High density graphene–carbon nanosphere films for capacitive energy storage

Highly packed films of reduced graphene oxide and sugar-based carbon nanospheres (CNSs) were prepared by a simple hydrothermal treatment. Under hydrothermal conditions, graphene oxide was partially reduced and self-assembled forming a monolith that effectively embedded the CNSs. The spheres were homogeneously distributed within the films, that had an apparent density of up to 1.40 g cm−3. The films thus synthesized were directly assembled into a cell and tested as free-standing electrodes for supercapacitors without using any binder or conductive additive. Electrodes with a mass loading similar to that of commercial devices showed very high values of volumetric capacitance (252 F cm−3) and also an excellent rate capability (64% at 10 A g−1) despite their highly packed microstructure. The homogeneous dispersion of the nanospheres was responsible for the improved ion diffusion when compared to the CNS-free counterpart. The use of a small CNS/graphene wt ratio is essential for achieving such good rate capability without compromising its performance in volumetric terms.