Showing papers on "Electrode potential published in 1997"

Electrochemical Reduction of CO at a Copper Electrode

Abstract: CO2 is electrochemically reduced to CH4, C2H4, and alcohols in aqueous electrolytes at Cu electrode with high current density. CO2 is initially reduced to adsorbed CO and further to hydrocarbons and alcohols. This paper describes macroscopic electrolytic reduction of CO at a Cu electrode in various electrolyte solutions in order to reveal the unique properties of Cu electrode in comparison with Fe and Ni electrodes. The reaction products from the Cu electrode are CH4, C2H4, C2H5OH, n-C3H7OH, CH3CHO, and C2H5CHO. Neither C2H6 nor CH3OH is produced. CH4 is favorably produced in aqueous KHCO3 of high concentrations (e.g. 0.3 mol dm-3), whereas C2H4 and C2H5OH are produced in dilute KHCO3 solutions (0.03 mol dm-3). Such product selectivity is derived from the electrogenerated OH- in the cathodic reaction, as is the case in the CO2 reduction. The partial current densities of CH4, C2H4, and C2H5OH are correlated with the electrode potential. Tafel relationships hold for C2H4 and C2H5OH irrespective of pH. The p...

Through-Bond and Chain-to-Chain Coupling. Two Pathways in Electron Tunneling through Liquid Alkanethiol Monolayers on Mercury Electrodes

Abstract: Formation, structure, and properties of alkanethiolate monolayers on micrometrically driven hanging mercury drop electrodes were investigated electrochemically. Alkanethiols with the chain length from C8 to C18 were shown to form densely packed (ca. 20.3 A2/molecule for C12SH), perpendicularly oriented monolayers on mercury in a process involving two electron oxidation of Hg to form mercuric thiolate, in agreement with earlier literature reports for a number of thiols. Electron tunneling rates across these films (due to Ru(NH3)63+ electro-reduction in aqueous 0.50 M KCl) exhibit characteristic exponential increase with the electrode potential (with transfer coefficient α = 0.25), and an exponential decay with the monolayer thickness (with a through-bond decay constant, βtb = 1.14 per methylene group or 0.91 A-1). Slow stepwise expansion of the mercury drop electrodes coated with alkanethiolates (C9−C14 only) results in an only small increase of the tunneling current maintaining the pin-hole free structure...

Spectroelectrochemistry of colloidal silver

Abstract: The spectroelectrochemical response of small silver particles was studied in aqueous solution using an optically transparent, thin layer electrode. The position of the surface plasmon band of the colloidal silver was found to depend on the applied electrode potential. It varied from 400 nm in air, corresponding to a redox potential of +0.15 V vs Ag/AgCl, to about 392 nm at −0.6 V vs Ag/AgCl. A value of 80 ± 10 μF cm-2 for the double-layer capacitance of the silver−water interface was obtained on the basis of the spectroelectrochemical shift. The equilibration kinetics of the particles with the electrode obeyed the Cottrell equation. However, the number of electrons transferred at each particle−electrode encounter was found to be potential dependent and reached 1600 ± 300 at potentials more negative than −0.4 V vs Ag/AgCl. The evidence suggests that this particle charging current occurs via electron tunneling across the particle and electrode double layers and not by contact electrification. Changes in the...

Ultrathin Films of Ruthenium on Low Index Platinum Single Crystal Surfaces: An Electrochemical Study

Abstract: We report on electrochemical properties of ultrathin films of ruthenium on platinum single crystal surfaces, Pt(100), Pt(111) and Pt(110), and demonstrate that such films can be obtained by spontaneous deposition. We also show that the spontaneously deposited ruthenium coverage is surface structure dependent. Using the spontaneous deposition process and a constant potential electrolysis, a variety of Pt/Ru surfaces up to ca. 0.4 monolayer of ruthenium were prepared. All such Ru films are stable in the electrode potential range that precedes platinum oxidation. A strong surface structure effect in the electrochemical properties of these thin films was found. On Pt(100)/Ru at a fixed Ru coverage, there is a transition from a reversible to irreversible surface redox behavior that is not observed on other platinum single crystal faces. In contrast to Pt(100)/Ru and Pt(110)/Ru, the individual voltammetric phases of the Pt(111)/Ru electrode are not resolved, and ruthenium surface oxides appear to be the most st...

Oxidation of β-nicotinamide adenine dinucleotide (NADH) at poly(aniline)-coated electrodes

Abstract: Poly(aniline)–poly(vinylsulfonate) composite-coated glassy carbon electrodes are found to give stable and reproducible electrocatlytic responses to NAD(P)H in citrate–phosphate buffer at pH 7. Analysis of these amperometric responses as a function of NAD(P)H concentration, film thickness, electrode potential and rotation speed show that the reaction occurs within the film, rather than just at the outside, and that the reaction is reversibly inhibited by the oxidation product [NA(P)D + ]. Based on a full kinetic analysis, rate constants for the various processes are determined.

Protonation and electrochemical redox doping processes ofpolyaniline in aqueous solutions: Investigations using insitu FTIR-ATR spectroscopy and a new dopingsystem

Abstract: Polyaniline (PANI), one of the most interesting members of the group of conducting polymers, has been the subject of intense experimental and theoretical studies because of its outstanding behaviour during electrochemical redox processes and during changes of pH. In this work the electrochemical doping and protonation processes of PANI are described by using in situ FTIR-ATR and a new doping system NaReO 4 /HReO 4 . Cyclic voltammetry shows that above ca. pH 4 only one wave response can be obtained. With increasing acidity of the electrolyte, a two-wave response develops, the splitting of the waves occurring at pH ca. 3.5–4. By in situ measurements during the base–acid transition process, a distinct picture of protonation of different forms of PANI has been obtained. A scheme for the dopant concentration, as a function of the electrode potential and pH, is presented. The results show that the amount of anions involved in the doping and dedoping processes depends on the pH and the potential. Maximum doping occurs at the intermediate oxidized metallic state. During both processes the electronic structure of the polymer and also the number of protons bound to nitrogen atoms are altered. The mechanisms of the changes under different redox and acid–base conditions are described.

Extending Surface Raman Spectroscopy to Transition Metal Surfaces for Practical Applications. 1. Vibrational Properties of Thiocyanate and Carbon Monoxide Adsorbed on Electrochemically Activated Platinum Surfaces

Abstract: By using confocal microprobe Raman spectroscopy and a unique electrochemical pretreatment procedure for the Pt surface, we are able to extend the detailed surface Raman studies, for the first time, to bare Pt electrodes in a wide potential region (e.g. −1.0 to +1.4 V vs SCE) and to more general adsorbates such as SCN- and CO having small Raman scattering cross sections. Taking advantage of being capable of observing the adsorbate−metal vibrational bands in the low-frequency region over IR and SFG techniques, the surface Raman spectroscopy has demonstrated the virtues of yielding detailed information on the surface bonding affected by surface coverage, coadsorbate, electrolyte ions, and electrode potential. The potential-dependent Raman spectra of SCN- reveal that the N-bound adsorbate is favored at the more negative potentials. The orientation conversion to the S-bound species occurs in the more positive region, depending on the SCN- concentration. In a solution of 10-5 M NaSCN and 0.1 M NaClO4, a new C−N...

Surface-enhanced raman spectroscopy of phosphate anions : adsorption on silver, gold, and copper electrodes

Abstract: The adsorption of phosphate anions on Ag, Au, and Cu electrodes from H2O and D2O solutions has been studied by means of surface-enhanced Raman spectroscopy (SERS). The interpretation of the spectra based on the solution Raman data and frequency shifts upon solution H2O/D2O exchange are presented. The prominent band at 1070−1100 cm-1, observed from adsorbed PO43- and HPO42- ions, exhibits downshifts of about 10 cm-1 in D2O solutions and has been assigned to the asymmetric P−O stretching mode. The corresponding asymmetric deformation mode has been assigned to the band located at ∼570 cm-1 which shows an upshift of 9−15 cm-1 in D2O solutions. Monodentate surface coordination of the PO43- and HPO42- ions is proposed. The dependence of the relative intensity of the internal modes on electrode potential was interpreted in terms of the migration of P−O groups from the surface as potential became more negative. Spectroscopic evidence was found for chemisorption of H2PO4- ion on the Cu electrodes, but no such evid...

An Electrochemical Coulomb Staircase: Detection of Single Electron-Transfer Events at Nanometer Electrodes

Abstract: The current-potential curve at a 2.8-nanometer-radius electrode immersed in a solution containing micromolar concentrations of a redox couple, [(trimethylammonio)methyl] ferrocene (Cp 2 FeTMA + )/Cp 2 FeTMA 2+ , shows discrete steps (a coulomb staircase) representing single electron-transfer events. Discrete potential steps are also found in the current-time curve as the electrode potential relaxes back to equilibrium after a coulostatic potential step.

Redox-Induced Orientation Change of a Self-Assembled Monolayer of 11-Ferrocenyl-1-undecanethiol on a Gold Electrode Studied by in Situ FT-IRRAS

Abstract: The potential dependent structure change of a self-assembled monolayer of 11-ferrocenyl-1-undecanethiol (FcC11SH) on a gold electrode surface during the redox reaction of the terminal ferrocene group was investigated in 0.1 M HClO4 solution by electrochemical in situ Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS). A number of bands were observed in the 3200−1200 cm-1 region with p-polarization measurement, but no band was observed by s-polarization measurement when the potential was kept more negative than +1.2 V. The intensity of these bands corresponded well to the degree of oxidation of the terminal ferrocene group in the monolayer. The potential dependent IRRAS behavior can be explained by considering an orientation change of the monolayer induced by the redox reaction of the terminal ferrocene moiety in the monolayer.

Fabrication and Characterization of a Solid State Reference Electrode for Electroanalysis of Natural Waters with Ultramicroelectrodes

Abstract: As an alternative to standard commercial reference electrodes, a solid state reference electrode is fabricated for in situ voltammetric analysis in solutions containing little or no added supporting electrolyte. In the fabrication process, a Ag/AgCl wire is coated with an electrolyte immobilized and protected with Nafion or polyurethane. The electrode potential is measured as a function of time, ionic strength, and pH. A stable potential is obtained for Nafion and polyurethane within 30−35 min in water. Both the polyurethane and Nafion solid state reference electrodes are stable for at least 90 days, but the Nafion electrode fluctuates more than the polyurethane electrode. The electrode is demonstrated using square wave anodic stripping voltammetry at a large electrode (3 mm), an ultramicroelectrode (10 μm), and an array of ultramicroelectrodes.

Photoelectrochemistry of films of quantum size lead sulfide particles incorporated in self-assembled monolayers on gold

Abstract: Films of AOT-capped (AOT = dioctyl sulfosuccinate) lead sulfide nanoparticles (Q-PbS) were prepared by incorporation into a self-assembled monolayer of hexanethiol on Au. The Q-PbS particles showed either cathodic or anodic photocurrents, depending on the existence of a hole scavenger or an electron scavenger in the contacting solution, on the state of the surface, and on the electrode potential. The electrochemistry of Q-PbS indicated that the anodic dissolution of Q-PbS occurred at about 0.25 V vs SCE and was not particle size dependent, while the cathodic corrosion potentials of Q-PbS ranged from −1.1 V vs SCE to beyond the hydrogen evolution potential and depended on the size of the Q-PbS particles.

Activation of Ruthenium Oxide, Iridium Oxide, and Mixed RuxIr1 − x Oxide Electrodes during Cathodic Polarization and Hydrogen Evolution

Abstract: Ruthenium, iridium, and mixed ruthenium/iridium oxide layers on titanium substrates have been obtained by thermal decomposition of chloride solutions. The decomposition temperature of RuO 2 and IrO 2 was varied from 300 to 500°C and from 400 to 500°C, respectively. That on the mixed Ru x Ir 1-x 0 2 layer was kept constant at 400°C. For the mixed oxide electrode, the Ru content was varied over the whole compositional range. Current-potential curves and cyclic voltammetry measurements were performed in 1 M H 2 SO 4 . It is shown that such oxide layers can be activated through cathodic polarization, leading to an increase of the electrocatalytic activity for hydrogen evolution. The ratio between the current density at a given electrode potential before and after completion of the activation process or the ratio between the exchange current density before and after the completion of the activation process was used to quantify this activation phenomenon. Values as high as 100 have been observed in some cases, but typical values are around ten. Through a series of specific measurements and comparison with data taken from the literature, it is shown that this activation phenomenon is not related to an increase of the electrochemically active surface area as determined through cyclic voltammetry measurements. An explanation is proposed whereby H-chemisorption within the oxide layer is ultimately responsible for the increase of the electrocatalytic activity of the oxide layer.

Redox Processes at the Manganese Dioxide Electrode II. Slow‐Scan Cyclic Voltammetry

Abstract: Slow-scan cyclic voltammetry was used to investigate the voltammetric behavior of electrolytic manganese dioxide (EMD), birnessite, chemically modified EMD (Bi-CMEMD), and birnessite (Bi-birnessite)electrodes under a variety of experimental conditions. During reduction, each electrode underwent a homogeneous reduction stage followed by a heterogeneous reduction stage. The composition at which the transition between these two stages occurred was dependent on the material under study. The initial cycle of the Bi-CMEMD electrode was similar to that of the EMD electrode. However, during the second cycle its behavior was similar to that of the Bi-birnessite electrode. This change in mechanism imparts rec argeable behavior to the Bi-CMEMD electrode. The end product of also dependent Mn(OH) 2 , except for the birnessite electrode, where Mn 3 O 4 was formed. The behavior of each electrode was also dependent on the graphite content used in the electrode, the electrolyte concentration, and the particle size of the manganese dioxide under study. The homogeneous reduction stage, which is a reaction involving a solid solution, favors a high proportion of graphite in the electrode and fine manganese dioxide particles. The heterogeneous reduction stage, which involves the formation of a soluble intermediate, was enhanced with concentrated KOH electrolytes and fine manganese dioxide particles. The influence of EMD surface area on electrode behavior was also investigated. The oxidation of Mn(OH) 2 in each electrode proceeded through a variety of solid Mn3 + intermediates to form a birnessite-like phase of manganese dioxide. The Bi 3+ ions in the chemically modified electrodes were incorporated into the structure of the oxidation product.

Redox Processes at the Manganese Dioxide Electrode I. Constant‐Current Intermittent Discharge

Abstract: The reduction of electrodeposited manganese dioxide (EMD), birnessite, and Bi-birnessite electrodes was investigated using a constant-current intermittent discharge technique under conditions where major kinetic effects had been minimized. The results suggested that each type of manganese dioxide underwent a homogeneous reduction followed by a heterogeneous reduction stage. The transition between these reaction mechanisms occurred at a composition that was dependent on the manganese dioxide used. The homogeneous reduction of each electrode was a complex process involving the reduction of Mn 4+ ions within the various manganese dioxide structures. The heterogeneous reduction of the electrodeposited manganese dioxide and Bi-birnessite electrodes resulted in formation of Mn(OH) 2 , whereas heterogeneous reduction of the birnessite electrode resulted in formation of Mn 3 O 4

Redox processes at the manganese dioxide electrode III. Detection of soluble and solid intermediates during reduction

Abstract: The soluble and solid-state intermediates formed during redox cycling of electrodeposited manganese dioxide (EMD), birnessite and chemically modified EMD (Bi-CMEMD), and birnessite (Bi-birnessite) electrodes were investigated using a stationary detector electrode (soluble intermediates) and x-ray diffraction (solid-state intermediates). Reduction of each electrode type can be divided into a homogeneous stage followed by a heterogeneous stage. For all electrode types, homogeneous reduction was a solid-state process involving proton and electron insertion into the manganese dioxide structure, causing a lattice expansion. Toward the end of homogeneous EMD reduction, soluble species were detected, presumably due to an equilibrium shift between solid and solution phase Mn 3+ species. The homogeneous/heterogeneous transition was also electrode dependent; i.e., ∼MnO 1.55 for EMD and Bi-CMEMD, ∼MnO 1.33 for birnessite, and ∼MnO 1.33 for Bi-birnessite. Heterogeneous electrochemical behavior was also electrode dependent. Initial heterogeneous reduction of EMD, Bi-birnessite, and Bi-CMEMD proceeded through a soluble Mn 3+ intermediate to form Mn(OH) 2 . Electrolyte concentration effects were more pronounced in this stage, since more concentrated KOH electrolytes lead to greater Mn 3+ solubility. The composition at which Mn(OH) 2 was first detected in the Bi-birnessite electrode suggested that the Mn(IV) to Mn(III) and Mn(III) to Mn(II) reduction processes occurred simultaneously. Heterogeneous reduction of birnessite was a solid-state process that resulted in Mn 3 O 4 , which is electrochemically inactive. Mn(OH) 2 oxidation resulted in formation of birnessite, the exact nature of which depended on the presence or absence of Bi 3+ ions. Under these deep discharge cycling conditions, the EMD electrode behaved poorly due to the eventual formation of Mn 3 O 4 . However, the Bi-birnessite and Bi-CMEMD electrodes are rechargeable due to the presence of Bi 3+ ions, which prevent Mn 3 O 4 formation

Vibrational coupling as a probe of adsorption at different structural sites on a stepped single-crystal electrode.

Abstract: Adsorption of carbon monoxide at step and terrace sites on a Pt(557) ≡ Pt(s)-[6(111) × (100)] electrode was detected with infrared spectroscopy. Vibrational coupling between adsorbates provided insights into the assembly of molecules at the different structural sites. The intermolecular coupling was weak at low coverages as CO ordered along the steps. For coverages between 40 and 70% of saturation, separate bands assignable to CO on steps and CO on terraces appeared. Coupling across this coverage range was markedly weaker on Pt(557) than on the structurally related Pt(335) ≡ Pt(s)-[4(111) × (100)] electrode surface. The results indicate that, after the steps fill, CO populates the terraces on Pt(557) at random rather than by ordering in alignment with the steps. At coverages below saturation, vibrational bands assignable to CO molecules at step and terrace sites are affected differently by changes in electrode potential. The potential-induced spectral changes for the terrace CO bands are similar to those of Pt(111)/CO, but the step CO bands show deviations from this trend at hydrogen adsorption potentials.

A Novel Way of Measuring Local Electrochemical Impedance Using A Single Vibrating Probe

Abstract: A novel method of measuring local electrochemical impedance using a single vibrating probe electrode has been developed by modification of a commercial device of scanning vibrating electrode technique (SVET). The small size of the vibrating probe used by the SVET minimizes the screening effect of the probe on the distribution of current in the region of analysis. Furthermore, the local impedance as defined here takes into account not only the local current density but also the local electrode potential, as opposed to the measurements proposed up to now in the literature. The signal processing used to obtain this impedance is explained in detail and tested on a dummy cell including a phototransistor to simulate a vibrating electrode. The local impedance of an electrode simulating pitting corrosion is also measured with this system.

Direct Measurement of Diffuse Double-Layer Forces at the Semiconductor/Electrolyte Interface Using an Atomic Force Microscope

Abstract: The forces between a silica probe and an n-type TiO2 single-crystal electrode were measured using an atomic force microscope in an aqueous electrolyte solution. These interactions were a strong function of the solution pH, the presence of specifically adsorbed anions, and the TiO2 electrode potential. For a series of pH values, a strong electrostatic repulsion was seen at high pH and decreased as the pH was reduced. At pH values below 5.5, the interaction became attractive. A series of force measurements between SiO2 and n-type TiO2 showed a repulsive interaction when TiO2 was held at negative electrode potentials, which transformed to an attractive force at positive potentials. The potential at which the interaction passed through a minimum, called the potential of zero force (pzf), corresponded closely to the flat-band potential ( Vfb) of the TiO2 electrode under conditions where the solution pH was held at the isoelectric point (iep) of titania. The Vfb measured by this method gave a value near -0.4 V vs SCE at pH 5.5, which was in good agreement with photoelectrochemical measurements made under similar conditions. At pH values deviating from the iep, the pzf and Vfb were not equivalent. This was illustrated by potential- and pH-dependent force curves taken at the same n-TiO2 electrode in the presence of the polymeric anion hexametaphosphate (HMP), which is known to specifically adsorb on TiO2. An increase in negative surface charge due to adsorbed HMP was observed by an increase in the repulsive force with respect to the silica probe at open circuit for a specific pH value. Potential-dependent force measurements determined that the pzf shifted toward more positive values in the presence of HMP, in direct opposition to the negative shift in Vfb. This apparent discrepancy was caused by the presence of both adsorbed and potential-induced surface charge, which could not be differentiated by simply measuring the diffuse double-layer charge.