Hiroaki Hatori

Extracting the Full Potential of Single‐Walled Carbon Nanotubes as Durable Supercapacitor Electrodes Operable at 4 V with High Power and Energy Density

Supercapacitors are electrochemical energy storage systems that store energy directly and physically as charge, whereas batteries, for example Li-ion cells, store energy in chemical reactants capable of generating charge. [ 1 ] Accordingly, the energy density of supercapacitors ( < 10 Wh kg − 1 ) is lower than batteries ( > 100 Wh kg − 1 ). However, their power is signifi cantly higher and their lifetime longer. As such, supercapacitors are expected to play a crucial role where superior power performance is required. The importance of supercapacitors is highlighted by a report from the US Department of Energy assigning equal importance to batteries and supercapacitors. [ 2 ] Examples of envisioned largescale applications of supercapacitors are load-leveling in solar, wind, and other energy sources and energy recovery from regenerative braking in automobiles. [ 2 , 3 ] To emerge as an important energy storage technology in the future, advanced supercapacitors must be developed with higher operating voltage and higher energy and power delivery, while maintaining high cyclability. Hitherto, activated carbon (AC) has been the electrode material of choice due to its high surface area (1000–2000 m 2

High-Power Supercapacitor Electrodes from Single-Walled Carbon Nanohorn/Nanotube Composite

A novel composite is presented as a supercapacitor electrode with a high maximum power rating (990 kW/kg; 396 kW/l) exceeding power performances of other electrodes. The high-power capability of the electrode stemmed from its unique meso-macro pore structure engineered through the utilization of single-walled carbon nanotubes (20 wt %) as scaffolding for single-walled carbon nanohorns (80 wt %). The novel composite electrode also exhibited durable operation (6.5% decline in capacitance over 100 000 cycles) as a result of its monolithic chemical composition and mechanical stability. The novel composite electrode was benchmarked against another high-power electrode made from single-walled carbon nanotubes (Bucky paper electrode). While the composite electrode had a lower surface area compared to the Bucky paper electrode (280 vs 470 m(2)/g from nitrogen adsorption), it had a higher meso-macro pore volume (2.6 vs 1.6 mL/g from mercury porosimetry) which enabled the composite electrode to retain more electrolyte, ensuring facile ion transport, hence achieving a higher maximum power rating (970 vs 400 kW/kg).

Structural changes in carbon aerogels with high temperature treatment

The structural change of carbon aerogels at high temperatures up to 2800°C has been investigated. Change in microtexture of fine particles, which constitute carbon aerogels derived from phenolic resin, was of a typical non-graphitized carbon. The microporosity decreased with an increase of heat-treatment temperature, and disappeared at 2000°C. The mesoporosity still remained even after heat-treatment up to 2800°C, though 50% of mesopore volume was lost because of the fusion of the particles with the change of carbon microtexture.

The mechanism of polyimide pyrolysis in the early stage

Abstract Pyrolysis mechanism of Kapton-type polyimide was investigated with special attention to the early stage. Two approaches were employed to reveal the pyrolysis reactions: one is an analysis of evolved compounds, the other is a hydrolysis method that selectively converts polyimide char into low-molecular-weight compounds. In the initial step of pyrolysis, bond cleavage around imide ring and following hydrogen transfer generates various intermediates on aromatic segments in polymer chains. It was derived from the quantitative analysis by the hydrolysis method that new linkages between the intermediates are formed efficiently instead of the cleaved bonds. Solid 13 C-NMR study showed that the packing structure of polyimide molecules is little changed by heat-treatment at the starting temperature of pyrolysis, though the structure is gradually disintegrated by reconstruction reactions at higher temperatures. It was also found that the thickness of sample films affects the weight loss and amounts of evolved compounds, while the difference of graphitizability has no appreciable effect.

Preparation and pore control of highly mesoporous carbon from defluorinated PTFE

Abstract Porous carbon having more than 2000 m2/g of BET specific surface area was synthesized by defluorination of polytetrafluoroethylene (PTFE) at 473 K using sodium metal. The porous carbon as-prepared had a large amount of narrow mesopores 2–3 nm in pore width, together with micropores. Control of the pore structure was attempted by simple heat-treatment of the carbon in nitrogen, and change of the porous structures was characterized by nitrogen adsorption techniques. As a result, it was found that the ratio between micro- and mesopores was easily varied. Electric double layer capacitance was measured as one of the applications for the mesoporous carbon with specific porosity, and the effect of pore control on capacitance was investigated.

Synthesis and Characteristics of Graphene Oxide-Derived Carbon Nanosheet−Pd Nanosized Particle Composites

Carbon nanosheet (CNS)-Pd nanosized particle (NP) composites were synthesized by using graphite oxide (GO) and bis(ethylenediamine)palladium(II) (Pd(en)(2)(2+)) as the precursors, and their structure and adsorption properties were examined. It was found that the Pd(en)(2)(2+) complex ions can be intercalated into GO layers highly efficiently to form a layered structure containing a large amount of Pd (approximately 12 wt %). By the subsequent chemical reduction, Pd NPs (2-6 nm in size) are well dispersed between CNS to form a CNS-Pd NP composite and serve as spacers to increase the porosity of the composite. Hydrogen adsorption results demonstrate that both Pd NPs and CNS play important roles in hydrogen adsorption, particularly at a lower temperature and for CNS with deficient sites, which bring about a H(2) adsorption greater than those on other Pd-loaded nanocarbon materials reported so far. The unique composite nanostructure having large contents of Pd NPs (20-25 wt %) stabilized by CNSs is hopeful to be applied to the fields of H(2)-related catalysis, sensing, and so forth.

Organic and carbon aerogels derived from poly(vinyl chloride)

Organic aerogels were derived from dimethylformamide solution of poly(vinyl chloride) (PVC) via dehydrochlorination using a strong base, 1,8-diazabicyclo[5,4,0]undec-7-ene, and supercritical drying using carbon dioxide. From these organic aerogels, carbon aerogels were yielded via stabilization and carbonization. Changes in the porous structure of the aerogels during the preparation process and influences of the preparation conditions on the porous structure were investigated. The framework of the aerogels composed the walls of the meso- and macropores. The volume and the size of these pores were reduced during stabilization and carbonization due to the shrinkage of the framework caused by the release of decomposition gases and densification of the material. Simultaneously, the release of decomposition gases produced additional micropores. The extent of dehydrochlorination, the concentration of PVC in the starting solution and the molecular weight of PVC were the factors with which the porous structure of the aerogels could be controlled over a wide range. In addition, the stabilization conditions notably influenced the carbonization behavior of the organic aerogels and the porous structure of the carbon aerogels. The optimum stabilization conditions that minimized the loss of mass and maximized the pore volume of the carbon aerogels were determined.

In-plane orientation and graphitizability of polyimide films

Abstract The relationship between in-plane orientation of polyimide film and the graphitizability was obtained. In-plane oriented films in which the molecular chains are aligned parallel to the film plane give graphitic carbons, whereas isotropic ones afford non-graphitic carbons. A thin region of graphitized carbon can be observed even on the non-graphitic film, the skin-core structure of which is derived from the non-uniform orientation of the polyimide film in the thickness direction.

In-plane orientation and graphitizability of polyimide films: II. Film thickness dependence

Abstract The relation between the in-plane orientation of polyimide film and graphitizability was investigated. The degree of in-plane orientation was estimated by means of optical birefringence and ESR technique. The polyimide film was found to have non-uniform orientation in the thickness direction because the thinner the film was, the greater the orientation. The inhomogeneity of orientation caused multiphase graphitization in a film with a composite profile of the X-ray diffraction peak.

Comparative examination of titania nanocrystals synthesized by peroxo titanic acid approach from different precursors

Titanium dioxide nanocrystalline particles were synthesized by peroxo titanium acid (PTA) approach from titanium alkoxide and inorganic salt precursors, and their structural and surface properties, porosities, and photocatalytic activities were comparatively examined by XRD, TG/DTA, DRIFT, UV-vis, low temperature N(2) adsorption, and methyl orange (MO) degradation. It was found that nanoparticles with single anatase phase can be obtained from alkoxide precursor even near room temperature if synthesis conditions are appropriately controlled. PTA-derived anatase nanoparticles from titanium alkoxide precursor have smaller crystalline sizes and better porosities, and contain less amount of peroxo group and no organic impurities as compared to those from TiCl(4) precursor. The advantages in structural property, porosity, and surface properties (few deficiencies) lead to a much better photocatalytic activity for TiO(2) nanoparticles from titanium alkoxide precursor in comparison with those from TiCl(4) precursor.