High-Energy MnO₂ Nanowire/Graphene and Graphene Asymmetric Electrochemical Capacitors

Abstract

In order to achieve high energy and power densities, we developed a high-voltage asymmetric electrochemical capacitor (EC) based on graphene as negative electrode and a MnO₂ nanowire/graphene composite (MGC) as positive electrode in a neutral aqueous Na₂SO₄ solution as electrolyte. MGC was prepared by solution-phase assembly of graphene sheets and α-MnO₂ nanowires. Such aqueous electrolyte-based asymmetric ECs can be cycled reversibly in the high-voltage region of 0−2.0 V and exhibit a superior energy density of 30.4 Wh kg⁻¹, which is much higher than those of symmetric ECs based on graphene//graphene (2.8 Wh kg⁻¹) and MGC//MGC (5.2 Wh kg⁻¹). Moreover, they present a high power density (5000 W kg⁻¹ at 7.0 Wh kg⁻¹) and acceptable cycling performance of ∼79% retention after 1000 cycles. These findings open up the possibility of graphene-based composites for applications in safe aqueous electrolyte-based high-voltage asymmetric ECs with high energy and power densities.