Filtering capacitor is a necessary component in the modern electronic circuit. Traditional filtering capacitor is often limited by its bulky and rigid configuration and narrow workable scope of applications. Here, an aqueous hybrid electrochemical capacitor is developed for alternating current line filtering with an applicable wide frequency range from 1 to 10,000 Hz. This capacitor possesses an areal specific energy density of 438 μF V 2 cm −2 at 120 Hz, which to the best of our knowledge is record high among aqueous electrochemical capacitors reported so far. It can convert arbitrary alternating current waveforms and even noises to straight signals. After integration of capacitor units, a workable voltage up to hundreds of volts (e.g., 200 V) could be achieved without sacrificing its filtering capability. The integrated features of wide frequency range and high workable voltage for this capacitor present promise for multi-scenario and applicable filtering capacitors of practical importance.

Capacitive energy storage has advantages of high power density, long lifespan, and good safety, but is restricted by low energy density. Inspired by the charge storage mechanism of batteries, a spatial charge density (SCD) maximization strategy is developed to compensate this shortage by densely and neatly packing ionic charges in capacitive materials. A record high SCD (ca. 550 C cm−3) was achieved by balancing the valance and size of charge‐carrier ions and matching the ion sizes with the pore structure of electrode materials, nearly five times higher than those of conventional ones (ca. 120 C cm−3). The maximization of SCD was confirmed by Monte Carlo calculations, molecular dynamics simulations, and in situ electrochemical Raman spectroscopy. A full‐cell supercapacitor was further constructed; it delivers an ultrahigh energy density of 165 Wh L−1 at a power density of 150 WL−1 and retains 120 Wh L−1 even at 36 kW L−1, opening a pathway towards high‐energy‐density capacitive energy storage.

window (around 3 V) are more favorable for practical applications. Up to date, only an ultrafast OEC based on CNT films has been reported in literature. [10] However, the preparation of CNT electrodes involved complicated processes of vacuum filtration and expensive chemical vapor deposition. Therefore, a facile and scalable method to fabricate OECs with excellent AC line-filtering performances still remains as a challenge.In this paper, we report the fabrication of OECs based on electrochemically reduced less defective graphene oxide (ERLGO) films with oriented 3D interconnected porous structures. For optimizing the performance of ERLGO electrode, LGO sheets with a small average size of 0.7 µm (sLGO) were used and the resulting ERsLGO electrode was further deeply reduced in an organic electrolyte. The typical OEC exhibited high areal specific energy density, excellent electrochemical stability, and superior rate capability, promising for AC line filtering. These OECs can also be connected in series or parallel to meet various demands in industrial levels. Results and DiscussionThe graphene electrode (Figure 1a) was prepared by electrochemical reduction of LGO or sLGO sheets in their aqueous dispersion (3 mg mL −1 ) at −1.1 V for 4 s, then further deeply reduced in an organic electrolyte at −2.0 V for 20 s. The organic electrolyte was 1 mol L −1 acetonitrile (AN) solution of tetraethylammonium-tetrafluoroborate (TEABF 4 ). LGO sheets were prepared by oxidation of graphite flakes at a low temperature of 0 °C. [19] They have an average lateral dimension of 7 µm (Figure 1b, Figure S1, Supporting Information). During the electrodeposition process, LGO sheets were selfassembled onto the surface of substrate electrode upon the driving by directional electric field. [20] Simultaneously, they were electrochemically reduced to ERLGO sheets to form a porous network because of π-π stacking and hydrophobic interactions. However, the ERLGO electrode has a relatively disordered porous structure ( Figure S2, Supporting Information) because of the steric obstacles of these large graphene sheets during their self-assembling process. Therefore, we pulverized LGO sheets in their aqueous dispersion by sonication, significantly reducing their average size to around 0.7 µm (Figure 1c, Less-defective graphene oxide sheets with a small average size of 0.7 µm are electrochemically reduced to form a hydrogel film with highly oriented porous structure. It is applied as the electrode of organic electrochemical capacitor (OEC) after solvent change with organic electrolyte and deep reduction in this organic medium. At 120 Hz, the typical OEC exhibits a high areal specific energy density of 472 µF V 2 cm −2 with a wide workable voltage window of 2.5 V, a phase angle of −80.5°, a resistor-capacitor time constant (τ RC ) of 0.219 ms, and an excellent electrochemical stability. Thus, it is promising to replace aluminum electrolytic capacitors for AC line filtering. Furthermore, two identical OECs connected in series keep the performance of single...

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As an emerging type of electrochemical energy storage devices, sodium-ion capacitors (SICs) are potentially capable of high energy density and high power density, as well as low cost and long lifespan. Unfortunately, the lack of high-performance capacitive cathodes that can fully couple with the well-developed battery-type anodes severely restricts the further development of SICs. Here, we develop a compact yet highly ordered graphene solid (HOGS), which combines the merits of high density and high porosity and, more attractively, possesses a highly ordered lamellar texture with low pore tortuosity. As the capacitive cathode of SICs, HOGS delivers a record-high volumetric capacity (303 F cm −3 or 219 mA h cm −3 at 0.05 A g −1 ), a superior rate capability (185 F cm −3 or 139 mA h cm −3 even at 10 A g −1 ), and an outstanding cycling stability (over 80% after 10 000 cycles). The material design and construction strategies reported here can be easily extended to other metal-ion-based energy storage technologies, exhibiting universal potentials in compact electrochemical energy storage systems.