Cyclic Voltametric Studies Research Articles

Low-temperature synthesis of perovskite-type oxides of lanthanum and cobalt and their electrocatalytic properties for oxygen evolution in alkaline solutions

A new hydroxide-solid-solution precursor route has been employed to synthesize perovskite-type oxides of lanthanum and cobalt at 500 °C. The oxide powders were used to obtain thin films on nicket supports by painting with a slurry of the oxide followed by sintering at 400 °C. The films were satisfactorily adherent and porous. They have been observed to exhibit p-type semiconducting behaviour in the potential region –50 to +300 mV in 1 mol I–1 KOH. Cyclic voltametric study indicated the formation of a diffusion-controlled quasi-reversible redox couple (E°= 417 mV) prior to the onset of O2 evolution at the oxide surface. It was found that Sr (or Ca) substitution enhanced greatly both the electrochemically active area as well the apparent electrocatalytic activity. The oxygen evolution reaction followed approximately first-order kinetics in OH– concentration, regardless of the nature of cobaltates. Values of the Tafel slope were 65 ± 5 mV decade–1 up to a current density of ca. 100 mA cm–2. Sr-(or Ca-) substituted cobaltates were more active for oxygen evolution than those prepared by other methods.

Polyaniline, a novel conducting polymer. Morphology and chemistry of its oxidation and reduction in aqueous electrolytes

The emeraldine salt form of polyaniline, conducting in the metallic regime, can be synthesized electrochemically as a film exhibiting a well defined fibrillar morphology closely resembling that of polyacetylene. Cyclic voltammograms of chemically synthesized and electrochemically synthesized polyaniline are essentially identical. Probable chemical changes which occur and the compounds which are formed when chemically synthesized poly-aniline is electrochemically oxidized and reduced between –0.2 and 1.0 V vs. SCE in aqueous HCl solutions at pH values ranging from –2.12 (6.0 mol dm–3) to 4.0 have been deduced from cyclic voltametric studies. These are shown to be consistent with previous chemical and conductivity studies of emeraldine base and emeraldine salt forms of polyaniline. It is proposed that the emeraldine salt form of polyaniline has a symmetrical conjugated structure having extensive charge delocalization resulting from a new type of doping of an organic polymer–salt formation rather than oxidation which occurs in the p-doping of all other conducting polymer systems.

Potentiodynamic and cyclic voltametric studies on the Passivity of Tin in neutral phosphate buffer

AbstractPassivity of tin has been investigated in phosphate buffer (pH 6.7) using potentiodynamic and cyclic voltammetric techniques. The formation of thin transparent electronically conducting passive film on which O2 is evolved in the transpassive range is indicated. The solution remains clear. Potentiodynamic measurements performed from −1.2 to 2.5 V (SCE) and backward at scan rates between 1 and 100 mV/sec give one anodic peak. In the reverse scanning, two cathodic peaks appear at low scan rates and disappear completely at higher rates. Values of the open dircuit potential, the corrosion potentials Ecor (forward and reverse), the anodic peak potential Ep and the anodic peak cd Ip are recorded. The results are compared with our previous work in neutral chloride, bromide, iodide, and sulphate solutions. Thermodynamic data are used to predict the possible oxidized species constituting the anode film.Cyclic voltammetric measurements have been carried out in the potential range between H2 and O2 evolution, and in limited potential ranges at scan rates between 1 and 200 mV/sec. One anodic peak is detected in all cases. While a short arrest and a cathodic peak appear in the results between H2 and O2 evolution, two distinct cathodic peaks appear up to 100 mV/sec in the results taken between –1.2 and −0.224 V. Plots of Ip vs the square root of scan rate are linear, and indicate diffusion‐controlled kinetics for the reactions initiating passivity. Linear relations are observed for Ep vs log scan rate indicating the irreversible nature of the anodic processes. While the formation of SnO and SnO2 is predicted from thermodynamics at potentials more negative than Ep, the formation of Sn(OH)4 is predicted at potentials more positive than Ep. The conclusions drawn from the results obtained with the two different techniques are generally the same. The results are compared with and discussed on the light of previous work using cyclic voltammetry.

Crystal and molecular structure of the complex [1,2-bis(diphenylphosphino)ethane](diethylphenylphosphine)tri-isothiocyanatorhenium(III) and the electrochemistry of isothiocyanato-derivatives of rhenium(III) and rhenium(IV)

Crystals of the title complex are triclinic, space group P, with unit-cell dimensions a= 9.680(3), b= 12.519(4), c= 19.072(8)A, α= 68.86(3), β= 71.82(3), γ= 75.40(2)°, and Z= 2. The intensities of 4 025 reflections above background have been collected and the structure refined by least-squares techniques to R′ 0.046. The complex exists as discrete pseudo-octahedral monomers. The rhenium atom in a given molecule is co-ordinated by three phosphorus and three nitrogen atoms in a mer configuration. Cyclic-voltametric studies on dichloromethane solutions of [Re(NCS)3(PEt2Ph)L′][L′= 1,2-bis(diphenylphosphino)ethane, 2,2′-bipyridyl, or 1,10-phenanthroline] show that these complexes exhibit three electrochemically reversible one-electron waves attributable to [Re]1+â†�[Re]0→[Re]1–→[Re]2–, thereby demonstrating the existence of an extensive redox chemistry for these species. The results of these electrochemical studies are compared with the related behaviour of [NBu4]2[Re2(NCS)8] and [NBu4]2[Re(NCS)6].