Microwave activation of electrochemical processes: High temperature phenol and triclosan electro-oxidation at carbon and diamond electrodes

The electrochemical oxidation of phenolic compounds in aqueous media is known to be affected by the formation of electro-polymerized organic layers which lead to partial or complete electrode blocking. In this study the effect of high intensity microwave radiation applied locally at the electrode surface is investigated for the oxidation of phenol and triclosan in alkaline solution at a 500 μm diameter glassy carbon or at a 500 μm × 500 μm boron-doped diamond electrode. The temperature at the electrode surface and mass transport enhancement are determined by calibration with the Fe(CN)₆^3−/4− redox system in aqueous 0.3 M NaOH and 0.2 NaCl (pH 12) solution. The calibration shows that strong thermal and mass transport effects occur at both glassy carbon and boron-doped diamond electrodes. The average electrode temperature reaches up to 390 K and mass transport enhancements of more than 20-fold are possible. For the phenol electro-oxidation at glassy carbon electrodes and at a concentration below 2 mM a multi-electron oxidation (ca. 4 electrons) occurs in the presence of microwave radiation. For the electro-oxidation of the more hydrophobic triclosan only the one-electron oxidation occurs. Although currents are enhanced in presence of microwave radiation, rapid blocking of the electrode surface in particular at high phenol concentrations still occurs.     

Temperature dependence of electrochemically promoted NO reduction by C3H6 under stoichiometric conditions using Me/YSZ/Au (Me = Rh, RhPt, Pt) electrochemical catalysts

Temperature dependence of electrochemical promotion in C₃H₆–NO–O₂ reaction under stoichiometric conditions was investigated using Me/yttria-stabilized zirconia (YSZ)/Au (Me = Rh, RhPt, Pt) electrochemical catalysts, wherein electrodes were deposited by a sputtering method. Influences of the applied potential, the sintering extent of YSZ substrate, and the precious metal used for the electrode were investigated.Based on the analysis of catalytic reaction and electrode surface state, the longer sintering of YSZ substrate induced a positive effect for non-Faradaic electrochemical promotion of C₃H₆ oxidation by favoring oxygen spillover, and a negative effect for Faradaic electro-reduction of NO due to decrease in electrical conductivity. We postulated that RhPt electrode showed catalytic activity using the synergistic effect of Pt and Rh; however, higher activity than pure Rh electrode was not observed.     

High rate performance of highly dispersed C60 on activated carbon capacitor

Fullerene-activated carbon composite electrodes were prepared and their charge/discharge characteristics were studied for use in a high power electric double-layer capacitor. The capacitance of the C₆₀-loaded activated carbon fiber (ACF) electrodes became greater than that of the unloaded ACF at charge/discharge current densities above 50 mA/cm². In order to obtain a highly dispersed C₆₀-loaded electrode, an ultrasonic treatment was performed. The size of the C₆₀ agglomerate decreased from 1-2 to 0.1 μm or less, and the capacitance of the C₆₀-loaded ACF electrodes increased with an increase in the ultrasonic treatment time. A higher capacitance of 172 F/g was obtained at 50 mA/cm² on a 1 wt% C₆₀-loaded electrode with ultrasonic treatment, and the C₆₀-loaded ACF electrode also showed a higher cycle performance.     

The kinetics of hydrogen transport through amorphous Pd82−yNiySi18 alloys (y=0-32) by analysis of anodic current transient

Hydrogen transport through amorphous Pd_82−yNi_ySi₁₈ alloys (y=0-32) was investigated in 0.1 M NaOH solution by analysis of the anodic current transient. It was found that the anodic current transient shows the non-Cottrell behaviour, but its shape and value remain nearly constant regardless of the hydrogen discharging potential. From the coincidence of the anodic current transient theoretically calculated with that experimentally measured, it is suggested that the change in surface concentration of hydrogen with time is uniquely given by the rate of hydrogen transfer from absorbed state at the electrode sub-surface to adsorbed state on the electrode surface. This means that neither the ‘constraint of constant concentration’ nor the ‘constraint by Butler-Volmer behaviour’ is effective at the electrode surface during hydrogen extraction. On the basis of the theoretical current-time relation under the ‘constraint by hydrogen transfer of absorbed state to adsorbed state’, the hydrogen diffusivity was determined to have an almost constant value of (1.3±0.4)×10⁻⁸ cm² s⁻¹, irrespective of the Ni content and in the absence of Ni. On the other hand, it is inferred that the rate constant of hydrogen transfer decreases markedly with increasing Ni content due to the Ni(OH)₂ layer formed on the electrode surface.     

Enhanced electrochemical response of carbamazepine at a nano-structured sensing film of fullerene-C60 and its analytical applications

In the present work, electrochemical behavior of carbamazepine (CBZ) at fullerene-C₆₀ modified glassy carbon electrode has been investigated. Cyclic voltammogram of CBZ showed two each of oxidation and reduction peaks at fullerene-C₆₀ modified glassy carbon electrode (GCE) in phosphate buffer of pH 7.2 at the scan rate of 100 mV s⁻¹. The fullerene film on GCE surface exhibited excellent enhancement effects on electrochemical response of CBZ. Marked negative shift in peak potential with enhanced peak current was noticed in the cyclic and differential pulse voltammograms of CBZ at fullerene-C₆₀ modified electrode. The effect of accumulation time, amount of fullerene-C₆₀ and pH on electrochemical behavior of CBZ has been investigated using differential pulse voltammetry (DPV). An analytical method was developed for the determination of CBZ employing DPV. The oxidation peak current of CBZ was observed to be linearly dependent on the concentration of CBZ in the range of 90 nM–10 μM. The values of limit of detection and limit of quantification were found to be 16.2 nM and 54.0 nM, respectively. The developed DPV method was satisfactorily applied to the determination of CBZ in pharmaceutical formulations, spiked human serum and urine samples.     

The bioelectrocatalytic properties of cytochrome C by direct electrochemistry on DNA film modified electrode

The direct electrochemistry of cytochrome C can be performed in weak acidic and basic aqueous solutions. Cytochrome C can be deposited as a stable and electrochemically active film on a deoxyribonucleic acid (DNA) modified glassy carbon electrode. These films can also be produced on gold, platinum, and transparent semiconducting tin oxide electrodes. Two-layer modified electrodes containing cytochrome C and a DNA film were prepared by the deposition of cytochrome C on a DNA film modified electrode. The cytochrome C/DNA film was electrocatalytically oxidation active for l-cysteine in a pH 8.3 tris(hydroxymethyl)aminomethane (TRIS)-buffered aqueous solution through both Fe^III and Fe^IV species. The electrocatalytic oxidation current developed from the anodic peak of the redox couple. The electrocatalytic oxidation properties of ascorbic acid, NH₂OH, N₂H₄, and SO₃²⁻ by a cytochrome C/DNA film were also determined, and shown to be electrocatalytically active. An electrochemical quartz crystal microbalance, cyclic voltammetry, and direct spectroelectrochemistry were used to study in situ DNA deposition on a gold disc electrode and cytochrome C deposition on DNA/Au and DNA/GC films. The direct electrochemistry of cytochrome C can also be performed, and it can be deposited as a stable and electrochemically active film on polyvinyl sulfonate, polystyrene sulfonate, TiO₂, and polyethylene glycol modified glassy carbon electrodes. The results show that cytochrome C interacts with, and deposits on, a DNA film modified electrode, and that the cytochrome C (Fe^III) oxidized form is more easily deposited on a DNA film than the cytochrome C (Fe^II) reduced form.     

Continuous glucose monitoring using enzyme-immobilized platinized diamond microfiber electrodes

A sensitive and stable glucose biosensor for in vivo monitoring has been developed using boron-doped diamond microfiber (BDDMF) electrodes. The electrodes were modified with platinum nano-particles to detect H₂O₂, which was enzymatically produced by glucose oxidase (GOx) immobilized on the electrode surface. The platinum-modified BDDMF (Pt-BDDMF) electrodes exhibited much higher sensitivity compared to Pt-microfiber electrodes, Pt electrodes and Pt-modified diamond thin film electrodes. Deposition conditions for Pt nano-particles on the BDDMF electrodes and immobilization of GOx were optimized. GOx/overoxidized polypyrrole (OPPy)/Pt-modified BDDMF electrodes were applied for continuous interference-free glucose monitoring. Amperometric measurements of glucose showed a linear response in the range of 1-70 mM, with an R.S.D. of 3.7% for five injections of 100 μM glucose. The electrodes exhibited good stability over 3 months with no detected anodic current for ascorbic acid (AA), which is an interfering compound.     

Cycling-induced stress in lithium ion negative electrodes: LiAl/LiFePO4 and Li4Ti5O12/LiFePO4 cells

In this work, we examined the electrochemical behaviour of lithium ion batteries containing lithium iron phosphate as the positive electrode and systems based on Li-Al or Li-Ti-O as the negative electrode. These two systems differ in their potential versus the redox couple Li⁺/Li and in their morphological changes upon lithium insertion/deinsertion. Under relatively slow charge/discharge regimes, the lithium-aluminium alloys were found to deliver energies as high as 438 Wh kg⁻¹ but could withstand only a few cycles before crumbling, which precludes their use as negative electrodes. Negative electrodes consisting solely of aluminium performed even worse. However, an electrode made from a material with zero-strain associated to lithium introduction/removal such as a lithium titanate spinel exhibited good performance that was slightly dependent on the current rate used. The Li₄Ti₅O₁₂/LiFePO₄ cell provided capacities as high as 150 mAh g⁻¹ under C-rate in the 100th cycle.     

Effects of substrates on the capacitive performance of RuOx·nH2O and activated carbon-RuOx electrodes for supercapacitors

The electrochemical energy storage and delivery on the electrodes composed of hydrous ruthenium oxide (RuO_x·nH₂O) or activated carbon-hydrous ruthenium oxide (AC-RuO_x) composites are found to strongly depend on the substrate employed. The contact resistance at the active material-graphite interface is much lower than that at the active material-stainless steel (SS) mesh interface. Thin films of gold plus RuO_x·nH₂O deposited on SS meshes (RuO_x/Au/SS) are found to greatly improve the poor contact between SS meshes and electrode materials. The maximum specific capacitance (C_S,RuOx) of RuO_x·nH₂O, 1580 F g⁻¹ (measured at 1 mV s⁻¹), very close to the theoretic value, was obtained from an AC-RuO_x/RuO_x/Au/SS electrode with 10 wt.% sol-gel-derived RuO_x·nH₂O annealed in air at 200 °C for 2 h. The highly electrochemical reversibility, high-power characteristics, good stability, and improved frequency response of this AC-RuO_x/RuO_x/Au/SS electrode demonstrate its promising application potential in supercapacitors. The ultrahigh specific capacitance of RuO_x·nH₂O probably results from the uniform size distribution of RuO_x·nH₂O nanoparticles, ranged from 1.5 to 3 nm which is clearly observed from the high-resolution transmission electron microscopy (HRTEM).     

Effects of ultrasound on electrochemical oxidation mechanisms of p-substituted phenols at BDD and PbO2 anodes

The effects of low-frequency (40 kHz) ultrasound are investigated with regard to the effectiveness and mechanisms of electrochemical oxidation of p-substituted phenols (p-nitrophenol, p-hydroxybenzaldehyde, phenol, p-cresol, and p-methoxyphenol) at BDD (boron-doped diamond) and PbO₂ anodes. Although ultrasound improved the disappearance rates of p-substituted phenols at both the BDD and PbO₂ anodes, the degree of enhancement varied according to the type of p-substituted phenol and type of anode under consideration. At the BDD anode, the %Increase values were in the range 73-83% for p-substituted phenol disappearance and in the range 60-70% for COD removal. However, at the PbO₂ anode, the corresponding %Increase values were in the range 50-70% for disappearance of p-substituted phenols and only 5-25% for COD removal, much lower values than obtained at the BDD anode. Further investigations on the influence of ultrasound on the electrochemical oxidation mechanisms at BDD and PbO₂ anodes revealed that the different increase extent were due to the specialized electrochemical oxidation mechanisms at these two anodes. The hydroxyl radicals were mainly free at the BDD electrodes with a larger reaction zone, but adsorbed at the PbO₂ electrodes with a smaller reaction zone. Therefore, the enhancement due to ultrasound was greater at the BDD anode than at the PbO₂ anode.     

On the changing electrochemical behaviour of boron-doped diamond surfaces with time after cathodic pre-treatments

The electrochemical response of the Fe(CN)₆^4−/3− redox couple on boron-doped diamond (BDD) electrodes immediately after a cathodic pre-treatment and as a function of time exposed to atmospheric conditions is reported here. After this pre-treatment the electrode exhibits a changing electrochemical behaviour, i.e., a loss of the reversibility for the Fe(CN)₆^4−/3− redox couple as a function of time. Raman spectra showed that neither important bulk structural differences nor significant changes in the sp²/sp³ content are introduced into the BDD film by the cathodic pre-treatment indicating that H-terminated sites play an important role in the electrochemical response of the electrodes. Thus, the changing behaviour reflected by a progressive decrease of the electron transfer rate with time must be associated to a loss of superficial hydrogen due to oxidation by oxygen from the air, as confirmed by X-ray photoelectron spectroscopy (XPS) analysis. Moreover, it was also found that this changing electrochemical behaviour is inversely proportional to the doping level, suggesting that the boron content has a stabilizing effect on the H-terminated surface. These results point out the necessity of doing the cathodic pre-treatment just before the electrochemical experiments are carried out in order to ensure reliable and reproducible results.     

Electrochemical behavior of cysteine at a CuGeO3 nanowires modified glassy carbon electrode

A CuGeO₃ nanowire modified glassy carbon electrode was fabricated and characterized by scanning electron microscopy. The results of electrochemical impedance spectroscopy reveal that electron transfer through nanowire film is facile compared with that of bare glassy carbon electrode. The modified electrode exhibited a novel electrocatalytic behavior to the electrochemical reactions of l-cysteine in neutral solution, which was not reported previously. Two pairs of semi-reversible electrochemical peaks were observed and assigned to the processes of oxidation/reduction and adsorption/desorption of cysteine at the modified electrode, respectively. The electrochemical response of cysteine is poor in alkaline condition and is enhanced greatly in acidic solution, suggesting that hydrogen ions participate in the electrochemical oxidation process of cysteine. The intensities of two anodic peaks varied linearly with the concentration of cysteine in the range of 1 × 10⁻⁶ to 1 × 10⁻³ mol L⁻¹, which make it possible to sensitive detection of cysteine with the CuGeO₃ nanowire modified electrode. Furthermore, the modified electrode exhibited good reproducibility and stability.     

Electroreduction of oxygen on glassy carbon electrodes modified with in situ generated anthraquinone diazonium cations

The electrochemical reduction of oxygen on glassy carbon (GC) electrodes modified with in situ generated diazonium cations of anthraquinone (AQ) has been studied using the rotating disk electrode (RDE) technique. The electrografting of the GC electrodes was carried out in two different media: in acetonitrile and in an aqueous acidic solution (0.5 M HCl). 1- and 2-Aminoanthraquinone were used as starting compounds for the formation of the corresponding diazonium derivatives. The anthraquinone diazonium cations were generated by reaction of the aminoanthraquinones with tert-butyl nitrite and sodium nitrite in acetonitrile and in 0.5 M HCl, respectively. For comparison purposes, the previously synthesised and crystallised diazonium tetrafluoroborates of anthraquinone were used for the GC surface modification. Cyclic voltammetry was employed to determine the surface concentration of AQ in O₂ free 0.1 M KOH. The electrocatalytic behaviour towards O₂ reduction was similar for all the AQ-modified electrodes studied. The kinetic parameters of oxygen reduction were determined using a surface redox catalytic cycle model. The rate constant of the reaction between the semiquinone radical anion of AQ and molecular oxygen was virtually independent of the point of attachment of the quinone to the electrode surface.     

On the fractal study of LiMn2O4 electrode surface

Fractal dimension of a LiMn₂O₄ electrode prepared by sol-gel method was determined using electrochemical techniques based on the phenomenon of “diffusion towards electrode surface”. A simple discussion was made on the methodology to understand what is really estimated as the fractal dimension. It was demonstrated that the value of fractal dimension determined based on electrochemical methods is strongly dependent on the electrochemical system situation. This is generally true for all real electrodes involving insertion/extraction processes. This comes from the fact that surface morphology of the electrode is subject of significant changes during the electrochemical experiment.     

Electrochemical characterisation of Ni/AlX hydrotalcites and their electrocatalytic behaviour

Some ‘conductive’ hydrotalcites containing nickel as divalent cation and Cl⁻, SO₄²⁻ or CO₃²⁻ as interlayer anion, have been synthesised and membranised with different types of polymeric matrices to modify glassy carbon electrodes. The mechanism responsible of the conductive properties has been deeply investigated by voltammetric techniques using both a stationary and a rotating disk electrode, demonstrating that the interlayer anion does not affect significantly the electrochemical behaviour of the material. The electrocatalytic properties have been also studied pointing out the key role of the steric hindrance of the oxidisable substrate. In particular, mono- and polyhydric compounds have been taken into account. As to the electrocatalytic efficiency, the nature of the interlayer anion and, hence, the dimension of the interlayer spacing, is important in determining the sensitivity of the measurement since it can affect the analyte diffusion inside the hydrotalcite structure.     

Electrochemical reduction of oxygen on gold and boron-doped diamond electrodes in ambient temperature, molten acetamide-urea-ammonium nitrate eutectic melt

The electrochemical reduction of oxygen has been studied on gold, boron-doped diamond (BDD) and glassy carbon (GC) electrodes in a ternary eutectic mixture of acetamide (CH₃CONH₂), urea (NH₂CONH₂) and ammonium nitrate (NH₄NO₃). Cyclic voltammetry (CV), differential pulse voltammetry (DPV), chronoamperometry and rotating disk electrode (RDE) voltammetry techniques have been employed to follow oxygen reduction reaction (ORR). The mechanism for the electrochemical reduction of oxygen on polycrystalline gold involves 2-step, 2-electron pathways of O₂ to H₂O₂ and further reduction of H₂O₂ to H₂O. The first 2-electron reduction of O₂ to H₂O₂ passes through superoxide intermediate by 1-electron reduction of oxygen. Kinetic results suggest that the initial 1-electron reduction of oxygen to HO₂ is the rate-determining step of ORR on gold surfaces. The chronoamperometric and RDE studies show a potential dependent change in the number of electrons on gold electrode. The oxygen reduction reaction on boron-doped diamond (BDD) seems to proceed via a direct 4-electron process. The reduction of oxygen on the glassy carbon (GC) electrode is a single step, irreversible, diffusion limited 2-electron reduction process to peroxide.     

Chitosan–Fe3O4 nanocomposite based electrochemical sensors for the determination of bisphenol A

This paper reports the application of chitosan–Fe₃O₄ (CS–Fe₃O₄) nanocomposite modified glassy carbon electrodes for the amperometric determination of bisphenol A (BPA). We observed that the CS–Fe₃O₄ nanocomposite could remarkably enhance the current response and decrease its oxidation overpotential in the electrochemical detection. Experimental parameters, such as the amount of the CS–Fe₃O₄, the accumulation potential and time, the pH value of buffer solution etc. were optimized. Under the optimized conditions, the oxidation peak current was proportional to BPA concentration in the range between 5.0 × 10⁻⁸ and 3.0 × 10⁻⁵ mol dm⁻³ with the correlation coefficient of 0.9992 and the limit of detection of 8.0 × 10⁻⁹ mol dm⁻³ (S/N = 3). The proposed sensors were successfully employed to determine BPA in real plastic products and the recoveries were between 92.0% and 06.2%. This strategy might open more opportunities for the electrochemical determination of BPA in practical applications. Additionally, the leaching studies of BPA on incubation time using the as-prepared modified electrode were successfully carried out.     

Antimonides (FeSb2, CrSb2) with orthorhombic structure and their nanocomposites for rechargeable Li-ion batteries

Intermetallic FeSb₂ and CrSb₂ and their nanocomposites (FeSb₂/C and Sb/Cr₃C₂/C) were prepared using solid-state routes, such as heat-treatment and high-energy mechanical milling, in order to enhance the electrochemical properties of Sb. These electrodes were tested as anode materials for rechargeable Li-ion batteries. The reaction mechanism of intermetallic FeSb₂ and CrSb₂ was investigated using ex situ X-ray diffraction and high resolution transmission electron microscopy. The FeSb₂/C and Sb/Cr₃C₂/C nanocomposite electrodes exhibited greatly enhanced electrochemical behaviors compared to the FeSb₂ and CrSb₂ electrodes. Additionally, the Sb/Cr₃C₂/C nanocomposite electrode showed a better electrochemical performance than the FeSb₂/C nanocomposite electrode.     

Electrochemical behaviors of graphene-ZnO and graphene-SnO2 composite films for supercapacitors

Graphene, graphene-ZnO and graphene-SnO₂ films were successfully synthesized and used as electrode materials for electrochemical supercapacitors, respectively. The screen-printing approach was employed to fabricate graphene film on graphite substrate while the ZnO and SnO₂ were deposited on graphene films by ultrasonic spray pyrolysis. The electrochemical performances of these electrodes were comparatively analyzed through electrochemical impedance spectrometry, cyclic voltammetry and chronopotentiometry tests. The results showed that the incorporation of ZnO or SnO₂ improved the capacitive performance of graphene electrode. Graphene-ZnO composite electrode exhibited higher capacitance value (61.7 F/g) and maximum power density (4.8 kW/kg) as compared with graphene-SnO₂ and pure graphene electrodes.     

Electrochemical studies of ganciclovir at boron-doped nanocrystalline diamond electrodes

The electrochemical oxidation of ganciclovir was investigated at boron-doped nanocrystalline diamond (BDND) electrodes by the use of cyclic voltammetry and differential pulse voltammetry. The optimization of the experimental variables including supporting electrolyte and pH value was studied, and the 0.04-M Britton-Robinson buffer solution (pH 2.5) was selected. The relationship of the oxidation peak potential to scan rate and pH value was also investigated, and 2 electron transfer and 2 proton participation for the oxidation process of ganciclovir at BDND electrode were obtained. Compared with boron-doped microcrystalline diamond and glassy carbon electrodes, the BDND electrode demonstrated the wider linear range of 0.5-350 μM, lower limit of detection of 0.2 μM, and higher reproducibility and stability for the determination of ganciclovir under the optimum conditions. For the analysis of ganciclovir in human serum at the BDND electrodes, precision and accuracy were checked by recovery experiments.