Phase controllable synthesis of three-dimensional star-like MnO2 hierarchical architectures as highly efficient and stable oxygen reduction electrocatalysts

To achieve high-performance fuel cells and metal-air batteries, inexpensive and earth-abundant catalysts with enhanced activity and durability for the oxygen reduction reaction (ORR) are currently sought after. Herein, three-dimensional (3D) alpha-MnO2 and epsilon-MnO2 hierarchical star-like architectures with tunable crystal phases and desirable ORR activity were readily prepared by a facile hydrothermal method with no surfactants or templates. The effects of reaction temperature, anion type, and dwell time on the morphologies of the MnO2 products were studied in detail, and the possible formation mechanism of the 3D MnO2 hierarchical stars was proposed. Due to the improved electrical conductivity and O-2 adsorption ability, the resulting alpha-MnO2 catalyst showed substantially enhanced ORR activity, compared to the epsilon-MnO2 and bulk MnO2 catalysts, with a more positive onset potential, a larger limiting current density, and better durability. Our results provide a facile chemical route towards the phase-controlled synthesis of 3D MnO2 architectures, which can serve as efficient catalysts for ORR-based applications.