In this study, the effect of manganese oxidation state and particle size on ORR catalytic activity and stability of manganese oxide nanoparticles in PEMFC environment were investigated. The Mn2O3 and Mn3O4 nanoparticles were characterized by field emission scanning electron microscopy (FE-SEM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction analysis (XRD). Thereafter, the ORR activity and durability of the manganese oxide were studied by cyclic voltammetry (CV) and accelerated stability test (AST) in N2 and O2 saturated 0.1 M HClO4 electrolyte using rotating disk electrode (RDE) technique. The results showed that the ORR activity of manganese oxide nanoparticles strongly depends on their metal oxidation states and particle size. The catalytic activity of manganese oxide nanoparticles increased with increase in the particle size. Moreover, the Mn2O3 nanoparticles catalyst with +3 oxidation state exhibited higher ORR activity compared to the Mn3O4 nanoparticles catalyst with mixed (+2, +3) oxidation state. The ORR activities of all catalysts, however, decreased after 5000 potential cycles from 0.6-1.0 V vs. RHE.
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