Room-temperature catalytic growth of hierarchical urchin-like MnO2 spheres on graphene to achieve silver-doped nanocomposites with improved supercapacitor performance

Abstract

Silver-doped graphene/MnO2 (SGM) hierarchical composites were readily fabricated by silver-ion-assisted room-temperature catalytic growth of urchin-like MnO2 spheres from the surface of graphene oxide (GO) followed by chemical reduction. This in-situ crafting strategy allows us to control the size of MnO2 spheres by tuning the concentration of silver ions in the reaction mixture. Graphene herein functions as a separator to prevent the agglomeration of MnO2 and a wrapper around MnO2 to avoid the electrochemical dissolution during charging/discharging cycles. MnO2 spheres grown from the surface of GO sheets ensure an intimate interface contact and large interface area between the two, and also prevent the stacking of graphene, thus affording open channels, high accessible surface, short diffusion paths of charges and high electrochemical utilization of the electrode. Furthermore, graphene sheets bridge a robust conductive network which facilitates fast transport of electrolyte ions and electrons throughout the electrode. As expected, an optimized SGM composite electrode delivers a much higher specific capacitance (∼273.1Fg−1 at 5mVs−1, and ∼260Fg−1 at 0.2Ag−1) compared to the reduced GO (119.4Fg−1) and MnO2 (140.9Fg−1) counterparts. The retention of initial capacitance reaches 77.8% after a 20-fold increase in the scan rate, and remains 83.5% after 1200 cycles, indicating high rate capability and excellent cyclability.