Recent Advances in Capacitive Deionization: Research Progress and Application Prospects

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

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.

Capacitive deionization has been developed as a promising desalination alternative for removing ions from aqueous solutions. In this study, the evaluation of capacitive performance was carried out by galvanostatic charge/discharge and cyclic voltammetry experiments. The good capacitive and electrosorption behaviors suggest carbon aerogel not only treated as an electrical double layer capacitor, but also as a potential electrode in capacitive deionization processes. Also, the capacitive deionization characteristics indicate that electrosorption/regeneration can be controlled by polarization and depolarization of each electrode. It implies that sodium and chloride ions are electrostatically held to form electrical double layer on the surface of charged electrodes. The electrosorption performance at different applied voltages and solution concentrations was investigated. It is found that the removal of sodium chloride increases with increasing applied voltage and solution concentration, resulting from stronger electrostatic interactions, higher concentration gradient, and less double layer overlapping effect. Based on Langmuir isotherm, the equilibrium electrosorption capacity at 1.2 V is determined as 270.59 μmol/g. Under this condition, due to the presence of micropores associated with the double layer overlapping, the effective surface area for electrosorption of ions at 1.2 V is estimated in the range of 12.18–14.25 % of the Brunauer–Emmett–Teller surface area. The results provide a fundamental understanding of electrosorption of ions and help promoting capacitive deionization technology for water purification and desalination.

Capacitive deionization (CDI) technology is a challenge on an economical and effective electrosorption desalination method. The paper analyses the CDI current status and progress of carbon electrode materials, and describes the types of CDI and its performances of testing materials. The electrosorption capacities are summarized on the carbon electrode materials and the current hurdles. The reported numbers from the literature vary in a wide range between 0.25 and 26.42 mg/g of both electrodes CDI cell, we suggest that the CDI electrodes should have an adsorption of at least 9.0 mg/g NaCl when the applyed voltage is 2.0 V. The potential capacitive deionization technologies are proposed here.

Recent progress in graphene-based and ion-intercalation electrode materials for capacitive deionization

Hierarchical hybrids with microporous carbon spheres decorated three-dimensional graphene frameworks for capacitive applications in supercapacitor and deionization

Capacitive deionization is proven to be a viable resource-efficient alternative to remove dissolved salts from water. Further improvements in electrode materials are, however, necessary to achieve a higher efficiency of water desalination. In this work, X-Fe (X = Mn, Co, Cu) Prussian blue analogues of homogeneous particle sizes were grown on electrically conducting activated carbon cloth electrodes by a simple and facile synthesis process. As two-dimensional/three-dimensional (2D/3D) nanostructured Prussian blue analogue materials present favorable electrochemical behavior and ion-exchange possibilities, they are promising for application in capacitive deionization. The modified carbon cloth electrodes exhibit higher salt adsorption capacity and better charge efficiency compared to those constructed with pristine carbon cloth. Cobalt hexacyanoferrate-modified electrodes show the most stable performance, reaching a salt adsorption capacity of 14.47 mg g–1 at 1.0 V, accompanied by the lowest energy consumption (0.389 kWh m–3). This simple strategy is expected to offer possibilities for designing and synthesizing advanced electrode materials with superior performances for application in capacitive deionization devices.

Application of capacitive deionization (CDI) technology to insulin purification process

Comparison of energy consumption in desalination by capacitive deionization and reverse osmosis

Water eutrophication driven by nitrates (NO3 -), phosphates (PO4 3-), and ammonium salts (NH4 +), along with groundwater contamination caused by fluorides (F-) and bromides (Br-), collectively poses significant threats to both ecological systems and human health. Capacitive Deionization (CDI) has demonstrated significant potential for selectively removing these pollutants due to its low energy consumption, absence of chemical byproducts, and cost-effectiveness. However, systematic mechanistic analyses and material design strategies focusing on this specific area remain insufficient. To this end, against the backdrop of water eutrophication and groundwater contamination, this paper presents a relatively comprehensive review of the research progress regarding the removal of NO3 -, PO4 3-, NH4 +, F-, and Br- using CDI technology. The review first discusses the advantages and limitations of conventional methods for removing these pollutants. It elaborates on the ion storage mechanism of CDI and addresses its fouling mechanisms and mitigation strategies. Subsequently, for different ions, the review systematically sorts out various types of electrode materials and explains in detail the intrinsic mechanisms through which they achieve ion selectivity. Finally, the review discusses current challenges in CDI technology, such as ion selectivity, electrode fouling, complexity of real water matrices, cost, and long-term stability, and proposes specific future research directions.

Capacitive deionization, CDI, is considered as a versatile and promising electrochemical technology useful for both water desalination and remediation.1 CDI works under operational conditions equivalent to the charging/discharging steps in a generic electric double-layer capacitor (EDLC). Thus, it allows reducing ion concentration of an aqueous stream by the electrosorption of ions in porous electrodes and formation of the EDL, resulting in simultaneous salt withdrawal and energy accumulation. Once the deionization (charging) step is completed, the ions can be desorbed in regeneration (discharging) step. This leads to energy release and leaves the electrode surface available for the next deionization cycle. Accordingly, CDI is considered a simple, non-energy intensive method to produce clean water.2 In this work, a novel current collector-free electrode for CDI devices is designed based on electrodes consisting of a porous metal oxide (MOx) network interpenetrated into porous fibres of carbon nanotubes (CNTf). The fabrication of electrodes is based on the continuous impregnation of CNTf with MOx precursors in-line as they are spun from the chemical vapour deposition reaction. SiO2 and γ-Al2O3 were chosen, which have the following main benefits: i) developing an electrostatic EDL surface potential in the pH range of seawater and drinking water; ii) increasing specific surface area; iii) enabling wetting of electrodes by aqueous electrolytes.3 The content of MOx can be varied by adjusting the concentration of sol in the dispersion. γ-Al2O3/CNTf and SiO2/CNTf electrodes from 6 wt.% to 60 wt.% of MOx have been manufactured and characterized (TG-analysis, electrical conductivity, N2 adsorption isotherms, SEM and TEM images and RAMAN spectroscopy). The hybrid structure consisting of two interconnected porous networks with a uniform distribution of MOx on the supporting CNTf (see Fig 1a and Fig 1b) leads to high capacitance while reducing internal resistance (see Fig 1c), as confirmed by the electrochemical characterization (cyclic voltammetry, charge-discharge and electrochemical impedance spectroscopy). The use of a relatively simple fabrication process, particularly the infiltration of fabrics with sol particles in-line during fibre spinning, enables fabrication of large electrode samples and testing of a full CDI flow cell for brackish water desalination, 2.0 gNaCl L-1 (see Fig 1d). The full current collector-free CDI cell (see Fig 1e and Fig 1f), comprising a stack of γ-Al2O3/CNTf and SiO2/CNTf anodes and cathodes, respectively, has a large salt adsorption capacity of 6.5 mg g-1 and very high efficiency of 86%, which translates into a low energy consumption per gram of salt removed, 0.26 W h g-1. This is an 80% improvement compared with reference devices based on activated carbon and titanium foil current collector electrodes. The exceptional desalination properties (see Fig 1g) are directly linked to the interconnected nanoparticle MOx/CNTf hybrid network. Accordingly, we believe that these CNTf-based electrodes fabricated in a large scale could contribute to a significant improvement in the engineering manufacturing process of desalination reactors through more energy-efficient systems. Considering the high flexibility in bending of these hybrid electrodes, the possibility to build current collector-free desalination devices with complex non-planar shapes has also been opened up. References M. A. Anderson et al. Capacitive deionization as an electrochemical means of saving energy and delivering clean water. Comparison to present desalination practices: Will it compete?, Electrochim. Acta, 55, 3845–3856 (2010). http://dx.doi.org/10.1016/j.electacta.2010.02.012.M. E. Suss et al., Water desalination via capacitive deionization: what is it and what can we expect from it?, Energy Environ. Sci., 8, 2296–2319 (2015).http://dx.doi.org/10.1149/2.0271805jes.J. J. Wouters et al., Influence of Metal Oxide Coatings, Carbon Materials and Potentials on Ion Removal in Capacitive Deionization J. Electrochem.Soc., 165, 148–161 (2018). http://dx.doi.org/10.1149/2.0271805jes. Figure 1

Controllable synthesis of a hollow core-shell Co-Fe layered double hydroxide derived from Co-MOF and its application in capacitive deionization

The application of electrochemical energy storage materials to capacitive deionization (CDI), a low-cost and energy-efficient technology for brackish water desalination, has recently been proven effective in solving problems of traditional CDI electrodes, i.e., low desalination capacity and incompatibility in high salinity water. However, Faradaic electrode materials suffer from slow salt removal rate and short lifetime, which restrict their practical usage. Herein, a simple strategy is demonstrated for a novel tubular-structured electrode, i.e., polyaniline (PANI)-tube-decorated with Prussian blue (PB) nanocrystals (PB/PANI composite). This composite successfully combines characteristics of two traditional Faradaic materials, and achieves high performance for CDI. Benefiting from unique structure and rationally designed composition, the obtained PB/PANI exhibits superior performance with a large desalination capacity (133.3 mg g-1 at 100 mA g-1 ), and ultrahigh salt-removal rate (0.49 mg g-1 s-1 at 2 A g-1 ). The synergistic effect, interfacial enhancement, and desalination mechanism of PB/PANI are also revealed through in situ characterization and theoretical calculations. Particularly, a concept for recovery of the energy applied to CDI process is demonstrated. This work provides a facile strategy for design of PB-based composites, which motivates the development of advanced materials toward high-performance CDI applications.

Developing cost-effective brackish water and seawater desalination technology is crucial. Capacitive deionization (CDI) and inverted capacitive deionization (i-CDI) have been recognized as a promising desalination technology with low energy consumption for brackish water.Both systems use an electric field between two porous electrodes to remove and release ions in water reversibly.Recently, CDI has started using pseudocapacitive or battery electrode materials to enhance the desalination performance.Different from the desalination principle of electric double layers, the energy storage mechanisms of pseudocapacitive and battery materials generally involve ion intercalation/adsorption or compound formation for charge balance, leading to the faradaic desalination.Low cost and high theoretical capacity make PBAs and conducting polymers be the potential materials for the faradaic desalination application.In our previous studies, two dissimilar pseudocapacitive materials show a memory effect during brackish water desalination. This allows them to retain ion capturing or releasing states without an electric field, aiding water purification and resource recovery.Based on above viewpoints, two materials with fundamentally different electrochemical properties are demonstrated to construct a high-capacity, hybrid, faradaic deionization system with CuHCF (battery type) as the positive electrode and PPy (pseudocapacitive type) as the negative electrode.The deionization performance of this CuHCF//PPy cell can be improved by reversing the appropriate cell voltage during the discharge process.The plot of specific SRC against time for the above CuHCF//PPy cell with variations in the charging cell voltage but a fixed discharging cell voltage of 0 V. In addition, Fig. 1(b) shows the plot of specific SRC against time for the same cell with variations in the discharging cell voltage but a fixed charging cell voltage of 1.2 V. From Fig. 1(a), at all charging voltages, the SRC generally decreases with the charging time, indicating the ion repelling process. The order of charging cell voltage with respect to increasing the SRC value is: 0.6 V < 0.8 V < 1.0 V < 1.2 V, revealing the impact of the charging cell voltage. From Fig. 1(b), the SRC obviously increase with prolonging the discharging time, suggesting the ion capturing process. However, the order of discharging cell voltage with respect to increasing the SRC value is: -0.4 V < 0 V < −0.1 V< −0.2 V < −0.3 V. Note that a little inverted cell voltage leads to a higher salt-removing capacity and rate in comparison with a discharge cell voltage of 0 V. However, when the inverted voltage is set at −0.4 V, the SRC profile exhibits the unstable performance, probably due to the presence of certain irreversible reactions at this cell voltage.In the stability test, Fig 2 shows the CuHCF//PPy cell still maintained more than 90% of its original SRC after 50 cycles of testing.The mean SRC values of this CuHCF//PPy system obtained from the 8, 15, and 30 mM solutions reached 35.536, 58.824, and 101.84 mg g−1, respectively.This positive correlation between SRC and solution concentration reveals the higher utilization of the electroactive materials in more concentrated solutions and the very high SRC of the hybrid faradaic CuHCF//PPy system.From Fig 3 shows the SRC values of a CuHCF//PPy cell with the charging/discharging times of 30/30 min at the charging/discharging cell voltages of 1.2/−0.2 V in the 8, 15, and 30 mM NaCl solutions.The memory effect of electrochemically active materials can further extend the application of this system to concentrating valuable ions while purifying water, showing another advantage.The results of ion-removing and salt-concentrating experiment with the discharge/charge times = 10 min/10 min for 10 cycles. In this 10-cycle test, the total amount of salts transferred is up to 152.9 mg g−1, revealing the dual function of purifying water and concentrating salts through this hybrid battery//pseudo-capacitive system.This hybrid cell showed high salt removal capacities in the media containing various monovalent and divalent cations. In this work, the suitable working potential windows of both CuHCF and PPy were systematically evaluated by CV and GCD methods with the charge balance application. Moreover, this cell provides the ability in capturing other cations such as Mg2+ and Ca2+, further broadening its future potential applications. This methodology is a promising strategy for constructing a high-performance desalination cells consisting of various active materials. Figure 1

Highly flexible, self-healable and conductive poly(vinyl alcohol)/Ti3C2Tx MXene film and it’s application in capacitive deionization