Electrochemical reduction of oxygen in gas-diffusion electrodes based on expanded natural graphite

Electroreduction of oxygen in gas-diffusion hydrophobized porous electrodes containing 8–10 wt % fluoroplastic, made of expanded natural graphite and acetylene black, in acidic (pH 0.6) and alkaline electrolytes was studied.     

Morphological and electrochemical characterisation of graphite electrodes coated with SPANI and Ni-cyclam

The preparation of the composite graphite–sulphonated polyaniline (SPANI)–Ni-cyclam was investigated with the purpose of dispersing a catalyst in a conducting matrix that allows a better electronic transfer from the surface of the electrode towards the reaction site. The graphite is a material with a high porosity, big area and cheaper than other materials. The electrochemical and morphological characterisations show that the best conditions for obtaining SPANI over graphite were reached with an anodic limit of 1,000 mV in H₂SO₄. The deposit of the Ni-cyclam over graphite–SPANI in basic medium allows the formation of a spongy and open material, which permits better accessibility to the catalytic sites, showing good catalytic properties. These results illustrate that the importance relays on the morphology and dispersion of the catalyst and not on the amount of catalytic material present in the composite. The electrocatalytical properties Ni-cyclam over graphite–SPANI was studied for the methanol oxidation in basic medium.     

Activation of glassy carbon electrodes by alternating current electrochemical treatment

Glassy carbon electrodes subjected to “severe” electrochemical treatment is saturated chloride solutions exhibit enhanced electrochemical reversibility compared to untreated electrodes. Apparently faster rates of electron-transfer result in lowering of the overpotential by 110–300 mV for eight electroactive species tested. Systematic alterations in the response in terms of peak potential, sensitivity, background current, and stability are reported. Scanning electron micrographs show the introduction of pits of random size distribution. From the practical point of view, the most significant improvement is observed in differential pulse measurements, that are highly sensitive to small changes in the rate of electron transfer. In amperometric detection for liquid chromatography, the pretreated electrodes are shown to facilitate the quantitation of different eluting species. It appears that the surface and bulk properties of the glassy carbon have profound effects on the observed behavior.     

Electrothermal atomic absorption spectrometry of elements after electrochemical deposition on graphite electrodes

The application of electrochemical deposition on graphite rods for separation and preconcentration prior to electrothermal atomic absorption spectrometry (a.a.s.) is examined. The metals to be determined are electrolyzed onto a graphite rod which is tehn transferred to a cup atomizer for a.a.s. Although only some of the element present in the solution is deposited on the surface of the graphite rod, favorable preconcentration rates are obtained. The method was tested on the determination of cadmium in aqueous solution. The precision is satisfactory for concentrations down to 5 × 10^?8 g l^?1 cadmium, and the detection limit is 4 × 10^?9 g l^?1.     

Reaction of toluene with ozone in the liquid phase

Reaction of toluene with ozone in acetic anhydride was studied. The dependence of the direction, selectivity, and extent of oxidation on the reaction temperature, solvent nature, and catalyst composition was examined.     

Detection of flavins by liquid chromatography using an electrochemical detector with two electrodes in series

A liquid chromatographic method for the selective detection of riboflavin (vitamin B₂) and flavin adenine dinucleotide (FAD) utilizing a thin-layer amperometric detector with two electrodes in series is described. The upstream electrode was held at ?0.4 V (vs. SCE) with the downstream electrode at +0.1 V (vs. SCE). The linear ranges are 40 ng?10 μg for riboflavin and 200 ng?8 μg for FAD and the limits of detection are 8 and 40 ng, respectively. The interference of thirteen different vitamins on the spectrophotometric or electrochemical detection of FAD and vitamin B₂ was studied and no interference was found using the two-electrode detector. The effects of light on riboflavin and FAD is discussed. The method is convenient, rapid and economic, and has high selectivity.     

Reaction of ozone with five-membered hetarenes in a liquid phase

Oxidation of pyrrole, furan, and thiophene by ozone in a solution of glacial acetic acid was studied. Peroxide compounds, which are, presumably, dimers of linear structure, were found as primary products. The kinetics of ozonolytic reactions with five-membered hetarenes was examined, and rate constants and experimental stoichiometric coefficients for ozone were determined.     

The electrochemical determination of phenolic derivates using multiple pulsed amperometry with graphite based electrodes

The electrochemical determinations of 4-chlorophenol (4-CP) and 4-nitrophenol (4-NP) by chronoamperometry (CA) and multiple pulsed amperometry (MPA) using expanded graphite-epoxy composite (EG-Epoxy) and rotating spectral graphite disc (SG) electrodes are reported. The electrochemical behaviours of both electrodes in the presence of organics informed about oxidation peak potential and the electrode fouling with organics concentration increasing. Setting up the oxidation peak potential as detection potential, only SG gave good electroanalytical performance using CA. However, by MPA applying both electrodes exhibited the capability to assess electrochemically and quantitatively the pollutants from aqueous solutions. UV spectrometric method detecting 4-CP and 4-NP at λ = 280 nm and λ = 398 nm wavelength, respectively was used for validation and parallel determinations.     

Regularities of solid phase crystallization on liquid metal electrodes

The measurement technique for the parameters of solid phase nucleation on liquid metal cathodes under electrolysis of molten salts is described. Their values are systematized and generalized for a wide range of temperatures, current densities, various deposited metals, liquid metal cathodes, emerging solid phases.     

Improvement in thermodynamic characteristics of sodium acetate trihydrate composite phase change material with expanded graphite

In this study, three different volume expansion ratios of expanded graphite (EG) are prepared and investigated to enhance the heat transfer efficiency of the sodium acetate trihydrate (SAT) composites. A series of SAT composite phase change materials (CPCMs) with EG were prepared. The influence of volume expansion ratio and mass fraction of EG on thermodynamic characteristics of SAT CPCMs was examined, including thermal conductivity, phase change temperature, enthalpy, latent heat storage and release time, and the degree of supercooling. Results showed that SAT CPCMs can be absorbed adequately by EG, and EG could enhance the heat transfer efficiency effectively. But it also brought some problems with the addition of all the three volume expansion ratios of EG, such as the poor enthalpy and serious supercooling. Particularly, the situation gets worse with the increase in mass and expansion ratio of EG. Therefore, it is better to choose the EG with proper expansion ratio or reduce the proportion of the EG which possesses higher expansion ratio. Besides, thermal cycling test and thermogravimetric analysis revealed that the SAT CPCMs with 3 mass% EG showed a good thermal stability.

     

Modification of platinum and graphite electrodes by cobalt hexacyanoferrate films

Methods for synthesizing cobalt hexacyanoferrate films on surfaces of the platinum and graphite electrodes are studied. The film properties depend on the modifying solution composition, including the nature of the supporting electrolyte cation, and the synthesis conditions. Reasons for the splitting of peaks in cyclic, voltammograms for the modified electrodes are put forth     

An electrochemical reactivation method for solid electrodes used in electrochemical detectors for high-performance liquid chromatography and flow injection analysis

An electrochemical reactivation method for solid electrodes used in electrochemical detectors is optimized for glassy carbon electrodes. Application of a voltage pulse train for 5 min is effective in restoring the response of the electrode after deactivation by organic compounds. Potassium hexacyanoferrate(II) and dl-synephrine were used to test the reactivation of electrodes; 2,6-dihydroxybenzoic acid and urine served to deactivate them. The effects of the amplitude, d.c. level, and frequency of the applied voltage pulse train, as well as the mode of termination, are discussed.     

Synchrotron-induced photoemission of GaAs electrodes after electrochemical treatment in aqueous electrolytes

Electrochemically-induced oxidation and reduction reactions of UHV-cleaved GaAs(110) surfaces have been studied after emersion under potential control using high resolution synchrotron-induced photoelectron spectroscopy. High quality spectra of the As and Ga core 3d lines and the valence band region have been obtained. The spectra of the anodic oxide show strong emission of bulk-like Ga(2)O(3) and some As(2)O(3) with the admixture of suboxides and hydroxides. Ga(2)O(3) and As(2)O(3) are cathodically reduced leaving the GaAs surface covered mostly with elemental As, some As-H and remnants of Ga-suboxides and -hydroxides.     

Electrooxidation of phenol by catalase immobilized on graphite electrodes

Electrocatalytic properties of catalase (CAT) immobilized on graphite and soot to mediate electrooxidation of phenol have been investigated. The kinetic parameters--Km, k(s), deltaG*Ar and Z0 of the process studied were calculated. Conclusions on a probable mechanism of the biocatalytic and electrochemical process observed were drawn from the calculated values of activation and kinetic parameters. A quantitative UV-spectrophotometrical approach was used as an analytical tool. The electrochemical oxidation of phenol was examined with potentiodynamic and polarization curves' method.     

Effect of expanded graphite lattice in exfoliated graphite nanofibers on hydrogen storage

A graphite exfoliation technique, using intercalation of a concentrated sulfuric/nitric acid mixture followed by a thermal shock, has successfully exfoliated a herringbone graphite nanofiber (GNF). The exfoliated GNF retains the overall nanosized dimensions of the original GNF, with the exfoliation temperature determining the degree of induced defects, lattice expansion, and resulting microstructure. High-resolution transmission electron microscopy indicated that the fibers treated at an intermediate temperature of 700 degrees C for 2 min had dislocations in the graphitic structure and a 4% increase in graphitic lattice spacing to 3.5 A. The fibers treated at 1000 degrees C for 36 h were expanded along the fiber axis, with regular intervals of graphitic and amorphous regions ranging from 0.5 to >50 nm in width. The surface area of the starting material was increased from 47 m(2)/g to 67 m(2)/g for the 700- degrees C treatment and to 555 m(2)/g for the 1000- degrees C treatment. Hydrogen uptake measurements at 20 bar indicate that the overall hydrogen uptake and operative adsorption temperature are sensitive to the structural variations and graphitic spacing. The increased surface area after the 1000- degrees C treatment led to a 1.2% hydrogen uptake at 77 K and 20 bar, a 3-fold increase in hydrogen physisorption of the starting material. The uptake of the 700- degrees C-treated material had a 0.29% uptake at 300 K and 20 bar; although low, this was a 14-fold uptake over the starting material and higher than other commonly used pretreatment methods that were tested in parallel. These results suggest that selective exfoliation of a nanofiber is a means by which to control the relative binding energy of the hydrogen interaction with the carbon structure and thus vary the operative adsorption temperature.     

New electrochemical and optical sensing techniques for the liquid phase

Three topics are summarized: advances in potentiometric sensors, in optical sensors, and in enzyme-based and enzyme-sensing biosensors. Received: 30 September 1998 / Accepted: 9 January 1999     

Preparation and thermal properties of expanded graphite/paraffin/organic montmorillonite composite phase change material

Expanded graphite (EG)/paraffin/organic montmorillonite (OMMT) composite phase change material (PCM) was prepared by using melt intercalation method. The microstructure of EG/paraffin/OMMT is observed by scanning electron microscope (SEM). The thermal properties are investigated by differential scanning calorimetry (DSC). The mass loss of EG/paraffin/OMMT after 50 heating cycles was measured for investigating the influence of EG and OMMT on the thermal properties of paraffin. The results show that EG and OMMT have the ability of adsorption and shape-stability. The melting point EG/paraffin/OMMT is decreased slightly with an addition of paraffin and the latent heat of EG/paraffin/OMMT is determined by the mass ratio of paraffin. The heat transfer efficiency of EG/paraffin/OMMT is strengthened and the heating time is decreased to one-sixth of that of paraffin by addition of EG and OMMT. The thermal stability of EG/paraffin/OMMT is improved by addition of OMMT.