High-performance symmetric electrochemical capacitor based on graphene foam and nanostructured manganese oxide

We have fabricated a symmetric electrochemical capacitor with high energy and power densities based on a composite of graphene foam (GF) with ∼80 wt% of manganese oxide (MnO₂) deposited by hydrothermal synthesis. Raman spectroscopy and X-ray diffraction measurements showed the presence of nanocrystalline MnO₂ on the GF, while scanning and transmission electron microscopies showed needle-like manganese oxide coated and anchored onto the surface of graphene. Electrochemical measurements of the composite electrode gave a specific capacitance of 240 Fg^-1 at a current density of 0.1 Ag^-1 for symmetric supercapacitors using a two-electrode configuration. A maximum energy density of 8.3 Whkg^-1 was obtained, with power density of 20 kWkg^-1 and no capacitance loss after 1000 cycles. GF is an excellent support for pseudo-capacitive oxide materials such as MnO₂, and the composite electrode provided a high energy density due to a combination of double-layer and redox capacitance mechanisms.