ELECTROCATALYSIS ON METAL-OXIDES

The electrocatalysis on metallic oxide for oxygen reactions (ORR, oxygen reduction reaction, and OER, oxygen evolution reaction) is reviewed. The emphasize is put on the spinel and perovskite families. The properties of the oxides depend on the preparation procedure. Some methods of preparation of powders and films of mixed oxides are briefly presented : thermal decomposition of salts, freeze drying, spray pyrolysis, sol-gel, sputtering and electrochemical. Characterization of the products by X-ray diffractometry is not sufficient. Surface optical methods like, for instance, near grazing X-ray diffraction are useful with very thin films, together with UV-vis-NIR reflectance spectroscopy for their identification. Transmission and/or reflexion IRFT spectroscopy is also useful, as the thickness detection lower limit can be as low as 5 nm in the best case with this technique. Optical spectra of Co3O4 and NiCo2O4 are presented. The knowledge of the cationic distribution in the mixed oxide is essential to a better understanding of the electrocatalytic properties. Cyclic voltammetry provides an electrochemical mean of characterization of the oxide/solution interface. A double electrode flow through cell has been designed to investigate the ORR on films and to determine the ratio of the direct reduction rate constant (k1) to the indirect (peroxide) reduction rate constant (k2). This will be exemplified with the copper manganite spinel Cu1+xMn2-xO4 on which k1/k2 increases from 2 to 9 when x increases from 0.2 to 0.4.