Hybrid Capacitive Deionization System Research Articles

Scalable NH4V4O10 clusters as functional network nodes on toughened porous carbon nanofibers for hybrid capacitive deionization

Layered vanadium-based compound (e.g., NH4V4O10) is acknowledged as an outstanding candidate for intercalation pseudocapacitance electrode of capacitive deionization (CDI) cells. However, the directed production of high-efficiency NH4V4O10-based self-supporting CDI electrodes and their structure–activity connection warrant more in-depth study. Herein, the delicate design and controllable construction of NH4V4O10 clusters on the freestanding porous carbon nanofiber (PCNF) scaffolds (the composite PCNFs/NH4V4O10 is defined as CNVO) using a new organic reducing agent of citric acid is proposed, where NH4V4O10 clusters as functional network nodes can improve the mechanical toughness, electrochemical capacitive performance, and desalination capability of the CNVO hybrid. The Na+ adsorption mechanism on CNVO is confirmed by ex-situ experimental characterizations and density functional theory (DFT) calculations. The hybrid CDI (HCDI) cell composed of the optimized CNVO cathode and PCNFs anode shows a desirable desalination capacity (45.07 mg g−1), a superior salt removal rate (16.4 mg g−1 min−1), an excellent energy utilization, and an acceptable cyclic regeneration ability in 500 mg L-1 NaCl solution. More importantly, the assembled HCDI system is promising for the desalination of reclaimed water from the printing and dyeing enterprise. This work highlights a scalable strategy that is experimentally and theoretically feasible to design and fabricate high-efficiency CDI electrodes.

Fast and stable lithium extraction enabled by less-defective graphene supported LiMn2O4 conductive networks in hybrid capacitive deionization

Hybrid capacitive deionization (HCDI) technology for lithium (Li+) extraction from brine has received growing concern owing to its easy operation, low energy consumption, and environmental friendliness. LiMn2O4 (LMO) as an affordable Li+ extraction material offers a high theoretical capacity, while the slow ion insertion kinetics and unavoidable Mn dissolution restrict its wide usage. Herein, an LMO-based electrode supported by less-defective (inherent lattice defects and oxygen-containing defects), interconnected graphene conductive networks is proposed for fast and stable Li+ extraction from brine using HCDI. The rGO/LMO electrode displays an excellent conductivity of 0.42 S·cm−1 and a high specific capacitance of 418.26 F·g−1. In the HCDI system, the rGO/LMO electrode delivers a Li+ adsorption capacity up to 4.34 mmol·g−1 with a rapid adsorption rate of 0.33 mmol·g−1·min−1 (0.05 mol·L–1 LiCl, 1.0 V), demonstrating both of the outstanding adsorption capacity and rate abilities as defined by Ragone plots. In a solution with a high Mg2+/Li+ molar ratio of 20, the separation factor can reach 71.32. By employing rGO/LMO electrode to simulated Atacama brine, the separation factor of Li+ from Na+, K+, Ca2+, and Mg2+ are calculated to be 473.89, 74.86, 63.07, and 38.55. Particularly, the rGO/LMO electrode shows remarkable cycling stability with a capacity retention rate of 90.73 % (50 cycles). This developed less-defective graphene-wrapped LMO electrode holds significant importance in enhancing selective Li+ extraction performance with fast rate and stability.

Structural stabilization of Cr-doped spinel LiMn2O4 for long-term cyclability towards electrochemical lithium recovery in original brine

Current explosive demand in raw lithium greatly boosts lithium mining, and electrochemical Li-extraction using spinel LiMn2O4 (LMO) from original brine has gained much attentions. In this study, Cr-doped LMO is prepared via a facile precipitation-calcination to elucidate the structural alterations and suppress Jahn-Teller distortion, and a novel hybrid capacitive deionization (HCDI) system of “Li-poor LiMn1.8Cr0.2O4||brine (recovery liquid)||activated carbon” is constructed for electrochemical Li-extraction. The results prove that Cr-doping effectively suppresses Mn3+ disproportionation induced by Jahn–Teller distortion, leading to a stabilized spinel framework for long lifespan. Importantly, the Cr-doping LMO realizes a stable (de)lithiation over 500 cycles with low manganese dissolution rate of 0.027 % per cycle. Meanwhile, the Li-extraction capacity can reach 22 mg g−1 in 1.0 M LiCl electrolyte with retention rate of 86.05 % after 500 cycles. In the assembled HCDI system, average Li-extraction capacity in brine reaches ∼15 mg g−1 after 15 cycles. Therefore, the HCDI system using Cr-doping LMO could introduce a new perspective for the industrial Li-extraction facing original brine.

Enhanced hybrid capacitive deionization performance by the mass balance of electrodes

The ion removal capacity of carbon based capacitive deionization (CDI) is limited by the double-layer mechanism. Replacing one electrode of the conventional carbon-based symmetric CDI with a battery-like ion insertion material to obtain hybrid capacitive deionization (HCDI) can significantly improve the adsorption capacity. Various ion insertion materials have been adopted for HCDI, however, how to optimize the performance of the HCDI system by matching the two electrodes with different ions removal mechanism remains challenging. In this work, a HCDI desalination cell was demonstrated by using NaTi2 (PO4)3/reduced graphene oxide composites (NTP-rGO-700) as the cathode and activated carbon (AC) as the anode. Based on the mass balance strategy of electrodes and the electrochemical behavior of hybrid capacitors with different AC:NTP-rGO-700 mass ratios, the HCDI desalination cell was assembled with the optimal AC:NTP-rGO-700 mass ratio of 2:1. At an operating voltage of 1.6 V and a current density of 125 mA·g−1, the specific adsorption capacity (SAC) of the HCDI cell can reach 51 mg·g−1 with an average salt adsorption rate (ASAR) of 2 mg·g−1·min−1, and the energy consumption (Em) is 136.53 kJ·mol−1. This work provides an efficient approach for developing high performance HCDI cell.

Three-dimensional (3D) MnMoO4@g-C3N4/CNT hybrid composite electrode for hybrid capacitive deionization

In this work, a hierarchical three-dimensional MnMoO4@g-C3N4/CNT hybrid ternary composite was developed through the facile hydrothermal method. MnMoO4@g-C3N4/CNT and g-C3N4/CNT electrodes were used as cathode and anode to fabricate the hybrid capacitive deionization (HCDI) system. Manganese molybdate (MnMoO4) rods are uniformly intertwined on the three-dimensional (3D) conductive network structure of graphitic carbon nitride/carbon nanotube (g-C3N4/CNT) heterojunction in the ternary composite. As-obtained electrode materials of MnMoO4@g-C3N4/CNT and g-C3N4/CNT show the specific capacitance of 252.4 and 89.2 Fg−1, respectively, at 1 Ag−1 in 1 M NaCl solution. Mn element in the MnMoO4@g-C3N4/CNT hybrid induces a reversible redox reaction that selectively intercalates/de-intercalates the Na+-ion in the salt solution, yielding a high salt adsorption capacity of 43.6 mg. g−1 at 1.2 V and 1000 ppm NaCl solution medium. In addition, MnMoO4@g-C3N4/CNT //g-C3N4/CNT-based HCDI cell was recycled for 10 cycles. The system reaches the highest SAC of 42.6 mg. g−1 after 10 consecutive charge–discharge cycles with a retention of 91%, indicating the excellent cycling stability of the MnMoO4@g-C3N4/CNT electrode. These results signify that the MnMoO4@g-C3N4/CNT is a promising electrode material, possessing a pseudocapacitive behavior and remarkable desalination ability, which can be a potential new candidate for the HCDI system.

Lithium-selective hybrid capacitive deionization system with a Ag-coated carbon electrode and stop-flow operation

Li-selective hybrid capacitive deionization technology comprising lithium manganese oxide and silver (Ag)-coated activated carbon electrodes.

Hybrid Capacitive Deionization with Ag-coated Activated Carbon Electrodes for Nickel Treatment

Nickel (Ni) is an essential resource in many industries. However, as the industrial use of Ni increases, its impact as an environmental pollutant has also increased. Accordingly, effective Ni treatment processes are required. In this study, hybrid capacitive deionization (HCDI) with an Ag-coated activated carbon (AC) electrode was proposed as a Ni treatment. The deionization capacity of the HCDI system with Ag coating was 66% higher than that of the membrane capacitive deionization system. In addition, as the applied potential increased from 0.5 V to 1.1 V, the deionization capacity also increased from 10 mg/g to 18 mg/g, although the charge efficiency decreased from 84.6% to 77.2%. However, when the applied potential was 0.9 V or more, the pH of the effluent exceeded the drinking water range, which appeared to be caused by the electron transfer reaction of Ni ions under a high applied potential (0.9 V or more). In terms of energy consumption, HCDI with Ag-coated electrodes achieved 0.24 Wh/g-NiCl2 as a minimum value. The results of this study demonstrate the potential of capacitive deionization (CDI) for Ni treatment.

Efficient dual-ions hybrid capacitive system for superior deionization with enhanced kinetics match

Capacitive deionization (CDI) is regarded as a promising water purification technology to tackle the global water crisis challenges. However, CDI technology is limited by the low capacity of harvesting more ions from highly concentrated feed water. This can be solved by coupling appropriate cathode with anode materials to enhance kinetics and capacities. In this study, a dual-ion hybrid capacitive deionization (HCDI) system was designed and constructed to enhance the charge efficiency and desalination capacity. The system consists of a BiOCl redox electrode and a capacitive electrode made of graphene-based MnO2 nanorods composite (MnGA). The BiOCl redox electrode was used to capture chloride ions and the MnGA capacitive electrode was employed to form an electric double layer for trapping sodium ions during discharging processes. It was found that a better capacitive electrode could realize the full potential of the counter redox electrode in the HCDI system. Consequently, the assembled dual-ion MnGA||BiOCl HCDI system showed enhanced kinetics characteristics in high-concentration NaCl solution with desalination capacity reaching 69.18 mg/g and charge efficiency of up to 75.21%. The satisfying kinetic match between anode and cathode has achieved optimized charge balance, which maximized the electrochemical performance of the HCDI system. This HCDI system was able to improve utilization efficiency, enhance desalination capacity, and achieve good stability at a low cost. Hence, it is promising for future practical deionization applications.

Surfactant-assisted self-assembly of flower-like ultrathin vanadium disulfide nanosheets for enhanced hybrid capacitive deionization

Increasing the salt adsorption capacity (SAC) and durability of electrode materials for hybrid capacitive deionization (HCDI) remain grand challenges. Herein, highly electro-adsorptive and durable vanadium disulfide (VS2) electrode material obtained by a surfactant-assisted hydrothermal method is reported. The distinct three-dimensional flower-like architecture and ultrathin thickness of VS2 nanosheets play a vital role in boosting HCDI performance by exposing a large number of accessible adsorption sites and facilitating the mass transfer of sodium ions. When used in the HCDI system, the flower-like VS2 electrode delivers a high salt adsorption capacity of 72 mg g−1 in 500 mg L−1 NaCl solution at 1.6 V, outperforming the bulk VS2 counterpart with a relatively increased thickness of nanosheets. Moreover, after 10 h of cycling test, the SAC of the flower-like VS2-based HCDI system remains at 93 % of the initial value, showing excellent operation stability. This surfactant-assisted morphology engineering of VS2 nanosheets with ultrathin thickness and unique three-dimensional architecture provides new insight into designing layered electrode materials for efficient HCDI.

Flexible self-supporting CoFe-LDH/MXene film as a chloride ions storage electrode in capacitive deionization

Excessive chloride ions (Cl-) are undesirable in water. However, most of the research on the application of capacitive deionization in desalination has focused on cation storage electrode materials, whereas less on the equally important Cl- storage electrode materials. Herein, the CoFe-Layered Double Hydroxide (CoFe-LDH) prepared by chemical co-precipitation was composited with MXene through simple vacuum filtration to form a self-supporting CoFe-LDH/Ti3C2Tx film electrode, and used as the anode material to store Cl- in the hybrid capacitive deionization system. The CoFe-LDH/Ti3C2Tx composite film has a hierarchical micro-mesoporous architecture, larger electrochemical capacity, and desalination capacity, reaching 149.25 ± 6.17 mg g−1 at a current density of 50 mA g−1 and desalination rate of 3.86 mg g−1 min−1 at 100 mA g−1. This work provides potential inspiration for the Cl- storage electrode for capacitive deionization technology.

Ultra-durable and highly-efficient hybrid capacitive deionization by MXene confined MoS2 heterostructure

Capacitive deionization (CDI) is an emerging desalination technology, offering an advisable route to fetch clean water due to its low energy consumption and environmental friendliness. Hybrid CDI (HCDI), proceeding through a faradic ion storage mechanism, is thought as the next generation of CDI due to its high desalination capacity and charge efficiency. However, current HCDI system is plagued by its low desalination rate and poor long-term stability due to the structural instability of the faradic materials. Herein, we developed an effective strategy via combining 2D MoS2 nanoflakes with 2D MXene to construct a 3D “mutually supported” network architecture, in which MoS2 nanoflakes could act as the “pillar” of the intrinsic structure of MXene, while MXene could offer structural reinforcement to prevent the structural agrgregation and enhance the electrical conductivity of the heterostructure. As a result, the HCDI system based on MoS2/MXene heterostructure electrode displays an outstanding desalination performance with a high desalination capacity of 23.98 mg g−1, outstanding desalination rate of 4.6 mg g−1 min−1 and superior cycling stability with only 4% desalination capacity degradation for over 100 cycles, indicating that our strategy should be a promising approach to achieve highly efficient HCDI.

Lithium-Selective Hybrid Capacitive Deionization System with Ag-Coated Carbon Electrode and Stop-Flow Operation

Lithium-Selective Hybrid Capacitive Deionization System with Ag-Coated Carbon Electrode and Stop-Flow Operation

Selective sorbents for recovery of lithium ions by hybrid capacitive deionization

Lithium is a critical element due to its use in lithium-ion batteries, required for electric vehicles and stationary energy storage devices. Various approaches have been applied to extract it from the available resources. One of the developed lithium-capturing methods is hybrid capacitive deionization (HCDI), where the cathode is made of spinel-type material. This paper presents our study on the most effective Li, Mn, and Ti spinel-type materials, for which we analysed their structure, efficiency for lithium capturing, and energy consumption in the HCDI system. We found spinel with Li:Mn:Ti ratio of 1:3:0.15 to be the best material for lithium recovery. The natural geothermal water was applied for testing lithium recovery by HCDI equipped with the investigated material. By applying the asymmetrical electric mode, the lithium capturing efficiency reached near 80% with 8 min. The separation factors for other cations such as sodium, potassium, calcium, magnesium, and strontium were over three-times smaller.

Hierarchical Co3O4/CNT decorated electrospun hollow nanofiber for efficient hybrid capacitive deionization

Hybrid capacitive deionization (HCDI) is emerging as an energy-efficient alternative for brackish water desalination with low expenditure cost and favourable ion removal effectiveness by adopting battery-type electrodes in the traditional capacitive deionization (CDI) system. Herein, an unscalable electrospinning method is introduced to contrive cathode material for HCDI system. Co3O4 and nitrogen-doped carbon nanotube decoration are formed successfully and uniformly distributed with the hollow structure of free-standing carbon nanofibers (Co3O4@CNF@CNT). The conductive “bridges” provided by the CNF matrix significantly shorten the diffusion length of Na+ and promote the electrical conductivity of the Co3O4 nanoparticles. Moreover, benefiting from the incorporation of nitrogen-doped CNTs, the electrical conductivity is further enhanced. The as-prepared Co3O4@CNF@CNT cathode shows excellent pseudocapacitive performance of 395F g−1 at a scan rate of 1 mV s−1 and superior rate performance of 279F g−1 at 100 mV s−1. The HCDI system delivers an outstanding salt adsorption capacity (SAC) of 58.6 mg g−1 and a highest salt adsorption rate (SAR) of 12.27 mg g−1 min−1 with a potential difference of 1.4 V, indicating the great potential of applying Co3O4@CNF@CNT in the practical HCDI system.

Beneficial synergy of adsorption–intercalation–conversion mechanisms in Nb2O5@nitrogen-doped carbon frameworks for promoted removal of metal ions via hybrid capacitive deionization

Enhanced removal of metal ions in a hybrid capacitive deionization system has been originally developed via the adsorption–intercalation–conversion mechanisms.

Selective removal of Cl− and F− from complex solution via electrochemistry deionization with bismuth/reduced graphene oxide composite electrode

Electro-adsorption is attracting increasing attention as an emerging technology for removing ionic species from water but suffer from low selectivity. In this work, a bismuth/reduced graphene oxide nanocomposite electrode was fabricated by a facile and green method. Based on this material, an electrode with improved selectivity by electrochemistry deionization system was successfully fabricated. The bismuth nanoparticles were uniformly covered with reduced graphene oxide plates and the ratio of Bi on the whole materials is 79.56%. Bismuth/reduced graphene oxide showed ions selectivity in the order of Cl− > F− ≫ SO42−. The average Cl− removal capacity can reach as high as 62.59 mg g−1. Moreover, bismuth/reduced graphene oxide electrodes have good regeneration performance. Typically, in the 10 adsorption-desorption multicycles, the salt absorption/desorption capacity of the hybrid capacitive deionization system is stable and reversible. This research opened a hopeful window to design and synthesize effective materials to selectively remove the ionic species to purify the water.

Ion removal performance and enhanced cyclic stability of SnO2/CNT composite electrode in hybrid capacitive deionization

Hybrid capacitive deionization (HCDI) is developed based on capacitive deionization (CDI) for water treatment, which combines CDI with a battery system. Developing the high-performing battery electrode materials is a significant point for HCDI. Herein, SnO2/CNT was fabricated according to hydrothermal method, which was denoted as SnO2/CNT-H, after calcination, the obtained product was denoted as SnO2/CNT-HC. The as-prepared SnO2/CNT-HC was used in HCDI system for the first time, which demonstrated a salt adsorption capacity of 9 mg g−1 and ultrafast salt adsorption rate of 2.5 mg g−1 min−1 at 500 mg L−1 initial NaCl concentration. The SnO2/CNT-HC electrode also had a good cyclic stability of keeping 120 % of salt adsorption capacity after 100 adsorption-desorption cycles at 1.0 V and 500 mg L−1 NaCl solution. Moreover, when this system was used to study the adsorption-desorption performance for different cations (Li+, K+ and Ca2+) and anions (F−, NO3− and SO42−), the phenomenon was revealed that the cation adsorption capacity of Li+ > K+ > Ca2+ captured by SnO2/CNT-HC and the anion adsorption capacity of F− > NO3− > SO42− adsorbed by activated carbon. These results show that SnO2/CNT-HC should be one of realistic electrode materials for HCDI.

Ag-doped hollow ZIFs-derived nanoporous carbon for efficient hybrid capacitive deionization

Desalination performance of hybrid capacitive deionization (HCDI) intensively depends on the electrode materials with redox activity and capacitance ability. Herein, Ag-doped hollow ZIFs-derived nanoporous carbon (Ag/ZCs) was employed as an anode material by the pyrolysis of Ag-doped hollow ZIFs precursor, which was fabricated by in situ tannic acid (TA) etching/reduction strategy. Provably, TA serves as not only an etchant for tailoring ZIFs structure, but also a reductant for Ag nanoparticles formation. The obtained Ag/ZCs possesses unique hollow cavities and high redox activity. As expected, the HCDI system with Ag/ZCs electrode exhibits excellent deionization capacity of 29.18 mg g−1 in 500 mg L−1 NaCl solution at 1.2 V, which is much higher than that of conventional carbon-based CDI system. Moreover, desirable regeneration stability of the Ag/ZCs HCDI was well maintained after 20 cycle experiments. The results demonstrate that Ag/ZCs is a potential anode material for HCDI applications.

Significantly improved stability of hybrid capacitive deionization using nickel hexacyanoferrate/reduced graphene oxide cathode at low voltage operation

Currently, hybrid capacitive deionization (HCDI) with faradaic material cathode and active carbon (AC) anode has attracted much attention due to its very excellent desalination performance even in highly-concentrated saline water. However, the carbon oxidation reaction occurring at a potential of 0.7–0.9 V in a CDI system still exists at the carbon anode in a HCDI system, which causes the performance degradation during long-term operation. Introducing costly ion exchange membranes has been reported to solve this issue. In this work, we report a more simple and cost-saving approach to improve the desalination performance of membrane-free HCDI cell by using nickel hexacyanoferrate/reduced graphene oxide (NiHCF/rGO) cathode which possesses high theoretic capacity and low Na+ intercalation/extraction potential at a low voltage operation (0.6 V). A high desalination capacity of 22.8 mg g−1 is achieved for this membrane-free HCDI cell, higher than those for the conventional AC//AC CDI cell. More importantly, the AC//NiHCF/rGO cell shows a stable cycling performance with a capacity retention of 78% at 0.6 V over 100 desalination-regeneration cycles, significantly improved compared with that at 1.2 V. The strategy should provide a helpful guidance for practical CDI operation.

Ag-doped sepiolite intercalated graphene nanostructure for hybrid capacitive deionization system

Hybrid capacitive deionization (HCDI) is one of the effective desalination technologies that consists of faradic and non-faradic carbon electrodes. Herein, an ion-exchange membrane-free HCDI system has been developed using sepiolite intercalated graphene (GS) and silver (Ag)-doped sepiolite intercalated graphene (AGS) as the cathode and anode, respectively. Electrochemical studies were revealed that the GS hybrid electrode provides an electrical double layer (EDL) with a specific capacitance of 73 F g−1, whereas the AGS exhibited pseudocapacitive behavior with a specific capacitance of 97.5 F g−1 at 10 mV s−1 in 2 M NaCl solution. AGS electrode was selectively intercalating the anions from the salt solution and prevented the co-ion repulsion, resulting in a significant salt adsorption capacity of 20.7 mg g−1 and charge efficiency of 87.1% at 1.2 V in 600 mg L−1 of NaCl solution. The presence of nanoclay in the hybrid exhibits intrinsic hydrophilicity and more stability of the electrode, resulting in a stable cycling performance over the 10 charge-discharge cycles. Moreover, by utilizing the advantage of the hybrid materials, ions are selectively captured in the HCDI cell via both the electrochemical faradic reaction and electrostatic EDL mechanism. Thus, the GS and the AGS hybrid materials exhibited good desalination performance in the HCDI application.