Microwave-Assisted Synthesis and Characterization of γ-MnO 2 for High-Performance Supercapacitors

Two hydrothermal techniques under microwave irradiation were used to synthesize γ-MnO 2 from 90°C to 150°C in 10−30 min. The first technique is based on reducing KMnO 4 with MnSO 4 , and the second one involves liquid-phase oxidation between MnSO 4 and (NH 4 ) 2 S 2 O 8 . The structures and morphologies of the samples were analyzed using X-ray diffraction, scanning electron microscopy, and N 2 physisorption measurements. The electrochemical properties were evaluated through cyclic voltammetry and electrochemical impedance spectroscopy. The γ-MnO 2 materials obtained by the first technique mainly exhibited nanorods with diameters of 40–60 nm, and the samples obtained by the second technique showed flower-like microspheres with diameters of 1−2 µm; each flower was composed of nanosheets with a thickness of 10−20 nm. The processing time directly depends on the size of the nanorods. The sample synthesized by the first technique at 150°C and 10 min has the highest specific surface area of up to 59.08 m 2 g −1 and mean pore diameter of 34.11 nm. Furthermore, this sample exhibits a near-rectangular cyclic voltammetry curves and high specific capacitance of 331.3 F g −1 in 0.1 M Na 2 SO 4 solution at 5 mV s −1 scan rate. Graphic abstract