Proton exchange membrane fuel cells (PEMFCs) are well known as high energy conversion efficient, low noise, and pollution-free devices to produce power and electricity from hydrogen. The preparation of electrocatalysts with high activity, poison tolerance and durability, however, remains a major challenge for the commercialization of PEMFCs [1, 2]. Recently, metal oxides have become a hot topic due to their high stability; additionally some of these oxide can act as co-catalysts in conjunction with platinum. As a search of new metal oxides supports or electrocatalysts, manganese oxides were studied for their high catalytic activity among non-precious metal oxides. Therefore, they might be a good candidate for oxygen reduction reaction (ORR) which occurs in the PEMFCs. It has been shown that manganese oxide can decompose the hydrogen peroxide ions, formed as an intermediate phase during ORR, by a disproportionation mechanism which conducts the ORR to follow the complete reduction reaction with a four electron transfer per O₂molecule [3]. Recently, Gorlin et al. [4] developed an active manganese oxide nanostructured thin film with high catalytic activity for ORR in alkaline media. The results showed that the manganese oxide thin film was more active for the ORR than that of carbon supported iridium and ruthenium nanoparticles with an activity very close to platinum nanoparticles supported on carbon. Furthermore, Roche et al. [5] indicated that manganese oxide catalyst, chemically deposited onto high specific surface area carbon, can have promising activity for the ORR in neutral pH solution. Although manganese oxide have shown favorable ORR activity in alkaline and neutral media, an extensive literature search did not yield any studies on ORR activity of manganese oxide catalysts conducted in acidic media of PEMFCs. Moreover, there appears to be no verdict on the stability of manganese oxides in PEMFC environment which further motivates the current study [6-9].

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.

References

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