Fast pseudocapacitive reactions of three-dimensional manganese dioxide structures synthesized via self-limited redox deposition on microwave-expanded graphite oxide

Abstract

Three-dimensional (3D) MnO₂ structures are deposited on microwave-expanded graphite oxide (MEGO) via a self-limited redox reaction between MEGO and KMnO₄. The 3D architecture consists of MnO₂ sheets lying uniformly on MEGO and walls protruding from MEGO, both with thickness in the range of 1–5 nm. The loading of MnO₂ and the height and density of walls in the 3D architecture can be controlled by tuning the reaction duration, leading to a balanced specific capacitance and power performance. Symmetric supercapacitors assembled using a MEGO–MnO₂ composite with 24.5 wt% MnO₂ can work at a voltage of up to 2 V in a 1 M Na₂SO₄ electrolyte, yielding an energy density of 14 W h kg⁻¹ (13.6 W h L⁻¹) at a power density of 250 W kg⁻¹ (243 W L⁻¹) or a power density of 7.67 kW kg⁻¹ (7.44 kW L⁻¹) at an energy density of 5.46 W h kg⁻¹ (5.3 W h L⁻¹). Asymmetric supercapacitors, consisting of the MEGO–MnO₂ (containing 24.5 wt% MnO₂) composite as the positive electrode and activated MEGO as the negative electrode in a 1 M Na₂SO₄ electrolyte, exhibit an energy density of 25.1 W h kg⁻¹ at a power density of 93 W kg⁻¹ with a working voltage of up to 1.8 V.