How key characteristics of carbon materials influence the ORR activity of LaMnO3- and Mn3O4-carbon composites prepared by in situ autocombustion method

Abstract

In this work we studied the effect of carbon in LaMnO3- and Mn3O4- carbon composites synthesized through the In Situ Auto Combustion (ISAC) route for the Oxygen Reduction Reaction (ORR). For this study, we prepared LaMnO3 perovskite and Mn3O4 spinel composites using different types of carbon materials: carbon blacks, pyrolytic carbons, and catalytic filamentous carbon. Various analytical methods such as cyclic voltammetry, rotating ring disc electrode, Brunauer-Emmett-Teller (BET) method, X-Ray Powder Diffraction (XRD), thermogravimetric analysis (TGA), X-ray Photoelectron Spectroscopy (XPS), Transmission Electron Microscopy (TEM) and Electron Energy Loss Spectroscopy (EELS) were used to shed light on the influence of carbon in LaMnO3- and Mn3O4 - carbon composites during the ORR. These allowed us to define key characteristics of carbon materials, which determine the ORR activity and selectivity of the composites, among them sub-structural characteristics of carbon materials and pore accessibility. We conclude that (i) the ISAC route allows to significantly increase the number of accessible oxide sites and the ensuing ORR activity, (ii) the oxidation degree of Mn in LaMnO3 depends on the type and wt.% of carbon material and sub-structural order of carbon. (iii) We further claim that sub-structural characteristics of carbon materials, their pore size and size distribution affect the ORR activity of LaMnO3- and Mn3O4 - carbon composites. (iv) We see that an optimum combination of the carbon and the metal oxide allows obtaining the highest ORR activity. (v) Vulcan XC72 carbon seems to be the best suited for the ISAC synthesis of metal oxide-carbon composites due to its high structure consisting of highly branched flexible reticulate agglomerates.