Benzoquinone derivatives are hybridized inside the pores of activated carbon via gas-phase adsorption. Alkylbenzoquinones have strong interaction with the carbon pore surface while the intermolecular interaction is dominant for halobenzoquinones.

Anthracene (ANT) is oxidized to anthraquinone (AQ) inside the nanopores of activated carbon (AC) without any metal catalysts. The resulting hybrid of AC and AQ shows high performance as an anode for aqueous electrochemical capacitors due to the reversible redox reaction of AQ inside the nanopores of AC. ANT is first adsorbed inside the nanopores of AC in the gas phase and finely dispersed at the nanolevel. Consequently, the adsorbed ANT has a large contact area with the conductive carbon pore surface. The adsorbed ANT is then electrochemically oxidized to AQ in aqueous H 2 SO 4 electrolyte. The oxidation of ANT requires conductive surfaces for charge transfer, and ANT is efficiently oxidized at the large contact interface between the conductive carbon pore surface and ANT by using AC. The hybrid is capable of fast charging and discharging through rapid charge transfer at the large contact interface. In addition, since the hybridization of AQ inside the AC pores does not expand the volume of AC particles, the volumetric capacitance is enhanced by the hybridization. Furthermore, the nanopores of AC prevent AQ from desorbing into the electrolyte during charging and discharging. Consequently, the hybridization endows the hybrid with a high volumetric capacitance, a high rate capability, and a long cycle lifetime. We demonstrate that nanopores can provide the reaction environment not only for efficient oxidation of ANT as nanoreactors but also for a fast redox reaction of AQ.

A pyrene dimer (PYD) is synthesized by electrochemical oxidation via homocoupling of pyrene (PY) inside the pores of MgO-templated mesoporous carbons without any metal catalysts or organic solvents. The resulting MgO-C/PYD hybrids can be used as high-performance aqueous electrochemical capacitor electrodes due to the reversible redox property of PYD and large contact area between the hybridized PYD and conductive carbon surfaces, which enable rapid charge transfer at the large contact interface. In our previous study, PY was considered to polymerize through electrochemical oxidation, and activated carbon with the pore sizes of ∼4 nm was used as a porous carbon substrate. In this study, the MgO-templated carbons have the average pore sizes of 5, 10, and 30 nm, and their large mesopore volumes can accommodate a large amount of PYD for enhancing the capacitance. To develop high-performance electrochemical capacitors, the dependence of the capacitance enhancement and the capacitance retention on the amount of PY and the pore sizes of MgO-templated carbons are studied. It is found that mesopores are necessary for fast charging/discharging, but the capacitance retention and capacitance enhancement decrease with increasing the mesopore sizes and the amount of PY due to the decreased utilization ratio of PY.

Norbornadiene (NBD) is adsorbed on activated carbon (AC), and the adsorbed NBD is polymerized within the pores of AC. Two kinds of ACs�AC-2 with only micropores of ∼2 nm and AC-4 with not only micropores but also mesopores below 4 nm�are examined to study the effects of the hybridized polynorbornadiene (PNBD) on the electric double-layer capacitor and hydrogen adsorption performance. Various measurements are performed to determine the form of the hybridized PNBD inside the pores of AC. Scanning and transmittance electron microscopy observations of the AC/PNBD hybrids confirm that PNBD is hybridized inside the pores of AC, and there is little PNBD on the surface of AC particles. The nitrogen adsorption/desorption measurement for the hybrids of AC-4 reveals that PNBD is not hybridized preferentially inside micropores rather than mesopores irrespective of the amount of PNBD. In addition, both micropore and mesopore volumes decrease at a constant rate with increasing amounts of PNBD. These results suggest that PNBD is hybridized not as a layer but as an agglomerate for both ACs, and the agglomerate delocalizes over the whole AC pores, which is supported by the results of electrochemical measurements and hydrogen adsorption behavior of the hybrids.

High utilization efficiencies of hybridized redox-active materials are of great importance for increasing the capacitance of electrochemical capacitor electrodes. Herein, a redox-active alkylbenzoquinone, tetramethyl-1,4-benzoquinone (TMBQ), is hybridized in the gas phase in activated carbon (AC) pores with sizes of ∼ 4 nm, and the electrochemical capacitor performance of the obtained hybrids is evaluated using aqueous 1 M H 2 SO 4 as electrolyte. The amount of hybridized TMBQ investigated herein is 4-10 mmol per gram of AC, and the utilization efficiencies of TMBQ in the resulting AC/TMBQ hybrids are ≥ 85%. The hybridization is not associated with the volume expansion of the AC particles, and therefore, the volumetric capacitance of the hybrids increases with increasing amount of TMBQ. Consequently, 1 mmol of TMBQ per gram of AC afforded the volumetric capacitance of ∼ 50 F cm −3 and the volumetric capacitance of the hybrid containing 10 mmol of TMBQ per gram of AC was 581 F cm −3 at the current density of 0.2 A g −1 , which is significantly higher than the volumetric capacitance of pristine AC (i.e., 99 F cm −3 ). Moreover, despite the substantial contribution of the reversible redox reaction of TMBQ to the increase in the capacitance, capacitance retentions of the hybrids up to a TMBQ amount of 8 mmol per gram of AC were higher at 10 A g −1 than that of AC. The gas-phase adsorption does not require any organic solvents, concomitant solvent removal, or the drying process, and enables complete adsorption of TMBQ for the TMBQ amounts below the saturation capacity of AC. Therefore, this hybridization method is promising not only for enhancing the electrochemical capacitor performance but also for waste minimization.