Physiochemical Investigation of Shape-Designed MnO2 Nanostructures and Their Influence on Oxygen Reduction Reaction Activity in Alkaline Solution

In this work, five types of MnO2 nanostructres (nanowires, nanotubes, nanoparticles, nanorods, and nanoflowers) were synthesized with a fine control over their a-crystallographic form by hydrothermal method. The electrocatalytic activities of these materials were examined toward oxygen reduction reaction (ORR) in alkaline medium. Numerous characterizations were correlated with the observed activity by analyzing their crystal structure (TGA, XRD, TEM), material morphology (FE-SEM), porosity (BET), inherent structural nature (IR, Raman, ESR), surfaces (XPS), and electrochemical properties (Tafel, Koutecky-Levich plots and % of H2O2 produced). Moreover, X-ray absorption near-edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) analysis were employed to study the structural information on the MnO2 coordination number as well as interatomic distance. These combined results show that the electrocatalytic activities are significantly dependent on the nanoshapes and follow an order nanowire > nanorod > nanotube > nanoparticle > nanoflower. a-MnO2 nanowires possess enhanced electrocatalytic activity compared to other shapes, even though the nanotubes possess a much higher BET surface area. In the ORR studies, a-MnO2 nanowires displayed Tafel slope of 65 mV/decade, n-value of 3.5 and 3.6% of hydrogen peroxide production. The superior ORR activity was attributed to the fact that it possesses active sites composed with two shortened MnO bonds along with a MnMn distance of 2.824 angstrom, which provides an optimum requirement for the adsorbed oxygen in a bridge mode favoring the direct 4 electron reduction. In accordance with the first principles based density functional theory (DFT), the enhancement in ORR activity is due to the less activation energy needed for the reaction by the (211) surface than all other surfaces.