| Summary: | The oxygen reduction reaction (ORR) is the rate-limiting reaction in the cathode side of fuel cells. In the quest for alternatives to Pt-electrodes as cathodes in ORR, appropriate transition metal oxide-based electrocatalysts are needed. In the present work, we have synthesized Co<sub>3</sub>O<sub>4</sub> and CoO/Co<sub>3</sub>O<sub>4</sub> nanostructures using flame spray pyrolysis (FSP), as electrocatalysts for ORR in acidic and alkaline media. A detailed study of the effect of (Co-oxide)/Pt ratio on ORR efficiency shows that the present FSP-made Co-oxides are able to perform ORR at very low-Pt loading, 0.4% of total metal content. In acid medium, an electrode with (5.2% Pt + 4.8% Co<sub>3</sub>O<sub>4</sub>), achieved the highest ORR performance (J<sub>max</sub> = 8.31 mA/cm<sup>2</sup>, E<sub>1/2</sub> = 0.66 V). In alkaline medium, superior performance and stability have been achieved by an electrode with (0.4%Pt + 9.6% (CoO/Co<sub>3</sub>O<sub>4</sub>)) with ORR activity (J<sub>max</sub> = 3.5 mA/cm<sup>2</sup>, E<sub>1/2</sub> = 0.08 V). Using XRD, XPS, Raman and TEM data, we discuss the structural and electronic aspects of the FSP-made Co-oxide catalysts in relation to the ORR performance. Cyclic voltammetry data indicate that the ORR process involves active sites associated with Co<sup>3+</sup> cations at the cobalt oxide surface. Technology-wise, the present work demonstrates that the developed FSP-protocols, constitutes a novel scalable process for production of co-oxides appropriate for oxygen reduction reaction electrodes.
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