G. Poillerat

Electrochemical Studies on Protective Thin Co3 O 4 and NiCo2 O 4 Films Prepared on Titanium by Spray Pyrolysis for Oxygen Evolution

Electrochemical studies were done on thin and films on Ti by cyclic voltammetric and potentiostatic polarization techniques. The films were quite conductive and prevented the underlying surface from playing a harmful role during measurements. The results indicated the formation of a surface quasi‐reversible redox couple, CoIV/CoIII, in solution. The reaction order with respect to OH− ion concentration was found to be nearly 2. The Tafel slopes decreased with increasing concentrations. was observed to evolve oxygen gas at a lower overpotential as compared to that of . Electrode kinetic parameters suggest similar mechanisms for the oxygen evolution for both the oxides.

Mixed valency spinel oxides of transition metals and electrocatalysis : case of the MnxCo3-xO4 system

Correlations between solid state chemistry, surface properties and electrocatalytical reactivities towards the reaction of oxygen are investigated on powder electrodes of cobalt and manganese spinel type oxides MnxCo3−xO4 (0≤x≤1). The cationic distributions have been computed from X-ray diffractometry, magnetism and oxidation power data, showing the occurrence of the Mn4+/Mn3+ solid state redox couple in octahedral sites. The change of the Mn4+/Mn3+ content as a function of x have been correlated to pH of zero charge, pHz, rest potential, Ei=0, activation energy of conductance, Ea, and to the electrocatalytical parameters of the oxygen reduction reaction (ORR) and evolution reaction (OER). Manganese catalyzes the ORR but inhibits the OER.

Thin films of Co3O4 and NiCo2O4 obtained by the method of chemical spray pyrolysis for electrocatalysis III. The electrocatalysis of oxygen evolution

The electrocatalytic properties of thin Co3O4 and NiCo2O4 films prepared on CdO-nonconductive glass by the method of chemical spray pyrolysis were investigated for oxygen evolution in varying KOH concentrations. The Tafel slopes were close to (2.3RT/F) V per decade for both oxides. The reaction order with respect to [OH−] was found to be approximately 1.3. It was observed that Co3O4 is more active than NiCo2O4 towards oxygen evolution. A mechanism for oxygen evolution involving the electrochemical adsorption of OH− as a fast step and the electrochemical desorption of OH forming an intermediate H2O2 as the rate-determining step has been suggested.

Electrochemical study of nickel-aluminium-manganese spinel NixAl1-xMn2O4. Electrocatalytical properties for the oxygen evolution reaction and oxygen reduction reaction in alkaline media

The oxygen reduction reaction and the oxygen evolution reaction have been studied on mixed oxides NixAl1−xMn2O4 (1≥x≥0) crystallising in the spinel structure. Two different preparation methods have been used: coprecipitation of metal hydroxides and sol–gel. The different activity that is observed is related to the solid-state couple in octahedral sites. AlMn2O4 shows the poorest activity due to intrinsic resistivity. The substitution of Al by Ni increases the catalytic activity, the maximum being that exhibited by NiMn2O4. These electrochemical reactions can be used to detect the nature of the electrodes by way of the electrocatalytic activity.

Characterization of spinel-type cobalt and nickel oxide thin films by X-ray near grazing diffraction, transmission and reflectance spectroscopies, and cyclic voltammetry

The application of three analytical techniques, x-ray near grazing diffraction, infrared transmission and reflectance spectroscopy, and UV-visible-near-infrared reflectance spectroscopy, to the characterization of cobalt cobaltite, Co 3 O 4 , and of nickel cobaltite, NiCo 2 O 4 films, is reported. Thin films of Co 3 O 4 and NiCo 2 O 4 , in a 10-1000 nm thickness range were prepared by two methods : spraying and sputtering. Characteristic spectra and thickness limits are discussed based on theoretical considerations. Corresponding cyclic voltammograms are presented and discussed. Among the four methods of investigation of the cationic composition of the films, cyclic voltammetry appears to be the least sensitive.

Oxygen electroreduction mechanism at thin NixCo3 − xO4 spinel films in a double channel electrode flow cell (DCEFC)

The electroreduction of O2 on pure thin oxide films of composition NixCo3−xO4 (0 ⩽ x ⩽ 1.8) sprayed onto nickel substrates has been studied with the help of a double channel electrode flow cell (DCEFC). The ratio of O2 molecules reduced to H2O via the direct 4e− pathway with respect to those reduced via the indirect 2 sx 2e− pathway depends on x, being maximum for Co3O4 (x = 0) and for oxides straddling NiCo2O4 (0.6 < x < 1.2). This ratio does not depend on the electrode potential.

Characterization of cobaltite spinels by reflectance spectroscopy

Abstract The longitudinal (LO) frequencies as well as the transversal (TO) frequencies are determined in a series of cobaltite spinels: Co3O4, ZnCo2O4, MnCo2O4, FeCo2O4, CuCo2O4, and NiCo2O4. The data were obtained from the infrared reflectance spectra of thin layers on cobalt substrate.

Active thin NiCo2O4 film prepared on nickel by spray pyrolysis for oxygen evolution

Thin films of NiCo2O4 were prepared on Ni at 370 °C by the method of spray pyrolysis. The films were uniform, adherent, free from cracks and electrocatalytically much more active for oxygen evolution than other NiCo2O4 films recently reported in the literature. This film electrode showed two Tafel slopes ~40 and ~120 mV decade−1 at low and high overpotential, respectively. The second-order observed kinetics in OH− concentration at lower overpotentials changed to first-order at higher overpotentials. A suitable mechanism involving the formation of a physisorbed hydrogen peroxide intermediate in a slow electrochemical step has been suggested.

Preparation and characterization of thin Co3O4 and MnCo2O4 films prepared on glass/SnO2:F by spray pyrolysis at 150 °C for the oxygen electrode

Abstract It is shown that pure Co 3 O 4 and MnCo 2 O 4 films can be prepared at 150 °C by spray pyrolysis of the metal nitrate aqueous solutions, provided that the flow rate and concentrations are lowered and optimized in comparison with classical procedures, and that undecomposed nitrates are further eliminated by prolonged washing. High roughness factors are obtained, yielding electrodes with a higher activity for the oxygen evolution reaction than electrodes prepared by conventional spray pyrolysis at higher temperatures.

Solvent effect on synthesis of perovskite-type La1−xCaxCoO3 and their electrochemical properties for oxygen reactions

The object of the present investigation is to compare some physico-chemical properties of two La1−xCaxCoO3 series (0.0 ≤ x ≤ 0.6), prepared by the sol-gel route using either water or propanol as solvents. The synthesized oxide powders were characterized by X-ray diffraction (XRD), energy-dispersive spectroscopy (EDS), BET, and electrical conductivity measurements. The morphology of the powders was examined using scanning electron microscopy (SEM). The results revealed that the use of propanol led systematically to the formation of homogeneous oxides with a single perovskite-structure phase, larger surface area, and higher conductivity values compared with those prepared using water. Such features, which seem to be directly related to the properties of the solvent of the starting solution, are undoubtedly favorable for a higher electrocatalytic activity of these oxides in oxygen electrode reactions.