Asymmetric Capacitive Deionization Research Articles

Polystyrene sulfonate coated activated graphene aerogel for boosting desalination performance using capacitive deionization

Capacitive deionization (CDI) has proven to be a cost-effective and environmentally friendly desalination process. However, the high salt adsorption capacity is hampered by the poor surface wettability of the electrode materials and a lack of charge in the surface electrode. Despite its unique chemical and physical properties, pure reduced graphene oxide is still far from the expected potential desalination performance. In this work, graphene aerogel (GA) and activated graphene aerogel (AGA) with an ultrahigh specific surface area were successfully synthesized. The AGA was functionalized using a charged polystyrene sulfonate (PSS) and utilized as a negative electrode for the asymmetric CDI configuration cell. The synthesized charged PSS-activated graphene aerogel (PSS-AGA) achieved outstanding hydrophilic behavior, a high specific surface area (1401.53 m2/g), and a typical specific capacitance of 178F/g, furthermore excellent capacitance retention of 97.2 % over 5000 cycles. The salt adsorption capacity of the fabricated electrodes is evaluated by conducting a single-pass mode (SP-Mode) experiment under various conditions. The fabricated PSS-AGA//AGA asymmetric cell configuration delivers an exceptional salt adsorption capacity of 26.33 mg/g, excellent charge efficiency of 88 % with low energy consumption at 1000 mg/L, and full regeneration with long-term stability.

Porous 3D-graphene functionalized with MnO2 nanospheres and NiO nanoparticles as highly efficient electrodes for asymmetric capacitive deionization: Evaluation by impedance-derived capacitance spectroscopy

Capacitive deionization (CDI) is a novel desalination technology for brackish water purification. Here, we report the rational design of an efficient asymmetric CDI system considering several steps. First, three-dimensional graphene nanostructures (3DG) were synthesized using the hydrothermal method, followed by the freeze-drying process, and then drop casting onto the carbon fiber paper (CFP) substrate. Second, a facile and controllable electrodeposition technique was used for the in-situ synthesis of MnO2 nanospheres and NiO nanoparticles onto the CFP/3DG surface to prepare CFP/3DG/MnO2 cathode and CFP/3DG/NiO anode. Third, the prepared nanocomposite electrodes were used to fabricate an asymmetric CDI cell. Here, MnO2 (pI = 4.5) and NiO (pI = 10) nanostructures provide opposite surface charges due to their dissimilar isoelectric points, enhancing cations and anions electrosorption. The surface morphology, chemical and electrochemical characteristics, as well as the capacitive performance of the fabricated CFP/3DG/MnO2 and CFP/3DG/NiO electrodes, were investigated using SEM, EDX, XRD, FTIR, TGA, BET/BJH, and electrochemical techniques. Besides, impedance-derived capacitive analysis was used to evaluate the ions retention time on the fabricated electrodes. Finally, the fabricated CFP/3DG/NiO||CFP/3DG/MnO2 asymmetric CDI cell was used in water desalination with high reversibility, good regeneration rate, and high electrosorption capacity (21.01 mg g−1) for the initial salt concentration of 1000 mg L−1 in 1.2 V. These results suggest that the proposed asymmetric CDI system is a promising candidate for the efficient capacitive desalination process.

Efficient and Durable Sodium, Chloride-doped Iron Oxide-Hydroxide Nanohybrid-Promoted Capacitive Deionization of Saline Water via Synergetic Pseudocapacitive Process.

Recently, the rational design and development of efficient faradaic deionization electrodes with high theoretical capacitance, natural abundance, and attractive conductivity have shown great promise for outstanding capacitive deionization (CDI)‐based desalination applications. Herein, the construction of novel FeOOH hybrid heterostructures with Na and Cl dopants (e.g., Na‐FeOOH and Cl‐FeOOH) via a robust hydrothermal strategy is reported, and an asymmetric CDI cell (Na‐FeOOH//Cl‐FeOOH) comprising Na‐FeOOH and Cl‐FeOOH working as the cathode and anode, respectively, is assembled. The multiple coupling effects of the specific structural features (e.g., enriched porosity, hierarchical pore alignment, and highly open crystalline framework), enhanced electrochemical conductivity, and optimized ion‐transfer property endow the FeOOH hybrid electrode with improved electrochemical performance. Impressively, the Na‐FeOOH//Cl‐FeOOH cell demonstrates a superior salt adsorption capacity (SACNaCl) of 35.12 mg g−1 in a 500 mg L−1 NaCl solution, a faster removal rate, and remarkable cycling stability. Moreover, the pseudocapacitive removal mechanism from the synergetic contribution of the Na‐FeOOH cathode and Cl‐FeOOH anode account for the significant desalination promotion of the Na‐FeOOH//Cl‐FeOOH cell.

Nickel hexacyanoferrate incorporated with reduced graphene oxide for highly efficient intercalation desalination

In this study, nickel hexacyanoferrate (NiHCF) was incorporated with reduced graphene oxide (rGO) as a pseudocapacitive intercalation cathode in asymmetric capacitive deionization (ACDI) for desalination. The electrochemical characterization results demonstrated that the NiHCF@rGO nanocomposite exhibited an excellent specific capacitance of 158.2 F g−1 at a scan rate of 5 mV s−1 and high electrical conductivity. The desalination experiments were carried out in single-pass mode for the performance analysis under different operational parameters. For desalting 4 g L−1 NaCl solution at 1.2 V, NiHCF@rGO-assembled ACDI exhibited a large salt adsorption capacity of 80.2 mg g−1 and a high charge efficiency of 86.6%, associated with an energy consumption of 0.053 kWh m−3. It is noted that the NiHCF@rGO-assembled ACDI significantly outperformed the activated carbon-based membrane capacitive deionization for desalination. More importantly, the incorporation of rGO can improve the 27.7% salt adsorption capacity of NiHCF. The significant improvement can be ascribed to rGO uniformly distributing NiHCF particles on its surface via conductive bridges, which enable a low charge transfer resistance and facilitate the transport of ions. The X-ray photoelectron spectroscopy analysis further verified the Na+ intercalation/deintercalation of the NiHCF@rGO electrode, accompanied by the Fe3+ to Fe2+ redox reaction in a cyclic charging–discharging process. Overall, this work demonstrates the great potential of NiHCF@rGO as a cathode for efficiently improving desalination performance.

Anion-kinetics-selective graphene anode and cation-energy-selective MXene cathode for high-performance capacitive deionization

Capacitive deionization (CDI) is one of the most promising energy-efficient technologies for water desalination, however its industrial translation is slow and impeded by limited electrosorption capacity, slow electrosorption rate and poor cycling stability. Herein we address the above challenges by developing and validating a new concept of unimpeded and selective full-cell anion-cation separation using custom-designed anion-kinetics-selective anode and cation-energy-selective cathode. Nanoporous graphene anode enables the selective anion kinetics, while cation selectivity on the functionalized MXene cathode is due to the difference in ion adsorption energy. These mechanisms are validated by electrochemical quartz crystal microbalance measurements. The finely-tuned balance between ion transport and adsorption causes unimpeded ion diffusion, leading to the higher electrosorption rates. These effects are guided by the atomistic and quantum chemistry simulations, and also confirmed experimentally by tuning the pore size of graphene anode and the functionalization of MXene cathode. The fabricated asymmetric CDI cell exhibits superior electrosorption capacity of 49 mg g−1 and high electrosorption rate of 2.92 mg g−1 min−1 in 5000 mg L−1 NaCl solution as well as excellent cycling stability (100 cycles), which are among the best of the current state-of-the-art CDI studies. These results demonstrate the design of advanced electrode materials for the effective control of ion kinetics and energies to achieve selective ion transport, thus providing new insights for a broader range of energy-related applications.

Facile fabrication of intercalation-type pseudocapacitive S-Ti3C2Tx/PANI/F-Ti3C2Tx cathode for asymmetric capacitive deionization

Although tremendous efforts have been paid on Ti3C2Tx as a cathode for capacitive deionization (CDI), its structural instability and easy agglomeration have always limited such a promising application. In this study, a hybrid of hollow Ti3C2Tx spheres and flakes was synchronously fabricated by a novel sacrificial-templated method for the first time, and then immobilized/bonded together by in-situ polymerization of polyaniline, which was labelled as S-Ti3C2Tx/PANI/F-Ti3C2Tx. The highest specific capacitance of S-Ti3C2Tx/PANI/F-Ti3C2Tx in 1 M NaCl solution was measured to be 102 F·g−1, and diffusion capacitance of the as-obtained S-Ti3C2Tx/PANI/F-Ti3C2Tx was calculated to be 63%. Thus, a high salt adsorption capacity of 76 mg·g−1 with a maximum salt adsorption rate of 11.45 mg·g−1·min−1 was obtained, which is much higher than that of Ti3C2Tx-related materials in literature for asymmetric CDI system. Moreover, such a superior desalination performance was verified stable at almost 100% after 30 cycles. Besides the ion intercalation and fluoro/hydroxyl groups of Ti3C2Tx, the high performance and stability of the as-obtained S-Ti3C2Tx/PANI/F-Ti3C2Tx should also be ascribed to the beneficial effects of immobilization/bonding role of conductive PANI between S-Ti3C2Tx and F-Ti3C2Tx. This study gives a new perspective to promote the real applications of CDI by designing highly capacitive and stable electrode materials.

Maximized ion accessibility in the binder-free layer-by-layer MXene/CNT film prepared by the electrophoretic deposition for rapid hybrid capacitive deionization

Two-dimensional (2D) pseudo-capacitive material MXene has attracted great consideration in the capacitive deionization (CDI) process for its prominent ion storage capacity and reversible ion intercalation/deintercalation mechanism. Unfortunately, the accessibility of active sites for ion intercalation is greatly limited by the self-stacking of MXene flakes for the strong van der Waals forces. What’s worse, the addition of binder in the MXene electrode prepared by the conventional coating method for CDI blocks ion accessibility channels and increases ion transfer resistance, hindering the efficient capture of ions by electrodes. Herein, the binder-free layer-by-layer MXene/CNT film electrode is flexibly constructed by the electrophoretic deposition (EPD) method to effectively alleviate the above situation. In this design, CNT not only acts as the conductive interlayer spacer to restrain MXene nanosheets self-stacking but also builds interconnected conductive channels to minimize the tortuosity of charge transport path. Furthermore, the binder-free EPD method for electrode preparation maximizes ion accessibility to ion intercalation sites, simplifies ion transfer path, accelerates ion transfer rate, and improves ion diffusion kinetics. Consequently, the resulting deposited MXene/CNT film electrode displays a dominant specific capacitance (178 F g−1), reduced charging resistance, rich Na+ diffusion channels, and maximized Na+ diffusion coefficient (5.8 × 10−8 cm2 s−1). Thus, the asymmetric hybrid CDI cell CNT//MXene/CNT adopting the deposited CNT electrode as anode and deposited MXene/CNT film electrode as cathode delivers the prominent desalination capacity (34.5 mg g−1), rapid average desalination rate (3 mg g−1 min−1), low energy consumption (0.26 kWh kg−1-NaCl), and excellent cyclic intercalation-deintercalation stability (89% retention rate). This rational design provides an alternative strategy to solve the tricky problems of MXene and develops new approach towards maximized ion accessibility in CDI field.

Enhanced capacitive deionization of a low-concentration brackish water with protonated carbon nitride-decorated graphene oxide electrode

Freshwater resources are one of the core elements that affect the harmonious development of mankind and society. Capacitive deionization (CDI) technology is one of the effective methods to transform brackish water into fresh water. The choice of material for a CDI electrode is critical to its electrosorption performance, which directly affects the electrosorption performance through interface optimization. Herein, protonated carbon nitride (H–C3N4)-modified graphene oxide (H–C3N4-mGO1/8) is fabricated by a simultaneous nucleophilic addition and amide reaction in order to enhance capacitive deionization of a very low concentration brackish water. Using activated carbon (AC) as the positive electrode and H–C3N4-mGO1/8 as the negative electrode, H–C3N4-mGO1/8 || AC asymmetric CDI devices are used to remove ions from a NaCl aqueous solution. The CDI test results indicate that the system has a high electrosorption capacity of 8.36 mg g−1 in the 50 mg L−1 NaCl solution with a low applied voltage of 1.2 V, which is 1.40 times than AC || AC symmetric. Moreover, the CDI device performs faster adsorption rate of 0.1879 mg (g·min)−1 and an excellent regeneration efficiency of 100%. The salt electrosorption capacity, electrosorption rate of the H–C3N4-mGO1/8 || AC asymmetric electrodes improve with increasing applied voltage due to the stronger Coulombic interaction between the electrode and charged ions with the formation of a more sufficient electric double layer principle.

Design of a novel asymmetric capacitive deionization device with high desalination performance

It is a serious challenge to develop a capacitive deionization device (CDI) with high electrosorption capacity, high desalination rate and low energy consumption.

Enhanced ammonium removal and recovery from municipal wastewater by asymmetric CDI cell equipped with oxygen functionalized carbon electrode

Following the concept of sustainable development and circular economy, the Capacitive Deionization (CDI) was examined for the removal and recovery of ammonium (NH4+) ions from municipal wastewater towards prevention of eutrophication and obtaining a valuable component for fertilizers production. The electrosorption study was conducted in the symmetric CDI cell, composed of activated carbon (AC) electrodes, and an asymmetric CDI cell, equipped with oxygen functionalized AC (AC-ox) cathode. The asymmetric CDI cell was demonstrated the enhanced performance in terms of the removal efficiency towards ammonium ions (83%) and the related energy consumption (and 0.085 kWh/mmol NH4+), compared to the symmetric one with the following results: 30% removal of NH4+ ions and 0.215 kWh/mmol NH4+. The beneficial performance of the AC-ox electrode also has been observed in terms of NH4+ recovery: 20% recovered NH4+ ions, compared to 9% NH4+ recovered ions by AC pristine electrode. The presence of competitive ions hindered the electrosorption efficiency. Nevertheless, the time-dependent selectivity of the AC-ox electrode towards NH4+ ions was observed. The specific capacities of the symmetric and asymmetric cells at 5 mV/s were calculated to be 10.4 F/g and 18.3 F/g, respectively. Material characterization of AC before and after oxidation explained the enhanced CDI performance of the AC-ox electrode due to the increased hydrophilicity and additional oxygen functionalities on the electrode surface.

Flexible structural engineering of PPy-NiCo-LDH@Mxene for improved capacitive deionization and efficient hard water softening process

Capacitive deionization (CDI), as a potential desalination technology, shows the prominent advantage in processing low-concentration brackish water. However, the traditional CDI electrodes suffer from the problems like the limited salt adsorption capacity and inferior practical application in selective ion separation process. Herein, the effective-designed three-dimensional Mxene-based composite PPy-NiCo-LDH@Mxene is constructed by integrating PPy, NiCo-LDH, and Mxene together through the hydrothermal method. Particularly, the excellent conductive PPy network can greatly alleviate the restacking of Mxene and NiCo-LDH flakes and provide effective charge diffusion and transfer channels. Moreover, the Mxene substrate can strengthen the structure stability of the composite and further reinforce the total electrical conductivity of the composite material. Meanwhile, the NiCo-LDH and Mxene provide rich ion intercalation sites, contributing to the high ion storage capacity and fast electrosorption kinetics. Consequently, the PPy-NiCo-LDH@Mxene achieves the significant specific capacitance (174 F g−1), superior charging and discharging property, and enhanced electrochemical cyclic stability. The asymmetric CDI cell (AC//PPy-NiCo-LDH@Mxene) displays the prominent salt adsorption capacity (31.5 mg g−1), rapid salt removal rate (4.7 mg g−1 min−1), fast electrosorption kinetics, and stable adsorption/desorption process. Furthermore, in the multi-salt solutions with different salt solution concentrations, the PPy-NiCo-LDH@Mxene electrode displays the superb adsorption selectivity to the Ca2+ and Mg2+ than Na+, which indicates its suitable application for hard water softening process. The effective structural design exhibits huge potential for applying effective electrode materials for fast CDI process and hard water softening.

Well-dispersed few-layered MoS2 connected with robust 3D conductive architecture for rapid capacitive deionization process and its specific ion selectivity

Capacitive deionization (CDI) has aroused significant concern for its application in treating low-concentration brackish water. However, some carbonaceous materials suffer from the limited salt adsorption capacity (SAC), sluggish adsorption kinetics, and inferior selectivity to particular ions. Hence, we develop the well-dispersed few-layered two-dimensional pseudocapacitive material MoS2 connected with a robust 3D conductive architecture constructed by carbon nanotubes (CNT) and carbon spheres (CS) (MoS2@CNT-CS). The enhanced dispersibility and expanded interlayer of MoS2 nanosheets provide sufficient faradic effective sites for ion storage and transfer, while the interlaced carbonaceous network affords improved hydrophilicity, convenient electron transport path, and low charge transfer resistance for fast charge transportation and permeation. Therefore, the asymmetric hybrid CDI cell (CNT-CS for anode and MoS2@CNT-CS for cathode) delivers a prominent SAC (25.35 mg g−1), fast SAR (3.9 mg g−1 min−1), enhanced desalination kinetics, and favorable cycling stability. In addition, the specific ion selectivity of MoS2@CNT-CS electrode was evaluated systematically in the equimolar multi-salt solutions, and a selectivity order of Ca2+ > Na+ > Mg2+ > K+ was obtained. The molecular dynamic (MD) calculation provides the lowest intercalation energy for Ca2+ (−2.55 eV), further confirming the strong interaction and preferable pseudocapacitive deionization of MoS2@CNT-CS electrode to Ca2+ over other cations.

Selective adsorption of phosphate by carboxyl-modified activated carbon electrodes for capacitive deionization.

Capacitive deionization (CDI) has been considered as a promising technology for removing phosphate from water but suffer inferior selectivity and electrosorption performances for phosphate of current carbon electrodes in CDI. Herein, we achieved highly selective phosphate removal from a ternary effluent of Cl-, PO43-, and SO42- by using nitric acid-treated activated carbon (AC) with various modification times and pure AC as the anode and cathode, a novel phosphate selective asymmetric CDI reactor. The results showed that carboxyl groups greatly grafted on the materials after modification (varying from 0.00084 to 0.0012 mol g-1). The phosphate selectivity of the present research was higher than that of unmodified CDI, and it increased with the increase of carboxyl groups content. The highest phosphate selectivity (2.01) in modified materials is almost six times higher than that of pure AC. Moreover, the modified electrodes exhibited good regenerative ability with a phosphate desorption efficiency of around 72.12% during the adsorption/desorption process and great stability during the cycling experiment. These results demonstrated that the innovative application of nitric acid-modified AC can effectively selectively remove phosphate from mixed anion solution, opening a hopeful window to selective adsorption in water treatment by CDI.

Using crude residual glycerol as precursor of sustainable activated carbon electrodes for capacitive deionization desalination

Capacitive deionization (CDI) is a promising electrochemical technology for water desalination that can contribute to reducing water scarcity. At the same time, appropriate routes for the disposal or reuse of liquid wastes are also a major current concern. Based on the water-waste nexus concept, this work demonstrates that crude glycerol from biodiesel plants can be successfully used to obtain a new sustainable activated carbon. After polymerization, the crude glycerol was carbonized and activated to obtain polyglycerol activated carbon (PGAC), which was employed as an electrode for CDI desalination of brackish water. Evaluation was made of the electrode performance and stability over cycles of electrosorption/desorption, using different cell configurations (symmetric, asymmetric, and membrane CDI) and cell voltages (Ecell). It was observed that maintaining the potential of zero charge of the negative and positive electrodes outside their working domains during the cycles enabled minimization of that part of the applied potential deviated to co-ion repulsion, consequently improving the salt adsorption capacity (SAC) and charge efficiency (QE). Furthermore, maintaining the potential of the positive electrode below the oxidation potential by controlling the applied Ecell could ensure electrode stability. The best desalination performance using the PGAC electrode was achieved using the membrane CDI configuration (at 1.6 V), resulting in stable SAC (∼27.1 mg g−1) and QE (∼100%) over 50 cycles. The low cost and high SAC and QE values suggested that the PGAC electrode could be considered a potential candidate for use in CDI desalination.

A novel “butter-sandwich” Ti3C2Tx/PANI/PPY electrode with enhanced adsorption capacity and recyclability toward asymmetric capacitive deionization

Capacitive deionization (CDI) is an environmentally-friendly-water-treatment technology, whereas its wide application has been limited by the low salt adsorption capacity and poor recyclability of electrode materials. In this study, a novel Ti3C2Tx/PANI/PPY electrode is invented with a “butter-sandwich” microstructure, where bread-like Ti3C2Tx slices are intercalated by polypyrrole (PPY) spheres, and then glued by butter-like polyaniline (PANI). Thus, the negatively charged PPY is fastened inside Ti3C2Tx slices by positively charged PANI for a superhigh structural stability and elevated specific surface area. When Ti3C2Tx/PANI/PPY is used as the electrode for CDI, a high salt adsorption capacity (39.62 mg g−1), a fast salt adsorption rate (2.64 mg g−1 min−1) and a high cycling stability of almost 100% for 30 times are obtained, surpassing those of the Ti3C2Tx electrodes and many other related materials in literature. Such an outstanding CDI performance can be ascribed to the intercalation of PPY, which enlarges the layer spacing of Ti3C2Tx as well as the ion storage space, and the binding of them by PANI, which provides more paths for electron transport and improves the ion adsorption rate. This work provides a new direction to enhance the performance of capacitive deionization by an attentive “butter-sandwich” new microstructural design strategy.

Controllable Design and Preparation of Hollow Carbon-Based Nanotubes for Asymmetric Supercapacitors and Capacitive Deionization.

Carbon-based materials are important desirable materials in areas such as supercapacitors and capacitive deionization. However, traditional commercial materials are heterogeneous and prone to agglomeration in nanoscale and have structural limitation of electrochemical and desalination performance due to poor transport channels and low capacitance of prepared electrodes. Here, we introduce the facile strategy for controllable preparation of two types of hollow carbon-based nanotubes (HCTs) with amorphous mesoporous structures, which are synthesized by employing a MnO2 linear template method and calcination of polymer precursors. The porous N-doped HCT (NHCT) shows a specific capacitance of 412.6 F g-1 (1 A g-1), with 77.3% rate capability (20 A g-1). The fabricated asymmetric MnO2//NHCT supercapacitor displays the energy density of 55.8 Wh kg-1 at a power density of 803.9 W kg-1. Furthermore, two typical MnO2//HCT and MnO2//NHCT devices both show the selective desalination performance of sulfate, and the MnO2//NHCT device possesses a high deionization value of 11.37 mg g-1 (500 mg L-1 Na2SO4). These fabricated hollow carbon-based architectures with functional characteristics promise potential applications in energy and environmental related fields.

Carbon electrode with cross-linked and charged chitosan binder for enhanced capacitive deionization performance

Capacitive deionization (CDI) has emerged an energy-efficient and operation-simple electrochemical desalination processes among water treatments. Polymeric binder as an important component of CDI electrode is desired to have good adhesion, wettability and stability. In order to reduce the chemical uses and further enhance the CDI performance, we have designed the preparation of the crosslinked chitosan (CS) with glutaraldehyde (GA), and coated it on the activated carbon (AC) electrodes for desalination. By adjusting different GA contents, the CS crosslinked with GA (CS + 3GA) binder with stable three-dimensional (3D) networks were fabricated due to the utilization of tertiary amines (C=N), which exhibited strong adhesive property. Moreover, the CS + 3GA not only possessed good hydrophilicity, but also carried more positive charges that function as ion anion exchange membrane. In the electrosorption tests, an asymmetric CDI system (CS + 3GA//PVDF) was assembled via pairing a composite carbon anode coated with CS + 3GA binder, and a PVDF-bound carbon cathode. The studies demonstrated that CS + 3GA//PVDF cell showed excellent salt adsorption capacity (SAC) of 16.0 mg g−1 over 2 times higher than PVDF//PVDF, and good charge efficiency (CE) of 62.5% at 1.2 V. More importantly, CS + 3GA//PVDF displayed a remarkable regeneration performance that retention rate of 82.8% and approached that of PVDF//PVDF-M (83.2%) over 80 regeneration cycles. In addition, CS + 3GA//PVDF cell could be implemented at 1.4 V for desalination, which showed the unprecedented SAC of 18.4 mg g−1. The cross-linked and charged CS + 3GA binder coated with the carbon anode possesses the merits of being inexpensive, green and stable, making it a promising candidate for future CDI applications.

3D hierarchical porous N-doped carbon quantum dots/vanadium nitride hybrid microflowers as a superior electrode material toward high-performance asymmetric capacitive deionization

3D hierarchical porous NCQDs/VN hybrid microflowers were first demonstrated as an advanced electrode for an asymmetric capacitive deionization of saline water.

Selective Capacitive Removal of Pb2+ from Wastewater over Redox-Active Electrodes.

Water pollution has become an environmental hazard. Diverse metal cations exist in wastewater; lead is the most common heavy metal pollutant among them. Selective removal of highly toxic and ultradiluted lead ions from wastewater is a major challenge for water purification. Here, selective capacitive removal (SCR) of lead ions from wastewater over redox-active molybdenum dioxide/carbon (MoO2/C) electrodes was developed by an environment-friendly asymmetric capacitive deionization (CDI) method. The MoO2/C spheres act as cathodes of an asymmetric CDI device and effectively reduce the concentration of Pb2+ from 50 ppm to <0.21 ppb. Moreover, the SCR efficiency of lead ions over redox-active MoO2/C electrodes is >99% in mixtures of 100 ppm Pb(NO3)2 and 100 ppm NaCl solutions. In addition, the electrodes exhibit high regeneration performance in mixtures of NaCl and Pb(NO3)2 and high SCR efficiency for lead ions from mixtures of heavy metal ions. The tetrahedral structure of the [MoO4] lattice is shown to be more favorable for the intercalation of lead ions. In situ Raman spectroscopy further shows that the transition of the crystal interface between [MoO6] and [MoO4] cluster lattice could be electrochemically controlled during SCR. Therefore, this study provides a new direction for the SCR of lead ions from wastewater.

A win-win strategy of β-cyclodextrin and ion-doped polypyrrole composite nanomaterials for asymmetric capacitive deionization

The hollow carbon sphere (HCS) coated with polypyrrole (PPy) layer was synthesized using β-cyclodextrin as the precursor compound with ion-doped (Cl-, DBS-) polypyrrole for the capacitive removal of salt ions from brackish water. HCS derived from β-cyclodextrin was firstly prepared using Pluronic F-127 as the soft template. The combination of HCS and ion-doped PPy effectively exerts the advantages of both to achieve a win-win effect. The supporting effect of HCS alleviates the agglomeration of PPy and enables the composite material to have a larger specific surface area. The electrochemical tests show that the HCS@PPy-Cl and HCS@PPy-DBS composites have improved electrochemical performance, with specific capacitances of 65.4F g−1 and 101.9F g−1, which are 1.62 times and 2.52 times that of HCS, respectively. Furthermore, the desalination tests indicate that the HCS@PPy-Cl||HCS@PPy-DBS (H-PC||H-PD) asymmetric cell exhibits a high electrosorption capacity of 31.77 mg g−1, which is much higher than that of the HCS||HCS (H||H) symmetric cell (26.35 mg g−1) in 500 mg L-1 NaCl solution. And the charge efficiency of H-PC||H-PD cell can reach 76.2%, which means a great improvement compared to 60.5% of the H||H cell. The improved performances indicate that ion-doped PPy can effectively improve the adsorption capacity of the composite material by providing pseudo-capacitance and suppressing the co-ion effect, while the total specific surface area can effectively increase by HCS and thereby improving the adsorption site.