Utilizing Activated Carbon Nanopores for Electrochemical Oxidation of Perylene to Redox-Active 3,10-Perylenedione: Application in Electrochemical Capacitor Electrodes.

We demonstrate that nanopores of activated carbon (AC) function as nanoreactors that oxidize perylene (PER) to a redox-active organic compound, 3,10-perylenedione (PERD), without any metal catalysts or organic solvents. PER is first adsorbed on AC in the gas phase, and the PER-adsorbed AC is subjected to electrochemical oxidation in aqueous H2SO4 as the electrolyte. Because gas-phase adsorption is solvent-free, PER is completely adsorbed on AC as long as the amount of PER does not exceed the saturated adsorption capacity of the AC, which enables accurate control of the amount adsorbed. PER is electrochemically oxidized to PERD in the nanopores of AC at above 0.7 V vs Ag/AgCl. The hybridized PERD undergoes a rapid reversible two-electron redox reaction in the nanopores owing to the large contact interface between the conductive carbon pore surfaces and PERD. The resulting AC/PERD hybrids serve as electrodes for electrochemical capacitors, utilizing the rapid redox reaction of PERD. The hybridization method is advantageous for quantitatively optimizing electrochemical capacitor performance by adjusting the amount of adsorbed PER. Moreover, because PERD hybridization in the AC nanopores does not expand the electrode volume, the volumetric capacitance increases with increasing hybridized PERD content. In three-electrode cell measurements, the volumetric capacitance at 0.05 A g-1 reaches 299 F cm-3, and 61% of this capacitance is retained at 10 A g-1 when 5 mmol of PER is used per gram of AC. Meanwhile, pristine AC delivers 117 F cm-3 at 0.05 A g-1 with a capacitance retention of 46% at 10 A g-1. Two-electrode cell measurements reveal that self-discharge is significantly suppressed by the hybridized PERD when AC/PERD hybrids and AC are used as cathodes and anodes, respectively, compared to that of a symmetrical AC cell. Moreover, PERD does not undergo cross-diffusion in the asymmetrical cells during self-discharge tests for 24 h.

Synthesis of Polynorbornadiene within the Pores of Activated Carbons: Effects on EDLC and Hydrogen Adsorption Performances.

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.

Effect of Heteroatoms in Ordered Microporous Carbons on Their Electrochemical Capacitance.

Micropores play a more important role in enhancing the electrochemical capacitance than mesopores and macropores; therefore, the effect of heteroatom doping into micropores on the electrochemical behavior is interesting. However, heteroatom doping into porous carbon materials would potentially change their pore structures and pore sizes, which also affect their electrochemical capacitive behaviors. To gain insight into the intrinsic effects of heteroatoms on the electrochemical capacitive behaviors, zeolite-templated carbon (ZTC) may be the most suitable candidate. ZTC is an ordered microporous carbon with a uniform micropore size of 1.2 nm, a high surface area, and a large micropore volume. In this work, a series of ZTCs containing oxygen, nitrogen, or boron as heteroatoms, with an ordered pore structure and the same pore size, are prepared. By examining their electrochemical capacitive behaviors in an organic electrolyte, the effect of heteroatom doping can be isolated and discussed without considering the effects of pore structure and pore size. Acid anhydride groups are found to generate pseudocapacitance in two potential ranges, -1.0 to -0.3 V (vs Ag/AgClO4) and -0.2 to 0.4 V. B is introduced into the ZTC framework solely as -B(OH)2, which is found to be an electrochemically inert species. N is introduced as pyridine (3.0%), pyridone/pyrrole (23.8%), quaternary (66.6%), and oxidized N (6.6%), and these species exhibit noticeable pseudocapacitance in the microporous carbon.

Reversible pore size control of elastic microporous material by mechanical force.

Nanoporous materials, such as zeolites, activated carbons, and metal-organic frameworks (MOFs), are peculiar platforms in which a variety of guest molecules are stored, reacted, and/or separated. The size of the nanopores is essential to realize advanced functions. In this work, we demonstrate a very simple but innovative method for the control of nanopore size, that is, reversible and continuous control by mechanical force loaded to soft nanoporous materials. The elastic properties of several microporous materials, including zeolites, zeolite-templated carbon (ZTC), activated carbon, and MOFs (e.g., ZIF-8), are examined and it is found that ZTC is a material that is suitable for the aforementioned idea thanks to its extraordinary soft properties compared to the others. The original pore size of ZTC (1.2 nm) can be contracted to 0.85 nm by using a relatively weak loading force of 135 MPa, whereas the other microporous materials barely contracted. To demonstrate the change in the physical properties induced by such artificial deformation, in situ gas adsorption measurements were performed on ZTC with and without loading mechanical force, by using CO2, CH4, and H2, as adsorbates. Upon the contraction by loading 69 or 135 MPa, CO2 adsorption amount is increased, due to the deepening of the physisorption potential well inside the micropores, as proved by the increase of the heat of adsorption. Moreover, the adsorption amount is completely restored to the original one after releasing the mechanical force, indicating the fully reversible contraction/recovery of the ZTC framework against mechanical force. The experimental results are theoretically supported by a simulation using Grand Canonical Monte Carlo method. The similar adsorption enhancement is observed also on CH4, whereas H2 is found as an exception due to the weak interaction potential.

High utilization efficiencies of alkylbenzokynones hybridized inside the pores of activated carbon for electrochemical capacitor electrodes.

Benzoquinone derivatives (BQDs) are hybridized inside activated carbon (AC) pores via gas-phase adsorption to prepare electrochemical capacitor materials. In this study, 2 mmol of BQDs are hybridized with 1 g of AC. The hybridization of alkylbenzoquinones (ABQs) with AC enhances the volumetric capacitances of the hybrids from 117 to 201 F cm-3 at 0.05 A g-1 and the capacitances are retained up to 73% at 10 A g-1. Meanwhile, the volumetric capacitances are increased to 163 F cm-3 at 0.05 A g-1 by the hybridization of halobenzoquinones (HBQs) and the capacitance retentions at 0.05 A g-1 are ∼62%, which are higher than that of AC (46%). The results of electrochemical measurements suggest that HBQs exist as agglomerates while ABQs are finely dispersed inside the pores. The ABQs have good contact with the conductive carbon pore surface compared to the HBQs. Consequently, most of the ABQ molecules undergo reversible redox reactions (i.e., high utilization efficiencies), and a large contact area facilitates charge transfer at the large contact interface, thereby endowing the hybrids of ABQs with fast charging and discharging characteristics. HBQ molecules can be finely dispersed by liquid-phase adsorption, but the finely dispersed HBQ molecules are mobile inside the pores at room temperature and gradually form agglomerates. The difference in the existing form of BQDs is explained by the dominant interaction affecting the BQD molecules. ABQs have a strong interaction with the carbon pore surface while the intermolecular interaction is dominant for HBQs.

Metal-Free Homocoupling of Pyrene inside the Pores of Mesoporous Carbons via Electrochemical Oxidation: Application for Electrochemical Capacitors.

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.

Three-dimensionally arrayed and mutually connected 1.2-nm nanopores for high-performance electric double layer capacitor.

Zeolite-templated carbon is a promising candidate as an electrode material for constructing an electric double layer capacitor with both high-power and high-energy densities, due to its three-dimensionally arrayed and mutually connected 1.2-nm nanopores. This carbon exhibits both very high gravimetric (140-190 F g(-1)) and volumetric (75-83 F cm(-3)) capacitances in an organic electrolyte solution. Moreover, such a high capacitance can be well retained even at a very high current up to 20 A g(-1). This extraordinary high performance is attributed to the unique pore structure.

Investigation of the ion storage/transfer behavior in an electrical double-layer capacitor by using ordered microporous carbons as model materials.

An ordered microporous carbon, which was prepared with zeolite as a template, was used as a model material to understand the ion storage/transfer behavior in electrical double-layer capacitor (EDLC). Several types of such zeolite-templated carbons (ZTCs) with different structures (framework regularity, particle size and pore diameter) were prepared and their EDLC performances were evaluated in an organic electrolyte solution (1 M Et(4)NBF(4)/propylene carbonate). Moreover, a simple method to evaluate a degree of wettability of microporous carbon with propylene carbonate was developed. It was found that the capacitance was almost proportional to the surface area and this linearity was retained even for the carbons with very high surface areas (>2000 m(2) g(-1)). It has often been pointed out that thin pore walls limit capacitance and this usually gives rise to the deviation from linearity, but such a limitation was not observed in ZTCs, despite their very thin pore walls (a single graphene, ca. 0.34 nm). The present study clearly indicates that three-dimensionally connected and regularly arranged micropores were very effective at reducing ion-transfer resistance. Despite relatively small pore diameter ZTCs (ca. 1.2 nm), their power density remained almost unchanged even though the particle size was increased up to several microns. However, when the pore diameter became smaller than 1.2 nm, the power density was decreased due to the difficulty of smooth ion-transfer in such small micropores.

Constraint spaces in carbon materials.

Nano-sized pores in carbon materials are recently known to give certain constraints to the encapsulated materials by keeping them inside, accompanied with some changes in their structure, morphology, stability, etc. Consequently, nano-sized pores endow the constrained materials with improved performances in comparison with those prepared by conventional processes. These pores may be called "constraint spaces" in carbon materials. Here, we review the experimental results related to these constraint spaces by classifying as nanochannels in carbon nanotubes, nanopores and nanochannels in various porous carbons, and the spaces created by carbon coating.

Study of the pore structure and size effects on the electrochemical capacitor behaviors of porous carbon/quinone derivative hybrids.

We demonstrate the hybridization of a redox-active quinone derivative, 2,5-dichloro-1,4-benzoquinone (DCBQ), and porous carbons with different pore structures for aqueous electrochemical capacitor electrodes. The hybridization is performed in the gas phase, which enables accurate porous carbon/DCBQ weight ratios. This method is advantageous over conventional liquid phase adsorption, in terms of facile optimization of the porous carbon/DCBQ weight ratio to obtain high-performance aqueous electrochemical capacitor electrodes, dependent on the kind of porous carbons; moreover, complete adsorption in the liquid phase cannot be achieved by the conventional liquid phase adsorption method. Their electrochemical capacitor performances are evaluated using an aqueous 1 M H2SO4 electrolyte, and the adsorbed DCBQ undergoes redox reactions inducing pseudocapacitance within the pores of porous carbons. To study the effect of the pore size on the electrochemical capacitor behavior, two kinds of activated carbon (AC) with different pore sizes are examined: the microporous AC and the AC with both micro- and mesopores. Additionally, we examine ordered microporous carbon with a uniform pore size of 1.2 nm and a three-dimensionally (3D) ordered and mutually connected pore structure. The results reveal that mesopores facilitate proton conduction inside the DCBQ-constrained carbon pores, whereas the 3D-ordered and mutually connected micropores balance high volumetric capacitance enhancement with excellent rate capability. Such high proton conduction inside such constrained spaces can be explained only by the Grotthuss mechanism.

Electrochemical polymerization of pyrene and aniline exclusively inside the pores of activated carbon for high-performance asymmetric electrochemical capacitors.

An asymmetric polymer capacitor was prepared from pyrene (PY), aniline (ANI), and commercially available activated carbon (AC) through a solvent-free preparation. PY and ANI were adsorbed into the AC host material in the gas phase and electrochemically polymerized exclusively inside the AC pores in an aqueous H2SO4 electrolyte (1 M). No volumetric expansion of the AC particles occurred upon the adsorption of monomers and their subsequent polymerizations; thus, the volumetric capacitance was enhanced by the inclusion of pseudocapacitive polypyrene (PPY) and polyaniline (PANI). The PPY and PANI structures formed inside the AC pores are very thin and have a large contact area with the conductive carbon surfaces. Therefore, the charge transfer distance between the polymers and the carbon surfaces was drastically shortened, significantly reducing the charge transfer resistance; i.e., high power density. The maximum volumetric capacitances for the PPY- and PANI-hybridized AC reached 314 and 299 F cm-3, respectively. Moreover, the strong adhesion derived from their large contact areas and adsorption capability of AC endow these materials with long cycle lifetimes. The PPY- and PANI-hybridized AC have different redox potentials and can be assembled into an asymmetric capacitor. The volumetric capacitance obtained for the asymmetric capacitor further surpassed that of the symmetric capacitor consisting of pristine AC, with high power density and long cycle lifetimes.

Electrochemical synthesis of polyaniline in the micropores of activated carbon for high-performance electrochemical capacitors.

Polyaniline (PANI) was synthesized exclusively inside the micropores of activated carbon (AC). This nanosized PANI was smaller than 2 nm in diameter and allowed for fast redox reactions, exhibiting superior pseudocapacitance in terms of power and energy densities over the electric double layer capacitance generated inside the micropores.

Conversion of silica nanoparticles into Si nanocrystals through electrochemical reduction.

The precise design of Si-based materials at the nanometer scale is a quite complex issue but of utmost importance for their present and potential applications. This paper reports the first attempt to address the electrochemical reduction of SiO2 at the nanometer scale. SiO2 nanoparticles are first covered with a uniform carbon layer with controlled thickness at an accuracy of a few nanometers, by pressure-pulsed chemical vapor deposition. With appropriate thickness, the carbon layer plays significant roles as a current path and also as a physical barrier against Si-crystal growth, and the SiO2 nanoparticles are successfully converted into extremely small Si nanocrystals (<20 nm) inside the shell-like carbon layer whose morphology is derived from the original SiO2 nanoparticles. Thus, the proposed electroreduction method offers a new synthesis strategy of Si-C nanocomposites utilizing the morphology of SiO2 nanomaterials, which are well known for a wide variety of defined and regular nanostructures. Owing to the volume difference of SiO2 and the corresponding Si, nanopores are generated around the Si nanocrystals. It has been demonstrated that the nanopores around the Si nanocrystals are effective to improve cycle performance of Si as a negative electrode for lithium-ion batteries. The present method is in principle applicable to various SiO2 nanomaterials, and thus, offers production of a variety of Si-C composites whose carbon nanostructures can be defined by their parent SiO2 nanomaterials.

Enhancement mechanism of electrochemical capacitance in nitrogen-/boron-doped carbons with uniform straight nanochannels.

Anodic aluminum oxide (AAO) with uniform straight nanochannels was completely coated with pure, N-doped, or B-doped carbon layer. Their electric double layer capacitances are measured in aqueous (1 M sulfuric acid) and organic (1 M Et4NBF4/polypropylene carbonate) electrolyte solutions in order to investigate the capacitance enhancement mechanisms caused by N- or B-doping. Since the three types of carbon-coated AAOs (pure, N-doped, or B-doped) have exactly the same pore structure, the observed capacitance enhancement was ascribable to only the following factors: (i) better wettability, (ii) the decrease of equivalent series resistance, (iii) the contribution of space-charge-layer capacitance, and (iv) the occurrence of pseudocapacitance. From the measurements of the wettability and the electrical resistance of the coated AAOs together with the electrochemical investigation (the cyclic voltammetry, the galvanostatic charge/discharge cycling, and the impedance analysis), it is concluded that the pseudocapacitance through faradic charge transfer (factor iv) is the most important factor to enhance the capacitance by N- or B-doping. This can be applied to not only the present carbon-coated AAOs but also any other porous carbons.