Abstract:
We present a study on the pseudocapacitive properties of birnessite-type MnO2 grafted on highly
graphitized onion-like carbon (OLC/MnO2). In a three-electrode setup, we evaluated two different
substrates, namely a platinum disc and nickel foam. The OLC/MnO2 nanohybrid exhibited a large specific
capacitance (Csp) of 295 and 323 F g 1 (at 1 A g 1) for the Pt disc and Ni foam, respectively. In addition,
the Ni foam substrate exhibited much higher rate capability (power density) than the Pt disc. A
symmetrical two-electrode device, fabricated with the Ni foam, showed a large Csp of 254 F g 1, a
specific energy density of 5.6 W h kg 1, and a high power density of 74.8 kW kg 1. These values have
been the highest for onion-based electrodes so far. The device showed excellent capacity retention
when subjected to voltage-holding (floating) experiments for 50 h. In addition, the device showed a very
short time constant (s ¼ 40 ms). This high rate handling ability of the OLC/MnO2 nanohybrid, compared
to literature reports, promises new opportunities for the development of aqueous-based pseudocapacitors.
