Showing papers on "Overpotential published in 2004"

Origin of the Overpotential for Oxygen Reduction at a Fuel-Cell Cathode

Abstract: We present a method for calculating the stability of reaction intermediates of electrochemical processes on the basis of electronic structure calculations. We used that method in combination with detailed density functional calculations to develop a detailed description of the free-energy landscape of the electrochemical oxygen reduction reaction over Pt(111) as a function of applied bias. This allowed us to identify the origin of the overpotential found for this reaction. Adsorbed oxygen and hydroxyl are found to be very stable intermediates at potentials close to equilibrium, and the calculated rate constant for the activated proton/electron transfer to adsorbed oxygen or hydroxyl can account quantitatively for the observed kinetics. On the basis of a database of calculated oxygen and hydroxyl adsorption energies, the trends in the oxygen reduction rate for a large number of different transition and noble metals can be accounted for. Alternative reaction mechanisms involving proton/electron transfer to ...

Electrochemical Characterization of La0.6Sr0.4Co0.2Fe0.8 O 3 Cathodes for Intermediate-Temperature SOFCs

Abstract: The electrochemical properties of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) have been assessed for its application as a cathode in intermediate-temperature solid oxide fuel cells. van der Pauw dc conductivity, two-electrode impedance, and three-electrode measurements were carried out to investigate the kinetics of the oxygen reduction reaction at various temperatures, oxygen partial pressures, and polarization values. A change in cathode behavior at temperatures around 600°C was observed. This is interpreted in terms of LSCF behaving as a mixed ionic electronic conductor at temperatures above around 600°C, oxygen reduction being stimulated by the formation of oxygen vacancies with increasing cathode overpotential. However, at temperatures below 600°C the contribution of mixed conductivity is low, and cathode behavior can then be interpreted in terms of the classical triple-phase-boundary model.

Electrodeposition of Copper in the SPS-PEG-Cl Additive System I. Kinetic Measurements: Influence of SPS

Abstract: 1to form a passivating film that inhibits the metal deposition rate by two orders of magnitude. Subsequent adsorption of short chain disulfide or thiol molecules with a sulfonate-end group~s! leads to the disruption and/or displacement of the passivating surface complex and acceleration of the metal deposition rate. The effect of submonolayer quantities of catalytic SPS is sustained even after extensive metal deposition, indicating that the catalyst largely remains segregated on the growth surface. Multicycle voltammetry reveals a significant potential dependence for SPS adsorption as well as its subsequent deactivation. Catalyst deactivation, or consumption, was examined by monitoring the quenching of the metal deposition rate occurring on SPS-derivatized electrodes in a SPS-free electrolyte. Catalyst consumption is a higher order process in terms of its coverage dependence and a maximum deactivation rate is observed near an overpotential of 20.1 V. Derivatization experiments are shown to be particularly effective in revealing the influence of molecular functionality in additive electroplating. Specifically, the charged sulfonate end group is shown to be central to effective catalysis. In the last three years, a curvature-enhanced accelerator coverage ~CEAC! mechanism has been shown to quantitatively describe superconformal film growth which is responsible for ‘‘bottom-up superfilling’’ of submicrometer features in damascene processing. 1-3 The mechanism has also been shown to apply to silver electrodeposition 4 as well as copper chemical vapor deposition. 5 A key characteristic of superfilling electrolytes, disclosed to date, is the competition between inhibitors and accelerators for electrode surface sites. According to the CEAC model, a thiol or disulfide accelerator, or catalyst, displaces an inhibiting halide-cuprouspolyether species from the interface and remains segregated at the surface during metal deposition. 1-3,6,7 A key consequence of these two stipulations is the possibility that local area change associated with metal deposition on a nonplanar surface may give rise to changes in the local catalyst coverage, ~e.g., increases on concave sections and decreases on convex segments! and thereby superconformal film growth. This process is particularly important for surface profiles with dimensions in the submicrometer regime and naturally provides an explanation for the beneficial effects induced by certain additives known as ‘‘brighteners.’’ 1,6 In this first of a series of papers, a more complete assessment of the electrochemical response of planar electrodes in copper superfilling electrolytes is presented. A typical electrolyte contains a dilute, i.e., micromolar, concentration of accelerator in the presence of an inhibitor concentration that is usually an order of magnitude greater. This configuration gives rise to hysteretic voltammetric curves, rising chronoamperometric transients, and decreasing chronopotentiometric traces, all of which reflect the competitive adsorption dynamics occurring between the two species. An underdeveloped aspect of this system is a quantitative description of the mass balance of the additives during plating. Of specific interest is the partitioning of the catalyst between segregation to the free surface vs. deactivation by either incorporation into the growing deposit or desorption into the electrolyte. Examination of the metal deposition kinetics on catalyst-derivatized electrodes in a catalyst-free electrolyte is shown to be particularly helpful in quantifying the deactivation process. These experiments also provide an avenue for exploring the impact of various additive functional groups on the metal deposition kinetics. Experimental

Effect of sp2-Bonded Nondiamond Carbon Impurity on the Response of Boron-Doped Polycrystalline Diamond Thin-Film Electrodes

Abstract: The physical and electrochemical properties of boron-doped polycrystalline diamond thin-film electrodes, prepared with varying levels of sp 2 -bonded nondiamond carbon impurity, were systematically investigated. This impurity was introduced through adjustment of the methane-to-hydrogen (C/H) source gas ratio used for the deposition. Volumetric gas ratios of 0.3, 0.5, 1, 2, 3, and 5% were employed. Proportional increases in the fraction of grain boundary, the extent of secondary nucleation, and the sp 2 -bonded carbon impurity content resulted in increasing C/H ratio. Variations in the morphology and microstructure were monitored using atomic force microscopy (AFM) and Raman spectroscopy, respectively. The electrode response was assessed using Fe(CN) 3-/4- 6 , Ru(NH 3 ) 3+/2+ 6 , Fe 3+/2+ . and 4-tert-butylcatechol (4-tBC). All were 1 mM in concentration and dissolved in either 1 M KCl or 0.1 M HClO 4 . While increased sp 2 -bonded carbon content had little effect on the cyclic voltammetric peak separation (ΔE p ) and peak current for the first two redox systems, the impurity had a significant impact on the latter two, as ΔE p decreased proportionally with increased sp 2 -bonded carbon content. The effect of the impurity on the reduction of oxygen in 0.1 M HClO 4 and 0.1 M NaOH was also investigated. A direct correlation was found between the relative amount of the impurity, as estimated from Raman spectroscopy, and the overpotential for oxygen reduction. The greater the nondiamond content, the lower the kinetic overpotential for the reduction reaction. Tafel plots yielded an apparent exchange current density that increased and a transfer coefficient that decreased with the increased nondiamond carbon content. The results demonstrate that the grain boundaries, and the sp 2 carbon impurity presumably residing there, can have a significant impact on the electrode reaction kinetics for certain redox systems and little influence for others.

Electrocatalytic detection of thiols using an edge plane pyrolytic graphite electrode.

Abstract: The first example of using an edge plane pyrolytic graphite electrode in electroanalysis is reported as the determination of homocysteine, N-acetylcysteine, cysteine and glutathione is studied. The response of the electrode in the direct oxidation of thiol moieties is explored and found to be electrocatalytic producing a reduction in the overpotential while having enhanced signal-to-noise characteristics compared to glassy carbon and basal plane pyrolytic graphite electrodes. The effectiveness of the methodology is examined in the determination of cysteine species in a growth tissue media that contains a high number of common biological interferences. The advantageous properties of this electrode for thiol determination lie in its excellent catalytic activity, sensitivity and simplicity.

Cathode Micromodel of Solid Oxide Fuel Cell

Abstract: A comprehensive micromodel considering all forms of polarization in the cathode of the solid oxide fuel cell was developed which governs the complex interdependency among the transport phenomena, electrochemical processes (charge-transfer and surface diffusion), and the microstructure of the electrode and their combined effect on the cathode overpotential under different operating conditions. To make the model more generalized, we consider possible oxygen reduction mechanisms, reactions at the cathode/ electrolyte interface, grain interior and grain boundary effects on the total resistance, both ordinary diffusion and Knudsen diffusion, active three-phase boundary length as a function of ionic/electronic particle size ratio and volume fraction, the exchange current density as a function of gases concentration, etc. Incorporated with reliable experimental data, the model can be used as a tool to design a high performance cathodes.

Phase field modeling of electrochemistry. II. Kinetics.

Abstract: The kinetic behavior of a phase field model of electrochemistry is explored for advancing (electrodeposition) and receding (electrodissolution) conditions in one dimension. We previously described the equilibrium behavior of this model [J. E. Guyer, W. J. Boettinger, J. A. Warren, and G. B. McFadden, Phys. Rev. E 69, 021603 (2004)]. We examine the relationship between the parameters of the phase field method and the more typical parameters of electrochemistry. We demonstrate ohmic conduction in the electrode and ionic conduction in the electrolyte. We find that, despite making simple, linear dynamic postulates, we obtain the nonlinear relationship between current and overpotential predicted by the classical "Butler-Volmer" equation and observed in electrochemical experiments. The charge distribution in the interfacial double layer changes with the passage of current and, at sufficiently high currents, we find that the diffusion limited deposition of a more noble cation leads to alloy deposition with less noble species.

Influence of Additives and Pulse Electrodeposition Parameters on Production of Nanocrystalline Zinc from Zinc Chloride Electrolytes

Abstract: Pulse electrodeposition was used to produce nanocrystalline zinc from an aqueous zinc chloride electrolyte with polyacrylamide and thiourea as additives. The influence of additive concentration and pulse electrodeposition parameters, namely, current-on time, current-off time, and peak current density on the grain size, surface morphology, and preferred orientation was investigated. The grain size and surface morphology of zinc deposits were studied by scanning electron microscopy and field emission scanning electron microscopy. The preferred orientation of zinc deposits was studied by X-ray diffraction. The optimum concentrations of polyacrylamide and thiourea in the bath that give the finest grains were 0.7 and 0.05 g/L, respectively. At constant current-off time and peak current density, the grain size decreased asymptotically with increasing current-on time. An increase in the current-off time at constant current-on time and peak current density resulted in grain growth. A progressive decrease of the grain size was observed with increasing peak current density at constant current-on and -off time. Nanocrystalline zinc with an average grain size of 50 nm was obtained at a peak current density of 1000 mA/cm 2 . The crystal orientations developed were correlated to the variation in the cathode overpotential accompanied with changing the electrodeposition parameters. A (1013) preferred orientation was developed at low overpotential while higher overpotential developed a dual (1122) (1010) orientation.

Numerical Modeling and Performance Study of a Tubular SOFC

Abstract: A quasi-2D model was proposed and made available for numerical studies on the performance of a single tubular solid oxide fuel cell (SOFC) under practical operating conditions. The model takes account of the air and fuel flow velocity fields, ohmic and thermodynamic heat generation, convective heat-transfer, mass transfer of participating chemical species including the electrochemical processes, and the electric potential and electric current in the electrodes and electrolyte. Numerical computation was carried out to test the proposed model for a single unit cell having a specific geometry being operated at a few different thermal and composition conditions for the inlet fuel and air flows. Obtained numerical results show that the quasi-two-dimensional approximation adopted in the model to mitigate the computational cost effectively can work reasonably well. At low electric current density, the cell terminal voltage was overpredicted. In order to improve the model on this point, the simple treatment adopted for the activation and concentration polarization in the model must be replaced by a more sophisticated approach in future studies. Discussions were further given conceming the obtained results for the overall cell performance and the detailed features of the velocity, thermal, and mass-transfer fields in the cell in addition to the local electrochemical characteristics. It is suggested that the air flow convective heat-transfer is important as a cooling means and that overpotential due to concentration polarization is more serious for the cathode side than for the anode side. All the presented results including the electricity conversion efficiency were observed to agree reasonably well with the popularly accepted cell performance.

Electrodes for sustained delivery of energy

Abstract: Electrodes include a coating that provides a barrier to fluids and ions within a biological environment. The coating increases the overpotential of the electrode. The coating permits the introduction and use of the electrodes into biological environments without the detrimental complications of electro-chemical reactions typically present with the use of metal and metal-alloy electrodes in such environments. The electrodes comprise: an electrically conductive substrate layer (101, 201, 301, 401, 612) an electrical conductor (103, 203, 303, 403) coupled to the conductive substrate layer, a coating layer (102, 202, 302, 402, 616) surrounding at least a portion of the substrate layer, wherein the coating layer provides an increased overpotential for the electrode in a fluid. Also disclosed are medical devices comprising such electrodes.

Electrochemical Determination of 4-Nitrophenol Using a Single-Wall Carbon Nanotube Film-Coated Glassy Carbon Electrode

Abstract: A single-wall carbon nanotubes (SWNT) film coated glassy carbon electrode (GCE) was fabricated for the direct determination of 4-nitrophenol (4-NP). The electrochemical behaviors of 4-NP at the SWNT-film coated GCE were examined. In 0.1 M phosphate buffer with a pH of 5.0, 4-NP yields a very sensitive and well-defined reduction peak at the SWNT-modified GCE. It is found that the SWNT film exhibits obvious electrocatalytic activity towards the reduction of 4-NP since it not only increases the reduction peak current but also lowers the reduction overpotential. Based on this, an electrochemical method was proposed for the direct determination of 4-NP. The reduction peak current varies linearly with the concentration of 4-NP ranging from 1 × 10−8 to 5 × 10−6 M, and the detection limit is 2.5 × 10−9 M after 3 min of open-circuit accumulation. The relative standard deviation at 2 × 10−7 M 4-NP was about 6% (n = 10), suggesting excellent reproducibility. This new method was successfully employed to determine 4-NP in several lake water samples.

Enhanced reduction of Cr(VI) by direct electric current in a contaminated clay.

Abstract: The probable relation between diffuse double-layer processes and redox reactions that enhance degradation or conversion of contaminants under an applied electric field were examined in a clay medium. Kaolinite clay, precontaminated with hexavalent chromium, was the test soil medium. Analyte, containing ferrous iron, was transported through the kaolinite clay using direct electric current. The Cr(VI) reduction to Cr(III) was followed by measuring the soil redox potential and pH at discrete locations in the clay bed. The post-test distribution of Cr showed significantly more Cr(III) than Cr(VI) at low to slightly acidic pH distribution (2 < pH < 6) in clay. The stoichiometric analyses of measured chromium and iron species concentrations versus the measured redox potentials were compared to Nernst equation predictions of an equivalent aqueous system. An average of +0.37 V shift was measured from the linear Nernstian prediction of cell potential. The applied electric field appeared to provide additional "cathodic current" to drive forth the redox reactions. The redox potential shift was explained by possible overpotential development at the clay-water interfaces due to double-layer polarization under the applied field.

Proton-Coupled Electron Transfer of an Osmium Aquo Complex on a Self-Assembled Monolayer on Gold

Abstract: A one-electron, one-proton (1e1H) redox couple based in an osmium aquo complex is attached to a self-assembled monolayer of alkanethiol on a gold electrode. The formal potential behavior of the Os aquo complex exhibits the expected pH dependence for a 1e1H system. The thermodynamic parameters are a formal potential of +0.30 V vs SCE for [Os(II/III)(H2O)], a formal potential of −0.11 V vs SCE for [Os(II/III)(OH)], a pKa of 2.4 for [Os(III)(H2O/OH)], and a pKa of 9.3 for [Os(II)(H2O/OH)]. A stepwise electron−proton transfer model, widely used in the electrochemical literature, predicts the effects of pH and overpotential on the kinetics of charge transfer for the 1e1H system. The kinetic behavior of the Os aquo complex deviates substantially from the predictions of the stepwise model. In particular, the standard rate constant and the transfer coefficient at zero overpotential are nearly independent of pH over the pH range at which proton-coupled electron transfer occurs, and Tafel plots are asymmetrical (st...