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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]

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]

Engineering strategies based on "nanostructuring" and "active/inactive composites" are commonly used separately to improve the performance of alkali‐ion battery electrodes. Here, these two strategies are merged to further enhance the performance of alloy‐type alkali‐ion battery anodes. Specifically, macroporous antimony (Sb)/magnesium fluoride (MgF2) active/inactive composite material is used as a high‐performance Na‐ion battery anode. The porous Sb phase with pore size in the sub‐micrometer range acts as the electrochemically active component and the electrochemically inactive dense MgF2 phase acts as a mechanical buffer. Na‐ion battery anodes made of porous Sb/MgF2 active/inactive composites are reversibly sodiated for over 300 cycles, delivering a capacity of ≈551 mAh g−1 after 300 cycles at a C‐rate of C/2. This performance is remarkable because the porous Sb/MgF2 composite is not made of mesoporous structures. Furthermore, the cycling longevity of this porous Sb/MgF2 composite outperforms the common nanostructured Sb‐based Na‐ion battery anode materials. This good performance is attributed to the "porous active/inactive" configuration, where the dense inactive mechanical buffer phase absorbs part of the phase transformation‐induced stresses, while porosity in the active phase helps to accommodate the phase transformation induced volume expansions and electrolyte transfer into the bulk of this composite. This work reports on a high‐performance Na‐ion battery anode consisting of macroporous Sb/MgF2 active/inactive composites in which dense MgF2 (blue in figure) acts as an electrochemically inert mechanical buffer that absorbs sodiation‐induced volume expansions in the macroporous Sb (red in figure), the active phase for Na storage. [ABSTRACT FROM AUTHOR]