Electrochemical assembly of MnO2 on ionic liquid-graphene films into a hierarchical structure for high rate capability and long cycle stability of pseudocapacitors

Hierarchical nanostructures are of prime importance due to their large surface area, easy accessibility to reaction sites, fast ion and electron transport, and mechanical integrity. Herein, we demonstrate the synthesis of hierarchically structured MnO2/ionic liquid-reduced graphene oxide (IL-RGO) nanocomposites through the electrochemical self-assembly. The structures of MnO2/IL-RGO nanocomposites and their formation mechanism are investigated by spectroscopic methods and as a consequence, correlated with the electrochemical behaviours. The specific capacitance (511 F g(-1)) of conformally MnO2-deposited IL-RGO composites is significantly higher than 159 F g(-1) of pure MnO2 film. High rate capability (61% retention at 30 A g(-1)) of the MnO2/IL-RGO composite is attributed to the facilitated ion diffusion and electron transport, whereas its long cycle life (95% retention after 2000 cycles) is related to the mechanical robustness. These results provide a new insight into the rational design of hierarchical and complex heterostructures consisting of carbon nanomaterials and metal oxides for applications in energy conversion and storage.