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• Activated carbon derived from agriculture by-products obtain high surface area, high porosity, and low density. • Deionization efficiency of CDI, MCDI, and FCDI reached 80–90 % and water recovery rate 90 %. • The roller, spray coating, and mill ball grinding are useful to fabricate electrodes for industry applications. • The adsorption capacities of AC and AC-modified electrodes were 25–40 mg/g. • Life span and deionization efficiency of the flow electrode are higher than electrode plate. Capacitive deionization (CDI), membrane capacitive deionization (MCDI), and flow capacitive deionization (FCDI) are emerging technologies applied to remove ions from brackish water or saline wastewater based on electrode double layer (EDL) mechanisms. The electrode itself can be seen as the most important factor that determines the deionization efficiency of the CDI system. Electrode fabrication using activated carbon derived from agriculture byproducts is a useful solution to decrease cost, restrict environmental pollution, and enable agriculture waste recovery. In this review, the primary parameter's impact on activated carbon (AC) and the electrode fabrication process from the agriculture byproducts is detailed in a discussion to find the most optimal conditions. With excellent characteristics for the activated carbon such as a surface area of up to 2500 m2/g, it can be used to fabricate electrodes that have a high adsorption capacity of approximately 25 mg/g, able to be advanced to 40 mg/g with a modified AC. It can achieve a cycle performance of 95–97 %. The deionization efficiency of the CDI, MCDI, and FCDI systems can be up to 90 % with an input salt concentration from 2000 to 3000 mg/L where the water recovery ratio achieved is 50–90 %, alongside a salt rejection rate of about 7–75 %. AC can be modified to increase its special properties such as surface area, conductivity, and ion selectivity using different methods. The drawbacks and challenges of the CDI technology are also mentioned along with useful solutions such as electrode coating techniques using an anilox roller and a spray, for use in industry applications. [Display omitted] [ABSTRACT FROM AUTHOR]

The influence of surface chemistry on heterogeneous electron transfer at activated carbon (AC) electrodes was studied using scanning electrochemical microscopy (SECM) and conventional electrochemical measurements, which allow for measuring respectively the local and the overall electrode electron transfer rate constants. Using a thermal treatment in air and under argon, commercial activated carbons were either oxidized or defunctionalized. Both treatments lead to a slight decrease of the specific surface area of the material, without altering its pore size distribution and morphology. The nanogap voltammetry and approach curves obtained by SECM were used to characterize the rate constants at the interface between an aqueous solution of ferrocenemethanol (FcMeOH) and the AC electrodes. The heterogeneous electron transfer (HET) rate constant for the untreated AC was 3.4 10−2 ± 0.2 cm s−1 and was increased by about 2.5 and 1.5 times, after oxidizing and defunctionalizing the surface, respectively. SECM results showed that the heat treatment increases the conductivity of AC, and the oxidation of the surface increases the wettability and accessibility of the pores, which are assumed to promote the electron transfer. The local heterogeneous electron transfer rate constant values obtained by SECM are higher, by an order of magnitude, than the global ones obtained from conventional electrochemical macroelectrode measurements. The local information obtained by SECM is valuable for understanding the nanoscale interfacial mechanisms in porous carbon electrodes as a better understanding of the charge storage mechanism is key to improve their electrochemical performances. [Display omitted] [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 Major challenges for effective implementation of the Electro-Fenton (EF) water treatment process are that conventional efficient cathodes are relatively expensive, and H 2 O 2 activation by Fe2+ may cause secondary pollution. Herein, we propose a low-cost activated carbon/stainless steel mesh (ACSS) composite cathode, where the SS mesh distributes the current and the AC simultaneously supports H 2 O 2 electrogeneration, H 2 O 2 activation, and organic compounds (OCs) adsorption. The oxygen-containing groups on the AC function as oxygen reduction reaction (ORR) sites for H 2 O 2 electrogeneration; while the porous configuration supply sufficient reactive surface area for ORR. 8.9 mg/L H 2 O 2 was obtained with 1.5 g AC at 100 mA under neutral pH without external O 2 supply. The ACSS electrode is also effective for H 2 O 2 activation to generate OH, especially under neutral pH. Adsorption shows limited influence on both H 2 O 2 electrogeneration and activation. The iron-free EF process enabled by the ACSS cathode is effective for reactive blue 19 (RB19) degradation. 61.5% RB19 was removed after 90 min and 74.3% TOC was removed after 720 min. Moreover, long-term stability test proved its relatively stable performance. Thus, the ACSS electrode configuration is promising for practical and cost-effective EF process for transformation of OCs in water. Graphical abstract Image 1 Highlights • A novel iron-free Electro-Fenton process is proposed for oxidation of chemicals in water. • A low-cost granular activated carbon/stainless steel mesh electrode is used for the process. • Effect of electrolyte pH and current on H 2 O 2 production are investigated. • Neutral pH could supported effective H 2 O 2 electrogeneration and H 2 O 2 activation. • Long-term stability for H 2 O 2 production, H 2 O 2 activation, OH production is verified. [ABSTRACT FROM AUTHOR]

Abstract In this work, we report the use of pseudocapacitive RuO 2 in a porous carbon-based substrate as a composite electrode for high-performance capacitive deionization (CDI). RuO 2 was electrodeposited onto inexpensive activated carbon (AC) via cyclic voltammetry to optimize the composite electrode (denoted as RuO 2 (20)-AC) fabrication. The electrochemical measurements indicate that the composite electrode with a specific surface area of 576 m2/g and hydrophilicity yields an improved specific capacitance and good cycling stability. The notably enhanced performance is attributed to the presence of RuO 2 , which allows rapid Faradaic charge-transfer reactions as well as pseudocapacitive charge storage. These results confirm that incorporating RuO 2 onto an AC electrode effectively reduces the electrical resistance and enhances the charge efficiency. Furthermore, batch-mode CDI experiments were conducted at 1.2 V in a 5 mM NaCl solution. As evidenced, the RuO 2 (20)-AC composite has a promising salt adsorption capacity of 11.26 mg/g, which is 3.7-fold higher than that of pristine AC. Therefore, using the RuO 2 (20)-AC composite as the cathode, an enhanced desalination performance can be achieved through a mixed capacitive-Faradaic process, resulting from two removal mechanisms of capacitive electrosorption and pseudocapacitive redox reactions. This work provides an efficient strategy to utilize RuO 2 on porous carbon-based substrates to improve CDI performance. Highlights • RuO 2 /activated carbon composite electrodes were prepared for CDI. • Electrodeposited RuO 2 showed good pseudocapacitive charge storage. • The metallic conductivity of RuO 2 ensures rapid ion transport. • The composite had a superior salt electrosorption capacity. • The combination of capacitive electrosorption and Faradaic reactions enhanced the CDI performance. [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]

The growing ubiquity of recalcitrant organic contaminants in the aqueous environment poses risks to effective and efficient water treatment and reuse. A novel three-dimensional (3D) electrochemical flow-through reactor employing activated carbon (AC) encased in a stainless-steel (SS) mesh as a cathode is proposed for the removal and degradation of a model recalcitrant contaminant p -nitrophenol (PNP), a toxic compound that is not easily biodegradable or naturally photolyzed, can accumulate and lead to adverse environmental health outcomes, and is one of the more frequently detected pollutants in the environment. As a stable 3D electrode, granular AC supported by a SS mesh frame as a cathode is hypothesized to 1) electrogenerate H 2 O 2 via a 2-electron oxygen reduction reaction on the AC surface, 2) initiate decomposition of this electrogenerated H 2 O 2 to form hydroxyl radicals on catalytic sites of the AC surface 3) remove PNP molecules from the waste stream via adsorption, and 4) co-locate the PNP contaminant on the carbon surface to allow for oxidation by formed hydroxyl radicals. Additionally, this design is utilized to electrochemically regenerate the AC within the cathode that is significantly saturated with PNP to allow for environmentally friendly and economic reuse of this material. Under flow conditions with optimized parameters, the 3D AC electrode is nearly 20% more effective than traditional adsorption in removing PNP. 30 grams of AC within the 3D electrode can remove 100% of the PNP compound and 92% of TOC under flow. The carbon within the 3D cathode can be electrochemically regenerated in the proposed flow system and design thereby increasing the adsorptive capacity by 60%. Moreover, in combination with continuous electrochemical treatment, the total PNP removal is enhanced by 115% over adsorption. It is anticipated this platform holds great promises to eliminate analogous contaminants as well as mixtures. [Display omitted] [ABSTRACT FROM AUTHOR]

Activated carbon (AC) has been widely used to prepare electrode sheet for capacitive deionization (CDI) owing to its high surface area for ionic adsorption. However, further modification of the AC electrode is still necessary to improve the CDI performance by enhancing the ionic transportation inside the electrode. Here, an asymmetric CDI cell was prepared using Nafion-AC composite with low PTFE content (3.5%) as the negative electrode and AC as the positive electrode. Nafion functions not only as a binder to improve the adhesion of electrode sheet, but also as a mediator to provide an ion conductive pathway for counterions. The morphology and elemental analysis of Nafion-AC were investigated via scanning electron microscopy (SEM) and elemental mapping. The electrochemical capacitance property of the Nafion-AC electrode was studied by cyclic voltammetry (CV). The desalination capacity of the asymmetric CDI cell arrived at 10.8 mg/g, compared with the symmetric AC-based cell (6.9 mg/g). Contribution of Nafion and the corresponding mechanism were analyzed by differential capacitance tests and inverse charging. It demonstrates that in the Nafion-AC composite containing low PTFE content, the co-ion repulsion was reduced and the electrode adhesion was improved. The electrode performance has been conveniently evaluated by monitoring the variation of the charging efficiency and desalination amount with time under an appropriate charge input, which is much helpful to guide the operation process in practical CDI applications. [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]

Flow electrode capacitive deionization (FCDI) has great advantages in high salinity wastewater desalination. However, due to the limited adsorption capacity of electrode material, membrane concentration polarization would form and increase energy consumption, and affect the continuity of desalination. In this work, milling technology was used to modify original activated carbon (AC), the obtained ball milled activated carbon (BAC) was used as FCDI electrode material. The milling process modified multiple physical and electrochemical characteristics of original AC. The highest average salt removal rate (ASRR) of BAC was 2.2 times of that of AC, which was attributed to the modified surface properties including more oxygenated functional group, larger electrostatic gravitation characteristics and better hydrophilicity. The reduced concentration polarization during the desalination was responsible for the increased adsorption capacity, which was due to larger electrolyte contact area, enhanced ions capacity by more suitable pore structures, and 5 times higher energy normalized removal salt (ENRS) of BAC flow electrode than that of AC. Because of its simple operation and remarkable modification effect, ball milling technology shows great potential in large scale modification of electrode materials. [ABSTRACT FROM AUTHOR]

• La-0.5% was less affected by the presence of competing anions than La-1.5%. • Chemisorption and electrosorption mechanisms in La-0.5% reduced the interference by other anions. • Charge and hydration played the major role in selectivity. • Hydroxyls from the La(III) clusters were exchanged by the studied anions. Here it is reported the electrosorption of fluoride in the presence of competing anions of environmental relevance (chloride, nitrate, sulfate, phosphate and arsenate) by two activated carbon electrodes impregnated with La(III), La-0.5% and La-1.5% (w/w %). The electrosorption kinetics were evaluated using equimolar initial concentrations (0.263 mM of each studied anion) in both binary solutions (F− vs. anion) and in a complex mixture comprising all anions simultaneously. For La-0.5%, the presence of nitrate and chloride did not significantly modify its fluoride removal (q = ∼19.25 µmol g−1), while these same anions decreased the fluoride removal of La-1.5% from 32.85 to 17.2 and 13.95 µmol g−1, for nitrate and chloride, respectively. In general, the divalent anions competed the most during the kinetics, decreasing the fluoride removal in both carbons; the lowest capacity values were 5.75 and 4 µmol g−1 for La-0.5% and La-1.5%, respectively. The increases in the pH of the solution corroborated that all anions anchored to the modified surface by a ligand-exchange mechanism involving the exposed hydroxyls from the La(III) clusters. The discussion of the results was not centered on the charge and the hydrated radius of the anions, but it was also expanded to other thermodynamic parameters that also denote selectivity, such as the hydration energies and mass-transport parameters. Electrochemical impedance spectroscopy was performed as a characterization technique in two electrolytes: 0.1 M NaF and the working solution, 0.263 mM NaF (5 ppm F− solution). The information obtained from the latter correlated the most with the electrosorption performance of the electrodes. [ABSTRACT FROM AUTHOR]

In capacitive deionization (CDI), co-ions will be repelled simultaneously from the electrodes when counterions are adsorbed onto the electrodes, leading to a diminished CDI performance. To reduce this so-called co-ion effect, carboxylic groups were grafted onto the surface of commercial activated carbon fiber (ACF) using a simple nitric acid treatment (ACF-HNO 3 ). ACF-HNO 3 exhibited good cation selectivity. An asymmetric CDI cell (N-CDI) was assembled, in which ACF-HNO 3 was used as the cathode and pristine ACF as the anode. The removal capacity of N-CDI at 1.2 V can be as high as 12.8 mg/g, whereas that of the symmetric CDI cell composed of pristine ACF (0-CDI) is only 8.0 mg/g. Accordingly, the charge efficiency of N-CDI stays above 74%, whereas that of 0-CDI is below 40% at various applied voltages. Moreover, N-CDI exhibited better cyclic stability than 0-CDI. These results indicate that use of ACF-HNO 3 as the cathode can effectively reduce co-ion expulsion, thereby enhancing the CDI performance. [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: 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]