Electrode Material For Capacitive Deionization Research Articles

Porous N, P co-doping Ti3C2Tx MXene for high-performance capacitive deionization

The emerging energy-saving and environmentally friendly capacitive deionization (CDI) technology has attracted more and more attention. However, it remains a great challenge to develop CDI electrode materials with excellent comprehensive properties. Herein, the porous N, P co-doping Ti3C2Tx MXene (N, P-Ti3C2Tx) was prepared successfully by combining simple flocculation with an annealing process. Benefitting from the synergistic effect of the combination of porous structure and co-doping of N and P heteroatoms, the N, P-Ti3C2Tx exhibits substantial specific surface area, which provides more surface bounding active sites for electrochemical reactions, thus assisting to boost the CDI performance. As a result, the N, P-Ti3C2Tx exhibited an admirable salt (Na+) adsorption capacity of 53.3 mg g−1 and exceptional recycling property. Impressively, the N, P-Ti3C2Tx also exhibited superior desalination performance of Pb2+, characterized by an exceptionally high desalination capacity of up to 168.2 mg g−1 at 1.2 V, and the corresponding desalination rate reached 0.047 mg g−1 s−1. Additionally, the deionization mechanism involved was elucidated through a series of characterizations. This work will furnish an effective avenue for the innovative design of MXene-based electrode materials toward high-performance CDI.

High-performance fluoride removal by Fe/N co-doped microporous carbon: Mechanism of capacitive deionization with FeNx sites

Fluoride pollution is becoming increasingly serious with industrial growth. Controlling fluoride pollution have become important environmental issues of global concern. Transition metal nitrides are excellent CDI electrode materials due to their excellent conductivity and electrochemical stability. In this study, FeNx/C was synthesized as an electrode material for using the capacitive deionization (CDI) technique to achieve the effective removal of fluoride ions from wastewater. Its high specific surface area (463.278 mg/g) and high capacitance (44.29 F/g) were demonstrated and it had a high adsorption capacity of 158.33 mg/g. FeN3 and FeN4 were used as the main sites as capacitive deionization for removing fluoride from the water by chemisorption and hydrogen bonding. The higher adsorption energy by density functional calculation also indicated that the existence of Fe-N coordination bond effectively improved its electrosorption capacity. This study is beneficial to provide references and research ideas for removing fluoride in the CDI field.

Liquid nitrogen quenching-induced grain boundary-rich intermetallic compound/metal/carbon composites for efficient capacitive deionization

In this work, a novel liquid nitrogen quenching treatment is introduced for the first time to design and prepare a grain boundary-rich intermetallic compound/metal/carbon composite, serving as an efficient electrode material for sodium ion removal from simulated seawater. The liquid nitrogen quenching method effectively enhances the grain boundary density within the composite due to the rapid cooling-induced forces, thereby creating abundant active sites. Consequently, Co7Fe3/Co/C-700 achieves optimal performance in sodium ion embedding and de-embedding, with a capacity of 207.5 mg/g. Furthermore, sodium ion adsorption/desorption mechanisms are substantiated through ex-situ techniques, revealing dynamic atomic and electronic structure evolutions under operational conditions. Density Functional Theory (DFT) calculations validate the pivotal role of grain boundary sites in enhancing activity, elucidating the correlation between sodium ion adsorption and grain boundaries. Overall, this work elucidates the significant correlation between adsorption behavior and grain boundaries, proposing an innovative strategy for developing electrode materials for capacitive deionization.

Rational design of MoS2 nanoflowers-decorated carbon nanofibers with enhanced electronic transmission for boosting capacitive deionization

Capacitive deionization (CDI) has sparked considerable interest for its promising application in handling brackish water. Unfortunately, most traditional carbonaceous materials are still plagued by the limited desalination capacity and sluggish adsorption kinetics with the single adsorption mechanism and irrational pore structure. Herein, we developed a facile and affordable strategy to integrate electrospinning with hydrothermal method to fabricate MoS2 nanoflowers-decorated carbon nanofibers (MoS2/CNF) composite for boosting desalination performance. CNF not only serves as a supportive framework to inhibit the agglomeration of MoS2 nanosheets favoring the full contact with salt solutions, but also typically diminishes the charge transfer resistance of the composite. By taking advantage of the double layer adsorption of CNF and the hierarchically micro-mesoporous structure of MoS2 nanoflowers with large interlayer spacer for salt ions intercalation, the optimized sample MoS2/CNF-1 employed as a cathode for the asymmetric hybrid CDI cell showed an eminent capacity for desalination of 30.9 mg·g−1, an ultrafast rate for desalination of 3.7 mg·g−1·min−1, and an attractive circulation stability for desalination with the capacity reservation remaining at 94.8 % after 30 cycles in the solution of 500 mg·L−1 NaCl at 1.2 V. This research sheds fresh light on the evolution of advanced CDI electrode materials for practical utilization.

Selective electrosorption of silver ions from wastewater by biomass-derived N, S co-doped hierarchical porous carbon

The recovery of precious silver (Ag) from wastewater, a dual pursuit of environmental sustainability and economic benefits, faces a formidable challenge in the selective extraction of Ag+ ions amidst a matrix of other metal ions. To address this, we developed a novel N, S co-doped hierarchical porous carbon (NSPC) derived from a renewable precursor of Camellia oleifera shell. As the electrode material for capacitive deionization (CDI), the NSPC exhibited a high electrosorption capacity of 480.5 mg g−1 for Ag+ ions under an operating voltage of 1.2 V. Owing to the strong reaction between Ag+ and S functional groups and the synergistic effect from a hierarchical porous structure and N doping, the formation of Ag2S and elemental Ag is facilitated, and the NSPC cathode could selectively remove nearly 100 % of Ag+ ions from saline water and a solution containing Ni2+, Co2+, Pb2+, Zn2+, and Fe3+ ions. Furthermore, the NSPC cathode showed a high anti-interference ability in the presence of anions (C10H14N2O82−, C4H4O62−, C5H7O5COO− and HCO3−) and excellent reusability until 9 cycles. The impressive results show that the biomass-derived NSPC has the potential for rapid and selective recovery of silver from wastewater and offers a solution for environmental remediation and metal resource recovery.

Selective electrosorption of heavy metal ions from wastewater with S-doped hierarchical porous carbon derived from waste Camellia oleifera shell

Capacitive deionization (CDI) has garnered significant attention in desalination and the removal of heavy metal ions. The quest for cost-effective and high-performance electrode materials is crucial to advance CDI applications. Herein, we report a sulfur-doped hierarchical porous carbon (SPC) derived from waste camellia oleifera shells via a sustainable hydrothermal carbonization process followed by KOH/ammonium salt activation. The optimized SPC-0.2 exhibits exceptional characteristics, featuring a high specific surface area of 1875 m2 g−1 and substantial sulfur doping at 8.65 %, leading to enhanced specific capacitance (161.3 F g−1) in a 1 M NaCl solution. Under optimal operating conditions (1.2 V, 10 mL min−1, 500 mg L−1 NaCl), the SPC-0.2 electrode achieves a notable desalination capacity of 26.64 mg g−1. In particular, the electrode can selectively remove >99 % of Cr3+ and Cu2+ ions (each 10 mg L−1) in a 100 mg L−1 NaCl solution. This high selectivity is attributed to the formation of sulfides via low adsorption energies between the doped sulfur atoms and the targeted heavy metal ions. Furthermore, the SPC-0.2 electrode demonstrates robust reusability and high efficiency in the actual water body. Collectively, our findings underscore the significant potential of the synthesized SPC as an electrode material for CDI, particularly in the selective capacitive removal of heavy metal ions from wastewater. This work not only contributes to the circular economy by using waste biomass, but also offers a viable solution for environmental remediation.

The novel α-TiO2@g-C3N4 heterostructure for ultra rapid ionic pumping and effective capacitive deionization

Anatase titanium dioxide (α-TiO2) has been widely identified as an efficient electrode material for capacitive deionization (CDI). However, this material usually suffers from limited exposure of active sites, poor electrical conductivity and significant obstacles in ion movement due to its tendency to severely aggregate during its preparation. A novel approach to fabricate α-TiO2@g-C3N4 composite materials was proposed in this work. In detail, the 2D material g-C3N4 serves as a high-speed electron-conducting scaffold, allowing uniform loading of nanoscale α-TiO2 and exposing abundant active sites. Furthermore, g-C3N4 facilitates electron transport to nanoscale α-TiO2, enhancing charge transfer and activating previously underutilized electrochemical centers. As a result, α-TiO2@g-C3N4 composites were synthesized for the first time. Through these improvements, the prepared α-TiO2@g-C3N4 exhibits excellent desalination performance, with a high degree of efficiency in water purification. It shows an improved salt adsorption capacity (SAC) of 50.4 mg g-1 and a salt adsorption rate (SAR) of 2.7 mg g-1 min−1 in a 1.2 V, 1 mm electrode spacing, and 500 mg L-1 NaCl solution. This outstanding CDI performance surpasses most recently reported electrode materials. More importantly, this electrode showed remarkable removal efficiencies for various heavy metal ions (Ni2+, Cr3+, Cu2+, Pb2+, Fe3+ and Cd2+), indicating its significant potential for wastewater purification. The superior CDI performance of the α-TiO2@g-C3N4 composite material endows it with practical application in future.

The morphologic dependence of MnO2 electrodes in capacitive deionization process

Manganese dioxide (MnO2) materials are one of promising cathode candidates for capacitive deionization (CDI) applications, with their morphology significantly impacting the performance of the electrode material itself. Therefore, this study systematically investigates the structure-performance relationships and the micro-interface ion storage mechanisms of four morphologies of MnO2 (1D nanorods (NNMO), nanowires (NWMO), 3D microspheres (MMO), and hollow urchin-like spheres (HUMO)) in relation to their capacitive deionization performance with typical heavy metal ions (Cd2+) through experimental and theoretical calculations. In terms of pore morphology, capacitance significantly increases with increasing surface curvature of materials, demonstrating a clear pore structure-dependent characteristic. NNMO, with the smallest pore size, has much lower surface capacitance (∼0.15 μF cm−2) than other materials (∼0.33 μF cm−2). As pore size increases, the capacitance distribution difference driven by pore structure size gradually disappears. Regarding surface and interface characteristics, high-energy crystal facets facilitate electron transfer ({310}>{200}≈{211}>{100}) and increase the proportion of surface active sites (Osur), thus promoting CDI absorption kinetics. During the capacitive deionization process, the pore structure (∼77 %) and surface-interface characteristics (∼23 %) exhibit highly coupled features and the driving force for interfacial ion storage among the four materials is in the order of HUMO>MMO>NWMO>NNMO. This work elucidates the morphology-dependent capacitive processes of MnO2 nanomaterials, enhancing the understanding of structure- electrochemical process at the nanoscale but also providing effective guidance for the design and development of practical, high-performance CDI electrode materials.

Wrinkled hierarchical porous carbon spheres with interconnected multi-cavity for ultrahigh capacitive deionization

As one of the most promising electrode materials for capacitive deionization (CDI), the development of carbon materials with controllable pore structure and continuous mass production is essential for their practical application. Herein, a facile ultrasonic spray pyrolysis method was developed to synthesize surface-functionalized wrinkled hierarchical porous carbon spheres (HCS) with unique interconnected multi-cavity structures. The wrinkled and interconnected multi-cavity hierarchical pores of the HCS play a crucial role in providing accessible ion adsorption sites and promoting ion diffusion and storage in the “multi-cavity warehouse”. The carboxyl groups on the surface of HCS generate a negative charge that promotes the adsorption of cations. The optimized HCS possesses outstanding desalination capacity (114.25 mg g−1), fast adsorption rate (6.57 mg g−1 min−1), and superior cycling stability (95%). Meanwhile, the HCS exhibited impressive desalination capacities in brackish water. Furthermore, the density functional theory calculation results confirmed that the synergistic effect of carboxyl groups and defects significantly enhanced the Na+ adsorption capacity and facilitated ion diffusion. This study extends the synthesis method of surface-functionalized hierarchical porous carbon, which is expected to facilitate the development of CDI electrode materials.

The novel Auricularia-like MoS2 as binder-free electrodes for enhanced capacitive deionization

Molybdenum disulfide (MoS2) has been proven to be an effective and promising electrode material for capacitive deionization (CDI) due to its unique architectures and excellent electrochemical activity. However, MoS2-based electrodes still suffer from the high contact electrical resistance between MoS2 and the current collectors because of the necessity of adding binder during their fabrication. In this work, a flexible MoS2-based electrode with no binders was successfully fabricated. It was found that the frameworks of stainless-steel mesh with surface treatment could serve as an ideal substrate to provide plenty of active nucleation sites for the growth of MoS2. Consequently, a novel morphology of MoS2 with Auricularia-like architectures was produced. Based on electrochemical impedance spectroscopy (EIS), the charge transfer resistance was reduced to nearly zero after the treatment, highly increasing the transportation of charges inside the electrodes compared to the common MoS2-based electrodes. As a result, the binder-free MoS2-based electrodes demonstrated a superior desalination performance of 28.76 mg g-1 (1.2 V) for NaCl solution, which was superior to that of common MoS2-based and many other advanced materials-based electrodes. The novel MoS2-based electrode not only facilitates the utilization of MoS2 in CDI but also paves the way for its application in other electrochemical domains.

A novel composite electrode with multiple pore structures for efficient treatment of heavy metal ions in capacitive deionization

Multiple porous carbon materials have great promise and potential in the capacitive deionization (CDI) field. Specific surface area (SSA), pore size distribution, and preparation method of CDI electrode materials are essential for the treatment of heavy metal ions. In this work, PPy composited porous carbon electrodes (hypercrosslinked polymers/polypyrrole, HCPs/PPy) were obtained by one-step crosslinked carbonization preparation and electro-deposition. The diverse pore structure gives the composite electrode a large SSA and excellent adsorption performance. HCPs/PPy-4 gives a high SSA of 251.26 m2/g. In the CDI process, the adsorption capacity of HCPs/PPy-4 for Fe3+, Cu2+, Pb2+, and Ag+ is 20.69 mg/g, 37.81 mg/g, 26.86 mg/g, and 40.95 mg/g. The negative electrode recoveries for the adsorption of the four ions were reached 81.2%, 89.2%, 85.5%, and 100%, respectively. It indicates that HCPs/PPy is a novel and potentially porous carbon electrode for high-performance CDI.

Exploring MXene’s role in capacitive deionization: Advances, challenges, and future directions

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.

Capacitive deionization: Capacitor and battery materials, applications and future prospects

Water scarcity all over the world attracts alternative methods to purify saline water and supplement the available dwindling freshwater resources. Capacitive deionization (CDI) is hopeful to supply water to the population due to operation at low potential along with low energy expenditure when low salinity (5 mM NaCl) feed water solutions are desalinated. Electrode material is the main controlling factor in CDI system and a lot of efforts are devoted to develop excellent materials for better CDI performance. So far, carbon materials are widely used as the electrode for CDI, though limitations such as co-ion expulsion and faradaic reactions hinder their full utilization. Alternatively, battery materials are used since their performance is great due to mitigation of co-ions ejection as well as faradaic reactions. In 2019 our group reviewed factors affecting the performance of activated carbon electrode materials and revealed lack of selectivity, co-ion expulsion, low electrical conductivity and inappropriate pore size distribution to largely contribute to its low salt removal capacity [1]. Therefore, herein, we extend the discussion beyond AC to include other carbons such as aerogels, nanotubes, graphenes etc., and battery materials such as MXenes, sodium super ionic conductors, and BiOCl to mention the few. This article also discusses the extent CDI is applied in the laboratory scale as well as in the field for desalination of real water, wastewater remediation and removal of harmful contaminants to substantiate what it can offer beyond the laboratory experiments. The work is expected to save as a complete reference for the progress of CDI electrode materials and its applications in water purification.

Multifunctional capacitive deionization removal of metal ions via crumpled phosphorus-doped Ti3C2Tx MXene nanosheets

Capacitive deionization (CDI) technology shows excellent development prospects in seawater desalination and the removal of heavy metal ions from wastewater due to its high efficiency, environmental protection, and energy saving. However, the lack of suitable electrode materials with excellent performance limits the development of CDI. Herein, a crumpled phosphorus-doped Ti3C2Tx MXene nanosheets (P-Ti3C2Tx) was synthesized by an in-situ phosphating strategy. The P heteroatom doping not only improves the electrical conductivity and modulates the surface properties of Ti3C2Tx MXene nanosheets, but also availably alleviates the self-stacking problem of Ti3C2Tx MXene nanosheets. Specifically, the P-Ti3C2Tx shows excellent multifunctional removal performance for various metal ions (Na+: 61.8 mg g−1, Pb2+: 84.3 mg g−1, and Cr3+: 99.7 mg g−1). Additionally, the removal mechanism of P-Ti3C2Tx is investigated by systematic characterizations and theory calculations. Therefore, this work provides new ideas for the development of multifunctional CDI electrode materials with excellent performance.

Electroadsorption desalination performance of carbon materials derived from coconut shell without membrane separation

This study investigated coconut shells, an abundant and low-cost raw material, as the precursor for activated carbon preparation, and applied a carbonization activation method to prepare carbon materials under different activation conditions. The differences in the performance of biomass derived carbon materials as capacitive deionization (CDI) electrodes were studied under conditions of different activators, activator doses, and activation temperatures. Compared with commercial coconut shell activated carbon, studies have shown that biomass derived carbon electrodes obtained under the optimal preparation conditions of NaOH as the activator, a carbonizer to activator ratio of 1:3, and an activation temperature of 800 ℃ have an excellent electro adsorption performance and the potential to be used as a CDI electrode material. The salt adsorption capacity (SAC) of activated carbon made with coconut shell derived carbon materials after 20 min of electro adsorption reached a maximum of 11.47 mg/g. After 10 repeated uses, its SAC remained at the original 90.58%, demonstrating an excellent cycling stability.

The Simpler the Better: Ultrafast Air-Plasma Synthesis of 3D Crosslinked Graphene Nanoscroll-Nanosheet Aerogels at Room Temperature for Capacitive Deionization.

Graphene nanoscroll (GNS) is an important 1D tubular form of graphene-derivative materials, which has garnered widely attention. However, conventional fabrication methods commonly suffer from complex processing and time-consuming. Herein, with graphene oxide (GO) as a precursor, the study puts forward a facile air-plasma synthesis strategy to fabricate 3D graphene nanoscroll-nanosheet aerogels (GSSA). It is demonstrated that without using any chemical additives, a highly efficient reduction-exfoliation-scrolling process can be achieved all-in-one at room temperature within 1 s. The GNSs "grew" from 2D graphene sheets and firmly cross-linked them together, and they not only provide a shortcut path for electron transport but also act as intrinsic spacers to prevent restacking of graphene sheets. When using as an electrode material for capacitive deionization (CDI), GSSA exhibits excellent merits of salt-removal performance. These findings open a new pathway to large-scale synthesis of high-quality and high-purity GNS-based materials with promising applications in CDI and beyond.

A novel ε-MnO2/MXene composite as an electrode with excellent conductivity and ultrahigh capacitive deionization

Manganese dioxide (MnO2) holds great promise as an electrode material for capacitive deionization (CDI); but its low electrical conductivity considerably restricts its desalination capacity. Nevertheless, its electrochemical performance can be enhanced by augmenting its conductivity. Hence, we propose a novel strategy for constructing a strongly coupled ε-MnO2/MXene composite, in which, MXene (Ti3C2Tx) serves as a network of electron conduction highways that enhance the charge transfer of ε-MnO2 and activates the underutilized electrochemically active sites of ε-MnO2. Based on these advantages, an ε-MnO2/MXene composite is prepared for the first time, comprising spherical ε-MnO2 nanoparticles grown on MXene layers. As demonstrated, ε-MnO2/MXene composite demonstrates exceptional desalination performance, exhibiting a maximum deionization rate of 17.54 mg g−1 min−1 and an ultrahigh desalination capacity of 114.66 mg g−1 in 1000 mg/L NaCl solution at 1.2 V, which is considerably higher than those observed for ε-MnO2 (74.93 mg g−1), MXene (29.26 mg g−1) and recently reported electrode materials. Such an outstanding CDI performance can be ascribed to a considerably high specific capacitance ε-MnO2/MXene composite of approximately 337.2F g−1 at a specific current of 1 A/g, which is approximately 2.16 times that of an ε-MnO2 electrode.The excellent performance of ε-MnO2/MXene composite makes it a potential practical application of CDI.

Optimization of Desalination Efficiency and Exploratory Applications of TiO2 Active Site Electrode Enhanced by Activated Carbon and Tween 80 in Capacitive Deionization Technology.

Capacitive deionization (CDI) is an emerging desalination technology for seawater desalination. The development of high-desalination and long-life electrode materials is a research focus in the global water treatment field. In this experiment, Tween T80 was used as a surface activator, and a modified electrode was prepared by facilitating the deposition of TiO2 active sites onto the surface of activated carbon through a sol-gel/hydrothermal two-step synthesis strategy. The morphology and specific surface area of the composite material were analyzed through scanning electron microscopy, specific surface area measurements, and contact angle tests. The results indicated that the sol-gel/hydrothermal two-step synthesis strategy played a crucial role in the homogeneous combination and performance enhancement of the composite material. Under constant voltage mode, when the working voltage was 1.2 V, the desalination capacity of this composite material in a NaCl solution with an initial conductivity of 3000 μS·cm-1 reached 23.8 mg·g-1 (26% higher than materials prepared by conventional sol-gel methods). After 150 cycles, the capacity retention rate was 78%, and the retention capacity was significant (87%). Overall, the results demonstrate the potential of the sol-gel/hydrothermal two-step synthesis strategy in preparing high-performance CDI electrode materials. The modified electrode prepared using this method offers enhanced desalination capacity and durability, making it a promising candidate for seawater desalination and other water treatment applications.

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.

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.