Many spinel cobaltite oxides meet the requirement for a wide range of electrochemical reactions. These materials have largely been investigated as electrocatalysts for the oxygen evolution reaction (OER) or oxygen reduction reaction (ORR). The objective of this article is to summarize the studies available in the literature on electrochemical and electrocatalytic properties of cobaltite spinel oxides towards the OER and ORR in alkaline media. The main focus of this review is on the recent investigations dealing with the performance of Co3O4 and Co- based spinel oxides as the oxygen electrodes in the light of the earlier published work.

Finely-dispersed nickel particles are electrodeposited on high surface-area perovskite-type La2-xSrxNiO4 (0≤x≤1) electrodes for possible use in a direct methanol fuel cell (DMFC). The study is conducted by cyclic voltammetry, chronoamperometry, impedance spectroscopy and anodic Tafel polarization techniques. The results show that the apparent electrocatalytic activities of the modified oxide electrodes are much higher than those of unmodified electrodes under similar experimental conditions; the observed activity is the greatest with the modified La1.5Sr0.5NiO4 electrode. At 0.550V (vs. Hg|HgO) in 1M KOH+1M CH3OH at 25°C, the latter electrode delivers a current density of over 200mAcm−2, whereas other electrodes of the series produce relatively low values (65–117mAcm−2). To our knowledge, such high methanol oxidation current densities have not been reported on any other non-platinum electrode in alkaline solution. Further, the modified electrodes are not poisoned by methanol oxidation intermediates/products.

Electronically conductive composite electrodes G/PPy/PPy(Ox)/PPy made of polypyrrole, PPy, and the perovskite oxide LaNiO3 on graphite, G, have been obtained following a sequential electrodeposition method and were investigated by electrochemical impedance spectroscopy, linear and cyclic voltammetries in an aqueous solution containing 0.5 M K(2)SO(4)plus 5 mM KOH at 25 degrees C. Results showed that the electrical conductivity of the composite electrode remained practically constant under the cathodic polarization condition. The electrocatalytic activity of LaNiO3 towards the oxygen reduction reaction (ORR) enhanced greatly when it was used in the form of the composite electrode. The incorporation of oxide particles has been observed to influence the morphology of the polymer film.

Composite G/PPy/PPy(La1−xSrxMnO3)/PPy electrodes made of the perovskite La1−xSrxMnO3 embedded into a polypyrrole (PPy) layer, sandwiched between two pure PPy films, electrodeposited on a graphite support were investigated for electrocatalysis of the oxygen reduction reaction (ORR). PPy and PPy(La1−xSrxMnO3) (0≤x≤0.4) successive layers have been obtained on polished and pretreated graphite electrodes following sequential electrodeposition technique. The electrolytes used in the electrodeposition process were Ar saturated 0.1moldm−3 pyrrole (Py) plus 0.05moldm−3 K2SO4 with and without containing a suspension of 8.33gL−1 oxide powder. Films were characterized by XRD, SEM, linear sweep voltammetry, cyclic voltammetry (CV) and electrochemical impedance (EI) spectroscopy. Electrochemical investigations were carried out at pH 12 in a 0.5moldm−3 K2SO4 plus 5mmoldm−3 KOH, under both oxygenated and deoxygenated conditions. Results indicate that the porosity of the PPy matrix is considerably enhanced in presence of oxide particles. Sr substitution is found to have little influence on the electrocatalytic activity of the composite electrode towards the ORR. However, the rate of oxygen reduction decreases with decreasing pH of the electrolyte from pH 12 to pH 6. It is noteworthy that in contrast to a non-composite electrode of the same oxide in film form, the composite electrode exhibits much better electrocatalytic activity for the ORR.

Some Cr-substituted manganese ferrites have been synthesized by a precipitation method in order to reduce the oxygen overpotential on the MnFe2O4 surface. The study showed that Cr substitution from 0.1 to 1.0 mol considerably improved the electrocatalytic activity of the oxide for the oxygen evolution reaction (OER). Values of the Tafel slope and the order for the OER with respect to OH- concentration were ∼2×2.303RT/3F and ∼2 and ∼2.303RT/F and ∼1.5 at low and high over potentials, respectively.

Multilayered composite electrodes on glassy carbon, GC, having the structure GC/PPy/PPy(Ox)/PPy, with PPy the polypyrrole and Ox a mixed valence oxide of transition metals, exhibit high reactivity and stability towards the oxygen reduction reaction (orr), when the orr proceeds electrocatalytically on the oxide particles dispersed throughout the inner layer, PPy(Ox). However the nature and concentration of the doping anions, A, of PPy have a profound effect on the resulting orr currents, due to their effects on the conductivity and morphology of the PPy layers. The paper shows and discusses these effects in the case of the composite electrode with Ox = Cu1.4Mn1.6O4 and A = Cl−, ClO4−, NO3−, PF6− and SO42−, in acid solution (pH 2.2). Optimal conditions were encountered with Cl−.

The oxygen reduction reaction (orr) was studied on composite electrodes GC/PPy/PPy(NixCo3−xO4)/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 (H2O2), which diffuses through the polymer layers to reach the bulk of electrolyte solution. The amount of H2O2 produced, determined indirectly by iodine spectrometry, depended strongly on the oxide stoichiometry. The effects of the orr and of the generation of H2O2 on the conductivity of PPy and on the behaviour of the embedded oxide particles are discussed.

The direct incorporation into a polypyrrole matrix Of NixCO3-xO4 (x=0.3 and 1) mixed valence oxide nanoparticles from their suspension in the electrolyte during the electropolymerization process led to the formation of composite electrodes having a good electrocatalytic reactivity towards the oxygen reduction reaction (ORR) and exhibiting remarkable stability.

Composite electrodes based on mixed valence oxide nanoparticles (Ox) embedded in an electrically conductive polymer (ECP) were investigated toward the oxygen reduction reaction (ORR). ECP was polypyrrole (PPy), and Ox was the spinel mixed valence oxide Cu1.4Mn1.6O4, which is known to be an electrocatalyst of the ORR in neutral and alkaline media. The main findings were that in such a composite electrode (i) the spinel lattice and the electrocatalytic activity of the oxide remained remarkably stable in acidic media where normally this oxide would be electrochemically reduced, and (ii) PPy retained its electrical conductivity at cathodic potentials where it would be normally in its insulating reduced state. Those composite electrodes sustained the ORR for hours without signs of deterioration. (C) 2002 The Electrochemical Society.

In this work we have studied the multilayered polypyrrole/oxide composite electrode on glassy carbon having the structure GC/PPy/PPy(Ni0.3Co2.7O4)/PPy, in which the spinel oxide Ni0.3Co2.7O4 is known to be an electrocatalyst of the oxygen reduction reaction in alkaline medium. The successive GC/PPy, GC/PPy/PPy(Ni0.3Co2.7O4) and GC/PPy/PPy(Ni0.3Co2.7O4)/PPy parts of the electrode were examined by X-ray photoelectron spectroscopy. The results indicate that the electronic structure of the PPy is independent of the thickness of the PPy used to prepare the electrodes. The XPS data also show that cobalt is present in both divalent and trivalent states. The PPy doping degree by Cl− ions (in terms of the Cl−/N) and the oxide/PPy mass ratio observed by XPS were 19 and 4.5%, respectively.

Films of mixed valency oxides NixCo3−xO4 (1≥x≥0.1) were studied in alkaline solutions for the indirect oxidation of ethylene glycol (EG) by peroxide anions HO2− resulting from the cathodic reduction of O2. It was found that the electrocatalytical activity of the oxides correlated to nominal Co3+/Co2+ content ratio in the spinel structure and that the oxide Ni0.3Co2.7O4, which presents the highest Co3+/Co2+ ratio, was the most effective in the series.

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.

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.

Thin cadmium selenide layers were deposited by chemical bath deposition (CBD) on various substrates: glass, gold, indium tin oxide (ITO), and poly(3-methylthiophene) (PMeT), an organic conducting polymer. In all cases, CdSe adhered strongly to the substrate. X-ray diffraction, scanning electron microscopy, and optical transmission revealed an effect of the substrate nature upon the structural and optical properties of CdSe films. Because CdSe is an n-type semiconductor and PMeT either a p-type semiconductor or a quasi-metallic conductor, attention was focused on the hybrid organic–inorganic photovoltaic junctions, which result either from CdSe chemical bath deposition on PMeT or, conversely, from PMeT electropolymerization on CBD CdSe. In the former case, a p-n type junction was obtained, whereas, in the latter case, a Schottky-type junction was obtained. The nonoptimized photovoltaic energy conversion efficiency of these junctions under 56 mW cm−2 white light from a Xenon lamp were 0.03 and 1.3%, respectively. When CdSe was deposited in the presence of silicotungstic acid (STA) in the chemical bath, localized energy levels were created in the forbidden band of CdSe, which improved the optical properties of the thin films, and hence, the energy conversion efficiency, which increased up to 2.7% for the Schottky-type photovoltaic organic–inorganic PMeT/CdSe(STA) photovoltaic junction.

Experiments have shown the feasibility of new composite electrodes ECP(Ox) resulting from the dispersion of nanoparticles of a metal oxide (Ox) into an electrically conductive polymer (ECP). In this preliminary work, Ox was a spinel mixed valence oxide of copper and manganese, Cu1.4Mn1.6O4, and ECP was polypyrrole. The intimate mixing of the two materials modified neither the ECP charge-transfer and transport behavior nor the electrocatalytical activity of the oxide. These composite electrodes exhibited a remarkable stability, even in acidic media, in contrast to noncomposite electrodes of the same oxide. (C) 2000 The Electrochemical Society. S1099-0062(99)12-039-X. All rights reserved.

Hybrid organic–inorganic all thin film photovoltaic junctions PMeT(Y)/CdS(X) were investigated, where PMeT(Y) is the conducting polymer poly(3-methylthiophene) doped with various anions Y=CF3SO3−, ClO4−, BF4−, PF6−, and CdS(X) cadmium sulfide doped with various elements X=Cu, Ni, Al, As and Sb. CdS(X) films were spray deposited on conducting and transparent indium-tin oxide (ITO) glass, and PMeT(Y) films were electrodeposited onto the CdS(X) film to form the junction. The electrochemical investigation of the mechanism of electrodeposition and growth of the PMeT(Y) films by means of chronoamperometry, and of the charge transfer behavior of the PMeT(Y)/CdS(X) junctions by means of cyclic voltammetry revealed a strong effect of the nature of Y and X. The same strong effect of Y was also found with PMeT(Y)/ITO junctions, and may have some generality. It was showed that the best quality of contact between the organic and inorganic phases, from an electrochemical viewpoint and in solution, was obtained with the PMeTPF6−CdS(Sb) junction, pointing towards a similarly better photovoltaic performance in solid state. This was indeed the case, and it was found that higher short-circuit current, open-circuit photovoltage and energy conversion efficiency, approaching 4%, could be obtained with this junction.

Two different procedures were used to prepare spinel-type NixAl1−xMn2O4 (0≤x≤1) compounds to study the effects of solid state properties of mixed oxides on their electrocatalytic properties. The oxalic route, coprecipitation of metal oxalates dissolved in propanol or ethanol, and the propionic route, hydrolysis of metal carboxylates in propionic acid, have been used. In both routes, thermal decomposition produces the corresponding oxides. X-ray diffraction patterns showed that the oxides crystallize in a cubic spinel phase with a unit cell parameter a that increases as aluminum is replaced by nickel. At low x values, compounds prepared by the propionic route showed a larger variation for parameter a than compounds prepared by the oxalic route, probably due to oxygen stoichiometric deficiency. This effect was estimated from the tetrahedral force constant (kt) values, which showed a fast decrease as x varied from 0 to 1. Electrical conductivity, conduction activation energy, hole mobility, and pHz of oxides prepared by the propionic route were also higher than those from oxides prepared by the oxalic route. Crystallinity grade and particle size were nearly 50′ higher in propionic-route samples than in oxalic-route samples. The apparent and real electrocatalytic activities of both types of oxides were compared for O2 evolution.

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Real current densities for the oxygen evolution reaction (OER) have been estimated taking into account the surface roughness in the case of films of perovskites LaNi1-xMxO3 (M = Fe, Co, Cu, 0 < x < 0.5). The oxides have been prepared in powder form via the malic acid route, involving decomposition at 650 degrees C of an amorphous malate gel. The films were characterized ex-sity by XRD and EDAX, and in-situ by AC impedance and cyclic voltammetry (CV). Electrocatalytic properties towards the OER were investigated by ordinary stationnary voltammetry. It was found that AC impedence and CV yielded similar roughness factors. The substitution of Fe, Co and Cu had practically no effect on the roughness, current densities and reaction mechanism of the OER, and only marginal effect on the electrocatalytic activity.