Supramolecular phenomena in organic redox films at electrodes: Part I. The methylene blue/leucomethylene blue redox couple at the platinum electrode

It is possible to generate conductive multilayer structures by reduction of methylene blue cations at the interface: clean surface of a platinum electrode/1 M aqueous electrolyte solution.The characteristics of the multilayer ordered phase generated in fluoride and nitrate solutions are presented and the mechanism of a fast charge transfer between electrode/organic film and organic film/aqueous solution interfaces is discussed. The conductivity of the film is interpreted by postulating formation of a mixed valence structure with the generation of a cation radical intermediate which is favoured in the solid state at characteristic potentials.     

On the reduction of platinum oxide layers by hydrogen in aqueous H2SO4 under open circuit conditions

This paper reports investigations on the mechanism of the open circuit (OC) reduction of platinum oxide layers by hydrogen in aqueous sulfuric acid. Special attention was given to the conditions of the oxide layer formation and the anodic oxidation of hydrogen thereupon. A twin-cell technique was developed which allows the instantaneous rate of the OC reduction to be determined. The experimental results show that the dissociative adsorption of hydrogen is the rate-determining step of the process and that the OC reduction follows, an island mechanism. Because of the time-dependent alteration of the oxide layer structure, the formation conditions affect the instantaneous rate of the OC reduction as well as the duration of the total reduction of the oxide layer.     

On the mechanism of hydrogen evolution at GaAs electrodes

The hydrogen evolution reaction at n- and p-GaAs electrodes has been reinvestigated. As in the case of metal electrodes, hydrogen evolution can occur in two ways: at ?0.5 V (SCE) hydronium ions are reduced, at ?1.25 V (SCE) reduction of water molecules takes place. It is confirmed that in both cases conduction band electrons are responsible for the two reduction steps, forming adsorbed hydrogen atoms in the first and hydrogen molecules in the second step. Hole injection can occur only to a negligible extent, although it appears energetically feasible.     

Differential double-layer capacitance of a mercury-drop electrode attached to a platinum wire: Frequency dispersion due to the measuring bridge and to the geometry at pendant and sessile electrodes

The frequency dependence of the double-layer capacitance has been studied at the interface Hg/0.116 M KCl in water at ?1.200 V vs. a reversible calomel electrode at 25°C. Two drop electrodes attached to 0.1-mm platinum wire have been used, one sessile with a narrow wedge, the other pendant with a widely open wedge between drop surface and support. The operation of the a.c. resistor-ratio arms bridge has been analysed in a manner generally applicable to series-R, C measurements. On that basis the effect of stray capacitances could be compensated and the frequency dispersion due to the measuring system reduced to a minimum. Both electrodes show a low-frequency dispersion of the measured series capacitance C(ω) and resistance R(ω), ω being the angular frequency. This dispersion has been analysed with a simple R, C network. In series with the bulk resistance R₃ this network has two mutually parallel series R, C's: R₁, C₁ of the drop itself and R₂, C₂ of its edge (neck). From the actual C(ω), R(ω) data follow correct capacitance data C₁, while R₂ and C₂ show a relatively stronger dependence on frequency and kind of electrode (pendant or sessile). Moreover, a sessile electrode exhibits a wedge or shielding effect above a much smaller ω (=ω_h >ω₁, the frequency above which the edge effect becomes insignificant) than a pendant electrode. Conclusions drawn from this information are: in the frequency domain (ω_h?ω₁) diffusive mass transport may be studied without interference from any geometric effect, which is probably also true at small electrolyte concentrations. This study should be performed with a pendant electrode, since it has the largest (ω_h?ω₁) domain.     

Transport and kinetics at microheterogeneous electrodes: Part III. Application to the photoelectrochemical decomposition of water

The transport and kinetics in a system involving two sets of microheterogeneous electrodes, which are designed to use solar radiation to split water into H₂ and O₂, are described. A reaction scheme involving fourteen different processes is considered. The conditions for an efficient system are derived. It is shown that the main problems are electrode selectivity and the loss of H₂ and O₂ by back reactions.     

Interfacial electrochemistry of cytochrome c at tin oxide,indium oxide,gold, and platinum electrodes

Cyclic voltammetry has been used to study the heterogeneous electron transfer kinetics of horse heart cytochrome c in pH 7 tris/cacodylate media at several electrode surfaces. Reversible voltammetric responses (formal heterogeneous electron transfer rate constant>10^?2 cm/s) were observed at bare gold electrodes and at tin-doped indium oxide semiconductor electrodes for certain experimental conditions. Quasireversible voltammetric responses were more typically observed at fluorine-doped tin oxide semiconductor electrodes, bare platinum electrodes, and at the indium oxide electrodes. Reaction rates at bare metal electrodes were strongly dependent on pretreatment procedures and experimental protocol. Reaction rates at metal oxide electrodes were strongly dependent on solution conditions, pretreatment procedures, and on the hydration state of the electrode surface. A general mechanistic scheme involving both interfacial electrostatic and chemical interactions is proposed for cytochrome c electrode reactions. The asymmetric distribution of surface charges on cytochrome c appears to play a dominant role in controlling electron transfer rates by its interaction with the electric field at the electrode surface. Electron transfer distances are also considered, and it is concluded that electron transfer between an electrode surface and the exposed heme edge of properly oriented cytochrome c molecules involves maximum distances of ca. 0.6–0.9 nm.     

The effect of pH on some outer-sphere electrode reactions at carbon electrodes

The apparent heterogeneous rate constant, k°_app, for three couples known to undergo outer-sphere electron transfer reactions has been investigated at carbon, gold, and platinum electrodes as a function of solution pH. The rates are found to be pH dependent at two different types of glassy carbon and at carbon paste but not at metallic electrodes. The rate is found to decrease with increasing pH for Fe(CN)^{$\text{3?}\text{4?}$}₆ and IrCl^{$\text{2?}\text{3?}$}₆, and the opposite effect is observed for the Ru(NH₃)^{$\text{3+}\text{2+}$}₆ couple. For the three compounds examined, the magnitude of the pH dependent rate changes can be associated with a change in the magnitude of the potential of the reaction layer induced by a change in surface charge of ～ 2 × 10^?7 C cm^?2. The pH dependence of the observed rate constant at glassy carbon can be attenuated by heat treatment, chemical reduction, or esterification of the surface. This behaviour is consistent with the theory that the pH dependence of k°_app arises from the presence of functional groups on the carbon surface.     

Catalytic effects of metal submonolayers formed by faradaic adsorption on the anodic oxidation of ascorbic acid at a platinum electrode

The catalytic effects of metal ions on the anodic oxidation of ascorbic acid on a Pt electrode in 1 M HClO₄ were studied by linear sweep voltammetry. The anodic peak due to a two-electron oxidation of ascorbic acid shifts to the negative potential side on the addition of Bi³⁺. This indicates the accelerating effect of Bi³⁺ on the oxidation of ascorbic acid. The presence of other metal ions, such as Pb²⁺, Hg²⁺, Tl⁺, Ag⁺ and Sb³⁺, also exerts similar effects. These metal ions were adsorbed on a Pt electrode at underpotentials and the adsorbed metals (denoted as M_ad) still remain on the electrode surface until the electrode potential goes up to and beyond the peak potential of the oxidation of ascorbic acid. On the other hand, metal ions forming no adsorbed layer on Pt, such as Co²⁺, Zn²⁺, Fe³⁺ and Ni²⁺, exhibit no catalytic effect. These facts suggest that the presence of a M_ad on Pt is essential for the promotion of the anodic oxidation of ascorbic acid. However, there is a difference in the catalytic action among the M_ad, for example, Cu_ad, Cd_ad, In_ad, Sn_ad and Mo_ad display no catalytic action.The catalytic activity depends on the degree of surface coverage by the M_ad. The maximal effect of the M_ad is attained in the submonolayer region. The effects of metal ions were discussed on the basis that the M_ad plays its major role in the removal of the adsorbed ascorbic acid occupying active sites on the electrode surface, and provides effective sites for the activation of adjacent water molecules. Furthermore, from the ¹³C NMR spectra for the oxidation products, the adsorbed water on the M_ad appears to function by promoting the subsequent hydration steps, following the electron-transfer step of ascorbic acid.     

Hydrodynamic voltammetry at channel electrodes: Part IV. Theory of chronopotentiometry for reversible electrode reactions

The theory of chronopotentiometric measurements at channel electrodes is developed for reversible electrode reactions, by rigorously solving the corresponding time-dependent boundary value problem, where the non-uniform accessibility of the channel electrode surface is taken into consideration. The theoretical equations of the transition time and the potential-time curve are derived as functions of (I_d/I), where I denotes the applied current intensity and I_d the limiting diffusion current obtained at steady-state. Finally, the time variation of the current density distribution at the electrode surface is given.     

Voltammetry at partially covered electrodes: Part I. Chronopotentiometry and chronoamperometry at model electrodes

Equations for chronopotentiometry and chronoamperometry at partially covered electrodes have been derived using a model of hexagonal array of cylidrical spaces terminated, at the electrode surface, by concentric active and inactive regions. The boundary value problem was shown to be analogous to that for a charge transfer preceded by a chemical reaction. Experiments with the reduction of ferricyanide on gold model electrodes partially covered with photoresist layer showed excellent agreement with the theory. Application of the equations to estimation of coverage and size of active sites distributed on a electrode surface is discussed.     

Synergistic effect in the electrocatalytic oxidation of methanol on platinum+palladium alloy electrodes

The electrocatalytic oxidation of methanol has been investigated on platinium+palladium alloy electrodes of different compositions in acid, neutral and alkaline aqueous solutions.The surface characteristics (composition and roughness factor) of the alloys and the stability of the electrodes in contact with different electrolytic solutions have been studied using cyclic voltammetry. In particular, a surface enrichment in platinum due to a preferential dissolution of palladium and an increase of the roughness factor with an increase of the palladium content has been shown.The electrocatalytic activity of different alloys for methanol oxidation has been characterized by exchange current densities obtained from extrapolation of Tafel lines of calculated equilibrium potential. The plot of these current densities vs. the surface composition leads to a synergistic effect, particularly important in alkaline medium. A reasonable explanation of this enhanced electroactivity at about 15 at.% in Pd is given on the basis of a decrease of electrode poisoning.     

Hydrodynamic voltammetry at channel electrodes: Part VII. Current transients at double channel electrodes

Expressions for the transient current at the downstream electrode in response to galvanostatic electrolysis at the upstream electrode in the channel flow cell were derived by applying double Laplace transformation when the electrode reaction at the upstream electrode is kinetically controlled. The ratio of the transient current to the steady state current or the transient collection efficiency was calculated as a function of electrode geometry and θ

, where U_m is the mean flow velocity in the channel cell, D the diffusion coefficient of the electroactive species, b the half height of the channel, x₁ the length of the upstream electrode and t the time elapsed since the beginning of the galvanostatic electrolysis at the upstream electrode. Curves for the transient collection efficiency can be applied to evaluating the amount of adsorption at the upstream electrode when metal at the electrode is anodically dissolved in solution. Digital simulation was carried out. Transient curves, obtained analytically, were in good agreement with those evaluated from the digital simulation. In order to allow one to draw transient curves readily, we derived a simple approximate equation.     

Adsorption phenomena in the NAD+/NADH system at glassy carbon electrodes

A variety of electrochemical approaches has been used to investigate the adsorption of NAD⁺, NADH and the NAD-NAD dimer from aqueous solution at glassy carbon electrodes (GCE) with supplementary studies of adsorption at pyrolytic graphite and platinum electrodes from aqueous media and at GCE from DMSO solution. The following hypotheses are advanced concerning the adsorption orientation: at carbon electrodes, on which NADH is not adsorbed, NAD⁺ produced by anodic oxidation of the NADH is first rapidly adsorbed in a planar configuration relative to the electrode surface, which is probably bound to the surface through the adenine moiety; there is then a relatively slow reorientation of the adsorbed NADH molecules to a perpendicular orientation relative to the electrode surface, which adsorbate is more tightly bound to the surface than the planar oriented adsorbate and which likely involves interaction between parallel adenine and pyridinium rings. Reduction (one-electron process) of NAD⁺ at the GCE produces the NAD-NAD dimer, which, at a clean electrode surface, involves a diffusion-controlled process and an adsorption-controlled process; the latter is due to formation of adsorbed dimer, which is more strongly adsorbed than NAD⁺. The dimer is oxidized at the GCE only if it is adsorbed. The factors controlling and involved in the adsorption processes have been examined with particular reference to the use of anodic voltammetry for the analytical determination of NADH.