In previous work, we introduced an elegant approach for bromide recovery from water by the introduction of a hybrid physical adsorption and capacitive deionization processes for selective removal and recovery of boron from water. In this paper, we show that the harsh environment of water contaminated with bromine-moieties adversely affects the longevity of relevant electrodes, with close to 100 consecutive work hours of bromides removal without noticeable degradation. To extend the lifespan of electrodes, we used an asymmetric CDI cell with a 1:5 positive/negative electrodes ratio in which a polarity switch between electrodes is applied every six adsorption-desorption cycles in a way that in each adsorption-desorption cycle, a different electrode of the six electrodes, functions as the positive electrode. We deduce that the polarity switch reduces oxidation and subsequent degradation of the positive electrodes, resulting in an extended lifecycle. After examining nine different carbonaceous materials, carbon cloth was chosen to be incorporated in the bromide- recovery cells because of its favorable kinetics and its physical and mechanical properties. We show that with a combination between endurance of the electrodes and asymmetric mode of operation, it is possible to overcome the main barrier that holds the technology from being practical.

Abstract: A reliable way to remove, retrieve and reuse bromide ions from brine is crucial for municipalities and industry, yet traditional methods such as reverse osmosis and direct distillation are costly and not selective. We introduce hybrid physical adsorption and capacitive deionization (HPA-CDI) integrated with polarity switch mode of operation suited for asymmetric cells, which builds and improves upon Cohen et al. ’s proof of concept selective bromide desalination, attaining a long-term stability milestone: 97 consecutive work-hours without noticeable degradation. The novelty is an asymmetric CDI cell with a 1:5 positive/negative electrode ratio in which a polarity switch of the positive charge is applied every six adsorption-desorption cycles. We deduce that the polarity switch reduces oxidation and subsequent degradation of the positive electrode, resulting in an extended lifecycle. Following 3-electrode procedures of chrono voltammetry and potentiometry on nine different carbonaceous materials, ACC-509215, Kynol carbon cloth with 1500 mA/g, was chosen and incorporated in the cell for its favorable kinetic, physical and mechanical properties. We show that with a combination between endurance of the electrodes and asymmetric mode of operation, it is possible to overcome the main barrier that holds the technology from being practical.

Capacitive deionization (CDI) has been considered as the most promising and environmentally friendly electrical desalination technology owing to its low energy consumption and no secondary pollution. CDI is based on the principle of electric double layer for salt ion adsorption, but the existence of co-ions repulsion reduce the charge efficiency, leading to the low salt adsorption capacity. To prevent the intrinsic “co-ion effect” inside the porous carbon electrodes, membrane capacitive deionization (MCDI) by applying an ion-exchange membrane (IEM) to the surface of electrode is of increasing interest. However, MCDI brings various resistances, such as the internal and interface resistances of membrane, as well as the contact resistance between membrane and carbon electrode. More recently, by integrating “membrane” with carbon electrode without the introduction of free-standing IEM, integrated-MCDI has shown great merits to enhance counter-ion selectivity of electrode and decrease resistance, resulting in improving adsorption rate and reducing energy consumption. In this review, the preparation methods of innovative electrodes, the working mechanisms and performances of integrated-MCDIs, and the advanced advantages are summarized. It is hoped that this work will provide insight into integrated-MCDI and shed light on the future direction of water desalination based on CDI technology.

The important phenomenon of electrical double layer (EDL) is often described by mathematical relations between surface charges, variation of electrostatic potentials with distance and distribution of ions across the interface between charged surfaces (or particles) and electrolyte solutions. A major advance was made in the last decade in understanding complex EDL relationships with an emphasis on nano-porous carbonaceous materials. These understandings were usually exploited for the interpretation of electro-sorption phenomena connected to capacitive deionization (CDI) processes. The aim of this short paper is to demonstrate, based on previous studies, how models of EDL in nano-porous carbons can be the basis for modification of carbonaceous materials for other applications, like sensors and energy extraction from salinity gradients.

Wastewater reclamation is becoming a top global interest as population growth and rapid industrialization pose a major challenge that requires development of sustainable cost-effective technologies and strategies for wastewater treatment. Carbon nanomembranes (CNMs)-synthetic 2D carbon sheets-can be tailored chemically with specific surface functions and/or physically with nanopores of well-defined size as a strategy for multifunctional membrane design. Here, we explore a bifunctional design for combined secondary wastewater effluent treatment with dual action of membrane separation and advanced oxidation processes (AOP), exploiting dissolved oxygen. The bifunctional membrane consists of a CNM layer on top of a commercial ultrafiltration membrane (Microlon™) and a spray-coated reduced graphene oxide (rGO) thin film as the bottom layer. The CNM/support/rGO membrane was characterized by helium ion and atomic force microscopy, FTIR, XPS with a four-point conductivity probe, cyclic voltammetry, galvanostatic measurements, and impedance spectroscopy. Combined treatment of water by nanofiltration and AOP was demonstrated, employing a unique three electrode-dead end filtration setup that enables concurrent application of potential and pressure on the integrated membrane. For the model organic compound methylene blue, oxidation (by the Fenton reaction) was evaluated using UV-vis (610 nm). The rejection rate and permeability provided by the CNM layer were evaluated by dissolving polyethylene glycol (400 and 1000 Da) in the feed solution and applying pressure up to 1.5 bar. This demonstration of combined membrane separation and AOP using an integrated membrane opens up a new strategy for wastewater treatment.

Capacitive deionization (CDI) has emerged as a novel desalination technology due to its cleanliness and low energy consumption. Despite being based on the principle of an electric double layer for ion adsorption on porous carbon materials, the inevitable faradaic reaction and the existence of co-ion repulsion reduce the charge efficiency (CE), leading to a low salt adsorption capacity (SAC). Herein, an integrated membrane electrode (IME), prepared by spray-coating a thin layer of an ion exchange polymer on the activated carbon (AC) electrode, effectively improves the selectivity of the electrode to the counter-ion, resulting in a high CE and high SAC with good stability. The results show that the CE of CDI performed using IMEs is between 70% and 98% during multiple adsorption-desorption cycles, and the SAC is up to 14-20 mg g(-1), which is much higher than CE (35-65%) and SAC (7-13 mg g(-1)) of traditional CDI with pristine AC electrodes. This study indicates that CDI using IMEs is advantageous for desalination with great application potential.

Capacitive deionization (CDI) is an emerging method for removal of charged ionic species from aqueous solutions, based on electrostatic interactions between (mostly) inorganic ions and porous carbon electrodes.Inspection of recent publications related to CDI processes, revealed that the majority of the publications are related to the removal of salt (NaCl) from the water (desalination) or electrosorption processes. However, such a water desalination is only one process in the improvement of the quality water, it is interesting to review the literature in the context of CDI processes for other water treatment processes. Herein wastewater treatments are discussed.In this paper, we critically review the last publications that relate to capacitive deionization with wastewater treatments. Since wastewater treatments may involve broad aspects, we address in this review four specific water treatment processes that are thought to be connected with CDI processes: organic fouling of CDI cells, removal of heavy metals by CDI processes, removal of organic micropollutants with CDI processes and disinfection with CDI processes. We also evaluate herein the status of several research efforts in this area and suggest future directions.

Capacitive deionization (CDI) is an alternative water desalination technology, which was investigated extensively in the last decade. The choice of electrodes' materials plays a major role in the electrosorption performance, affecting the whole desalination process. Graphene-based nanostructures in various types were extensively studied owing to their superior inherent physico-chemical properties. Whereas excellent electrosorption performance was reported - expressed in terms of salt adsorption capacity (SAC) or average salt adsorption rate (ASAR) - the cost-benefit of graphene-based electrodes, considering total production cost and much lower price of commercial activated carbon, is still controversial. Here, we explore partially exfoliated thermally reduced graphene oxide (GO) - denoted as PE-rGO - prepared by scalable low-temperature thermal exfoliation of GO under air atmosphere. PE-rGO displays a "paper-like" structure with nanoscale pores. By the construction of a lab-scale system, a few grams of product were produced in one batch. A PE-rGO electrode assembled in membrane-CDI three-electrode configuration showed moderate to high SAC of around 13 mg/g under potential window of 0-550 mV versus Ref. electrode in 2000 ppm NaCl solution. However, the energy consumption was shown to be nearly constant with increasing ASAR. This has significant implications for the energy consumption and the projected capital costs.

This paper suggests an effective approach to evaluate carbon electrodes that can be effective in capacitive de-ionization processes of salty aqueous solutions. The first assessment of any electrode material, before its assembly in a CDI cell, includes its specific capacitance and its electric conductivity among other physical properties. These properties may bring a sense whether the electrode can be suitable for electro-adsorption processes, but can hardly tell about the efficiency of the process which is related to the charge utilization. A new term is discussed - the ratio between the specific capacity of an electrode and the specific charge related to the electrode's surface groups. A close inspection of the amphoteric modified Donnan model of the electric double layer shows that the ratio between the surface charge density and the integral specific capacitance, namely, the overall charge density, of the carbon electrodes is a useful term for predicting the CDI process efficiency. Such a ratio is obtained by regular electrochemical analysis of carbon electrodes and a simple titration of the functional surface groups. For demonstration, electrodes comprising commercial activated carbon cloth were tested in CDI cells. The theoretical calculations were correlated to the performance of their performance in CDI processes. (C) 2019 The Electrochemical Society.

Carbon Xerogel electrodes' materials were prepared by polycondensation of resorcinol and formaldehyde. They show accommodation of anions in preference to cations by their porous structure. In addition, their selectivity toward anion electroadsorption was found to be pH dependent, with an increase in electrodes' capacitance in acidic solutions. This dependence of the capacitance on the pH of the solution is explained by the presence of functional carboxylic surface groups in the Xerogel carbon micropores, determined by Boehm titration to be 3.26 meq/g. These functional surface groups give rise to the Donnan exclusion effect, whereby they cause partial repulsion of anions from the microporous structure. When the pH of the electrolyte solution is acidic (<2), most of these acidic functional surface groups are in their non-ionized state, and hence anions can freely penetrate the Xerogel carbon microporous structure. The pH dependent capacitance of these carbons was utilized to examine the feasibility of energy extraction from mixing of saline and acidic solutions following the four basic steps of capacitive mixing processes - charge, solution exchange, discharge, and another solution exchange. As opposed to what we expected, a potential rise was observed when the solution was exchanged from acidic to saline, rather than during the other exchange, from saline to acidic solution. It appears that upon exchange of the solutions from saline to acidic, the expected potential rise resulting from the change in the electrodes capacitance, is cancelled by the new quinone-hydroquinone equilibrium that is established on the electrodes by their surface groups. The ability of these surface redox functional groups to store charge provides a basis for energy extraction from the neutralization of acidic solutions, as described herein. (C) 2019 Elsevier Ltd. All rights reserved.

The use of sodium manganese oxide as an intercalation electrode for water treatment was recently explored, and referred to as a "desalination battery" and "hybrid capacitive deionization". Here, we examine the feasibility of using such a desalination battery, comprising crystalline Na4Mn9O18 as the cathode and Ag/AgCl/Cl- electrode as the anode, to extract energy from low-grade waste heat sources. Sodium manganese oxide electrode's material was produced via a solid-state synthesis. Electrodes were produced by spray-coated onto graphite foils, and showed a temperature dependence of the electrode potential, namely, partial derivative E partial derivative T, of -0.63 mV/K (whereas, the Ag/AgCl/Cl- mesh electrode showed much lower temperature dependence, < 0.1 mV/K). In order to demonstrate ion-removal capabilities together with the feasibility of thermal-energy conversion, a flow battery system was constructed. Thermally regenerative electrochemical cycles (TREC) were constructed for the flow battery cell. The thermal energy conversion, in this particular system, was shown to be feasible at relatively low C-rate (C/19) with temperatures varying between 30 degrees C and 70 degrees C.

Removal and recovery of bromide ions by electro-oxidation and electro-reduction are presented using hybrid physical adsorption and capacitive deionization cells, which contain activated carbon cloth electrodes. This is a proof of concept research with results, which indicate that when comparing the removal and recovery quantities of bromide and chloride ions (starting with the same initial concentration of 0.05 M for both salts), the desalination capacity of the bromide ions is larger by almost 2 orders of magnitude than that of the chloride ions; thus, we obtained specific desalination of bromide ions from a solution containing chloride ions. Removal and recovery of 3.5 mmol of bromide ions were achieved by a working electrode with 1 g of activated carbon cloth, and the calculated energy consumption for the removal and recovery of 1 g of bromide ions was about 2.24 kJ/g.

Capacitive mixing is a newly emerging technique for the production of renewable energy from the controlled mixing of river water and seawater. Energy extraction is provided by the potential rise in electrodes held in a fixed charge, as a response to the concentration change. Therefore, electrodes that exhibit high potential variation as a response to concentration change (negative rise for the cation-capturing electrode and vice versa) are highly desirable. In this work, electrodes that can accommodate mostly cations within their porous structure are discussed. In accordance to the modified Donnan (mD) model of the electrical double layer, it is expected that such electrodes will display the highest potential change as a response to concentration change (while being held with a fixed charge). Using appropriate selective activated carbon electrodes, high potential rise, around 50 mV, was observed as a response to concentration change, when the concentration of NaCl solutions was changed from 1M to 10(-1) M and from 10(-1) M to 10(-2) M. Such a capability of potential rise of selective electrodes can serve as a good basis for energy extraction by capacitive mixing. (C) The Author(s) 2017. Published by ECS.

Capacitive mixing is a newly emerging technique for the production of renewable energy from differences in salinity, usually of wastewater streams. The method is based on the controlled mixing of two streams with different salt concentrations, which are alternatingly brought into contact with precharged porous electrodes, thus taking advantage of the fact that modification of the electrical double layer of the electrodes results in changes in the solution salinity. Usually, the renewable energy resources are seawater and river water streams. Here, we demonstrated that electrical energy can be extracted by capacitive mixing of acidic wastewater and seawater. This concept is proven by the use of proton-selective carbon as the cation-capturing electrode, fabricated by carbonization of cellulose filter paper followed by mild activation in concentrated nitric acid. Considerable energy extraction was demonstrated even if the concentration of the NaCl solution was tenfold higher than that of the acidic solution.

This paper aims at analyzing the impact of irreversible faradaic side reactions and, in particular, oxygen reduction on capacitive deionization (CDI) processes. As opposed to electrochemical supercapacitors, the presence of dissolved oxygen in the feed stream is unavoidable and requires appropriate attention. By constructing simple two- and three-electrode cells, comprising untreated activated carbon cloth, aerogel, and hydrogen-treated activated carbon cloth, we show that the rate at which oxygen is reduced at the active site of the electrodes has significant impact on the cell parameters and, in particular, the potential distribution with respect to its initial state. This has impact on the endurance of the electrodes during long-term cycling. The rate and mechanism of the oxygen catalytic reaction on the carbon electrode is evaluated by adopting the self-discharge model suggested by Conway [1].

Cellulose-based carbon electrodes with pores of different widths (~ 2.6 and ~ 4 nm) have been fabricated by the pyrolysis of cellulose filter paper, followed by mild activation and surface treatment in nitric acid. Such electrodes were introduced into solutions with different pH values and their potential was measured vs. a reference electrode. Nernstian behavior between measured potentials and solution pH with a slope value of 57 mV per decade was only demonstrated for the carbon with the smaller pore size. This carbon also has proton-selective properties, unlike the carbons with wide pores. Based on the modified Donnan model of the electrical double-layer structure, it is hypothesized that the micropores of proton-selective carbons can hold a fixed amount of protons, regardless of the composition of the solution outside the micropores, when a Donnan potential between the carbon-electrode micropores and the external solution is established.

The demand for potable water is continuously increasing. Therefore energy-efficient water desalination methods are the focus of intensive research. Capacitive deionization (CDI) is an energy-efficient water desalination technology. This study focuses on solving the problem of electrode oxidation and degradation in CDI cells. The effect of the geometric flow regime was investigated. Comparison of flow-through vs. flow-by in CDI cells indicates that geometry has an impact on the electro-oxidation rates of the positively polarized electrodes. We examined operation with periodic potential (difference) application by alternating the electrodes polarization. Whilst operating in such a way, the life of CDI cells could be pronouncedly extended without any drops in the desalination level. We investigated the effect of oxygen, which is unavoidably dissolved in the aqueous solutions, on the stability of the electrodes in CDI processes, with the aid of prolonged experiments under nitrogen atmosphere. We determined that the inevitably dissolved air in regular brackish water significantly impacts the oxidation rate of the positively charged electrodes in CDI. Stabilization means for CDI cells are discussed.

Corrosion of the positive electrodes, in capacitive deionization (CDI) cells for water desalination processes, is a major problem that may prevent them from becoming practically important. This paper deals with the consequence of the corrosion of the positive electrodes in CDI processes on the desalination performance, in terms of capacity and the ratio between adsorption of counter-ions and desorption of co-ions. The detrimental effect of the positive electrodes oxidation on the de-ionization efficiency is demonstrated and discussed. The role of the potential difference applied to CDI cells on the electrodes’ stability was explored as well. We used for this study CDI cells comprising several pairs of activated carbon electrodes and 3 electrodes cells containing reference electrode. The interrelated parameters measured included potential, current, concentration (translated from conductivity measurements) and pH vs. time. The present study and the understanding gained herein, will enable the development of durable, long term and effective CDI processes.

The worldwide demands for potable water are continuously increasing due to population growth, global warming, and contamination of fresh-water sources. When dealing with Brackish Water, (BW) Capacitive Deionization (CDI) is a water treatment process that holds the promise of obtaining potable water by high energy efficiency and low expenses. The technology is based on polarizing high surface area electrodes under saline water ( electrolyte) by a constant potential; in which an electrical double layer forms and adsorption of the solution ions is obtained. This paper reviews CDI challenges that have been found, analyzed and dealt with by using electrochemical techniques in order to analyze new CDI advantages and enhance the charge efficiency of the regular CDI cell.

In this paper, we report on attempts to improve the charge efficiency of electrochemical capacitive deionization (CDI) processes without limiting the range of applied potentials, by using surface-treated (oxidized or reduced) activated carbon fiber (ACF) electrodes, and by means of a third, auxiliary electrode. For oxidizing the ACF electrodes, we etched ACFs for different periods of time with, a concentrated nitric acid solution. For reduction of the ACFs, several surface treatments were considered: reaction with hydrogen at high temperatures, removal of oxygen surface groups by heating under vacuum at high temperatures, reaction with a concentrated aqueous solution of a sodium borohydride solution, and reaction with a concentrated sodium borohydride solution after oxidation with a concentrated nitric acid solution. To examine the charge efficiency, we elaborated a special flow-through cell (where the solution flows through the ACF electrode) with a silver/silver chloride mesh reference electrode. The feasibility of using surface-treated carbon electrodes and/or of using a third, auxiliary electrode (with which the potential applied to each electrode can be controlled) for enhancing the charge efficiency is discussed and examined. We were able to demonstrate an increase in the charge efficiency of the CDI process by 30% without the need to reduce the potential range of operation.