Tuning the Surface Morphology and Pseudocapacitance of MnO₂by a Facile Green Method Employing Organic Reducing Sugars

In the present work, three different MnO2 nanostructures, nanoneedles, hollow tubes, and nanorods of MnO2, have been synthesized by a simple redox reaction between permanganate and organic sugars at room temperature. The MnO2 samples were characterized by a variety of analytical techniques. The results illustrate that the organic reducing sugars of mannose, galactose, and glucose effectively tune the morphology, crystallinity, and pore structure of the MnO2 material. The nanoneedles and hollow tubes were found to be β-MnO2, while the nanorods were α-MnO2. The formation of different MnO2 nanostructures appears to be a kinetically driven process that proceeds in a quite distinctive way in the presence of different organic reducing sugars. Cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) tests were conducted to evaluate the charge storage behavior of the α- and β-MnO2 nanostructures. Among all three MnO2 samples, β-MnO2 composed of nanoneedles delivered a large specific capacitance, CS (∼365 F g–1 at 0.5 A g–1) with improved rate capability (56% retention at 12 A g–1) and excellent cyclability (82% retention at 2000 cycles). The elegant combination of the high specific surface area (∼146 m2 g–1) and 1D-nanoneedle structure of β-MnO2, enhances the electrode–electrolyte contact area and hence provides a number of active sites for fast charge–discharge propagations.