Facile shape-controlled growth of hierarchical mesoporous δ-MnO₂for the development of asymmetric supercapacitors

Abstract

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 δ-MnO₂ and its supercapacitive performance. The mesoporous δ-MnO2 is synthesized by the thermodynamically favourable redox reaction of MnO4− with HBr. The growth of δ-MnO2 involves the facile reduction of MnO₄^-. to Mn²⁺. and the subsequent reaction of in situ generated Mn²⁺. with unreacted Mn₄^- 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 δ-MnO₂ of 300 nm size. It has a three-dimensional mesoporous structure with a large surface area of 238 m² g^-1. It has an average pore size and pore volume of 36.14 Å and 0.567 cc g^-1, respectively. The concentration of Br^-1 controls the growth of δ-MnO₂ and a large excess of Br^-1 completely reduces MnO₂ to Mn²⁺. The δ-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 δ-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.