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

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.