Effect of raw material ratio on the crystal structure and electrochemical performance of nano-MnO₂ synthesized by hydrothermal method

Manganese oxides with different structures and morphologies were synthesized by hydrothermally treating KMnO₄ and MnSO₄ with varying molar ratio. As the molar ratio of KMnO₄ and MnSO₄ is 6:1, the urchin-like δ-MnO₂ superstructures with the diameter of 0.5-1 μm were prepared, which consist of agglomerate petal-shaped thin nanosheet. By adjusting the molar ratio of KMnO₄ and MnSO₄ to 2:3, the uniform α-MnO₂ nanowires were fabricated with the diameter of 30 to 50 nm and the length up to 1 μm. With a decrease molar ratio of 1:3, the short rod-like β-MnO₂ were obtained with the diameter of 500 nm and the length ranging from 2 to 4 μm. The electrochemical properties of obtained MnO₂ samples as supercapacitor electrode materials were examined by cyclic voltammetry and galvanostatic charge-discharge in a three electrode system, the results indicated that spherical urchin-like δ-MnO₂ possessed excellent supercapacitive performance. The specific capacitance of δ-MnO₂ was about 162 F/g at a constant current of 1 A/g, which was much higher than the specific capacitance of α-MnO₂ nanowires (62 F/g) and β-MnO₂ nanorods (8 F/g).