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 MnO ₂/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.