A. Damjanovic

Electrode kinetics of oxygen reduction on oxide-free platinum electrodes☆

The cathodic V log i curves for O2 reduction on oxide-free Pt electrodes were determined in oxygen-saturated purified acid (HClO4, pH 1–3) and alkaline (NaOH, pH 13) solutions. It was found that ∂V/∂ log i = −2·3RT/F, ∂V/∂ log pO2 = −2·3RT/F and ∂V/∂ pH = −100 mV. Coverage of electrodes by oxygen-containing species was determined by superimposing a constant cathodic current over the working electrode. Intermediate values of coverages were found to be nearly linearly dependent on the potential in the Tafel region. The kinetics mechanism of O2 reduction on oxide-free Pt electrode are different from that reported for the oxide-covered electrode. Reaction mechanism are discussed in terms of Langmuir and Temkin adsorption isotherms. It is suggested that under Temkin conditions the rate-determining step in the over-all four-electron process is the first electron transfer.

Distinction between Intermediates Produced in Main and Side Electrodic Reactions

A diagnostic criterion was developed to distinguish between an intermediate formed in an over‐all reaction path and a product in a parallel electrochemical reaction. It utilizes the method of the rotating‐disk electrode with a concentric ring, first proposed by Frumkin and Nekrassov. From the dependence of currents at the disk and at the ring electrode on the rate of disk rotation, individual rates of parallel reactions can be obtained.

Kinetics of oxygen evolution and dissolution on platinum electrodes

Different pre-treatment of Pt electrodes, provided they are well anodized, does not affect the mechanism of the oxygen-electrode reaction. Due to the low exchange current density for the reaction, measurements at low (< 10−7 A/cm2) current densities are possible only if the solution is highly purified. Reproducible cathodic V/ln i curves can be obtained by fast measurements on pre-anodized electrodes. The instability with time of V/ln i curves at higher cathodic η is due to gradual reduction of the surface oxides. The reaction mechanism in acid solution includes the primary water discharge as the rate-controlling step. In alkaline solution, at low anodic η, a rate-controlling chemical step follows a fast OH− discharge. The same step is rate-controlling in O2 dissolution. At high anodic η, the mechanism changes and, for the same path, the primary discharge of OH− becomes the rate-controlling step. Phase oxides cover the electrode surface during O2 evolution. In dissolution, oxides already present reduce (at potentials < V (he)). Total oxygen coverage (and hence the average thickness of the oxides) increases linearly with applied potential both in acid and in alkaline solution. The same type of oxides forms in acids and in alkaline solution.

On the Deposition and Dissolution of Zinc in Alkaline Solutions

The zinc/KOH‐zincate electrode reaction was investigated under high purity conditions with galvanostatic and potentiostatic transient techniques in the 0.1–3.0M and 0.0001–0.5M zincate concentration range. The exchange current density was found to be between 8 and 370 mA/cm2, with 40 mV/decade anodic and 120 mV/decade cathodic nominal Tafel slopes; an overpotential range of ± 100 mV was covered. The cathodic reaction orders were 1 for zincate, and −1 for hydroxyl ions. A four‐step mechanism, consistent with the kinetic data, is suggested. It consists of four consecutive dissociation reactions of the zincate complex, with two of them incorporating a single electron charge transfer. The rate‐determining step is The mechanism of the anodic and cathodic reactions is the same. The transients indicate only double layer charging and charge transfer processes. No surface diffusion effects or intermediate build‐up was observed (no pseudocapacitance). A rationalization is given for the mechanism. The effect of zinc surface preparation is discussed.

Hydrogen peroxide formation in oxygen reduction at gold electrodes: II. Alkaline solution

Abstract Oxygen reduction at Au electrodes in acid solution has been studied with a rotating disk electrode with a concentric ring. Two linear regions in the V-logi plot are observed, both with Tafel slope of −0.115 V. From the Id/Irvs. ω− 1 2 plots it is concluded that (i) oxygen reduction at and below 0.3 V (H.E.) proceeds along a single reaction path with hydrogen peroxide as a reaction intermediate which further reduces at an increasing rate as the potential becomes more cathodic; and (ii) at potentials anodic to 0.30 V, the reduction proceeds along two parallel reaction paths. Below 0.55 V, the major reaction is that in which oxygen is reduced to hydrogen peroxide. Above 0.55 V, the major reaction is the reduction to water without hydrogen peroxide intermediate. Linear V-log i regions correspond to these two situations.

Adsorption and kinetics at platinum electrodes in the presence of oxygen at zero net current

The rest potentials at Pt electrodes were determined in 1 N2SO4. Solutions saturated at 0°, 20u°, 50° And 80° with various partial pressures of oxygen ranging from 0.005 to 1 atm. Oxygen adsorption isotherms were obtained by cathodic reduction transient technique. The nature of the rest potentials is analyzed. The behavior of the Pt-O2-H2O system under open circuit conditions, appears to be best interpreted in terms of the following model. Oxide-free Pt electrodes immersed in an aqueous solution of H2SO4 acquire a mixed potential due to cathodic reduction of oxygen and oxidation of impurities as the anodic component. The value of this potential is simply dependent upon the oxygen pressure. The potential in turn determines the coverage of oxygen atoms, which arises from an equilibrium of the water discharge reaction with the Pt surface. According to this model, this reaction, and not the dissociative adsorption of oxygen, is the origin of oxygen coverage at the surface. The lack of reversibility of the O2 electrode would therefore result from the presence of impurities. Further analysis of the Behavior of noble metal electrodes In O2-saturated solutions is in progress.

Dependence of the Rate of Electrodic Redox Reactions on the Substrate

The steady‐state velocities of an electrodic redox reaction have been determined on noble metals and some of their alloys. They are expressed in terms of the exchange current densities i0. Correspondingly, measurements of i0s as a function of temperature are recorded. The heats of activation at the reversible potential are independent of the properties of the substrate. However, log (reaction rate at the reversible potential) is linear with the work function of the substrate. Reactions in Cl− containing solutions have velocities greater than those in SO4– – solutions, and the ratio υCl– / υSO4– – depends upon the substrate in the order Pd > Au > Pt. The lack of dependence of the heat of activation upon the substrate is consistent with an electron transfer and not an atom‐transfer model for the electrodic redox reactions. It is consistent with a weak interaction model. The dependence of the exchange current densities upon the substrate can be shown quantitatively to depend on the change of electrokinetic ...

Differential surface tension measurements at thin solid metal electrodes

A new technique has been developed for measuring the differential interfacial tension as a function of potential at solid-solution interfaces. The technique utilizes the bending experienced by a thin electrode, insulated on one side, when the interfacial tension is changed. Potentials of zero charge are reported for gold in KCl solution and platinum in H2SO4 solution.

The Kinetics of the Electrodeposition and Dissolution of Metal Monolayers as a Function of Dislocation Density

The mechanism and the kinetics of the short time metal deposition and dissolution has been analyzed on the basis of a model which includes transfer of ions across the double layer, diffusion of adions over the metal surface, and incorporation of ions into the lattice at steps one atom high. It is shown that it is possible to obtain data on the exchange current density of the reaction and on the equilibrium adion concentration from the initial portion of the potentiostatic transients. Steady‐state current density is analyzed. Conditions necessary for the surface diffusion of adions or for the transfer of ions across the double layer to be the rate‐controlling step in the over‐all reaction are formulated and discussed. Current density at each potential depends on distances between the steps suitable for the incorporation of adions into the lattice, which distances are related in turn to the dislocation density. On an ideal surface, without nucleation, the steady‐state current density should be zero. Conditions for two‐dimensional nucleation are analyzed and discussed.

Catalysis of the electrodic hydrogen evolution and dissolution reactions on rationally chosen substrates

Summary The rate of the hydrogen evolution (h.e.r.) and dissolution reactions were measured on noble metals, noble and non-noble metal alloys, tungsten bronzes, etc. The substrates were chosen to examine the effects of Fermi level, chemi-bonding, surface defects, internuclear distance and the inclusion of polyvalent atomic species. The h.e.r. decreases as the Fermi level decreases. The effects of chemi-bonding are complex but the rate follows the change of % d character in most systems. Surface defects increase the rate by up to an order of magnitude. Increase of internuclear distance gives a faster reaction. These effects are mainly due to changes in the heat of activation (ΔH 0# ) and not to changes in surface concentration. The Fermilevel effects arise from a dependence of M−H bond strength on work function. Chemi-bonding effects are the principal ones to govern Δ H 0# : using only three well supported rate-determining steps, most of the data on the variation of rate with % d character of the substrate can be rationalized. Surface defects effects arise by means of local Φ changes, which are associated with corresponding M−H changes. Internuclear distance changes do not have primary effects on the rate. The effects of inclusions of polyvalent atoms is to be related more to local work function changes than to the partaking of redox intermediate ions in the rate.