Oxygen evolution reaction on thermally treated iridium oxide films

The properties of electrochemically grown and thermally treated oxide films on iridium were examined by cyclic voltammetry and potentiostatic polarization at potentials of the oxygen evolution reaction in 0.5 mol dm^–3 sulphuric acid. The oxide was grown by square wave pulses from –0.25 to +1.25 V vs SCE, a procedure much faster in comparison with potentiodynamic activation at the same frequency. The activated electrode, exhibiting low corrosion resistance during oxygen evolution, was subsequently stabilized by heat treatment. Optimal conditions between stability and electrocatalytic activity have been determined to be between 200 and 300°C.     

Electrocatalytic oxidation of benzyl alcohol in alkaline medium on RuO2-coated titanium electrode

The heterogeneous electrocatalytic redox behaviour of RuO₂ electrodes fabricated by thermal decomposition is investigated with and without benzyl alcohol using cyclic voltammetric and potentiodynamic techniques. The cyclic voltammetric results show that benzyl alcohol oxidation is mediated by perruthenate ion electrogenerated at the electrode surface. Evaluation of kinetic parameters in relation to Tafel lines allows the postulation of a plausible reaction scheme in which benzyl alcohol adsorbs on the RuO₂ electrode surface and the rate determining step is chemical reaction between perruthenate and adsorbed species. The reaction orders with respect to benzyl alcohol and OH^– concentrations are 0.85 and 1, respectively. The results in galvanostatic electrolysis show that the major product for benzyl alcohol oxidation in an aqueous solution is benzaldehyde, and the organic yield is affected by such electrolysis conditions as t-butanol concentration, electrolysis current density, KOH concentration and electrolysis temperature.     

The Bromine Electrode. Part I: Adsorption Phenomena at Polycrystalline Platinum Electrodes

A cyclic voltammetric study of the behaviour of Br^– and Br^– ₃ at Pt electrodes, in the potential range between hydrogen and oxygen evolution, is described. Different experiments were carried out, in the presence of Br^– and Br^– ₃, in which the ratio between the species has been kept constant and equal to 1. The halide concentration was varied between 4 × 10^–6 and 1 × 10^–3 and mol dm^–3, at constant ionic strength, in 1 M HclO₄ as well as in 1 M NaClO₄ adjusted to a pH of 2. Underpotential deposition of Br is observed at potentials as low as –0.125 V vs SCE. The adsorption parameters of Br species were determined from the adsorption/desorption peak pair in the hydrogen adsorption/desorption region, and from the oxide reduction peak data. In the absence of oxygen adsorption, a relatively high coverage of the electrode surface is attained. A Langmuir-type adsorption is observed under the different experimental conditions.     

Electrochemical synthesis of Cr(II) at carbon electrodes in acidic aqueous solutions

The electrochemical synthesis of Cr(II) has been investigated on a vitreous carbon rotating disc electrode and a graphite felt electrode using cyclic voltammetry, impedance spectroscopy and chronoamperometry. The results show that in 0.1 M Cr(III) + 0.5 M sulphuric acid and in 0.1 M Cr(III) + 1 M hydrochloric acid over an electrode potential range of –0.8 to 0.8 V vs SCE, the electrochemical reaction at carbon electrodes is essentially a surface process of proton adsorption and desorption, without significant hydrogen evolution and chromium(II) formation. At electrode potentials more negative than –0.8 V vs SCE, both hydrogen evolution and chromium(II) formation occurred simultaneously. At electrode potentials –0.8 to –1.2 V vs SCE, the electrochemical reduction of Cr(III) on carbon electrodes is controlled mainly by charge transfer rather than mass transport. Measurements on vitreous carbon and graphite felt electrodes in 1 M HCl, with and without 0.1 M CrCl₃, allowed the exchange current density and Tafel slope for hydrogen evolution, and for the reduction of Cr(III) to Cr(II), to be determined. The chromium(III) reduction on vitreous carbon and graphite electrodes can be predicted by the extended high field approximation of the Butler–Volmer equation, with a term reflecting the conversion rate of Cr(III) to Cr(II).     

Fundamental study of the anodic and cathodic reactions during the electrolysis of a lithium carbonate-lithium chloride melt using a carbon anode

Cyclic voltammetry, in conjunction with the chromatographic analysis of the anode product, has been used to elucidate the reactions occurring during the electrolysis of lithium carbonate-lithium chloride melts. At a carbonate ion concentration of 0.033 mole fraction the peak anodic current densities were 3100 A m^–2 on vitreous carbon and 6900 A m^–2 on graphite with the product being carbon dioxide. The cathodic reduction of carbonate at low concentrations was found to occur at –1.0 V to –1.2 V vs a Ag/Ag(I) reference electrode which is 1.2 V less negative than the potential at which lithium ions were reduced. Voltammetric studies of the reduction of the carbonate ion indicated that the reaction mechanism involved an irreversible charge transfer.     

In situ infrared spectroscopic characterization of a bismuth–ethanol interface

The structure and composition of substances adsorbed to a Bi(0 0 1) electrode in ethanolic LiClO₄ solution were studied by cyclic voltammetry, electrochemical impedance and infrared reflectance spectroscopic methods. An analysis of the results demonstrates that at negative surface charge densities, there are no chemisorbed particles at the bismuth–ethanol solution interface. In solutions containing dissolved oxygen, an insoluble surface compound was detected at positive surface charge densities (E > −0.38 V vs. Ag|AgCl). In a behavior very different from that of the platinum–electrolyte interface, no ethanol oxidation products were detected on the Bi electrode. Absorption peaks measured in the infrared spectra are mainly caused by the variation of solvated perchlorate anion adsorption resulting from changes in surface charge density with the variation of the bismuth electrode's potential.     

Electrooxidation of benzyl alcohol at high surface area nickel (NiS x ) electrodes in alkaline solution

The heterogeneous catalytic redox behaviour of NiS _x deposited electrodes was investigated with and without benzyl alcohol in KOH solution using cyclic voltammetry and linear sweep voltammetry. The limiting current density for benzyl alcohol oxidation on a NiS _x electrode was 22 times larger than that on a polished nickel electrode. The experimental results in galvanostatic electrolysis using fractional factorial design showed that the main and interaction effects of benzyl alcohol concentration, temperature, and OH^– concentration are the key variables influencing the selectivity of benzaldehyde formation during electrolysis.     

Kinetics of oxygen reduction at RuO2-coated titanium electrode in alkaline solution

Oxygen reduction at RuO2-coated titanium electrodes prepared by thermal decomposition was investigated by employing cyclic voltammetry and rotating-disc electrode techniques. Cyclic voltammetric results indicated that oxygen reduction was catalysed by at least some hydrous oxyruthenium species, (i.e., Ru(iii)) at low polarization range (E >–0.45V) and by at least some low oxide states of ruthenium species at high polarization (E<–0.45V). On the basis of measurements using a rotating disc electrode together with polarization curves, Tafel slopes and stoichiometric number determinations, two mechanisms for oxygen reduction on an RuO2-coated titanium electrode are proposed.     

Anodically deposited manganese–molybdenum–tungsten oxide anodes for oxygen evolution in seawater electrolysis

Ternary manganese–molybdenum–tungsten oxides were anodically deposited on to IrO₂-coated titanium substrates at current densities of 60–600 A m^–2 in 0.4 M MnSO₄ solutions containing 0.003 M Na₂MoO₄ and 0.003 M Na₂WO₄ at pH 0–1.5 and at 30–90 °C. The effect of anodic deposition conditions on the activity, selectivity and durability of the anodes for oxygen evolution in 0.5 M NaCl solution was investigated. Most of the oxide anodes prepared initially gave an oxygen evolution efficiency of almost 100%. When the anodic deposition was performed at temperatures lower than 90 °C, gradual oxidative dissolution occurred during the electrolysis in the NaCl solution, due to formation of poorly crystalline oxides. In contrast, the oxide deposited at 90 °C revealed no obvious dissolution during electrolysis for more than 1500 h. The oxygen evolution efficiency, however, decreased gradually with time of electrolysis because of partial detachment of the deposited oxide. The anodic deposition in electrolytes at lower pH and at higher current density resulted in the formation of oxides with better adhesion to the substrate, resulting in improved anode durability. The durability was further improved by repeated anodic deposition with the oxide surface washed during intervals.     

Simultaneous determination of chlorine and oxygen evolving at RuO2/Ti and RuO2–TiO2/Ti anodes by differential electrochemical mass spectroscopy

Chlorine and oxygen evolving at RuO2/Ti and RuO2–TiO2/Ti anodes have been simultaneously determined at electrode potentials from 1.0 to about 2V (vs Ag/AgCl) by differential electrochemical mass spectroscopy (DEMS). On the RuO2/Ti anodes, the threshold electrode potential for oxygen evolution increased with a decrease in RuO2 loading, while the chlorine evolution potential was unchanged. Low RuO2 loading anodes gave a high chlorine evolution ratio under various constant electrolysis potentials. On the RuO2–TiO2/Ti anodes, the threshold electrode potential for oxygen evolution increased with an increase in the TiO2 content more remarkably than that for chlorine evolution. High TiO2 content anodes gave a high chlorine evolution ratio at various constant electrolysis potentials. The combination of RuO2 and TiO2 exhibits a remarkable effect with respect to the enhancement of chlorine evolution selectivity.