel and facile synthesis strategy to design 2D MXene (Ti 3 C 2 T x) nanosheets (NSs) coated cellulose fibers (CLF) based 3D nanostructures (CLF@Ti 3 C 2 T x) to overcome the drawbacks of co-ion expulsion in carbon-based, commonly used CDI electrodes and restacking of MXene NSs due to van der Waals forces in the pure MXene based electrodes. CLF extracted from facial tissue paper were used as a porous carbon core-substrate to coat shell of exfoliated two-dimensional (2D) Ti 3 C 2 T x NSs in order to prepare unique CLF@Ti 3 C 2 T x nanostructures composite by an improved dip-coating method. After appropriate structural and chemical characterization, the designed CLF@Ti 3 C 2 T x material was assembled in symmetric capacitive deionization (CDI) cell as an active electrode and the electrochemical properties and desalination capacity were studied in detail. Interestingly, the CLF@Ti 3 C 2 T x based active electrodes displayed good specific capacitance of 142 F·g−1 in 1 M sodium chloride electrolyte, and high salt adsorption capacitance of 35 mg·g−1 compared with the pure MXene and carbon-based electrodes at an applied voltage of 1.2 V, with considerable cycling stability of 10 cycles. Inspired by the conventional chemical gilding process, the proposed unique and low-cost synthesis strategy and unique design open a new way to develop MXene-carbon based composite nanostructures for CDI and energy storage applications. [ABSTRACT FROM AUTHOR]

• Five operational modes regarding the flow of electrodes and feed water were studied. • pH of the effluent stream maintained stable for all kinds of FCDI operational modes. • SCC/single-pass operational mode can be deemed optimal. • Demand for constant effluent salt concentration can be achieved in single-pass mode. Flow-electrode capacitive deionization (FCDI) has attracted growing attention due to its superior desalination capacity and continuous operation. This study, for the first time, systematically investigated the desalination behaviors of FCDI under a variety of operational modes and made a detailed comparison of relevant operation to provide useful information for selecting appropriate FCDI operational mode. Five operational modes with respect to the flow of electrodes and feed water were studied with both constant voltage and constant current applied. Results revealed that the effluent conductivity during charging decreased continuously in batch mode while decreased quickly and then leveled off in single-pass mode. pH of flow-electrodes fluctuated differently for the operational modes of isolated closed-cycle (ICC), short-circuited closed-cycle (SCC) and open cycle (OC), while pH of the effluent stream maintained stable for all operational modes. The variation of current or voltage could be explained by the change in the resistance of FCDI mainly induced by the middle-chamber salt concentration. Based on the three performance indicators of average salt removal rate, charge efficiency and removed salt normalized energy consumption, ICC/single-pass and SCC/single-pass are the two most superior operational modes, followed by OC/single-pass, ICC/batch-mode and SCC/batch-mode. Further considering the advantage of SCC in the continuous charge neutralization and electrode regeneration, SCC/single-pass operational mode can be deemed optimal. [ABSTRACT FROM AUTHOR]