S.Lj. Gojković

Kinetic study of methanol oxidation on carbon-supported PtRu electrocatalyst

Methanol electrooxidation was investigated on the carbon-supported PtRu electrocatalyst (1:1 atomic ratio) in acid media. X-ray diffraction measurement indicated alloying of Pt and Ru. Cyclic voltammetry of the sample reflects the amount of Ru in the catalyst and its ability to adsorb OH radicals. Tafel plots for the oxidation of 0.02–3 M methanol in the solutions containing 0.05–1 M HClO4 and in the temperature range 27–40 °C showed reasonably well-defined linear region with the slope of about 115 mV dec−1 at the low currents, irrespective of the experimental conditions employed. Reaction order with respect to methanol was found to be 0.5. A correlation between methanol oxidation rate and pseudocapacitive current of OH adsorption on Ru sites was established. It was proposed that bifunctional mechanism is operative with the reaction between methanol residues adsorbed on Pt sites and OH radicals adsorbed on Ru sites as the rate-determining step.

Methanol oxidation on an ink type electrode using Pt supported on high area carbons

Methanol oxidation was investigated on the following Pt catalysts: bare Pt electrode, Pt covered by Nafion® film, and Pt catalysts supported on high area carbons in the form of an ink. The reaction was carried out using slow linear potential sweep and potentiostatic steps. Active surface area of Pt particles was estimated from hydrogen desorption charge on cyclic voltammograms. The influence of Nafion film, Pt loading, type of carbon support, and methanol concentration were studied. No influence of Nafion® film on kinetic parameters of methanol oxidation was found. Tafel slope on smooth Pt was found to be about 90 mV dec−1, while on supported Pt catalysts time dependent values between 140 and 90 mV dec−1 were estimated from the potentiostatic decays by using data between 0.5 and 5 min. Specific activity was slightly dependent on Pt loading with flat maximum at about 20% Pt. Activity of Pt/SAB catalyst was two to three times higher than that of Pt/BP and Pt/XC-72 catalysts.

Oxygen reduction on iron—Part VI. Processes in alkaline solutions

The kinetics of oxygen reduction were studied on passivated iron in borate buffered and unbuffered solutions of pH ranging from 7.4 to 13.8. A rotating disk electrode was employed. The O2 reduction on iron under these conditions was found to occur on the oxidized surfaces. The reaction orders with respect to O2 and OH− are 1 and −1, respectively, in solutions of pH > 11 and 1 and 0 in solutions of pH < 11. The OH− reaction order was explained as being due to the acid—base properties of the passive film. The Tafel slope value is about −85mV. In solutions of pH < 11 there are two Tafel regions, whereas at higher pH values Tafelian behavior is observed only in the high current density region. Two reaction mechanisms are proposed having the same rate determining step which is the adsorption of O2 molecules on Fe(II)ox surface sites which act as the active centers.

Oxygen reduction on a duplex stainless steel

O2 reduction on duplex stainless steel was investigated in neutral and alkaline solutions with and without the presence of chloride ions. All polarization curves showed hysteresis with higher reaction rate in positive going sweep than in the negative one. Tafel slopes of about −90 mV dec−1 in neutral and alkaline solutions without chloride ions and about −150 mV dec−1 in NaCl solution were obtained. Reaction rate is almost independent on pH in solutions of pH < 12 while in more alkaline solutions reaction order with respect to OH− ions of about −1 was obtained. Reaction rate and kinetic parameters for O2 reduction on duplex stainless steel were compared to data for O2 reduction on pure Fe and great similarity was found.

Electrochemically deposited thin films of amorphous Fe–P alloy: Part I. Chemical composition and phase structure characterization

Abstract Thin Fe–P alloy films (0.5 to 25.0 μm thick) were electrodeposited from sulfate solutions. The influences of various deposition conditions (current density, time, pH and temperature) on the composition and structure of the deposited films were examined. These Fe–P films were first characterized by the ALSV. The voltammograms obtained show several current peaks, indicating the presence of several different phase structures in the deposit. Chemical composition of the phase structures in the film was determined by EDX analysis and the amorphous nature of the alloy was verified by XRD and DTA techniques. Chemical and phase structure compositions of the alloys (containing 15–50 at.% P in the form of phosphides FeP, Fe2P and/or Fe3P) were found to depend on deposition conditions. Appearance and morphology of the film alloys were also found to depend on deposition conditions, particularly current density, as illustrated by the SEM technique.

Hydrogen peroxide oxidation on passive iron in alkaline solutions

Abstract The kinetics of hydrogen peroxide oxidation were studied on passive iron in alkaline solutions of pH ranging from 9.8 to 13.8, using the rotating disk electrode. Measurements were taken using the linear sweep technique. The kinetic parameters of H 2 O 2 oxidation were determined: the Tafel slope is 110 mV dec −1 and the reaction order with respect to H 2 O 2 is 0.5 and with respect to the OH − ion 0.7. It is assumed that the rate determining step is the transfer of the first electron to the HO − 2 ion adsorbed under Temkin conditions. The pH dependence of the reaction rate is attributed to the pH dependent properties of the passive iron surface. There are no indications for the semiconducting behaviour of the passive film. The rate of H 2 O 2 chemical decomposition was estimated. It was found that the passive film on iron was a poor catalyst for H 2 O 2 chemical decomposition.

Oxygen reduction on iron. Part VII. Temperature dependence of oxygen reduction on passivated iron in alkaline solutions

Abstract The temperature dependence of the kinetics of O2 reduction was studied on passivated Fe in NaOH solutions having a concentration ranging from 0.010 to 1.0 mol dm−3 and in the temperature range from 2 to 42 °C. The apparent enthalpy of activation was determined and was found to be pH dependent. The Tafel slpe increased with temperature in the range −69 to −92 mV dec−1. The enthalpic and entropic components of the transfer coefficient were calculated and these values were also pH dependent. As a consequence of the pH dependence of the apparent enthalpy of activation and of the transfer coefficient components, the reaction order with respect to OH− could not be determined precisely. Its value depends on the potential and temperature and was estimated to be about −0.8. Such complex temperature and pH behavior of O2 reduction on passive Fe was attributed to the pH, temperature and potential dependence of the passive film properties.

Morphology and Capacitive Properties of [RuOxHy/Low Surface Area Carbon Black] Composite Materials Prepared by Sol Gel Procedure

The morphology and the capacitive properties of carbon supported hydrous ruthenium oxide (RuOxHy/C), prepared by impregnation of low-surface area carbon black VulcanÒ XC-72 R with solid phase of RuOxHy sol, were investigated by scanning electron microscopy, cyclic voltammetry and electrochemical impedance spectroscopy. The oxide sols of different ageing times, prepared by forced hydrolysis of RuCl3, were used for composite preparation. The prepared composites show considerably higher capacitance values in acid electrolyte if compared to carbon black support. Microscopic investigations show that the extent of impregnation and the composite surface appearance depend on ageing time. The composite capacitance increases with ageing time of oxide sol. A gradual decrease in capacitance of thermally untreated composite with charging/discharging cycles was registered, which appears to be related to the extent of impregnation, too. Electrochemical impedance spectroscopy measurements indicate an in-depth capacitance distribution through the composite layer. It was found that NafionÒ layer, used to ensure the adhesion of the composite layer to the current collector, influenced the composite impedance behavior, especially on the oxide-rich parts of the composite surface.

The Influence of Oxide Sol Properties on the Capacitive Behaviour of Carbon Supported Hydrous Ruthenium Oxide

Carbon supported hydrous ruthenium oxide was prepared by an ultrasonic threatment of carbon black-RuOxHy sol mixture (impregnation). Black Pearl 2000 was used as a supporting material, while an oxide sol was prepared by forced hydrolysis of hydrous RuCl3. The influence of the RuOxHy sol aging time and the amount of oxide solid phase in the impregnating medium on the capacitive behavior of prepared carbon supported hydrous ruthenium oxide was investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The oxide amount in the composite increases with increasing concentration of oxide particles in the impregnating medium. The composite specific capacitance decreases with increasing aging time of oxide sol, while increases with increasing concentration of oxide particles in the impregnating medium. Transition line model fits well the composite impedance behavior indicating a distinctive capacitive response of the inner and the outer composite active surface. This distinction becomes more pronounced as the amount of oxide in the composite increases.

Structural Effects in Electrocatalysis: Formic Acid Oxidation at Model and Real Pt Catalysts

Formic acid oxidation was studied at low-index Pt single crystals (model systems) as well as at the platinum catalyst supported on high area carbon (real catalyst) in HClO4. The Pt single crystals were characterized by LEED. The LEED patterns obtained after a mild heating of flame-annealed crystals have shown clean, well ordered unreconstructured surfaces. Pt-C supported catalyst was analyzed by AFM and STM in air and by XRD. AFM and STM images revealed the presence of Pt-C agglomerates of several tenth of nm consisting of Pt particles ranged from 2 nm to 6 nm. The electrocatalytic activity of these catalysts in formic acid oxidation increased in a sequence: Pt(100) < Pt(110) < Pt-C/GC < Pt(111).