Showing papers on "Overpotential published in 1995"

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-058{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}=12%, which corresponds tomore » Z=15{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

Electrooxidation of CO and H2/CO Mixtures on a Well-Characterized Pt3Sn Electrode Surface

Abstract: The electrochemical oxidation of hydrogen, carbon monoxide, and dilute mixtures of carbon monoxide in hydrogen on a well-characterized Pt{sub 3}Sn alloy electrode with a Sn surface concentration of {approx}20 at.% was examined using the rotating disk electrode technique in 0.5 M H{sub 2}SO{sub 4} at 62{degree}C. The potential for the onset of carbon monoxide oxidation on this surface was nearly 0.5 V lower than for pure Pt and approximately 0.15 V lower than for pure Ru or any Pt Ru alloy and represents the lowest overpotential for CO oxidation ever seen in this laboratory. The potential for the onset of the oxidation of hydrogen in the presence of 0.1-2% CO coincides with the potential for the onset of CO oxidation on the alloy surface. The polarization curve for the oxidation of these gas mixtures is shifted cathodically by {approx}0.35 V with respect to that for pure Pt. 20 refs., 4 figs.

Mass transfer and electrocrystallization analyses of nanocrystalline nickel production by pulse plating

Abstract: A comparison between the experimental process parameters employed for the pulse plating of nanocrystalline nickel and the solution-side mass transfer and electrokinetic characteristics has been carried out. It was found that the experimental process parameters (on-time, off time and cathodic pulse current density) for cathodic rectangular pulses are consistent and within the physical constraints (limiting pulse current density, transition time, capacitance effects and integrity of the waveform) predicted from theory with the adopted postulates. This theoretical analysis also provides a means of predicting the behaviour of the process subject to a change in the system, kinetic and process parameters. The product constraints (current distribution, nucleation rate and grain size), defined as the experimental conditions under which nanocrystalline grains are produced, were inferred from electrocrystallization theory. High negative overpotential, high adion population and low adion surface mobility are prerequisites for massive nucleation rates and reduced grain growth; conditions ideal for nanograin production. Pulse plating can satisfy the former two requirements but published calculations show that surface mobility is not rate-limiting under high negative overpotentials for nickel. Inhibitors are required to reduce surface mobility and this is consistent with experimental findings. Sensitivity analysis on the conditions which reduce the total overpotential (thereby providing more energy for the formation of new nucleation sites) are also carried out. The following lists the effect on the overpotential in decreasing order: cathodic duty cycle, charge transfer coefficient, Nernst diffusion thickness, diffusion coefficient, kinetic parameter (γ) and exchange current density.

Electrocatalysis of Anodic Oxygen‐Transfer Reactions: The Electrochemical Incineration of Benzoquinone

Abstract: The electrochemical performance of several anode materials is compared for the electrochemical incineration of p-benzoquinone in acetate buffer media (pH 5). The chemical oxygen demand (COD) estimated for benzoquinone by titration with standard permanganate solution can be decreased to virtually zero by electrolysis at electrodes comprised of Fe(III)-doped {beta}-PbO{sub 2} films on Ti substrates. Carbon dioxide is a product of the electrochemical process; however, the possibility of other volatile products cannot be dismissed. Addition of solid benzoquinone to acetate media is followed by slow formation of a brownish black color that is concluded to result from one or more humic compounds produced by condensation of benzoquinone; however, it cannot be concluded whether the condensation reaction is a necessary prerequisite to successful electrochemical incineration of benzoquinone. Optimal operating conditions suggested for electrochemical incineration of benzoquinone in acetate buffer include heating of the anode (e.g., 60 C) to increase the rate of anodic discharge of H{sub 2}O and, thereby, decrease the anodic overpotential. Evidence also is presented that the rate of electrochemical incineration is enhanced slightly by the addition of a traces of Fe(III) with reduction of dioxygen (O{sub 2}) to hydrogen peroxide (H{sub 2}O{sub 2}) at a cooled stainless steel cathodemore » (e.g., 15 C) in an undivided cell.« less

Investigation of a “Two‐State” Tafel Phenomenon for the Oxygen Evolution Reaction on an Amorphous Ni‐Co Alloy

Abstract: The anodic polarization for the oxygen evolution reaction of many metals and metal alloys results in the measurement of two Tafel regions, one for low overpotentials and one for high overpotentials. The polarization of the amorphous Ni{sub 50}Co{sub 25}P{sub 15}B{sub 10} alloy resulted in the measurement of a Tafel plot which appeared to have two different states linked by a transition potential region that was dependent on electrode pretreatment. A ``limiting current`` phenomenon in the transition region, in which the current appears to be independent of the potential, on the amorphous Ni{sub 50}Co{sub 25}P{sub 15}B{sub 10} alloy was observed. The limiting-current phenomenon in the transition region may reflect changes of the electrodes surface oxidation states and the oxygen reaction mechanisms in the two Tafel regions. This limiting current transition region was investigated by dc anodic polarization, electrochemical impedance spectroscopy, and X-ray photoelectron spectroscopy (XPS). A time-dependent impedance behavior was observed coincident with a limiting current in the Tafel plot within the same potential region. This was further characterized by means of a fixed frequency impedance measurement. XPS spectra of oxidation species of Ni were obtained from the amorphous Ni{sub 50}Co{sub 25}P{sub 15}B{sub 10} alloy and were compared with the spectramore » of chemically synthesized Ni oxide and hydroxides. The XPS results from the amorphous Ni-Co alloy electrode polarized in the two Tafel regions revealed different surface chemistries. The ``limiting current`` phenomenon could thus be attributed to a slow, surface place exchange between Ni and Co sites and a further oxidation of the surface from the low overpotential Tafel region to the high overpotential Tafel region.« less

Electrochemical deposition of hydrogen on platinum single crystals studied by infrared‐visible sum‐frequency generation

Abstract: Under‐ and overpotential deposition of hydrogen on polycrystalline platinum, Pt(100), Pt(110), and Pt(111) surfaces in sulfuric acidic medium is monitored in situ by infrared‐visible sum‐frequency generation (SFG). In the underpotential range, the vibrational signature between 1800 and 2020 cm−1 reveals hydrogen bonding between the stable hydrogen adsorbed in terminal sites and the water molecules in the electrolyte. For Pt(100) and Pt(111), the observed double SFG resonance is most consistent with a bonding configuration where water dimers are each bounded to three hydrogen atoms adsorbed on the relaxed substrate. In the overpotential range, an additional hydrogen adspecies appears with a similar vibrational fingerprint on all the investigated platinum surfaces and is tentatively assigned to the intermediate of the hydrogen evolution reaction. Its SFG resonance frequency near 1770 cm−1 suggests a dihydride adsorption configuration. The detection at under‐ or overpotential of high frequency SFG resonances...

Remarkable decrease in overpotential of oxalate formation in electrochemical CO2 reduction by a metal–sulfide cluster

Abstract: Triangular metal–sulfide clusters, [{Ir(C5Me5)}3(µ3-S)2]2+ and [{Co(C5H4Me)}3(µ3-S)2]2+, catalyse the electrochemical CO2 reduction to selectively produce oxalate at –1.30 and –0.70 V (vs. Ag/AgCl), respectively, in MeCN.

Galvanic Copper Deposition on Thiol-Modified Gold Electrodes

Abstract: Galvanic deposition of copper from 10 -2 M CuSO 4 on self-assembled X-terminated alkanethiol monolayers (HS(CH 2 ) n-1 X) on gold is studied. The influence of the chain length (X = CH 3 , n = 12, 18, 22) and of the type of terminal group (X = OH (n = 12)) on the deposition process is determined using cyclovoltammetry and optical and scanning electron microscopy. The presence of a thiol layer on gold, independent of the terminal group or thiol chain length, results in deposition of hemispherical nuclei, whereas deposition on bare gold gives rather homogeneous flat copper films. The difference in morphology is ascribed to the difference in surface tension between copper and the substrate. Nucleation occurs on top of the thiol layer when the self-assembled monolayer is highly ordered. For nucleation to occur, an overpotential is required due to the potential drop across the dielectric of the thiol layer. As a consequence, the overpotential is found to increase with increasing thiol chain length : it is about -210 mV for a HS(CH 2 ) 17 CH 3 layer and -270 mV for a HS(CH 2 ) 21 CH 3 layer. The structure of the monolayer affects the potential drop across the thiol layer, and therefore the magnitude of the deposition overpotential. This makes cyclovoltammetry to a very sensitive method to test the quality of self-assembly. Although the overpotential for nucleation on OH-terminated thiol is about the same as for CH 3 -terminated thiol, about 100 times more particles are deposited on the OH-terminated thiol. This is ascribed to a combination of a smaller potential drop across the OH-thiol layer and a higher chemical affinity of Cu adatoms for OH groups.

Electrocatalytic Surface for the Oxidation of NADH and Other Anionic Molecules of Biological Significance

Abstract: A simple electrochemical treatment of a carbon fiber electrode surface has been found to dramatically improve the voltammetry of NADH and several other anionic molecules under steady-state and fast scan (100 V/s) conditions. The electrocatalytic surface is generated through the electrochemical oxidation of NADH on a carbon fiber electrode that exhibits product adsorption. The oxidative product is reacted with ascorbic acid at elevated temperatures to create a surface which has very little overpotential for the oxidation of dopamine and many metabolites such as NADH, DOPAC, uric acid, and ascorbate. The electrochemical properties of the modified surface were examined voltammetrically at both slow and fast scan rates. The surface shown in this paper shifts the oxidation overpotentials different magnitudes for each analyte tested, thus allowing discrimination between analytes of interest and their major interferences. Another benefit of this new electrocatalytic wave is that it decreases the limit of detection for NADH by approximately 1 order of magnitude. Therefore, this new carbon surface not only gives better discrimination between two analytes but also gives better detection limits for certain analytes of interest.

Electrochemical Coupling Effects on the Corrosion of Silicon Samples in HF Solutions

Abstract: Multilevel metallization commonly used in semiconductor manufacturing for VLSI and ULSI circuits production requires low defect densities. In particular, we could observe in some circumstances, a new defect generation owing to a pitting corrosion of the polysilicon substrate during wet processing. Polysilicon pitting corrosion has been investigated in hydrofluoric solutions (DHF and BHF). The mechanism of corrosion can proceed by two different modes. First, a purely electroless mechanism can appear on a nearly homogeneous surface of silicon where a few sites play the role of microcathodes; in that case, surface metallic contamination by metal impurities from hydrofluoric solutions is a source of silicon corrosion. A study to characterize all intervening parameters is made using total reflectance x-ray fluorescence (TXRF). Second, a spontaneous difference of potential between Pt-Si plots and polysilicon electrode enhances the short-circuit current. A procedure is developed to study this corrosion in relation to real conditions existing during the wet process. This corrosion is analogous to the anodic dissolution very near the thermodynamic equilibrium value ; then, a very small defect on the structure results in a local variation of potential, favorable to pitting or intergranular corrosion. Such a mechanism, at a low value of the overpotential and low current density, is known to be responsible for a preferential etching on crystal lattice defects and leads to surface roughening and pinhole formation.

In-Situ Photoelectron-Spectroscopy of Oxygen Electrodes on Stabilised Zirconia

Abstract: Evaporated silver films as an oxygen electrode material were studied in-situ and ex-situ by XPS and UPS under UHV conditions. The experimental set-up consisted of a miniaturised solid state galvanic cell with yttria stabilised zirconia as solid electrolyte and a solid oxide reference electrode. The oxygen activity at the working electrode was changed by controlling the electrode potential versus the reference electrode consisting of a metal/metal oxide system Me/MeO (with Me = Pd or Fe). The electrode surface was investigated by XPS and UPS under cathodic polarisation with low oxygen activity as well as under anodic polarisation with steady-state oxygen evolution. The measurements have been performed between 720 K and 1100 K. With XPS, several oxygen species have been detected at the silver surface. Their concentration depends on the applied electrode potential. A species with low XPS binding energy was found whose intensity increased with anodic polarisation. It is attributed to oxygen species at the surface and at subsurface sites. The latter corresponds to oxygen atoms dissolved in the bulk. Under cathodic polarisation, the oxygen species at low binding energy disappeared, but a second oxygen species at higher XPS binding energy grew in intensity with increasing cathodic polarisation. In view of the reducing conditions and the presence of water, this latter oxygen signal is attributed to a steady state concentration of hydroxyl species at the surface. With UPS, the work function of electrons at the silver surface was determined as a function of the applied overpotential. The work function of silver increased by about 1.6eV in changing from a cathodic polarisation of -1.5 V to an anodic polarisation of +1.5 V. This pronounced change is caused by a concentration decrease of adsorbed OH and a corresponding increase of adsorbed O atoms leading to a higher surface electron affinity. Applying high cathodic as well as anodic polarisation led to the formation of insulated silver particles within open pores of the silver layer on the surface of the zirconia. These particles which are electronically coupled to the Fermi level of the underlying zirconia cause a superposed but slightly shifted second UPS spectrum with the shift depending on the effective overvoltage. The as determined overvoltage at the silver electrode depends linearly on the cell voltage.

Electrochemical enzyme biosensor

Abstract: In the case of the electrochemical enzyme biosensor comprising noble metal electrodes, an electrochemical oxidation of pyridine nucleotides takes place, in particular of NADH. The noble metal electrodes have a microtexture rich in surface pores and having catalytic properties, as a result of which the overpotential required for electrochemical oxidation of the pyridine nucleotides is reduced. Owing to these microtextures, the surface area of an electrode is from 3 to 10 times larger than its geometric surface area.

Oxygen evolution on electrochemically generated cobalt spinel coating

Abstract: The oxygen evolution reaction (OER) has been investigated in 40 wt % KOH at 80°C on a thick cobalt oxide coating obtained by potential cycling of a cobalt electrode for 0, 2, 4, 10 and 17h in the same electrolyte with and without dissolved strontium. The kinetic parameters of the OER were determined after preanodization for 1 h at 1 A cm−2. Improved electrocatalytic activity for the OER, better mechanical strength of the coating and lower variation of the oxygen overpotential with time were noticed up to 70 h of polarization as the coating built up in the presence of dissolved strontium in the electrolyte. The beneficial effect of dissolved strontium on the electrocatalytic activity is ascribed to the accumulation of Co3O4, which results in a lower Tafel slope for the OER.