Emerging MXene-based electrode materials for efficient capacitive deionization: A comprehensive review

Membrane capacitive deionization (MCDI) is a modification of capacitive deionization (CDI) using ion-exchange membranes (IEM) in front of the electrodes. Electrode properties, especially the specific surface area, are known to be strongly related with desalination performance in CDI, but the effects of other properties in MCDI are not fully understood. The objective of this study was to investigate the effect of hydrophilicity in activated carbon electrodes on desalination performance in MCDI. Two types of activated carbon (P60 and YS-2) whose specific surface areas were similar were used as electrode materials, but they had different hydrophilicity (i.e., P60 was originally hydrophobic and YS-2 was relatively hydrophilic due to its nitrogen-containing surface chemistry). These hydrophilic electrodes (either the electrode itself or modified with polydopamine (PDA)) led to an increase in the salt adsorption capacity (SAC) in MCDI because they facilitated the access of both ions and water molecules into the electrode pores. In particular, the SAC of the P60 electrode displayed a large increase to almost reach that of the YS-2 electrode due to the improved hydrophilicity with PDA modification and the insignificant effects of PDA modification on an already hydrophilic YS-2 electrode. Additionally, PDA-modified IEM in MCDI reduced the SAC as a result of the additional insulating PDA layer with little changes in hydrophilicity.

Three-dimensional cross-linked sugarcane bagasse carbon material: A substitute for graphene with excellent performance in capacitive deionization and highly efficient Cu2+ removal

Capacitive deionization (CDI) is a class of electrochemical desalination technologies which desalinate via ion storage in electric double-layers. CDI has received renewed attention in recent years due to the ability to couple energy storage with salt separation. During galvanostatic operation, a current is applied between porous carbon electrodes until a limiting voltage is reach. The cell is then discharged by applying a reverse current, generating brine and recovering stored charge. However, CDI desalination and energy efficiency can be limited by parasitic side reactions, and selective adsorption of counter-ions (anions at the positive electrode, and cations at the negative electrode). Several material additions and electrode configurations have been proposed to overcome these limitations, with the most prominent being the addition of ion exchange membranes (IEMs) promote counter ion flux out of the desalination chamber and incorporation of carbon slurry electrodes to increase system adsorption capacity. Likewise, functionalization of carbon electrodes has been studied to improve counter-ion adsorption within EDL micropores. While the incorporation of functionalized carbon, IEMs in membrane capacitive deionization (MCDI), and the use of slurry electrodes in flow capacitive deionization (FCDI) have successfully reduced energy consumption or increased ion adsorption capacity in CDI systems, these modifications are often evaluated under limited conditions on the basis of specific performance enhancements. Additional clarity is necessary to evaluate the associated performance and cost tradeoffs across the design space. In this study, an equivalent circuit (M)CDI model, with porous electrode sub-models, was used to measure the sensitivity of CDI performance to material selection, design, and operating choices. In order to investigate the performance of FCDI, pulse-flowed electrodes of high capacity and electronic resistances were incorporated into the existing model. Similarly, fixed charge in the anodic and cathodic micropores was studied to investigate functionalized carbon. Constrained system parameters were randomly selected via latin hypercube sampling (LHS) across multiple electrode geometries and influent salt concentrations. The resultant model outputs were then correlated with input values to quantify parameter sensitivity. Our sensitivity analysis shows that operating current density, electrode specific capacitance, contact resistance, and fixed charge density were the parameters which most significantly dictated (M)CDI performance. These parameters where then used to construct an operational space for CDI, MCDI, and FCDI. The results of the sensitivity analysis and technology comparison were used to evaluate the current effectiveness of CDI technologies as well as near-term projections for improvements in material performance. Through this analysis we are able to set targets for performance and material properties necessary for each CDI configuration to favorably operate.

With the gradually increasing requirement for freshwater, capacitive deionization (CDI) as a burgeoning desalination technique has gained wide attention owing to its merits of easy operation, high desalination efficiency, and environmental friendliness. To enhance the desalination performance of CDI, different CDI architectures are designed, such as membrane CDI, hybrid CDI, and flow-electrode CDI. However, these CDI systems have their own drawbacks, such as the high cost of membranes, capacity limitation of carbon materials and slurry blockage, which severely limit their practical application. Notably, rocking-chair CDI (RCDI) composed of symmetric electrode materials delivers excellent desalination performance because of its special dual chamber structure, which can not only break through the capacity limitations of carbon materials, but also deliver a continuous desalination process. Although RCDI showcases high promise for efficient desalination, few works systematically summarize the advantages and applications of RCDI in the desalination field. This review offers a thorough analysis of RCDI, focusing on its electrode materials, structure designs and desalination applications. Furthermore, the desalination performances of RCDI and other CDI architectures are compared to demonstrate the advantages of RCDI and the prospect of RCDI is elucidated.

Optimizing the energetic performance of capacitive deionization devices with unipolar and bipolar connections under constant current charging

Nitrogen-doped nanostructured carbons: A new material horizon for water desalination by capacitive deionization

Brackish groundwater is an invaluable yet overlooked water resource which can be leveraged to mitigate the unprecedented water scarcity experienced around the globe. Capacitive Deionization (CDI) represents a highly efficient and economical alternative to the common, energy intensive desalination methods for low salinity water streams. In CDI a small voltage is applied to separate ionic species from aqueous solutions and adsorb them into a highly porous material. Extraction of these ions lowers the solution concentration at the exit. The electrodes can then be discharged and regenerated by desorbing the adsorbed ions. CDI is a complex system comprised of several highly coupled transport mechanisms taking place at different length and time scales. In this work, a comprehensive theoretical and experimental study is conducted to further understand the underlying physics of CDI and to improve its the performance. First, I conduct a parametric study to investigate the interrelationship between the multiscale transport phenomena in CDI. The findings demonstrate that despite the superiority of the systems with high mass Péclet number in the desalination performance, units at low mass Péclet number show higher energetic efficiency. Based on these findings, next, I introduce an operational method aimed at enhancing the overall output of CDI. The numerical and experimental data prove the improved performance of the proposed configuration. I then present a new multi-cycle arrangement and investigate the effects of the regeneration feed stream in this cyclic configuration. The results indicate trade-offs between the desalination performance and energy efficiency of the proposed arrangements. Additionally, a dynamic two-dimensional model is developed incorporating the adsorption/desorption resistance in the porous electrodes. The obtained results accentuate the significant effect of this resistance limiting the transfer rate of the ionic species, especially in systems subject to ion starvation. Moreover, to achieve systems with better desalination and energetic performance, activated carbon electrodes with engineered microtopology are manufactured by employing magnetic field manipulation and are tested for desalination using a CDI cell. This work provides deeper insights into the transport mechanisms of CDI and allows for better guidelines on design and implementation of efficient systems with improved overall desalination performance.

Biomass-derived N-doped porous carbon as electrode materials for Zn-air battery powered capacitive deionization

Improvement of water softening efficiency in capacitive deionization by ultra purification process of reduced graphene oxide

MXenes, as a novel two-dimensional lamellar material, has attracted much attention. However, MXenes lamellar are prone to collapse and stacking under hydrogen bonding and interlayer van der Waals forces, which affects their electrochemical and capacitive deionization performance. A three-dimensional Ni-1,3,5-benzenetricarboxylate/Ti3C2Tx (Ni-BTC/Ti3C2Tx) composite electrode material was developed to enhance the electrochemical and capacitive deionization performance. The uniformly decorated Ni-BTC can prevent MXenes from aggregation and provide a large specific surface area and rich pore structure. As a substrate supporting Ni-BTC, MXenes can effectively disperse the growth of Ni-BTC and enhance the ion transport rate. In addition, the unique three-dimensional structure of Ni-BTC/Ti3C2Tx provides horizontal charge transfer paths like two-dimensional nanosheets and has unique vertical charge transfer paths between nanosheets. Therefore, the Ni-BTC/Ti3C2Tx exhibits an exceptional chromium(VI) removal rate of 94.1%. The electrosorption capacity of the Ni-BTC/Ti3C2Tx for chromium(VI) is 124.5 mg g−1, much higher than that of the pure Ti3C2Tx (55.5 mg g−1). The superior CDI efficiency accomplished through the Ni-BTC/Ti3C2Tx electrode is due to the unique three-dimensional network structure and synergistic effect of the pseudocapacitance generated by the unique assembly of Ni-BTC and Ti3C2Tx. Ni-BTC/Ti3C2Tx is a promising CDI electrode material that can be used for capacitive deionization.

Capacitive deionization (CDI) has shown great promise as a water treatment and conditioning technique. However, a large majority of studies using this technology have focused solely on NaCl removal, while other ions commonly found in natural waters might pose unaddressed challenges for CDI, such as specific adsorption, diffusional limitations and scaling precipitation. For instance, precipitation of inorganic salts such as calcium carbonate is an issue associated with brackish waters and RO plants. Similarly, high concentrations of sulfate and silicate salts in groundwater may lead to membrane scaling due to their low solubility [1]. Therefore, there is still a need to identify the effect of both individual ions and mixtures on CDI performance. In this work, calcium and sulfate sorption was studied using a CDI reactor outfitted with low-cost, high surface area carbons coated with two different metal oxides (SiO2 on the cathode and Al2O3 on the anode). The metal oxide coatings were chosen because of previous efforts in the literature showing improved electrode performance in the CDI process with the addition of these coatings [2-4]. Two calcium and two sodium salts (CaSO4, CaCl2, NaCl and Na2SO4) were tested to examine and compare the sorption characteristics of different anions and cations. Previous testing of this CDI system in a “Single Pass” (passing the water through the device only once) operational mode using a CaSO4 solution showed reductions in both Ca2+ and SO4 2- concentrations [5]. However, issues related with ion desorption and faradaic reactions (due to high cell potentials) were also observed. In this work, cycles of consecutive adsorption/desorption tests were performed to evaluate the long-term performance of this CDI system. Ion sorption and electrode stability were analyzed in terms of variables such as influent concentration, flow rate, and regeneration potential. These analyses also included performance metrics related with charge efficiency and energy consumption. For comparison purposes, CDI cells with uncoated carbon electrodes were evaluated following similar protocols. The results showed that increasing the concentration of CaSO4 in the feed solution (from 1.5 to 4.5 mM) increased the ion removal to values up to 3.29 ± 0.25 mg·g-1 and reduced energy consumption from 0.16 down to 0.09 kWh·mol-1. It was also observed that electric charge efficiencies increased by 15-30 % when asymmetrically coated configurations were employed. Nevertheless, the presence of SO4 2- significantly hindered salt desorption; conversely, recovery values close to 95 % were achieved using NaCl electrolyte. Finally, the evaluation of the long-term CDI performance revealed a slight but progressive loss in the ion electrosorption capability, probably due to the oxidation of the positive electrodes.

Co-Co3O4 encapsulated in nitrogen-doped carbon nanotubes for capacitive desalination: Effects of nano-confinement and cobalt speciation

Many studies have shown that capacitance deionization (CDI) has great potential in salt‐water treatment, one of the issues of great concern in many countries, especially Vietnam. The electrode material in CDI is one of the essential factors contributing to the desalination efficiency of this technology, so it is of research interest. In this study, TiO2 and TiO2/carbon nanotubes (CNTs) were synthesized from the sol‐gel process and utilized as an electrode for desalination. The composite materials were intensively characterized by X‐ray diffraction, Raman spectroscopy, scanning electron microscopy, energy‐dispersive X‐ray spectroscopy, Brunauer–Emmett–Teller and thermal analysis. The electrochemical properties were investigated using electrochemical impedance spectroscopy, cyclic voltammetry, and galvanostatic charge/discharge. The fabricated TiO2/CNTs nanocomposite electrode consisting of 1% CNTs (electrode T1) exhibited remarkable capacitance, conductivity, and durability; thus, it was employed as an electrode for desalination. With this electrode T1, the maximum salt adsorption capacity of 17.5 mg g−1, together with the highest charge efficiency of 90%, was achieved. Therefore, TiO2/CNTs can be considered a suitable electrode candidate for CDI technology.

Preparation optimization on the coating-type polypyrrole/carbon nanotube composite electrode for capacitive deionization

Design and optimization of electrode materials plays the pivotal role on the performance of capacitive deionization (CDI). Activated carbon (AC) has been a workhorse material for electrode fabrication in capacitive technologies. Several modification methods have been reported with enhanced activity and versatility attributes. Undeniably, tuning and tailoring AC properties have opened avenues for broadening the scope of applications, by meeting necessary features of electrodes for a given CDI cell configuration. This review traces the beneficial and also detrimental effects from various modifiers on AC electrodes with respect to CDI performance. Furthermore, a comprehensive classification of CDI cells based on different architectural aspects with a comparative performance is presented. On this basis, the tradeoff between physical, chemical, electrochemical properties in the course of electrode modification and the interdependence between electrode design and CDI cell configuration are discussed with disclosing some prospective guidelines on AC electrode design. It is important to evaluate the electrode materials and modifications in the way of practical including not only the electrode design, but also the cell architecture and operational parameters. This review aims to raise the attention on the rational electrode design by taking into account all necessary features of electrode in a given cell configuration.