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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]

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]

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-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]

A constant phase element (CPE) is often used in electrochemical impedance spectroscopy to describe the non-ideal impedance of solid/electrolyte interfaces. The generalization of its mathematical description leads to the concept of a variable or generalized phase element α g p e , which, in association with Brug's expression, can be used to determine the interfacial capacitance of metallic electrodes, whose time-constant distribution is restricted to a bidimensional plane (C 2 D). In this work, a heuristic approach is used to extend the generalization principle even further, in order to arrive at an expression of capacitance, C g p e , that is valid for electrodes with time-constant dispersions that also spread into the electrode bulk, causing a 2D+1D time-constant distribution. As a proof of concept, the proposed method is applied in the analysis of the electrochemical impedance of n -doped hydrogen-terminated Si(100) electrode in 100 mM Na 2 SO 4 , in a potential range that extends from accumulation to depletion. The single crystalline structure and polished surface of the electrode grants a quasi-ideal capacitive behavior to the system that allows comparison of the proposed analysis with the traditional representation of effective capacitance C e f f. Both methods yield similar results in the depletion region, where the impedance response is predominantly defined by the ideal behavior of the silicon space charge region. In accumulation, however, when impedance is influenced by the somewhat larger distribution of relaxation times of the electric double layer, C e f f shows frequency dispersion, whereas C g p e does not. A dimensional analysis of the functional elements in C g p e reveals the underlying reason for this intriguing behavior and also shows that C 2 D is, in fact, just an approximate expression of the more general C g p e. It is also shown that the generalized capacitance representation allows for a clear detection of the onset of size border effects, and these appear at a higher frequency than what is observed in the C e f f representation. [Display omitted] [ABSTRACT FROM AUTHOR]

Here we report on a research effort towards full cell potential utilization during the elimination of organic pollutants under environmentally relevant conditions. Meropenem was selected as a representative recalcitrant antibiotic and a newly-developed Co and Bi co-doped TiO 2 nanotube array (Co/Bi/TiO 2 NTA) electrode was employed as both anode and cathode. While electrochemical reduction of Meropenem appeared to be much more energy efficient (0.042 kWh/m3) than electrochemical oxidation (0.813 kWh/m3) in synthetic electrolyte, the degradation was significantly inhibited in environmental matrices (secondary effluent/SE and RO concentrate) due to the competition and/or inhibition effects of other species present in the matrices as well as electrode fouling. Calcium carbonate and magnesium oxide were identified to be main mineral foulants. The elimination of Meropenem was then examined in coupled treatment schemes (e.g. sequential and simultaneous), taking advantages of the "self-regeneration" characteristic of TiO 2 NTA based electrodes through cathodic polarization. While a simultaneous oxidation & reduction with regular polarity switching seems to be a preferred treatment scheme especially for challenging matrices such as RO concentrate, the bipolar stability of the nanostructured electrode may limit the electrode lifetime. A hybrid electrolysis system coupling anodic oxidation of organic pollutants and cathodic hydrogen generation as a renewable energy source was also tested in a flow-through reactor, where consistent and steady treatment performance was demonstrated for a period of 3 days. We think the results obtained from this study have underlined the importance of matrix characteristics on the design and selection of electrochemical treatment processes. It also provides insights into future development of coupled anodic and cathodic processes for water purification. Image, graphical abstract [ABSTRACT FROM AUTHOR]

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]