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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 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]

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]

• 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]

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]

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]