High-performance aqueous asymmetric supercapacitors based on K+ and Na+ co-preinserted δ-MnO2 nanocrystals

Layed d-MnO2 nanocrystals are fabricated through a facile precipitation reaction at ambient conditions with Na2SO3 as reductant. The high content of co-preinserted K+ and Na+ and water content in d-MnO2 are responsible for the enhanced electrochemical performance. Specifically, d-MnO2 delivers a high specific capacitance of 303.7 F g(-1) at 0.5 A g(-1), together with steady cycle property. Moreover, an asymmetric supercapacitor (ASC) is fabricated with d-MnO2 as the positive electrode and reduced graphene oxide (RGO) as the negative electrode. The device displays a large energy density of 53.2 Wh kg(-1) at 250 W kg(-1) with a potential window of 0-2.0 V in aqueous electrolyte, and 91.1% capacitance retention after 5000 cycles. These results can well demonstrate the potential application of layed d-MnO2 nanocrystals as the active electrode material in energy storage systems.