Determination of transport and electrochemical kinetic parameters of M-H electrodes

Electrochemical and transport properties of La_0.65Ce_0.35Ni_3.55Co_0.75Mn_0.4Al_0.3 electrode were investigated in alkaline solution. The exchange current density, polarization resistance and the symmetry factor were determined from polarization curves obtained at low overpotentials. The symmetry factor was estimated to be 0.55 ± 0.01 and is independent of the state of charge. The equilibrium potential of the electrode was found to depend upon the hydrogen content in the alloy. The constant current discharge technique was used to determine the hydrogen diffusion coefficient in the alloy. The estimated value of /a ² at 0.1 C discharge rate was 1.39 × 10^–4s^–1.     

A kinetic study of pyrolysis in pitch impregnated electrodes

Carbon electrodes impregnated with three different pitches have been studied and compared. The electrode samples were heated up to 893 K in a thermogravimetric analyser at different heating rates. The weight loss was recorded with respect to time and temperature. A special technique and a calculation procedure have been developed to handle the pitch dripping from the electrode at low temperatures. A kinetic model assuming a first order reaction rate was used to interpret the data. This paper presents the experimental technique and the results of the study. The model predictions are compared with the data which shows a good agreement.     

Electrochemical studies of pyrite oxidation and reduction using freshly-fractured electrodes and rotating ring-disc electrodes

Oxidation and reduction processes on coal- and mineral-pyrite surfaces have been investigated to better understand the reactions that control the hydrophobicity and flotation behavior of pyrite. The incipient oxidation and reduction reactions were studied using fresh surfaces of pyrite that were created by in situ fracturing electrodes potentiostated at a predetermined potential. Chronoamperometry immediately after fracture and subsequent cyclic voltammetry have established that fresh fracture surfaces of pyrite instantaneously assume a unique potential (referred to as the “stable” potential) at which neither oxidation nor reduction takes place. For Peruvian and Chinese pyrites, the stable potential is −0.28 V (standard hydrogen electrode, SHE) at pH 9.2 and 0 V at pH 4.6. The initial oxidation of pyrite begins at potentials slightly positive of the stable potential and is believed to produce a hydrophobic sulfur-rich species, most likely a polysulfide or metal-deficient sulfide. A rotating ring-disc electrode (RRDE) was employed to study the kinetics and mechanisms of surface reactions on pyrite over moderate potential ranges. Two distinct soluble reduction products (ferrous hydroxide and HS⁻) and one distinct soluble oxidation product (ferrous hydroxide) were observed on pyrite in alkaline solutions. It is concluded that the initial oxidation of pyrite and the oxidation of ferrous to ferric hydroxide occur in a similar potential range. When the electrode is oxidized, e.g. by polishing, prior to experiments, the initial oxidation of pyrite is masked by the oxidation of ferrous hydroxide, making it difficult to study the oxidation of pyrite itself.     

Conductivity and atmospheric aging studies of polypyrrole-coated cotton fabrics

Polypyrrole (PPy) is one of the preferred alternatives among the intrinsically conductive polymers (ICPs). In this study, PPy-coated cotton (PPy-CT) fabrics were synthesized by two step in situ chemical polymerization. The reaction parameters, such as monomer concentration and temperature, were studied in detail. The surface resistivity of PPy-CT fabrics ranged ∼ 15–5000 Ω⁻². To assess long-term usage potential, the atmospheric aging of conductivity characteristics of treated fabrics was monitored over a period of 6 months. It was found that the synthesis temperature had a significant impact on conductivity and atmospheric aging of PPy-CT fabrics. Furthermore, various sulfonic acid sodium salts added as external doping agents during polymerization also had a positive effect. The scanning electron microscopy revealed smoother morphology of sulfonic acid salt doped PPy coatings. The overall study addresses the durability aspect of PPy-CT fabrics in potential applications areas. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Isothermal and temperature programmed kinetic studies of thermosets

Differential Scanning Calorimetry (DSC) is a popular method for the characterization of the cure kinetics of thermosetting materials. The experiments may be carried out under isothermal or temperature program modes. Several authors have pointed out the fact that the kinetic rate expressions for non-isothermal experiments must be different from the isothermal ones; however, this fact has been neglected in practice. In this work, we present a kinetic characterization of a thermosetting system using a phenomenological model for the reaction rate expression and DSC data from isothermal and non-isothermal experiments. In general, thermosetting materials exhibit the vitrification phenomenon which stops the reaction before complete conversion is achieved. Vitrification is taken into account in both Isothermal and temperature programmed experiments.     

A feasible kinetic model for the hydrogen oxidation on ruthenium electrodes

The hydrogen oxidation reaction (hor) was studied on a polycrystalline ruthenium electrode in H₂SO₄ solution at different rotation rates (ω). The experimental polarization curves recorded on steady state show the existence of a maximum current with a non-linear dependence of the current density on ω^1/2. On the basis of the Tafel-Heyrovsky-Volmer kinetic mechanism, coupled with a process of inhibition of active sites by the reversible electroadsorption of hydroxyl species, it was possible to appropriately describe the origin of the maximum current. The corresponding set of kinetic parameters was also calculated from the correlation of the experimental results with the proposed kinetic model.     

The magnesium and magnesium amalgam electrodes in aprotic organic solvents a kinetic study

Cathodic discharge of magnesium into mercury and the anodic dissolution of magnesium and magnesium amalgam in N,N-dimethylformamide (DMF) have been studied. The magnesium electrode is intrinsically, kinetically irreversible (k₀ ∼ 10⁻⁸ cm s⁻¹ at 22°C) in DMF and the reaction is further severely retarded by insoluble material formed in the reaction of magnesium ions with the reduction products of molecular oxygen. The effect of water as an impurity in the electrolyte is less severe than that of oxygen but at high concentrations water too inhibits the dissolution of magnesium. Additional experiments in other solvents (acetonitrile, AN and propylene carbonate, PC) are described.     

Scanning probe microscopy studies of Ebonex® electrodes

The porosity of Ebonex® electrodes is known to have a marked affect on their electrochemical properties. Atomic force microscopy (AFM) and scanning tunnelling microscopy (STM) have been used to investigate the topography of porous and fully-hardened (nonporous) Ebonex® at high resolution. AFM has also been used to study the early stages of copper electrodeposition on porous Ebonex® electrodes. Initial copper nucleation and growth were found to occur preferentially at surface pores.     

Parametric studies of uranium deposition and dissolution at solid electrodes

In pyroprocessing, most of uranium is separated from used nuclear fuel by depositing at a solid electrode of an electrorefiner. In the present study, a model, incorporating both diffusion and reaction kinetics of electrorefining, is developed. The model is applied to conduct parametrical studies on uranium deposition at an inert solid electrode to investigate properties of uranium deposition and dissolution and their dependence on the operating conditions. Both linear potential sweep and cyclic voltammetry processes are considered.     

A novel-design camera for simultaneous X-ray and kinetic studies

A novel-designed camera for simultaneous X-ray and kinetic studies was developed. The camera was used for studying the phase transformation and reaction kinetics in the hydrogenation of acetylene-ethylene mixture on Pd/Al₂O₃ catalysts.     

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.     

Conductivity and viscosity studies of ethylene carbonate based solutions containing lithium perchlorate

Specific conductivities and viscosities of lithium perchlorate at four different concentrations (0.5, 1.0, 1.5 and 2.0 M) in ethylene carbonate (EC) based binary mixed solvent systems at 25°C are reported. The co-solvents chosen were tetrahydrofuran (THF), 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL). Viscosity variations in all the three mixed solvent systems without electrolyte showed negative deviation from ideal behaviour thereby indicating the occurrence of a structure breaking effect in these three different binary systems. The increase in viscosity with increase in concentration of LiClO₄ is attributed to the structural enhancement through the formation of a solvated complex which occupies interstitials in the solvent mixtures. 1 M LiClO₄ solution shows maximum specific conductivity at 30 vol % EC for EC + DME and EC + DOL mixtures and at 50 vol % EC for EC + THF mixtures. Conductivity variations are explained on the basis of preferential solvation of lithium perchlorate by co-solvents (THF, DME and DOL) in their respective mixtures with ethylene carbonate.     

Conductivity studies of solid polymer electrolytes based on polyethers and polyphosphazene blends

In this work, electrical characteristics of several polymer electrolytes based on polyether and polyphosphazene blends are reported by means of complex impedance spectroscopy. In addition, a statistical analysis was conducted applying a mathematical model to a previously designed pattern to the purpose of gaining insight into the effect exerted on the conductivity of the electrolyte by the portion of each component in the blend. Evidence was obtained to prove that the dependence of conductivity on blend composition adjusts to a reduced cubic model, whose regression coefficients are determined in this work. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2181–2186, 1998     

Non-uniform accessibility and the use of hydrodynamic electrodes for mechanistic studies: A comparison of wall-jet and rotating disc electrodes

The merits of non-uniformly accessible electrodes for discriminating between electrode reaction mechanisms are established. In particular a comparison of the theoretical behaviour of the uniformly accessible rotating disc electrode and the highly non-uniformly accessible wall-jet electrode towards a wide range of different types of electrode process shows that mechanistic resolution is better achieved with the latter electrode geometry.     

Biological applications of diamond electrodes; electrochemical studies of riboflavin

The electrochemical study of flavines is a challenge using many solid-state electrode materials, since pronounced adsorption effects are observed which have deleterious effects on the recorded observations. In the present work three distinctive types of diamond electrode are employed to measure electrochemical charge transfer to riboflavin. At nanodiamond, it is found that rapid charge-transfer kinetics are observed, between the electrode and an adsorbed layer of around 2 monolayers thickness which rapidly forms at the electrode surface. In contrast, high phase purity microcrystalline diamond electrodes, show little adsorption, enabling solution phase diffusion controlled electrochemistry to be observed. Studies of the pH dependence of the observed electrochemistry are presented, which show results comparable to those observed at dropping mercury electrodes, and electrochemical measurements in the presence of power ultrasound are also described.     

Capacitive studies of the semiconducting properties of passive tin electrodes

The semiconducting properties of passive films grown on tin in slightly alkaline solutions have been studied by means of capacitance measurements. The results confirm that different types of films grow at low and high potentials. Thin films follow a Mott-Schottky behavior; both flat band potentials and donor concentrations have been estimated; the location of the donor level has also been determined. The behavior of thick films is more complex, probably due to cracks in the film.     

Haematoxylin sorption onto yak hair: kinetic and thermodynamic studies

The kinetic and thermodynamic behaviours of haematoxylin sorption onto yak hair as a model system were investigated. It was found that the sorption kinetics of haematoxylin on hair followed a pseudo‐second‐order kinetic model. The equilibrium sorption capacity increased as the temperature increased from 303 to 313 K, but dropped gradually as the temperature increased beyond 313 K. The maximum equilibrium sorption capacity was 24.04 mg g^?1 at T = 313 K. Meanwhile, the sorption isotherm data were ingood agreement with the Freundlich isotherm, presenting high coefficients (R² > 0.99). The free energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS) terms for haematoxylin dyeing were also determined, the negative values of ΔG and ΔH obtained indicated that the haematoxylin dye sorption process is a spontaneous and an exothermic one. Also, positive values of ΔS showed an increase in disorder of the system of haematoxylin sorption onto hair. Haematoxylin could be a good candidate as a natural green dye for hair fibre.     

Xylose hydrogenation: kinetic and NMR studies of the reaction mechanisms

Hydrogenation of xylose over Raney nickel was studied in a batch reactor. A pseudo-homogeneous kinetic model was able to prognose the xylose and xylitol concentrations rather well. The obtained fit for the activation energies suggests that external diffusion limitations are absent in our experimental conditions. The sugar equilibria studies gave new information about the temperature dependence of the –β-pyranose equilibria. It was found that the equilibria in D₂O follows an S-shaped curve, the equilibria being shifted towards the -form at higher temperatures.     

Comparative kinetic studies of polypyrrole electrogeneration from acetonitrile solutions

The formation and growth of polypyrrole films on platinum electrodes from acetonitrile media has been followed by microgravimetricex situ determination of the polymer grafted on the electrode. Kinetic parameters were also obtained from the charge consumed during polymerization. The electrogenerated polymer films were checked in the background electrolyte by voltammetry and chronoamperometry. Assuming a constant oxidation or reduction charge per unit of polymer weight, these charges were used to obtain the reaction order during the polymerization processes. The kinetics were found to be dependent on [pyrrole]^0.5 [LiClO₄]^0.5 from gravimetric determination; [pyrrole]^0.4 [LiClO₄]^0.5 from the polymerization charge; [pyrrole]^0.4 [LiClO₄]^0.5 from the anodic charge of the control voltammograms and [pyrrole]^0.4 [LiClO₄]^0.4 from the anodic and cathodic charge of the control chronoamperograms. Good agreement was found between the different methods. The good agreement between order dependence was due to the low water content.