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 degrees 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 m(2) 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. (C) 2011 Elsevier By. All rights reserved.