Showing papers on "Overpotential published in 2001"

The Electrochemistry of Ni Pattern Anodes Used as Solid Oxide Fuel Cell Model Electrodes

Abstract: Ni pattern electrodes with well-defined triple phase boundary (TPB) lengths were prepared in order to investigate the reaction mechanisms at solid oxide fuel cell (SOFC) anodes The anode microstructures were stable during thermal treatment and electrochemical measurements Electrochemical impedance spectroscopy was used to study the influence of the overpotential of the gas atmosphere, of the temperature, and of the pattern geometry on the electrochemical behavior of SOFC anodes It is found that the reaction kinetics are dominated by one main process This process is thermally activated with an activation of energy of E A = 088 ± 004 eV At overpotentials higher than 300 mV, a second process becomes relevant The partial pressure of water in the fuel gas atmosphere has a catalytic effect on the anode performance, whereas variations of the patrial pressure of hydrogen in the fuel gas atmosphere have no significant influence on the electrode behavior A model was established in order to explain the catalytic effect of water The direct proportionality between the relaxation frequency and the TPB length suggests a TPB limitation of the anode kinetics

In Situ Scanning Tunneling Microscopy Study of the Anodic Oxidation of Cu(111) in 0.1 M NaOH

Abstract: In situ electrochemical scanning tunneling microscopy (STM) measurements of the anodic oxidation of Cu(111) in 0.1 M NaOH are reported. Anodic oxidation is preceded, in the underpotential range, by adsorption of an ordered layer assigned to OH species. This ordered adlayer is a precursor of the oxide growing at higher potential with the copper surface reordering to mimic the structural arrangement of a (111) oriented Cu2O oxide. In the potential range of Cu(I) oxidation, a Cu2O(111) oxide film is formed with a faceted, and most likely hydroxylated, surface. The nucleation, growth, and crystallization of this Cu(I) oxide depend on the overpotential of oxidation. At low overpotential, poorly crystallized and one-monolayer-thick islands partially covering the substrate are formed after preferential nucleation at step edges. At higher overpotential, well crystallized and several-monolayer-thick films are formed, and the step edges are not preferential sites of nucleation. In the potential range of Cu(II) oxid...

The Mechanism of Porous Sm0.5Sr0.5CoO3 Cathodes Used in Solid Oxide Fuel Cells

Abstract: Overpotential and ac impedance spectra were measured to construct a model to describe porous Sm 1-x Sr x CoO 3 (SSC) cathodes for solid oxide fuel cells (SOFCs). Analysis of the impedance spectra revealed that there are three processes involved in the overall electrochemical reaction; (i) the adsorption/desorption process on the surface of the electrode. (ii) the ionic conduction in the bulk SSC, and (iii) the diffusion of oxygen in the gas phase. It was found that in air atmosphere, the reaction processes (i) and (ii) were dominant, while the diffusion process of gaseous oxygen was fast enough not to limit the overall reaction rate. A reaction model for the porous SSC cathodes used in SOFCs was proposed to determine the electrode resistance hy taking processes (i) and (ii) into account. It was found that our model explained the experimental results well. These results suggested the possibility of using our model to describe the cathodes with high ionic conductivity and to design the high-performance cathode systematically. The validity of the analysis hy conventional equivalent circuit was also discussed.

Electrodeposition of a thin germanium film on gold from a room temperature ionic liquid

Abstract: The electrodeposition of germanium on flame annealed Au(111) films was investigated by cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) in the room temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate containing GeI4 as germanium source The CV shows two quasireversible redox processes at 485 ± 5 mV and − 30 ± 5 mV, one peak couple at − 430 ± 20 mV and two irreversible diffusion controlled reduction peaks at − 910 ± 10 mV and − 1510 ± 10 mV s Pt quasi reference At potentials positive from the open circuit potential (ocp), gold oxidation starts at the steps with detectable rates up to 1 nm s−1 At − 2000 mV s Pt quasi reference, germanium overpotential deposition is observed The surface shows a remarkable reordering: an almost disordered surface structure transforms to a layered structure on the time scale of about 1 h with an averaged terrace height of 330 ± 30 pm indicative of a (111) oriented germanium bilayer At − 1500 mV the reoxidation starts both at the interior and at the edges of the deposited layers, leading to random wormlike nanostructures which heal on the time scale of about 2 h The healing is a complex process comprising (electro)dissolution/electrodeposition and surface diffusion phenomena A chemical attack of GeI4 on the deposits is clearly observed

Reliability and accuracy of measured overpotential in a three-electrode fuel cell system

Abstract: Numerical simulation was conducted to study the potential and current density distributions at the active electrode surface of a solid oxide fuel cell. The effects of electrode deviation, electrolyte thickness and electrode polarization resistance on the measurement error were investigated. For a coaxial anode/electrolyte/cathode system where the radius of the anode is greater than that of cathode, the cathode overpotential is overestimated while the anode overpotential is underestimated. Although the current interruption method or impedance spectroscopy can be employed to compensate/correct the error for a symmetric electrode configuration, it is not useful when dealing with the asymmetric electrode system. For the purpose of characterizing the respective overpotentials in a fuel cell, the cell configuration has to be carefully designed to minimize the measurement error, in particular the selection of the electrolyte thickness, which may cause significant error. For the anode-support single fuel cell, it is difficult to distinguish the polarization between the anode and cathode with reference to a reference electrode. However, numerical results can offer an approximate idea about the source/cause of the measurement error and provide design criteria for the fuel cell to improve the reliability and accuracy of the measurement technique.

Effect of the Metal on Electron Transfer across Self-Assembled Monolayers

Abstract: Electron-transfer kinetic measurements are made on one type of self-assembled monolayer {HS(CH2)15COOH with attached [RuII(NH3)5(4-aminomethylpyridine)]} on gold, platinum, and silver polycrystalline electrodes in one electrolyte at room temperature Cyclic voltammograms over a wide range of scan rates are analyzed to extract the rate constants as a function of overpotential Tafel plots are fitted to theoretical curves based on the Marcus DOS theory to obtain the standard rate constants and the reorganization energies A reorganization energy of 08 ± 01 eV is obtained on all three metals The standard rate constants for Au, Pt, and Ag are 10 ± 03, 17 ± 04, and 06 ± 02 s-1 The results confirm the theoretical prediction that the electronic coupling of “d” band states in the metal to remote redox centers is weak

Separation of anodic peaks of ascorbic acid and dopamine at an α-alanine covalently modified glassy carbon electrode

Abstract: A covalently modified glassy carbon electrode with an α-alanine monolayer was fabricated and was applied to the electrocatalytic oxidation of ascorbic acid, reducing the overpotential by about 280 mV with an obviously increased current response. The catalytic current obtained from square-wave voltammetry was linearly dependent on ascorbic acid concentration in the range 2.0 × 10−5–5.0 × 10−3 mol L−1 with a correlation coefficient of 0.997. Owing to the catalytic effect of the modified film towards ascorbic acid, the modified electrode resolved the overlapped voltammetric responses of ascorbic acid and dopamine into two well-defined square wave voltammetric peaks with peak-to-peak separation in potentials of about 180 mV. This can be used to allow the determination of ascorbic acid in the presence of dopamine. The modified electrode showed good selectivity, stability and anti-fouling properties.

Electrode Reaction of La1 − x Sr x CoO3 − d Cathodes on La0.8Sr0.2Ga0.8Mg0.2 O 3 − y Electrolyte in Solid Oxide Fuel Cells

Abstract: The electrode reaction mechanism was investigated at the porous La 1-x Sr x CoO 3-d (LSC)/La 0.8 Sr 0.2 Ga 0.8 Mg 0.2 O 3 (LSGM) interface (x = 0.2, 0.3, 0.4). Direct current (dc) polarization curves showed a linear relationship between the overpotential and the current density at the lower cathodic polarization. The interface conductivity was adopted to examine the activity for oxygen reduction at the LSC/LSGM interface, which was measured both from the de polarization curves and the alternating current impedance, The interface conductivity increased with Sr concentration in LSC and cathodic applied voltage. The higher Sr concentration of LSC showed the lower oxygen partial pressure dependence of the interface conductivity. The activation energy for the interface conductivity was almost the same for different Sr concentration of LSC (137-142 kJ mol -1 ). A similar reaction mechanism is assumed for the examined LSC. The interface conductivity has a relation with oxide ion diffusion in LSC, which can be the main factor to determine the cathode reaction rates under cathodic polarization.

Theory of Coupled Electron−Proton Transfer with Potential-Dependent Transfer Coefficients for Redox Couples Attached to Electrodes†

Abstract: A theoretical study of coupled electron−proton transfer is presented for heterogeneous electron transfer to redox centers attached to electrodes. Proton transfer steps are assumed to be at equilibrium and the transfer coefficients are assumed to exhibit the potential dependence predicted by Marcus Density-of-States theory. Five cases are considered: 1e1H, 1e2H, 2e, 2e1H, and 2e2H. Procedures are given for calculating the apparent standard rate constant, the apparent transfer coefficient, and the path of electron transfer as a function of overpotential and pH. The behavior of these parameters are compared with a previous theoretical treatment in which the transfer coefficient for each electron transfer step was assumed to be 0.5 independent of overpotential. The predictions of the two treatments are qualitatively similar but quantitatively different. In particular, when the transfer coefficient depends on the electrode potential, apparent standard rate constants are smaller, asymmetrical Tafel plots appea...

Effect of Lanthanide Oxide Additives on the High-Temperature Charge Acceptance Characteristics of Pasted Nickel Electrodes

Abstract: For the purpose of increasing high-temperature charge acceptance of nickel-metal hydride (NiMH) secondary batteries, we studied effect on the charge acceptance of pasted nickel electrodes by adding the oxides of a series of lanthanides (Pr to Lu) with the nickel hydroxide active material in the temperature range 20-60°C. The oxides of heavy lanthanides Er, Tm, Yb, and Lu or their mischmetal oxides, were found to shift the oxygen evolution overpotential of nickel electrodes to more noble potentials than other lanthanides and to he particularly effective in raising the high-temperature charge acceptance of nickel electrodes. The higher oxygen overpotential was correlated with the uniform distribution of the heavier lanthanides having amphoteric character. Application to sealed NiMH batteries yielded a charge acceptance of about 80% at 70°C.