Showing papers on "Overpotential published in 1996"

The Performance and Stability of Ambient Temperature Molten Salts for Solar Cell Applications

Abstract: Room temperature molten salt systems based on methyl-hexyl-imidazolium iodide (MHImI) have been used to scrutinize the performance characteristics, the stability and the mass-transfer effects in a photoelectrochemical regenerative device, as the latter is influenced and can even be limited by local concentration and mass-transport of the electroactive redox mediator species in the electrolyte phase. These salts appear to afford particular advantages over organic liquids as solvents for solar cell electrolytes. Cell performance showed outstanding stability, with an estimated sensitizer turnover in excess of 50 million. An investigation has been carried out on the physical-electrochemical properties of MHImI and its mixtures with organic solvents such as n-methyl-oxazolidinone, acetonitrile and with other lower viscosity molten salts such as methyl-butyl-imidazolium triflate. The repercussions of these properties on solar cells is described experimentally by the performance of practical application devices. Simulation models of mass transport in the nanocrystalline solar cell help illustrate operational aspects such as concentration profiles, limiting currents, anticipated mass-transfer overpotential as a function of current density, and they help to make projections as to how the properties of molten salt electrolytes can be better exploited toward this practical end.

Electrocatalytic Oxidation of NADH at Glassy Carbon Electrodes Modified with Transition Metal Complexes Containing 1,10-Phenanthroline-5,6-dione Ligands

Abstract: The preparation and electrochemical characterization of glassy carbon electrodes modified with 1,10-phenanthroline-5,6-dione (phen-dione) complexes of transition metals including [M(phen-dione)3]2+ (M = Fe, Ru, Co, Cr, Ni), [Re(phen-dione)(CO)3Cl], and [Ru(v-bpy)2(phen-dione)](PF6)2 (v-bpy is 4-vinyl-4‘-methyl-2,2‘-bipyridine) as well as their behavior as electrocatalysts toward the oxidation of NADH are described. The above-mentioned materials could be deposited by electrochemical deposition or electropolymerization. The resulting modified electrodes exhibited redox responses very similar to those of the complexes in solution, including the pH-dependent response ascribed to the pendant o-quinone groups. The deposited films also showed potent and persistent electrocatalytic activity toward NADH oxidation. In all cases, NADH oxidation took place at potentials around 0.0 V vs SSCE, which represents a dramatic diminution in the overpotential. In addition, interference effects due to ascorbate could be signif...

Electrocatalytic properties of ternary oxide mixtures of composition Ru0.3Ti(0.7−x)CexO2: oxygen evolution from acidic solution

Abstract: Mixed oxide electrodes of general composition Ru0.3Ti(0.7−x)Ce x O2 were prepared by thermal decomposition of the respective chlorides using Ti as a support. x was varied between 0 and 70 mol %. Oxygen evolution was used as a model reaction to investigate the dependence of the electrocatalytic properties on oxide composition. Kinetics was studied by quasistationary current-potential curves and reaction order determination. A minimum Tafel slope of about 30 mV has been found in the 10–30% CeO2 composition range. On the basis of a zero reaction order at constant overpotential, a reaction mechanism has been proposed accounting for the composition dependence of the Tafel slope. It has been concluded that the replacement of TiO2 with CeO2 brings about an increase in the electrocatalytic activity for oxygen evolution, while the layer becomes more prone to mechanical erosion.

Comparison of Hydrogen Electroadsorption from the Electrolyte with Hydrogen Adsorption from the Gas Phase

Abstract: This paper focuses on fundamental aspects associated with H adsorption at the solid/liquid and solid/gas interfaces and compares H electroadsorption from the electrolyte with dissociative H chemisorption from the gas phase. At the solid/liquid interface, two distinguishable electroadsorbed H species are observed, the underpotential deposited H and the overpotential deposited H , and their roles in the hydrogen evolution reaction (HER) and H absorption are discussed. At the solid/gas interface, there is only one distinguishable H species, chemisorbed H , which can undergo interfacial transfer into the metal. Three distinct mechanisms of H electroabsorption into the metal electrode are discussed in relation to the adsorption sites of and . The paper discusses thermodynamic methodology used in determination of , and for and compares the value of with . The authors demonstrate a new theoretical formalism which is applied to the determination of the bond energy . New data demonstrate that and are bonded to Rh with the same energy; this points to the same binding mechanism and the same adsorption sites of and . The chemical potentials of , and subsurface H, , are defined, and the chemical‐potential gradient of H associated with its interfacial transfer across the liquid/solid or gas/solid interfaces is formulated in terms of the surface H coverage, θH, and the lattice occupancy fraction .

Power Conversion Efficiency, Electrode Separation, and Overpotential in the Ferricyanide/Ferrocyanide Thermogalvanic Cell

Abstract: Thermogalvanic cells are electrochemical cells in which thermal energy is converted to electrical energy by maintaining the two (usually identical) electrodes at different temperatures. The effect of electrode separation on the power conversion efficiency of a ferricyanide/ferrocyanide thermogalvanic cell containing platinum electrodes was studied at electrode separations up to 150 cm, using the ``cold-above-hot`` electrode configuration. The open-circuit potential difference is almost independent of electrode separation, but the short-circuit current density falls continuously with increasing electrode separation, while the power conversion efficiency increases asymptotically to a plateau value of ca. 0.04% at a separation near 150 cm. This corresponds to an efficiency of ca. 0.6% relative to that of a Carnot engine operating between the temperatures of 293.1 and 313.1 K used in the present study. A separation of 10 cm provides efficiencies which are ca. 83% of the plateau value and is a convenient separation to use if minimal cell volume is required. The power conversion efficiency of the cell was largely limited by ohmic overpotential. Measured exchange current densities showed that activation overpotential has negligible effect on cell current. Mass transport does not limit the cell current because of the beneficial convection caused by the ``cold-above-hot`` electrode configuration.

Morphology and utilization of smooth hydrogen-evolving Raney nickel cathode coatings and porous sintered-nickel cathodes

Abstract: The utilization of the inner surfaces of hydrogen-evolving, porous, sintered-nickel electrodes and Raney nickel-coated electrodes was investigated and compared by steady-state voltammetry, impedance spectroscopy, coulometric determination of catalyst surface, and scanning electron microscopy. Porous, sintered-nickel electrodes are shown to be utilized only to approximately 10%. On the time average, roughly 90% of the inner surface of these electrodes is gas-blanketed. Nanoporous, smooth Raney nickel coatings are divided by micron-scale cracks. The essential part of the catalytically active electrode surface of Raney nickel coatings is represented by the walls of nanopores whose diameter is around 2 nm. Tafel slopes of less than {minus}120 mV/dec, namely, {minus}50 to {minus}70 mV/dec, are measured at 50 {micro}m thick smooth Raney nickel coatings. These low Tafel slopes are explained by an increasing degree of nanopore utilization with increasing current density rising from less than 0.6 to {approximately}10% if the overpotential rises from {minus}30 to {minus}120 mV. The effect can be modeled for nanopores and is at variance with known micropore behavior under concentration polarization known for increased Tafel slopes. From pore modeling it follows also that in another type of Raney nickel coatings, the so-called composite coating composed of micrometer particles of Raney nickel, differentmore » from smooth Raney nickel coatings, the utilization of that part of particles which is contacted by the electrolyte is almost 100%. Since, as in sintered electrodes, only 10% of the particle surface are expected not to be gas-blanketed, the total utilization of composite coated nanoporous catalysts amounts to {approximately} 10%, independent of overpotential and current density.« less

Kinetics of the Hydrogen Evolution Reaction on RuO2 and IrO2 Oxide Electrodes in H 2 SO 4 Solution: An AC Impedance Study

Abstract: The hydrogen evolution reaction on RuO 2 and IrO 2 electrodes in 1 M H 2 SO 4 at room temperature was studied by performing current-potential and ac impedance measurements. It is shown that IrO 2 is more active than Ru0 2 . Following correction of the electrode potential for the uncompensated resistance, the Tafel plot of both electrode materials shows two distinct slopes. The hydrogen evolution reaction on these metallic oxide electrodes was assumed to proceed according to three different mechanisms. The corresponding kinetic equations were derived and were used to simulate the log i = log i(η) and log A = log A(η) curves. The first and second mechanism involve three steps and two surface intermediates. Step one and two involve the electroreduction of surface species while step three occurs as a result of a chemical reaction. These two mechanisms differ only in that the first step of the overall reaction leading to hydrogen evolution is assumed to be at equilibrium at all electrode overpotentials for the second mechanism. They both fail to give satisfactory approximation of the experiment data, especially in the high overpotential range. The third mechanism proposed is based on the Volmer-Heyrovsky reaction sequence used to describe the hydrogen evolution reaction on metallic surfaces. This mechanism involves only one intermediate species. A more logical fit of the experimental data is obtained assuming this last reaction scheme.

A Review of Power Generation in Aqueous Thermogalvanic Cells

Abstract: The literature on aqueous thermogalvanic cells has been reviewed. Wherever possible, the power conversion efficiency, {phi}, relative to that of a Carnot engine operating between the same temperatures and the figure of merit, Z, of the cell have been extracted from the literature data in order to assess the cell`s possible use for solar energy conversion. The determination of {phi} and Z in such a cell requires the temperature dependence of its open-circuit potential difference and the knowledge of the current delivery characteristics of the cell. The current delivery characteristics depend on the effects of activation overpotential, ohmic overpotential, and mass transport overpotential on the cell`s current. The determination of the cell performance is hindered because a number of researchers apply an external potential to their cells or use forced electrolyte stirring, both of which introduce unknown amounts of energy into the energy balance for the cell. The best performance found for an aqueous thermogalvanic cell which does not have such external energy inputs, is {phi}=.50% and Z-0.58{times}10{sup {minus}4}K{sup {minus}1}. Estimates of the maximum {phi} likely to be obtained from an aqueous thermogalvanic cell suggest that it would be difficult to obtain values in excess of {phi}=1.2%, which corresponds tomore » Z=1.5{times}10{sup {minus}4}K{sup {minus}1}. These values are somewhat lower than the typical values for metal and semiconductor thermocouples. The main cause of the low efficiencies of aqueous thermogalvanic cells is the presence of high concentrations of water molecules which conduct heat from the hot to the cold electrode but which are not themselves charge carriers. It is shown that the replacement of aqueous electrolytes by molten salts would not provide a sufficiently large increase in {phi} to justify the very high temperatures required for such systems.« less

Durable electrode coatings

Abstract: Durable electrolytic cell electrodes having low hydrogen overpotential and performance stability. A highly porous electrocatalytic primary phase and an outer, secondary phase reinforcement coating are provided on an electrically conducting transition metal substrate to make the electrodes. Durability is achieved by the application of the outer secondary phase to protect the primary phase electrocatalytically active coating. A process is also disclosed for catalizing a substrate surface to promote electroless deposition of a metal.

High‐Performance Electrode for Medium‐Temperature Operating Solid Oxide Fuel Cells Polarization Property of Ceria‐Based Anode with Highly Dispersed Ruthenium Catalysts in Gas

Abstract: Polarization properties of catalyzed anode, which the authors developed for medium-temperature operating solid oxide fuel cells, were examined in a synthetic gas containing H{sub 2}, CO{sub 2}, and H{sub 2}O. The overpotential was greatly decreased by loading only a small amount of Ru microcrystals onto the mixed conducting oxide, samaria-doped ceria (SDC), anode layer in both synthetic gas and wet hydrogen atmospheres. It was found that the Ru-SDC anode exhibited the highest electrocatalytic activities at an optimum oxygen partial pressure in the fuel gas due to high conductivities of both electrons and oxide ions in the SDC. The maximum current density on the Ru-SDC anode exceeded 1 A/Cm{sup 2} at all the operating temperatures between 800 and 1,000 C at the potential of {minus}0.9 V vs. air reference electrode, when 0.1 mg/cm{sup 2} of Ru microcatalysts were loaded on the SDC surface and the appropriate fuel gas was used.

Studies of Niobium Electrocrystallization Phenomena in Molten Fluorides

Abstract: The electrodeposition of niobium from NbCl{sub 5} or K{sub 2}NbF{sub 7}, dissolved in molten alkaline fluorides (LiF/NaF), in the 700--800 C temperature range has been studied by using cyclic voltammetry, chronopotentiometry, and chronoamperometry. A linear relationship between the Nb{sup IV} diffusion coefficient and the temperature was found in the cyclic voltammograms and chronopotentiograms. Scanning electron micrography shows that the shape of the nuclei is hemispherical. Chronoamperometric results show that the deposition process involves instantaneous nucleation with diffusion-controlled growth of the nuclei. The influence of temperature, overpotential, and substrate (copper, stainless steel, vitreous carbon) on the nuclear site densities is also considered.

Silver electrocrystallization from a nonpolluting aqueous leaching solution containing ammonia and chloride

Abstract: Silver electrocrystallization from aqueous solutions at pH 11, pC10 and pNH3 - 0.2, where Ag(NH3) + is the dominant Ag(i) species, has been studied. In spite of the complexities of this medium, the experimental results can be satisfactorily described in terms of multiple nucleation and diffusioncontrolled growth of hemispherical nuclei. Nucleation rates, A, and number densities of active sites on the electrode surface, No, were determined from potentiostatic current transients as a function of overpotential. Saturation number densities of silver nuclei on the electrode surface obtained from the A and No values were found to be in excellent agreement with those obtained from the direct, microscopic observation of the electrode surface. Spatial distributions of nuclei were also analysed for silver electrodeposited at different potentials. It was found that nuclei were uniformly distributed when electrodeposited at low overpotentiats, whereas inhibition of nucleation close to already established nuclei occurred at higher overpotentials. From the change of the true nucleation rate with overpotential, it was found that the critical nucleus is formed by a single atom within the - 100 to -300 mV overpotential range.

Ph modulated heterogeneous electron transfer across metal/monolayer interfaces

Abstract: Dense monolayers of [Os(bpy)2(p3p)2]2+, where bpy is 2,2‘-bipyridyl and p3p is 4,4‘-trimethylenedipyridine, have been formed by spontaneous adsorption onto clean platinum microelectrodes. Cyclic voltammetry of these monolayers is nearly ideal, and the area occupied per molecule suggests that only one of the p3p ligands binds to the electrode surface, the other being available for protonation. Chronoamperometry conducted on a microsecond time scale has been used to measure the heterogeneous electron transfer rate constant k for the Os2+/3+ redox reaction. For electrolyte concentrations above 0.1 M, heterogeneous electron transfer is characterized by a single unimolecular rate constant (k/s-1). Tafel plots of the dependence of ln k on the overpotential η show curvature, and larger cathodic than anodic rate constants are observed for a given absolute value of η. This response is consistent with electron transfer occurring via a through-space tunneling mechanism. Plots of k vs pH are sigmoidal, and the standa...

Mathematical Modeling of Electroless Nickel Deposition at Steady State Using Rotating Disk Electrode

Abstract: Mathematical modeling of electroless nickel deposition was performed to predict the phosphorus content in Ni-P alloy film at steady state using the rotating disk system. The model consists of steady-state convective diffusion equations with nonlinear boundary conditions and overpotential equations satisfying the mixed potential theory. The weight percent of phosphorus predicted in Ni-P alloy agrees well with the experimental values within the experimental conditions carried out.