Alpha-MnO₂ nanofibers/nitrogen and sulfur-co-doped reduced graphene oxide for 4.5 V quasi-solid state supercapacitors...

α-MnO₂ nanofibers combined with nitrogen and sulfur co-doped reduced graphene oxide (α-MnO₂/N&S-rGO) were prepared through simple hydrothermal and ball milling processes. Structural characterization results by X-ray diffraction, X-ray photoemission spectroscopy, electron microscopy and Raman spectroscopy demonstrated that α-MnO₂ nanofibers with the average diameter of ~40 nm were well dispersed on N&S-rGO nanoflakes. The synthesized material was incorporated into supercapacitor (SC) electrodes and assembled with the quasi-solid-state electrolyte comprising N,N-Diethyl-N-methyl-N-(2-methoxy-ethyl)ammonium bis (trifluoromethyl-sulfonyl)amide [DEME][TFSA]/polyvinylidene fluoride-hexafluoropropylene (PVDF-co-HFP) to produce coin-cell SCs. Electrochemical performances of SCs were measured by cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy. From the electrochemical data, SC using α-MnO₂/N&S-rGO exhibited a good specific capacitance of 165F g^-1 at 0.25 A g^-1 with a wide potential window of 0-4.5 V, corresponding to a high energy density of 110 Wh kg^-1 and a power density of 550 W kg^-1. In addition, it exhibited good electrochemical stability with a capacitance retention of 75% after 10,000 cycles at 1 A g^-1 and a low self-discharge loss. The attained energy-storage performances indicated that the α-MnO₂/N&S-rGO composite could be highly promising for high-performance ionic liquid-based quasi solid-state supercapacitors.