P. Chartier

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.

Sol-gel derived spinel MxCo3−xO4 (M=Ni, Cu; 0≤x≤1) films and oxygen evolution ☆

Spinel-type binary transition metal oxides of cobalt and nickel (or copper) with composition MxCo3−xO4 (where M=Ni, Cu; 0≤x≤1) were synthesized in film forms by a sol-gel route and their physicochemical and electrocatalytic properties have been investigated using SEM, XRD, cyclic and stationary voltammetries. The roughness factors, RF, of electrodes made of these films were determined by AC-impedance and cyclic voltammetry (CV). The two procedures compared well, and enabled the discrimination between apparent and true (real) electroactivity towards the oxygen evolution reaction (OER). Intermediate compositions showed a higher activity despite a lower real surface area, and laboratory tests at 70°C in 30 wt% KOH good practical performances.

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.

Surface properties of Ni+Co mixed oxides: a study by X-rays, XPS, BET and PZC

Abstract Samples of Ni and Co mixed oxide in the form of powders were synthesised by thermal decomposition of the nitrates at 400°C. The samples were characterized by XRD for the crystal structure, XPS for the surface composition, the BET method for the surface area and by potentiometric titration to measure the point of zero charge (pzc). Lattice parameters and average crystallite size have been obtained from the X-ray analysis, the surface composition shows strong enrichment with Ni, while the pzc increases from 0 to about 40% Ni to remain constant up to pure NiO. Inner layer capacitance and electrochemical (wet) surface area have also been derived from potentiometric titration curves. The results have been discussed in comparison with other few data in the literature.

Oxygen reduction on NixCo3-xO4 spinel particles/polypyrrole composite electrodes: Hydrogen peroxide formation

Abstract The oxygen reduction reaction (orr) was studied on composite electrodes GC/PPy/PPy(Ni x Co 3− x O 4 )/PPy with x =0.3 and 1, in 2.5×10 −3 M KOH+0.8 M KCl at room temperature. The orr takes place on the oxide particles with formation of hydrogen peroxide (H 2 O 2 ), which diffuses through the polymer layers to reach the bulk of electrolyte solution. The amount of H 2 O 2 produced, determined indirectly by iodine spectrometry, depended strongly on the oxide stoichiometry. The effects of the orr and of the generation of H 2 O 2 on the conductivity of PPy and on the behaviour of the embedded oxide particles are discussed.

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.

Oxygen evolution electrocatalysis in alkaline medium at thin MnxCo3-xO4 (0 ≤ x ≤ 1) spinel films on glass / SnO2: F prepared by spray pyrolysis

Films of a series of manganese-cobalt mixed valency spinel oxides of theoretical formula MnxCo3-xO4, (x = 0; 0.25; 0.5; 0.75 and 1), have been prepared by spray pyrolysis at 150 °C on conductive glass in order to relate solid state and surface chemistry to electrocatalytical activity towards the oxygen evolution reaction (OER). X-ray diffraction (XRD) and IR spectroscopy characterized their bulk crystalline properties. Both techniques have confirmed that all oxides belong to the spinel family. Their surface properties were characterized by XPS yielding the Co3+ / Co2+ surface ratio. The kinetics of the OER was investigated in 1 M KOH by means of steady state current-potential curves, obtained in the potentiostatic mode and by reaction order determination. The order of reaction ranging from 1.0 to 1.2, and the Tafel slopes ranging from 67 to 69 mV dec−1 can be tentatively interpreted by a mechanism involving a Temkin type of electrosorption of OH intermediates associated with an electrochemical rate-determining step. Replacing progressively Co by Mn, the electrodes were increasingly electrocatalytically less active, surface Co3+ ions being the active sites, and making Co3O4 the most active in the series.

Indirect oxidation of ethylene glycol by peroxide ions at Ni0.3Co2.7O4 spinel oxide thin film electrodes

Abstract Films of mixed valency oxides Ni x Co 3− x O 4 (1≥ x ≥0.1) were studied in alkaline solutions for the indirect oxidation of ethylene glycol (EG) by peroxide anions HO 2 − resulting from the cathodic reduction of O 2 . It was found that the electrocatalytical activity of the oxides correlated to nominal Co 3+ /Co 2+ content ratio in the spinel structure and that the oxide Ni 0.3 Co 2.7 O 4 , which presents the highest Co 3+ /Co 2+ ratio, was the most effective in the series.