Deionization Performance Research Articles

Optimizing the Micro/Mesoporous Structure of Hierarchical Porous Carbon Synthesized from Petroleum Pitch Using the Solvent-Free Method for Ultra-Fast Capacitive Deionization.

In this work, a solvent-free ZnO-template method is used to synthesize hierarchical porous carbons (denoted as HPC-X; X = 1, 1.5, 2, and 4 g of ZnO) via the pyrolysis of petroleum industrial-residual pitch with ZnO. The proposed method allows precise control of the micro/meso/macroporous structure of the HPC by adjusting the amount of ZnO. The results show that the average pore size of HPCs prominently increases from 2.4 to 3.7 nm with the increase in the ZnO/pitch ratio. In addition, it is shown that HPCs have a high surface area between 1141 and 1469 m2 g-1, a wide-range pore size distribution (micro-, meso-, and macropores), and a tap density ranging from 0.2 to 0.57 g cm-3. The capacitive deionization performances of HPCs for sodium and chloride ions are investigated. The results show that HPC-2 exhibits the highest electrosorption capacity of 9.94 mg g-1 within 10.0 min and a maximum electrosorption capacity of 10.62 mg g-1 at 1.2 V in a 5.0 mM NaCl solution. Hence, HPC-2 is a highly promising candidate as an electrode material for rapid deionization.

Capacitive deionization performance of asymmetric nanoengineered CoFe2O4 carbon nanomaterials composite.

Capacitive deionization (CDI) is a relatively new technique that uses electric double layer (EDL) effects, high-affinity chemical groups, redox-active materials, and membrane capacitive electrosorption principle for the desalination. In this paper, hydrothermal synthesis of cobalt ferric oxide (CFO) metal oxide nanoparticles (NPs) coupled with the vacuum filtration method, or the freeze-drying method is used to fabricate high-performance nanocomposites: CFO-graphene, CFO-CNTs, and CFO-3DrGO. Two times of hydrothermal reaction methods were conducted to fabricate the CFO-3DrGO nanoengineered as a pseudocapacitive/EDL electrode. The results have demonstrated that the SAC of CFO-3DrGO/CFO (64.5mgg-1) is greater than that of the CFO-graphene/CFO (55.16mgg-1) and CFO-CNTs/CFO (21.5mgg-1) due to the better surface area of the CFO-3DrGO nanocomposite (330 m2g-1). The higher surface area of the CFO-3DrGO is due to the porous and interconnected 3D structure of the 3DrGO, and it provides a larger surface area to form EDL capacitance. In addition, the added porous 3DrGO entangled with the spinel crystals (CoFe2O4) in the composite allowed for a quick ion diffusion across the interconnected open macroporous structures.

Magnetic-assisted strategy for performance enhancement of flow-by capacitive deionization

Capacitive deionization is an environment friendly electro-sorption desalination technology that has broad application prospects in high-efficiency seawater desalination and brackish water purification. However, the insufficient ion separation performance and desalination capacity restrict the large-scale application of capacitive deionization with flow-by structure. Since the separation of cations and anions is intensified via the synergetic manipulation of the electric field force and Lorentz force, a magnetic-assisted strategy was proposed to enhance the performance of capacitive deionization. The effects of magnetic field strength and layout on the desalination performance of capacitive deionization were experimentally investigated. The results confirmed the feasibility of performance enhancement via the effect of the magnetic field. The performance of capacitive deionization under a vertical magnet layout was increased by 25.2 % at a magnetic field strength of 0.33 T, compared to that without a magnetic field. However, the deionization performance decreased when the magnetic field strength was further increased to 0.45 T, which indicates the existence of a critical magnetic field strength. Benefitting from the superposition of magnetic fields, the horizontal layout of magnets can provide a stronger average magnetic field than the vertical layout. This study provides insights into a new design of flow-by capacitive deionization.

Aerosol Synthesis of Vanadium Oxide-Carbon Hybrid Nanoparticle Clusters for High-Performance Lithium Extraction via Electrochemical Deionization

An aerosol-based synthetic approach is demonstrated for the development of vanadium oxide-carbon hybrid nanoparticle clusters (VOx-C-NPCs) used for the fabrication of a unique electrochemical deionization (ECDI) cell. Hybrid ECDI cells are constructed with a positive electrode of pseudocapacitive VOx-C-NPCs and a negative electrode of battery-like silver–carbon nanoparticle clusters (Ag-C-NPCs). The reversible extraction of Li+ ions and the storage of Cl– ions are conducted through Faradaic reactions on VOx-C-NPCs and Ag-C-NPCs, respectively. The surface of the VOx-C-NPC electrodes can be successfully protected by an anion-doped polypyrrole (PPy) film, by which the structural stability of VOx-C-NPC, in terms of the salt adsorption capacity (SAC) retention, is enhanced to 92%. Remarkably high SAC values of the hybrid ECDI cells are achievable in comparison to the reported values in the field: up to 27.5 mg/g for the bare VOx-C//Ag-C cell and 49.3 mg/g for the PPy-protected VOx-C//Ag-C cell. This work provides a prototype study for the rapid and continuous production of high-performance VOx-C-NPCs using aerosol-based synthesis supported by complementary material characterization. The mechanistic understanding of the material synthesis and the corresponding ECDI process shows promise for achieving an optimal deionization performance in terms of SAC and stability.

Ion Exchange Conversion of Na-Birnessite to Mg-Buserite for Enhanced and Preferential Cu2+ Removal via Hybrid Capacitive Deionization.

Layered manganese oxides (LMOs) have recently been demonstrated to be one of the most promising redox-active material platforms for electrochemical removal of heavy metal ions from solution via capacitive deionization (CDI). However, the impact of interlayer spacing of LMOs on the deionization performance of electrodes in a hybrid capacitive deionization (HCDI) system with an LMO cathode and a carbon anode (i.e., LMO/C electrodes), and their phase transformation behaviors, particularly during the desalination operations, have yet to be extensively evaluated. In this study, we thoroughly evaluate Mg-buserite obtained by ion exchange of fresh Na-birnessite and Na- and K-birnessite as HCDI electrodes to remove copper ions (Cu2+) from saline solutions. Among the three LMO/C electrodes, the Mg-buserite/C (MgB/C) electrodes demonstrate the best deionization performance in terms of salt adsorption capacity (SAC), electrosorption rate, and cycling stability, followed by K-birnessite/C (KB/C) and Na-birnessite/C (NaB/C). More importantly, MgB/C exhibits the highest Cu2+ ion adsorption capacity (IAC) of 89.3 mg Cu2+ per gram electrode materials at a cell voltage of 1.2 V in 500 mg L-1 CuCl2 solution, with an IAC retention as high as 96.3% after 60 charge/discharge cycles. Given that electrosorption of Cu2+ ions is often competed by alkali and alkaline earth metal ions, our data reveal that the MgB/C electrodes demonstrate selectivities of 4.7, 7.7, and 8.1 for Cu2+ over Na+, Ca2+, and Mg2+, respectively. Moreover, X-ray diffraction and spectroscopic analyses show that the enhanced deionization performance and preference for Cu2+ are mainly attributed to the expanded interlayer spacing of LMO minerals. This study provides a promising strategy for tailoring LMO minerals for improving their electrosorption capacity and preference for copper ions from a multivalent-ion solution via an HCDI platform.

(Invited) A Decade Effort for Advancing Capacitive Deionization at University of Kentucky Center for Applied Energy Research (UK CAER)

Even though two-thirds of our world's surface is covered by water, less than 1% of that water can be directly consumed to satisfy the rapid growth in population, urbanization, and industrialization.[1] Water quality and scarcity have become some of the most important global challenges of our time. Current desalination technologies such as multi-stage flash distillation and reverse osmosis can be costly to implement and operate, requiring significant pretreatment and consistent maintenance procedures.[2] Thus, investigations into alternative desalination options are being explored toward building more sustainable water treatment systems.Capacitive deionization (CDI) is a desalination technology using highly porous carbon electrodes that can reversibly adsorb dissolved ions. By regulating applied voltages to a CDI cell, ionized salts are trapped in the electric double layers (EDLs) at carbon electrodes, thereafter desalinating water in the CDI cell.[3] CDI technology may have potential advantages over current desalination methods in that no heat treatment or high pressure is required, potentially leading to a significant decrease in the operational and energy costs compared to current desalination processes and aiding in the production of clean/fresh water.Since 2011, researchers from the University of Kentucky Center for Applied Energy Research (UK CAER) have contributed to ongoing efforts to advance CDI technology from theoretical studies to applied process research.[3-22] Works primarily include the improvement of desalination capacity, mitigation of performance degradation, and technology commercialization. In this talk, one of the presenters will provide key milestones of the CDI technology developed at UK CAER in honor of Prof. D. Noel Buckley for his 50-year experience in electrochemistry research.Ref:[1] M. Elimelech, W.A. Phillip, The future of seawater desalination: energy, technology, and the environment, Science, 333 (6043) (2011), pp. 712-717[2] J.-J. Yan, S.-F. Shao, J.-H. Wang, J.-P. Liu, Improvement of a multi-stage flash seawater desalination system for cogeneration power plants, Desalination, 217 (1) (2007), pp. 191-202[3] A. Omosebi, X. Gao, J. Rentschler, J. Landon, K.-K. Liu, Continuous operation of membrane capacitive deionization cells assembled with dissimilar potential of zero charge electrode pairs, J. Colloid Interf. Sci., 446 (2015), pp. 345-351[4] J. Landon, X. Gao, A. Omosebi, K. Liu, “Local pH Effects on Carbon Oxidation in Capacitive Deionization Architectures” Environmental Science: Water Research & Technology, 7, 861 – 869 (2021)[5] A. Omosebi, Z. Li, N. Holubowitch, X. Gao, J. Landon, A. Cramer, K. Liu, “Energy recovery in capacitive deionization systems with inverted operation characteristics”, Environmental Science: Water Research & Technology, 6, 321-330 (2020)[6] X. Gao, A. Omosebi, Z. Ma, F. Zhu, J. Landon, M. Ghorbanian, N. Kern, K. Liu, “Capacitive Deionization Using Symmetric Carbon Electrode Pairs”, Enviro. Sci.: Water Res. Tech., 5, 660-671 (2019).[7] J. Landon, X. Gao, A. Omosebi, K. Liu, “Progress and outlook for capacitive deionization technology”, Current Opinion in Chemical Engineering, 25, 1-8 (2019)[8] N. Holubowitch, A. Omosebi, X. Gao, J. Landon, K. Liu, “Membrane-Free Electrochemical Deoxygenation of Aqueous Solutions Using Symmetric Activated Carbon Electrodes in Flow-Through Cells”, Electrochim. Acta., 297, 163-172 (2019).[9] X. Gao, A. Omosebi, J. Landon, K. Liu, “Voltage-Based Stabilization of Microporous Carbon Electrodes for Inverted Capacitive Deionization”, J. Phys. Chem. C, 122, 1158-1168 (2018).[10] A. Omosebi, X. Gao, N. Holubowitch, Z. Li, J. Landon, K. Liu, “Anion Exchange Membrane Capacitive Deionization Cells”, J. Electrochem. Soc., 164, E242-E247 (2017).[11] N. Holubowitch, A. Omosebi, X. Gao, J. Landon, K. Liu, “Quasi-Steady-State Polarization Reveals the Interplay of Capacitive fand Faradaic Process in Capacitive Deionization”, ChemElectroChem, 4, 2404-2413 (2017).[12] X. Gao, A. Omosebi, N. Holubowitch, J. Landon, K. Liu, “Capacitive Deionization Using Alternating Polarization: Effect of Surface Charge on Salt Removal”, Electrochim. Acta, 233, 249-255 (2017).[13] X. Gao, A. Omosebi, N. Holubowitch, A. Liu, K. Ruh, J. Landon, K. Liu, “Polymer-Coated Composite Anodes for Efficient and Stable Capacitive Deionization”, Desalination, 399, 16-20 (2016).[14] X. Gao, S. Porada, A. Omosebi, K. Liu, P. M. Biesheuvel, J. Landon, “Complementary Surface Charge for Enhanced Capacitive Deionization”, Water Res., 92, 275-282 (2016).[15] X. Gao, A. Omosebi, J. Landon, K. Liu, “Enhanced Salt Removal in an Inverted Capacitive Deionization Cell Using Amine Modified Microporous Carbon Electrode”, Environ. Sci. Tech., 49, 10920 (2015).[16] X. Gao, A. Omosebi, J. Landon, K. Liu, “Surface Charge Enhanced Carbon Electrodes for Stable and Efficient Capacitive Deionization Using Inverted Adsorption-Desorption Behavior”, Energy Environ. Sci., 8, 897 (2015)[17] X. Gao, A. Omosebi, J. Landon, K. Liu, “Dependence of the Capacitive Deionization Performance of Potential of Zero Charge Shifting of Carbon Xerogel Electrodes during Long-Term Operation”, J. Electrochem. Soc., 161, E159 (2014).

Importance of the conductivity and specific surface area of electrode particles to the desalination performance and energy efficiency of flow-electrode capacitive deionization

Different from electrodialysis for desalination, flow-electrode capacitive deionization (FCDI) removes ions via electrosorption on the surface of charged flow-electrode particles. In this study, the exact effects of the conductivity and surface area of carbon electrode particles on the desalination performance and energy efficiency of the FCDI were investigated under the constant voltage (CC) and constant current (CV) power-supply modes. The low-cost activated carbon (AC) with a large specific surface area and carbon black (CB) with a high conductivity were used as the model electrode particles. In the CC mode, increasing the CA content from 0.5 wt% to 5 wt% achieved a limited enhancement of the desalination rate and efficiency (50.2%-54.5% in 2 h), but dramatically reduced the energy consumption from 1.39 kWh/kg to 0.73 kWh/kg. In the CV mode at 1.2 V, doubling the AC content from 5 wt% to 10 wt% or mixing a small amount of CB in AC (AC:CB = 9:1 at 5 wt%) significantly improved the desalination efficiency from 65.6% to over 90%, but the energy consumption remained similarly at 0.64–0.68 kWh/kg. The conductivity of the electrode particles greatly affects the energy efficiency in the CC mode and the desalinization rate in the CV mode. The CB addition can effectively enhance the electric charge transfer between the current collectors and electrode particles. The surface area of electrode particles largely regulates the FCDI capacitance, as well as the back diffusion during the process. Accordingly, the flow-electrode property can be optimized, e.g., CB is mixed with AC at an appropriate ratio, to ensure both a high conductivity (to facilitate electron transfer) and sufficient surface area (to increase the capacitance and alleviate back diffusion) for cost-effective desalination.

Enhanced electrochemical and capacitive deionization performances of single-layer graphene oxide/nitrogen-doped porous carbon/activated carbon fiber composite electrodes

Capacitive deionization (CDI) is an electrosorption desalination technology based on electrode charge surface, where CDI electrode materials play important roles in efficient desalination processes. This study created and manufactured innovative single-layer graphene oxide (SGO), nitrogen-doped porous carbon (NPC), and activated carbon fiber (ACF) composites to produce SGO/NPC/ACF electrode materials with electrochemical and CDI performances. The SGO/NPC/ACF electrode combined the stability of NPC with the high conductivity of SGO to yield enhanced electrochemical and CDI activities. The samples have been characterized using XRD, SEM, and XPS. The electrochemical performance was tested by cyclic voltammetry and electrochemical impedance spectroscopy methods. As a result, SGO/NPC/ACF showed a high specific capacitance of 323.08 F g−1. The specific surface area of SGO/NPC/ACF is 1154.91 m2 g−1. The salt adsorption capacity (SAC) of the SGO/NPC/ACF composite electrode is 16.25 mg g−1, which is about 1.5 times (10.70 mg g−1) that of the ACF electrode. After fifty adsorption/desorption treatments, the SAC retention of the SGO/NPC/ACF electrode is 1.6 times higher than that of ACF. Our work also provides a theoretical basis for the efficient development of green carbon-based composites. Therefore, the SGO/NPC/ACF electrode has good cycle stability and reproducibility, which is expected to become a practical CDI electrode material for desalination.

An open-source deep learning model for predicting effluent concentration in capacitive deionization

To effectively evaluate the performance of capacitive deionization (CDI), an electrochemical ion separation technology, it is necessary to accurately estimate the number of ions removed (effluent concentration) according to energy consumption. Herein, we propose and evaluate a deep learning model for predicting the effluent concentration of a CDI process. The developed deep learning model exhibited excellent prediction accuracy for both constant current and constant voltage modes (R2 ≥ 0.968), and the accuracy increased with the data size. This model was based on the open-source language, Python, and the code has since been distributed with proper instructions for general use. Owing to the nature of the data-oriented deep learning model, the findings of this study are not only applicable to conventional CDI but also to various types of CDI (membrane CDI, flow CDI, faradaic CDI, etc.). Therefore, by referring to the examples shown in this study, we hope that this open-source deep learning code will be widely used in CDI research.

Metal-organic framework–derived carbon/N-doped three-dimensional reduced graphene oxide composite with high capacitive deionization performance

Capacitive deionization (CDI) is an electrostatic adsorption water treatment technology based on capacitor structure and charge-discharge principle, which has the advantages of pollution free, low energy saving, and high value added. Herein, a novel metal-organic framework (MOF)-derived carbon (PC)/nitrogen-doped three-dimensional reduced graphene oxide composite was prepared for CDI electrode material. In this nanoarchitecture, PC was attached to the N-doped graphene sheet, formed a hierarchical porous structure, and provided more active sites and ion transfer channels, which featured with enhanced the salt adsorption capacity (SAC) and rate. The carbonized products (NHG/PC-700) showed a high nitrogen content of 10.78 at.% and large surface area (432.3 m2/g), which was conducive to improve electric conductivity. Accordingly, the specific capacitance of 370.9 F/g and excellent long charge-discharge cycling stability were obtained at 1 A/g current density. In CDI experiments, the NHG/PC-700 electrodes displayed an outstanding SAC of 28.17 mg/g with an initial concentration NaCl solution of 500 mg/L, and the SAC was reduced 4.6% after six CDI regeneration cycles. It was found that the NHG/PC-700 could be considered as a promising electrode material in the field of water treatment.

Effect of Surface Charge on the Fabrication of Hierarchical Mn-Based Prussian Blue Analogue for Capacitive Desalination.

Multiple and hierarchical manganese (Mn)-based Prussian blue analogues obtained on different substrates are successfully prepared using a universal, facile, and simple strategy. Different functional groups and surface charge distributions on carbon cloth have significant effects on the morphologies and nanostructures of Mn-based Prussian blue analogues, thereby indirectly affecting their physicochemical properties. Combined with the advantages of the modified carbon cloth and the nanostructured Mn-based Prussian blue analogues, the composite with negative surface charge formed by the electronegativity differences shows good electrochemical properties, leading to improvement in charge efficiency during capacitive desalination. An asymmetric device fabricated with Mn-based Prussian blue analogue-modified F-doped carbon cloth as the cathode and acid-treated carbon cloth as the anode presents the highest salt adsorption capacity of 10.92 mg g–1 with a charge efficiency of 82.28% and the lowest energy consumption of 0.45 kW h m–3 at 1 V due to the main influencing factor from the negative surface charge leading to co-ion expulsion boosting the capacitive deionization performance. We provide insights for further exploration of the relationship between second-phase materials and carbon cloth, while offering some guidance for the design and preparation of electrodes for desalination and beyond.

Improving capacitive deionization performance by using O2 plasma modified carbon black

Previous research on capacitive deionization (CDI) mainly focuses on electrode active materials, while the research on conductive additive and their modification for improving CDI performance are rarely reported. In this work, we mainly investigate the effect and mechanism of oxygen (O2) plasma-modified carbon black (PCB) to enhance CDI performance. The CB treated with O2 plasma at 100 W for 10 min (CB-100 W-10 min) was assembled with activated carbon (AC) to form a CB-100 W-10 min/AC electrode with excellent desalination performance (32.54 mg g−1), 6.9 times that of the original CB/AC electrode (OCB/AC, 4.91 mg g−1). The roles of the specific surface area, hydrophilicity, electrical conductivity, and specific capacitance of CB in enhancing the performance of CDI were explored, and the interaction mechanism between H2O molecules and CB was investigated using molecular dynamics (MD) simulations. The advantages of PCB are mainly attributed to the synergy of the following aspects: (i) The excellent three-dimensional network structure acts as a “bridge” between activated carbon and increases the contact sites for allowing more PCB and AC to participate in the electrosorption process; (ii) Shorter electron transmission paths and more electron transmission channels between PCB and AC promote the electrosorption rate; (iii) High surface roughness and oxygen-containing functional groups of PCB boost hydrophilicity and dispersibility. Interestingly, we found that PCB possesses universal applicability in the enhancement of deionization performance of various active materials for purification of brine, heavy metals and radionuclides. This work unprecedentedly applies plasma technology to the modification of conductive additive, providing new ideas for material modification and improvement of CDI technology.

HCl-activated porous nitrogen-doped carbon nanopolyhedras with abundant hierarchical pores for ultrafast desalination

Exploring an emerging carbon-based material with optimized structure and controlled porosity is of significance for further heightening the capacitive deionization (CDI) performance and solving the problem of emergency fresh water supply. Herein, a porous nitrogen-doped carbon nanopolyhedra with hierarchical pores prepared via using zeolite-type metal-organic framework (ZIF-8) as precursor is reported and used for CDI. In order to prepare the nanomaterials with abundant hierarchical pore structure, the synthetic route of carbonization followed by HCl-activation is adopted. The resulting nitrogen-doped carbon materials exhibit a bimodal porosity containing micro- and meso-pores, high specific surface area, and numerous exposed adsorption active sites. The excellent performance in structure ensures the ultrahigh desalination capacity of 37.52 mg g−1 and excellent recyclability (retained 90% over 30 cycles) of the as-prepared carbon electrode material. Notably, the above electrode demonstrates ultrafast desalination rate of 16.01 mg g−1 min−1, which is 2–8 times faster than the conventional carbon materials. This present work may provide a new insight for developing efficient MOF-derived CDI electrode materials and realizing rapid water resource supply in emergencies such as outdoor survival or unexpected natural disasters.

Enhanced capacitive deionization of toxic metal ions using nanoporous walnut shell-derived carbon

Capacitive deionization (CDI) technology differs from traditional demineralization technology. Its desalination effect is mainly dependent on the electrode material and the porous nanostructure which can enhance capacitive deionization performance. Here, a low-cost and eco-friendly CDI technology has been developed to remove divalent ions from water. Using different activators such as KOH, ZnCl2 and K2C2O4 to reconstruct the biomass carbon skeleton, porous carbon material derived from walnut shells was used as symmetric electrodes. The maximum removal capacities in the CDI electrosorption were 17.1 mg/g for Mg2+, 11.99 mg/g for Ca2+, 18.56 mg/g for Pb2+ and 15.52 mg/g for Cd2+. The best effect on formation of micropores and construction of adsorption channels was observed using KOH as activator. The KOH-C had a specific surface area of 1600.34 m2/g and a cumulative pore volume of 0.3561 cm3/g, which provides favorable conditions for ion storage. These results demonstrate that KOH-activated carbon biomass can improve electrosorption performance and has potential as an advanced, inexpensive electrode material for capacitive deionization.

Removal and recovery of ammonia from simulated wastewater using Ti3C2Tx MXene in flow electrode capacitive deionization

Flowing electrode capacitive deionization systems (FE-CDI) have recently garnered attention because of their ability to prevent cross contamination and operate in uninterrupted cycles ad infinitum. Typically, FE-CDI electrodes suffer from low conductivity, reducing deionization performance. Utilization of higher mass loadings to combat this leads to poor rheological properties. Herein, Ti3C2Tx MXene was introduced as 1 mg mL−1 slurry electrodes in an FE-CDI system for the removal and recovery of ammonia from simulated agricultural wastewater. The electrode performance was evaluated by operating the FE-CDI system with a feed solution of 500 mg L−1 NH4Cl running in batch mode at a constant voltage of 1.20 and −1.20 V in charging and discharging modes, respectively. Despite the low loading, Ti3C2Tx flowing electrodes showed markedly improved performance, achieving 60% ion removal efficiency in a saturation time of 115 min with an adsorption capacity of 460 mg g−1. To understand the high adsorption performance of the electrodes, physiochemical and structural analysis was done via a variety of characterization techniques such as SEM, TEM, XRD, DLS, and Raman spectroscopy. Cyclic voltammetry and galvanostatic charge/discharge profiles were obtained to evaluate the electrochemical properties of the electrodes. The system proved to be an energy-saving technology by exhibiting a charge efficiency of 58–70% while operating at an energy consumption of 0.45 kWh kg−1. A 92% regeneration efficiency showed that the electrodes were stable and suitable for long term and scalable usage. The results demonstrate that MXenes have the potential to improve the FE-CDI process for energy-efficient removal and recovery of ammonia.

Hierarchical N-Doped porous 3D network electrode with enhanced capacitive deionization performance

Capacitive deionization (CDI) has emerged as a promising method of desalination owing to its outstanding regeneration efficiency, low energy consumption, and without secondary contamination. The electrosorption capacity is largely determined by the electrode material. Nevertheless, the bottlenecks of present carbon-based CDI electrode materials are their limited desalination capability and single pore channel, which are unfavorable for salt ion diffusion and access to the internal space. Herein, we synthesized nitrogen-doped porous carbon (NPC) and performed a facile activation to obtain hierarchical porous carbon (KNPC) for CDI desalination. The optimized KNPC showed a remarkable desalination performance of 28.40 mg/g in 500 mg/L NaCl solution, with an exceptional regeneration performance of 94.44% after 15 consecutive cycles. The enhanced CDI performance could be attributed to the following aspects: (1) KNPC was introduced plenty of microporous and mesoporous structures, which provides convenient transport channels and accelerated salt ion transport. (2) The enlarged specific surface area and pore volume provide sufficient adsorption sites for salt ions. (3) The modified KNPC exhibited excellent electrochemical properties and wettability, consisting of higher specific capacitance, decreased internal resistance, and ultra-high hydrophilicity. (4) KOH activation promoted the capacitive contribution of the KNPC electrode, thus improving its CDI performance.

Chemical bonding of flexible graphene to carbon paper: A new synthetic paradigm for freestanding electrode with high capacitive deionization performance

Capacitive deionization (CDI) has become a promising desalination and water treatment technology due to its low-cost, environment-friendliness and energy-efficient. In this work, it is successful to achieve the preparation of high-quality three-dimensional flexible graphene frameworks (3DFGFs) from commercial carbon paper (CP) by simply combining the modified Hummer's method with freezing drying technology. The electroactive area, conductivity and hydrophilicity are greatly improved, the areal specific capacitance of 3DFGFs electrode reaches 1.43 F/cm2 at 10 mV/s in 1 M NaCl solution, and the areal adsorption capacity is up to 322.21 (50 mg/L NaCl) and 581.51 mg/m2 (500 mg/L NaCl), respectively. Interestingly, the overall shape of the 3DFGFs electrode remain intact after 500 times of crimping (capacity retention rate is 96.85% for >500 crimping), suggesting that 3DFGFs have outstanding flexibility owing to the strong interaction between the fibers and the graphene sheets in the frameworks. Finally, we propose a concept of how-to recovery the electrical energy stored in CDI cell. This work opens up a promising direction for the preparation of flexible CDI electrodes, and the simple and mild method developed can be easily applied to the preparation of other flexible electrodes.

Capacitive heavy metal ion removal of 3D self-supported nitrogen-doped carbon-encapsulated titanium nitride nanorods via the synergy of faradic-reaction and electro-adsorption

Recently, the highly efficient, low-cost and environment-friendly capacitive deionization (CDI) hold great promise in the field of wastewater purification. In this paper, we report an N-doped carbon nanolayer (N-C) encapsulated ultrafine titanium nitride (TiN) nanorod-assembled flower-like framework anchored on the carbon cloth (CC) matrix (N-C@TiN@CC) as the self-supported electrode for robust CDI-based heavy metal ion wastewater purification. Benefitting from the synergistic effects of specific structure features (i.e., high porosity, 3D open framework and hierarchical pore arrangement), surface doping strategy of high-conductivity carbon nanolayer and integrated electrode processing, the resultant N-C@TiN@CC hybrid electrode proves superior deionization performance and excellent cycling stability, yielding an outstanding average removal efficiency up to ca. 98 % in the simulated high-concentration single, binary and multi-component wastewater systems. Moreover, it is further revealed the effective electro-adsorption and faradic reaction coupling ion removal process in the deionization system, which may account for the significant removal efficiency promotion of N-C@TiN@CC hybrid electrode.

Enhancing performance of capacitive deionization under high voltage by suppressing anode oxidation using a novel membrane coating electrode

High voltage operation is effective to improve salt adsorption capacity (SAC) of capacitive deionization (CDI), while this will cause performance loss during long-term operation. Herein, to address this issue, we report a simple approach to prepare novel membrane coating electrode (MCE) that polyvinyl alcohol/polyethyleneimine membrane was deposited onto activated carbon fiber (ACF) and glutaraldehyde (GA) was used for crosslinking. Compared with ACF (SAC: 13.2–20.9 mg/g, charge efficiency: 63.6%–43.9%), MCE with optimized 5 wt% GA concentration (MCE (5)) showed an improved SAC from 14.6 to 28.4 mg/g and stable charge efficiency at 80.2% when voltage increased from 0.8 V to high voltage of 1.8 V due to suppressed anode oxidation. After forty cycles experiments at 1.8 V, SAC retention of ACF declined to 71.6% caused by degradation of electrode properties, while SAC retention of MCE (5) increased to 124.7%. The deposited membrane was found to act as a barrier and inhibitor to restrict ACF oxidation, and conversions of N-containing groups and reduced charge transfer resistance of cycled MCE (5) led to superb long-term performance. It is reasonable to believe that our work provides new strategy for enhancing CDI performance by using newly developed MCE.

Boosting Capacitive Deionization Performance of Commercial Carbon Fibers Cloth via Structural Regulation Based on Catalytic‐Etching Effect

Monolithic carbon electrodes with robust mechanical integrity and porous architecture are highly desired for capacitive deionization but remain challenging. Owing to the excellent mechanical strength and electroconductivity, commercial carbon fibers cloth demonstrates great potential as high‐performance electrodes for ions storage. Despite this, its direct application on capacitive deionization is rarely reported in terms of limited pore structure and natural hydrophobicity. Herein, a powerful metal‐organic framework‐engaged structural regulation strategy is developed to boost the desalination properties of carbon fibers. The obtained porous carbon fibers features hierarchical porous structure and hydrophilic surface providing abundant ions‐accessible sites, and continuous graphitized carbon core ensuring rapid electrons transport. The catalytic‐etching mechanism involving oxidation of Co and subsequent carbonthermal reduction is proposed and highly relies on annealing temperature and holding time. When directly evaluated as a current collector‐free capacitive deionization electrode, the porous carbon fibers demonstrates much superior desalination capability than pristine carbon fibers, and remarkable cyclic stability up to 20 h with negligible degeneration. Particularly, the PCF‐1000 showcases the highest areal salt adsorption capacity of 0.037 mg cm−2 among carbon microfibers. Moreover, monolithic porous carbon fibers‐carbon nanotubes with increased active sites and good structural integrity by in‐situ growth of carbon nanotubes are further fabricated to enhance the desalination performance (0.051 mg cm−2). This work demonstrates the great potential of carbon fibers in constructing high‐efficient and robust monolithic electrode for capacitive deionization.