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Capacitive Deionization (CDI) technology achieves ion immobilization through a reversible electrochemical process, gradually emerging as a substitute for conventional desalination techniques due to its simplicity, efficiency, and energy-saving advantages. Materials design has played a pivotal role in the continuous refinement and development of the CDI theoretical framework, with substantial efforts directed towards the design of high-performance novel electrode materials. Among them, Covalent Organic Frameworks (COFs) have been explored as exceptionally promising candidates. COFs represent a burgeoning class of porous crystalline materials, characterized by pronounced intrinsic features encompassing high tunability, highly ordered pore structures, elevated surface areas, and an abundance of functional sites. Consequently, it becomes more facile to tailor channels with facile ion transport and immobilization, facilitating the seamless integration of ion-selective electrochemical redox-active sites. This ultimately fulfills the design objectives for CDI materials. This comprehensive review aims to systematically and comprehensively explore the underlying design fundamentals, principles and frontiers of COFs materials for CDI. Specifically, discussions encompass the three major research challenges of enhancing intrinsic conductivity, suppressing co-ion effects and Faradaic side reactions, and delving into mechanistic studies. These insights provide valuable guidance for the future design of sustainable, high-performance, and ion-selective capacitive deionization electrode materials. [ABSTRACT FROM AUTHOR]

Novel two-dimensional carbon nitride hexagonal-like sheets were fabricated from thermal condensation, which uses hexaazatriphenylene-based precursor prepared microporous and oxidation resistant carbons materials (HAT-CN). The results show that the as-prepared HAT-CN is the first electrode utilized in capacitive deionization, due to the rich heteroatom and border-group content, as well as the potential collaborative effect of C and N atoms, displaying the quick ion diffusion and strong charge transfer performance. Additionally, the capacitive deionization performance of HAT-CN is improved by the synergistic contribution of a large accessible surface area, large pore volume, and high graphitization. The electrode of HAT-CN was carbonized at 550 °C which display an excellent specific capacitance of 179.2 F g−1 in 1-M NaCl solution at a scan rate of 5 mV s−1. Subsequently, a high salt adsorption capacity of 24.66 mg g−1 was attained for an applied voltage of 1.2 V in 500 mg L−1 NaCl solution, showing good cyclic stability at 30 cycles. Considering those excellent properties of the prepared HAT-CN, the capacitive deionization electrode should be an ideal candidate for high-performance deionization application. [ABSTRACT FROM AUTHOR]

Capacitive deionization (CDI) has been used to remove salt ions with carbon electrodes by the electrical double layer (EDL) mechanism. Other non-carbon materials based on pseudocapacitive mechanism resulting from faradaic reactions with fast electrosorption/desorption properties have emerged as new promising electrode materials. Until now, pseudocapacitive behavior was limited to thin film electrodes with nanometer thickness and was not yet suitable for practical application. In this work, we employed a two-times of hydrothermal synthesis process constructing MnFe 2 O 4 /Porous rGO (MFO/PrGO) nanostructure as pseudocapacitive electrodes. In this carefully designed architecture, the dense structure of MFO/rGO prepared by first-time hydrothermal process provided good connections of MFO nanoparticle with rGO framework for efficient redox charge transfer; the chemically-etched porous rGO used in the second-time hydrothermal process offered hierarchical pores for easier access of the electrode materials, both contributed in enhanced pseudocapacitive electrosorption. HRTEM verified the MnFe 2 O 4 spinel structure and measured the inter-planar lattice parameter which functioned for ion intercalating in and out of the electrode. The MFO/PrGO electrode resulted in 237 F/g in electrosorption of ions from aqueous solution which was higher than the MFO only electrode. Finally, in the CDI experiment, the great salt electrosorption capacity of 8.9 mg/g and 100% regeneration in multiple cycles were achieved by the MFO/PrGO nanocomposite electrode. These results paved the way for using spinel compounds and graphene nanocomposite electrodes for efficient electrosorption and desalination. [ABSTRACT FROM AUTHOR]

Abstract In capacitive deionization (CDI) systems, more Faradaic reactions generally occur at a higher cell voltage, resulting in the poor ion-removal efficiency and high energy consumption, but a low cell voltage may lead to a low deionization capacity. In this study, an integrated strategy, consisting of (1) selecting the acceptable charge/discharge time ratio, (2) finding the potential windows of both positive and negative electrodes, and (3) using the charge-balanced method for determining the optimal mass ratio between the positive and negative electrodes (denoted as (m+)/(m−)), is proposed to find the optimal cell voltage of a CDI system with a high desalination efficiency and low energy consumption. According to this strategy, two newly designed cells using two types of activated carbon (AC) show excellent CDI performances; cell 1 with AC = ACS679, (m+)/(m−) = 1:1.4, and cell voltage = 1.4 V: salt adsorption capacity of 12 mg g−1, charge efficiency of 62%, and energy consumption of 109 kJ mol−1 and cell 2 with AC = ACS25, (m+)/(m−) = 1:1, and cell voltage = 1.2 V: salt adsorption capacity of 13 mg g−1, charge efficiency of 81%, and energy consumption of 72 kJ mol−1 although the potential window of water decomposition for such AC-coated electrodes in 8 mM NaCl is about 2 V. This universal strategy is applicable to all CDI systems utilizing various electrode materials. Graphical abstract Image 1 Highlights • We demonstrate a universal strategy for constructing the AC//AC cell with high CDI performances. • The same charge/discharge time ratio with high reversibility on both electrodes is the key. • Influences of two-electrode mass ratio on the desalination performances are studied. • The pH change is affected by the Faradaic reactions from the potential windows of electrodes. [ABSTRACT FROM AUTHOR]

Abstract Covalent triazine-based frameworks (CTFs) with high nitrogen content, porous structures and low electric resistance were synthesized and applied as membrane capacitive deionization (CDI) electrodes for the first time. The obtained CTFs exhibit abundant micropore structure which provide adsorption site for ion and certain hydrophilicity ensure the whole pore volume is participating. The electrodes of p -CTFs show ideal capacitive behavior with the maximum specific capacitance values of 122.63 F g−1 at a scan rate of 1 mV s−1 under 1 M NaCl solution. Besides, the further membrane capacitive deionization by batch mode experiment exhibits excellent electrosorption capacity of 29.34 mg g−1 under 1.2 V in a 1000 mg L−1 NaCl solution and stable regeneration performance. With the admirable properties mentioned above, the prepared CTFs can offer great opportunities as promising CDI electrodes for application of high-performance desalination. [ABSTRACT FROM AUTHOR]

Capacitive deionization (CDI) is an emerging electrochemical desalination technique for the energy- efficient removal of dissolved ions from aqueous solution. This is a first research attempt which describes the influence of dye molecules on capacitive deionization of salt solution. In this regard, a CDI flow cell has been fabricated and tested in order to scrutinize the electrosorptive removal of three different dye molecules such as amido black 10B (AB) (acidic dye), eosin yellow (EY) (neutral dye) and methyl violet (MV) (basic dye) from synthetic aqueous solutions. The electrosorption capacitance was analyzed by cyclic voltammetry cell and CDI flow cell using activated carbon cloth (ACC) electrodes with 1 cm 2 and 24 cm 2 surface areas respectively. The capacitance values of 106 and 99 F/g correspondingly were obtained for a steady-state CV and CDI flow cell with 50 mM Na 2 SO 4 electrolyte solution. In addition to this, the dye removal efficiency was also examined by a CDI flow cell for the solution containing 10 ppm of dye and 500 ppm of Na 2 SO 4 . The experimental results substantiate that EY exhibits strong adsorption during charging and strong desorption during discharge cycle when compared with other two dye molecules (AB & MV). Conclusively, electrosorption of dye molecules at the carbon cloth electrodes surface was found in the following order: EY > AB > MV. [ABSTRACT FROM AUTHOR]

Desalination performance of capacitive deionization (CDI), a promising deionization technology, depends significantly upon the compositions and internal structures of electrode materials. Herein, hollow ZIFs-derived nanoporous carbons (HZCs) were prepared via chemical etching method and subsequent pyrolysis. Particularly, the tannic acid as an etching agent was employed to tailor hollow structure. The resultant HZCs possess high surface area, wide pore size distribution, obvious hollow cavity and high nitrogen content. Moreover, HZCs electrodes exhibited a significantly improved CDI performance over the solid ZIFs-derived nanoporous carbons (SZCs) electrodes. The improved electrosorption rate and capacity (15.31 mg g -1  at 1.2 V) of HZCs electrode imply that hollow structure facilitates the transportation for both ions and mass on CDI process. Besides, the further cycle experiment exhibits a great regeneration stability of HZCs electrode over 20 adsorption-desorption cycles. All these results indicate that HZCs should be a promising candidate for CDI application. [ABSTRACT FROM AUTHOR]

Capacitive deionization (CDI) is a promising technology for the desalination of brackish water due to its potentially high energy efficiency and its relatively low costs. One of the most challenging issues limiting current CDI cell performance is poor cycling stability. CDI can show highly reproducible salt adsorption capacities (SACs) for hundreds of cycles in oxygen-free electrolyte, but by contrast poor stability when oxygen is present due to a gradual oxidation of the carbon anode. This oxidation leads to increased concentration of oxygen-containing surface functional groups within the micropores of the carbon anode, increasing parasitic co-ion current and decreasing SAC. In this work, activated carbon (AC) was chemically modified with titania to achieve additional catalytic activity for oxygen-reduction reactions on the electrodes, preventing oxygen from participating in carbon oxidation. Using this approach, we show that the SAC can be increased and the cycling stability prolonged in electrochemically highly demanding oxygen-saturated saline media (5 mM NaCl). The electrochemical oxygen reduction reaction (ORR) occurring in our CDI cell was evaluated by the number of electron transfers during charging and discharging. It was found that, depending on the amount of titania, different ORR pathways take place. A loading of 15 mass% titania presents the best CDI performance and also demonstrates a favorable three-electron transfer ORR. [ABSTRACT FROM AUTHOR]

Capacitive neutralization deionization with flow electrodes (FCND) was proposed by combining neutralization dialysis (ND) and flow-electrode capacitive deionization (FCDI). Its key property is that by employing flow electrodes, capacitive adsorption of salt ions occurs simultaneously during the neutralization dialysis process. To compare the desalination performance between FCND and FCDI, a series of experiments were conducted by changing salt concentration and applied electrical voltage. The hybrid exhibits a promising average salt adsorption rate (ASAR) and salt removal efficiency (SRE) compared to either ND or FCDI process. The ASAR and SRE of FCND are 203.27 mmol m −2 min −1 and 72.20% in 0.1 mM NaCl solution at 1.2 V after 120 min, whereas the values of FCDI are 120.86 mmol m −2 min −1 and 42.92%, respectively. In order to facilitate the understanding of the ion transportation in this complex system, a theory was also set up based on simplified Nernst-Planck equation and Donnan equilibrium, which helps to understand the experimental data. [ABSTRACT FROM AUTHOR]

Graphene is a promising electrode material for high performance supercapacitors and capacitive deionization applications. Our present work is focused on the synthesis of graphene nanosheets by the reduction of graphite oxide using focused solar irradiation. The as prepared graphene nanosheets are characterized by Transmission Electron Microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy, X-ray Photoelectron Spectroscopy (XPS) and Fourier transform infrared spectroscopic (FTIR) techniques for surface, structural and functional group analysis. Electrochemical properties are studied using cyclic voltammetry and galvanostatic charge-discharge method. The fabricated supercapacitor cell shows high specific capacitance of 265.9 and 187.1 F g −1 with 6 M KOH and ionic liquid electrolyte (EMIMBF 4 ), respectively. High energy density of 58.5 Wh Kg −1 has been obtained for EMIMBF 4 electrolyte. The electrosorption studies were also performed to study the effect of deionization behaviour of the prepared graphene under flow in between electrodes technique. The electrosortion capacity of 22.4 mg/g has been obtained for 100 mM NaCl (∼5844 mg/l) solution at the applied voltage of 1 V. [ABSTRACT FROM AUTHOR]

Capacitive deionization (CDI) is an emerging technology that supplies deionized water to resolve the fresh water shortage. CDI electrodes are mainly made up of carbon materials, of which the deionization performance is closely related to their physical properties and structures. Hence, a rational design of electrode material structure is essentially significant. Functionalized graphene (fG) in particular has recently been regarded as characteristic CDI electrode material. However, preparation of fG based on graphene oxide usually results in serious secondary pollution due to usage of highly poisonous chemicals, and thus still cannot meet the demand of practical application. It is feasible that environmentally-friendly activated carbon (AC) and small amounts of fGs can be combined rationally, and used as CDI electrodes. Here, sono-assembled AC/m-phenylenediamine (mPEA) or p-phenylenediaminefG inter-connected network architecture has been constructed for the first time successfully. The specific capacitances of the AC/fG composites were found to be significantly higher than that of the AC electrode owing to mesoporous generation. Also, among all the samples, the AC composite with 5 wt % mPEA-fG exhibited an ultrahigh electrosorption capacity of 12.58 mg/g (or 0.22 mmol/g) in NaCl solution. These observations indicate that fG can serve as an efficient conductive bridge to decrease the aggregation of AC particles, and improve the electron transfer with the composite electrode. This work provides an effective strategy for the environmental and economical electrode architectures for general applications in CDI and energy storage. [ABSTRACT FROM AUTHOR]

• La-doping HAP (La-HAP) was synthesized by hydrothermal method in the presence of LaCl 3. • LaHAP/3D-rGO composites with various ratios were obtained by simple ultrasonic mixing. • 50LaHAP/3D-rGO possesses good conductivity and high hydrophilicity. • 50LaHAP/3D-rGO delivers F− adsorption of 1132 μmol g−1 with high selectivity by CDI. • The selective removal of F- in actual high-fluorine groundwater is achieved. Excessive fluoride in water is severely hazardous to environment and human health. It is of great significance to exploit effective methods for defluoridation from water. In this paper, lanthanum-doped hydroxyapatite modified three-dimensional reduced graphene oxide (LaHAP/3D-rGO) was successfully synthesized with LaHAP uniformly distributed on the 3D network. Electrochemical characterization demonstrates the improvement of the conductivity and ion diffusion rate after doping appropriate amount of LaHAP. With LaHAP/3D-rGO as positive electrode (activated carbon as negative electrode), the corresponding capacitive deionization (CDI) system is successfully applied for the selective adsorption of fluoride. With the appropriate LaHAP/3D-rGO ratio (50%), the obtained LaHAP/3D-rGO electrode achieves the optimal F− adsorption amount of 1132 μmol g−1. The corresponding CDI cell shows excellent selectivity and high cycle stability for adsorption of fluoride. When applied to the fluoride removal in the fluoride-containing groundwater through a 3-cell CDI stack, the electrode attains the high F− removal rate of 90.5%, with the effluent concentration down to 0.32 mg L−1, much lower than the permissible limit of World Health Organization. This LaHAP/3D-rGO based CDI system is much promising for water purification of the high F− groundwater. [Display omitted] [ABSTRACT FROM AUTHOR]

Here a facile strategy was proposed to prepare graphene (GE) sheets with unique mesoporous morphology (denoted as mGE) via a simple carbothermal reaction by using KMnO 4 as the etching agent. The morphology, structure and electrochemical properties of mGE were investigated by scanning electron microscopy, transmission electron microscopy, Raman spectra, nitrogen adsorption-desorption, cyclic voltammetry and electrochemical impedance spectroscopy. The electrosorption performance of mGE in NaCl solution was studied and compared with GE. The results show that mGE electrode exhibits a high electrosorption capacity of 6.38 mg g −1 when the initial NaCl concentration is ∼75 mg L −1 and applied voltage is 1.2 V, which is about 2.3 and 1.5 times of those of GE (2.76 mg g −1 ) and recently reported microporous GE (∼4 mg g −1 ), respectively, and also higher than those of other carbon materials in the literatures, due to short ion diffusion path, large accessible surface area and low charge transfer resistance offered by its novel mesoporous structure. Therefore, it is believed that mGE is a promising candidate for practical capacitive deionization applications. [ABSTRACT FROM AUTHOR]

Carbon aerogels doped with manganese or iron oxides were prepared by the resorcinol-formaldehyde method as potential electrodes for the capacitive deionization of sodium chloride from saline water. The solution containing the metal precursors was mixed with the hydrogel before the supercritical drying step ensuring a highly homogeneous dispersion of the nanometric metal oxides as determined by X-ray diffraction and electron microscopy. XPS spectra revealed the increased contribution of hydroxyl and carboxylic groups after the activation process. Also, an enhanced metal oxidation at the outer particle surface was observed. Nitrogen isotherms have revealed a decrease of the pore volume in doped samples ascribed to the partial blocking of the pore structure by the embedded nanometric metal oxide particles. Capacitance values as large as 99 F/g and 91 F/g were respectively recorded for CAGDFeAct and CAGDMnAct by cyclic voltammetry. The large capacitance detected for the iron containing activated aerogel was confirmed by deionization experiments. An electrosorption capacity of 0.133 mmol/g was recorded for CAGDFeAct in a 0.025 M NaCl solution when 1.5 V were applied during the charge period. The study of the kinetic response of the electrodes showed a poor behavior for CAGDMnAct and was correlated to the partial blocking of pore structure exerted by the embedded nanometric metal oxide particles. [ABSTRACT FROM AUTHOR]

• The first high entropy oxides based multifunctional electrode material is synthesized by simple grinding technique. • Nano-sized (< 10 nm) high entropy oxides are incorporated in surface functionalized biowaste derived activated carbon. • High salt adsorption capacity of 241 mg g−1 and cycle repeatability upto 20 cycles is demonstrated. • The excellent desalination capacity is attributed to the combined electric-double layer and pseudocapacitive mechanism. Nano-sized high entropy oxides (particle size < 10 nm) are expected to possess enhanced stability, electrochemical properties and increased potential in versatile applications than bulk metal oxides. In this study, nano-sized high entropy oxides incorporated surface functionalized coconut shell derived activated carbon (HEO/f-CSAC) is synthesized, characterised and used as an advanced multifunctional material for electrochemical energy storage and desalination. Attributed to its unique microstructure and extraordinary properties, HEO/f-CSAC shows an excellent salt adsorption capacity of 241 mg g−1 at 1.2 V in 500 mg L−1 concentration of NaCl solution, which is very high as compared to previous reports. The reason behind high salt adsorption capacity is its outstanding specific capacitance of 147.5 F g−1 at current density 1 A g−1. Moreover, the assembled lab-scale CDI unit demonstrates extremely good performance upto 20 adsorption-desorption cycles due to the long cycle life with good capacitance retention of fabricated HEO/f-CSAC based two-electrode supercapacitor. Additionally, a red light-emitting diode illumination by connecting three supercapacitors in series, indicates the high efficiency and potential of HEO/f-CSAC for real life energy-related application. These results prove that HEO/f-CSAC can be implemented for water desalination with enhanced supercapacitive performance manifesting its multifunctionality for energy and environmental applications. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: An oxygen plasma treatment was employed to modify the surface of carbon electrodes used in capacitive deionization (CDI). X-ray photoelectron spectroscopy analysis of samples showed that oxygen plasma is mainly attaching oxygenated groups on the PTFE binder used in these electrodes. By functionalizing the binder it can increase the hydrophilicity of the electrode surface and increase the available specific surface area. 2.5min of plasma treatment resulted in the largest improvement of CDI performance of electrodes. Thermodynamic study of CDI performance showed that the modified electrodes followed Langmuir and Freundlich isotherms resulting from the increased interaction between the enhanced electrodes and water. The kinetic study showed that the CDI process followed a pseudo-first order adsorption kinetics. The calculated adsorption rate constants suggested that plasma modification can accelerate ion adsorption of electrodes. [Copyright &y& Elsevier]

Abstract: The nanostructured anatase titanium dioxide/activated carbon composite material for capacitive deionization electrode was prepared in a short time by a lower temperature two-step microwave-assisted ionothermal (sol–gel method in the presence of ionic liquid) synthesis method. This method includes a reaction and a crystallization step. In the crystallization step, the ionic liquid plays a hydrothermal analogy role in driving the surface anatase crystallization of amorphous titanium dioxide nanoparticles formed in the reaction step. The energy dispersive spectroscopic study of the composite indicates that the anatase titanium dioxide nanoparticles are evenly deposited in the matrix of activated carbon. The electrochemical property of the composite electrode was investigated. In comparison to the pristine activated carbon electrode, higher specific capacitance was observed for the nanostructured anatase titanium dioxide/activated carbon composite electrode, especially when the composite was prepared with a molar ratio of titanium tetraisopropoxide/H2O equal to 1:15. Its X-ray photoelectron spectroscopic result indicates that it has the highest amount of Ti-OH. The Ti-OH group can enhance the wetting ability and the specific capacitance of the composite electrode. The accompanying capacitive deionization result indicates that the decay of electrosorption capacity of this composite electrode is insignificant after five cycle tests. It means that the ion electrosorption–desorption becomes a reversible process. [Copyright &y& Elsevier]

Abstract: Capacitive deionization (CDI) is an emerging water desalination method, which employs high surface area porous electrode materials for electro-sorption of ions. We used an asymmetric CDI cell constructed with alumina and silica nanoparticle (NP) coated electrodes and KCl as a probe electrolyte to gain insights into electro-sorption behavior and elucidate underlying process mechanisms. This CDI system is efficient for use in desalination and up to 15 to 60μmol/g (total electrode) sorption capacity was achieved. Higher removal of K+ compared to Cl− was obtained attributable to competition between OH− and Cl−. The presence of NPs not only creates highly accessible surface area but also increases the charge efficiency by shifting the applied potential to a high efficiency range due to protonation/deprotonation occurring on metal oxide surfaces. Data were described using both mechanistic electrical double layer (EDL) based Gouy–Chapman–Stern (GCS) formulation and empirical Freundlich equations. Our results suggest that the presence of metal oxide NPs can effectively modify the isoelectric points and an increase in planar charge efficiency of up to 20% could be achieved. However, global charge efficiency was still severely constrained by backward thermal diffusion and mass transfer limitations. EDL over-lapping effect plays an important role in determining critical pore size for electro-sorption. The GCS model revealed that only BJH associated (pore size: 1.5–50nm) surface area is effective for electro-sorption. Though the Freundlich equation adequately described the sorption data, we attribute this sorption behavior to reduction in micropore overlapping effects and an increase in mass transfer caused by higher concentration gradients. Our results are expected to guide the design and development of appropriate electrode materials for CDI. [Copyright &y& Elsevier]

Abstract: In capacitive deionization (CDI) water is desalinated by applying an electrical field between two porous electrodes placed on either side of a spacer channel that transports the aqueous solution. In this work we investigate the equilibrium salt adsorption and the dynamic development of the effluent salt concentration in time, both as function of spacer and electrode thicknesses. The electrode thickness will be varied in a symmetric manner (doubling both electrodes) and in an asymmetric manner, by doubling and tripling one electrode but not the other. To describe the structure of the electrostatic double layer (EDL) which determines the salt adsorption in the micropores of activated carbons, a modified Donnan-model is set up which successfully describes the data, also for situations of very significant electrode thickness ratios. We develop a generalized CDI transport model accounting for thickness variations, which compares favorably with experimental data for the change of the effluent salt concentration in time. These experiments are aimed at further testing our equilibrium and transport models, specifically the assumption therein that in first approximation, for electrodes made of chemically unmodified activated carbon particles, the EDL structure is independent of the sign of the electronic charge. To investigate the relevance of chemical surface charge we also varied pH of the salt solution flowing into the cell. [Copyright &y& Elsevier]

Abstract: Activated carbon fiber (ACF) webs with a non-woven multi-scale texture were fabricated from polyacrylonitrile (PAN), and their electrosorption performance in capacitive deionization for desalination was investigated. PAN nanofibers were prepared by electrospinning, followed by oxidative stabilization and activation with carbon dioxide at 750–900°C, resulting in the ACF webs that were characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy and nitrogen adsorption. The results show that the as-made ACFs have a specific surface area of 335–712m2/g and an average nanofiber diameter of 285–800nm, which can be tuned by varying the activation temperature. With the ACF webs as an electrode, an electrosorption capacity as high as 4.64mg/g was achieved on a batch-type electrosorptive setup operated at 1.6V. The ACF webs made by electrospinning are of potential as an excellent electrode material for capacitive deionization for desalination. [Copyright &y& Elsevier]

Capacitive deionization (CDI) is an emerging technology as a sustainable low energy process for desalination of brackish water. Activated carbon electrodes are often used in the CDI devices. Electrosorption capacity was found to be improved on asymmetric electrode configuration using activated carbon cloth doped with fluorine due to redistribution of electric potential. This led to improvement in desalination performance up to 12.4 mg/g for a desalination cycle of 6 min employing an asymmetric fluorinated electrode as cathode (ACC//F-ACC). A relatively high charge efficiency of 77 % was obtained representing 92 % charge efficiency neglecting the leakage currents. Furthermore, the ion adsorption rate was found to improve substantially due to an increased surface conductivity of the fluorinated electrode confirmed by Mott-Schottky analysis. Energy consumption during desalination of 1000 ppm sodium chloride solution of 0.71kWh/m3 for symmetric electrode configuration was found to reduce by 36 % upon employing asymmetric configuration. This study shows some of the benefits of asymmetric configuration to achieve an optimal operation of CDI device, as well as improvements related to energy consumption. Image, graphical abstract [ABSTRACT FROM AUTHOR]

While Electrical Double-Layer Capacitors (EDLC's) have been proven to achieve round-trip efficiencies of over 95%, electrical efficiencies of Capacitive Deionization (CDI) systems are still far from those values. Since this issue seems to be crucial for CDI to commercially compete with other water treatment processes including Reverse Osmosis (RO) it is essential that these efficiencies be improved. In this work, the electrical response of a commercial EDLC's and a CDI system are compared. Results of asymmetric constant current experiments reveal that the main electrical inefficiency occurs during the discharge stage (i.e. in CDI this refers as salt desorption or electrode's regeneration). Here, we propose an operational mode of CDI focused on improving performance of regeneration by simply conducting this stage in a more concentrated solution (3.5M NaC) brine water. The experimental findings suggest that this operational strategy led to 1) an increase from ≈40% to 75% in the round-trip efficiency of CDI, approaching (although admittedly not reaching) those of EDLC's; 2) a drop in ohmic resistance in the discharge cycle of 92% (from 19.77 Ω to 1.45 Ω) consequently increasing the amount of energy that could be potentially recovered from 0.24 J to 0.50 J, and thus, reducing significantly the energy consumption of the process; 3) a wider range of deionization/regeneration current density ratios (0.1–1.25) in which high efficiency values (65–75%) are still maintained. In general, we find that this operational mode might have practical relevance as it implies that CDI systems may be operated with a higher degree of flexibility allowing them to be coupled more readily with renewable energy technologies. Furthermore, the use of a highly concentrated electrolyte (i.e. not deionized water) during regeneration offers an innovative solution for brine concentration. Image 1 • The electrical performance of a CDI flow-cell and an EDLC were compared. • Low round-trip efficiency of CDI cell was mainly associated to discharge step. • New operational mode based on electrolyte replacement was successfully tested. • Round-trip efficiency of CDI can be boosted up to 75%. • Wider range of charge/discharge current density ratios can be used. [ABSTRACT FROM AUTHOR]