Design and synthesis of hierarchically porous MnO2/carbon hybrids for high performance electrochemical capacitors.

In this study, various morphologies of manganese dioxide (MnO2), including solid spheres, yolk-shell structures, and hollow spheres, are synthesized through an ambient reaction between KMnO4 and carbon sphere. The diversity in crystal structure and morphology of these forms of MnO2 are investigated using detailed X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) observations, and a possible formation mechanism called "inward-outward growth" is proposed. Based on our experimental results, we believe that the final products form through a synergetic effect of outward diffusing carbonaceous fragments and inward diffusing aqueous KMnO4 solution. The rate of inward KMnO4 diffusion relative to the rate of outward carbon diffusion determines the morphology of the MnO2 product. Furthermore, a coated core structure consisting of an internal graphitized carbon sphere and an external MnO2 layer is successfully synthesized, which not only possesses high surface area and hierarchical porosity, but also has improved electrical conductivity. Such structural characteristics enable the obtained composite to show a specific capacitance of 583 F g(-1) at a current density of 1 A g(-1) in 0.1 M Na2SO4 electrolyte.