Development of 3D Urchin-Shaped Coaxial Manganese Dioxide@Polyaniline (MnO₂@PANI) Composite and Self-Assembled 3D Pillared Graphene Foam for Asymmetric All-Solid-State Flexible Supercapacitor Application

We have fabricated high-energy-density all-solid-state flexible asymmetric supercapacitor by using a facile novel 3D hollow urchin-shaped coaxial manganese dioxide@polyaniline (MnO₂@PANI) composite as positive electrode and 3D graphene foam (GF) as negative electrode materials with polyvinyl alcohol (PVA)/KOH gel electrolyte. The coaxial MnO₂@PANI composite was fabricated by hydrothermal route followed by oxidation without use of an external oxidant. The formation mechanism of the 3D hollow MnO₂@PANI composite occurs first by nucleation and growth of the MnO₂ crystal species via dissolution-recrystallization and oriented attachment mechanisms followed by the oxidation of aniline monomers on the MnO₂ crystalline template. The self-assembled 3D graphene block was synthesized by hydrothermal route using vitamin C as a reducing agent. The microstructures of the composites are analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The morphology is characterized by field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM), which clearly showed the formation of urchin-shaped coaxial MnO₂@PANI composite. The electrochemical studies are explored by cyclic voltammetry, electrochemical impedance spectrometry, and cyclic charge-discharge tests. The symmetric all-solid-state flexible MnO₂@PANI//MnO₂@PANI and GF//GF supercapacitors exhibit the specific capacitance of 129.2 and 82.1 F g^-1 at 0.5 A/g current density, respectively. The solid-state asymmetric supercapacitor shows higher energy density (37 Wh kg^-1) with respect to the solid-state symmetric supercapacitors MnO₂@PANI//MnO₂@PANI and GF//GF, where the obtained energy density are found to be 17.9 and 11.4 Wh kg^-1, respectively, at 0.5 A/g current density. Surprisingly, the asymmetric supercapacitor shows a high energy density of 22.3 Wh kg^-1 at a high current density of 5 A g^-1. The solid-state asymmetric supercapacitor shows a good cyclic stability in which ∼11% capacitance loss was observed after 5000 cycles.