Shape influence of β-MnO2 on catalytic activity in the oxygen reduction reaction in alkaline media

Abstract

Abstract The catalytic activity of the oxygen reduction reaction (ORR) is important in energy conversion devices. Transition metal oxides have been identified as promising alternatives. Rutile-phase β-ΜnO2 nanostructures were produced using the hydrothermal method. The nanostructures were in the form of rods, and their hierarchical architecture to those resembling a dandelion flower were compared. The microstructural surface was examined through scanning electron microscopy (SEM) and transmission electron microscope (TEM), the Rietveld refinement technique, and surface area analysis, while the oxidation states were determined using X-ray Photoelectron Spectroscopy-Ultraviolet Photoelectron Spectroscopy (XPS-UPS). Both nanostructures were evaluated as catalysts for the ORR in alkaline environments. The results suggest that introducing shape increased the specific surface area and the Mn4+/Mn3+ ratio. This change can be attributed to the observed microstructural changes. The ORR was facilitated by a four-electron mechanism, increasing current density. This enhancement was observed in nanocrystalline β-MnO2, as well as in rod-shaped and dandelion-shaped structures. The production rates of H2O2 were determined using a rotating ring-disk electrode (RRDE). Hydrogen peroxide (H2O2) production was less than 20% in dandelion structures compared to β-MnO2 nanorods. This study enhances our understanding of β-ΜnO2 catalysts and highlights their significant potential in energy conversion, particularly in alkaline anion-exchange membrane fuel cells (AEMFCs).