Preparation and electrochemical properties of lamellar MnO2 for supercapacitors

Lamellar birnessite-type MnO2 materials were prepared by changing the pH of the initial reaction system via hydrothermal synthesis. The interlayer spacing of MnO2 with a layered structure increased gradually when the initial pH value varied from 12.43 to 2.81, while the MnO2, composed of α-MnO2 and γ-MnO2, had a rod-like structure at pH 0.63. Electrochemical studies indicated that the specific capacitance of birnessite-type MnO2 was much higher than that of rod-like MnO2 at high discharge current densities due to the lamellar structure with fast intercalation/deintercalation of protons and high utilization of MnO2. The initial specific capacitance of MnO2 prepared at pH 2.81 was 242.1 F g−1 at 2 mA cm−2 in 2 mol L−1 (NH4)2SO4 aqueous electrolyte. The capacitance increased by about 8.1% of initial capacitance after 200 cycles at a current density of 100 mA cm−2.