Microwave–hydrothermal synthesis of birnessite-type MnO2 nanospheres as supercapacitor electrode materials

Birnessite-type MnO2 (Bir-MnO2) nanospheres have been successfully synthesized by the microwave–hydrothermal (M–H) method at 75 °C for 30 min under low pressure. The properties and electrochemical performance of the as-prepared MnO2 are analyzed and evaluated by the field emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) measurements and electrochemical tests. The Bir-MnO2 appears mesoporous nanosphere structure with 70–90 nm in diameter, and it exhibits a large specific surface (SS) of 213.6 m2 g−1 by the results of FE-SEM, XRD and BET. The electrochemical test results show that the specific capacitance (SC) is 210 F g−1 at 200 mA g−1 in 1.0 M Na2SO4 electrolyte, and the SC retention and coulombic efficiency are over 96% and 98% respectively after 300 cycles at 1.6 A g−1. Compared with the conventional syntheses of MnO2, the performance of the Bir-MnO2 nanospheres synthesized by M–H method is significantly improved.

► Birnessite-type MnO2 nanospheres are synthesized by microwave–hydrothermal method.
► Synthesis process is fast (30 min), at low temperature (75 °C) under low pressure.
► As-prepared MnO2 has interesting nanosphere structure.
► The specific surface of the as-prepared MnO2 is as large as 213.6 m2 g−1.
► The nanospheres show high coulombic efficiency (98%) and excellent cycle stability.