Figure 4.

(a) Cyclic voltammograms of MnO₂ nanotubes in a 1 M Na₂SO₄ aqueous electrolyte. (b) Charge-discharge curves of MnO₂ nanotubes at different current densities. (c) Specific capacitance of MnO₂ nanotubes measured under different current densities. (d) Electrochemical impedance spectrum of the MnO₂ nanotubes electrodes at open circuit potential in the frequency range from 0.01 Hz to 100 kHz. The inset shows the equivalent circuit. (e) Cycling performance of MnO₂ nanotubes at the current density of 5 A g⁻¹. The inset shows the charge-discharge curves of the last 10 cycles of the MnO₂ nanotubes electrode. (f) The corresponding SEM image of the electrode after 3000 electrochemical cycles.