Regeneration of expanded graphite electrodes by joined electrochemical and ozone treatment in liquid phase

The researches presented in this work were devoted to electrochemico-chemical regeneration of exhausted electrode made of expanded graphite (EG). The aimed process was conducted by electrochemical treatment and ozone flow performed together in wet environment. EG was covered with insoluble products of incomplete oxidation of phenol formed during cyclic voltammetry measurement. The same electrochemical technique was applied for evaluation of regeneration efficiency. To understand the process of EG regeneration, the electrode was characterized by calculating of BET surface, FTIR and XPS analysis. Moreover, SEM images of the investigated samples were also done. Obtained results have showed the success of regeneration treatment, which led to significant enhancement of electrode activity compared to original EG. The present work also revealed that the mechanism of phenol electrooxidation is changed after the regeneration treatment of electrode material. This effect is probably caused by the modification of chemical composition of EG surface due to its interactions with OH radicals intensively generated during the process of regeneration.

     

Improved electrooxidation of phenol at exfoliated graphite electrodes

In the presented paper, we report on electrochemical oxidation of phenol occurring at exfoliated graphite (EG) in alkaline solution. The mechanism of the electrocatalytic reaction of phenol oxidation was modified on adding methanol to the phenol-containing electrolyte. Using the voltammetry method, the influence of methanol additive on cyclic behavior of EG electrode was examined. A particular attention has been paid to the first two cycles when an abrupt decrease in electrocatalytic activity of various electrode materials has been observed. The results obtained showed that in the presence of methanol EG, electrode preserves its electrocatalytic activity for a longer time of phenol oxidation. In the absence of methanol in a phenol/KOH electrolyte, the charge of phenol oxidation peaks decreases sharply on cycling, whereas in the presence of methanol, the observed drop is considerably inhibited. The anodic charge attained for the 15th cycle of phenol oxidation in methanol-admixed electrolyte is the same as that for the third cycle recorded in methanol-free electrolyte. The thermogravimetric analysis (TG), Fourier-transformed infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) data showed that an improved electrocatalytic activity of EG can be accounted for by new chemical composition of oligomer film built on the EG surface with the participation of methanol and/or the products of its oxidation.Presented at the 4th Baltic Conference on Electrochemistry, Greifswald, March 13–16, 2005.     

Electrochemical derivatization of carbon surface by reduction of diazonium salts in situ generated from nitro precursors in aqueous solutions and electrocatalytic ability of the modified electrode toward hydrogen peroxide

The carbon electrode was covalently modified by electrochemical reduction of nitro precursor in the presence of NaNO₂ in aqueous solutions. The nitro precursor used is p-nitrophenyl phosphate, a well-known chromogenic substrate for the determination of acid and alkaline phosphatases. It is the first method for the covalent modification of carbon surface with a phosphate group. The modified electrode was characterized via cyclic voltammetry, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy. It displays good electrocatalytic activity toward hydrogen peroxide reduction.     

Biomimetic Sensor for Dobutamine Employing Nano‐ TiO2/Nafion/Carbon Nanoparticles Modified Electrode

A novel voltammetric biosensor based on nano‐TiO₂/nafion/carbon nanoparticles modified glassy carbon electrode (TiO₂/N/CNP/GCE) was developed for the determination of dobutamine (DBA). Characterization of the surface morphology and property of TiO₂/N/CNP layer was carried out by the scanning electron microscopy and atomic force microscopy. The electrochemical performance of the modified electrode was investigated by means of the cyclic voltammetry, differential pulse voltammetry and electrochemical impedance spectroscopy techniques. Effective experimental variables, such as the scan rate, pH of the supporting electrolyte, drop size of the casted modifier suspension and accumulation conditions of DBA on the surface of TiO₂/N/CNP/GCE were optimized. Under the optimized conditions, a significant electrochemical improvement was observed toward the electro‐oxidation of DBA on the surface of TiO₂/N/CNP/GCE compared to the bare GCE. Under the optimized conditions, a wide linear dynamic range (6 nM–1 µM) with a low detection limit of 2 nM for DBA was resulted. The prepared modified electrode shows high sensitivity, stability and good reproducibility in the determination of DBA concentrations. Satisfactory results were obtained for DBA analysis in the pharmaceutical and clinical preparations using TiO₂/N/CNP/GCE.     

Electrocatalytic Investigations of Cu(II) and Fe(III) Complexes of Salophen Derivative Schiff Bases on the Pencil Graphite Electrode

This present study describes a pencil graphite electrode surface covered with Cu(II) and Fe(III) complexes based on Salophen derivative Schiff bases in acetonitrile solution containing LiClO₄ as a supporting electrolyte. Cyclic voltammetry method was used for the surface modification procedure with 25 cycle at a sweep rate of 50 mV s^?1. Some characterization methods were used to identify of the prepared modified surfaces including cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), Ultraviolet‐visible Spectroscopy (UV‐Vis), and Scanning Electron Microscopy/Energy Dispersive X‐ray Spectroscopy (SEM/SEM‐EDX). The catalytic activity of these modified surfaces on the electrochemical oxidation of catechol (CC) was investigated and they compared with each other. The results demonstrated that these modified electrodes showed perfect electrocatalytic activity on the catechol determination, however the modified electrode prepared with the Cu(II) complex has higher catalytic activity than this prepared with the Fe(III) complex thanks to its the lower detection limit.     

Electrochemical Sensor Based on Thioether Oligomer Poly(N‐vinylpyrrolidone)‐modified Gold Electrode for Bisphenol A Detection

Presently, bisphenol A (BPA) has been added to the list of substances of very high concern as endocrine disruptors. According to the literature, exposure to bisphenol A even at low doses may result in adverse health effects. In this study, electrochemical sensor of Bisphenol A based on thioether DDT‐Poly(N‐vinylpyrrolidone) oligomer has been developed. The thioether oligomer, which is capable of recognizing BPA, was prepared and used for gold electrode modification. The characterization of the modified gold electrode and the synthesized thioether oligomer were carried out by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), atomic force microscopy (AFM), Fourier‐transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance spectroscopy (¹H NMR) and Size exclusion chromatography (SEC). Obtained results indicate that the modified electrode shows good electrochemical activity, good sensitivity and reproducibility for BPA detection. It exhibited a good linear relationship ranging from 1 to 20 pg/mL, and the detection limit was found to be 1.9 pg/mL at S/N=3. Several interfering species such as hydroquinone, phenol and resorcinol were used and their behaviors on the modified gold electrode were investigated.     

Electrooxidation of phenol at exfoliated graphite electrode in alkaline solution

The present paper reports on exfoliated graphite (EG) used for the cyclic electrochemical process of phenol oxidation in alkaline solution. It is shown that the electrochemical activity of anode-produced EG decreases considerably in the second cycle due to the deposition of an oligomer film, composed of the products of phenol oxidation, on the EG surface. Thermal treatment of the inactive graphite anode in air at 500 °C provided a regenerated material of activity three times higher for the first cycle and 2.6 times higher for three cycles as compared to the original anode. The reason for such a behavior is assigned to a carbon film formed on the EG surface during the carbonization/oxidation processes involving the products of phenol oxidation. Comparative studies showed that electroactivity of the original EG can also be enhanced if before the process of phenol oxidation the original EG is activated by heat treatment. Unfortunately, the electrochemical activity of the product of such a treatment is higher only for the first cycle of phenol oxidation and drops dramatically in the following cycles.Contribution to the 3rd Baltic Conference on Electrochemistry, Gdask-Sobieszewo, 23–26 April 2003. Dedicated to the memory of Harry B. Mark, Jr. (28 February 1934–3 March 2003)     

A Novel Sensitive Electrochemical Sensor Based on Nickel Chloride Solution Modified Glassy Carbon Electrode for Curcumin Determination

In this research, the high conductivity of nickel chloride solution as well as the ability of nickel ions in establishing particular bonds with curcumin was benefited to fabricate a new electrochemical sensor based on nickel chloride solution modified glassy carbon electrode (NiCl₂/GCE) for detection and measurement of curcumin in human blood serum. Atomic force microscope (AFM), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) methods indicated that using nickel chloride solution for the modification of the glassy carbon electrode (GCE) surface had a significant effect on improvement of the electrode performance. Differential pulse voltammetry (DPV) was used for quantitative measurement of curcumin, which exhibited the linear response of NiCl₂/GCE toward curcumin within the concentration range of 10–600 μM and provided the detection limit of 0.109 μM for curcumin in human blood serum.     

Investigation on structural,morphological, and electrochemical properties of mesoporous cobalt oxide-infiltrated NaY zeolite

We report the synthesis and characterization of a new mesoporous cobalt oxide-infiltrated NaY zeolite prepared by ion-exchange route. The scanning electronic microscopy (SEM) image shows homogenous and uniform grains size distributions smaller than 1 μm, unlike to CoO_x particles, elaborated under the same conditions. The energy dispersion spectroscopy (EDS) data confirm the presence of cobalt, oxygen, silicon, and aluminum. The X-ray diffraction indicates a partial crystallization of cobalt oxide and the formation of new phases. N₂ adsorption-desorption measurement shows a high-specific surface area for the modified material (579 m² g^?1), with Barrett-Joyner-Halenda (BJH) pore diameters in the range (3–8 nm). The cyclic voltammetry indicates a typical faradic process, and the electrochemical impedance spectroscopy exhibits Warburg diffusion at low frequencies. The charge-discharge curve shows a clear improvement in the charge capacity of the modified material compared to CoO_x, due to the increased specific surface area. The galvanostatic charge-discharge tests of the modified electrode exhibit a typical battery behavior preceded by a pseudo-capacitive phenomenon.     

An Exfoliated Graphite Based Electrochemical Sensor for As(III) in Water

In this work, we present the application of an exfoliated graphite electrode modified with gold nanoparticles (AuNPs) for the detection of As(III) in acidic media. Gold nanoparticles were deposited on the surface of an exfoliated graphite electrode by electrodeposition at a potential window of ?0.2 V to 1.2 V. This was followed by activation in 0.5 M H₂SO₄ with 10 cycles from 0.6 V to 1.4 V. The modification of exfoliated graphite (EG) showed an increased electroactive surface area of the electrode and improved peak current output in a Fe(CN)₆^3?/4? redox probe. EG‐AuNPs electrode was used to detect As(III) in 1.0 M HNO₃ using square wave anodic stripping voltammetry (SWASV) technique at optimum conditions of pH 3, deposition potential of ?0.8 V, deposition time of 180 s, frequency of 5 Hz and pulse amplitude of 50 mV. The EG‐AuNPs electrode detected As(III) in solution to a limit of 0.58 ppb with regression of 0.9993. The method reported is simple, cheap and possesses good reproducibility. The developed electrochemical sensor was applied in the detection of As (III) in an industrial real water sample. The results of the real water sample analysis from the developed method are comparable with the inductively coupled plasma – optical emission spectroscopy (ICP‐OES) results.     

Electrochemical Behavior and Ultrasensitive,Simple and Effective Voltammetric Determination of Acetaminophen Using Modified Glassy Carbon Electrode Based on 4-Hydroxyquinoline-3-carboxylic Acid

An electrochemical oxidation of acetaminophen (ACOP) has been successfully performed by using glassy carbon electrode covered with 4-hydroxyquinoline-3-carboxylic acid (4HQ3CA) to reinforce electrode's feature. To characterize the modified electrode (4HQ3CA/GC), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and Fourier transform infrared spectroscopy (FT-IR) techniques were used. The finding optimum conditions (supporting electrolyte, pH) and the electrochemical determination studies were performed with differential pulse voltammetry (DPV). It was decided that the supporting electrolyte medium suitable for ACOP determination was Britton-Robinson (BR) buffer and the effect of pH change on the oxidation peak of ACOP in this media was investigated. The effect of changing scan rate on the oxidation peak of ACOP was examined and this study showed that the oxidation process of ACOP on the 4HQ3CA/GC modified electrode surface was diffusion and adsorption controlled process. A wide concentration range from 0.0025 μM to 141 μM with a limit of detection (LOD) of 5.98×10⁻¹⁰ M (3 s/m) was obtained. This prepared sensor was carried out for the determination of ACOP in pharmaceutical sample.     

Traitement électrochimique d' eaux usées chargées de phénol : étude comparative sur des électrodes de dioxyde de plomb et de platine

Electrochemical treatment of waste water containing phenol: a comparative study on lead dioxide and platinum electrodes. The objective of this work was to study the efficiency of the Pb/PbO₂ electrode for decomposing the molecule of phenol, then to compare it to a platinum model electrode. Preliminary investigations by cyclic voltammetry showed that the Pb/PbO₂ anode presents a good chemical and electrochemical stability and possesses a high oxygen overvoltage. The study also showed that the electrochemical oxidation of phenol on Pt and PbO₂ in acidic media is a complex process. Long-time electrolysis was carried out using a three potential-plateau program with different values of the oxidation potentials and different concentrations of phenol. The obtained results showed that the transformation of phenol is total on the Pb/PbO₂ anode and that it is partial on Pt. On the other hand an increase in the phenol concentration decreases the rate of its conversion on the electrodes.     

A novel electrochemical sensor for determination of hydroquinone in water using FeWO4/SnO2 nanocomposite immobilized modified glassy carbon electrode

A novel electrochemical sensor based on iron tungstate doped tin oxide nanocomposite Nafion (FeWO₄/SnO₂/Nf) immobilized modified glassy carbon electrode (GCE) is fabricated to determine hydroquinone (HQ) in this present study. The structural morphology and phase of FeWO₄/SnO₂ nanocomposite are characterized by X-ray powder diffraction (XRD), energy dispersive X-ray analysis (EDX), Fourier transform infrared spectroscopy (FT-IR), high transmission electron microscopy (HR-TEM) and Field emission scanning electron microscopy (FE-SEM), Brunauer-Emmett-Teller (BET) and X-ray photoelectron spectroscopy (XPS) respectively. Electrochemical methods such as cyclic voltammetry (CV), difference pulse voltammetry (DPV) and amperometric (i-t curve) are used to describe the electrochemical performance of the surface modified electrode for HQ sensing studies. The FeWO₄/SnO₂/Nf immobilized GCE is exhibited excellent catalytic activity with the increasing current signal during HQ sensing. The linear range of response is obtained between 0.01 µM and 50 µM for HQ detection under optimized conditions and the low detection limit (LOD) is found to be 0.0013 µM. Moreover, the present modified electrode shows good reproducibility and excellent anti-interference behavior. In addition, the present electrochemical sensor is applied to the real samples of collected waters from various sources and the obtained experimental results are quite satisfactory.     

Multiple anodic regeneration of exfoliated graphite electrodes spent in the process of phenol electrooxidation

The present paper deals with the studies concerning anodic regeneration of exfoliated graphite (EG) electrodes coated with oligomer products of incomplete phenol electrooxidation. The electrochemical activity of regenerated samples depends on the concentration of regenerating electrolyte and potential applied during the regeneration. The effective method of a single-step regeneration of spent EG electrodes was found to be anodic treatment proceeded in 6 M KOH at the potential of 1.3 V vs. Hg/HgO/0.5 M KOH. During the oxidative regeneration of EG, the physical and chemical interactions between intensively generated active oxygen and/or OH· radical and oligomer covering the EG surface take place. As has been proved by FTIR and XPS analyses, active oxidative agents cause significant modification of chemical composition of oligomer that resulted in enhanced concentration of surface functionalities mainly involving C=O bonds. The results of multiple regeneration of EG electrode, considered as promising method for practical application, showed that the highest efficiency of regeneration is attained due to potentiostatic treatment carried out in 6 M KOH at the potential of 1.2 V. The electrochemical activity of EG regenerated at this potential increases gradually with the number of regenerating loops and after the third regeneration is almost fourfold higher compared to that of the original EG.     

A novel nitrite sensor fabricated through anchoring nickel-tetrahydroxy-phthalocyanine and polyethylene oxide film onto glassy carbon electrode by a two-step covalent modification approach

In this paper, we present a two-step covalent modification approach to fabricate a novel nitrite sensor through anchoring nickel-tetrahydroxy-phthalocyanine (NiPc(OH)₄) and polyethylene oxide (PEO) onto a glassy carbon electrode (GCE). The surface morphology of the prepared NiPc(OH)₄/PEO composite films under different dry conditions was characterized by scanning electron microscopy (SEM). The electrochemical behavior of NiPc(OH)₄/PEO composite film modified GCE toward the catalytic oxidation of nitrite in pH 7.0 phosphate buffer solution (PBS) was investigated by cyclic voltammetry (CV). After drying under an infrared lamp, the fabricated sensor showed a pronounced electrocatalytic activity improvement toward the oxidation of nitrite and led to a significant decrease in the anodic overpotentials compared with bare GCE, which should be ascribed to the synergistic effect of NiPc(OH)₄ and PEO, as well as the enlarged electrochemical effective surface area after drying. Using differential pulse voltammetry (DPV), the sensor gave a linear response to nitrite over the concentration range of 0.1–5,300 μM, with a detection limit of 0.0522 μM. The nitrite sensor exhibits good sensitivity, selectivity, and stability and has been applied for the determination of nitrite in water samples.     

Synthesis,characterization, and application of silver nanoparticle‐thiophenol nanocomposite film on the glassy carbon surface

In this study, 4‐thiophenol modified glassy carbon electrode was prepared by the reduction of 4‐diazothiophenol tetrafluoroborate salt. Silver nanoparticles were attached to the thiophenol modified surface to prepare a thiophenol‐silver nanoparticle composite film. 4‐Aminothiopenol molecules were deposited by self‐assembling technique to form multi‐layered nanofilms of TP/SNP/PhNH₂ on glassy carbon substrate. These surfaces were characterized by cyclic voltammetry, electrochemical impedance spectroscopy, X‐ray photoelectron spectroscopy, reflectance‐absorption infrared spectroscopy, and ellipsometry at each multilayer film growth process. Atomic force microscopic images of GC/TP/SNP/PhNH₂ surfaces were also acquired. The characterization methods show that the amine group containing surface permits the subsequent modification by a variety of coupling reactions for the immobilization of more complex systems. An application of the electrode modification for the determination of uric acid with a significantly lower detection limit is described. Copyright © 2013 John Wiley & Sons, Ltd.     

Modified multiwalled carbon nanotubes as an electrode reaction catalyst for an all vanadium redox flow battery

Different modified multiwalled carbon nanotubes (MWCNTs) are prepared by heat treatments in the air and in the H₂SO₄?+?HNO₃ (1:1) mixed acids which are investigated by transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Brunaur–Emmett–Teller, and cyclic voltammetry measurements. The results show the physicochemical properties of MWCNTs change significantly after these different modification processes, especially the electrochemical catalytic activity towards the VO₂ ⁺/VO²⁺ and V³⁺/V²⁺ redox pairs. The MWCNTs treated in the air at 600 °C for 30 min shows better electrochemical performances for the VO₂ ⁺/VO²⁺ redox reactions (58.8 and ?32.4 μA for the oxidation and reduction peaks at 10 mV?s^?1, respectively) than any other samples. Compared with the V³⁺/V²⁺ redox couple, the VO₂ ^+/VO²⁺ redox reactions are more easily affected by the physicochemical property changes of the MWCNTs. The enhanced electrochemical catalytic activity of the modified MWCNTs is not only related to the surface oxygen content, but also to the specific surface area, conductivity and the unique structure variations of the MWCNTs. The investigation demonstrated that the modified MWCNTs have a promising future application in the vanadium redox flow battery.     

Gold nanoparticle-modified glassy carbon electrode for electrochemical investigation of aliphatic di-carboxylic acids in aqueous media

The electrochemical reduction of di-carboxylic acids; oxalic, succinic, malic, and tartaric have been studied on the gold nanoparticles modified electrode in aqueous media solution of 0.1 M KCl. Gold nanoparticle (Au_NPs)-modified electrodes were prepared by the electrodeposition with cyclic voltammetric method onto glassy carbon electrode in acidic media. The surface morphology of the electrodeposited gold nanoparticles was examined by SEM. Also, the electrochemical properties of the prepared electrodes were investigated with different electrochemical techniques; cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy. Cyclic voltammetric, chronoamperometric, and electrochemical impedance spectroscopic techniques were used for investigating the electrochemical behavior of the particulate acids. The modification of the electrode with Au nanoparticles (Au_NPs) enables the appearance of cyclic voltammogram peaks completely clear and sharp for the acids under investigation in comparison with the poor behavior of them in absence of the modification. All acids undergo totally irreversible redox reaction in neutral and acid media. The cyclic voltammetric response of the investigated acids is sensitive to pH, as well as of the scan rate. Each acid has a different reduction peak position from the other acids depending on the structure of the acid undergo the electroreduction process. Further, the lowest unoccupied molecular orbital energies of the investigated acids have been theoretically evaluated and are compared with their electroreduction potential peaks.     

Study on surface acid-base property of carboxylic acid-terminated self-assembled monolayers by cyclic voltammetry and electrochemical impedance spectroscopy

Cyclic voltammetry and electrochemical impedance spec-troscopy were used to study the surface acid-base property of carboxylic acid-terminated self-assembled monolayers(SAMs).A carboxylic acid-terminated thiol,such as thioctic acid(1,2-dithiolane-3-pentanoic acid),was self-assembled on gold electrodes.Electron transfer between the bulk solution and the SAM modified electrode was studied at different pH using Fe(CN)63-as a probe.The surface pka of thioctic acid was determined by cyclic voltammetry and electrochemical impedance spectroscopy to be 5.6 ±0.1 and 5.8±0.1,respectively.The method is compared with other methods of monolayer pKa measurement.     

Cyclic voltammetry and impedance spectroscopy analysis for graphene-modified solid-state electrode transducers

The transducer of solid-state electrodes based on an epoxy-graphite composite was modified by two different methods, such as direct mixed and layer deposition of graphene (commercial and synthesized by electrochemical exfoliation of graphite). The modified electrodes were characterized by cyclic voltammetry and impedance spectroscopy. Also, scanning electron microscopy (SEM) was carried out to acquire information concerning the morphology of the composite electrode. Voltammetric measurements, in presence of [Fe(CN)₆]^3? as electroactive standard, determined a quasi-reversible electrochemical behavior under linear diffusion control. Electronic transference for modified and unmodified electrodes was compared. Solid-state electrode modified by inclusion of synthesized graphene showed a better electronic transference at electrode surface, due to the lower potential difference between anodic and cathodic peaks (ΔE = 125 mV) with respect to unmodified electrode (ΔE = 160 mV). Impedance spectroscopy characterization of electrode bodies in solid-state it was revealed a higher electronic conductivity and a supercapacitive behavior for the modified composites (values of intrinsic capacitances in the order of nanofarads) due to inclusions of graphite and graphene in the epoxy matrix. These inclusions were verified through SEM microscopy. The electronic conductivity and the supercapacitive character contributed both to the enhancement of electronic transference at electrode surface.