Porous graphene/CNT@metal (hydr)oxide composite films achieving fast ion and electron kinetics for asymmetric supercapacitors with ultra-high volumetric performance

Abstract

With broad application prospects in electronic systems, supercapacitors are under ongoing development to have higher flexibility, portability, and integration. Although flexible supercapacitors have received widespread attention, their further applications are limited as a consequence of low volumetric energy densities. Developing dense film materials with a high specific capacitance and employing them to assemble asymmetric supercapacitors (ASCs) for expanding the working voltage serve as a strategy to break through the limitation. Herein, we report a universal method for preparing porous graphene nanosheets/carbon nanotubes@metal (hydr)oxide (PGNs/CNT@metal (hydr)oxide) composite films with fast ion and electron kinetics via the vacuum-assisted filtration self-assembly technique. The designed channels on the graphene surface accelerate electrolyte ion diffusion in the direction perpendicular to the graphene film. The introduction of CNT@metal (hydr)oxide inhibits the agglomeration of graphene sheets, thereby speeding up electrolyte ion diffusion in the direction parallel to the graphene film. For pseudocapacitive materials with poor conductivity, the internal and external conductance paths established by CNTs and PGNs can facilitate the electron kinetics of the composite. For the asymmetric supercapacitor assembled with PGNs/CNT@Ni(OH)₂ and PGNs/CNT@Fe₂O₃ as the positive and negative electrodes, respectively, its volumetric energy density reached 114.3 W h L⁻¹ in 1 M KOH aqueous electrolyte and even 45 W h L⁻¹ in a solid electrolyte. This rational structure design of electrode materials can provide guidance for the development of energy storage materials with outstanding volumetric performance for flexible supercapacitors.