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Traditional filtering capacitors suffer from low volumetric energy density (Ev), 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 Ev 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 Ev of 14.9 F V2 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. [ABSTRACT FROM AUTHOR]

α-Form Co xNi1- x hydroxides with different Co/Ni ratios were synthesized by a chemical co-precipitation method under mild conditions. The effects of Co/Ni ratio on the structure, morphology, and supercapacitive properties of α-form Co xNi1- x hydroxides were investigated in detail. α-Form Co xNi1- x hydroxides had structures similar to those of hydrotalcite and α-Co(OH)2, and they exhibited better electrochemical performance than α-Co(OH)2 on testing by XRD, FTIR, and electrochemical performance (when x ≠ 0). The highest specific capacitance at 1 A g-1 for α-form Co xNi1- x hydroxides (for x = 0.2) is 1696.7 F g-1. An asymmetric supercapacitor packaged with Co0.6Ni0.4 hydroxide and activated carbon as positive and negative material, respectively, had an energy density of 23.5 Wh kg-1 at a power density of 412.5 W kg-1. [ABSTRACT FROM AUTHOR]

Supercapacitors: Ultrahigh-Rate Supercapacitor Based on Carbon Nano-Onion/Graphene Hybrid Structure toward Compact Alternating Current Filter (Adv. Keywords: alternating current filters; carbon onions; supercapacitors; ultrahigh rate capability EN alternating current filters carbon onions supercapacitors ultrahigh rate capability 1 1 1 11/19/20 20201117 NES 201117 In article number 2002132, Chenguang Zhang and co-workers report on an ultrahigh-rate supercapacitor fabricated based on a carbon onion/graphene hybrid structure for high-performance alternating current line filtering. Alternating current filters, carbon onions, supercapacitors, ultrahigh rate capability. [Extracted from the article]