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

Abstract

We describe a new facile chemical route for the synthesis of hierarchical mesoporous α-MnO₂ and the development of an aqueous asymmetric supercapacitor. The hierarchical α-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 α-MnO₂ has a three-dimensional (3D) flower-like mesoporous hierarchical structure with an average size of 500 nm and a large surface area (206 m² g^-1) with a pore size of 48.34 Å and a pore volume of 0.543 cm³ 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 α-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 α phase even after 3000 charge–discharge cycles, though a partial disintegration of the hierarchical structure was observed.