Fe₃O₄ hard templating to assemble highly wrinkled graphene sheets into hierarchical porous film for compact capacitive energy storage

Highly wrinkled graphene film (HWGF) with high packing density was synthesized by combining an electrostatically self-assembling process, a vacuum filtration-induced film assembling process and capillary compression. Fe₃O₄ nanoparticles were used as a low-cost and environment-friendly hard template. Hierarchical porosity and high packing density were achieved with the aid of capillary compression in the presence of Fe₃O₄ nanoparticles. This strategy enables integration of highly wrinkled graphene sheets to form highly compact carbon electrodes with a continuous ion transport network. The generated HWGF exhibited a high packing density of 1.53 g cm^-3, a high specific surface area of 383 m² g^-1 and a hierarchically porous structure. The HWGF delivered a high capacitance of 242 F g^-1 and 370 F cm^-3 at 0.2 A g^-1 in 6 M KOH aqueous electrolyte system with excellent rate capability (202 F g^-1 and 309 F cm^-3 retained at 20 A g^-1). The capacity retention rate reached 97% after 10000 cycles at 1 A g^-1. The HWGF-based supercapacitor exhibited a high energy density of 17 W h kg^-1 at the power density of 49 W kg^-1. Such high capacitive performances could be attributed to the highly dense but porous graphene assemblies composed of highly wrinkled graphene sheets.