Hirotomo Nishihara

Templated Nanocarbons for Energy Storage

The template carbonization method is a powerful tool for producing carbon materials with precisely controlled structures at the nanometer level. The resulting templated nanocarbons exhibit extraordinarily unique (often ordered) structures that could never be produced by any of the conventional methods for carbon production. This review summarizes recent publications about templated nanocarbons and their composites used for energy storage applications, including hydrogen storage, electrochemical capacitors, and lithium-ion batteries. The main objective of this review is to clarify the true significance of the templated nanocarbons for each application. For this purpose, the performance characteristics of almost all templated nanocarbons reported thus far are listed and compared with those of conventional materials, so that the advantages/disadvantages of the templated nanocarbons are elucidated. From the practical point of view, the high production cost and poor mass-producibility of the templated nanocarbons make them rather difficult to utilize; however, the study of their unique, specific, and ordered structures enables a deeper insight into energy storage mechanisms and the guidelines for developing energy storage materials. Thus, another important purpose of this work is to establish such general guidelines and to propose future strategies for the production of carbon materials with improved performance for energy storage applications.

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.

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.

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.

Production of Colored Pigments with Amorphous Arrays of Black and White Colloidal Particles

There are many technical and industrial applications for colored pigments with nonfading properties. The development of a low-cost, high-volume production method for nonfading pigments with low toxicity and minimal environmental impact may promote their widespread use. To accomplish this goal, pigments need to be prepared using abundant and environmentally friendly compounds. Here, we report on the variously colored aggregates formed by spraying fine, submicrometer-sized spherical silica particles. The microstructure of the aggregate is isotropic with a shortrange order on a length scale comparable to optical wavelengths, and exhibits an angle-independent structural color as a result of wavelength-specific constructive interference. Interestingly, the color saturation of these aggregates can be controlled by the incorporation of a small amount of conventional black particles, such as carbon black (CB). We demonstrate that a Japanese-style painting can be successfully drawn with this method. Silicon dioxide, which is a major component of silica particles, is chemically stable and used in scientific glassware suitable for chemical experiments. It is also a primary component of soil and found in abundant supply in nature. Furthermore, in vivo toxicity of silica particles that are greater than 300 nm in diameter has not been detected. Therefore, submicrometer-sized silica particles are one of the best candidates for fabricating environmentally friendly materials. Fine submicrometer-sized spherical silica particles usually appear white to the human eye when they are in powdered form. However, assemblies of these particles can appear colored because of wavelength-specific optical interference, 5, 7] despite the absence of light-absorbing pigments and dyes. Such color is generally referred to as structural color, because it is essentially caused by the microstructure through optical phenomena, such as interference, diffraction, and scattering. 9] Crystalline arrays of fine submicrometersized spherical silica particles (colloidal crystals) are well known examples of assembled particles that have structural colors as a result of a very high reflectance at a certain wavelength of light. However, the structural colors produced by colloidal crystals show distinct variations, which depend on viewing and light illumination angles. Such iridescence makes the use of colloidal crystals as pigments difficult, because typical pigments generally require a constant color at different viewing angles. The iridescences of the colloidal crystals originate from Bragg reflection, which is the reflection mechanism that occurs as a result of the long-range order in the particle arrangement. Thus, if the arrangement is changed from the crystalline structure to the amorphous state, which has only a short-range order, iridescence is expected to be suppressed. In fact, amorphous aggregates of colloidal particles have been reported to exhibit angle-independent structural colors. 4, 5,12] However, amorphous colloidal arrays are difficult to fabricate because submicrometer-sized particles have a strong tendency to crystallize. Previously, amorphous colloidal arrays have been prepared by mixing two different kinds of submicrometer-sized silica particles. 4, 5, 13] These mixtures exhibit structural colors, but the colors are very pale. 4,5] Therefore, such amorphous colloidal arrays are unsuitable for use as brightly colored pigments. A simple synthetic method for the preparation of assemblies of submicrometer-sized particles with angle-independent brilliant structural colors for use as pigments has not yet been reported. Herein, we report a simple and reproducible synthetic procedure for the preparation of pigments that exhibit angleindependent, bright structural colors from amorphous colloidal arrays by spraying fine submicrometer-sized spherical silica particles of uniform size. We added a small amount of black particles to the colloidal amorphous array to enhance the saturation of the structural color by reducing incoherentlight scattering across the entire visible spectrum. Variously [*] Prof. Y. Takeoka, Prof. A. Takano, M. Teshima, Y. Ohtsuka, Prof. T. Seki Graduate School of Engineering, Nagoya University Furo-cho, Chikusa-ku, Nagoya, 464-8603 (Japan) E-mail: ytakeoka@apchem.nagoya-u.ac.jp

Formation of monolithic silica gel microhoneycombs (SMHs) using pseudosteady state growth of microstructural ice crystals

Monolithic silica gel microhoneycombs, which have an array of straight macropores within their structure and developed micro/mesopores inside their walls, were prepared using pseudosteady state growth of ice crystals which occurs during the unidirectional freeze-gelation of freshly gelled aqueous silica hydrogels, followed by a pore-protecting drying method, freeze drying.

Porous properties of silica gels with controlled morphology synthesized by unidirectional freeze-gelation

Abstract In this work, the unidirectional freezing method was applied to a wide variety of silica gel precursors; from silica sols which do not readily gel, to thoroughly aged silica hydrogels. It was found that in addition to the well-known structure of fibers with polygonal cross-sections which are commonly obtained in the unidirectional freezing method, porous silica gels having unique morphologies such as honeycomb, lamellar and flat fiber structures could also be obtained by changing the state of the precursor sol or gel. The obtained silica hydrogels were freeze-dried after exchanging the water included in its structure to t -butanol, and finally dry samples maintaining their wet state structures were obtained. The morphology and the porous properties of the obtained silica gels were systematically analyzed and the influences of preparation conditions, including pH, aging time before freezing, SiO 2 concentration, freezing temperature, and storage time at the frozen state on both factors were examined in detail. It was found that the simultaneous controlling of both factors could be easily conducted by simply adjusting preparation conditions.

Preparation of monolithic SiO2–Al2O3 cryogels with inter-connected macropores through ice templating

Macroporous monoliths of SiO2–Al2O3 cryogels were prepared. Macropores were generated by using ice crystals as the template, while the walls which surround the macropores were tailored as porous cryogels by freeze drying. Macropores and walls formed honeycomb-like structures, which were confirmed from scanning electron microscopy images of cross-sections of the samples. It was confirmed that the sizes of the macropores and the wall thicknesses were respectively in the ranges of 10–20 µm and 200–500 nm. Al mapping analysis by energy dispersive X-ray diffractometry showed that Al atoms were homogeneously dispersed throughout the samples without local aggregation. Moreover, Raman spectroscopy and 27Al NMR spectroscopy indicated that Al atoms were incorporated into the silica framework by forming an Al–O–Si polymeric network. Nitrogen adsorption–desorption measurements indicated that the walls were micro/mesoporous with high BET surface areas (>700 m2 g−1) and large pore volumes (>0.45 cm3 g−1). Moreover, NH3-TPD measurements revealed that the samples had acid sites, which allowed this material to be used as a solid acid catalyst.

Synthesis of silica-based porous monoliths with straight nanochannels using an ice-rod nanoarray as a template

An attempt was made to synthesize a large-size silica porous monolith with straight and parallel nanochannels using an ice-rod nanoarray as a template. Since the previously reported ice templating methods allowed only the formation of micrometer-sized channels, special efforts were made to decrease the channel size down to the nanometer scale. Furthermore, such nanoporous silica monoliths were prepared not from an often-used and unstable hydrogel but from a stable colloidal silica solution. By increasing both the ice growing rate and the temperature gradient in the ice–water interface as much as possible, the channel size was reduced to 530 nm. Moreover, with the addition of the water-soluble polymers such as γ-cyclodextrin and dextran, the channel size was further decreased. Finally, using dextran polymer, the channel size reached as small as 180 nm, which is close to the theoretical lower limit (120 nm) in the present silica solution. This noticeable effect of the polymers may be explained from their strong interaction with water molecules through hydrogen bonding, but the most necessary condition for the polymers is that they never induce the aggregation of the silica particles in the solution. The methodology of obtaining much narrower channels using the ice templating method was discussed on the basis of the present results.

Influence of surfactants on porous properties of carbon cryogels prepared by sol–gel polycondensation of resorcinol and formaldehyde

Abstract The role of surfactants on carbon cryogels is investigated by using three different surfactants, nonionic (SPAN80), cationic (trimethylstearylammonium chloride; C18) and nonionic polymeric fluorinated (FC4430) surfactants. By using different SPAN80 concentrations (10.0, 5.0, 2.5, 1.0 and 0.5 vol.%), double-structure carbon microspheres with SBET (630–700 m2/g) and Vmes (0.51–0.93 cm3/g) are obtained. Mesoporous carbon cryogels with different SBET and Vmes are prepared by using C18 with different volume ratios of cyclohexane to water in a C18/water/cyclohexane mixture. Carbon cryogels with SBET (690–810 m2/g) and Vmes (0.83–1.74 cm3/g) are obtained when cyclohexane is contained in the mixture, on the contrary, when there is no cyclohexane in the mixture, a water-based carbon cryogel with low SBET (480 m2/g) and Vmes (0.29 cm3/g) is obtained. Carbon cryogels prepared by using C18 have larger mesopore size and broader mesopore size distribution compared with carbon cryogels prepared by using other surfactants. Microcellular (sponge-like) carbon cryogels with mesoporous surface, SBET (210–660 m2/g) and Vmes (0.37–0.92 cm3/g), are obtained by introducing FC4430 (two concentrations) to two starting RF solutions (C/W=6,45). Low FC4430 concentration leads to carbon cryogels with higher SBET (610 and 660 m2/g) and narrower mesopore size distributions compared to the high concentration counterpart. Hence, it is found that different surfactant types have interesting effects on morphologies and porous properties of RF carbon cryogels.