Nanostructured manganese dioxide with adjustable Mn3+/Mn4+ ratio for flexible high-energy quasi-solid supercapacitors

Abstract Manganese dioxide (MnO2) is attracting much attention recently due to the wide potential window as the electrode in aqueous supercapacitors. However, low electrical conductivity of MnO2 significantly hinders its further development. The obstacle may be circumvented by introducing a proper concentration of trivalent Mn ions in MnO2. Herein, nanostructured MnO2 with adjustable Mn3+/Mn4+ ratio is deposited to conductive carbon cloth substrate (CC) by potentiometric electrochemical deposition method. MnO2 electrode with Mn3+/Mn4+ ratio of about 0.99 deposited at 70 °C displays a specific capacitance of 408.1 F g−1 at 1 A g−1, and maintains a 99% capacity after 2000 cycles at 10 A g−1. In this paper, we explain the influence of the introduction of Mn3+ ions on the electrochemical performance of MnO2 electrode according to the Mn3+-O-Mn4+ double-exchange interaction mechanism and oxygen vacancy. Furthermore, an asymmetric supercapacitor (ASC) with nanostructured MnO2/CC as positive electrode and carbon coated FeOOH (FeOOH/C) as negative electrode is assembled. The ASC can exhibit a maximum energy density of 55.9 Wh kg−1 and power density of 6.87 kW kg−1. Moreover, the prepared ASC shows excellent flexibility and great potential applications by lighting blue LEDs.