Showing papers on "Overpotential published in 1998"

Electrochemical Behavior of Highly Conductive Boron‐Doped Diamond Electrodes for Oxygen Reduction in Alkaline Solution

Abstract: Highly conductive boron-doped polycrystalline diamond thin films (ρ ≃ 10 -3 Ω cm) reduction prepared via microwave plasma chemical vapor deposition (CVD). The electrochemical behavior for oxygen reduction was examined in 0.1 M KOH using linear sweep voltammetry. Oxygen reduction was found to be highly inhibited, the cathodic voltammetric peak being observed at ∼ -1.2 V vs. Ag/AgCl, compared with the standard potential for the two-electron reduction of oxygen (O 2 + H 2 O + 2e - = HO 2 - + OH - , E°' = -0.234 V vs. Ag/AgCl at pH 13). This demonstrates that, even in the presence of dissolved oxygen, diamond retains a relatively wide potential window, which could be advantageous in certain types of analytical applications. Possible interpretations for the high overpotential for oxygen reduction include a lack of adsorption sites for oxygen and/or reduced intermediates, a low density of states or a potential drop within a thin were nm) surface layer, all of which have also been proposed for highly ordered pyrolytic graphite. The experimental data were fitted using digital simulation, which showed that the reduction peak appearing at ca. -1.2 V also consistent with an nantly due to the reduction of oxygen to peroxide. Rotating disk electrode measurements were also consistent with an overall two-electron process. Experiments involving the addition of superoxide dismutase also supported this conclusion. The oxygen reduction reaction is proposed to occur on the sp 3 carbon component of the surface, with a very small contribution from sp 2 carbon impurities at smaller overpotentials.

Gas Conversion Impedance: A Test Geometry Effect in Characterization of Solid Oxide Fuel Cell Anodes

Abstract: The appearance of an extra arc in impedance spectra obtained on high performance solid oxide fuel cell (SOFC) anodes is recognized when experiments are conducted in a test setup where the working and reference electrodes are placed in separate atmospheres. A simple continuously stirred tank reactor (CSTR) model is used to illustrate how anodes measured with the reference electrode in an atmosphere separate from the working electrode are subject to an impedance contribution from gas conversion. The gas conversion impedance is split into a resistive and a capacitive part, and the dependences of these parameters on gas composition, temperature, gas flow rate, and rig geometry are quantified. The fuel gas flow rate per unit of anode area is decisive for the resistivity, whereas the capacitance is proportional to the CSTR volume of gas over the anode. The model predictions are compared to actual measurements on Ni/yttria stabilized zirconia cermet anodes for SOFC. The contribution of the gas conversion overpotential to dc current-voltage characteristics is deduced for H{sub 2}/H{sub 2}O and shown to have a slope of RT/2F in a Tafel plot.

Fast-scan cyclic voltammetry of protein films on pyrolytic graphite edge electrodes: characteristics of electron exchange.

Abstract: The rapid electron-exchange characteristics of metalloproteins adsorbed at a pyrolytic graphite “edge” electrode have been studied by analog dc cyclic voltammetry at scan rates up to 3000 V s-1. The voltammetry of four proteins, azurin (a “blue” copper protein) and three 7Fe ferredoxins, reveals oxidation and reduction peaks that display only modest increases in width and peak separation as the scan rate is raised. This is indicative of a substantially homogeneous population of noninteracting centers which undergo rapid electron exchange with the electrode. Both the Butler−Volmer and Marcus models have been tested. The electrochemical kinetics, as reflected by k0 (the rate at zero overpotential), are too fast to allow the determination of reorganization energies by this method. Nonetheless, the rapid and energetically coherent nature of the electron transfer enables the cyclic oxidation and reduction of protein redox centers to be examined on a time scale sufficiently short to recognize coupled processes ...

Electrooxidation of H2, CO, and H2/CO Mixtures on a Well-Characterized Pt70Mo30 Bulk Alloy Electrode

Abstract: The electrochemical oxidation of hydrogen (H2), carbon monoxide (CO), and their mixtures (500 ppm−2%) on a well-characterized Pt70Mo30 bulk alloy was examined using the rotating disk electrode technique in 0.5 M H2SO4 at 333 K. The electrodes were transferred to and from a UHV chamber, where surface analyses were conducted using a combination of low-energy ion scattering (LEIS), Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). The surface composition of this alloy after sputter-etching and annealing in UHV was slightly enriched in Pt to a composition of Pt77Mo23. The kinetics of H2 oxidation are not measurably affected by the presence of the Mo in the surface. The shapes of the polarization curves for the oxidation of the H2/CO mixtures are qualitatively similar to those for the Pt50Ru50 alloy examined previously: a high current−potential slope (ca. 0.5 V/dec) at low overpotential followed by a transition to a highly active state where the current approaches the diffusion-li...

A new method for electrochemical screening basedon the rotating ring disc electrode and its applicationto oxygen reduction catalysts

Abstract: A new method for electrochemical characterization of composite electrode materials is reported. A paste of the catalytic material in Nafion® is coated on a rotating ring disc electrode (RRDE) to partially simulate the working environment of a proton exchange membrane (PEM)/electrode composite as used in, for example, water electrolysis or PEM fuel cell operation. This allows direct comparison of a wide range of candidate electrocatalysts in a reproducible manner. Problems specific to these volumic electrodes are accommodated satisfactorily by rational modification of the standard expressions used in RRDE analysis. The value of the method is illustrated in studies of various cobalt complexes which show promise in dioxygen reduction; namely, cobalt tetramethoxyphenylporphyrin (CoTMPP), cobalt phthalocyanine (CoPC), and cobalt cyclam (CoCy), supported on a range of particulate carbons BP2000, Printex XE 2 and Vulcan XC-72. Typical electrochemical parameters have been measured or estimated, including half-wave potentials (E1/2), Tafel slopes (b), ‘activation currents’ (Ia) and the average number of electrons transferred (n). The nature of the complex itself and the carbon support have a strong influence on electrode behaviour. Ligands with more aromatic character give better performance. Dramatic improvements in performance result from heat pretreatment, which is tentatively attributed to the formation of dimeric cobalt species via thermally-induced aggregation. In terms of the four electron reduction (to water), the best result was obtained for CoTMPP on Printex XE2 and rationalized on the basis of popular current views on the mechanism and catalyst functionality. CoPC on BP2000 is unusual in showing a strong change in n with reduction potential. Product selectivity ranges between mainly hydrogen peroxide (n=2) and water (n=4) with increasing overpotential.

Gradual Internal Methane Reforming in Intermediate‐Temperature Solid‐Oxide Fuel Cells

Abstract: Gradual internal reforming is based on local coupling between steam reforming of the fuel which occurs on a catalyst and hydrogen electrochemical oxidation which occurs at the electrode triple-phase perimeter. In order to demonstrate the feasibility of this strategy, the catalytic and electrochemical properties of lanthanum chromite, pure and impregnated with ruthenium, were investigated. Ruthenium supported on lanthanum chromite exhibits very good catalytic activity for the steam reforming of methane. Full conversion of steam is obtained for ratios H{sub 2}O/CH{sub 4} even lower than 1 at 700 C. No carbon deposition could be detected after 100 h of operation. Electrochemical measurements, carried out by impedance spectroscopy on cone-shaped microelectrodes of lanthanum chromite, show that the overpotential resistance under H{sub 2}/H{sub 2}O is lower than under CO/CO{sub 2} and much lower than under CH{sub 4}/H{sub 2}O. In the presence of ruthenium, impedance diagrams under hydrogen and methane are fairly similar and gas analysis shows that some methane is reformed. This observation demonstrates that gradual internal reforming can be implemented. A detailed analysis of the electrode impedance diagrams shows that the so-called high-frequency semicircle is virtually independent of the nature of the atmosphere. This indicates that it is not directly related tomore » any chemical or electrochemical step of the electrode reaction.« less

High‐Performance Electrode for Medium‐Temperature Solid Oxide Fuel Cells Effects of Composition and Microstructures on Performance of Ceria‐Based Anodes

Abstract: Polarization properties of ceria-based anodes dispersed with nanometer-sized Ru catalysts, which the authors developed for medium-temperature solid oxide fuel cells, were greatly improved by controlling the composition and microstructure. Among samaria-doped ceria (SDC) anodes with compositions of (CeO{sub 2}){sub 1{minus}x}(SmO{sub 1.5}){sub x} (0 {le} x {le} 0.4) a SDC anode with x = 0.2 was found to exhibit the maximum current density at a given overpotential at temperatures of 800 to 1,000 C, when operating under a hydrogen atmosphere. This high current density is a direct result of improved conductivities of both oxide ions ({sigma}{sub ion}) and electrons ({sigma}{sub e}). Attaching a very thin film of SDC onto a yttria-stabilized zirconia electrolyte before coating the Ru-dispersed SDC layer appreciably lowered the anodic overpotential for the SDC. The current densities on the improved Ru-SDC anode at a potential of {minus}0.9 V vs. an air reference electrode were 0.8 and 1.4 A/cm{sup 2} at 800 and 900 C, respectively.

First Determination of the Standard Potential for the Dissociative Reduction of the Antimalarial Agent Artemisinin

Abstract: The reduction of the known antimalarial agent Artemisinin (ART) has been studied in N,N-dimethylformamide (DMF) by cyclic voltammetry and other electrochemical techniques. ART undergoes an irreversible, dissociative reduction with an anodic peak potential (Ep) that varies with scan rate and is −1.68 V vs SCE at 1 V/s. Direct electrochemical reduction of ART is subject to a large activation overpotential, and thus the Ep's do not provide an accurate value of the standard reduction potential (E°diss(ART)) required for the determination of the free energy of electron transfer from possible biological electron donors (ΔG°ET = F(E°D•+/D − E°diss(ART))). Using careful heterogeneous electrochemical methods with convolution analysis, the standard potential of the dissociative reduction of ART (E°diss(ART)) has been determined for the first time to be −0.89 V versus SCE in DMF. This value is ca. 0.8 V more positive than the irreversible direct reduction observed using cyclic voltammetric measurements. In addition,...

Effect of brighteners on hydrogen evolution during zinc electroplating from zincate electrolytes

Abstract: Hydrogen evolution during zinc electrodeposition on a steel substrate from zincate electrolytes containing different additives was studied using various experimental techniques.The hydrogen evolution reaction is limited by the electron transfer step. Hydrogen evolution is most intensive during the first seconds from the beginning of electrodeposition due to the lower overpotential of hydrogen on steel as compared with that on zinc. The evolved hydrogen is dissipated in three ways. Most is dissipated to the atmosphere via gas bubbles at a constant rate. Some is dispersed in the electrolyte some diffuses into the steel substrate, predominantly at the commencement of deposition. The additives affect both the total amount of evolved hydrogen and its distribution. The highest amount of hydrogen is evolved in the presence of the anisaldehyde bisulphite containing composite additive. The highest amount of hydrogen included in the substrate and remaining in the electrolyte corresponds to the use of the Na–N-benzylnicotinate containing additive. In this case blistering is observed.

Electrochemical Scanning Tunneling Microscopy Investigation of HOPG and Silver Electrodeposition on HOPG from the Acid Room-Temperature Molten Salt Aluminum Chloride−1-Methyl-3-butyl-imidazolium Chloride

Abstract: The electrodeposition of silver on highly oriented pyrolytic graphite (HOPG) from the acid room-temperature molten salt aluminum chloride−1-methyl-3-butyl-imidazolium chloride (molar ratio of 55/45 containing AgCl in a concentration of 0.05 mol/L) has been investigated with electrochemical scanning tunneling microscopy, cyclic voltammetry, and potential-step experiments. HOPG can be viewed on an atomic scale; at electrode potentials above +1000 mV vs the Ag/Ag+ reference it is oxidized, leading to both hole etching and protrusions. Silver electrodeposition requires an overvoltage of −300 to −350 mV. With rising overpotential, the mechanism changes from three-dimensional progressive nucleation at a finite number of active sites to three-dimensional instantaneous nucleation. In the far overpotential range the deposition mainly takes place at steps and defects between different basal planes of graphite and to a lower extent on the basal planes themselves.

Catalytic Electrooxidation of 2-Mercaptoethanol on Perchlorinated Iron Phthalocyanine Adsorbed on a Graphite Electrode

Abstract: In this article we show that iron perchlorinated phthalocyanine (FePcCl16), adsorbed on an OPG electrode, acts as a catalyst for the electrooxidation of 2-mercaptoethanol (R-SH), lowering the overpotential of the reaction by near 0.3 V, compared to the unmodified OPG. A Tafel slope close to 0.12 V/decade was observed, indicating that the first electron-transfer is rate determining. Chemical orders of one in R-SH and in OH− are obtained. The linearity of a plot of log I vs. log [RSH] indicates that this system could have potential applications in the amperometric detection of thiols.

Amperometric determination of ascorbic acid at a novel ‘self-doped’ polyaniline modified microelectrode

Abstract: A ‘self-doped’ polyaniline modified microelectrode, prepared by cyclic potential sweep on a microdisk gold electrode from –0.2 to 0.85 V in 0.5 mol/L sulfuric acid containing aniline and o-aminobenzoic acid, has been developed. The copolymerized process and the resulting polymer characteristics were investigated in detail. This composite film indicated a good electrochemical activity in a wide pH range even in basic solution. Meanwhile, the redox couple exhibited an excellent electrocatalytic activity for the oxidation of ascorbic acid. The oxidation overpotential of ascorbic acid was decreased over 200 mV at this modified electrode compared with a bare gold one. Moreover, the effects of film thickness and pH on the catalytic efficiency were further studied. The dependence of catalytic currents on the concentration of ascorbic acid was linear in the range of 1.2 × 10–5∼ 2.4 × 10–3 mol/L with a correlation coefficient of 0.996. Also, the determination of ascorbic acid in actual samples was evaluated and the results are satisfactory.

Redox Properties of Ground and Electronically Excited States: [Ru(bpy)2Qbpy]2+ Monolayers

Abstract: Dense monolayers of [Ru(bpy)2Qbpy]2+, where bpy is 2,2‘-bipyridyl and Qbpy is 2,2‘:4,4‘‘:4‘4‘‘-quarterpyridyl, have been formed by spontaneous adsorption onto clean platinum microelectrodes. Cyclic voltammetry of these monolayers is nearly ideal, and five redox states are accessible over the potential range from +1.3 to −2.0 V. Chronoamperometry conducted on a microsecond time scale has been used to measure the heterogeneous electron-transfer rate constant, k, for both metal- and ligand-based redox reactions. Heterogeneous electron transfer is characterized by a single unimolecular rate constant (k/s-1). Standard heterogeneous electron-transfer rate constants, k°, have been evaluated by extrapolating Tafel plots of ln k vs overpotential, η, to zero driving force to yield values of (5.1 ± 0.3) × 105 s-1, (3.0 ± 0.1) × 106 s-1, and (3.4 ± 0.2) × 106 s-1 for k°3+/2+, k°2+/1+, and k°1+/0, respectively. Temperature-resolved measurements of k reveal that the electrochemical activation enthalpy, ΔH⧧, decreases f...