Ultrahigh‐Rate Supercapacitor Based on Carbon Nano‐Onion/Graphene Hybrid Structure toward Compact Alternating Current Filter

Abstract Traditional filtering capacitors suffer from low volumetric energy density ( E v ), which hinders their integration in miniaturized electronics and their high‐frequency response ability to alternating current (AC). Here, a supercapacitor based on a carbon nano‐onion (CNO)‐graphene hybrid structure concurrently possessing high E v and ultrahigh rate capability is reported for AC filtering. The hybrid structure is synthesized on etched nickel foil using preformed monodispersed nanocrystals through a chemical vapor deposition method. The supercapacitor device shows capacitive behavior independent of voltage scan rate up to 5000 V s −1 in both aqueous and organic electrolytes, which represents a record high among the reported filtering capacitors to date. It also exhibits a high E v of 14.9 F V 2 cm −3 at 120 Hz and 28.8 mWh cm −3 at a current density of 0.25 mA cm −2 . Moreover, the supercapacitor is capable of filtering AC with different waveforms at high frequencies into direct current. The hybrid structure holds promise for applications in compact filtering units. The ultrafast ion transportation performance is enabled by the highly positive surface curvature of CNOs, hierarchical interconnection of CNO particles, and the covalent interfacial bonding between CNOs and graphene.