Preparation and electrochemical properties of lamellar MnO 2 for supercapacitors

Abstract

Lamellar birnessite-type MnO 2 materials were prepared by changing the pH of the initial reaction system via hydrothermal synthesis. The interlayer spacing of MnO 2 with a layered structure increased gradually when the initial pH value varied from 12.43 to 2.81, while the MnO 2, composed of α-MnO 2 and γ-MnO 2, had a rod-like structure at pH 0.63. Electrochemical studies indicated that the specific capacitance of birnessite-type MnO 2 was much higher than that of rod-like MnO 2 at high discharge current densities due to the lamellar structure with fast intercalation/deintercalation of protons and high utilization of MnO 2. The initial specific capacitance of MnO 2 prepared at pH 2.81 was 242.1 F g −1 at 2 mA cm −2 in 2 mol L −1 (NH 4) 2SO 4 aqueous electrolyte. The capacitance increased by about 8.1% of initial capacitance after 200 cycles at a current density of 100 mA cm −2.