Facile synthesis of manganese oxide nanostructures with different crystallographic phase and morphology for supercapacitors

Abstract

The charge storage performances of manganese oxide (MnO2) nanostructures highly depend on their crystallographic phase and structure. In this work, MnO2 nanostructures (α-MnO2, β-MnO2 and δ-MnO2) with high crystallinity and different morphology are prepared through a simple, mild and controllable hydrothermal method at 120 °C. [Mn(C8H4O4) (H2O)2]n, a water insoluble metal-organic framework (Mn-MOF), can efficiently react with KMnO4 at different pH, which results in the generation of α-MnO2, β-MnO2 and δ-MnO2. The size of MnO2 nanostructures can be easily adjusted by controlling the ratio of Mn(II)/Mn(VII). The as-prepared MnO2 nanostructures with different structures and sizes are characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), X-ray photoelectron spectroscopy (XPS), and N2 adsorption/desorption. Among three samples, the layered δ-MnO2 nanostructures with Mn(II)/Mn(VII) = 2.0 show a specific surface area of 240 m2·g−1, specific capacitance of 416 F·g−1 at 0.5 A·g−1, which can be explained as both adsorption/desorption and intercalation/deintercalation process. The energy density of the asymmetric supercapacitor constructed by MnO2 and activated carbon (AC) is 23.2 W·h·kg−1 at a power density of 425 W·kg−1. The controllable synthesis of MnO2 nanostructures with different crystallographic phases based on Mn-MOF may give further insights into supercapacitors.