It can be seen from eqn (1) and (2) that both the cations and
the electrons take part in the redox reactions of the MnO2
nanostructures. Therefore it can be concluded that the degree
of diffusion of the cations on the surface affects the pseudocapacitive performance of the electrodes.30,31
Fig. 6 compares the electrochemical performance of three
different types of MnO2 nanostructures used as electrode
materials. Fig. 6a shows the Nyquist plots of the MnO2 electrode
in KOH aqueous solution. These measurements were carried
out at room temperature in the frequency range 0.1 Hz to
100 kHz with an open circuit voltage at an AC amplitude of
5 mV. In the Nyquist plot, the intercept at the real part (Z0
), Rs,
which indicates the resistance of the electrochemical system, is
a combination of the ionic resistance of the electrolyte, the
intrinsic resistance of the substrate, the intrinsic resistance of
the active material and the contact resistance at the active
material/current collector interface.14 From the intercepts at the
real axis at high frequency it can be seen that Rs decreases in theorder a-MnO2 z d-MnO2 > b-MnO2. All the MnO2 structures
show a low impedance as pseudocapacitors (