Hierarchical three-dimensional mesoporous MnO2 nanostructures for high performance aqueous asymmetric supercapacitors

We describe a new facile chemical route for the synthesis of hierarchical mesoporous alpha-MnO2 and the development of an aqueous asymmetric supercapacitor. The hierarchical alpha-MnO2 is synthesized by the thermodynamically favorable redox reaction between metallic Zn and MnO4- in mild acidic solutions without any template. Zn and the in situ generated nascent hydrogen efficiently reduce MnO4- to MnO2. The growth mechanism is studied with time-dependent electron microscopic measurements. The alpha-MnO2 has a three-dimensional (3D) flower-like mesoporous hierarchical structure with an average size of 500 nm and a large surface area (206 m(2) g(-1)) with a pore size of 48.34 angstrom and a pore volume of 0.543 cm(3) g(-1). It has significantly high electronic conductivity with respect to traditional/commercial MnO2. A specific capacitance as high as 322 F g(-1) at a current density of 1 A g(-1) with excellent cycling stability (90% after 8000 cycles) is achieved. An aqueous asymmetric supercapacitor (ASC) is developed by pairing alpha-MnO2 and reduced graphene oxide-based electrodes. An ASC could deliver a specific capacitance of 63.5 F g(-1) at 2 A g(-1) with a potential window of 0-2 V. The ASC retains 100% initial specific capacitance even after 3000 continuous charge-discharge cycles. It has an energy density of 35.28 W h kg(-1) at a power density of 2.0 kW kg(-1) and could retain 27.78 W h kg(-1) at a power density of 16.67 kW kg(-1). The three-dimensional mesoporous structure favors the facilitated transport of the electrolyte across the electrode. The post-mortem XRD analysis shows that the MnO2 nanostructure retains its initial alpha phase even after 3000 charge-discharge cycles, though a partial disintegration of the hierarchical structure was observed.