Capacitive Deionization Device Research Articles

Enhanced construction of 3D carbon skeleton through molten salt coupling activation effect for high efficiency capacitive deionization of lead (II) ions removal in wastewater

Efficiently eliminating of highly toxic ultra-dilute lead ions (Pb2+) from wastewater is a challenging task. Capacitive deionization (CDI) technology, based on the electrochemical capacitance mechanism, has emerged as a pivotal approach to address this challenge. In this work, a KNO3-mediated molten salt-coupled activation strategy was proposed to prepare nitrogen-doped hierarchical porous biomass carbon electrode material (3DCF-650-0.2) with a three-dimensional carbon skeleton, which was integrated into a symmetrical CDI device for Pb2+ removal. The 3DCF-650-0.2 electrode exhibited a high specific capacitance of 330.3 F g-1 at a current density of 1 A g-1, with a capacity retention rate of 98.45% after 10,000 cycles at 10 A g-1. Furthermore, the electrochemical adsorption capacity of the CDI device for Pb2+ reached 72.86 mg g-1 (1.2 V, 50 mg L-1), and the CDI device retained a high adsorption capacity of 91.14% after twenty adsorption/desorption cycles. These excellent adsorption properties are attributed to the in-situ activation and gas foaming of KNO3, which favors the production of reasonable pore structure (electric double-layer capacitance) and an appropriate pyrrolic nitrogen doping amount (selective adsorption) in the carbon materials. This work provides insight into the mechanism of the molten salt-coupled activation strategy, offering new ideas for the targeted design of high-performance CDI biomass carbon materials.

Bringing multiscale dot-sheet heterostructured assembly into lignin valorization for efficient water desalination and zinc-ion hybrid capacitors

The notion of biomass valorization has been widely adopted to alleviate low-carbon-emission anxieties with the attributes in energy-environmental sustainability via groping multiscale-structured composite, yet still suffering from the dilemma of lignin upcycling. In this context, we propose a novel dot-sheet-assembled heterostructure featuring all lignin-derived bifunctional carbon composites (Co-N-C@CDs) that includes porous Co, N co-modified nanosheets and solvent-extracted carbon quantum dots. The photo-physical properties of lignin-based CDs are refined into multicolor green- and red-emissive fractions (GCDs and RCDs) via a facile biphasic fractionation. Owing to the high-graphite and conductivity, the red-emitting RCDs are installed with lamellar Co-N-C to favor the enlarged interlayer spacing, fast ionic intercalation-transfer and reversible chemical adsorption, thereby resulting in high-performance capacitive deionization (CDI) and zinc-ion hybrid capacitors (ZIHCs). While assembling the CDI device, the Co-N-C@CDs possess a maximum adsorption capacity of 33.64 mg g−1 NaCl in water desalination with stable restorable behavior. As the cathode of ZIHCs, it delivers an operating voltage of 2.0 V and superior energy density of 209.72 Wh kg−1, posing a long-term cycling durability at high current rates. Detailed hierarchical structure readily enables the large specific surface area, excellent percolating porosity, intercalated hydrogen bonds and redox active sites, which are conducive to not only expedite ionic infiltration and charge reaction kinetics but also reduce the energy barriers of aqueous ion adsorption. The multiscale 0D-2D supramolecular assembly breaks the frontier of lignin valorization to connecting energy-environment functions, thereby meeting the needs of carbon–neutral society.

Selective electro-capacitive deionization of Yb(III) by nanospherical N-doped carbon supported nickel‑iron bimetallic oxide, nitride and phosphide

The separation of rare earth elements from the mining waste water is highly important but still facing challenge. In this study, three different porous composite electrode materials were well-designed by decorating the nickel‑iron bimetallic oxide, nitride or phosphide onto nitrogen-doped carbon nanosphere, respectively (termed as NiFeO@NC, NiFeN@NC and NiFeP@NC), and utilized in capacitive deionization device for electrosorption of Yb(III) from water under the low-voltage electric field. The characterization and electrochemical results indicated that the incorporation of N or P atom lead to the reduced specific surface area and the enhanced electrochemical properties (i.e., larger specific capacitance, lower charge transfer resistance). In addition, NiFeN@NC and NiFeP@NC exhibited the excellent electrosorption capacities of Yb(III) with 105.24 mg·g−1 and 90.31 mg·g−1, respectively, under conditions of a flow rate of 20 mL·min−1, a voltage of 1.2 V, and pH of 5.0, which was extremely higher than NiFeO@NC (i.e., 50.7 mg·g−1). Moreover, all these three materials also demonstrated the remarkable electrosorption selectivity of Yb(III) from the mixed solution, and the robust durability with over 90 % of initial capacities after ten consecutive electrosorption-desorption cycles. Furthermore, the XPS characterizations and electrosorption results revealed that the electrosorption mechanism mainly depended on the synergistic effects of intrinsic Faraday redox reaction, electric double layer capacitance and active binding sites. Therefore, this work provided a feasible strategy for the separation of rare earth elements from aqueous solution, and the prospective applications related to the recovery of rare earth elements from mining wastes or spent permanent magnets in future life.

Enhanced Capacitive Deionization of Hollow Mesoporous Carbon Spheres/MOFs Derived Nanocomposites by Interface-Coating and Space-Encapsulating Design.

Exploring new carbon-based electrode materials is quite necessary for enhancing capacitive deionization (CDI). Here, hollow mesoporous carbon spheres (HMCSs)/metal-organic frameworks (MOFs) derived carbon materials (NC(M)/HMCSs and NC(M)@HMCSs) are successfully prepared by interface-coating and space-encapsulating design, respectively. The obtained NC(M)/HMCSs and NC(M)@HMCSs possess a hierarchical hollow nanoarchitecture with abundant nitrogen doping, high specific surface area, and abundant meso-/microporous pores. These merits are conducive to rapid ion diffusion and charge transfer during the adsorption process. Compared to NC(M)/HMCSs, NC(M)@HMCSs exhibit superior electrochemical performance due to their better utilization of the internal space of hollow carbon, forming an interconnected 3D framework. In addition, the introduction of Ni ions is more conducive to the synergistic effect between ZIF(M)-derived carbon and N-doped carbon shell compared with other ions (Mn, Co, Cu ions). The resultant Ni-1-800-based CDI device exhibits excellent salt adsorption capacity (SAC, 37.82mg g-1) and good recyclability. This will provide a new direction for the MOF nanoparticle-driven assembly strategy and the application of hierarchical hollow carbon nanoarchitecture to CDI.

Effective electro-removal of glyphosate and glufosinate from aqueous solutions via capacitive deionization

Both glyphosate (Gly) and glufosinate (Glu) are commonly used as herbicides, which has caused rising concerns about their impact on the environment and human health. Capacitive deionization (CDI) is a newly developed energy-efficient water treatment technique that effectively removes inorganic and small organic molecules under the electric field. In this work, a carbon electrode-based CDI device was used in the electro-removal of Gly and Glu in an aqueous solution with an excellent removal rate of up to 95 % under various conditions. Furthermore, maximum electro-removal rates of 1.99 μmol∙cm−2∙min−1 for Gly and 1.33 μmol∙cm−2∙min−1 for Glu were also achieved, except with super-high removal rates of 100 % (Gly) and 97.6 % (Glu) in actual Gly and Glu-containing lake water, demonstrating the feasibility of the CDI device in real aqueous systems. In-situ electro-oxidation that occurred during the electrosorption process was confirmed by analyzing the three-electrode system-based electrochemical behaviors and tracing the time-dependent changes of the suspicious degradation products. Transition state (TS) search and molecular dynamics (MD) simulations were employed to study the microscopic processes theoretically, revealing the degradation pathways and thermodynamic characteristics of Gly and Glu. This research presents a novel avenue for the decomposition and removal of herbicides from water, leveraging the inherent side reactions of CDI systems as an advantage to decontaminate organic pollutants.

Nitrogen-doped porous carbon with regulated pore structure and surface wettability as flow electrodes for efficient capacitive deionization

Nitrogen-doped porous carbon with controlled pore structure and wettability was synthesized using spent mushroom substrate (SMS) via physical activation process. The nitrogen-rich feature of precursor enables the carbon material to possess both electrical conductivity and surface wettability, creating an effective conducting network in flow electrodes. These materials have been proven to be highly efficient for deionization due to their ability of maintaining surface wettability and electrical conductivity. Our results show that the SMS-1.5 sample has a high adsorption capacity of 8.38 mg/g, which is 1.8 times greater than the commercial YP-50. The corresponding charge efficiency was up to 90.66 % after being exposed to 1.6 g/L NaCl feed solution for 120 min in flow electrode capacitive deionization device. This study highlights the significance of the inherent nitrogen element in biomass for the synergistic promotion of electrical conductivity and surface wettability, providing a strategy for designing porous carbons with excellent desalination performance.

Bendable, lamellar MXene membrane electrodes with diverse diffusion pathways for potential application in miniaturized capacitive deionization devices

Flexible devices have shown great application potential in portable and miniaturized devices. However, work on flexible electrodes for capacitive deionization (CDI) has not been well developed due to the issues of desalination property, processability, and availability under actual bending conditions. Herein, we constructed a kind of flexible lamellar MXene membrane by assembling Ti3C2Tx sheets with both crumpled and perforated structures and studied their desalination performance in conventional planar and bent configurations. The crumpled and perforated features in Ti3C2Tx (C-P-TC) nanosheets could optimize the in-plane and out-of-plane diffusion channels of the stacked samples, respectively, effectively improving the accessibility of membranes. As a result, the desalination performance of such C-P-TC electrodes was superior to that of those assembled from crumpled or perforated sheets alone. Significantly, this flexible C-P-TC electrode still showed excellent performance at real bend deformation, which possessed not only almost the same CDI capacity as the traditional planar configuration but also good cycling stability. This work not merely proposes an alternative strategy to improve the CDI performance of lamellar MXene electrodes by regulating the nanostructures of channels but also confirms their effective deionization ability under real bent state, greatly facilitating the potential use of flexible electrodes in miniaturized CDI devices.

Sodium lignosulfonate/graphene composites for efficient desalination by incorporating CoS to control pore size

The phenomenon of overlapping double layers due to micropores inhibits capacitive deionization performance, which is improved by increasing the pore size. In this study, a novel ternary composite electrode (sodium lignosulfonate/reduced graphene oxide/cobalt sulfide, LGC) was designed using a two-step hydrothermal method. CoS with high pseudocapacitance modifies sodium lignosulfonate and graphene connected by hydrogen bonding, benefiting from the constitutive steric structure. The electrochemical performance was significantly enhanced, and the desalination capacity substantially improved. The LGC electrode specific capacitance was as high as 354.47 F g−1 at a 1 A g−1 current density. The desalination capacity of the capacitive deionization device comprising LGC and activated carbon in 1 M NaCl electrolyte reached 28.04 mg g−1 at an operating condition of 1.2 V, 7 mL min−1. Additionally, the LGC electrodes degraded naturally post the experiment by simply removing the CoS, suggesting that the LGC composites are promising material for capacitive deionization electrodes.

La-doped CeO2/rGO hybrid with high oxygen vacancy to enhance phosphorus adsorption by capacitive deionization

Cerium oxide is one of the most attractive electrode materials in capacitive deionization (CDI) technology for selective phosphate adsorption due to the specific interaction. However, Ce3+/Ce4+ ratio and material conductivity of cerium oxide electrode determine the adsorption performance. Herein, a dual strategy was exploited with polyol-solvothermal reaction in the presence of graphite oxide and cerium salt followed by hydrothermal La-doping to obtain the three-dimensional (3D) assembly of La-doped CeO2 at the reduced graphene oxide (La-CeO2/rGO). The as-prepared La-CeO2/rGO hybrid exhibited interconnected structure with CeO2 uniformly distributed on the rGO support. La3+, by partially replacing Ce4+ in the lattice, was evenly fused into CeO2 nanosphere, enhancing the content of Ce3+ and oxygen vacancies and inhibiting lattice distortion. As a CDI positive electrode, the La0.15-CeO2/rGO with optimal La doping achieved the maximum theoretical adsorption capacity of 118.7 mg P/g with high separation factor (αSO42-P = 13.9), superior to the CeO2/rGO (67.8 mg P/g, αSO42-P = 10.1). In addition, the La-CeO2/rGO electrode is extremely robust, with excellent adsorption performance in wide pH range and good regeneration ability, compared to CeO2/rGO. This work proves that the synergy of trace La doping and 3D rGO support effectively improves the adsorption performance of La-CeO2/rGO electrodes and enhances the cycling stability of CDI devices. The corresponding adsorption mechanism was demonstrated.

Pine-derived porous carbon for efficient capacitive deionization and the role of its hierarchical pore structure

Capacitive deionization (CDI) is a low-energy, eco-friendly, and economically viable technology for seawater desalination. In the desalination process of CDI, the pore structure characteristics of electrode materials play a crucial role. In this study, the carbon materials with a hierarchical pore structure were prepared through high-temperature carbonization and KOH activation using pine powder as raw material. When the pine-derived porous carbon (PPC) was used as an electrode material, it was found that its specific capacitance is remarkably consistent with the specific surface area of the mesopores. This discovery provides guidance for further optimization and regulation of biomass-derived carbon electrode materials. The desalination performance of the PPC electrode was studied in detail by CDI and flow-electrode capacitive deionization (FCDI) devices. In NaCl solution with an initial concentration of 700 ppm, the equilibrium adsorption capacities in CDI and FCDI systems are 24.96 mg g−1 and 23.12 mg g−1, respectively. Particularly in FCDI tests, the average adsorption rate reaches 23.3 μg cm−2 min−1, and the desalination rate reaches 98.96 %. The PPC shows an excellent desalination performance, making it a promising efficient electrode material for electrochemical desalination.

MoS2/TiO2/graphite felt electrode preparation for capacitive deionization device performance improvement

In order to improve the desalination effectiveness of a capacitive deionization system, graphite felt (GF) electrodes were modified by TiO2 and MoS2 thin films deposition to enhance the wettability and electrochemical characteristics. The influence of varied synthesis concentration ratios of precursor on the proportion of the 1T and 2H phases in MoS2 was investigated by electrochemistry and desalination tests. The results demonstrated that depositing MoS2 on GF effectively alleviated the agglomeration problem. An enhanced effect was found when the 1T phase to 2H phase ratio was 1:1. This ratio facilitated in the rapid transport of electrolytic ions and electrons across the electrode surface, resulting in high electrochemical performance. The MoS2/TiO2/GF electrodes exhibited a specific capacitance value of 257.9 F/g, a desalination efficiency of 34.3 % and a desalination capacity of 40.96 mg/g. After 50 repeated desalination tests, a desalination retention rate of 92.7 % was obtained. The research results verified that the MoS2/TiO2/GF electrodes have high desalination efficiency and long-term stability, meriting its great potential in the future development of water purification.

Sustained Phosphorus Removal and Enrichment through Off-Flow Desorption in a Reservoir of Membrane Capacitive Deionization.

In this study, we used a membrane capacitive deionization device with a reservoir (R-MCDI) to enrich phosphorus (P) from synthetic wastewater. This R-MCDI had two small-volume electrode chambers, and most of the electrolyte was contained in the reservoir, which was circulated along the electrode chambers. Compared with conventional MCDI, R-MCDI exhibited a phosphate removal rate of 0.052 μmol/(cm2·min), approximately double that of MCDI. This was attributed to R-MCDI's utilization of OH- alternative adsorption to remove phosphate from the influent. Noticing that around 73.9% of the removed phosphate was stored in the electrolyte in R-MCDI, we proposed a novel off-flow desorption operation to enrich the removed phosphate in the reservoir. Exciting results from the multicycle experiment (∼8 h) of R-MCDI showed that the PO43--P concentration in the reservoir increased all the way from the initial 152 mg/L to the final 361 mg/L, with the increase in the P charge efficiency from 5.5 to 22.9% and the decrease in the energy consumption from 28.2 to 6.8 kW h/kg P. The P recovery performance of R-MCDI was evaluated by viewing other similar studies, which revealed that R-MCDI in this study achieved superior P enrichment with low energy consumption and that the off-flow desorption proposed here considerably simplified the operation and enabled continuous P enrichment.

3D interconnected porous graphene architecture as a high-performance capacitive deionization electrode

Capacitive deionization (CDI) is a promising technology for brackish water desalination because of its high efficiency, no secondary pollution and low cost. However, the ion removal capacity of CDI system for brackish water desalination is still insufficient. Herein, a new in-situ porous Cu template method is used to prepare a unique open three-dimensional mesoporous graphene foam (3DMG) with high CDI performance. The developed 3DMG material has rich mesoporous structure, interconnected pore structure, high specific surface area, good hydrophilicity and excellent electrical conductivity, which can achieve effective solution/pore contact, rapid ion migration and high electrosorption capacity. Therefore, the specific capacitance of 3DMG as high as 141.37F/g at 1 M NaCl and 10 mV/s, and the maximum salt adsorption capacity is as high as 32.08 mg/g (1000 mg/L, 1.2 V), which is significantly higher than that of graphene-based electrode materials reported in most literatures. It also shows excellent regeneration performance and stability after repeated electrosorption-desorption cycles. The developed 3DMG undoubtedly provides a promising electrode material platform for CDI devices, effectively solving the urgent problem of freshwater resource shortage.

Maximizing active site utilization in carbocatalysts for high-performance oxygen reduction reactions and zinc–air battery-powered capacitive deionization

A structure-property relationship is disclosed between electrochemical surface area and active site utilization in ORR. The optimized NPC–Zn exhibits excellent zinc–air battery performance, serving as a power to drive capacitive deionization devices.

Molten salt etching synthesis of Ti3C2Tx/Ni composites for highly efficient capacitive deionization

Capacitive deionization (CDI) has garnered significant attention as a promising solution to address the issue of freshwater scarcity. Developing novel electrodes with high salt removal capacity and good stability is crucial for the high-performance CDI devices. In this study, Ti3AlC2 is employed as a precursor to prepare Ti3C2Tx/Ni composite for the CDI anode and cathode using the molten salt etching method. The Ti3C2Tx/Ni composite exhibits a well-preserved accordion-like morphology of Ti3C2Tx, with Ni nanoparticles uniformly distributed on the surface and between layers. The Ti3C2Tx/Ni composite possesses a large interlayer spacing and high specific surface area, which can effectively trap Cl− and Na+ ions. Moreover, the Ti3C2Tx/Ni interface can improve electronic conductivity and enhance ion transport. As a result, the Ti3C2Tx/NiǁTi3C2Tx/Ni CDI symmetry device exhibits a salt removal capacity of 49.4 mg g−1 in a 500 μS cm−1 NaCl solution at an applied voltage of 1.2 V with a removal rate of 3.00 mg g−1 min−1. After 30 cycles, the Ti3C2Tx surface remains unoxidized, and the salt removal capacity increases, indicating that the surface functional groups (−Cl and −O) and Ni nanoparticles can alleviate oxidation and facilitate the cycle stability of the Ti3C2Tx/Ni composites.

UiO-66-NH2 modified thin film nanocomposite (TFN) membranes for selective separation of Li+/Co2+ in a flow electrode capacitive deionization system

As key metal elements in lithium-ion batteries (LIBs), the recovery of lithium (Li) and cobalt (Co) showed both economic and environmental benefits. In this study, a thin-film nanocomposite (TFN) membrane with high selectivity was fabricated based on nanoscale UiO-66-NH2 particles, and the modified membranes were applied in the flow-electrode capacitive deionization device (FCDI) for the separation of monovalent and divalent cations. UiO-66-NH2 provided hydrophilic water channels and increased the permeability of Li+. TFN exhibited a rejection rate of Co2+ over 95%, and the separation factor of Li+/Co2+ was 27.44 during the operation of 150 min in FCDI with the addition of 10 mg UiO-66-NH2, which was about 1.52 times greater than that of the commercial cation-selective membrane (CSO). The size repulsion effect caused by size difference between hydrated ions and membrane pores and Donnan electrostatic repulsion force generated by positively charged membranes were evaluated to be the main basis for the ion separation of prepared membranes. The novel system using TFN membrane in FCDI had a better ability to resist Co2+ fouling and a good Li+/Co2+ separation effect. We believe these modified membranes have promising potential for the selective separation of heavy metal ions and the recycling of metal resources.

A novel flow electrode capacitive deionization device with spindle-shaped desalting chamber

A novel flow electrode capacitive deionization device with spindle-shaped desalting chamber

Oxygen-vacancy abundant δ-Bi2O3@PCNF anode for selective phosphate removal with exceptional capacity

Excessive phosphate concentration in waterbody will lead to eutrophication and deterioration of water quality. Capacitive deionization (CDI) is an environmental-friendly technique for ion removal, and the phosphate ions can be selectively removed and accumulated by designing the CDI electrode materials with specific phosphate bonding capability. Herein, a δ-Bi2O3-modified porous carbon nanofiber (δ-Bi2O3@PCNF) was constructed by solvothermal reaction in the presence of polyhydric alcohols. Due to abundant oxygen vacancies and high flexibility, the δ-Bi2O3@PCNF was used directly as a self-supporting CDI anode for selective phosphate adsorption. Under the optimized conditions, the adsorption capacity of phosphate arrived at 99.6 mg P/g in 50 mg P/L solution, and the adsorption maximum of 127.5 mg P/g was achieved according to Langmuir adsorption model. In the dual-anion solution containing PO43– and Cl–, the δ-Bi2O3@PCNF electrode still maintains the high selectivity, with a P separation factor of 22 (CPO43–/CCl– = 1:5, molar ratio) in PO43–/Cl– mixture. By using the δ-Bi2O3@PCNF-based CDI device, the actual paddy drainage has realized the ultralow phosphate concentration down to 0.03 mg P/L. The related mechanisms involved in phosphate adsorption on δ-Bi2O3@PCNF were proposed. This work demonstrates the great potential of CDI technology for low-concentration phosphorus recovery, owing to the high selectivity of the δ-Bi2O3@PCNF electrode material.

Efficient Hybrid Capacitive Deionization Device with High Operating Voltage Based on Layered Sodium Manganese Oxide

Efficient Hybrid Capacitive Deionization Device with High Operating Voltage Based on Layered Sodium Manganese Oxide

Pore engineering in robust carbon nanofibers for highly efficient capacitive deionization

Capacitive deionization (CDI) is considered as a revolutionary desalination method to alleviate the global water crisis. However, the current carbon-based CDI cells are constrained by the inferior desalination efficiency, resulting from the undesirable pore structure and sluggish electron/ion transport. Herein, a new porogen of silicalite-1 is proposed for the tailorable fabrication of flexible porous carbon nanofibers (PCNFs) by electrospinning, carbonization and alkali etching, where the surface area, total pore volume, mechanical robustness, specific capacitance, and electrosorption performance of the developed PCNFs are associated with the silicalite-1 dosage. The symmetric CDI unit assembled by the freestanding PCNF membrane electrodes shows a state-of-the-art desalination capacity (38.64 mg g−1), a remarkable salt removal rate (12.1 mg g−1 min−1), a desirable charge efficiency (82.36 %), an acceptable energy consumption (0.668 Wh g−1), and a satisfactory cyclic durability, and such excellent deionization performance can be explained by that the abundant hierarchical pores, the large accessible surface area, and the interconnected conductive carbon network effectively expedite the electron/ion transport. Additionally, the home-made CDI device has a favorable desalination capability toward the high-ion concentration seawater from the Yellow Sea (Xiaomai Island). This work not only validates the feasibility of silicalite-1 as the pore-forming agent for the targeted preparation of flexible PCNFs, but also it highlights the potential of pore engineering in freestanding PCNFs for advanced CDI.