Capacitive Deionization Of Water Research Articles

Casein/starch composites: novel binders for green carbonaceous electrodes applied in the capacitive deionization of water

Schematic illustration of the effect of casein/starch composite binder surface chemistry and its role as an ion-exchange membrane on the salt removal process under the application of an electric field.

Knowledge and Technology Used in Capacitive Deionization of Water.

The demand for water and energy in today’s developing world is enormous and has become the key to the progress of societies. Many methods have been developed to desalinate water, but energy and environmental constraints have slowed or stopped the growth of many. Capacitive Deionization (CDI) is a very new method that uses porous carbon electrodes with significant potential for low energy desalination. This process is known as deionization by applying a very low voltage of 1.2 volts and removing charged ions and molecules. Using capacitive principles in this method, the absorption phenomenon is facilitated, which is known as capacitive deionization. In the capacitive deionization method, unlike other methods in which water is separated from salt, in this technology, salt, which is a smaller part of this compound, is separated from water and salt solution, which in turn causes less energy consumption. With the advancement of science and the introduction of new porous materials, the use of this method of deionization has increased greatly. Due to the limitations of other methods of desalination, this method has been very popular among researchers and the water desalination industry and needs more scientific research to become more commercial.

Designing capacitive deionization module for water treatment systems at car washers

It is impossible to effectively use water with a high salt content at car washes. In many places, access to water with a high salt content is almost unlimited but its utilization requires deionization. For this purpose, several methods are used, the main of which are reverse osmosis, electrodialysis, ion exchange methods, and distillation. However, they have significant drawbacks. Recently, the technology of capacitive deionization of water has been widely used, based on the removal of salt ions from the solution during the charge/discharge of "double" electric layers of carbon materials with a significant active surface (800‒2,000 m2/g). Theoretically, this process should be more energy efficient by using a low potential voltage (1–2 V). This paper considers the interrelation of physical parameters that affect the process of capacitive deionization of water. The dependences of voltage drop on serial internal resistance on different concentrations of sodium chloride and the distance between electrodes for electrodes based on the material SAUT-1S (Belarus) have been investigated. It is shown that the main contribution to the sequential internal resistance is introduced by the resistance of the electrolyte. As the distance between the electrodes increases, the voltage drop on the serial internal resistance increases linearly. A decrease in the concentration of ions leads to a decrease in the conductivity of the solution, which causes an increase in energy consumption and a decrease in the efficiency of sorption. It has been demonstrated that the voltage drop at the serial internal resistance when the voltage on the electrodes is limited, which is set in order to avoid the transition of the electrode charging mode to the electrolysis of water, causes a significant drop in the efficiency of the capacitive deionization process

Highly conductive colloidal carbon based suspension for flow-assisted electrochemical systems

SummaryCarbon suspension electrodes are promising for flow-assisted electrochemical energy storage systems. They serve as flowable electrodes in electrolyte solutions of flow batteries, or flow capacitors. They can also be used for other applications such as capacitive deionization of water. However, developments of such suspensions remain challenging. The suspensions should combine low viscosity and high electronic conductivity for optimized performances. In this work, we report a flowable aqueous carbon dispersion which exhibits a viscosity of only 2 Pa.s at a shear rate of 5 s−1 for a concentration of particles of 7 wt%. This suspension displays an electronic conductivity of 65 mS/cm, nearly two orders of magnitude greater than previously investigated related materials. The investigated suspensions are stabilized by sodium alginate and arabic gum in the presence of ammonium sulfate. Their use in flowable systems for the storage and discharge of electrical charges is demonstrated.

Exceptional Reduction of Faradaic Redox Reactions of Activated Carbon-Based Electrodes in the Capacitive Deionization of Water through a Facile Gold Impregnation Method

Activated carbon-based capacitive deionization (CDI) electrodes have been coated with gold nanoparticles through a facile impregnation method. Activated carbon (AC) in the original and nitric acid treated form had been used. Au addition, irrespective of AC being acid treated or not, increases the salt adsorption capacity (SAC), increases charge efficiency and reduces specific energy consumption (SEC) in the CDI process. While applying both acid treatment and gold deposition result in maximum SAC (6.1 mg g−1), sole gold decoration is accompanied with least SEC (0.7 KWh (kg adsorbed ion)−1). Interestingly, gold addition significantly reduces cathodic reduction and anodic oxidation reactions in the case of untreated and treated AC, respectively. A theoretical explanation has been proposed.

Highly Conductive Colloidal Carbon Based Suspension for Flow-Assisted Electrochemical Systems

Colloidal carbon based suspension electrodes are gaining increased attention for large-scale energy storage applications, and particularly for flow-assisted electrochemical energy storage systems. In the latter, they serve as flowable electrodes in an electrolyte solution of flow batteries, or flow capacitors. They can also be used for applications other than energy storage such as capacitive deionization of water. However, developments of such suspensions remain challenging. The suspensions should combine low viscosity and high electronic conductivity for optimized performances. In this work, we report a flowable aqueous carbon dispersion which exhibits a viscosity of only 2 Pa.s at a shear rate of 5 sec-1 for a concentration of particles of 7 wt%. At this concentration, the suspension forms a conductive network and displays an electronic conductivity of 65 mS/cm, nearly two orders of magnitude greater than previously related materials. The investigated suspensions are stabilized by sodium alginate and arabic gum in the presence of ammonium sulfate. Experiments under flow demonstrate that the suspensions remain highly conductive upon shearing. Their use in flowable systems for the storage and discharge of electrical charges is demonstrated.

Capacitive Deionization of Water with Electrodes Based on Nanoporous Activated Carbon and a Mosaic Cation–Anion Exchange Membrane

The capacitive deionization of water (CDW) was investigated with the purpose to obtain pure water. To this end, mosaic cation–anion-exchange membranes and activated carbon electrodes were used. The mosaic membranes contained cation- and anion-exchange components embedded in a synthetic-fiber-based matrix. The means of preparation for the pressed mosaic membranes included pressing the cation- and anion-exchange membranes into each other. Another method was via the subsequent formation of cation- and anion-exchange bands in the fibrous matrix (in a banded membrane). The activated carbon electrodes and mosaic membranes possessed sufficient specific ion surface conductivities even in clean water. The specific energy consumption was 31.9 and 111.7 W mol–1 for the CDW devices containing banded and pressed membranes, respectively. Therefore, the banded membrane was preferable for obtaining pure drinking water. It was found that the CDW with the banded mosaic membrane exhibited the best performance at a voltage of 2 V and a solution flow rate of 15 cm3/min.

Electrical conductivity of wood biochar monoliths and its dependence on pyrolysis temperature

Biochar is traditionally used as solid fuel and for soil amendment where its electrical conductivity is largely irrelevant and unexplored. However, electrical conductivity is critical to biochar’s performance in new applications such as supercapacitor energy storage and capacitive deionization of water. In this study, sugar maple and white pine were carbonized via a slow pyrolysis process at 600, 800 and 1000 °C and conductivities of monolithic biochar samples along the radial direction were measured using the 4-probe method. Biochars were characterized using an elemental analyzer, scanning electron microscopy, X-ray diffraction and Raman spectroscopy. The solid carbon in biochar samples was found to consist primarily of disordered carbon atoms with small graphitic nanocrystallites that grow with increasing temperature. The bulk conductivity of biochar was found to increase with pyrolysis temperature—1 to ~ 1000 S/m for maple and 1 to ~ 350 S/m for pine, which was accompanied by an increase in carbon content—91 to 97 wt% and 90 to 96 wt% for maple and pine, respectively. The skeletal conductivity of biochar samples carbonized at 1000 °C is about 3300 S/m and 2300 S/m for maple and pine, respectively (assuming solid carbon is amorphous); both values are above that of amorphous carbon (1250–2000 S/m). This work demonstrated the importance of carbonization and graphitization to electrical conductivity and suggested electron hopping as a likely mechanism for electric conduction in biochar—an amorphous carbon matrix embedded with graphitic nanocrystallites.

Capacitive deionization of water using mosaic membrane

Capacitive deionization of water using membrane-electrode assembly was researched. The approach that allows to decrease energy consumptions of water purification was considered. The method provides usage of mosaic membrane containing both cation and anion exchange fragments instead of glass spacer. Counter-ions inside the membrane ensure rather high ionic conductivity even in pure water. Mosaic membranes based on polyethylene (film type) and phenol-formaldehyde (fibrous type) matrices were studied, their electric conductivity and exchange capacity were determined. Deionization in static and dynamic electrochemical cells, which were filled with deionized water and 0.005M KCl solution respectively, was researched. The mechanism of electric double layer charging inside pores of the electrodes impregnated with pure water has been proposed. Specific energy consumptions for deionization of very diluted solutions are sufficiently lower for the cell containing mosaic membrane than those for the cell with inert glass spacer. Minimum energy consumptions and maximum deionization degree are reached at cell voltage of 1.4V. The value of specific energy consumptions is 12Whmol−1 for the laboratory cell containing the mosaic membrane, when degree of deionization reaches 50%, cell voltage is 1.4V, electrode area is 50cm2, initial concentration of the KCl solution is 0.005M.

Time-dependent ion selectivity in capacitive charging of porous electrodes

In a combined experimental and theoretical study, we show that capacitive charging of porous electrodes in multicomponent electrolytes may lead to the phenomenon of time-dependent ion selectivity of the electrical double layers (EDLs) in the electrodes. This effect is found in experiments on capacitive deionization of water containing NaCl/CaCl2 mixtures, when the concentration of Na+ ions in the water is five times the Ca2+-ion concentration. In this experiment, after applying a voltage difference between two porous carbon electrodes, first the majority monovalent Na+ cations are preferentially adsorbed in the EDLs, and later, they are gradually replaced by the minority, divalent Ca2+ cations. In a process where this ion adsorption step is followed by washing the electrode with freshwater under open-circuit conditions, and subsequent release of the ions while the cell is short-circuited, a product stream is obtained which is significantly enriched in divalent ions. Repeating this process three times by taking the product concentrations of one run as the feed concentrations for the next, a final increase in the Ca2+/Na+-ratio of a factor of 300 is achieved. The phenomenon of time-dependent ion selectivity of EDLs cannot be explained by linear response theory. Therefore, a nonlinear time-dependent analysis of capacitive charging is performed for both porous and flat electrodes. Both models attribute time-dependent ion selectivity to the interplay between the transport resistance for the ions in the aqueous solution outside the EDL, and the voltage-dependent ion adsorption capacity of the EDLs. Exact analytical expressions are presented for the excess ion adsorption in planar EDLs (Gouy-Chapman theory) for mixtures containing both monovalent and divalent cations.

Preparation and Characterization of Carbon Nanotubes-Based Composite Electrodes for Electric Double Layer Capacitors

E-mail: sjpark@inha.ac.krReceived November 26, 2011, Accepted February 3, 2012In this work, we prepared activated multi-walled carbon nanotubes/polyacrylonitrile (A-MWCNTs/C)composites by film casting and activation method. Electrochemical properties of the composites wereinvestigated in terms of serving as MWCNTs-based electrode materials for electric double layer capacitors(EDLCs). As a result, the A-MWCNTs/C composites had much higher BET specific surface area, and porevolume, and lower volume ratio of micropores than those of pristine MWCNTs/PAN ones. Furthermore, somefunctional groups were added on the surface of the A-MWCNTs/C composites. The specific capacitance of theA-MWCNTs/C composites was more than 4.5 times that of the pristine ones at 0.1 V discharging voltageowing to the changes of the structure and surface characteristics of the MWCNTs by activation process.Key Words : Multi-walled carbon nanotubes, Polyacrylonitrile, Composites, Activation, ElectrochemicalpropertiesIntroductionIn recent years, electrochemical capacitors have beenstudied in terms of their practical application as high powerdevices, since they have higher power density than batteriesand higher energy density than ordinary capacitors, as wellas a long cycle life. Electrochemical capacitors can bedivided into two types according to the energy storage mech-anism: electrochemical double-layer capacitors (EDLCs)and redox supercapacitors. EDLCs are being considered fora variety of applications such as the capacitive deionizationof water and as pulse power sources for cellular devices.