Abstract
A γ-radiation induced synthesis method is used to fabricate manganese oxide catalysts through both reduction and oxidation routes. It is shown that the morphology, composition and electrochemical performance of the produced manganese oxide particles can be tuned by altering the redox conditions. The catalysts prepared via radiolytic oxidation have a hollow spherical morphology, possess γ-MnO2 structure and show high catalytic activity for the complete four-electron reaction pathway of the oxygen reduction reaction (ORR) in alkaline electrolyte. Meanwhile, the catalysts synthesized via radiolytic reduction possess a rod-like morphology with a Mn3O4 bulk structure and favour the incomplete two-electron reaction pathway for ORR. The high catalytic activity of the manganese oxide synthesized via the oxidation route can be attributed to high electrochemical surface area and increased amount of Mn3+ on the surface as compared to those in the sample obtained via the reduction route.
Highlights
Manganese oxides are known for their multiple applications for energy storage and conversion [1,2,3,4]
This crystal system can undergo spontaneous symmetry breaking that leads to several structures such as MnO-Mn2O3 (Mn2+ and Mn3+) and even 2MnO-MnO2 (Mn2+, Mn4+) with a weak interaction between different ion sites can exist for Mn3O4 compound [42]
In this study we demonstrated that by varying redox condition of a precursor solution in the c-radiation induced synthesis one can tune the morphology, composition and electrochemical performance of manganese oxide catalysts
Summary
Manganese oxides are known for their multiple applications for energy storage and conversion [1,2,3,4]. In earlier studies it was shown that catalytic activity of manganese oxide was dependent on their polymorphs and morphology, where ORR catalytic performance was displayed in the following sequence: a-MnO2 > d-MnO2 > c-MnO2 > b-MnO2 [6,14,15]. The electro-catalytic activity for ORR of different types of Mn oxides can be dependent on the amount of accessible Mn3+ species on the surface [15]. It was shown previously that the structural properties of manganese oxides can be tuned by choosing appropriate synthesis routes [10]. In the current study we apply c-radiation induced approach to synthesize free-standing MnOx nanomaterials but manganese oxide based nanocatalysts deposited on a carbon support, aiming to substantiate how morphology, composition and catalytic activity of the obtained material can be tuned by the redox conditions. Structural properties of the obtained material and its electrochemical activities for the ORR in alkaline electrolyte were investigated
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