Phase and morphology controlled polymorphic MnO2 nanostructures for electrochemical energy storage

In this work, a series of MnO2 nanostructures with different crystallographic structures, including δ-MnO2 nanotubes and α-MnO2 nanorods, were prepared using polycarbonate membrane as a template. The crystallographic structure and morphology were controlled by adjusting the pH of the KMnO4 solutions via a one-step hydrothermal method. The crystal form of the samples changed from birnessite to α-MnO2 with the decrease in the applied pH values and a precise pH value for the critical point was found. When used as positive electrode materials in the supercapacitor, the structure prepared in the higher pH value solutions (MnO2-12, pH = 12) with a δ-type crystallographic structure gave an ideal specific capacitance of 364.1 F g−1 at a current density of 0.5 A g−1, a good rate capability, and a favorable cycling stability. An asymmetric supercapacitor assembled with MnO2-12 as the positive electrode and activated graphene (AG) as the negative electrode produced an energy density of 29.4 W h kg−1 at a power density of 248.9 W kg−1. The excellent electrochemical properties were attributed to the novel tubular structure composed of poor crystalline δ-MnO2 nanosheets, resulting in a high ionic conductivity and two-sided reaction surfaces.