Yasushi Soneda

Effects of Nitric Acid and Heat Treatment on Hydrogen Adsorption of Single-Walled Carbon Nanotubes

The effects of nitric acid treatment and subsequent heat treatment on the hydrogen adsorption properties of single-walled carbon nanotube (SWCNT) samples were investigated. The hydrogen adsorption and desorption isotherms of SWCNTs treated with nitric acid alone and SWCNTs treated with nitric acid and then heated were measured at 77 and 303K over the hydrogen pressure range 0 - 3.5 MPa. Nitric acid treatment increased hydrogen adsorption by opening the nanotubes and changing the bundle structure. Subsequent heat treatment of the acid-treated SWCNT sample further increased hydrogen adsorption by removing functional groups introduced by the acid treatment.

Electronic properties and structure of stage-4 MoCl5 GICs prepared from highly crystallized graphite films

Abstract Films of stage-4 MoCl 5 graphite intercalation compound (GIC) were synthesized from laboratory-prepared high-quality graphite films and their electronic properties such as electrical conductivity, Hall coefficient and transverse magnetoresistance were measured at 300, 77 and 1.3 K. The graphite films were prepared from polyimide films of Kapton (25 μm thick) and pyromellitic dianhydride- p -phenylenediamine-3,3′,4,4′-tetraaminobiphenyl (PMDA-PPD-TAB, molar ratio 100/92/4, hereafter called PPT) (45 μm thick) by heating them up to 3200 °C. The stage-4 MoCl 5 GICs were made by heating the graphite films in a glass tube with a mixture of natural graphite flakes and MoCl 5 powder at 450 °C for 7 days. Majority carriers in GICs were holes, the density of which was estimated to be about 5.2 × 10 26 m −3 at 1.3 K. A Shubnikov-de Haas oscillation with a period of 7.84 × 10 −3 T −1 and slight modulation of the amplitude was observed in the dependence of the transverse magnetoresistance on magnetic field at 1.3 K for both GIC films. The electronic and structural parameters suggested that the GIC film from a polyimide PPT has less perfect crystallinity than that from a polyimide Kapton, even though the pristine graphite film from PPT has better crystallinity.

Preparation and characterization of sodium-graphite intercalation compounds

Preparation and characterization of sodium-graphite intercalation compounds were investigated. It was confirmed that the resulting compound (NaC x ) had the stage 8 structure with identity period of 2.8 nm. The Raman spectroscopy showed that the G-band signal of NaC x was observed at 1582 cm −1 and at 1608 cm −1 , which is a typical indication of the formation of intercalation compounds. The g-value of NaC x was determined to be 2.0018 by the ESR measurement, which was very close to the reported values for the other alkali metal-graphite intercalation compounds. The electrical resistivity of NaC x was about 1/3 of that of host graphite at room temperature and showed metallic temperature dependence.

Phase transition in porous electrodes. III. For the case of a two component electrolyte.

The electrochemical thermodynamics of electrolytes in porous electrodes is qualitatively different from that in the bulk with planar electrodes when the pore size is comparable to the size of the electrolyte ions. In this paper, we discuss the thermodynamics of a two component electrolyte in a porous electrode by using Monte Carlo simulation. We show that electrolyte ions are selectively adsorbed in porous electrodes and the relative concentration of the two components significantly changes as a function of the applied voltage and the pore size. This selectivity is observed not only for the counterions but also for the coions.

Host Effect on the Properties of AM-GICs

Abstract Carbon materials (A-1, A-2 and A-3 derived from pitch cokes) with different graphitization degree, were allowed to react with potassium. The hydrogen-sorption behavior at 77 K, electrical resistivity. ESR and Raman spectra of the resulting compounds with the composition of KC60 were determined. The sorbed amount at saturation, (n H2/n K)sat, was 0.55 and 1.43 for KC60s prepared from A2 (d 002 = 0.3377 nm) and A3 (d 002 = 0.3361 nm), respectively. No H2 sorption was observed for KC60 from A-1 (d 002=0.342 nm). Temperature dependence of the resistivity of KC60s from A-2 and A-3 showed metallic behavior, contrary to semiconductive one for the host materials. However, KC60 (A-1) showed semiconductive temperature dependence, similarily to the host material. Raman spectra of KC60s from A-2 and A-3 showed doublet structure, similarly to that of K-GICs from HOPG, characteristic for graphite intercalation compounds with stage n > 2. On the contrary, KC60 (A-1) gave single peak at around 1597 cm−1. Those facts suggest that potassium exists in the interlayer spaces for KC60s (A-2, A-3), but not for KC60 (A-1). It was also shown that g-factor of ESR spectra of KC60 can be useful to predicting the H2-sorption behavior.

TEM and Electron Tomography Imaging of Pt Particles Dispersed on Carbon Nanospheres

Three-dimensional (3D) observation of Pt nanoparticles on the surface of carbon nanoparticles, one of the important catalyst systems for energy storage such as fuel cells, was carried out by using an electron tomography system attached to a transmission electron microscope (TEM). In the Pt/carbon sample preparation process, pristine and graphitized carbon nanospheres (CNSs, 200 nm average diameter) were immersed into H2PtCl6 solution, followed by a heat-treatment at 493 K. According to the electron tomography technique, we confirmed that crystalline Pt particles (2-3 nm) were homogeneously dispersed on the whole surface of pristine CNSs, while on the graphitized CNSs Pt was aggregated on the ridgeline of the polyhedronized CNS particles or on the connecting surface between two CNS particles. Dependence of Pt dispersion as shown was discussed with the distribution of surface defects of CNS particles.

Ferroelectric Phase Behaviors in Porous Electrodes

The phase behavior of ions in porous electrodes is qualitatively different from that in the bulk because of the confinement effect and the interaction between the electrode surface and the electrolyte ions. We found that porous electrodes of which the pore size is close to the size of the electrolyte ions can show ferroelectric phase behaviors in some conditions by Monte Carlo simulations of simple models. The phase behavior of the porous electrodes dramatically changes as a function of the pore size of the porous electrode and that is compared to the phase behavior of typical ferroelectric materials, for which the phase behavior changes as a function of the temperature or the composition. The origin of the phase behavior is discussed in terms of the molecular interaction and the ionic structure inside the porous electrodes. We also found that the density of counterions and that of co-ions inside porous electrodes changes in a nonlinear fashion as a function of the applied voltage, which is in agreement with the experimental results.

Carbons for Supercapacitors

Electric double-layer capacitors (EDLCs) using carbon electrodes have been anticipated as an energy storage device for electric vehicle, distributed energy system applications, and so on. High surface-area carbons were employed for the electrode materials of EDLC to store energy in the electric double layer, and the control of the pore size distribution of its carbons was recognized to be very important. On the other hand, when the electrode material shows Faradic reactions at the electrode surface in a wide range of potential window, the energy stored by the charge transfer can be accounted as pseudocapacitance. The investigation of novel carbon materials is directly connected to the improvement of EDLC performance, since carbon is essential for capacitor electrode materials. This paper presents recent developments in carbon materials for EDLC electrodes in which the enhancement of various characteristics was achieved by various techniques such as chemical and structural control and hybridization with heteroatoms. Recently, capacitors using these novel carbon materials, having Faradic reactions and novel mechanisms to enhance the total performance, have become to be known as supercapacitors.

Hydrogen adsorption properties of single-walled carbon nanotubes treated with nitric acid

The single-walled carbon nanotube sample (SWCNT) was treated with nitric acid. In addition, the SWCNT sample treated with nitric acid was heat-treated, and the hydrogen adsorption and desorption isotherms of these SWCNT samples were measured at 77 and 303 K over the hydrogen pressure range 0-3.5 MPa. The nitric acid treatment enhanced the amount of hydrogen adsorbed on SWCNT due to opening up the nanotubes and changing the bundle structure. The heat treatment further increased the hydrogen adsorption capacity of acid-treated SWCNT sample owing to the removal of the functional groups introduced by the nitric acid treatment.