Ladislav Kavan

Perfluoro anion-exchange polymeric films on glassy carbon electrodes

Abstract Solutions of the perfluoro anion-exchange membrane TosHex® in a solvent mixture composed of methanol + isopropanol + water (1:1:1) were prepared and applied in coating glassy carbon electrodes. The evaporated films were used to accumulate the Fe(CN) 3− 4− 6 redox couple on the electrode surface. The magnitude of the electrochemical response of the loaded films is comparable with that for Nafion® incorporated cationic redox species. The multicharged Fe(CN) 3− 4− 6 couple accumulated in Tosflex® film causes an ion cross-linking of the polymeric backbone, thus decreasing ion transport in the film substantially.

Electrochemical, IR and XPS study of Nafion films prepared from hexamethylphosphortriamide solution

Abstract Membranes of 1100 ew Nafion 117 were dissolved in hexamethylphosphortriamide at 200°C. This solution was used to coat glassy carbon electrodes with a Nafion film. The electrochemical properties of these electrodes and their ion-exchange selectivity for methylviologen cations were found to be similar to those in the case of films prepared from alcoholic solvents. Electrochemical measurements were combined with IR spectroscopy and XPS study of the original Nafion membranes and of the prepared Nafion films. They are in support of a close similarity between the Nafion film and the original Nafion membrane. The existence of the hydrolysis of HMPA and the necessity of drying of the film vacuum are pointed out.

On the stability of polyyne

Abstract Results of first-principles, density-functional calculations on two interacting, parallel chains of polyyne are reported. The total energies indicate the existence of a (meta) stable configuration consisting of two crosslinked linear chains. This agrees with experimental findings that polyyne undergoes slow crosslinking if the interchain reactions are sterically possible. For slightly larger interchain distances the calculations predict that the chains prefer a zigzag structure and this may be the stable structure for a multichain system. The band structures indicate that the system changes from semiconducting to metallic upon reduction of the interchain distance. This agrees with the experimentally observed decrease of the energy onset of UV—VIS absorption and increase of electronic conductivity upon aging.

XPS study of carbon in electrochemical reduction products of poly(tetrafluoroethylene)

Abstract XPS spectra of reaction products of poly(tetrafluoroethylene) with lithium amalgam were studied. The primary reaction product is a mixture of LiF and elementary carbon in the sp- state in a molar ratio of 2:1. This carbon is very reactive, among others also with respect to air oxidation at room temperature leading to the formation of surface oxides with a well-defined chemical shift of the C 1s photoemission band, which can be attributed to COOH groups. The binding energy of 1s electrons in C atoms of the basic skeleton hidden in LiF is markedly higher than with other known modifications of carbon. The carbonaceous materials formed by the leaching out of LiF with water, or by the removal of LiF by melting, contain, after air oxidation, various types of surface oxides. The binding energy of C 1s photoelectrons in the resulting skeletons is comparable with that of other carbonaceous materials.

The role of ion transport in the electrochemical corrosion of fluoropolymers. Preparation and properties of n-doped polymeric carbon with mixed ion/electron conductivity

Abstract Electrochemical reductive corrosion of fluoropolymers by alkali metal amalgams leads to the formation of a compact uniform layer of reaction products on the fluoropolymer surface. This layer behaves as a solid electrolyte with mixed electronic-ionic conductivity; the electron/ion-conductive component in the layer is elemental carbon that is n -doped with alkali metal. The rate of the layer growth is controlled by transport of alkali metal cations, which increases substantially in the sequence K + + + .

Electronic resistivity of carbon in the product of electrochemical reduction of poly(tetrafluoroethylene), I

Abstract The product of electrochemical reduction of poly(tetrafluoroethylene) (a mixture of elemental carbon and LiF) is an electronic conductor with a time-dependent resistivity, whose value at room temperature decreases by one to two orders of magnitude within one year after the preparation. This decrease is accompanied by recrystallization of LiF and probably by gradual cross-linking of linear carbon chains in the sp state, which are the primary reduction product of the polymer. The temperature dependence of the resistivity satisfies the Mott “ T − 1 4 law,” evidence for the electron-hopping mechanism. The electronic resistivity increases abruptly on exposition to air and also by injection of Li + ions, the latter effect being reversible.

Study of chemical reactivity of carbon prepared by electrochemical reduction of poly(Tetrafluoroethylene) against NH3, NO and S in the gas phase

The product of electrochemical reduction of poly(tetrafluoroethylene) with lithium amalgam contains a very reactive form of carbon in elemental form. To initiate its chemical reaction with chosen compounds in the gas phase, it is necessary to destroy the compact inert matrix formed by LiF present in the mentioned product. This can be done either by dissolving with water under exlusion of air or by recrystallization of the LiF at 350°C. The products obtained by reaction of the carbon with NH3, NO or S in the gas phase contained 0.2–15 at.% of nitrogen or sulphur and were sensitive towards oxidation with air. They were studied by EPR and XPS methods.

Electrodes modified by perfluoro ion exchange polymeric films prepared from aqueous solutions of Nafion and Tosflex

Abstract A preparation of aqueous solutions of the perfluorinated ionomers Nafion ® and Tosflex ® is described. It is shown that Nafion and Tosflex films on electrodes prepared by evaporation of their aqueous solutions display very similar properties to those prepared from non-aqueous solvents. A simple procedure for the preparation of Nation films modified with enzymes is proposed.

The effect of pulsed laser irradiation on the properties of glassy carbon electrodes

Abstract The effect of irradiation by the pulsed light of an iodine laser (λ = 1.315 μm, pulse length 150 (μs, pulse intensities 0.037, 0.13 and 0.4 MW cm−2) on glassy carbon, graphite and platinum was studied in air. Scanning electron microscopy indicated removal of the upper layers of the materials and roughening of the surface. The treatment leads to an approximately 40% decrease in the content of chemisorbed oxygen, as demonstrated by X-ray photoelectron spectroscopy. Cyclic voltammetry of the ferricyanide/ferrocyanide couple on irradiated glassy carbon demonstrated an increase in the charge-transfer rate which deteriorates on ageing of the electrode. A decrease in the differential capacity of the electrode is compensated by an increase in surface roughness. The method is promising for electrode treatments prior to further chemical modification, as the state of the electrode surface can be finely regulated by varying the irradiation conditions (pulse intensity, duration and wavelength).

EPR study of carbon in the products of the electrochemical reduction of poly(tetrafluoroethylene)

Abstract The EPR spectra of elemental carbon in the products of the electrochemical reduction of poly(tetrafluoroethylene) by lithium amalgam were studied. The primary product of the reduction, C(LiF) 2 Li 0,2 , contains 2.1 · 10 20 spins/mol. The concentration of the spin centers in partially reacted poly(tetrafluoroethylene) is in agreement with the stoichiometry of the corrosion process in a wide range of degrees of conversion. The spin centers are reactive to atomospheric oxygen, but only in a thin surface layer of the sample. Deeper layers of the compact material are stable to atmospheric oxidation. By washing out the LiF with water under completely oxygen-free conditions, elemental carbon can be prepared with no apparent decrease in the concentration of spin centres, but the EPR signal becomes completely sensitive to atmospheric oxidation. The structural bonding aspects of the observed facts are discussed.