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

Abstract

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) α-MnO₂ and ε-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 O₂ adsorption ability, the resulting α-MnO₂ catalyst showed substantially enhanced ORR activity, compared to the ε-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.