Capacitive Deionization Electrodes Research Articles

Flow electrode capacitive desalination of industrial RO reject.

Flow electrode capacitive deionization (FCDI) is a promising technology for efficiently treating industrial brine with high salt content. However, its desalination performance is currently limited by internal resistance. Achieving an effective FCDI system relies on active electrode materials with high conductivity. This study compares the desalination performances of the widely used flow electrode activated carbon (AC) with more conductive materials, reduced graphene oxide (rGO), and ZnO/rGO composite. Additionally, the lack of particle-to-particle contact in the flow electrode contributes to internal resistance and to address this, a cationic surface-active agent is introduced. This agent forms a stable dispersion, creating a space for enhanced mass loading of the active material. This modification enhances the conductive network and particle contact, reducing the diffusion path and promoting rapid ion transport. With a 5 wt% loading, ZnO/rGO achieved a 73% salt removal efficiency, surpassing AC at 63%. Furthermore, the surfactant-modified ZnO/rGO flow electrode with a 7 wt% loading demonstrated an 81% salt removal efficiency.

Simultaneously improving surface area and hydrophilicity of biomass activated carbon for achieving superior desalination performance in CDI

Super activated carbon is a promising electrode material for capacitive deionization (CDI), an emerging sustainable desalination technology. Increasing its CDI performance requires improving hydrophilicity while maintaining high specific surface area. Traditionally this is challenging, but modifying super activated carbon with earth-abundant metal ions at room temperature can achieve both aims through a low-energy process aligned with sustainability. The modified super activated carbon showed higher specific surface area, improved hydrophilicity, and enhanced desalination performance. Particularly, the salt adsorption capacity of Cu2+-modified super activated carbon in 500 mg/L NaCl solution at 1.2 V increased from 35.4 mg/g to 49.3 mg/g, among the highest for sustainable carbon materials. The capacity increase was enabled by simultaneously increasing specific surface area and hydrophilicity through themetal modification process. Along with high charge efficiency (93.4%), fast adsorption rate, and good cycling stability (80% capacity retention over 50 cycles), the modified super activated carbon displayed superior overall desalination performance. This study exemplifies how sustainable design principles can be applied to synthesize high-performance CDI electrode materials, through green chemistry and engineering for optimized efficiency.

Constructing a hierarchical MoS2/MXene heterostructure for efficient capacitive deionization of saline water

Well-dispersed and few-layered molybdenum disulfide (MoS2) has been considered as a suitable electrode material for pseudocapacitive capacitive deionization (CDI) due to its lamellar structure, flexible interlayer intercalation, and high theoretical capacity. However, the serious aggregation, low electrical conductivity and poor hydrophilicity of MoS2 have limited its desalination application. To address these issues, few-layered MXene is used as a conductive and hydrophilic skeleton to support MoS2 nanosheets. In this study, a hierarchical MoS2/MXene heterostructure is fabricated for highly efficient CDI of saline water. The designed heterostructure effectively inhibits the agglomeration of MoS2 and MXene nanosheets, exposes more active electrochemical sites, and improves the wettability, thereby enhancing ion/charge transport. The MoS2/MXene heterostructure electrode exhibits low electrical resistance (3.2 Ω), along with a high specific capacitance of 171.8 F·g-1 at 2 A·g-1. Furthermore, the MoS2/MXene-based CDI electrode demonstrates an excellent desalination capacity of 55.8 mg·g-1 and a fast desalination rate of 13 mg·g-1·min-1. This study paves the way for novel applications in the CDI field.

Critical review of capacitive deionization: design, operation consideration and real-world environmental applications

Capacitive Deionization (CDI) has emerged as a promising and environmentally sound solution for addressing pressing water treatment challenges. This critical review meticulously examines the design, operational considerations, and performance metrics of CDI systems, particularly highlighting their significance in environmental applications under real-world conditions for handling different water sources such as groundwater, surface water, sea water. Various CDI configurations, electrode materials, and operational parameters are scrutinized through an extensive analysis of literature, with their direct influence on system efficiency, ion removal capacities, and long-term stability being elucidated. The reduced energy consumption, minimal chemical usage, and potential for sustainable water treatment of CDI, as compared to traditional methods, are emphasized from an environmental perspective. Moreover, this review underscores the pivotal role of CDI in practical environmental applications, including desalination, wastewater treatment, and remediation of inorganic pollutants (heavy metals and hardness) and resource recovery (nutrient, uranium, lithium…). By presenting compelling case studies, CDI's efficacy in mitigating environmental impacts is illustrated, offering a cleaner and more cost-effective alternative to conventional treatment methods. By outlining challenges and charting future directions, this review serves as a roadmap for researchers, engineers, and policymakers, facilitating the development of sustainable and efficient water treatment strategies. In an era marked by escalating environmental concerns, CDI emerges as a potential technology for responsible and effective water management, poised to shape a more sustainable future.

Constructing ionic highway in capacitive deionization via integrated electrodes based on bifunctional ionomer

In this work, bifunctional ionomeric capacitive deionization (BICDI) electrode are constructed by using bifunctional ionomer as electrode binder and ion exchange material for the enhancement of desalination performance. Compared with traditional membrane capacitive deionization (MCDI) electrodes, the BICDI electrodes were fabricated by using the poly(arylene alkylene) based ionomer to substitute the PVDF binder and ion exchange membrane (IEM). The BICDI electrode based on bifunctional ionomer exhibits higher hydrophilicity, special ion exchange capability, as well as excellent electrochemical properties in comparsion with that of MCDI electrode counterpart. The built-in ionic groups in poly(arylene alkylene) based ionomers could create interconnected ionic highway between different electrode particles, which encourages effective ion transport. The desalination results validated that the salt adsorption capacity (SAC) of carbon-based BICDI electrodes reached 19.2 mg g−1, which is 2.2 times that of MCDI electrodes during adsorption-desorption cycles. Meanwhile, BICDI electrode possesses exceptional charge efficiency (96.6 %), which is much higher than that of MCDI (76.8 %). In the case of BICDI device, the ionomer utilization rate increased by a factor of 88 compared to that of ion exchange membrane in MCDI device. Especially, the economic cost of the integrated BICDI electrodes module is only $35 m−2, which is much cheaper than that in conventional MCDI system ($520 m−2). It is reasonable to believe that the integrated BICDI electrode incorporated with charged bifunctional ionomer proposed in this work provide novel insights and strategies for the high-performance and large-scale CDI applications.

Selection of graphene as a conductive additive for biomass-based activated carbon electrode in capacitive deionization: acid-treated as a practical approach to reduce graphene content

Abstract The use of graphene (GE) as a conductive additive for activated carbon (AC) electrode in capacitive deionization (CDI) has attracted much attention due to its high electrical conductivity. However, self-agglomeration by π–π interactions between individual GE sheets required the use of high content to ensure an efficient conductive network in the electrode, which limited its practical application. This work proposed an approach to reduce the GE content in the fabrication of biomass-based activated carbon electrode for CDI application. The stacking effect of GE sheets is inhibited when GE sheets functionalize with oxygen groups via acid treatment under ultrasonic condition. This helps to reduce GE content to less than 2 wt% in the fabrication of CDI electrodes. The electrode fabricated using 2 wt% treated GE (T-GE) achieved the superior capacitance of 54 F/g, twice that of the pristine AC. The desalination process using as-prepared electrodes was evaluated by batch-mode CDI system with the initial NaCl solution of 2000 ppm and 3000 ppm at various applied voltages. The AC/T-GE 2 % electrode showed excellent performance with a desalination capacity greater than 10 mg/g, and reached an adsorption rate of 1.93 mg/g.min in both solutions under an applied voltage 1.2 V.

Advancing capacitive deionization via intelligent optimization approach part 1: Development of tunable low-cost carbon electrodes

This work introduces a novel approach based on utilizing intelligent data-based optimization techniques to fabricate high-quality and low-cost carbon electrodes for capacitive deionization (CDI). The effects of synthesis parameters of peanut shell-derived porous carbon (PSPC) (KOH to Raw ratio, pyrolysis temperature and time) on five factors (production yield, BET surface area, electrode specific capacitance, and ion transfer resistances, including interface and diffusion) were investigated using combined design-of-experiment (DOE), response-surface-methodology (RSM), and Sobol's sensitivity analysis (SA). Optimal scenarios providing maximum production yield, surface area, and specific capacitance, and minimum ion transfer resistances were found using a multi-objective genetic algorithm (GA). Results uncover that increasing the impregnation ratio, pyrolysis temperature and time enhances the PSPC porosity that causes the carbon surface area and electrode specific capacitance to promote, the production yield to reduce and ion transfer resistances to raise. From Sobol's SA, pyrolysis temperature was the most impacting parameter on all decision factors. The optimal PSPC prepared at 750 °C and minimum KOH:Raw = 1 and time = 30 min offered a developed mesoporous structure with BET surface area of 685 m2/g and the resulting electrode had an outstanding specific capacitance (127.13 at 5 mV/s), with reliable durability after 100 cycles and showing the lowest possible ion transfer resistances.

Activated carbon cloth electrodes for capacitive deionization: a neutron imaging study

Neutron imaging was employed to track the uptake of Gd3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document} ions by the sub 2 nm micropores of charged activated carbon cloth electrodes from an aqueous Gd(NO3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_3$$\\end{document})3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_3$$\\end{document} solution. The transmitted neutron intensity evinces the persistent presence of Gd3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document} in the micropores during the discharge cycle, which is caused by the adsorption of oppositely charged ions. The charge efficiency of the activated carbon cloth system was determined by direct comparison with the imaged Gd3+\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{3+}$$\\end{document} concentration changes, with which the influence of ion swapping and resistive losses on capacitive deionization cells can be ascertained.

Characteristics of mesoporous activated carbon prepared from oxygen annealed pitch using dry air and its application to capacitive deionization electrode

A two-step process was used to develop high specific surface area mesoporous activated carbon for capacitive deionization (CDI) electrodes. Annealing petroleum pitch in an oxygen atmosphere followed by steam activation produces the desired mesoporous structure. Oxygen annealing creates oxygen on the pitch surface while reducing the crystallinity of the carbon. The oxygen thus forms acts as an active site during the steam activation process and forms pores in the petroleum pitch. A comparison of activated carbon from pitch carbonized in a nitrogen atmosphere and pitch heat-treated with oxygen reveals a distinct advantage for the latter. OAP-AC6 shows remarkable properties, including a specific surface area of 1748 m2/g and mesoporosity of 59.2 %. It exhibits an excellent capacitance of 99.2F/g at a scan rate of 5 mV/s. The prepared activated carbon was used as a CDI electrode material. OAP-AC6 shows a salt adsorption capacity of 17.6F/g and an amazing salt removal efficiency of 94.2 % in a 250 ppm NaCl aqueous solution. Based on these results, OAP-AC6 is highly recommended as a promising active material for CDI electrodes.

Integrated hierarchical porous lignin-based carbon electrode for boosting membrane-free capacitive deionization areal adsorption capacity

The development of high value-added lignin-based functional porous carbon electrodes with excellent properties from sustainable industry lignin powder remains a challenge. This work aims to create robust, binder-free, conductive additives-free, and current collector-free monolithic porous carbon electrodes using industrial lignin powder for membrane-free capacitive deionization (CDI). The material exhibits high mechanical strength, hierarchical porosity structure, large uniform size, and thickness of just a few millimetres (<2.6 mm). In a three-electrode supercapacitor system, the areal specific capacitance of CLCA300-3-1.0 reaches 5.03–1.02 F cm−2 when the scan rate between 1 and 20 mV s−1 in 1 M NaCl solution. As CDI electrodes, the charge efficiency of CLCA300-3-1.0 at different voltages of 1.2 V, 1.4 V and 1.6 V is 0.53, 0.72 and 0.71, respectively. The energy consumption of CLCA280-3-1.0, CLCA300-3-1.0 and CLCA320-3-1.0 tested at 1.2 V are 3.27, 3.40 and 3.25 Wh m−3, respectively. In addition, with thickness increasing to 1.5 mm, the developed CLCA300-3-1.5 electrode exhibits an areal adsorption capacity of 0.46 mg cm−2, and relative highly capacity retention of 84.78 % after 70 cycles. The impressive desalination performance is attributed to the well-designed hierarchical porosity, superhydrophilicity and robust monolithic structure.

Fabrication of Fe-CoS2@N-doped C electrode for highly efficient and durable electrosorption of Yb(III) from aqueous solutions

In this study, the novel Fe-doped CoS2@nitrogen-doped carbon (Fe-CoS2@NC) composites were manufactured and investigated as the capacitive deionization (CDI) electrode materials for electrosorption of Yb(III). The as-prepared materials were characterized with X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, N2 adsorption–desorption test with Brunauer-Emmett-Teller method, Raman spectroscopy, and X-ray photoelectron spectroscopy. The electrochemical tests, including cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy, revealed that Fe-CoS2@NC exhibited the better electrochemical performance with a specific capacitance of 58.41 F g−1 than Fe-CoS2@0.5NC and Fe-CoS2. The electrosorption results demonstrated that Fe-CoS2@NC possessed the Yb(III) uptake of 129.4 mg·g−1 within 60 min at a voltage of 1.2 V and a flow rate of 20 mL·min−1. After five successive sorption–desorption cycles, the electrosorption efficiency of Fe-CoS2@NC remained at 98 %, exhibiting the robust durability for future industrial application. Furthermore, Fe-CoS2@NC also showed the favorable electrosorption performance towards various rare earth ions, especially for heavy rare earth ions. The mechanism analysis indicated that the impressive electrosorption performance is attributed to the synergistic contribution by the electric double layers of carbonaceous matrix and the pseudo-capacitance that induced by the redox reactions of Fe atom.

Enhancing copper recycling through synergistic valence conversions in MnOx-modified capacitive deionization electrodes

The copper present in industrial waste has significant value for environmental resource recovery. Nonetheless, in electrosorption employed for ion separation, electrode materials commonly exhibit limited adsorption capacities, inadequate cycling stabilities, and insufficient selectivities toward specific ions for their single adsorption mechanism. In this study, a strategy was developed for optimizing the design of electrode materials to substitute zirconium atoms in organometallic frameworks (UiO-66, abbreviated as ZO) for manganese atoms from manganese oxide precursors to form a robust three-dimensional conductive structure (MZO). MZO has a stable and sizable adsorption region for electric adsorption, and manganese atoms with a dot-like dispersion provide Faraday active sites. Notably, in an external power supply circuit, Mn3+ fluently transforms to Mn2+ and then facilitates the deposition of copper ions by capturing electrons from Mn2+. Manganese regains electrons by utilizing the power source, which facilitates the continuous transfer of electrons to copper ions, enhancing the pseudocapacitance process. The combined effects of electrodeposition, a double-layer reaction, and a pseudocapacitive reaction improve the adsorption performance of MZO for copper ions, with an adsorption capacity of 249.5 mg·g−1 (at 1.2 V, 40 mg·L−1 Cu2+). Meanwhile, MZO based on ZO has acceptable cyclic stability, with a capacity retention rate of over 75 % after 32 cycles and 3 times of performance recovery experiments. Moreover, the MZO electrode exhibits positive selectivity for copper ions in mixed multiple salt solutions (separation factor α = 0.68). Density functional theory (DFT) calculations confirmed the selectivity of the MZO electrode for Cu2+ by providing the lowest binding energy (−2.4065 eV) for Cu2+.

Redox flow deionization using Prussian blue and functionalized ion exchange membrane for enhanced selective ion recovery

Flow electrode capacitive deionization (FCDI) is a newly developed water treatment technology that offers better performance than the original, solid electrode capacitive deionization (CDI). By eliminating the need for discharging process, FCDI provides much higher desalination capacity with a continuous desalination operation. Along with applications for salt removal, selective removal and recovery of specific ions are also prominent features of FCDI technology. Characteristics of electrodes and ion exchange membranes play a crucial role in controlling the selectivity of ions in an FCDI system. In this study, we demonstrated selective and continuous ion recovery by combining redox-active Prussian blue as flow electrodes and functionalized ion exchange membranes in FCDI configuration. Prussian blue selectively absorbs monovalent Na+ ions from Na+/Ca2+ mixtures through intercalation reaction, in which a smaller Stokes radius of Na+ ion is more favored to fit in the crystal lattice. Furthermore, such selectivity is much more enhanced through functionalization of a cation-exchange membrane (CEM) with polymer multilayers. A layer-by-layer method was used to deposit poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate) (PSS) on the CEM. Results showed that polymer multilayers changed the CEM from divalent to monovalent ion-affinity due to different ion sizes and charge density. The combination of Prussian blue and functionalized CEM raised the selectivity for sodium around 17 times compared to a control system (0.2 to 3.35). We believe that our approach can provide insight into various areas including resource production, recovery, removal of toxic ions, and material recycling using an FCDI system.

Research progress of sodium super ionic conductor electrode materials for capacitive deionization

The world is suffering from water shortage and the hope relies on the desalination of salty water. Several technologies are available for desalination such as reverse osmosis and electrodialysis. However, the application of these technologies in desalination is limited by the higher energy consumption, high investment and maintenance costs. Capacitive deionization (CDI), an up-coming technology at an infant stage is able to supplement the dwindling freshwater resources and provide water to the population. The performance of CDI is directly affected by the electrode materials. Since its inception carbon has been the principal electrode materials for CDI technology. However, low salt removal capacity due to inaccessibility of nanopores and ejection of co-ions hinder the application of carbon materials. Battery materials have been developed as an alternative to accompany carbon materials. Specifically, sodium superionic conductor (NASICON) materials have been used in brackish water desalination by CDI due to having the diffusion channels forming a three dimensional open structure and availability at low cost. However, the NASICON materials have not yet applied in the desalination of water with high salinity such as sea water. This paper reviews the present status of NASICON materials as CDI electrode materials and fairly compares it with carbon materials. It further compares the cost of NASICON and carbon materials, highlights the possibility of NASICON to desalinate high ionic strength solutions such as seawater. Moreover, the paper, discuss the methods which can be used to enhance the salt removal capacity of NASICON materials and provide examples of NASICON materials in the field of energy storage that can be borrowed and used in CDI. From the analysis of this work, NASICON materials are great companion of the carbon materials, are cost effective and are promising for the desalination of sea water.

Insight into the scaling behavior of capacitive deionization for the removal of multiple cation components in the single-pass mode

Capacitive deionization (CDI) can effectively remove various ions in seawater and brackish water. However, due to the complex cation components in natural water, the desalination stability of CDI is challenged by cation scaling. In this study, the scaling behavior of CDI for the removal of multiple cation components in the single-pass mode was investigated. Three feed solutions containing various compositions (FS1 for Na+, FS2 for Na+/Ca2+/Mg2+, and FS3 for Na+/Ca/Mg2+/Fe3+) were tested. The results showed that the desalination stability was affected by cation components. Especially, the salt adsorption capacity for FS3 dramatically decreased from 7.16 to 3.53 mg/g over multiple cycles. Characterization demonstrated that the scaling of Ca and Mg for FS2 was slight while the scaling of Fe for FS3 was noticeable. Additionally, compared to FS2 scale, Fe significantly aggravated the scaling of Ca and Mg. The severe scaling behavior led to a decrease in specific capacitance, and an increase in charge transfer resistance and ion diffusion resistance of CDI electrodes. Finally, the correlation between scaling behavior and Faradaic reactions was comprehensively explored. The study provides insight into the scaling behavior of CDI, which is crucial for desalination stability in actual operation.

Performance and mechanism of chromium removal using flow electrode capacitive deionization (FCDI): Validation and optimization

The contamination of wastewater with hexavalent chromium (Cr (VI) poses significant health and environmental risks. To alleviate this challenge, our research explores the potential of flow electrodes capacitive deionization (FCDI) technique, an energy-efficient and environmentally friendly approach, for Cr (VI) removal and detoxification. We incorporated varying weight loadings of carbon nanotubes (CNTs) into activated carbon (AC) flowable electrodes, and assessed the system’s performance under different operational conditions. Our findings revealed that by adding just 1.5 wt% CNT to AC and applying a voltage of 0.9 V, the current response increased 3.4 times compared to that of pure AC flowable electrode, resulting in a remarkable Cr (VI) removal efficiency of 99.5 %. This study also revealed that the presence of high concentrations of co-existed chloride ions (Cl−) did not significantly influence the efficiency of Cr (VI) removal. Incorporation of CNT in AC flow electrodes reduced the inter-tube aggregation and enhanced the contact efficiency between the AC particles, forming a connectingpercolationnetwork and enhancing the conductivity of the slurry electrodes for Cr (VI) removal. In addition to Cr (VI) removal, FCDI technique transformed extremely dangerous Cr (VI) to less toxic Cr (III) from 5.4 to 49.8 % during multiple cycles. In summary, we presented an innovative, efficient, eco-friendly approach for Cr (VI) removal and detoxicity by using CNTs incorporated electrodes, underscoring the potential FCDI technology in heavy metal disposal.

Na2MnSiO4/C as hybrid capacitive deionization electrode material to enhance desalination performance

The utilization of Na2MnSiO4 as a Faraday electrode in hybrid capacitive deionization (HCDI) is investigated to achieve efficient desalination. The Na2MnSiO4/C (NMSO) materials were fabricated via a simple sol–gel method, in which the synthesis process was modulated by adjusting the volume ratio of ethanol to water. When maintaining the volume ratio of water to ethanol at 3:1, the resultant NMSO-3/1 exhibited expected salt adsorption capacity of 31.06 mg g−1 and salt adsorption rate of 20.43 mg g−1 min−1. This distinguished desalination performance was mainly attributed to its inherent multiple redox pairs, as well as the integration of ethanol, which enhanced both specific capacitance and hydrophilicity of the material. This study opens a new perspective for the development of highly efficient materials in HCDI.

Synthesis of novel 2D/2D Ti3C2Tx MXene / 1T-MoS2 heterostructure enhanced with carbon nanotubes as a highly-efficient electrode for hybrid capacitive deionization

MXenes are a prominent group within 2D materials with extensive utilization in various electrochemical applications due to their notable specific capacitance, swift ion intercalation kinetics, and favorable hydrophilicity. Despite this, they suffer from low oxidation resistance and ease of layer restacking. Herein, an effective approach to overcome this inherent weakness of MXenes was employed by embedding MoS2 nanopetals as pillars into the backbone of Ti3C2Tx MXene by a one-step hydrothermal technique. It not only alleviates the restacking problem of Ti3C2Tx but also shorten the diffusion path of sodium ions. Meanwhile, ammonium bicarbonate as a guest ion agent was utilized to achieve the 1 T phase of MoS2 with numerous edge-active sites to overcome the low electrical conductivity of the 2 H phase derived from the semiconducting nature. The incorporation of carbon nanotubes (CNTs) has strengthened the structural integrity of the Ti3C2Tx/1 T-MoS2/CNTs composite by linking 2D material components. Moreover, it elevates the heterostructure’s overall electrical conductivity by establishing multiple routes for efficient electron transfer. As a result, the membrane-free established hybrid capacitive deionization cell with Ti3C2Tx/1 T-MoS2/CNTs // activated carbon demonstrates a remarkable salt removal capacity of 25.82 mg.g−1 along with an outstanding high removal rate of 5.25 mg.g−1.min−1 and superior cyclic stability (98.1% desalination capacity retention rate). This work represents a simple approach for rational designing low-dimensional heterostructure materials with high connectivity and straightforward electron transferring for electrochemical desalination.

Flow electrode capacitive deionization with iron-based redox electrolyte

Flow electrode capacitive deionization (FCDI) employs carbon-based materials for electrochemical separation. Redox additives like vanadium, ferricyanide, and organic molecules have boosted desalination rates in FCDI. However, these additives must be low-cost, have chemical stability in electrolytes, and have low toxicity. We investigate two iron-based redox couples, iron chloride, and iron citrate, for their cost-effectiveness and reduced health risks. We incorporate them with activated carbon (AC) in flow electrodes, resulting in a significant enhancement of the salt removal rate, with iron chloride increasing it by 100 % and iron citrate by 23 % compared to standard AC slurries. In our system, the AC exhibits a salt removal rate of 7.8μmolcm−2min−1, while the iron chloride system reaches 15.6μmolcm−2min−1. Iron citrate flow electrodes achieve a desalination rate of 9.6μmolcm−2min−1, maintaining a pH suitable for potable water. We observe that iron chloride concentration in the flow electrode influences iron crossover and pH in the water streams, leading to acidity. Conversely, the iron citrate-based slurry maintains minimal iron crossover and a neutral pH. This study highlights iron-based redox additives as promising for FCDI slurries and the need for tailored flow electrode composition based on specific ion removal requirements.

Selective fluoride removal from groundwater using CNT-CeO2 electrodes in capacitive deionization (CDI)

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-CeO2 (CNT-CeO2) 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-CeO2 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 CeO2-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-CeO2 towards fluoride is attributed to the intercalation Faraday capacitance initiated by the reaction between F− with CeO2, 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-CeO2 electrode. The addition of CNTs substantially increases the conductivity of CeO2 electrodes and restricts the aggregation of CeO2, 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.