Template-free rapid sonochemical synthesis of spherical α-MnO2 nanoparticles for high-energy supercapacitor electrode

Abstract Spherical MnO2 nanoparticles were synthesized at room temperature via template-free sonochemical reduction of potassium permanganate using a reducing agent of polyethylene glycol under two different time durations (i.e., 15 min (M1) and 30 min (M2)). The M2 sample exhibited the α-phase of the tetragonal MnO2 structure, whereas the M1 sample showed the δ-MnO2 phase. For microstructural analyses, the M2 sample revealed a spherical structure of the nanoparticles with an average grain size of 20–30 nm. When using spherical MnO2 nanoparticles (i.e., M2) as an electrode material for the supercapacitor device, the maximum specific capacitance of 136 F/g was observed at the current density of 0.5 A/g. The device displayed an excellent cyclic stability of 72% after 5000 charge/discharge cycles, and showed a high energy density up to 38.3 Wh/kg. These results suggest that the sonochemically-synthesized spherical MnO2 nanoparticles could be used as a suitable electrode material for high-energy electrochemical supercapacitors.