Facile shape-controlled growth of hierarchical mesoporous δ-MnO2 for the development of asymmetric supercapacitors

Synthesis of pseudocapacitive mesoporous transition metal oxides with hierarchical structures is of great interest in the development of high performance energy storage devices. Herein, we demonstrate a facile single-step, template-free chemical route for the synthesis of hierarchical mesoporous delta-MnO2 and its supercapacitive performance. The mesoporous delta-MnO2 is synthesized by the thermodynamically favourable redox reaction of MnO4- with HBr. The growth of delta-MnO2 involves the facile reduction of MnO4- to Mn2+ and the subsequent reaction of in situ generated Mn2+ with unreacted MnO4- in one pot at room temperature. Br- has dual roles of reducing MnO4- and controlling the growth of MnO2 by surface etching. The possible Ostwald ripening and self-assembling of the nanoseeds formed at the initial stage of the reaction and the ensuing surface etching of the urchin-like MnO2 by Br- produce hierarchical flower-like delta-MnO2 of 300 nm size. It has a three-dimensional mesoporous structure with a large surface area of 238 m(2) g(-1). It has an average pore size and pore volume of 36.14 angstrom and 0.567 cc g(-1), respectively. The concentration of Br- controls the growth of delta-MnO2 and a Large excess of Br- completely reduces MnO2 to Mn2+. The delta-MnO2 nanostructure shows excellent supercapacitive performance with a specific capacitance of 364 F g(-1) at a current density of 1 A g(-1). An aqueous asymmetric supercapacitor (ASC) is developed by pairing the delta-MnO2-based cathode with an activated carbon anode. ASC delivers a specific capacitance of 86.5 F g(-1) at 1 A g(-1) with a wide potential window of 0-2 V. It retains 100% initial specific capacitance even after 3000 continuous charge-discharge cycles. The device has an energy density of 48.06 W h kg(-1) at the power density of 1.0 kW kg(-1) and it retains 24.44 W h kg(-1) at a power density of 20 kW kg(-1). The favourable access of the electrode material to the electrolyte due to the mesoporous structure enhances the overall performance of the device.