High-performance 2.5 V flexible aqueous asymmetric supercapacitors based on K+/Na+-inserted MnO2 nanosheets

The voltage of aqueous asymmetric supercapacitors is usually limited within 2 V, which hampers their practical application as energy storage device. Herein, three-dimensional porous alkaline ions-inserted δ-MnO2 nanosheet arrays are prepared via hydrothermal synthesis on a modified carbon cloth substrate with N-doped carbon nanowire network (N-CNN). Interestingly enough, the potential window of the MnO2 nanosheet arrays can be extended to 1.2 V versus saturated calomel electrode (vs. SCE). The charge storage mechanism of δ-MnO2 for serving as the cathode is discussed which reveals the behavior of insertion/extraction for alkaline ions in electrolyte solution. Carbon-coated Fe2O3 nanorod arrays supported on bare carbon cloth are also fabricated successfully, which can stably function as the anode in a broad negative potential window of −1.3-0 V (vs. SCE). Consequently, a flexible aqueous asymmetric supercapacitor with a high voltage of 2.5 V is constructed by using MnO2@N-CNN as the cathode and C@Fe2O3 as the anode. The MnO2@N-CNN//C@Fe2O3 device displays a large energy density up to 1.43 mWh/cm3 as well as excellent rate capability and cycling performance, which is among the best supercapacitors based on MnO2 materials. This work paves a novel way to develop high-voltage aqueous asymmetric energy storage devices with large capacitance and energy density.