Microstructural and Morphological Effects on Charge Storage Properties in MnO₂-Carbon Nanofibers Based Supercapacitors

MnO 2 -carbon nanofibers (CNFs) composites were developed by dedicated synthetic routes and characterized as positive electrode materials for electrochemical capacitors. A series of four MnO 2 allotropic phases were investigated and commented on through MnO 2 microstructures, morphologies and their electronic and ionic conductivities. The MnO 2 -CNFs morphologies were imaged by transmission electron microscopy (TEM). Both MnO 2 birnessite and cryptomelane –CNFs nanostructures show a nanoflakes-like morphology. For Octahedral Molecular Sieves –CNFs (OMS-5-CNFs), a homogeneous MnO 2 film was grafted at the entire carbon surface, while the spinel-CNFs composite shows the formation of agglomerated nanoparticles at the carbon surface. These various fiber decoration designs greatly influence the charge storage of the resulting electrodes. The electrochemical performances of MnO 2 -CNFs were studied by cyclic voltammetry, galvanostatic charge-discharge cycling, and electrochemical impedance spectroscopy. Independently on the applied current density, the capacitances were found to increase in the following order: spinel-CNF < OMS-5-CNF < cryptomelane-CNF < birnessite-CNF.