The synergism of large surface area, multiscale porous structure, and good conductivity endows hierarchical carbon nanocages with high-level supercapacitive performances. Further nitrogen doping greatly improves the hydrophilicity, which boosts the supercapacitive performances to an ultrahigh specific capacitance of up to 313 F g(-1) at 1 A g(-1).

The mesostructured NiO/Ni composites boost the electrochemical energy storage performance of NiO to its theoretical limit, which results from the synergism of high accessibility to electrolyte, short solid-state ion diffusion length and high conductivity owing to the unique mesostructure.

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