Shape-Controlled Synthesis of 3D Hierarchical MnO2 Nanostructures for Electrochemical Supercapacitors

A simple hydrothermal method has been used to fabricate sea urchin shaped α-MnO2 without using any template and surfactant. When other conditions were kept the same and Al3+ exists in the solution, the product is α-MnO2 nanorods clusters; when Al3+ was substituted by Fe3+ in the solution, the phase of the products was transformed from α-MnO2 to ε-MnO2; relatively, the morphology of the product was changed from urchinlike clusters to 3D clewlike. The sea urchin shaped α-MnO2 and α-MnO2 nanorods clusters consist of radially grown single crystalline MnO2 nanorods of ca. 30−40 nm in diameter and ca. 0.87−1.2 μm in length. The 3D clewlike ε-MnO2 nanostructures are composed of nanosheets with a thickness of about 17 nm. The electrochemical properties of the prepared MnO2 materials were studied using cyclic voltammetry (CV) and ac impedance measurements in aqueous electrolyte. 3D clewlike ε-MnO2 nanostructures have a specific capacitance of 120 F g−1 and lower charge-transfer resistance. The results indicate that metal ions (Fe3+, Al3+) have great effects on the structure and morphology of MnO2.