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• Different carbon-based electrodes following a nature-inspired solution were tested by CV and EIS. • Glass-carbon electrodes (GCE) and carbon paper-based electrodes (CPE) with and without polytetrafluoroethylene (PTFE) were evaluated. • The electrodes' performance was assessed by employing the ferri/ferrocyanide ([Fe(CN) 6 ]3-/4−) electrochemical probe. • Surface properties of the commercial electrodes materials were evidenced from AFM and water contact angle measurements. • Best results were achieved with CPE without PTFE, given its hierarchical porous structure and absence of the insulating binder. Carbon electrodes are widely accepted as very versatile platforms, with applications ranging from electrocatalysis to sensors and other devices, like fuel cells and water electrolyzers. However, there are still difficulties given that over time, at high potentials, the oxidation of carbon materials (as a catalyst and/or catalyst support) can play a detrimental role, undermining the efficiency and stability of the electrochemical processes and devices performance. In this paper, it is reported the research work followed by resourcing to electrochemical analytical techniques, like cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), along with complementary atomic force microscopy (AFM) and water contact angle (WCA) measurements. These techniques were used to characterise glass-type and paper-based carbon electrodes. On a nature-inspired solution, we took advantage of the different interfacial carbon-support hierarchical porous structures to boost the carbon electrocatalytic effect towards sensing the ferri/ferrocyanide redox couple ([Fe(CN) 6 ]3-/4−) in aqueous solution. It is shown that the best results were achieved with carbon paper electrodes without wet proofing, given its hierarchical porous structure and absence of the insulating binder. This research endeavors to contribute to the ongoing advancements in the field of electrochemical green energy conversion by exploring innovative approaches and materials, with the ultimate aim of developing carbon substrates that not only enhance performance but also promote environmental sustainability. [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]

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 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. [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 a promising technology for the desalination of brackish water due to its potentially high energy efficiency and its relatively low costs. One of the most challenging issues limiting current CDI cell performance is poor cycling stability. CDI can show highly reproducible salt adsorption capacities (SACs) for hundreds of cycles in oxygen-free electrolyte, but by contrast poor stability when oxygen is present due to a gradual oxidation of the carbon anode. This oxidation leads to increased concentration of oxygen-containing surface functional groups within the micropores of the carbon anode, increasing parasitic co-ion current and decreasing SAC. In this work, activated carbon (AC) was chemically modified with titania to achieve additional catalytic activity for oxygen-reduction reactions on the electrodes, preventing oxygen from participating in carbon oxidation. Using this approach, we show that the SAC can be increased and the cycling stability prolonged in electrochemically highly demanding oxygen-saturated saline media (5 mM NaCl). The electrochemical oxygen reduction reaction (ORR) occurring in our CDI cell was evaluated by the number of electron transfers during charging and discharging. It was found that, depending on the amount of titania, different ORR pathways take place. A loading of 15 mass% titania presents the best CDI performance and also demonstrates a favorable three-electron transfer ORR. [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]

• 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]

• 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]

Capacitive deionization (CDI) which removes ionic species from solution by applying electric energy to carbon based electrodes is one of excellent convergence technologies combined with energy storage technology and environmental systems. In recent decades, the technology for synthesizing new carbon materials has advanced, the electrical double layer capacitance in porous structures has become better understood, and novel deionization systems have been developed. Nevertheless, achieving a higher deionization performance is required for CDI to compete with reverse osmosis for deionization. The recently introduced Hybrid CDI (HCDI), which utilizes sodium manganese oxide (Na 4 Mn 9 O 18 ) and carbon material, successfully demonstrated its superior deionization performance over conventional CDI systems. Despite the great promise of the HCDI system, the limited availability of aqueous based intercalation materials and the lack of information regarding the characteristics of the HCDI system operations are obstacles to the advancement of HCDI. Thus, we report a new HCDI system with sodium iron pyrophosphate (Na 2 FeP 2 O 7 ), which is a promising material for sodium ion batteries due to its high capacity, low cost and environmentally benign nature. The major results of the HCDI system with Na 2 FeP 2 O 7 showed a superior maximum deionization rate performance (0.081 mg g −1 s −1 ) with a comparable deionization capacity (30.2 mg g −1 ) compared to the previous HCDI system with Na 4 Mn 9 O 18 . Furthermore, the analysis of the CDI Ragone plot revealed the hybrid behavior characteristics of this HCDI system and that the high deionization capacity originated from the high capacity of Na 2 FeP 2 O 7 at a low current density, whereas the fast deionization rate originated from the supercapacitor system at a high current density. Consequently, this study on a new HCDI system with Na 2 FeP 2 O 7 contributes to expanding the understanding of the kinetic properties of the HCDI system with respect to its diverse operations. [ABSTRACT FROM AUTHOR]

• New insights on the electrochemical behavior of fluometuron were proposed. • Modification-free electroanalytical approach for herbicide sensing. • Low LOD was achieved using the bare GCE. • The proposed method presented high sensitivity, excellent selectivity, and accuracy. • Novel strategy for the quantification of fluometuron in environmental samples. For the first time, both theoretical and electrochemical behavior of the phenylurea herbicide fluometuron (FTN) were investigated using the glassy carbon electrode (GCE). The cyclic voltammetry (CV) performed in the presence of FTN showed an irreversible profile for the analyte with one well-defined anodic peak at ca. +1.2 V (vs. Ag/AgCl). The linear dependence of pH and peak potential was evaluated using the Nernst equation through CV measurements. The slope of 46 mV s−1 suggests an oxidation mechanism involving an equal number of protons and electrons. High-level quantum-mechanical calculations based on the density functional theory showed that the presence of defects on the GCE surface induces strong interaction leading to FTN dissociation and/or its adsorption with orientation parallel to the substrate which compromises the electroanalysis. Raman spectroscopy confirmed the surface fouling. Differential pulse voltammetry (DPV) was used for the electroanalytical determination of FTN under optimized conditions. The oxidation peak increased with increasing concentrations of FTN in the range of 207–3846 µg L−1; the theoretical detection limit was estimated as 63 µg L−1. To avoid surface fouling, the calibration curve was applied for the determination of FTN. The unmodified electrode offered proper selectivity in the presence of other pesticides and potential interfering species. Intra and inter-day precision was established. Recoveries were obtained in the range of 81.8–98.9% (n = 3) with the absence of matrix effects. The results showed that electroanalysis through the bare GCE might be used as a novel strategy to determine the persistent organic compound FTN in spiked river water and simulated serum samples and preliminary evaluations such as screening purposes. [ABSTRACT FROM AUTHOR]

• Electrochemical separations can play a role in improving PFAS removal efficiency. • Electrosorption using functionalized carbon materials offers tunable selectivity. • Electrooxidation and plasma-based methods are promising electrically-driven methods for PFAS destruction. • Coupling of separation and reaction through electrochemical approaches can lead to process intensification in PFAS remediation. Organic anthropogenic pollutants such as per- and polyfluorinated alkyl substances (PFAS) present a severe threat to bodies of water and neighboring ecosystems, due to their toxicity and recalcitrance to current treatment methods. PFAS have highly stable carbon-fluorine bonds that contribute to the persistence of these compounds in the environment. Conventional separation methods based on membranes or adsorption materials can often be inefficient at targeting PFAS, or require significant energy or chemical consumption for regeneration.. Electrochemical approaches offer a sustainable and potentially energy-efficient solution to PFAS capture and degradation, with rapid development of new electrode materials and electrochemically-driven processes for the separation and reaction of these compounds. Here, we review electrochemical approaches for PFAS removal and degradation, including electrosorption and electrochemical oxidation/reduction processes. In particular, we discuss investigations using porous carbon electrodes and functional polymers for PFAS electrosorption, as well as electrodes used for PFAS defluorination such as boron-doped diamond. Finally, we present a perspective on emerging topics in electrochemical PFAS remediation, such as the integration of electrochemical reaction and separations, and the urgent need for remediation studies of short-chain PFAS. [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]

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]

CoMoO 4 with oxygen vacancy (CoMoO 4-x) porous nanosheet is prepared by hydrothermal synthesis and hydrogenation reduction method. The CoMoO 4-x porous nanosheet has a higher specific surface area together with a more diverse pore size distribution than CoMoO 4 nanosheet. The density functional theory calculation result exhibits that CoMoO 4-x has lower energy band gap (0.041 eV) than CoMoO 4 (1.683 eV). The electronic density of states plots reveals that CoMoO 4-x has more electron state distribution at Fermi energy level than CoMoO 4. CoMoO 4-x porous sheet achieves higher specific capacitance (1989 g−1) than CoMoO 4 (917 F g−1) at 2 mA cm−2. It also achieves superior capacitance retention rate (91.3%) than CoMoO 4 (83.9%) with the current density increasing from 2 to 20 mA cm−2. The introduction of oxygen vacancy can increase the carrier density, accelerate the electron transfer, promote the electrical conductivity, and accordingly strengthen the redox reactivity of CoMoO 4-x. An asymmetric supercapacitor is also constructed by using CoMoO 4-x as positive electrode and activated carbon as negative electrode. It achieves high energy density of 62.3 W h kg−1 at a power density of 800 W kg−1, together with a good cyclic life. Both experimental measurement and theorical calculation are applied to prove the promotive role of oxygen vacancy in supercapacitive performance of CoMoO 4-x. [ABSTRACT FROM AUTHOR]

We report on the development of a lattice model to predict structural, dynamical and capacitive properties of electrochemical double layer capacitors. The model uses input from molecular simulations, such as free energy profiles to describe the ion adsorption, and experiments, such as energy barriers for transitions between lattice sites. The model developed is approximately 10,000 times faster than common molecular simulations. We apply this model to a set of carbon structures with well-defined pore sizes and investigate the solvation effect by doing simulations with neat ionic liquids as well as acetonitrile-based electrolytes. We show that our model is able to predict quantities of adsorbed ions and capacitances in a range compatible with experimental values. We show that there is a strong dependency of the calculated properties on the pore size and on the presence or absence of solvent. In particular, for neat ionic liquids, larger capacitances are obtained for smaller pores, while the opposite trend is observed for organic electrolytes. Image 1 • A mesoscopic model to predict quantities of adsorbed ions and capacitances of porous carbons has been developed. • As a charging mechanism, ionic exchange is enhanced in porous carbons with small pores. • Addition of solvent in the electrolyte improves the ionic exchange mechanism. • Carbon electrodes store more ionic charge at the interface with neat ionic liquids. • Higher capacitance is obtained when combining neat ionic liquids with small pores or organic electrolytes with large pores. [ABSTRACT FROM AUTHOR]