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[Display omitted] • Discussed MXene-based capacitive deionization (CDI) electrode optimization. • Presented forefront applications of MXene-based CDI across multiple fields. • Envisaged challenges and future directions for MXene-based CDI. MXene, hailed for its hydrophilicity, conductivity, and modifiable interlayer spacing, emerges as a frontrunner in capacitive deionization (CDI) electrode materials. Yet, pristine MXene grapples with suboptimal ion removal and stability. This review delves into inventive strategies to fortify MXene-based CDI electrodes, spotlighting their avant-garde applications in combatting ion pollution, reclaiming resources, softening water, and thwarting bacteria. Future prospects for MXene in CDI are also deliberated, aimed at igniting inspiration in the CDI arena. Some strategies proposed herein can be applied to other electrode materials in the CDI domain, thus fostering advancement across the entire spectrum of CDI electrode materials. [ABSTRACT FROM AUTHOR]

[Display omitted] • BiOCl/MXene composite was synthesized via a hydrothermal reaction. • The effect of MXene on the morphology of BiOCl was studied. • PDDA was used to adjust the electronegativity of the MXene surface. • PDDA modified BiOCl/MXene has high dichlorination ability and cycling stability. • Capacitive deionization (CDI) was simulated using COMSOL Multiphysics. Currently, chloride ions in industrial water limits its reuse when extending the circulating water cycle. Therefore, it is necessary to remove Cl− to ensure its effective reuse. Considering the development of new materials for efficient electrochemical chloride removal is a key issue limiting the development of capacitive deionization technology for chlorine removal. In this study, BiOCl was grown in situ on the surface of V-MXene, and then a nanoflower-like BiOCl/MXene composite was synthesized by utilizing the properties of MXene as a structure-directing agent to modulate the morphology, which exhibits superior pore structure and specific capacitance. The poly (diallyl dimethylammonium) (PDDA) modification improved the materials' specific capacitance and reduced energy consumption for dichlorination. The optimal BMP-10 exhibited superior performance for dichlorination, the maximum dichlorination capacity and rate were 161.64 mg g−1 and 2.20 mg g−1 min−1. We found that MXene has excellent properties for exfoliation and morphology modulation of layered materials. Additionally, the electrical properties have an important influence on the electrochemical removal of chloride ions. Here we successfully synthesized an efficient electrode material with unique morphological structure for CDI dichlorination and provided ideas for structure-directing agents and electrical modulation in material design for electrochemical dichlorination. [ABSTRACT FROM AUTHOR]

[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]

• The leaf-liked structure of CoP/FeP is in situ constructed by a simple strategy. • The CoP/FeP enables strong adsorption of H and weak adsorption of O 2 on Co. • The synergy between FeP and CoP induces abundant active sites and efficient mass transfer. • The CoP/FeP exhibits superior OER/HER performance in alkaline solution. Development of non-noble bifunctional electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are of great significance under alkaline electrolytes for overall water splitting. In this work, CoP/FeP has been designed and constructed in situ on carbon fiber paper by using the bimetallic metal organic frameworks (MOFs) synthesis strategy. The heterogeneous CoP/FeP displays excellent catalytic activities in both HER and OER of the fully hydrolyzed semi-reaction. Remarkably, CoP/FeP is applied as the bifunctional electrode in anode and cathode to construct an overall water splitting apparatus, resulting in the low cell voltage and excellent stability. The exceptional electrocatalytic performance may be mainly attributed to the heterogeneous interface, which constructs a built-in electric field to provide electron to Co with electron-rich structure, resulting in the strong adsorption of H and weak adsorption of O 2 on Co. This study provides an effective and controllable method for the design of self-supporting bimetal phosphide catalysts. [ABSTRACT FROM AUTHOR]

[Display omitted] • A CNT-CeO 2 electrode is fabricated to selectively remove F− in mixed ion solutions. • CNTs increase the conductivity of the electrode and hinder the aggregation of CeO 2. • Pseudo-capacitance behavior of F– entering CeO 2 was observed first time by EQCM. • The energy consumption for treating actual groundwater using scaled-up equipment is low at approximately 0.2 kWh/m3. Selective capacitive deionization (SCDI) is a promising process for preferentially removing specific ions from waters with complex compositions. The selectivity towards certain species in CDI is most frequently achieved through novel electrode materials with high affinities towards targeted species. In this study, we investigate the selective removal of fluoride ions from groundwater containing concentrated co-existing chloride ions. A carbon nanotube-CeO 2 (CNT-CeO 2) electrode is employed for the electro-sorption of fluoride ions. Our findings are compelling: when processing a mixed F−/Cl− solution comprising 10 mg/L F− and 100 mg/L Cl−, the CNT-CeO 2 electrode is seen to reduce the concentration of F− ions to 1.5 mg/L in just 150 min, amounting to an 85 % F− removal efficiency, while the Cl− removal efficiency remains below 2 %. Importantly, this translates to a F−/Cl− separation factor of up to 4.16 when using the CeO 2 -based electrodes, which is 40 times higher than that achieved with conventional activated carbon (AC) electrodes. Furthermore, the energy consumption for treating actual groundwater using scaled-up equipment is impressively low at approximately 0.2 kWh/m3. The high affinity of CNT-CeO 2 towards fluoride is attributed to the intercalation Faraday capacitance initiated by the reaction between F− with CeO 2 , as verified by the electrochemical quartz crystal microbalance (EQCM). Moreover, EQCM results show a substantial increase in both mass and current as the potential increased beyond 0.8 V vs Ag/AgCl, implying that the current surge is not a result of water splitting but rather the adsorption of F− onto the CNT-CeO 2 electrode. The addition of CNTs substantially increases the conductivity of CeO 2 electrodes and restricts the aggregation of CeO 2 , thereby accelerating ion diffusion and promoting selective adsorption characteristics. Importantly, our electro-driven approach demonstrates excellent adsorption–desorption over 20 cycles. This comprehensive study advances the technological development of selective CDI, while providing new insights for fluoride removal in groundwater. [ABSTRACT FROM AUTHOR]

[Display omitted] • The development history of capacitive deionization for defluorination is introduced. • Recent advances of electrode materials have been classified and reviewed. • Defluorination mechanisms have been summarized. • Challenges for engineering applications are presented. • Future perspectives on defluorination by capacitive deionization are provided. The combined need for advanced treatment of fluoridated water and low-energy technologies has inspired researchers to develop more efficient and green methods of fluoride removal. Capacitive deionization (CDI), a novel water treatment technology, has garnered significant attention in the defluorination field due to its exceptional ion selectivity. Recent decades, CDI has achieved remarkable advancements in electrode materials, F− storage mechanisms, and the potential for engineering applications in the realm of F− removal. However, there has been a lack of reviews that highlight design strategies for electrode materials in terms of fluoride ion properties in water and provide insights into key factors for designing CDI structures with excellent performance and promise for scale applications. Herein, starting from the background of fluorine pollution in surface water, and the selective removal of fluorine heavily relies on the crucial role played by electrode materials. Accordingly, all kinds of defluorinated electrode materials are reviewed. Secondly, the defluorination mechanism is summarized especially the distinctions among these processes. Additionally, the challenges existing in the promotion of CDI defluorination to large-scale applications are firstly analyzed in depth. Finally, future research opportunities and prospects by CDI for defluorination are proposed. In summary, CDI shows great potential for removing fluoride ions from water, offering a fresh approach to sustainable water resource management and environmental protection. [ABSTRACT FROM AUTHOR]

[Display omitted] • Various morphologies of core–shell tungsten oxide@carbon composite electrodes were synthesized. • The urchin-like W 18 O 49 @C HCDI cathode achieved high SAC of 32.25 mg g−1 at 1.2 V. • Excellent cycling stability over 200 cycles with negligible metal leaching was displayed. • Such core@carbon shell structure with particular morphology provides inspirations for future research. W 18 O 49 has been considered as a promising candidate for electrochemical applications, but it suffers from unsatisfactory electrical conductivity, low specific surface area, poor structural stability, and the corresponding morphology dependent capacitive deionization (CDI) performance has not been reported. Herein, we explored the novel composites of various morphologies of W 18 O 49 encapsulated by a carbon shell (W 18 O 49 @C) as hybrid capacitive deionization (HCDI) cathode for water desalination. The as-obtained W 18 O 49 @C exhibited unique structures, allowing synergistic effects of ion intercalation and electric-double-layer capacitance, fast ion penetration, rapid electron transfer, and dissolution prevention of the active core materials. Among the three morphologies of W 18 O 49 @C, urchin-like W 18 O 49 @C (UWC) presented appropriate pore size distribution, the largest specific surface area and specific capacitance, the highest degree of graphitization and carbon content, consequently achieving excellent desalination capacity. Furthermore, an outstanding cycling stability over 200 cycles as well as a high charge efficiency and a low energy consumption were displayed for the UWC electrode. This study illustrates the great promise of urchin-like core–shell tungsten oxide@carbon composite in HCDI and paves a way for rational design of highly efficient and stable electrode materials for water desalination. [ABSTRACT FROM AUTHOR]

Highlights • The feasibility of perchlorate selective removal by batch-mode CDI was investigated. • A one-dimensional CDI process model was developed to describe the process. • A favorably strong preferential adsorption of ClO 4 − over Cl− was observed. Abstract Capacitive deionization (CDI) has attracted increasing attention over the past decade for the facile removal of ions from aqueous solutions. Nowadays, CDI is rapidly growing and evolving, and has been applied in many aspects. One important application field is water remediation, i.e., removing ionic contaminants of concern from water. Herein, we investigated the feasibility of perchlorate removal from brackish waters by batch-mode CDI technology. Effects of various operating parameters on dynamic electrosorption processes of both perchlorate and chloride were examined. Meanwhile, a one-dimensional CDI process model for dual-anions was developed to quantitatively describe the electrosorption kinetics of perchlorate and chloride, and an excellent agreement between the modeling results and the experimental data was observed. Experimental results revealed a favorably strong preferential adsorption of ClO 4 − over Cl− in the studied CDI system under various conditions. Moreover, only slight discrepancy between adsorption-desorption cycles was found, demonstrating the good regeneration of electrodes and operational stability of the CDI cell. Finally, the scale-up studies indicated that the CDI stack with multiple pairs of electrodes could achieve a much more superior ion removal performance than the CDI cell with only one pair of electrodes. To conclude, these results showed that the CDI system was effective for perchlorate selective removal in the presence of other major ions and had potential in treatment of perchlorate-contaminated brackish water. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Over-potential membrane capacitive deionization (OP-MCDI) improves desalination. • The OP-MCDI system shows 8 times higher salt capacity than a typical CDI system. • OP-MCDI with reversed polarity (OP-MCDI-RP) works well for high salinity water. • OP-MCDI-RP enhances desalination with either mesoporous carbon or carbon aerogel. • The OP-MCDI-RP technology can desalinate natural seawater into freshwater. Abstract The salt removal capacity (SRC) of the carbon electrodes in membrane capacitive deionization (MCDI) is limited by the applied potential (<1.6 V). Thus, enhancement of the SRC is essential before applying MCDI to seawater desalination. This study aims at developing a novel MCDI system for seawater desalination by applying over-potential (OP) to enhance ion-adsorption capacity and kinetics, and by reversing the polarity (RP) of electrodes to improve ion-desorption kinetics (hereafter named OP-MCDI-RP) for seawater desalination. At 2.4 V, the OP-MCDI system with mesoporous carbon demonstrated a SRC value of 64.7 mg g−1 when desalting 0.5 M NaCl solution, while the OP-CDI system could only remove 8.2 mg g−1 under identical conditions. Similarly, an OP-MCDI cell using carbon aerogel demonstrated a greater SRC value of 43.4 mg g−1 in comparison with 12.5 mg g−1 achieved in OP-CDI. These results indicate that the OP-MCDI system can directly desalt high salinity water regardless of the electrode material. In contrast to the OP-MCDI system, the OP-MCDI-RP system yields a higher regeneration efficiency by applying a reversed low-voltage. It is shown that, under appropriate operating conditions, the OP-MCDI-RP system reduced the electrical conductivity of a seawater sample of 37 practical salinity units by 99.9%, suggesting that the OP-MCDI-RP system can be employed for seawater desalination. Due to the dramatic increase in energy demand and insufficient long-term stability caused by high potential and high salt concentration, however, the OP-MCDI-RP process should be further investigated for potential practical applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • Scaling up preparation of modified catalysts by direct calcination in air. • Both defects and oxygen-containing groups contribute to performance enhancement. • Electro-Fenton for high pollutant removal kinetics in a wide pH range. • Industrializable case of 30 g catalyst synthesis theoretically for 5.4 m2 electrode. • Lab-scale demonstration for wastewater treatment with capacity of 7.2 L/h. Electro-Fenton (EF) regarded as a green substitution to traditional homogeneous Fenton, produces highly oxidizing OH through in-situ H 2 O 2 electrosynthesis and electrochemical Fe2+ regeneration. One of obstacles that hinders translation of bench-scale EF performance to water treatment practices is the difficulty of scaling up catalyst/electrode preparation in low cost while producing high performance. Here we report scalable preparation of modified carbon catalysts for high-efficiency EF by directly calcinating commercial carbonaceous materials in air. The EF with modified carbon cathode shows high kinetics of 0.296–0.569 min−1 for levofloxacin removal in much wider pH range of 3.0–6.6 using less Fe dosage compared with homogeneous Fenton. This EF system can reduce chemical oxygen demand (COD) of coking wastewater from 312.0 to 99.5 mg L−1 to satisfy wastewater discharge standard in China while consuming low electricity of 1.8–4.9 kWh kg−1 COD−1. We demonstrated the scalability of this EF process by scaling up preparative throughput of catalyst (30 g that can be processed into electrodes with total working areas of 54000 cm2), enlarging electrode size (230 cm−2) and reactor volume with wastewater treatment capacity of 7.2 L/h. This scalability demonstration of EF technology shows the promising potential of achieving low-cost advanced wastewater treatment. [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] • The preparation process of the CDI electrode is simplified. • Laser-induced graphene (LIG) can be directly used as a CDI electrode. • LIG electrode is modified by one-step electrodeposition of Co 4 S 3. • The LIG6/Co 4 S 3 -15 presents a high adsorption capacity of 2702.79 mg g−1. • UO 2 2+ adsorption is attributed to the synergistic action of EDLs and pseudocapacitance. It has been proved that capacitive deionization (CDI) technology possesses a significant potential for uranium capture. However, the CDI performance has been limited by the complex preparation process of the electrodes and the poor adhesion between electrode materials and collectors. Here we combine the laser-induced graphene (LIG) and the electrodeposition to develop the LIG/cobalt sulfide (LIG/Co 4 S 3) electrodes with good adsorption performance. The obtained LIG6/Co 4 S 3 -15 electrode has both electrical double layers (EDLs) and pseudocapacitance properties, with a specific capacitance of 24.27 F g−1, which is 1.82 times higher than that of LIG6 (13.36 F g−1). The LIG6/Co 4 S 3 -15 electrode exhibited a high adsorption capacity of 2702.79 mg g−1. The advantages of this electrode preparation method are mainly attributed to the synergistic effect of the following aspects: (i) simplified graphene preparation by avoiding wet chemistry and post-treatment steps; (ii) simplified tedious electrode preparation such as electrode slurry distribution and sheet coating; (iii) the three-dimensional structure of LIG provides not only the conductive network but also the site for Co 4 S 3 nanosheet growth, thus avoiding nanosheet aggregation. The abundant pore structure of the conductive graphene substrate and the layered structure of the Co 4 S 3 nanosheets enable the LIG6/Co 4 S 3 -15 electrode with fast charge/ion transport, high hydrophilicity and superior pseudocapacitance, allowing uranyl (UO 2 2+) to be firstly electrosorbed, then physicochemically adsorbed, and finally electrocatalytically reduced/deposited onto the electrode. This study will offer a simple and environmentally friendly method for the synthesis of electrodes and provide a reference for the design of efficient electroadsorption materials. [ABSTRACT FROM AUTHOR]

• A graphene-based asymmetric electrode configuration was rationally designed. • This electrode pair exhibited a removal rate of 91.3% and super-fast kinetic rate. • The mechanism involves complexation, electro-adsorption, and photocatalytic reduction. • The method is superior to traditional photocatalysis and electro-adsorption methods. Recently, capacitive deionization has become a promising approach for uranium extraction owing to its energy-efficiency and environmental benign. However, the adsorbed charged uranium (VI) (U(VI)) would repulse the incoming U(VI) ions, and might re-enter into solution, vastly impeding the improvement of electro-adsorption performance. Reducing the adsorbed U(VI) ions into U(IV) precipitation is an ideal way to address above issues. Here, a coupling approach of photocatalytic reduction and electro-adsorption (PEA) was proposed. An asymmetric electrode configuration, composed of phosphate functionalized graphene (GP) cathode and graphene/TiO 2 nanocomposite (GT) anode was rationally designed, and originally proved its enhanced U(VI) extraction via the PEA method. The asymmetric electrode device enables the fast transport of photo-electron and the rapidly directed migration of U(VI) ion to the GP cathode. More importantly, it combines three synthetic mechanisms of complexation, electro-adsorption, and photocatalytic reduction to extract U(VI) ions. As a consequence, the as-designed PEA method shows a higher removal rate of 91.3% in comparison with conventional photocatalytic reduction (PA) and electro-adsorption (EA) methods. Meanwhile, its kinetics rate is 225% and 50% faster than PA and EA methods. Furthermore, an enhanced reduction efficiency of U(VI) to U(IV), good selectivity as well as reusability for PEA method were also obtained. The results provide a potential approach to combine multiple mechanisms for efficient U(VI) extraction from aqueous solution by purposeful design of asymmetric electrodes. [ABSTRACT FROM AUTHOR]

[Display omitted] • A Ti/RuO 2 -IrO 2 -RhO x has been designed for periodic bipolar operation. • The super bipolar operation stability arose from RhO x -bridged pseudocapacitance. • The PP adsorption inhibited the interfacial charge transfer instead of mass transfer. • The bipolar operation could effectively in-situ desorb PPs from the electrode. • A BCM model has been established to simulate the PP desorption during degradation. Anodic oxidation has been recognized as an attractive technique for wastewater treatment. However, polymeric product (PP) adsorption still remains to be an obstacle because currently available exogenous PP removal methods are either laborious or costly. To overcome such a problem, a Ti/RuO 2 -IrO 2 -RhO x has been artfully designed for periodic bipolar operation to realize in-situ PP desorption herein. Due to the RhO x -bridged pseudocapacitance enhancement, the electrochemical stability reduction problem of the conventional electrode in bipolar operation could be substantially eliminated. The PP adsorption mainly impacted the interfacial charge transfer of the electrode instead of the mass transfer of contaminant from the solution toward the electrode surface. Delightedly, the bipolar operation could effectively in-situ regenerate the poisoned electrode from PP adsorption due to the combined effects of OH– dissolution and H 2 stripping, thereby essentially promoting the deep mineralization of degradation intermediates rather than further polymerization, with this promotion degree influenced by four factors (current density, initial contaminant concentration, bipolar frequency, and contaminant types). This further encouraged the successful establishment of a new bipolar completely mixed (BCM) model for theoretical guidance of efficient PP desorption during various contaminant degradation. This work might open up a brand new avenue for poisoned electrode in-situ regeneration from detrimental PP adsorption and provide mathematical model guidance for PP desorption. [ABSTRACT FROM AUTHOR]

[Display omitted] • Energy stored in bipolar MCDI is recovered via direct connection manner. • Direct connection of two bipolar MCDI induce their partial adsorption and desorption. • By using this phenomenon, operation with direct connection reduced energy up to 47% • Appropriate time combination can reduce energy with the same desalination performance. In the practical implementation of capacitive deionization (CDI), membrane CDI with a bipolar electrode (bipolar MCDI) is emerging as one of the alternative CDI platforms due to its favorable cell configuration for scale-up and low current originating from the serial connection of electrodes. Nevertheless, one obstacle to practical use is that there are few studies about the energy recovery process for the high energy efficiency of bipolar MCDI, requiring further research. Therefore, in this study, we propose a bipolar MCDI process with energy recovery and assess its potential by analysis of a lab-scale module with a single stack and nine stacks of the bipolar electrode (i.e., 2.4 V and 12 V system, respectively) and a pilot-scale module with 250 stacks (i.e., 300 V system). As a result, the energy consumption of the bipolar MCDI systems was reduced by 43% and 41% in the lab-scale modules with 2.4 V and 12 V systems, respectively, via energy recovery. Furthermore, the energy recovery led to a 40% reduction in the energy consumption of bipolar MCDI, even in the pilot-scale modules of the 300 V system. The results suggest that energy recovery in bipolar MCDI can be one of the essential processes and it has a strong potential for implementation in real industrial and environmental applications. [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] • Waste pomelo peel converted into a novel adsorbent for use in CDI. • Removal of Pb and Cu ions strongly enhanced by functionalizing with pyrrolic-N. • Strong selectivity of Pb2+ from water containing multiple competing ions. • Easy regeneration of adsorbent on no loss in performance over 300 cycles. Water is not only a valuable resource that is needed to sustain life, but also an essential ingredient in many engineering processes, which unavoidably leads to large volumes of water being contaminated. To achieve safe discharge and also recover valuable "pollutants", better performing sorbents are needed to rapidly and efficiently decontaminate water while generating minimal secondary wastes. Bio-sorbents derived from pomelo peel were functionalized with pyrrolic-N (BNC-5 electrode) and pyridinic-N (BNC-6 electrode) to enhance electroadsorption and selectivity of Pb2+ and Cu2+. The interaction between soft acid ions (Pb2+) and soft base sites (pyrrolic-N) contributed to a strong chemisorption that elevated the electroadsorption capacity to 2.0 mmol g−1 for Pb2+ at an applied voltage of 1.2 V. With fast removal kinetics (0.077 g mg−1 min−1 of Pb2+), the BNC-5 sorbent performed comparably to other N-doped sorbents prepared using graphene. The large adsorption–desorption hysteresis of BNC-5 in responding to the applied electric voltage confirmed the nature of chemisorption. The results showed only 32.4% of the adsorbed ions being desorbed from the sorbent by reducing the applied voltage to 0 V, but reached almost complete desorption (98.5% of adsorbed ions) at –0.8 V. When operated in adsorption–desorption cycle mode, BNC-5 after ∼ 400 cycles maintained a capacity retention ≥ 80%. After 400 cycles, the electrode capacity was almost fully restored (98.7%) by only mild chemical washing (0.1 M HNO 3) of the sorbent and the cycling performance maintained. The study demonstrated the robustness of the sorbent over 1200 cycles and hence the potential to successfully convert waste into high-performance materials for large-scale remediation strategies of contaminated wastewater using CDI. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fungal hypha activated carbon pad (FhACPad) is a promising electrode for desalination. • FhACPad is a freestanding electrode readily prepared from the fungus suspension for CDI. • The continuous and porous structure of FhACPad facilitates both electron and ion transfer. • FhACPad with hierarchical pores exhibits a high desalination capacity of 35.6 mg NaCl /g. Capacitive deionization (CDI) is an emerging technology for water desalination, especially low-salinity water. Freestanding porous carbon-based electrodes with rapid electron transfer rate and effective ion diffusion features are desired to achieve high desalination performance. Here, a porous carbon pad derived from the fungal hyphae of Aspergillus niger was prepared as a precursor of the electrode. After carbonization and activation, the intertwined and hierarchical porous structure of the fungal hypha pad was well-preserved, producing a freestanding fungal hypha activated carbon pad (FhACPad) electrode for MCDI. The interconnected carbon fibers and open pore structure of the FhACPad yielded an electrode with high electrical conductivity and rapid ion transport properties. The novel FhACPad exhibited a gravimetric salt adsorption capacity of 35.6 ± 2.3 mg NaCl /g with an average desalination rate of 1.2 mg NaCl /g electrode /min from a 585 mg/L NaCl aqueous solution at a cell voltage of 1.2 V and a flow rate of 1 mL/min. As a result, the FhACPad outperformed conventional powdered activated carbon (PAC) electrodes and the most recently reported porous carbon electrodes under similar desalination conditions. This work also provides insights into the structure-performance relationships of CDI electrodes and shows a great potential of fungal biomass-derived porous carbon electrodes for high-performance MCDI applications. [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]

[Display omitted] • Polysaccharides-bound porous graphene electrodes were rationally designed. • Oppositely charged groups at cathode and anode weakened the co-ions exclusion effect. • The asymmetrical configuration gave rise to 97.9% removal ratio for U(VI). • The mechanism involves improved EDL, extra micro electric field and complexation. As an energy-saving and facile technology, the potential of capacitive deionization (CDI) starts to surface in enrichment and recovery of uranium (U(VI)) from aqueous solutions. However, co-ion expulsion effect and poor surface wettability of electrode materials limit the eletrosorption performance. Here, polysaccharides xanthan gum (XG)- and chitosan (CS) -bound porous reduced graphene (RGH) electrodes were rationally designed, and then assembled into asymmetrical electrode configuration for U(VI) eletrosorption. Owing to the introduction of polysaccharide binders, the oppositely charged groups on the surface of cathode and anode weaken the co-ions exclusion effect as well as endow their superior surface hydrophilicity and electroconductivity, which facilitates the electrosorption for U(VI). Meanwhile, the negatively charged carboxylate groups could act as extra micro electric fields to attract U(VI) cations, and form stronge complexation between them. As a consequence, asymmetrical polysaccharide-bound RGH configuration gave rise to a larger removal ratio of 97.9% within 4 h at 1.2 V, as well as 2.5 times faster kinetics than symmetrical PVDF-bound RGH electrode configuration. Moreover, the accumulated adsorption capacity in six adsorption–desorption cycles of the former is up to 1413.0 mg g−1, 75% higher than the latter, which is of practical significance and economic benefit for electrosorption application. [ABSTRACT FROM AUTHOR]

[Display omitted] • CM-CNT/graphene electrode was directly constructed by core-sheath 3D printing. • Electrode features hierarchical pores and highly interconnected conductive network. • Electrode enables adequate channel and pathway for ultrafast ion/electron transport. • Pseudocapacitor exhibits prominent stability and electrochemical performance. Pseudocapacitance holds great promise for energy density improvement of supercapacitors. However, how to enable a pseudocapacitor with combined feature of high mass loading of pseudocapacitive material, efficient ion/electron transport, as well as facile and scalable manufacturing, still remains a significant challenge. Herein, we demonstrated an efficient and scalable extrusion-based core-sheath 3D printing strategy for directly and controllably constructing pseudocapacitive carbon nanotube (CNT)- MoS 2 (CM)-CNT/graphene electrode, with high mass loading up to 55% and uniform distribution of MoS 2 , hierarchically porous configuration and highly interconnected conductive network framework built by coupling graphene scaffold located in shell layer and CNTs situated in core layer. The unique architecture featuring adequate channels and pathways can act as a "superhighway" for ultrafast ion diffusion and electron transport throughout the entire device, thereby enabling fast kinetics of Faradaic reactions and abundant availability of electrochemical active sites. A symmetric pseudocapacitor assembled with 3D-printed electrode and in situ electrospun poly(vinylidene fluoride) (PVDF) nanofiber separator delivered high specific capacitance (558 mF cm−2) and energy density (49 µWh cm−2), remarkable cycling stability (87% after 8000 cycles), and superior capacity even at large electrode thickness. This work has shed light on new strategies for designing and fabricating high-performance pseudocapacitors toward future uses. [ABSTRACT FROM AUTHOR]

This work provides a new synthetic conception for the synergistic effect of ZIF and LDH on the construction of unique hollow nanoarrays, simultaneously proposes a feasible means for designing dual-functional catalysts for efficient overall water splitting. [Display omitted] • Ordered self-supported CuO@CoZn-LDH nanoarrays have been successfully prepared. • The hollow structure with defect-rich pores facilitates electrocatalytic. • The CuO@CoZn-LDH exhibits outstanding electrocatalytic activity and stability. Driving the electrochemical water splitting is considered as a green and sustainable method to produce hydrogen energy. Herein, a novel strategy to fabricate the dual-metal zeolitic imidazolate framework (CoZn-ZIF) nanocrystals on Cu(OH) 2 nanowires supported by Cu foam (CF) as a precursor to change the conventional LDH growth has been proposed. It can obtain highly ordered hollow CuO@CoZn-LDH nanoarrays with defect-rich porous surface for efficient overall water splitting. Specifically, the appropriate adjustment of Co/Zn metal ratios in the ZIF precursor could lead to a well-defined morphology to create the defect-rich porous surface, which exposes more active sites and the accessibility of electrolyte to promote the electrocatalytic efficiency. The obtained hollow CuO@CoZn-LDH/CF nanoarrays catalyst exhibits an excellent activity in alkaline media with the low overpotentials of 194 mV and 124 mV at the current density of 10 mA cm−2 for OER and HER, respectively. Remarkably, as bifunctional electrodes for overall water splitting, it displays an alkali-electrolyzer with a low cell voltage of 1.55 V at the current density of 10 mA cm−2, and can be comparable to commercial the IrO 2 @CF||Pt/C@CF couple catalyst. The work provides a new prospect for the rational design and fabrication of advanced hierarchical multifunctional electrocatalysts in electrochemical energy device application. [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]

In this work, three-dimensional hierarchical porous NiO/NF composites were fabricated by hydrothermal method combined with subsequent annealing process. First, nickel hydroxide (Ni(OH) 2) nanosheets were grown vertically on flexible and porous nickel foam (NF) to form Ni(OH) 2 /NF. Second, three-dimensional porous NiO/NF were obtained by thermal decomposition of Ni(OH) 2 /NF in different atmosphere containing various oxygen content (Ar, Air, O 2) at 400 °C for 3h, accompanied by the emergence of numerous mesopores. It is found that O 2 -annealed NiO/NF possesses more Ni3+ and exhibiting superior adsorption performance for Cr(VI)/OH− anions. Furthermore, NiO-X/NF(X represents annealing atmosphere) was used as anode to remove Cr(VI) accompanied with hydrogen generation by electrocoagulation. Compared with bare nickel foam, the presence of NiO nanosheets promote the generation of Ni(OH) 2 flocculant further accelerate Cr(VI) removal and H 2 production. Besides, the NiO/NF exhibited excellent removal performance for other coexisting metal ions. Removal mechanism was investigated and ultimately demonstrated three routes consist of reduction of Cr(VI) to Cr(III) at the cathode followed by the coprecipitation with Ni(OH) 2 floc in the form of Cr(OH) 3 , direct complexation reactions with the freshly produced amorphous Ni hydroxides, and adsorption on NiO-X/NF electrodes. • Three-dimensional hierarchically porous NiO/NF composites were fabricated. • O 2 -annealed NiO/NF enhances the content of Ni3+ promoting the adsorption of Cr(VI). • NiO/NF exhibits excellent Cr(VI) removal efficiency via electrocoagulation. • Simultaneous Cr(VI) removal and hydrogen generation achieve resource utilization. 3D hierarchically porous NiO/NF (Ni foam) was synthesized by a facile hydrothermal method and a subsequent annealing process in different atmospheres for the simultaneous Cr(VI) removal and hydrogen generation from wastewater by the electrocoagulation method. O 2 atmosphere for annealing increased the Ni3+ content in NiO, thus led to an enhanced electrostatic interaction with Cr(VI)/OH− anions and subsequently increased the Cr(VI) adsorption and Ni hydroxide flocculant production. Thus, the NiO-O 2 /NF electrode exhibited the best Cr(VI) removal performance of 99.5% within 20 min at the applied potential of 0.97 V vs. RHE, along with a hydrogen generation rate of 1.1 mmol g−1 h−1. The removal mechanism was suggested, including the reduction of Cr(VI) to Cr(III), direct complexation reactions with the freshly produced amorphous Ni hydroxides, and adsorption on NiO-X/NF electrodes. [ABSTRACT FROM AUTHOR]

• Floriform WO 3 /C electrode were developed for electrosorption of U(VI) from water. • The charge/discharge process of WO 3 /C primarily belongs to capacitive controlled behavior. • WO 3 /C electrodes achieve a maximum U(VI) electrosorption capacity of 449.9 mg g−1 at 1.2 V. • Electrosorption of U(VI) is credited to synergistic effect of EDLs and pseudocapacitance from WO 3 /C. Decontamination and recovery of uranium(VI) [U(VI)] from radioactive wastewater is vital to sustainable nuclear power production and reclamation of radioactive waste. As a promising technology of water treatment, capacitive deionization for adsorption of U(VI) started to attract attention. Herein, for the first time, a floriform WO 3 /C composite was developed as a pseudocapacitive electrode material for electrosorption of U(VI) from water. The as-prepared WO 3 /C composite was constructed from dozens of 2D nanosheets with a thin amorphous carbon layer, resulting in uniform flower-like particles with hierarchical nanostructure and superior open pore network. The charge/discharge process of WO 3 /C was demonstrated as prominent capacitive controlled behavior by quantization of capacitance contributions. As a result, the WO 3 /C electrodes achieved a maximum U(VI) electrosorption capacity of 449.9 mg g−1 at 1.2 V and good cycling stability after consecutive adsorption-desorption, which is superior to the conventional physical adsorption processes. The superior CDI performance of the WO 3 /C electrode is credited to the synergistic effect of EDLs and pseudocapacitance based on the combination of WO 3 and carbon layer with hierarchical nanostructure. Overall, the facile preparation strategy, the superior electrosorption performances, and the high cycling stability, endow the floriform WO 3 /C electrodes to be a promising electrode material for electrosorption of U(VI) from wastewater. [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]