High voltage asymmetric supercapacitor based on MnO₂ and graphene electrodes

Asymmetric supercapacitor cells based on MnO₂ as positive electrode and graphene as negative electrode have been constructed with 1 M Na₂SO₄ aqueous electrolyte. Amorphous MnO₂ nanoparticles were prepared by reducing KMnO₄ in N,N-dimethylformamid (DMF). Graphene was prepared by hydrothermal reduction of exfoliated graphite oxide sheets. The MnO₂/graphene asymmetric supercapacitor could operate reversibly at a high cell voltage of 2.0 V, showing an energy density of 25.2 Wh kg^-1 at a power density of 100 W kg^-1, which is much higher than those of symmetric supercapacitors based on MnO ₂/MnO₂ (4.9 Wh kg^-1) and graphene/graphene (3.6 Wh kg^-1). Moreover, the asymmetric system exhibits capacitance retention of 96% after 500 cycles.