Fig. 4 shows the Raman spectra of the three types of MnO2
structures. The diffraction peaks at 645 cm1 correspond well
with the Raman spectrum of MnO2 and can be attributed to the
MnO6 symmetric stretching vibrations.26 d-MnO2 consists of
layered sheets of MnO6 octahedra and the peak at 573 cm1
might come from Mn–O stretching in the basal plane of the
MnO6 sheet.27
The electrochemical properties of the three different types
of MnO2 nanostructures were studied by cyclic voltammetry
(CV), galvanostatic CD and electrochemical impedance spectrometry (EIS) in a three-electrode conguration. Fig. 5 shows
the cyclic voltammograms and corresponding CD curves of
the three different MnO2 nanostructures used as electrode
materials in aqueous KOH electrolyte. As the current density
is one of the most important parameters controlling the
capacitive performance of the electrode, the CD curves of the
three types of MnO2 were recorded at different current
densities. All the CV curves deviate from a rectangular shape
because of the overpotential and the corresponding CD curves
deviate from symmetrical behavior under different current
densities. The CD curves at low current densities show a
platform corresponding to the redox reactions. There are two
mechanisms for charge storage by MnO2. The rst involves
the intercalation/extraction of protons (H3O+
) or alkali cations
such as Li+
, Na+ or K+ into the bulk of the oxide particles with
the concomitant reduction/oxidation of Mn ions:28,29