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Last Updated :2024/11/09

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Master

Affiliation (Master)

  • Faculty of Engineering　Materials Science and Engineering　Energy Materials

Affiliation (Master)

  • Faculty of Engineering　Materials Science and Engineering　Energy Materials

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Profile and Settings

Degree

  • PhD（Hiroshima Univ）

Profile and Settings

  • Name (Japanese)

    ISOBE

  • Name (Kana)

    SHIGEHITO

  • Name

    201301036740486697

Achievement

Research Interests

  • Hydrogen Storage   Energy Materials   Materials Science   

Research Areas

  • Nanotechnology/Materials / Structural and functional materials

Research Experience

  • 2009/04 - Today Hokkaido University
  • 2008/04 - 2009/03 The University of Oxford
  • 2006/04 - 2008/03 Hiroshima University JSPS PD

Education

  • 2004/04 - 2006/03  Hiroshima University  Doctoral course
  • 2002/04 - 2004/03  Hiroshima University  Master course
  • 1998/04 - 2002/03  Hiroshima University  FIAS

Published Papers

  • Shigehito Isobe, Kentaro Shibata
    International Journal of Hydrogen Energy 2024/01
  • Hirotada Hashimoto, Shigehito Isobe, Naoyuki Hashimoto, Hiroshi Oka
    Journal of Alloys and Metallurgical Systems 4 100037 - 100037 2949-9178 2023/12
  • Haotian Sun, Naoyuki Hashimoto, Hiroshi Oka, Shigehito Isobe, Tian Liu
    Materials Characterization 206 113443 - 113443 1044-5803 2023/12
  • Haotian Sun, Zairan Luo, Shuai Wang, Naoyuki Hashimoto, Hiroshi Oka, Qian Liu, Shigehito Isobe
    Materials Letters 347 134644 - 134644 0167-577X 2023/09
  • Peng Bi, Naoyuki Hashimoto, Shigenari Hayashi, Hiroshi Oka, Shigehito Isobe
    Corrosion Science 208 110697 - 110697 0010-938X 2022/11
  • Fangqin Guo, Hikaru Oyama, Hiroyuki Gi, Kyohei Yamamoto, Shigehito Isobe, Tomoyuki Ichikawa, Hiroki Miyaoka, Takayuki Ichikawa
    Journal of Alloys and Compounds 918 0925-8388 2022/10/15 
    The corrosion of carbide/nitride/oxide(C/N/O)-based reactor materials were investigated to solve the corrosion problems in sodium redox reactions for thermochemical hydrogen production in this work. Various types of carbides, nitrides, and oxides were used as the potential materials of reaction containers for the sodium redox cycle to expect strong corrosion resistance. Comparison of corrosion of materials was performed by heat treatment with Na2O at 500 °C and corrosive phases were identified by X-ray diffraction. All the carbides materials showed the low corrosion resistance, while AlN among all the nitrides and Al2O3 among all the oxides were proved to possess high corrosion resistance with a very less corrosive phase from XRD results and non-discoloration of the test plate. The order of corrosion resistance of various materials was determined from the width of the corrosive layer observed by SEM and from the evaluation of Gibbs energy corresponding to the corrosion reaction from the thermodynamic point of view. The order of corrosion resistance was found to be AlN > Al2O3> ZrO2> TiO2. As a result, AlN was the most corrosive-resistant material according to SEM, XRD and EDS methods.
  • Hironobu Arashima, Yusuke Yanagisawa, Ryo Ikeda, Shigehito Isobe, Naoyuki Hashimoto
    INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 47 (59) 25057 - 25065 0360-3199 2022/07 
    Four heats of commercially available JIS SNCM439 steel are prepared, and fatigue tests are conducted in air and hydrogen gas. The materials evaluated are all martensitic steel with a tensile strength of 900 MPa or less and contain nonmetallic inclusions of different sizes. A decrease in the fatigue limit is observed in the specimens with large nonmetallic inclusions, but the fatigue limit in air is approximately equal to the fatigue strength at 300,000 cycles in hydrogen. However, in the finite life region, the fatigue life in hydrogen significantly decreases owing to the presence of large nonmetallic inclusions. It was observed that hydrogen considerably affects the fatigue life even at low stress amplitudes close to the fatigue limit. This effect is considered to be dependent on the size of the initial crack originating from the nonmetallic inclusions; large nonmetallic inclusions accelerate the hydrogen-induced fatigue crack growth rate. (C) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
  • Keita Shinzato, Yuki Nakagawa, Subing Yang, Shigehito Isobe, Tamaki Shibayama, Hiroki Miyaoka, Takayuki Ichikawa
    ACS Applied Energy Materials 5 (1) 951 - 957 2574-0962 2022/01/24
  • Hironobu Arashima, Satoru Masada, Shigehito Isobe, Naoyuki Hashimoto
    ISIJ INTERNATIONAL 62 (7) 1540 - 1547 0915-1559 2022 
    To investigate the effect of high-pressure hydrogen gas on the fracture of high-strength low-alloy steels, rising load tests were conducted on JIS SNCM439 steel in high-pressure (20 MPa) hydrogen gas at room temperature (20-25 degrees C), and its hydrogen-induced crack initiation behavior was investigated. The load-crack opening displacement curve obtained for rising load tests in hydrogen began to deviate from that obtained in air at very low loads, indicating that the stress intensity factor at crack initiation was significantly smaller in hydrogen. Scanning electron microscopy observations of the fractured surfaces of the specimens unloaded during the middle of the rising load test confirmed that hydrogen-induced cracks had already occurred at a load lower than the deviation point. The stretch zone that appeared in the rising load test in air was not observed for the test in hydrogen, and the hydrogen-induced cracks were found to directly initiate from the tip of the fatigue pre-crack. The hydrogen-induced cracks were initiated at almost the same stress intensity factor value as that at which the stretch zone was observed in air, indicating that plastic slip and the resulting hydrogen ingress from the new surface were the causes of the hydrogen embrittlement. In addition, it was shown that the stress intensity factor for crack initiation in hydrogen increased and the effect of hydrogen decreased with the increase in loading rate, inferring that dislocation migration and hydrogen penetration into the steel are key factors for hydrogen embrittlement.
  • Nozomi Noto, Shigehito Isobe, Naoyuki Hashimoto
    International Journal of Energy Research 45 (12) 18237 - 18244 0363-907X 2021/10/10
  • Keita Shinzato, Hiroyuki Gi, Toru Murayama, Masahiro Sadakane, Yongming Wang, Shigehito Isobe, Takayuki Ichikawa, Hiroki Miyaoka
    ACS Omega 6 (36) 23564 - 23569 2470-1343 2021/09/14 
    Five types of niobium(V) oxides (Nb2O5) were synthesized by hydrothermal and heat treatment processes, and their structural properties and catalytic activities for the hydrogen absorption/desorption reactions of magnesium were characterized. The synthesized Nb oxides were dispersed on magnesium hydride (MgH2), a typical hydrogen storage material, using the ball-milling method. All the synthesized Nb oxides improved the reaction kinetics of the hydrogen desorption/absorption reactions. The catalytic activities for the hydrogen desorption were comparable, while the hydrogen absorption rates were significantly different for each synthesized Nb oxide. This difference can be explained by the structural stability of Nb2O5, which is related to the formation of a catalytically active state by the reduction of Nb2O5 during the ball-milling process. Notably, the highest catalytic effect was observed for Nb2O5 with a highly crystalline pyrochlore structure and a low specific surface area, suggesting that pyrochlore Nb2O5 is a metastable phase. However, only the amorphous Nb oxide was out of order, even though there is a report on the high catalytic activity of amorphous Nb oxide. This is attributed to the initial condensed state of amorphous Nb oxide, because particle size affects the dispersion state on the MgH2 surface, which is also important for obtaining high catalytic activity. Thus, it is concluded that Nb2O5 with lower stability of the crystal structure and smaller particle size shows better catalysis for both hydrogen desorption and absorption reactions.
  • Yuki Nakagawa, Tomoharu Kimura, Takao Ohki, Shigehito Isobe, Tamaki Shibayama
    Solid State Ionics 365 115656 - 115656 0167-2738 2021/07 
    The effect of mechanical milling on the lithium-ion conductivity of lithium alanate (LiAlH4) is investigated. Although pristine LiAlH4 is a poor ion conductor in its highly crystalline state, short-time mechanical milling for 0.1-2 h is shown to significantly improve the conductivity. Hand-milling treatment in an argon-filled glove box is also shown to improve the conductivity. The activation energy (E-a) for ion conduction of milled LiAlH4 is similar to 0.70 eV, which is attributed to Li-ion site vacancy-mediated conduction. However, the Ea of pristine LiAlH4 shows a large value of 0.84 eV. The Li-7, Li-6, Al-27 nuclear magnetic resonance and infrared spectra suggest that surface Li hydroxide species would exhibit Li-ion conduction in pristine LiAlH4. Thus, the milling treatment can eliminate the effect of surface hydroxide species and increase the vacancy concentration at the Li-ion site of inner LiAlH4 grains, which results in the enhancement of the conductivity.
  • Jingming Shi, Naoyuki Hashimoto, Shigehito Isobe
    Journal of Nuclear Materials 549 0022-3115 2021/06/01 
    A systematic study based on density functional theory (DFT) simulation on the cohesion properties of incoherent Fe/W interfaces, Fe(100)/W(100), Fe(110)/W(110) and Fe(100)/W(110) interfaces, is presented. Results show that Fe(100)/W(110) interface is the most stable with the lowest interface energy, while Fe(110)/W(110) interface has the highest strength. In all three Fe/W interfaces, the disorder of lattice can be found mainly in Fe lattice. In Fe(100)/W(100) interface, a pseudomorphic structure is formed at the 1st Fe layer close to W layer. The formation energy of monovacancy in Fe/W interfaces is lower than it in bulk Fe or W, suggesting the formation of vacancy is easy at these Fe/W interfaces region. Moreover, Fe(100)/W(110) interface shows a significant sink effect to both H and He. Furthermore, the density of states and the charge transfer of Fe/W interfaces are investigated to give a deep understanding of cohesion properties.
  • Peng Bi, Naoyuki Hashimoto, Shigenari Hayashi, Hiroshi Oka, Shigehito Isobe
    MATERIALS TRANSACTIONS 62 (12) 1716 - 1723 1345-9678 2021 
    The oxidation behavior of CuNi, CuFeNi, CrCu0.3FeNi, and Al0.4CrCuFeNi2 Co-free Cu-containing concentrated solid solution alloys was investigated in steam conditions under 500, 600, and 700 degrees C for 25 h. All the alloys have a face-centered cubic structure. The kinetic curves of oxidation were measured, and the microstructure and elemental distribution of oxide scales were analyzed. The oxidation of all the Cu-containing alloys indicated parabolic behavior, and those appeared to have better corrosion resistance than normal 316 SS. The parabolic rate constant increased with increasing temperature. The oxidation resistance of CuNi and CuFeNi were relatively poor due to the formation of unprotective NiO and Fe3O4 oxide scales, which were thicker than that of OCu0.3FeNi and Al0.4CrCuFeNi2 at all testing temperatures. Cross-sectional electron probe microanalysis (EPMA) and X-ray diffraction (XRD) results revealed that, by the addition of Cr and Al, CrCu0.3FeNi and Al0.4CrCuFeNi2 had the enhanced oxidation resistance, which is probably due to the formation of Cr2O3 and Al2O3 inner oxide layer.
  • Jingming Shi, Naoyuki Hashimoto, Shigehito Isobe
    Journal of Nuclear Science and Technology 57 (11) 1223 - 1230 0022-3131 2020/11/01 
    The dissolution behavior of hydrogen (H) at Fe/W interface and the effect of H on the strength of Fe/W interface are investigated by means of first-principle calculation. The dissolution of H atom at Fe/W interface would be easier than in bulk Fe or W due to the low charge density at Fe/W interface caused by the distortion of lattice. Hydrogen atom prefers to stay at Fe side of Fe/W interface due to its lower dissolution energy in Fe lattice at the interface region. However, H atom could be trapped by vacancy at Fe/W interface. In addition, H and vacancy reduce the strength of the interface however, H atom at the specific site seems to increase the strength of Fe/W interface slightly. Furthermore, the electronic structures of the interface are analyzed to give a physical understanding of the effect of H on Fe/W interface.
  • Hiroyuki Gi, Keita Shinzato, Ratna Balgis, Takashi Ogi, Masahiro Sadakane, Yongming Wang, Shigehito Isobe, Hiroki Miyaoka, Takayuki Ichikawa
    ACS OMEGA 5 (34) 21906 - 21912 2470-1343 2020/09 
    Magnesium is a promising hydrogen storage material but requires an efficient catalyst to enhance the sluggish kinetics of its hydrogen desorption/absorption reactions. Niobium catalysts have been shown to accomplish this, but the effective factors for catalysis on hydrogen desorption/absorption of Mg are not well understood. In order to investigate these aspects, various types of Nb oxides were synthesized and mixed with Mg, and their catalytic properties were investigated. The spray pyrolysis synthesis of Nb oxides at different temperatures produced homogeneous spherical particles with different degrees of crystallinity, while Nb oxide particles synthesized by simple calcination of ammonium niobium oxalate were nonuniform. These Nb oxides show significant catalytic activities for the hydrogen desorption/absorption of Mg, with amorphous oxides being more effective catalyst precursors than crystalline precursors. Metastable, amorphous Nb oxide is more easily converted to the reduced state, which is the catalytically active state for the reactions. In addition, Nb in the deactivated sample is in the oxidized state compared with the initially activated sample, and the catalytically active (reduced) state is recovered by reactions with hydrogen. Based on these findings, it is concluded that the chemical state of Nb is an important factor in catalyzing the desorption/absorption of hydrogen by Mg, and the catalytically active state can be preserved without further treatments.
  • Keita Shinzato, Yuki Nakagawa, So Hamamoto, Yuya Hayashi, Hiroki Miyaoka, Shigehito Isobe, Tamaki Shibayama, Norio Ogita, Takayuki Ichikawa
    CHEMICAL COMMUNICATIONS 56 (53) 7237 - 7240 1359-7345 2020/07 [Refereed]
     
    Surface modification effects of graphite and organic solvents on Ti were investigated by thermogravimetry (TG), Raman spectroscopy, and transmission electron microscopy (TEM) observations to improve its hydrogen absorption properties. As a result, Ti ball-milled with graphite showed high reactivity and selectivity for hydrogen with high durability.
  • Yuki Nakagawa, Tomoharu Kimura, Shigehito Isobe, Tamaki Shibayama
    The Journal of Physical Chemistry C 124 (19) 10398 - 10407 1932-7447 2020/05 [Refereed][Not invited]
     
    The lithium-ion conductivity and hydrogen-desorption properties of lithium alanate (LiAlH4)/boron nitride (BN) composites were investigated. The effects of boron nitrides with different structures (hexagonal (h-BN), turbostratic (t-BN), and cubic (c-BN)) on the properties were demonstrated. Compared with milled LiAlH4, the ion conductivity was improved in the h-BN and t-BN composites. Among the composites, the t-BN composite showed the highest conductivity and lowest activation energy (E-a) for ion conduction. In the 40 wt % t-BN composite, the conductivity at 80 degrees C reached as high as 1.1 x 10(-3) S cm(-1). Impedance plots that correspond to the interfacial conductivity were observed, which implies the appearance of LiAlH4/t-BN interfacial ion conduction. Ball-milling of LiAlH4 with BN caused the partial replacement of aluminum with boron to form LiBH4 phase, which would cause the E-a decrease. The LiAlH4 melting and hydrogen-desorption temperature decreased in the t-BN composite. In the c-BN composite, a large amount of stainless steel impurity during ball-milling caused desorption without melting. This study suggests that the addition of defective boron nitride like t-BN can be a useful strategy to improve the lithium-ion conductivity of complex hydrides. The t-BN additive effects on the hydrogen-desorption properties, which are different from transition metal catalysts, were also demonstrated.
  • Kohta Asano, Hyunjeong Kim, Kouji Sakaki, Yumiko Nakamura, Yongming Wang, Shigehito Isobe, Masaaki Doi, Asaya Fujita, Naoyuki Maejima, Akihiko Machida, Tetsu Watanuki, Ruud J. Westerwaal, Herman Schreuders, Bernard Dam
    Inorganic Chemistry 59 (5) 2758 - 2764 0020-1669 2020/03/02 [Refereed][Not invited]
     
    Magnesium-based transition-metal hydrides are attractive hydrogen energy materials because of their relatively high gravimetric and volumetric hydrogen storage capacities combined with low material costs. However, most of them are too stable to release the hydrogen under moderate conditions. Here we synthesize the hydride of Mg2FexSi1-x, which consists of Mg2FeH6 and Mg2Si with the same cubic structure. For silicon-rich hydrides (x < 0.5), mostly the Mg2Si phase is observed by X-ray diffraction, and Mössbauer spectroscopy indicates the formation of an octahedral FeH6 unit. Transmission electron microscopy measurements indicate that Mg2FeH6 domains are nanometer-sized and embedded in a Mg2Si matrix. This synthesized metallographic structure leads to distortion of the Mg2FeH6 lattice, resulting in thermal destabilization. Our results indicate that nanometer-sized magnesium-based transition-metal hydrides can be formed into a matrix-forced organization induced by the hydrogenation of nonequilibrium Mg-Fe-Si composites. In this way, the thermodynamics of hydrogen absorption and desorption can be tuned, which allows for the development of lightweight and inexpensive hydrogen storage materials.
  • Fuga Tanaka, Yuki Nakagawa, Shigehito Isobe, Naoyuki Hashimoto
    INTERNATIONAL JOURNAL OF ENERGY RESEARCH 44 (4) 2941 - 2951 0363-907X 2020/03 [Refereed][Not invited]
     
    In light metal hydroxide systems, hydrogen desorption generally occurs in exothermic reactions; therefore, these reactions are irreversible in principle. However, according to theoretical calculation of thermodynamics, some of these systems can desorb hydrogen in endothermic reactions. In this research, several hydroxide systems were studied to clarify the reversibility of these systems. Six types of hydroxide systems (XH + Y(OH)(n), X: Li, Na, Y: Li, Na, Mg) were prepared by ball-milling, then the desorption properties of these systems were evaluated. As a result, NaH + LiOH and NaH + NaOH systems desorbed hydrogen in endothermic reactions. Also, NaH + NaOH system absorbed hydrogen almost 100% reversibly under 300 degrees C and 1.0 MPa H-2 pressure. The reversibility in NaH + LiOH system was not accomplished in our experimental condition, because high pressure of GPa order is required for the rehydrogenation. In order to modify the desorption kinetics, we added KH catalyst to NaH + LiOH and NaH + NaOH systems. The hydrogen desorption temperature of the two systems was decreased by addition of 1 mol% of KH. The catalytic effect on hydrogen desorption properties was sustained even after the rehydrogenation of KH-added NaH + NaOH system.
  • Toshiro Yamaguchi, Keita Shinzato, Kyohei Yamamoto, Yongming Wang, Yuki Nakagawa, Shigehito Isobe, Tomoyuki Ichikawa, Hiroki Miyaoka, Takayuki Ichikawa
    International Journal of Hydrogen Energy 45 (11) 6806 - 6812 0360-3199 2020/02 [Refereed][Not invited]
     
    In this work, nitrogenation, ammonia generation, regeneration reactions of lithium-tin alloy is investigated as pseudo catalytic process of ammonia synthesis. Li17Sn4 synthesized by thermochemical method at 500 °C can react with 0.1 MPa of N2 below 400 °C. Nano or amorphous lithium nitride would be formed by the nitrogenation. By reaction of the nitrogenated samples and H2, ammonia is generated at 300 °C under 0.1 MPa. The initial alloy phase Li17Sn4 is regenerated below 350 °C from the products after the ammonia generation process. Based on the above three step process, ammonia can be pseudo-catalytically synthesized from N2 and H2 below 400 °C under ambient pressure. Furthermore, the reactivity for the ammonia synthesis using Li–Sn alloy is preserved during the NH3 synthesis cycles due to the characteristic reaction process based on the Li extraction and insertion.
  • Jingming Shi, Yu Lei, Naoyuki Hashimoto, Shigehito Isobe
    Materials Transactions 61 (4) 616 - 621 1345-9678 2020 
    The dissolution behavior of H, He, C and N in CrCoFeNi high entropy alloy (HEA) is studied by density functional theory calculation. Results show that the site preference of H, C and N in CrCoFeNi HEA is the octahedral site, while the site preference of He is the tetrahedral site. The dissolution energy of H in CrCoFeNi HEA is the lowest and that of He is the highest. The high dissolution energy of He is considered to be due to its closed-shell electronic structure. Moreover, results suggest that the H, He and N are more stable at the site with more Cr atoms, and C is more stable at the site with less Ni atoms. Furthermore, It is found that H, He, C and N have the effect in decreasing the magnetic moments of solutes in CrCoFeNi HEA. Electronic structure analysis shows that, for C and N, there is strong hybridization between them and solutes. It implies the chemical bonding between C, N and solutes is strong.
  • Yu Lei, Naoyuki Hashimoto, Shigehito Isobe
    MATERIALS TRANSACTIONS 61 (7) 1247 - 1251 1345-9678 2020 
    Face-centered cubic Co-free Cu-containing solid solution concentrated alloys, Cu, CuNi, CuNiFe, Cu0.3NiFeCr, Al0.4CuFeCrNi2 were prepared, and their microstructure, hardness, and tensile strengths were investigated in order to develop a new high entropy alloy with a high irradiation resistance, which is applicable for nuclear reactor components. All the as-cast alloys were identified as single-phase FCC alloys by X-ray diffraction analysis. While, the SEM observation indicated a new Cr-rich phase with Cu-rich phase in the annealed Cu0.3NiFeCr alloy, which is probably due to low solubility of Cr and Cu in the alloy. After annealing at 1076 degrees C for 120 hours, Cu0.3NiFeCr alloy became a single-phase FCC. Mechanical property examinations indicated the highest Vickers hardness, the highest Tensile strength and the smallest elongation in the Al0.4CuFeCrNi2. The results indicate that the Al0.4CuFeCrNi2 alloy would have the potential to be a Co-free high-entropy alloy applicable to nuclear reactor components. In order to improve the elongation of Al0.4CuFeCrNi2, the detailed analysis of fracture surface and the optimization of annealing condition would be needed.
  • Pratibha Pal, Pooja Kumari, Yongming Wang, Shigehito Isobe, Manoj Kumar, Takayuki Ichikawa, Ankur Jain
    Journal of Materials Chemistry A 8 (48) 25706 - 25715 2050-7488 2020 

    The addition of chalcogenides destabilized LiBH4 efficiently and the lowest onset temperature was found to be 75 °C for LiBH4–50 wt% bulk Bi2S3. The detailed mechanism associated with this destabilization is proposed herein.

  • Lee Chung-Hyun, Nakagawa Yuki, Isobe Shigehito, Hashimoto Naoyuki, Sugino Seiki, Miyaoka Hiroki, Ichikawa Takayuki
    JOURNAL OF ALLOYS AND COMPOUNDS 801 645 - 650 0925-8388 2019/09/15 [Refereed][Not invited]
     
    Sodium magnesium amidoborane, which can be regarded as one of the ammonia borane (NH BH ) based materials, has superior hydrogen storage properties. In this study, we investigated the hydrogen release process and ammonia absorption phase of sodium-magnesium amidoborane. The sodium-magnesium amidoborane was prepared by ball-milling of NH BH , MgH and NaNH with two kinds of molar ratios (NH BH : MgH : NaNH = 4:1:2 and 3:1:1). As a result, a single hydrogen release step from Na Mg(NH BH ) was clearly observed in the molar ratio (4:1:2), and multiple hydrogen release steps were observed in the molar ratio (3:1:1). The synthesized Na Mg(NH BH ) released hydrogen of 7.5 wt% in an endothermic reaction. Also, we found an ammonia absorption phase of Na Mg(NH BH ) by using sodium amide as a starting material. The Na Mg(NH BH ) absorbs 5 mol of ammonia in the form of solid as the pressure of ammonia gas reaches 100 kPa. After that, a total of 15 mol of ammonia is absorbed up to 600 kPa in the liquid phase. 3 3 3 3 2 2 3 3 2 2 2 2 3 4 2 2 3 4 2 2 3 4 2 2 3 4
  • Yuki Nakagawa, Chung-Hyun Lee, Kouki Matsui, Kohei Kousaka, Shigehito Isobe, Naoyuki Hashimoto, Shotaro Yamaguchi, Hiroki Miyaoka, Takayuki Ichikawa, Yoshitsugu Kojima
    Journal of Alloys and Compounds 734 55 - 59 0925-8388 2018/02/15 [Refereed][Not invited]
     
    Hydrogen desorption properties of α-AlH3 doped with Nb species (Nb, Nb2O5 and NbF5) were investigated. Doping Nb species improved the desorption properties of AlH3. In particular, 1 mol% NbF5-doped AlH3 showed the lowest onset desorption temperature at 60 °C. Compared with Nb- or Nb2O5-doped AlH3, the fine distribution of dopant was successfully achieved in NbF5-doped AlH3. The apparent activation energy for hydrogen desorption of AlH3 was slightly decreased with the dopant of NbF5. The improvement of desorption properties might be due to the finely dispersed Nb and/or AlF3, which are formed by the reaction between NbF5 and AlH3 (surface Al2O3).
  • Suganthamalar Selvaraj, Ankur Jain, Sanjay Kumar, Tengfei Zhang, Shigehito Isobe, Hiroki Miyaoka, Yoshitsugu Kojima, Takayuki Ichikawa
    International Journal of Hydrogen Energy 43 (5) 2881 - 2889 0360-3199 2018/02/01 [Refereed][Not invited]
     
    The development of a suitable hydrogen compressor plays one of the key roles to realize the fuel cell vehicle as well as for many other stationary and mobile applications of hydrogen. V-Ti-Cr BCC alloys are considered as promising candidates for effective hydrogen storage. The cyclic durability of hydrogen absorption and desorption is very important for these alloys to be realized as practical options. In connection to this, two alloys of V-Ti-Cr, (1) V40Ti21.5Cr38.5 and (2) V20Ti32Cr48, were selected and their cyclic hydrogen absorption-desorption performance was evaluated up to 100 cycles for temperature and pressure ranges of 20–300 °C and 5–20 MPa, respectively. It has been found that the cyclic hydrogen storage capacity continuously decreased for one composition while it was stable after 10 cycles for another composition. This performance difference of the alloys was studied in terms of their structural and microscopic properties and the results are presented in this paper.
  • Tengfei Zhang, Yongming Wang, Tao Song, Hikaru Miyaoka, Keita Shinzato, Hiroki Miyaoka, Takayuki Ichikawa, Siqi Shi, Xiaogang Zhang, Shigehito Isobe, Naoyuki Hashimoto, Yoshitsugu Kojima
    Joule 2 (8) 1522 - 1533 2542-4351 2018 [Refereed][Not invited]
     
    Lithium borohydride ammoniates can be readily generated by LiBH4 absorbing ammonia at room temperature. Li(NH3)nBH4 (0 < n ≤ 2) performed high ionic conductivity near room temperature, e.g., 2.21 × 10−3 S cm−1 for mono-ammoniate at 40°C. A drastic increase in ionic conductivity occurs around 38°C due to the structural change resulting from ammonia desorption. Moreover, the jump of the ionic conductivity is reversible because ammonia absorption/desorption acts as a switch for the structural change. The experimental results show that the stable electrochemical window for the sample is ∼4 V. The crystal structure of Li(NH3)nBH4 (0 < n ≤ 1) can be easily changed while the equilibrium dynamically changes with temperature. First-principles calculation results also indicate that Li(NH3)BH4 exerts an expected compatibility in contact with lithium iron phosphate cathode material. Finally, by introducing a gas instead of substituting an element, a dynamic change of crystal structure is achieved, accompanied with the improvement of lithium ionic conductivity. The progress of lithium-ion battery technology has been inseparable from the innovation of electrolyte materials. Compared with the traditional organic liquid electrolytes, stable solid-state electrolytes are safe, easy to assemble, and inexpensive. However, the large-scale application of solid-state electrolytes is currently limited owing to its low ionic conductivity at room temperature. Generally, lithium ionic conductivity of a solid electrolyte is easily influenced by its crystal structure and defect density, rather than others, and the gas absorption/desorption reaction is an easy way to induce the structural change in solids. Here, we first propose a new concept of utilizing the gas absorption/desorption reaction to improve the ionic conductivity of solids, and demonstrate the ionic conductivity jump of lithium borohydride ammoniates switched by ammonia desorption and absorption at around room temperature. Lithium ionic conductivity up to 10−3 S cm−1 can be achieved from solid-state lithium borohydride ammoniates near room temperature. The gradual change of crystal structure from Li(NH3)BH4 to Li(NH3)xBH4 (0 < x < 1) with the increase of temperature indicates a dynamic equilibrium formed in a closed system. The electrochemical properties of lithium borohydride ammoniates have been investigated. The amount of ammonia as a switch is a key factor controlling the lithium ionic conductivity.
  • Yuki Nakagawa, Shigehito Isobe, Takao Ohki, Naoyuki Hashimoto
    Inorganics 5 (4) 2304-6740 2017/12/01 [Refereed][Not invited]
     
    Hexagonal boron nitride (h-BN) is known as an effective additive to improve the hydrogen de/absorption properties of hydrogen storage materials consisting of light elements. Herein, we report the unique hydrogen desorption properties of LiAlH4/h-BN composites, which were prepared by ball-milling. The desorption profiles of the composite indicated the decrease of melting temperature of LiAlH4, the delay of desorption kinetics in the first step, and the enhancement of the kinetics in the second step, compared with milled LiAlH4. Li3AlH6 was also formed in the composite after desorption in the first step, suggesting h-BN would have a catalytic effect on the desorption kinetics of Li3AlH6. Finally, the role of h-BN on the desorption process of LiAlH4 was discussed by comparison with the desorption properties of LiAlH4/X (X = graphite, LiCl and LiI) composites, suggesting the enhancement of Li ion mobility in the LiAlH4/h-BN composite.
  • Shigehito Isobe, Hao Yao, Yongming Wang, Hiroshi Kawasaki, Naoyuki Hashimoto, Somei Ohnuki
    INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 42 (31) 20398 - 20398 0360-3199 2017/08 [Refereed][Not invited]
  • Ankur Jain, Sanjay Kumar, Hiroki Miyaoka, Tengfei Zhang, Shigehito Isobe, Takayuki Ichikawa, Yoshitsugu Kojima
    INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 42 (35) 22388 - 22394 0360-3199 2017/08 [Refereed][Not invited]
     
    The decomposition of NaNH2 has been reported, mainly decomposing into NaH, N-2 and H-2. Ammonia is also produced in addition to N-2 and H-2. To the best of our knowledge, very few scattered reports on the effect of alkali hydrides on NaNH2 exist in literature. Thus, we choose NaNH2 MH (M = Li, Na, K, Mg, Ca) system to be investigated in detail. Since NaNH2 NaH is the simplest combination due to same cation, it was tested for the establishment of reaction mechanism using transmission electron microscopy (TEM). It is observed that the entire process follows NH3 mediated reaction similar to LiNH2 LiH system. Sodium amide first decompose into Na metal and NH3, then generated NH3 reacted with added NaH to form NaNH2 and release H-2. This process continues until the consumption of NaH, thus suppresses NH3 evolution to a great extent. The investigation has been extended further to the other metal hydrides and it is found that the addition of other metal hydride i.e. LiH, KH, MgH2, and CaH2 have also effectively suppressed the NH3 evolution. The detailed reaction mechanism has been elucidated for all the amide hydride systems. It is observed that the decomposition takes place through an intermediate step of double-cation amide formation. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
  • Tengfei Zhang, Shigehito Isobe, Ankur Jain, Yongming Wang, Shotaro Yamaguchi, Hiroki Miyaoka, Takayuki Ichikawa, Yoshitsugu Kojima, Naoyuki Hashimoto
    JOURNAL OF ALLOYS AND COMPOUNDS 711 400 - 405 0925-8388 2017/07 [Refereed][Not invited]
     
    Lithium metatitanate (Li2TiO3) doped magnesium hydride (MgH2) has been investigated in this paper. Desorption properties of the sample with catalyst are compared to the pure MgH2. Particularly, MgH2 doped with 5 mol % Li2TiO3 started to desorb hydrogen at 170 degrees C with a peak temperature at 211 degrees C, which is 100 degrees C and 80 degrees C lower than that of the as-milled MgH2. The reversibility and cyclability of sample with catalyst have also been investigated. Compared with the raw material, the desorption activation energy was reduced from 113 kJ/mol to 84 kJ/mol. Furthermore, the catalytic mechanism was discussed according to the experimental results. (C) 2017 Elsevier B.V. All rights reserved.
  • Shigehito Isobe, Akifumi Ono, Hao Yao, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    Applied Physics Letters 96 (22) 0003-6951 2017/06/26 [Refereed][Not invited]
     
    In situ observation on dehydrogenation of MgH2 was performed by using transmission electron microscope (TEM). The dehydrogenation of MgH 2 with 1 mol % Nb2 O5 and formation of nanosized Mg particles were observed at 150 °C. Nb2 O5 was not confirmed in diffraction patterns and TEM images probably due to wide dispersion. On MgH2 with 10 mol % Nb2 O5, the high resolution TEM could recognize the dehydrogenation at the interface between MgH2 and Nb2 O5, proceeding with increasing temperature. This suggests that hydrogen atoms could diffuse from MgH 2 phase to the interface between Mg and Nb2 O5, resulting in formation of hydrogen molecules at the interface. © 2010 American Institute of Physics.
  • Shigehito Isobe, Akifumi Ono, Hao Yao, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    APPLIED PHYSICS LETTERS 110 (26) 0003-6951 2017/06 [Refereed][Not invited]
  • Shotaro Yamaguchi, Takayuki Ichikawa, Yongming Wang, Yuki Nakagawa, Shigehito Isobe, Yoshitsugu Kojima, Hiroki Miyaoka
    ACS Omega 2 (3) 1081 - 1088 2470-1343 2017/03/31 [Refereed][Not invited]
     
    Lithium alloys are synthesized by reactions between lithium metal and group 14 elements, such as carbon, silicon, germanium, and tin. The nitrogenation and denitrogenation properties are investigated by thermal and structural analyses. All alloys dissociate the nitrogen triple bond of gaseous molecules to form atomic state as nitrides below 500 °C, which is lower than those required for conventional thermochemical and catalytic processes on nitride syntheses. For all alloys except for germanium, it is indicated that nanosized lithium nitride is formed as the product. The denitrogenation (nitrogen desorption) reaction by lithium nitride and metals, which is an ideal opposite reaction of nitrogenation, occurs by heating up to 600 °C to form lithium alloys. Among them, the lithium-tin alloy is a potential material to control the dissociation and recombination of nitrogen below 500 °C by the reversible reaction with the largest amount of utilizable lithium in the alloy phase. The nitrogenation and denitrogenation reactions of the lithium alloys at lower temperature are realized by the high reactivity with nitrogen and mobility of lithium. The above reactions based on lithium alloys are adapted to the ammonia synthesis. As a result, ammonia can be synthesized below 500 °C under 0.5 MPa of pressure. Therefore, the reaction using lithium alloys is recognized as a pseudocatalyst for the ammonia synthesis.
  • Yuki Nakagawa, Keita Shinzato, Tessui Nakagawa, Keita Nakajima, Shigehito Isobe, Kiyotaka Goshome, Hiroki Miyaoka, Takayuki Ichikawa
    INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 42 (9) 6173 - 6180 0360-3199 2017/03 [Refereed][Not invited]
     
    Na[Al(NH2BH3)(4)], a mixed-metal amidoborane, was synthesized by ball-milling (solid method) and the chemical reaction in THF (solution method). Solid method has a tendency to remain unreacted NaAlH4 and AB. In the solution method, the partial decomposition of Na[Al(NH2BH3)(4)] would proceed during mixing in THF. The local structural characterization of as-synthesized material was performed by MAS NMR and FT-IR. While Na[Al(NH2BH3)(4)] desorbed hydrogen in two steps as reported, the results of structural characterization suggested that the hydrogen desorption in the 2nd step would originate from the Al-N-B H phase. Effect of hydrogen pressure during ball-milling was also investigated for nNH(3)BH(3)-NaAlH4 (n = 1, 4) composites. In the case of n = 4, Na[Al(NH2BH3)(4)] was formed under both Ar and H-2 atmosphere. However, in the case of n =1, Na[Al(NH2BH3)(4)] only formed under H-2 atmosphere, whereas most of H-2 was desorbed during ball-milling under Ar atmosphere. Thus, the hydrogen pressure is necessary for the synthesis in the case of n = 1. Potential energy diagram of AB-NaAlH4 system was described. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
  • Yingqiong Yong, Mai Thanh Nguyen, Tetsu Yonezawa, Takashi Asano, Masaki Matsubara, Hiroki Tsukamoto, Ying-Chih Liao, Tengfei Zhang, Shigehito Isobe, Yuki Nakagawa
    JOURNAL OF MATERIALS CHEMISTRY C 5 (5) 1033 - 1041 2050-7526 2017/02 [Refereed][Not invited]
     
    A method for producing Cu films with low resistivity, based on low temperature sintering, is demonstrated. The Cu inks for preparing conductive Cu films consisted of Cu particles that were coated with a decomposable polymer (poly(propylenecarbonate), PPC) as well as a self-reducible copper formate/1-amino-2- propanol (CuF-IPA) complex as an additive. The sintering temperature used in this study was as low as 100 degrees C. Following sintering at a temperature of 100 degrees C, the lowest reported resistivity (8.8 x 10(-7) Omega m) was achieved through the use of Cu-based metal-organic decomposition (MOD) inks. This was due to the dual promotional effects of the aminolysis of PPC with IPA and the pyrolysis of the CuF-IPA complex.
  • Hao Yao, Hiroshi Kawasaki, Shigehito Isobe, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    MATERIALS TRANSACTIONS 58 (6) 971 - 971 1345-9678 2017 [Refereed][Not invited]
  • Tengfei Zhang, Shigehito Isobe, Yongming Wang, Chaomei Liu, Naoyuki Hashimoto, Keisuke Takahashi
    PHYSICAL CHEMISTRY CHEMICAL PHYSICS 18 (39) 27623 - 27629 1463-9076 2016/10 [Refereed][Not invited]
     
    High efficiency catalysts are needed to improve the kinetics of complex hydrides for practical applications. In this study, lithium metatitanate (Li2TiO3) is introduced in lithium alanate (LiAlH4), and the catalytic effect for notable complex/metal hydrides, such as LiAlH4, is investigated. Experiment results indicate that Li2TiO3 improves the kinetics of LiAlH4. In particular, Li2TiO3 dramatically improves the onset temperature of LiAlH4, which decreases to 75 degrees C and is within the temperature range for use in proton exchange membrane fuel cells. Transmission electron microscopy (TEM) observations help understand the catalytic effect of Li2TiO3 in the nanoscale. First principles calculations also show the improvement of H- and Li+ mobility by doping Li2TiO3, where calculations indicate that the physical origin of the catalytic effect is due to two factors: charge transfer and minor surface relaxation. Thus, experimental and theoretical evidence reveals the catalytic mechanism of Li2TiO3 in LiAlH4.
  • Tengfei Zhang, Yuki Nakagawa, Takenobu Wakasugi, Shigehito Isobe, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    ACS APPLIED MATERIALS & INTERFACES 8 (23) 14548 - 14551 1944-8244 2016/06 [Refereed][Not invited]
     
    A window type of the environmental cell system for a high-voltage electron microscope was developed and applied to in situ observation of a palladium (Pd) thin film. For in situ hydrogenation of Pd thin films, the distances of the lattice fringes were 0.20 and 0.23 nm, which correspond to the lattice d spacings of beta-phase (200) and (111) planes. Expansion of the Pd lattice happened as a result of phase transformation from the alpha phase to the beta phase. In particular, the lattice fringes were clearly distinguished, and the dislocation behavior during Pd hydrogenation was easily recognized according to the corresponding inverse fast fourier transform images. Furthermore, significant growth in the number of dislocations was observed at the grain boundary during increasing hydrogen pressure in the cell.
  • Keisuke Takahashi, Shigehito Isobe, Kengo Omori, Torge Mashoff, Domenica Convertino, Vaidotas Miseikis, Camilla Coletti, Valentina Tozzini, Stefan Heun
    JOURNAL OF PHYSICAL CHEMISTRY C 120 (24) 12974 - 12979 1932-7447 2016/06 [Refereed][Not invited]
     
    Hydrogen adsorption on graphene-supported metal clusters has brought much controversy due to the complex nature of the bonding between hydrogen and metal clusters. The bond types of hydrogen and graphenesupported Ti clusters are experimentally and theoretically investigated. Transmission electron microscopy shows that Ti clusters of nanometer size are formed on graphene. Thermal desorption spectroscopy captures three hydrogen desorption peaks from hydrogenated graphene-supported Ti clusters. First -principles calculations also found three types of interaction: two types of bonds with different partial ionic character and physisorption. The physical origin for this rests on the charge state of the Ti clusters: when Ti clusters are neutral, H2 is dissociated, and H forms bonds with the Ti cluster. On the contrary, H2 is adsorbed in molecular form on positively charged Ti clusters, resulting in physisorption. Thus, this work clarifies the bonding mechanisms of hydrogen on graphene-supported Ti clusters.
  • Yuki Nakagawa, Tengfei Zhang, Masaumi Kitamura, Shigehito Isobe, Satoshi Hino, Naoyuki Hashimoto, Somei Ohnuki
    JOURNAL OF CHEMICAL AND ENGINEERING DATA 61 (5) 1924 - 1929 0021-9568 2016/05 [Refereed][Not invited]
     
    The effects of metal-based additives in a wide range of elements doping into ammonia borane (AB) were systematically investigated. Pure transition metals in period 4 and total of 17 metal chlorides were selected as additives and ball-milled with AB. Metal chloride additives were effective to decrease H-2 desorption temperature and the amounts of by product gas emissions of AB, whereas pure transition metals did not show any ability because they did not work as Lewis acids. The strong correlation between H-2 desorption temperature and the Pauling electronegativity of metal (chi(p)) was observed. Mn+ of MCln would work as Lewis acid to initiate the AB dehydrocoupling reaction. The amount of NH3 emission was correlated with the ionic radius of M, suggesting MCln center dot mNH(3) complexes be formed in this system. AB-CuCl2 and AB-AgCl mixtures showed the similar H-2 desorption processes. The Cl-containing intermediate phase would destabilize AB during reaction.
  • Keisuke Takahashi, Yuki Nakagawa, Lauren Takahashi, Shigehito Isobe
    NEW JOURNAL OF CHEMISTRY 40 (9) 7303 - 7306 1144-0546 2016 [Refereed][Not invited]
     
    Key descriptors representing the desorption temperature of ammonia borane against hydrides and chlorides are investigated and applied to machine learning. The desorption temperature of ammonia borane with hydrides and chlorides is experimentally measured and a database is constructed. The database indicates a correlation between the desorption temperature and electronegativity. A support vector machine with selected descriptors from the database reveals the optimal amounts of CuCl2 and AgCl needed for decreasing the desorption temperature of ammonia borane. Prediction of the electronegativity that would achieve the desorption temperature is also revealed where the crystal structure is also found to be a key descriptor. Thus, electronegativity and crystal structures are revealed to be key descriptors that predict the desorption temperature of ammonia borane with a chemical compound. This would essentially lead towards finding the optimal amount of dopants and ideal dopants with a minimum number of experiments.
  • Ammonia borane - Metal hydride mixture materials
    Shigehito Isobe, Yuki Nakagawa
    WHEC 2016 - 21st World Hydrogen Energy Conference 2016, Proceedings 659 - 660 2016
  • SHINZATO Keita, NAKAGAWA Yuki, ISOBE Shigehito, NAKAGAWA Tessui, NAKAJIMA Keita, GOSHOME Kiyotaka, MIYAOKA Hiroki, ICHIKAWA Takayuki, KOJIMA Yoshitsugu
    Proceedings of the Annual Conference of The Japan Institute of Energy 一般社団法人 日本エネルギー学会 25 260 - 261 2423-8325 2016 

    Ammonia borane (AB: NH3BH3) is a promising solid state hydrogen storage material because it contains >19 mass% of hydrogen. However, the thermal decomposition of AB kinetically requires high temperature, and considerable amount of harmful by-product gases emit from AB. Recently, the decomposition properties of AB have been improved by the composite with alanates (MAlH4, M = Na, Li). It was reported that the new compound NaAl(NH2BH3)4 formed during mechanical milling of 4AB-NaAlH4. In this work, we have tried to synthesize NaAl(NH2BH3)4 in THF solvent (Solution method) to obtain the single phase with high-purity. The powder X-ray diffraction pattern of the product obtained by the solution method was similar to that obtained by the solid method, suggesting that NaAl(NH2BH3)4 can be synthesized by the solution method as well. However, the 23Na-nuclear magnetic resonance (NMR) spectrum suggests that a different phases from NaAl(NH2BH3)4 coexist. Thus, in order to obtain the single phase of NaAl(NH2BH3)4, further optimization of reaction conditions would be necessary.

  • Kenji Hirai, Shigehito Isobe, Kazuki Sada
    SCIENTIFIC REPORTS 5 18468 - 18468 2045-2322 2015/12 [Refereed][Not invited]
     
    Central in material design of metal oxides is the increase of surface area and control of intrinsic electronic and optical properties, because of potential applications for energy storage, photocatalysis and photovoltaics. Here, we disclose a facile method, inspired by geochemical process, which gives rise to mesoporous anion-doped metal oxides. As a model system, we demonstrate that simple calcination of a multinuclear coordination cluster results in synchronic chemical reactions: thermal oxidation of Ti8O10(4-aminobenzoate)(12) and generation of gases including amino-group fragments. The gas generation during the thermal oxidation of Ti8O10(4-aminobenzoate)(12) creates mesoporosity in TiO2. Concurrently, nitrogen atoms contained in the gases are doped into TiO2, thus leading to the formation of mesoporous N-doped TiO2. The mesoporous N-doped TiO2 can be easily synthesized by calcination of the multinuclear coordination cluster, but shows better photocatalytic activity than the one prepared by a conventional sol-gel method. Owing to an intrinsic designability of coordination compounds, this facile synthetic will be applicable to a wide range of metal oxides and anion dopants.
  • Nobuko Hanada, Tessui Nakagawa, Hirotaka Asada, Masayoshi Ishida, Keisuke Takahashi, Shigehito Isobe, Itoko Saita, Kohta Asano, Yumiko Nakamura, Akitoshi Fujisawa, Shinichi Miura
    JOURNAL OF ALLOYS AND COMPOUNDS 647 198 - 203 0925-8388 2015/10 [Refereed][Not invited]
     
    LaNi5-based AB(5) type alloy has high tolerance to CO2 poisoning for hydrogen purification and storage from 20 to 25% CO2 mixed gas. To elucidate the CO2 poisoning factors of AB(5) type alloys, which are LaNi5, CaNi5, LaCo5, and MmNi(4.025)Co(0.4)Mn(0.275)Al(0.3) (Mm-Ni), the dependence of the constituent elements has been investigated on hydrogenation degradation by CO2 poisoning. The tendency of CO2 poisoning magnitude is CaNi5 < LaNi5 << Mm-Ni < LaCo5, which was evaluated by the hydrogenation rate and capacity under CO2 partial pressure and after CO2 exposure. The Ni element of B site in CaNi5 and LaNi5 is an important role to maintain higher tolerance of CO2 poisoning compared to Co element in LaCo5. Moreover, the element of A site effects on CO2 poisoning magnitude in AB(5) type alloy. The experimental tendency of CO2 poisoning magnitude is consistent with the theoretical CO2 adsorption energy on the (1010) surface plane of -1.39, -2.05, and -2.68 eV for CaNi5, LaNi5, and LaCo5, respectively. CO2 adsorbs on B site with electron charge transfer from AB(5) alloys to carbon. Not only Ni element in B site but also Ca element in A site decreases the energy of CO2 adsorption on B site in AB(5) type alloys. (C) 2015 Elsevier B.V. All rights reserved.
  • Hiroyuki Oguchi, Shigehito Isobe, Hiroki Kuwano, Susumu Shiraki, Shin-ichi Orimo, Taro Hitosugi
    APL MATERIALS 3 (9) 2166-532X 2015/09 [Refereed][Not invited]
     
    We report on the epitaxial thin film growth of an air-sensitive hydride, lithium hydride (LiH), using pulsed laser deposition (PLD). We first synthesized a dense LiH target, which is key for PLD growth of high-quality hydride films. Then, we obtained epitaxial thin films of [100]-oriented LiH on a MgO(100) substrate at 250 degrees C under a hydrogen pressure of 1.3 x 10(-2) Pa. Atomic force microscopy revealed that the film demonstrates a Stranski-Krastanov growth mode and that the film with a thickness of similar to 10 nm has a good surface flatness, with root-mean-square roughness RRMS of similar to 0.4 nm. (C) 2015 Author(s).
  • Shigehito Isobe, Katsuhiro Kudoh, Satoshi Hino, Kenji Hara, Naoyuki Hashimoto, Somei Ohnuki
    APPLIED PHYSICS LETTERS 107 (8) 0003-6951 2015/08 [Refereed][Not invited]
     
    In this letter, catalytic efficiency of Nb, NbO, Nb2O3, NbO2, and Nb2O5 for dissociation and recombination of hydrogen were experimentally investigated. On the surface of Nb and Nb oxides in a gas mixture of H-2 and D-2, H-2 and D-2 molecules can be dissociated to H and D atoms; then, H-2, D-2, and HD molecules can be produced according to the law of probability. With increase of frequency of the dissociation and recombination, HD ratio increases. The ratio of H-2 and HD gas was analyzed by quadrupole mass spectrometry. As a result, NbO showed the highest catalytic activity towards hydrogen dissociation and recombination. (C) 2015 AIP Publishing LLC.
  • Hiroki Miyaoka, Yongming Wang, Satoshi Hino, Shigehito Isobe, Kazuhiko Tokoyoda, Takayuki Ichikawa, Yoshitsugu Kojima
    MATERIALS 8 (7) 3896 - 3909 1996-1944 2015/07 [Refereed][Not invited]
     
    Various synthesis and rehydrogenation processes of lithium hydride (LiH) and magnesium amide (Mg(NH2)(2)) system with 8:3 molar ratio are investigated to understand the kinetic factors and effectively utilize the essential hydrogen desorption properties. For the hydrogen desorption with a solid-solid reaction, it is expected that the kinetic properties become worse by the sintering and phase separation. In fact, it is experimentally found that the low crystalline size and the close contact of LiH and Mg(NH2)(2) lead to the fast hydrogen desorption. To preserve the potential hydrogen desorption properties, thermochemical and mechanochemical rehydrogenation processes are investigated. Although the only thermochemical process results in slowing the reaction rate due to the crystallization, the ball-milling can recover the original hydrogen desorption properties. Furthermore, the mechanochemical process at 150 degrees C is useful as the rehydrogenation technique to preserve the suitable crystalline size and mixing state of the reactants. As a result, it is demonstrated that the 8LiH and 3Mg(NH2)(2) system is recognized as the potential hydrogen storage material to desorb more than 5.5 mass% of H-2 at 150 degrees C.
  • Tengfei Zhang, Shigehito Isobe, Motoaki Matsuo, Shin-ichi Orimo, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    ACS CATALYSIS 5 (3) 1552 - 1555 2155-5435 2015/03 [Refereed][Not invited]
     
    This paper presents a relationship between ionic mobility and reaction kinetics for the Li-N-H system after doping LiTi2O4. The structural characteristic of this fast ionic conductor was introduced to the complex-hydride system. On one hand, the properties of the dehydrogenation process were improved significantly. On the other hand, the relationship between lithium ionic conductivity and the catalytic effect on the dehydrogenation was investigated according to the alternating current (AC) impedance results. The lithium ionic conductivity of samples with catalyst was higher than the samples without catalyst. Especially, the conductivity of LiNH2 and LiH mixtures with LiTi2O4 was almost 1.5 times higher than that of LiNH2 and LiH. The mobility of the Li+ ions between LiH and LiNH2 solid phases was enhanced by adding LiTi2O4.
  • Tengfei Zhang, Shigehito Isobe, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    RSC ADVANCES 5 (24) 18375 - 18378 2046-2069 2015 [Refereed][Not invited]
     
    We have decreased the end temperature of the Li-N-H system, a hydrogen storage material developed in 2002, to below 260 degrees C, and obtained a lowest peak temperature of 223 degrees C.
  • An In-Situ, Environmental Cell-Holder of Conventional Transmission Electron Microscope and Its Applications
    Y. Wang, T. Wakasugi, H. Nagakura, S. Isobe, N. Hashimoto, S. Ohnuki
    MICROSCOPY Oxford 64 I119  2015 [Refereed][Not invited]
  • Development of High Pressure Gas Environmental Cell and its Application to Hydrogen Reaction
    H. Nagakura, T. Wakasugi, K. Ohkubo, T. Tanioka, T. Endo, S. Isobe, Y. Wang, N. Hashimoto, S. Ohnuki
    MICROSCOPY Oxford 64 I119  2015 [Refereed][Not invited]
  • Yongming Wang, Takenobu Wakasugi, Shigehito Isobe, Naoyuki Hashimoto, Somei Ohnuki
    MICROSCOPY 63 (6) 437 - 447 2050-5698 2014/12 [Refereed][Not invited]
     
    Transmission electron microscope (TEM) observation of light metal hydrides is complicated by the instability of these materials under electron irradiation. In this study, the electron kinetic energy dependences of the interactions of incident electrons with lithium, sodium and magnesium hydrides, as well as the constituting element effect on the interactions, were theoretically discussed, and electron irradiation damage to these hydrides was examined using in situ TEM. The results indicate that high incident electron kinetic energy helps alleviate the irradiation damage resulting from inelastic or elastic scattering of the incident electrons in the TEM. Therefore, observations and characterizations of these materials would benefit from increased, instead decreased, TEM operating voltage.
  • Takahashi K, Isobe S
    Physical chemistry chemical physics : PCCP 16 (31) 16765 - 16770 1463-9076 2014/08 [Refereed][Not invited]
     
    The titanium iron (TiFe) alloy is a notable hydrogen storage material which can operate at ambient temperature. However, low hydrogen storage capacity is a major drawback that is needed to be overcome. Enhancement of the hydrogen capacity of TiFe is considered by utilizing TiFe clusters within the density functional theory. Calculations reveal that TiFe clusters can absorb large amounts of hydrogen. Furthermore, the desorption energies of Ti1Fe1H6 are lower than that of bulk TiFeH where the physical origins of low desorption energies are considered to be due to the closed shell structure of Ti1Fe1. This indicates that the Ti1Fe1H6 has the possibility to operate at near ambient temperature; therefore, only hydrogen gas pressures are required to control the hydrogen storage and release.
  • Shigehito Isobe, Ayaka Umeda, Takenobu Wakasugi, Tao Ma, Ryo Yamagami, Satoshi Hino, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    MATERIALS TRANSACTIONS 55 (8) 1175 - 1178 1345-9678 2014/08 [Refereed][Not invited]
     
    We propose the microstructural change model of magnesium hydride catalyzed by Nb2O5 during hydrogenation. The ball-milled composites, MgH2 and 1 mol% Nb2O5, were dehydrogenated and then rehydrogenated for varied time at room temperature under 0.1 MPa H-2 atmosphere. The crystallite size of Mg and MgH2 was evaluated by powder X-ray diffraction (XRD) measurement and confirmed by transmission electron microscopy (TEM) observation. The crystallite size of generated MgH2 was smaller than that of Mg and did not change significantly with increasing time of hydrogenation. It is suggested that the number density of MgH2 crystallites increases during the hydrogenation process.
  • Shigehito Isobe, Yudai Ikarashi, Hao Yao, Satoshi Hino, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    MATERIALS TRANSACTIONS 55 (8) 1138 - 1140 1345-9678 2014/08 [Refereed][Not invited]
     
    Recently, LiAlH4 has attracted attention as one of the most promising hydrogen storage materials, because LiAlH4 is able to release large amount of hydrogen (7.9 mass%H-2) below 250 degrees C. However, the kinetics of the dehydrogenation reaction of LiAlH4 is too slow for applications to fuel cell vehicles. To improve the dehydrogenation kinetics, the dopant effect of TiCl3 has been investigated in this research. Here, LiAlH4 doped with various ratios (0, 0.1, 0.2, 0.5, 1.0, and 2.0 mol%) of TiCl3 were prepared by ball milling for 30 min under a 1.0 MPa H-2 atmosphere. The decomposition of LiAlH4 proceeds via a two-step reaction and the dehydrogenation kinetics of each step were compared to determine the optimum amount of TiCl3 that would assist the process. With increasing of TiCl3 amount, the dehydrogenation temperature for both of the reactions decreased. Activation energies decreased with increasing TiCl3 amount, however the amount of desorbed hydrogen decreased. Considering the kinetics and hydrogen capacity in the both steps, the results suggest that the optimum amount of doped TiCl3 for the dehydrogenation of LiAlH4 is around 0.2 mol%.
  • Keisuke Takahashi, Shigehito Lsobe
    PHYSICAL CHEMISTRY CHEMICAL PHYSICS 16 (31) 16765 - 16770 1463-9076 2014/08 [Not refereed][Not invited]
     
    The titanium iron (TiFe) alloy is a notable hydrogen storage material which can operate at ambient temperature. However, tow hydrogen storage capacity is a major drawback that is needed to be overcome. Enhancement of the hydrogen capacity of TiFe is considered by utilizing TiFe clusters within the density functional theory. Calculations reveal that TiFe clusters can absorb large amounts of hydrogen. Furthermore, the desorption energies of Ti1Fe1F16 are lower than that of bulk TiFeH where the physical origins of low desorption energies are considered to be due to the closed shell structure of Ti1Fe1.. This indicates that the Ti1Fe1F16 has the possibility to operate at near ambient temperature; therefore, only hydrogen gas pressures are required to control the hydrogen storage and release.
  • Keisuke Takahashi, Yongming Wang, Shotaro Chiba, Yuki Nakagawa, Shigehito Isobe, Somei Ohnuki
    SCIENTIFIC REPORTS 4 4598 - 4598 2045-2322 2014/04 [Refereed][Not invited]
     
    Hydrogenation of iron nanoparticles was performed both computationally and experimentally where previously chemically-bonded iron hydride is considered to be unachievable under ordinary conditions. Density functional theory (DFT) calculations predict that hydrogenated iron nanoparticles are stabilized on a single-layer graphene/Cu substrate. Experimentally, iron nanoparticles were deposited onto a graphene/Cu substrate by vacuum deposition. Hydrogenation was done at 1atm of hydrogen gas and under liquid nitrogen. Mass spectrometry peak confirmed the hydrogen release from hydrogenated iron nanoparticles while a scanning transmission electron microscopy is used in order to link a geometrical shape of iron hydride nanoparticles between experimental and theoretical treatments. The hydrogenated iron nanoparticles were successfully synthesized where hydrogenated iron nanoparticles are stable under ordinary conditions.
  • Tengfei Zhang, Shigehito Isobe, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    CHEMCATCHEM 6 (3) 724 - 727 1867-3880 2014/03 [Refereed][Not invited]
     
    In this study, LiTi2O4 was synthesized as a possible catalyst in the Li-N-H system. The properties of hydrogen desorption in the Li-N-H system with a homogeneous catalyst have been investigated. The X-ray diffraction and X-ray photoelectron spectroscopy results indicated that the single phase of LiTi2O4 was successfully synthesized and it was stable in the sample after high energy ball-milling and heat treatment. LiTi2O4 exhibited a catalytic effect in the Li-N-H system according to the thermogravimetry differential thermal analysis results. During dehydrogenation, a storage capacity of 5.7wt% was obtained under moderate temperature. A sharp peak of thermal gas desorption mass spectrometry curve occurred at 227 degrees C. Furthermore, the catalytic mechanism of LiTi2O4 in the Li-N-H system was discussed in accordance with the experimental results.
  • Yuki Nakagawa, Yudai Ikarashi, Shigehito Isobe, Satoshi Hino, Somei Ohnuki
    RSC ADVANCES 4 (40) 20626 - 20631 2046-2069 2014 [Refereed][Not invited]
     
    Hydrogen desorption properties and decomposition processes of NH3BH3-MAlH4 (M = Na, Li) composites were investigated by using thermogravimetry-differential thermal analysis-mass spectrometry (TG-DTA-MS), powder X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analyses. We prepared the composites by ball-milling and the mixtures by hand-milling. The ball-milled composites desorbed 4-5 wt% hydrogen at three exothermic steps below 260 degrees C. The emissions of by-product gases, NH3, B2H6 and B3H6N3, were effectively suppressed. From XRD analysis, the formation of a mixed-metal (Na(Li), Al) amidoborane phase was suggested. Very different results were obtained using hand-milling. They showed only one exothermic reaction at 80-90 degrees C. The emission of by-product gases was not suppressed. By comparing the differences between ball-milled composites and hand-milled mixtures, the importance of mixed-metal amidoborane in this system was proposed.
  • Development of the non-electric power type hydrogen collection system with hydrogen storage materials
    Satoshi Hino, Ken-Ichi Yano, Shigehito Isobe, Naoyuki Hashimoto, Kazuhiko Tokoyoda, Ryutaro Hino, Yasunari Maekawa, Yoshitsugu Kojima
    20th World Hydrogen Energy Conference, WHEC 2014 2 792 - 795 2014 
    The goal of this work is to develop a hydrogen gas collection system by using hydrogen storage materials. In case of hydrogen leakage, the system is intended to work as the safety apparatus that can prevent hydrogen explosion by collecting hydrogen gas without electric power supply. The composite of magnesium powder with 1 mol% niobium oxide (activated-Mg), which can absorb hydrogen at room temperature, is promising material for the system. To accumulate fundamental knowledge regarding hydrogen collection properties of the system, ∼ 10 g of activated-Mg was placed in a closed container (8L) as simulated experiments. The atmosphere inside the container was controlled (gas mixture of hydrogen/nitrogen/oxygen) and change in concentrations of hydrogen gas in the container was monitored. The activated-Mg absorbed hydrogen gas of low concentration (< 4 vol%) indicating that the hydrogen gas collection system using the activated-Mg has capability to maintain hydrogen concentration in a room below lower explosive limit.
  • Yuki Nakagawa, Shigehito Isobe, Yudai Ikarashi, Somei Ohnuki
    JOURNAL OF MATERIALS CHEMISTRY A 2 (11) 3926 - 3931 2050-7488 2014 [Refereed][Not invited]
     
    Dehydrogenation properties of AB-MH (Ammonia Borane-Metal Hydride, M - K, Na, Li, Ca, Mg, Al) composites were systematically investigated by thermal and mass analyses. The results suggest that the Pauling electronegativity of M, chi(p), is a good indicator to predict the phases of composites, the dehydrogenation temperature and the amount of by-product gases (NH3 and B2H6). The phases of composites were classified by chi(p) as follows. MBH4 was formed for M = K, Na (chi(p) <= 0.9), MNH2BH3 was formed for M = Na, Li (0.9 <= chi(p) <= 1.0) and no new compounds were formed for M = Ca, Mg, Al (1.0 <= chi(p)). The 1st dehydrogenation temperatures of the samples (M = Na, Li, Ca, Mg) were 10-20 degrees C lower than that of AB itself (chi(p) <= 1.2). The amount of NH3 was decreased as chi(p) increased. On the other hand, the amount of B2H6 was decreased as chi(p) decreased. The emission of B3H6N3 could occur by the reaction of NH3 and B2H6. Finally, AB-MAlH4 (M = Na, Li) composites, which were prepared based on the indicator, showed superior potential as hydrogen storage materials because they did not desorb any by-products NH3, B2H6 and B3H6N3.
  • Tengfei Zhang, Shigehito Isobe, Yongming Wang, Hiroshi Oka, Naoyuki Hashimoto, Somei Ohnuki
    JOURNAL OF MATERIALS CHEMISTRY A 2 (12) 4361 - 4365 2050-7488 2014 [Refereed][Not invited]
     
    In this study, LiTi2O4 was synthesized as a possible catalyst for complex metal hydrides. LiTi2O4 was stable in the sample after high-energy ball milling and heat treatment. LiTi2O4 exhibited a catalytic effect among the samples of MgH2, LiAlH4 and LiNH2. The desorption kinetics and the purity of the desorbed hydrogen gas have been improved by doping LiTi2O4. Furthermore, the catalytic mechanism of LiTi2O4 was discussed in accordance with the experimental results.
  • Yuki Nakagawa, Shigehito Isobe, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki, Liang Zeng, Shusheng Liu, Takayuki Ichikawa, Yoshitsugu Kojima
    JOURNAL OF ALLOYS AND COMPOUNDS 580 (Supplement 1) S163 - S166 0925-8388 2013/12 [Refereed][Not invited]
     
    Dehydrogenation processes of alpha- and gamma-AlH3 were investigated by in situ transmission electron microscopy observations. The relationship between Al2O3 thickness and dehydrogenation kinetics was also clarified. The initial shape of alpha-AlH3 particle was cubic and that of gamma-AlH3 particle was rod-shaped. The process of gamma-AlH3 was quite similar with alpha-AlH3. The precipitation and growth of Al was observed in both processes. The dehydrogenation kinetics did not depend on Al2O3 thickness. It was found that milling effect on the dehydrogenation kinetics was larger than doping effect. The dehydrogenation process was discussed in terms of both microscopic and kinetic studies. (C) 2013 Elsevier B.V. All rights reserved.
  • Keisuke Takahashi, Shigehito Isobe, Somei Ohnuki
    JOURNAL OF ALLOYS AND COMPOUNDS 580 (Supplement 1) S25 - S28 0925-8388 2013/12 [Refereed][Not invited]
     
    The catalytic effect of Nb, Nbo and Nb2O5 with different surface planes on dehydrogenation of MgH2 clusters was investigated by using a density functional theory. The adsorption sites of Mg and H are different depending on different surface planes. In particular, NbO(111) shows very high adsorption energy which is considered to be due to electron pairing instead of a charge transfer. Electronic structure reveal that NbO(111) has a strong interaction between the s-state of H and d-state of Nb. This thermo-dynamically suggests a low dissociation barrier of Mg-H bond. Mg becomes fully oxide on a oxygen-rich planes of NbO(100) and all planes of Nb2O5. Those planes prevent rehydrogenation of MgH2. These results should help understand the catalytic effects of Nb, NbO and Nb2O5 and develop a better catalyst for MgH2 systems. (C) 2013 Elsevier B.V. All rights reserved.
  • Takenobu Wakasugi, Shigehito Isobe, Ayaka Umeda, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    JOURNAL OF ALLOYS AND COMPOUNDS 580 (Supplement 1) S127 - S130 0925-8388 2013/12 [Refereed][Not invited]
     
    A close type of an environmental cell was developed for a high voltage electron microscope. Using this cell allowed an in situ observation of hydrogenation in Pd particles under H-2 gas of 0.05 MPa at RT. Two types of window films, Tri-Acetyl-Cellulose (TAC) and Silicon Nitride (SiN), were used for testing the contamination on the sample, as well as the strength for pressure. We confirmed the hydrogenation in diffraction patterns and images, and additionally the image resolution of 0.19 urn was obtained by using a SiN film with a thickness of 17 nm. (C) 2013 Elsevier B.V. All rights reserved.
  • 山上亮, 礒部繁人, 王永明, 橋本直幸, 大貫惣明
    日本金属学会講演概要(CD-ROM) 153rd ROMBUNNO.6  1342-5730 2013/09/03 [Not refereed][Not invited]
  • Shigehito Isobe, Sumito Yamada, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki, Hiroki Miyaoka, Takayuki Ichikawa, Yoshitsugu Kojima
    JOURNAL OF APPLIED PHYSICS 114 (9) 093509-093509-7  0021-8979 2013/09 [Refereed][Not invited]
     
    Li-C-H system, which can store about 5.0 mass% of rechargeable H-2, has been reported as a promising hydrogen storage system by Ichikawa et al. [Appl. Phys. Lett. 86, 241914 (2005); Mater. Trans. 46, 1757 (2005)]. This system was investigated from the thermodynamic and structural viewpoints. However, hydrogen absorption/desorption mechanism and the state of hydrogen atoms absorbed in the composite have not been clarified yet. In order to find new or better hydrogen storage system, graphite powder and nano-structural graphite ball-milled under H-2 and Ar atmosphere were prepared and milled with Li and Mg under Ar atmosphere in this study. Microstructural analysis for those samples by transmission electron microscope revealed that LiC6 and/or LiC12 were formed in Li-C-H system. On the other hand, MgC2 was found in Mg-C-H system ball-milled under H-2 atmosphere, but not in the system ball-milled under Ar atmosphere. These results indicated that nano-structure in composites of nano-structural graphite is different from that of alkali (-earth) metal. For these reasons, metal-C-H system can be recognized to be a new family of hydrogen storage materials. (C) 2013 AIP Publishing LLC.
  • Keisuke Takahashi, Shigehito Isobe, Somei Ohnuki
    LANGMUIR 29 (38) 12059 - 12065 0743-7463 2013/09 [Refereed][Not invited]
     
    Niobium-based oxide nanoparticles have proven to be catalytically effective toward hydrogenation of Mg where H-2 dissociation over the niobium-oxides is considered to be a crucial reaction step. However, the role of niobium oxides toward H-2 dissociation still remains unclear as to what atomic configurations are responsible for the catalytic activity. H-2 dissociation over different surface planes of Nb, NbO, and Nb2O5 as well as small NbO clusters is performed by using a density functional theory. The calculations reveal that H-2 dissociation, adsorption energy, and the bond type between H and surfaces (clusters) depend on the atomic configurations of Nb and O. In particular, H-2 adsorption on NbO(111) is enhanced by O atoms without forming O-H bond where the bond type of H and surface is found to be an electron pairing. Thus, NbO(111) could not only be a effective catalyst but also potentially prevent the formation of MgO during the hydrogenation of Mg. The results should be helpful in developing and tailoring the efficient catalyst toward H-2 dissociation and hydrogenation of Mg.
  • Tao Ma, Shigehito Isobe, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    JOURNAL OF PHYSICAL CHEMISTRY C 117 (20) 10302 - 10307 1932-7447 2013/05 [Refereed][Not invited]
     
    The state of Nb-contained catalysts in MgH2 nanocomposites was investigated during the full cycle. X-ray diffraction (XRD) results showed that Nb2O5 and Nb reacted with MgH2 during ball-milling, forming NbH2 and NbH, respectively. In the following dehydrogenation, the (di)hydride decomposed, and Nb was produced. Then NbH was generated in both samples after rehydrogenation. Similar composition in both samples suggests that the catalytic effect follows the same mechanism, the Nb-gateway model, in which Nb facilitates the hydrogen transportation from MgH2 to the outside. By contrast, NbO remained during the full cycle. Scanning and transmission electron microscopy (SEM and TEM) observations revealed that the Nb2O5-doped sample tended to be refined in size, compared to the Nb-doped and NbO-doped ones. Nb crystals in the Nb2O5-doped sample were observed to be highly dispersed in the sample, with 10-20 nm in size. Given all that, tiny Nb crystals distributed in the composites worked as the gateway facilitating hydrogen transportation and improving dehydrogenation properties.
  • Tessui Nakagawa, Takayuki Ichikawa, Keiji Shimoda, Masami Tsubota, Hiroki Miyaoka, Shigehito Isobe, Tetsuo Honma, Shinji Michimura, Fumitoshi Iga, Somei Ohnuki, Yoshitsugu Kojima
    INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 38 (16) 6744 - 6749 0360-3199 2013/05 [Refereed][Not invited]
     
    The mixtures of scandium hydride ScH2 and metal boride MBn, which is MgB2 or CaB6, were hydrogenated by mechanical milling under hydrogen pressure at room temperature. ScH2-MgB2 and ScH2-CaB6 desorbed 3.4 and 2.3 mass% of H-2, respectively, with peaks below 300 degrees C. The results of synchrotron radiation X-ray powder diffraction and X-ray absorption spectroscopy at the Sc K-edge indicated that ScB2 was produced by milling. Fourier-transform infrared spectroscopy suggested that hydrogen was stored as B-H bonds in the as-milled samples. Nuclear magnetic resonance spectroscopy clarified the presence of metal borohydrides M(BH4)(2) (M = Mg and Ca) in the as-milled ScH2-MBn mixtures. These results indicate that M(BH4)(2) is synthesized by milling the ScH2-MBn mixtures under hydrogen pressure at room temperature, and hydrogen was desorbed from M(BH4)(2). The by-products of M(BH4)(2) are MgH2 in the M = Mg case, which was observed by transmission electron microscopy, and ScB2 in both cases. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
  • Keisuke Takahashi, Shigehito Isobe, Somei Ohnuki
    APPLIED PHYSICS LETTERS 102 (15) 0003-6951 2013/04 [Refereed][Not invited]
  • Keisuke Takahashi, Shigehito Isobe, Somei Ohnuki
    Applied Physics Letters 102 (11) 113108-113108-4  0003-6951 2013/03/18 [Refereed][Not invited]
     
    The interaction of H and Fe clusters of up to nine atoms were investigated within a density functional theory. Calculations indicate that one gas-phase Fe atom can absorb ten H atoms, an amount 2.5 times more than methane (CH 4). The magnetic state of Fe atoms non-uniformly decrease by increasing the number of H. The bonding of Fe-H in FeH clusters consists of charge transfer and electron pairing. Thus, two types of bondings are involved. The bond mechanism is general in nature within transition metal clusters, bringing insight for the development of heterogeneous catalyst and hydrogen storage materials. © 2013 American Institute of Physics.
  • Ankur Jain, Erika Kawasako, Hiroki Miyaoka, Tao Ma, Shigehito Isobe, Takayuki Ichikawa, Yoshitsugu Kojima
    JOURNAL OF PHYSICAL CHEMISTRY C 117 (11) 5650 - 5657 1932-7447 2013/03 [Refereed][Not invited]
     
    Lithium hydride has high hydrogen capacity (12.7 mass %), but could not be considered as practical hydrogen storage media because of being very stable (required 900 degrees C for 0.1 MPa desorption pressure). Recently, C and Si have been found suitable to reduce the stability of LiH. This motivates us to investigate the properties of other alloys of Li, formed with the other elements. In the present work, Li3.75Ge (Li15Ge4) alloy was synthesized by mechanical milling, which transformed into Li42Ge (Li21.1875Ge5) and Li3.5Ge (Li7Ge2) phases during the vacuum heating at 400 degrees C. Hydrogenation of thus formed alloys at 400 degrees C under 3 MPa hydrogen pressure during PCI experiment transforms this mixed phase into Li2GeH0.5 (Li4Ge2H) and LiH phase. A remarkable decrease in the desorption temperature (similar to 300-450 degrees C) is observed by preparing the above alloy with Ge as observed from TG-DTA-MS experiment. The enthalpy of the reaction has also been calculated using the van't Hoff plot. The present work concluded with the establishment of a direct relationship between hydrogen storage parameters and electrochemical parameters using the Nernst equation and van't Hoff equation. A good agreement is found between the values of required potential for lithiation/delithiation as obtained by two methods.
  • Hao Yao, Shigehito Isobe, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 38 (9) 3689 - 3694 0360-3199 2013/03 [Refereed][Not invited]
     
    The thermal decomposition process of lithium alanate (LiAlH4) was investigated by TEM, TG-DTA and XRD. It was shown that LiAlH4 decomposes through a two-step reaction: a liquid-to-solid phase transition in the first step of the decomposition and a solid-to-solid reaction in the second step of the decomposition, both steps accompanied by hydrogen release. The particle size of the aluminum (Al), which formed in the first decomposition step, was much larger than that in the second decomposition step. In addition, Al particles formed in the liquid phase of LiAlH4 in the first decomposition step, while, they form in solid phase of Li3AlH6 in the second decomposition step, resulting in the kinetics of the first decomposition step being faster than that of the second decomposition step. The investigation clearly demonstrated the reaction model of the decomposition of LiAlH4 in the nano-scale, showing different diffusion processes of Al in each of the decomposition steps. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
  • Shuai Wang, Keisuke Takahashi, Naoyuki Hashimoto, Shigehito Isobe, Somei Ohnuki
    SCRIPTA MATERIALIA 68 (5) 249 - 252 1359-6462 2013/03 [Refereed][Not invited]
     
    The effect of hydrogen in body-centered cubic iron is explored by using the density function theory. Hydrogen atoms increase the concentration of free electrons in the simulation cell and undergo bonding interaction with Fe atoms. Elastic interaction between hydrogen and screw dislocations was found, caused by the anisotropic strain components of the hydrogen atoms in the tetrahedral sites. The dependence of the hydrogen-screw dislocation interaction on hydrogen concentration is confirmed by repeated stress relaxation tests. (c) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
  • Tengfei Zhang, Shigehito Isobe, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    RSC ADVANCES 3 (18) 6311 - 6314 2046-2069 2013 [Refereed][Not invited]
     
    The reaction mechanism of the (de) hydrogenation of a LiNH2 + LiH mixture with a nanoscale catalyst has been investigated. In this research, the position of each solid phase was examined by transmission electron microscopy (TEM). The observation showed that LiH particles, around 100 nm in size, formed around LiNH2 in the hydrogenation process. According to in situ TEM observation of the dehydrogenation process, the LiH particles became smaller as they reacted with LiNH2. Fine particles of Li2NH with crystallites of size 30-40 nm formed on the surface of the LiNH2. It indicated that H+ from the LiNH2 moved to the interface and combined with H- from the LiH in the dehydrogenation process, consequently H-2 was released. At the same time, Li+ ions diffused from the LiH to the LiNH2 through the interface and Li2NH formed. On the other hand, it was confirmed that the catalyst was located at the interface between the LiH and LiNH2. It was found that the catalyst had the effect of improving the migration of Li+ from LiH to LiNH2.
  • Keisuke Takahashi, Shigehito Isobe, Somei Ohnuki
    RSC Advances 3 (44) 21841 - 21847 2046-2069 2013 [Refereed][Not invited]
     
    The interaction of H< inf> 2< /inf> and small Fe, Ru, and Os clusters of up to nine atoms was investigated through the density functional theory. The ground state structures of H< inf> 2< /inf> adsorbed Fe, Ru, and Os clusters were explored by using the basin-hopping algorithm. Calculations indicate that Os clusters show relatively high stability and reactivities upon H< inf> 2< /inf> adsorption while bare and hydrogenated Fe clusters show relatively high magnetic moments. The thresholds upon stability change between bare and hydrogenated clusters were also discovered. © The Royal Society of Chemistry 2013.
  • A metal-oxide catalyst for hydrogen storage in Li-NH nanocomposites
    Zhang, Tengfei, Isobe, Shigehito, Wang, Yongming, Hashimoto, Naoyuki, Ohnuki, Somei
    第五届世界氢能技术大会 (WHTC2013) 1 - 1 2013
  • Shigehito Isobe, Miki Dohkoshi, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki, Satoshi Hino, Takayuki Ichikawa, Yoshitsugu Kojima
    JOURNAL OF THE JAPAN INSTITUTE OF METALS The Japan Institute of Metals and Materials 77 (12) 571 - 574 0021-4876 2013 [Refereed][Not invited]
     
    In order to clarify the reaction mechanism of the hydrogenation process in the Li(2)NHsystem, we have observed the partially hydrogenated samples by transmission electron microscopy (TEM). From the TEM images of partially hydrogenated samples, it was shown that the LiNH2 phase was located between Li2NH phase, which was almost at the center of the particle, and LiH particles with the size of 100 nm. This result indicated that LiH was generated and grew up at the outside of the complex particle of Li2NH and LiNH2 with Li moving from the center to the surface of the particle.
  • Keisuke Takahashi, Shigehito Isobe, Somei Ohnuki
    CHEMICAL PHYSICS LETTERS 555 26 - 30 0009-2614 2013/01 [Refereed][Not invited]
     
    The ground state structures and several isomers of osmium clusters of up to 14 atoms were obtained from the basin-hopping algorithm within a density functional theory. The small Os clusters favor cubic-based configurations which are similar to those found in Ru clusters. In contrast, Os clusters show much higher binding energy and stability than Ru clusters, which suggests that Os clusters have the potential to be better catalysts than Ru clusters. Magnetic property of small Os clusters also studied. These results provide an elementary understanding of small osmium clusters and provide a catalyst alternative to other group VIII clusters. (C) 2012 Elsevier B.V. All rights reserved.
  • Tao Ma, Shigehito Isobe, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    CATALYSTS 2 (3) 344 - 351 2073-4344 2012/09 [Refereed][Not invited]
     
    We report a study on the desorption properties, crystallography and chemical state of MgH2 and 1 mol% Nb2O5 ball-milled composites. Desorption temperatures of the composites decreased with increase of ball-milling time. Size of MgH2 crystallites decreased during ball-milling. Reduction of Nb2O5 after ball-milling was confirmed by tracing the chemical state of Nb and was further supported by TEM observation. The reduced phases may act as more effective catalysts improving the desorption properties.
  • Akitaka Michigoe, Takumi Hasegawa, Norio Ogita, Takayuki Ichikawa, Yoshitsugu Kojima, Shigehito Isobe, Masayuki Udagawa
    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN 81 (9) 094603.1-094603.11  0031-9015 2012/09 [Refereed][Not invited]
     
    Raman scattering spectra of LiNH2 have been measured from 3.4 to 673 K. Precise polarization dependence of the single crystalline LiNH2 and first principles calculation have successfully assigned all observed peaks. Li vibration with the lowest energy at 133 cm(-1) shows the anomaly, where its energy decreases with decreasing temperature. This anomaly shows that the Li vibration is highly anharmonic with large amplitude. In addition, the energy of 133 cm(-1) gives the very small force constant of 0.05 mdyn/angstrom between Li and NH2 in the diatomic model. This weak interaction suggests that LiNH2 easily decomposes to Li and NH2. Below 100 K, we have found new peaks in the energy range from 100 to 700 cm(-1). No additional degrees of freedom for vibrations conclude that rotational motion of NH2 molecule freezes below 100 K. With increasing temperature, the bond angle of H-N-H and bond length N-H in NH2 become narrow and long, respectively. At the reaction temperature region, the correlation between the integrated intensity of N-H vibration in Li2NH and back ground slope has been observed and the background change is originated from the desorption of NH3 molecules. Final materials due to chemical reactions are different between single crystal and powder samples. In the single crystal, the spectrum similar with that at room temperature appears at 673 K and this suggests that the reaction finishes only at the thin surface layer, not entire volume. However, the powder sample of LiNH2 decomposes to Li2NH and finally change to the Li3N for the sake of NH3 desorption.
  • Tao Ma, Shigehito Isobe, Keisuke Takahashi, Yongming Wang, Shuai Wang, Naoyuki Hashimoto, Somei Ohnuki
    JOURNAL OF PHYSICAL CHEMISTRY C 116 (32) 17089 - 17093 1932-7447 2012/08 [Refereed][Not invited]
     
    Mg-Nb2O5 evaporated composites, which were prepared by evaporating Mg thermally on single crystals of Nb2O5, were investigated in this work. We attempted to hydrogenate the sample under 5 bar H-2 atmosphere at 250 degrees C for 2 h. The electron microscope was used to observe the microstructure of the as-prepared and hydrogenated samples. It was found that the phase transition occurred along the particular orientation where Mg(002) is parallel to MgH2(101) or MgO(200). Density functional theory calculations were carried out on MgH2 and MgO slabs, showing that MgH2(101) and MgO(200) have the lowest surface-formation energy. On the basis of the observation and calculation, the phase-transition process of Mg during hydrogenation and oxidation was discussed. Finally, a structural model including Mg-Mg distance adjustment and layer shift was proposed to demonstrate the phase-transition process.
  • ISOBE Shigehito, OHNUKI Somei, KOJIMA Yoshitsugu
    化学工学 = Chemical engineering 化学工学会 76 (1) 19 - 21 0375-9253 2012/01/05 [Not refereed][Not invited]
  • 礒部 繁人
    水素エネルギーシステム = Journal of the Hydrogen Energy Systems Society of Japan 水素エネルギー協会 37 (4) 314 - 317 1341-6995 2012 [Not refereed][Not invited]
  • Hao Yao, Shigehito Isobe, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    JOURNAL OF ELECTRON MICROSCOPY 60 (6) 375 - 378 0022-0744 2011/12 [Refereed][Not invited]
     
    A plastic bag method was developed to observe air-sensitive samples on microstructure and phase distribution without exposure to air during the holder transfer process into the transmission electron microscope (TEM). As an example, a type of lithium aluminum hydride (Li(3)AlH(6)) was observed in the TEM to demonstrate the effectiveness of this method. Results show that the plastic bag method is a simple and practical TEM transfer method utilized to reduce air contact for a series of air-sensitive materials.
  • Tao Ma, Shigehito Isobe, Eri Morita, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki, Toru Kimura, Takayuki Ichikawa, Yoshitsugu Kojima
    INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 36 (19) 12319 - 12323 0360-3199 2011/09 [Refereed][Not invited]
     
    We reported on the hydrogen desorption properties, microstructure, kinetics, and chemical bonding state of catalyst surface for composites of MgH(2) and 1 mol% Nb(2)O(5) ball-milled for 0.02 h, 0.2 h, 2 h, 20 h under 1 MPa H(2) atmosphere, as well as hand-mixed (HM) one. Hydrogen desorption properties were significantly improved by ball-milling with Nb(2)O(5). Then, we estimated by Kissinger Method the activation energy (E(a)) of hydrogen desorption reaction that decreased with the increase of ball-milling time. Especially, E(a) of the sample ball-milled for 0.2 h was drastically decreased, compared with that of the sample ball-milled for 0.02 h. TEM observations revealed that the distribution of Nb(2)O(5) in MgH(2) was gradually improved during ball-milling. On the other hand, we confirmed by XPS that in the sample ball-milled for 0.2 h, Nb(2)O(5-x) phase(s) existed at least on the surface. It can be suggested that these deoxidized Nb(2)O(5-x) phases eventually decrease E(a) as substantial catalyst rather than Nb(2)O(5) itself. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
  • Shigehito Isobe, Somei Ohnuki, William I. F. David, Matthias Gutmann, Martin O. Jones, Peter P. Edwards, Takayuki Ichikawa, Yoshitsugu Kojima
    INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 36 (13) 7909 - 7913 0360-3199 2011/07 [Refereed][Not invited]
     
    We have synthesized a single crystal of lithium amide (LiNH2, LiND2) by melting method, and performed neutron diffraction of the single crystal at variable temperature. LiND2 is tetragonal structure and I-4 space group. Lattice parameters and unit cell volume of LiND2 at room temperature, 50 degrees C, 100 degrees C, 150 degrees C and 200 degrees C were determined. Both of the lattice parameters and the unit cell volume increase with increase of temperature. From these results, we have estimated coefficient of volumetric thermal expansion alpha(v) of LiND2 to be 222 x 10(-6)/K. With increase of temperature, all thermal ellipsoids gradually expand because of thermal vibration. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
  • Shigehito Isobe, Satoshi Hino, Takayuki Ichikawa, Yoshitsugu Kojima
    APPLIED PHYSICS LETTERS 99 (1) 0003-6951 2011/07 [Refereed][Not invited]
     
    Chemical bonding states of titanium compounds in LiH and LiNH(2) mixture, which have been a candidate for a hydrogen storage material, have been examined by x-ray absorption spectroscopy measurement as the characterization of the catalysts. The results of x-ray absorption near-edge structure indicated that the Ti atoms in the Ti compounds, which had the catalytic effect on the kinetics of the hydrogen desorption properties, had a common electronic (chemical bonding) state. Additionally, this common electronic state of the Ti catalysts agrees with that of TiCl(3)center dot 5NH(3). These results indicated that TiCl(3)center dot 5NH(3) could act as the catalyst. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3606534]
  • Akitaka Michigoe, Takumi Hasegawa, Norio Ogita, Masayuki Udagawa, Masami Tsubota, Takayuki Ichikawa, Yoshitugu Kojima, Shigehito Isobe
    CHINESE JOURNAL OF PHYSICS 49 (1) 294 - 300 0577-9073 2011/02 [Refereed][Not invited]
     
    Phonon Raman spectra of single crystalline LiNH2 have been measured in the temperature range between 3.5 K and 300 K. The observed peaks are well assigned by the polarization dependence of the single crystal, comparing with the first principles calculation. Below 100K five new peaks appear, and it is concluded that these are caused by the freezing of the rotational motion of NH2 molecule. It is found that the vibration energy of the Li motion decreases with decreasing temperature. This result shows that the vibration of Li is highly anharmonic with the large amplitude.
  • Hiroki Miyaoka, Takayuki Ichikawa, Tatsuo Fujii, Wataru Ishida, Shigehito Isobe, Hironobu Fuji, Yoshitsugu Kojima
    JOURNAL OF ALLOYS AND COMPOUNDS 507 (2) 547 - 550 0925-8388 2010/10 [Refereed][Not invited]
     
    On the synthesis of nano-structural hydrogenated graphite by ball-milling under H(2) atmosphere, iron contamination was mingled from steel balls during ball-milling. It is clarified by spectroscopic measurements that the mingled iron formed a non-stoichiometric iron-carbon (Fe-C) compound. The Fe-C phase was transformed to a well-ordered phase with H(2) desorption at 450 degrees C, suggesting that the hydrogen atoms were anomalously trapped at the Fe-C phase. With respect to hydrogen absorbing properties, the mingled iron enhanced the hydrogen capacity by about 50% compared with iron free hydrogenated graphite, where H/Fe was about 13 mass%. Therefore, if the hydrogen absorption site originated in the Fe-C phase could be synthesized independently, it should be recognized as a promising hydrogen storage system. (C) 2010 Elsevier B.V. All rights reserved.
  • Shigehito Isobe, Hao Yao, Yongming Wang, Hiroshi Kawasaki, Naoyuki Hashimoto, Somei Ohnuki
    INTERNATIONAL JOURNAL OF HYDROGEN ENERGY 35 (14) 7563 - 7567 0360-3199 2010/07 [Refereed][Not invited]
     
    In-situ transmission electron microscopy (TEM) has been performed to observe decomposition process of sodium alanate (NaAlH4) in this work. NaAlH4 was ground in a glove box under inert gas, and then it was transferred into microscope without exposed to air by Plastic Bag Method. The results of in-situ electron beam diffraction showed that NaAlH4 decomposed to Na3AlH6 + Al, and NaH + Al during heated up to 150 and 200 degrees C, respectively. Moreover, we obtained the result of high-resolution (HR) TEM images about the decomposition of NaAlH4 by high voltage electron microscopy (HVEM) of 1250 key. It showed that the porous structures appeared with increase of temperature. This should be from structural defects and/or cavities due to volume change of the phases. It was also shown that Na3AlH6 and Al particles with the grain size of several 10 nm were irregularly distributed near the pores. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
  • Shigehito Isobe, Akifumi Ono, Hao Yao, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    APPLIED PHYSICS LETTERS 96 (22) 223109  0003-6951 2010/05 [Refereed][Not invited]
     
    In situ observation on dehydrogenation of MgH2 was performed by using transmission electron microscope (TEM). The dehydrogenation of MgH2 with 1 mol % Nb2O5 and formation of nanosized Mg particles were observed at 150 degrees C. Nb2O5 was not confirmed in diffraction patterns and TEM images probably due to wide dispersion. On MgH2 with 10 mol % Nb2O5, the high resolution TEM could recognize the dehydrogenation at the interface between MgH2 and Nb2O5, proceeding with increasing temperature. This suggests that hydrogen atoms could diffuse from MgH2 phase to the interface between Mg and Nb2O5, resulting in formation of hydrogen molecules at the interface. (C) 2010 American Institute of Physics. [doi:10.1063/1.3442910]
  • Hao Yao, Hiroshi Kawasaki, Shigehito Isobe, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    MATERIALS TRANSACTIONS 51 (5) 1016 - 1019 1345-9678 2010/05 [Refereed][Not invited]
     
    Sodium alanate (NaAlH4) was ground in a dove box in inert gas and transferred to a microscope without exposing the samples to air, using a plastic bag method. The results of in-situ electron beam diffraction showed that NaAlH4 decomposed to Na3AlH6 and Al, and then to NaH and Al when heated to 150 degrees C and then 200 degrees C. The decomposition of NaAlH4 was observed by ex-situ transmission electron microscopy (TEM) at 1250 keV. Porous structures appeared when the samples were heated and dehydrogenated. These are likely due to structural defects or are cavities due to volume changes between phases. The Na3AlH6 and Al particles were found to distribute around the pores. [doi:10.2320/matertrans.MBW200902]
  • Akifumi Ono, Shigehito Isobe, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    JOURNAL OF THE JAPAN INSTITUTE OF METALS 日本金属学会 74 (3) 205 - 208 0021-4876 2010/03 [Refereed][Not invited]
     
    In situ observation of the catalytic action of MgH2-1 mol%Nb2O5 and MgH2-10 mol%Nb2O5 was carried out by using transmission electron microscopy (TEM). In case of MgH2-1 mol%Nb2O5, TEM analysis indicated that MgH2 started to decompose at 150 degrees C and nano size particle of Mg formed. However, Nb2O5 was not observed in the diffraction pattern and images. In case of MgH2-10 mol%Nb2O5, high resolution images, FFT and IFFT, revealed that the decomposition of MgH2 started with the contact region in Nb2O5 and Mg formed. The result suggested that the reaction of dehydration could proceed due to hydrogen diffusion to the Mg-Nb2O5 interface.
  • Akifumi Ono, Shigehito Isobe, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    Nippon Kinzoku Gakkaishi/Journal of the Japan Institute of Metals The Japan Institute of Metals and Materials 74 (3) 205 - 208 0021-4876 2010/03 [Not refereed][Not invited]
     
    In-situ observation of the catalytic action of MgH2-1 mol%Nb2O5 and MgH2-10 mol%Nb2O 5 was carried out by using transmission electron microscopy (TEM). In case of MgH2-1 mol%Nb2O5, TEM analysis indicated that MgH2 started to decompose at 150°C and nano-size particle of Mg formed. However, Nb2O5 was not observed in the diffraction pattern and images. In case of MgH2-10 mol%Nb 2O5, high resolution images, FFT and IFFT, revealed that the decomposition of MgH2 started with the contact region in Nb 2O5 and Mg formed. The result suggested that the reaction of dehydration could proceed due to hydrogen diffusion to the Mg-Nb 2O5 interface. © 2010 The Japan Institute of Metals.
  • In-situ TEM observation for reaction mechanism in MgH2 hydrogen storage material
    Akifumi Ono, Shigehito Isobe, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    Materials Research Society Symposium Proceedings 1216 159 - 163 0272-9172 2010 
    In-situ observation on the catalytic effect of Nb2O5 in MgH2 was carried out by using transmission electron microscopy (TEM). We prepared two kinds of samples, because we tried to observe the reaction from two kinds of viewpoints. MgH2 catalyzed with 1 mol% of Nb2O5 was prepared for an overall viewpoint on the desorption process of MgH2 with catalyst by conventional TEM. The dehydrogenation of the 1 mol% sample started at 150 °C and Mg nano-size particles were formed. However, Nb2O5 was not confirmed in diffraction patterns and images, because it was highly dispersed by ball-milled. So MgH2 catalyzed with 10 mol% of Nb2O 5 was prepared for local viewpoint to focus the boundary between the catalyst and the Mg phase by high voltage electron micro scope (HVEM). The sample mixed in mortar was prepared for this, because it was difficult to find the boundary in the sample ball-milled. The high resolution images of the 10 mol% sample revealed that the dehydrogenation started from the interface of MgH2 and Nb2O5. The result suggested that the dehydrogenation could proceed with hydrogen diffusion from MgH2 phase to the interface between Mg and Nb2O5. © 2010 Materials Research Society.
  • Eri Morita, Akifumi Ono, Shigehito Isobe, Yongming Wang, Naoyuki Hashimoto, Somei Ohnuki
    PRICM 7, PTS 1-3 654-656 (Pt.3) 2867 - 2870 0255-5476 2010 [Refereed][Not invited]
     
    We carried out in-situ observation on the catalytic effect of Nb(2)O(5) in MgH(2) by using a high voltage transmission electron microscope (HVEM). We synthesized two kinds of samples, which were prepared by ball milling and by hand mixing. In milled sample, Nb(2)O(5) was not confirmed from high resolution images, however, NbO was identified. As the temperature increased, the decomposition of MgH(2) proceeded, while Mg formed and grew. It suggested that NbO had the catalytic effect to promote the dehydrogenation of MgH(2). In mixed sample, which was prepared to clearly observe the boundary between the catalyst and Mg phase, it was revealed that the dehydrogenation started from the boundary of MgH(2) and Nb(2)O(5). This result suggested that the dehydrogenation could proceed with hydrogen diffusion from MgH(2) through Mg phase to the boundary.
  • 礒部 繁人, 王 永明, 奥寺 公也, 平澤 寛子, 橋本 直幸, 大貫 惣明
    まてりあ : 日本金属学会会報 The Japan Institute of Metals and Materials 48 (12) 616 - 616 1340-2625 2009/12/01 [Refereed][Not invited]
  • 礒部繁人, WANG Yongming, 奥寺公也, 平澤寛子, 橋本直幸, 大貫惣明
    まてりあ The Japan Institute of Metals and Materials 48 (12) 616 - 616 1340-2625 2009/12/01 [Not refereed][Not invited]
  • Yoshitsugu Kojima, Kyoichi Tange, Satoshi Hino, Shigehito Isobe, Masami Tsubota, Kosei Nakamura, Masashi Nakatake, Hiroki Miyaoka, Hikaru Yamamoto, Takayuki Ichikawa
    JOURNAL OF MATERIALS RESEARCH 24 (7) 2185 - 2190 0884-2914 2009/07 [Refereed][Not invited]
     
    We show a drastically improved gas-solid reaction between NH(3) and LiH by mechanical treatment for LiH, generating a hydrogen gas even at room temperature. The results Of x-ray photoelectron spectroscopy showed that the mechanical pretreatment was effective in reducing a hydroxide phase from the surface of LiH. It was also possible to successfully recycle back LiNH(2), which is the byproduct of this hydrogen desorption reaction, to LiH under 0.5-MPa H(2) flow at 573 K. Thus, the LiH-NH(3) system provides a recyclable H, storage system to generate H(2) at room temperature with 8.1 mass% and 4.5 kg/100 L hydrogen capacity.
  • Nobuko Hanada, Takayuki Ichikawa, Shigehito Isobe, Tessui Nakagawa, Kazuhiko Tokoyoda, Tetsuo Honma, Hironobu Fujii, Yosbitsugu Kojima
    JOURNAL OF PHYSICAL CHEMISTRY C 113 (30) 13450 - 13455 1932-7447 2009/07 [Refereed][Not invited]
     
    A valence state and a local structure of transition metals (Nb, V, and Ti) in MgH2 doped with metal oxides (Nb2O5, V2O5, and TiO2nano) by ball milling were examined by X-ray absorption spectroscopy (XAS). The main edge regions of the Nb, V, and Ti K-edges in the X-ray absorption near edge structure (XANES) profiles are located between 0 and +5 in the oxidation states. Since these spectra coincide with those of NbO, VO, and Ti2O3, respectively, the additives are reduced by MgH2 to the metal oxides, which have lower oxidation states than those of the starting materials. Furthermore, in order to examine the local structures around the transition metal atoms, the extended X-ray absorption fine structure (EXAFS) spectra were analyzed. In the Fourier transformation curves of the EXAFS spectra, all samples doped with the metal oxides show two peaks corresponding to metal-oxygen and metal-metal bonds, being the same as the references of NbO, VO, and Ti2O3. The local structure formed after ball milling or dehydrogenation is close to that of each of the reduced metal oxides (NbO, VO, and Ti2O3) but in a more disarrangement state.
  • Shigehito Isobe, Takayuki Ichikawa, Haiyan Leng, Hironobu Fujii, Yoshitsugu Kojima
    JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 69 (9) 2234 - 2236 0022-3697 2008/09 [Refereed][Not invited]
     
    Li-Mg-N-H systems composed of Mg(NH2)(2) and LiH with various ratios can reversibly store a large amount of hydrogen under the temperature condition above 150 degrees C. These composites with 3:6, 3:8 and 3:12 ratio of Mg(NH2)(2) and LiH have been independently reported by four groups as promising candidates of high performance hydrogen storage materials possessing the reversibility, and the high capacity. In any cases, an interaction between NH3 and LiH plays an important role for the progress of hydrogen desorbing and absorbing reactions. For the hydrogen desorption process, the NH3 molecule generated from Mg(NH2)(2) reacts with LiH, forming LiNH2 and H-2. Especially, under an equilibrium condition, in situ diffraction results indicated that the single phase of LiNH2 MgNH (LiMgN2H3) could be generated other than the separated two phases. As a next step, the NH3 molecule generated from LiNH2 reacts with LiH, desorbing H-2. As a result, the dehydrogenated phase was evaluated to be Li2NH - MgNH (Li2MgN2H2) or separated two phases, in which the final phase should depend on the experimental conditions. Thus, if the amount of LiH is enough to react with NH3, the hydrogen desorption processes are described by the NH3 generation from the corresponding amides and the imide. (C) 2008 Elsevier Ltd. All rights reserved.
  • Shigehito Isobe, Takayuki Ichikawa, Kazuhiko Tokoyoda, Nobuko Hanada, Haiyan Leng, Hironobu Fujii, Yoshitsup Kojima
    THERMOCHIMICA ACTA 468 (1-2) 35 - 38 0040-6031 2008/02 [Refereed][Not invited]
     
    Enthalpy change (Delta H) due to hydrogen desorption (H-desorption) for the lithium amide/imide system was evaluated by differential scanning calorimetry (DSC) measurement. In order to obtain the accurate and precise value of Delta H, we have paid special attention to following two points for correcting raw experimental data. One is to determine a cell constant of DSC equipment, which was evaluated by using the TiO2-doped MgH2 compound as a reference because of its quite similar hydrogen desorption properties to that of the lithium amide/imide system. The other is to estimate the sample amount corresponding to the H-desorption reaction from weight loss in the thermogravimetric (TG) analysis. By performing both the corrections, the Delta H value due to the H-desorption reaction from LiNH2 + LiH to Li2NH + H-2 was evaluated to be 67 kJ/mol H-2. (c) 2007 Elsevier B.V. All rights reserved.
  • Takayuki Ichikawa, Shigehito Isobe
    ZEITSCHRIFT FUR KRISTALLOGRAPHIE 223 (10) 660 - 665 0044-2968 2008 [Refereed][Not invited]
     
    We discuss crystal structure of amides and imides, which are focusing on promising hydrogen storage materials. They are ionic crystal with anion of (NH(2))(-) or (NH)(2)- and cation of Li(+), Na(+), Mg(2+), Ca(2+), and so forth. We also discuss reaction mechanisms on hydrogen ab/desorption of amide/imide hydrogen storage materials.
  • Shigehito Isobe, Takayuki Ichikawa, Yoshitsugu Kojima, Hironobu Fujii
    JOURNAL OF ALLOYS AND COMPOUNDS 446 360 - 362 0925-8388 2007/10 [Refereed][Not invited]
     
    A lithium-nitrogen-hvdrouen (Li-N-H) system has been reported as one of the attractive hydrogen storage systems. So far, it was clarified that some catalysts, especially titanium-based compounds, were significantly effective for improving the hydrogen storage kinetics in the Li-N-H system. In this research, the chemical bonding states of the titanium compounds in the mixture of LiH and LiNH2 were examined by X-ray absorption spectroscopy (XAS) as the characterization of the catalysts. The results of X-ray absorption near-edge structure (XANES) indicated that the Ti atoms in the titanium compounds, which revealed the catalytic effect on the hydrogen desorption (H-desorption) proper-ties, had a common electronic (chemical bonding) state. (C) 2007 Elsevier B.V. All rights reserved.
  • 礒部繁人, 花田信子, 市川貴之, 小島由継, 藤井博信
    ケミカルエンジニヤリング 化学工業社 52 (4) 258 - 261 0387-1037 2007/04/01 [Not refereed][Not invited]
  • MIYAOKA Hiroki, ISOBE Shigehito, ICHIKAWA Takayuki, FUJII Hironobu
    Tanso 炭素材料学会 0 (226) 2 - 7 0371-5345 2007/01/15 [Not refereed][Not invited]
     
    Hydrogen storage properties of the ball-milled mixtures composed of hydrogenated nano-structural graphite (CnanoHx) and lithium hydride (LiH) were examined from thermodynamic and structural points of view, where the CnanoHx was synthesized by ball-milling graphite powder under hydrogen atmosphere. Since hydrogen atoms in the CnanoHx and LiH are strongly bonded with each host atom, heating up to >600°C is necessary to release hydrogen from each solid product. The CnanoHx and LiH composites desorbed hydrogen and hydrocarbon gases below 400°C with about 9.4 mass% weight loss. The 2: 1 composite of the CnanoHx and LiH reversibly stored hydrogen with an effective capacity of about 4.5 mass % at 350°C. No evident peaks were observed in XRD profiles after the dehydrogenation, indicating that the nano-structural feature remained after dehydrogenation due to formation of (CLi) nano clusters. However, only the LiH phase was crystallized after rehydrogenation at 350°C. From these results, it is concluded that hydrogen in the composites is destabilized by a novel interaction between the CnanoHx and LiH in a nanometer scale, and is desorbed at lower temperatures than each of the components. Therefore, this Li-C-H system can be recognized to be a new family of hydrogen storage materials.
  • Shigehito Isobe, Takayuki Ichikawa, Hironobu Fujii
    JOURNAL OF THE JAPAN INSTITUTE OF METALS 日本金属学会 70 (11) 865 - 869 0021-4876 2006/11 [Refereed][Not invited]
     
    We have investigated hydrogen storage materials composed of light elements such as MgH2, Metal-N-H, Metal-C-H and Metal-B-H (Metal=Li, Na, Mg, Ca) for automobile applications. Particularly, our recent research results on H-storage properties of the metal-N-H system are reviewed in this paper. The mixture of LiH and LiNH2 catalyzed with titanium compound desorbed similar to 6 mass% of hydrogen in temperature ranges from 150 to 250 degrees C under a He gas flow. However, the hydrogen desorption (H-desorption) temperature at P-H2 = 0.1 MPa was 250 degrees C which is too high for on-board applications. We investigated the H-desorption mechanism in the reaction from LiH + LiNH2 to Li2NH + H-2 by Thermal Desorption Mass Spectroscopy (TDMS) and Fourier Transform Infrared (FT-IR) analyses for the products replaced by LiD or LiND2 for LiH or LiNH2, respectively. The results indicated that the H-desorption reaction progresses through two-step elementary reactions mediated by ammonia. On the basis of the ammonia mediated model, we successfully designed a new Li-Mg-N-H system composed of 8LiH and 3Mg (NH2)(2). The mechanically milled composite desorbed similar to 7 mass% H-2 in range from 120 to 200 degrees C and the H-desorption pressure was higher than 5 MPa at 200 degrees C, being suitable for on-board applications. Moreover, for understanding the role of titanium compounds as catalysts, the chemical state of the titanium compounds doped in the mixture was examined by X-ray Absorption Near-Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) measurements.
  • T. Ichikawa, H. Y. Leng, S. Isobe, N. Hanada, H. Fujii
    JOURNAL OF POWER SOURCES 159 (1) 126 - 131 0378-7753 2006/09 [Refereed][Not invited]
     
    In this paper, we review our recent results on hydrogen storage properties in light metals(M)-nitrogen(N)-hydrogen(H) systems prepared by mechanochemical method. At first, the composite mixture of LiH and LiNH2 doped with TiCl3 as a catalyst was prepared by ball milling for 2 h under a H-2 gas pressure of 1 MPa. The TDS profile indicated that similar to 6 mass% H-2 was desorbed by the reaction LiH + LiNH2 <-> Li2NH + H-2 in the temperature range from 150 to 250 degrees C under a He gas flow at a heating rate of 5 degrees C min(-1), but the H-desorption equilibrium pressure P-H2 was similar to 0.1 MPa at 250 degrees C. This temperature is too high for onboard use, indicating that further improvement is necessary to destabilize the above Hstorage reaction. For that, we clarified the H-desorption mechanism by the isotopic exchange experiments, on the basis of which we designed a new Li-Mg-N-H composite system with the reaction 8LiH + 3Mg(NH2)(2)<-> 4Li(2)NH + Mg3N2 + 8H(2). This composite materials desorbed similar to 7 mass% H-2 in the range from 120 to 200 degrees C and the H-desorption equilibrium pressure P-H2 was higher than 5 MPa at 200 degrees C, indicating that this system has an excellent potential for onboard applications. (c) 2006 Elsevier B.V. All rights reserved.
  • Hiroki Miyaoka, Takayuki Ichikawa, Shigehito Isobe, Hironobu Fujii
    PHYSICA B-CONDENSED MATTER 383 (1) 51 - 52 0921-4526 2006/08 [Refereed][Not invited]
     
    Thermodynamic and structural properties of some ball-milled mixtures composed of the hydrogenated nanostructural carbon ((CHx)-H-nano) and metal hydride (MH; M = Li, Na, Mg and Ca) were examined from thermal desoroption mass spectroscopy and powder X-ray diffraction, respectively. The results showed that the hydrogen desorption temperatures are significantly lowered from those of each hydride ((CHx)-H-nano, MH) in the composites. This indicates that a new type of interaction exists between (CHx)-H-nano and MH, which destabilizes C-H and/or M-H bonding as well. Therefore, the above Metal-C-H system would be recognized as a new family of hydrogen storage materials. (c) 2006 Elsevier B.V. All rights reserved.
  • HY Leng, T Ichikawa, S Hino, N Hanada, S Isobe, H Fujii
    JOURNAL OF POWER SOURCES 156 (2) 166 - 170 0378-7753 2006/06 [Not refereed][Not invited]
     
    The synthesis and decomposition properties of some metal amides M(NH2)(x) such as LiNH2, NaNH2, Mg(NH2)(2) and Ca(NH2)(2) were investigated, which play important roles for designing a new family of metal-N-H hydrogen storage systems. Both the gas chromatographic examination and X-ray diffraction measurement indicated that the reaction between alkali or alkaline earth metal hydride MHx (such as LiH, NaH, MgH2 and CaH2) and gaseous NH3 could quickly proceed at room temperature by ball milling and the corresponding metal amides were easily synthesized in high quality. The kinetics of these kind of reactions is faster in the order of NaH > LiH > CaH2 > MgH2, which is consistent with the inverse order of electronegativity of those metals, i.e. Na < Li = Ca < Mg. The thermal decomposition properties indicated that both Mg(NH2)(2) and Ca(NH2)(2) decomposed and emitted NH3 at lower temperature than LiNH2. (c) 2005 Elsevier B.V. All rights reserved.
  • E Gomibuchi, T Ichikawa, K Kimura, S Isobe, K Nabeta, H Fujii
    CARBON 44 (5) 983 - 988 0008-6223 2006/04 [Not refereed][Not invited]
     
    Nano-structural graphite prepared by ball milling under H-2 or Ar atmosphere was studied as an electrode for electric double layer capacitors (EDLCs) by means of a conventional 2-electrode galvanostatic method. Especially, the product prepared under H-2 atmosphere using zirconia balls revealed 500 m(2) g(-1) surface area and showed 12 F g(-1) specific capacitance, which was comparable to that of an activated carbon with large specific surface area of 3000 m(2) g(-1) examined as a reference. A proper condition of the milling time is rather a shorter time than similar to 8 h, where the graphitic feature is remained in the ball milled product. On the other hand, for the sample prepared by using steel balls, the specific capacitance per surface area was several hundreds times smaller than the others, indicating that the small amount of Fe contamination during milling played a negative role for the EDLC properties. (c) 2005 Elsevier Ltd. All rights reserved.
  • Mechanism of hydrogen desorption in Li-N-H hydrogen storage system catalyzed by titanium compounds
    Shigehito Isobe, Takayuki Ichikawa, Satoshi Hino, Hironobu Fujii
    TMS Annual Meeting 2006 207 - 218 2006 
    The Li-N-H system composed of LiH and LiNH2 catalyzed by titanium compound desorbs ∼6 mass% hydrogen and a quite small amount of ammonia in temperature range from 150 to 250°C under He flow atmosphere. In this work, hydrogen desorption (H-desorption) mechanism in the reaction from LiH+LiNH2 to Li2NH+H2 in a non-equilibrium condition was clarified by TDMS, TG and FT-IR analyses for the products replaced by LiD or LiND2 for LiH or LiNH2 respectively. The results suggested that H-desorption reaction progresses through two-step elementary reactions mediated by ammonia. As a next step, we carefully determined the amount of the ammonia containing in desorbed hydrogen for the Li-N-H system in a closed system from the practical point of view. The results indicated that a quite small of ammonia thermodynamically coexisted in the desorbed hydrogen in closed system, although ammonia could not be almost detected in emitted hydrogen gas by TDMS under some non-equilibrium conditions. Moreover, for understanding the role of titanium, the chemical bonding state of the titanium compounds doped as catalyst in the mixture was examined by XAFS measurement. The results indicate that the Ti atoms, which can induce catalytic effect, are in a particular chemical bonding state.
  • Takayuki Ichikawa, Hiroki Miyaoka, Zhong Yong, Shigehito Isobe, Hironobu Fujii
    TMS Annual Meeting 2006 67 - 76 2006 
    Hydrogen storage properties of ballmilled mixtures composed of the hydrogenated nano-structured carbon (C H ) and lithium hydride (LiH) were examined by thermal and optical spectroscopic analyses. LiH needs higher temperature than 600°C for hydrogen desorption even under vacuum condition, while C H , which was mechanochemically synthesized from graphite powder by ballmilling under hydrogen atmosphere, reveals hydrogen and hydrocarbons desorption in wide temperature range from 300 to 800°C. Nevertheless, the composite of C H and LiH prepared by ballmilling desorbs hydrogen in relatively lower temperature range from 150 to 450°C. These phenomena indicate that the thermally liberated hydrocarbon radicals immediately react with LiH and release hydrogen instead of hydrocarbons emission, because C H contains hydrocarbon groups at the edge of nano-meter sized graphenes, which are recognized as IR active C-H stretching mode, and LiH is well-known as an ionic crystal. Furthermore, it is of interest that a nanostructural feature persists in this composite even after annealing at 350°C, which shows excellent cycle properties as hydrogen storage media. nano nano nano nano x x x x
  • HY Leng, T Ichikawa, S Isobe, S Hino, N Hanada, H Fujii
    JOURNAL OF ALLOYS AND COMPOUNDS 404 443 - 447 0925-8388 2005/12 [Refereed][Not invited]
     
    Three metal amides LiNH2, NaNH2 and Mg(NH2)(2) were synthesized by ball milling the metal hydrides under gaseous ammonia NH3 at room temperature. The decomposition behaviours from these metal amides were investigated by thermal desorption mass spectroscopy and thermogravimetry analysis methods. The results showed that LiNH2 decomposed at T > 230 degrees C and was transformed into the imide Li2NH with emitting NH3, while Mg(NH2)(2), decomposed at T > 180 degrees C and was transformed into MgNH and finally into Mg3N2 with emitting NH3 within 500 degrees C. Then, a new metal-N-H system composed of Mg(NH2)(2) and LiH with a molar ratio of 3:8 was designed by ball milling treatment and examined the hydrogen storage properties. The results showed that this system could reversibly absorb/desorb a large amount of hydrogen (similar to 7 wt.%) at a moderate temperature and pressure, which was better than the system of LiNH2 and LiH for hydrigen storage. (c) 2005 Elsevier B.V. All rights reserved.
  • T Ichikawa, N Hanada, S Isobe, HY Leng, H Fujii
    JOURNAL OF ALLOYS AND COMPOUNDS 404 435 - 438 0925-8388 2005/12 [Refereed][Not invited]
     
    The Li-N-H system expressed by LiNH2 + LiH <-> Li2NH + H-2 can be expected as a promising candidate for the hydrogen storage materials because of possessing a large amount of reversible hydrogen (6.5 wt.%), a satisfactorily fast kinetics and a relatively small enthalpy change. In this work, we investigated the hydrogen storage properties of the Li-N-H system from three different points of view. Firstly, we claim that the ball milled 1: 1 mixture of lithium amide (LiNH2) and lithium hydride (LiH) containing a small amount (I mol %) of titanium chloride (TiCl3) shows superior hydrogen storage properties; a large amount of H-2 gas desorbs in the temperature range from 150 to 250 degrees C at a heating rate of 5 degrees C/n-dn and it reveals an excellent reversibility. Secondly, we clarify that the above hydrogen desorption reaction is composed of two kinds of elementary reactions: The one is that 2LiNH(2) decomposes to Li2NH and emits ammonia (NH3). The other is that the emitted NH3 reacts with LiH and transforms into LiNH2 and H-2, indicating that NH3 plays an important role on this H-2 desorption reaction. Finally, we examined the reaction of LiH and LiOH to clarify the influence of exposing the product to air. This is because due to the fact that LiOH is easily produced by exposing LiH and LiNH2 to air. The reaction between LiH and LiOH indicated better kinetics but worse durability and an extra H2 desorption due to transforming into Li2O. (c) 2005 Elsevier B.V. All rights reserved.
  • S Isobe, T Ichikawa, N Hanada, HY Leng, M Fichtner, O Fuhr, H Fujii
    JOURNAL OF ALLOYS AND COMPOUNDS 404 439 - 442 0925-8388 2005/12 [Refereed][Not invited]
     
    The effect of some different type Ti additives on kinetics of the reaction, LiH + LiNH2 <-> Li2NH + H-2, was intensively investigated in this work. The mixture of LiH and LiNH2 powders with the 1: 1 molar ratio and Ti additives with different chemical form were mechanically ball milled under a hydrogen gas atmosphere of I MPa at 400 rpm for 2 It and the measurements of thermal hydrogen desorption spectrum (TDS), thermogravimetry (TG) and X-ray diffraction (XRD) were performed. Here, we used Ti (nano particle), Ti (micro particle), TiCl3, TiO2 (nano particle) and TiO2 (micro particle) as the additives. The results indicated that the Ti-nano, TiCl3 and TiO2nano doped composites revealed a superior catalytic effect on the TDS properties, while the Ti-micro and TiO2micro did not show so good catalytic effect being similar to the sample without any additives. In the XRD profiles, there are traces of Ti and TiO2 phases in the Ti-micro and TiO2micro doped composites, respectively, whereas no trace of Ti, TiCl3 and TiO2 was found in the Ti-nano TiCl3 and TiO2nano doped composites. These results indicate that the uniform distribution of nano particle Ti metal between LiH and LiNH2 plays an important role for catalytic effect. (c) 2005 Elsevier B.V. All rights reserved.
  • T Ichikawa, S Isobe, H Fujii
    MATERIALS TRANSACTIONS 46 (8) 1757 - 1759 1345-9678 2005/08 [Not refereed][Not invited]
     
    Hydrogen desorption properties of a mixture of hydrogenated nanostructural graphite (CHx)-H-nano and lithium hydride LiH are demonstrated in this paper, where (CHx)-H-nano was synthesized from graphite by ballmilling under 1 MPa hydrogen for 80h. First of all, we clarified the hydrogenated properties of (CHx)-H-nano synthesized under four different milling conditions. The hydrogen desorption profile with typical two-peak structure was caused by iron contamination in (CHx)-H-nano from steel balls during ballmilling, while the products prepared by zirconia balls showed the broad single peak in hydrogen desorption. The amount of desorbed hydrocarbon gas from the products using a rocking (vibrating) mill estimated by the thermogravimetry was larger than that using a rotating (planetary) one. Next, the destabilization properties of extremely stable LiH was examined, indicating that LiH was destabilized by mixing with another component LiOH or NaOH, and then, the mixture easily released hydrogen gas at lower temperature compared with LiH, LiOH and NaOH themselves. On the analogy of this result, we examined hydrogen desorption properties of the ballmilled mixture of LiH and (CHx)-H-nano. The hydrogen desorption started from about 200 degrees C and showed a peak at 350 degrees C, although each product needs more than 400 degrees C to release hydrogen. Since this hydrogen storage system is specially based on lithium, carbon and hydrogen in the mixture, it can be regarded as Li-C-H hydrogen storage system.
  • S Isobe, T Ichikawa, S Hino, H Fujii
    JOURNAL OF PHYSICAL CHEMISTRY B 109 (31) 14855 - 14858 1520-6106 2005/08 [Refereed][Not invited]
     
    The hydrogen desorption mechanism in the reaction from LiH + LiNH2 to Li2NH + H-2 was examined by thermal desorption mass spectrometry, thermogravimetric analysis, and Fourier transform IR analyses for the products replaced by LiD or LiND2 for LiH or LiNH2, respectively. The results obtained indicate that the hydrogen desorption reaction proceeds through the following two-step elementary reactions mediated by ammonia: 2LiNH(2) -> Li2NH + NH3 and LiH + NH3 - LiNH2 + H-2, where hydrogen molecules are randomly formed from four equivalent hydrogen atoms in a hypothetical LiNH4 produced by the reaction between LiH and NH3 according to the laws of probability.
  • T Ichikawa, H Fujii, S Isobe, K Nabeta
    APPLIED PHYSICS LETTERS 86 (24) 241914.1-241914.3  0003-6951 2005/06 [Not refereed][Not invited]
     
    A hydrogen storage ability caused by the interaction between nanostructured carbon (CnanoHx) and lithium hydride (LiH) is demonstrated, which should be recognized as Li-C-H system in the H-storage materials. Especially, the 2: 1 mixture of CnanoHx and LiH exhibited promising hydrogen storage properties with a rechargeable hydrogen capacity of more than 4 mass% below 350 degrees C, preserving the nanostructural feature in the mixture even after hydrogen release. On the other hand, the 1: 2 and 1: 1 mixtures exhibited the crystal growth of Li2C2 after hydrogen desorption, leading to poorer hydrogen rechargeability. (c) 2005 American Institute of Physics.
  • T Ichikawa, N Hanada, S Isobe, HY Leng, F Hironobu
    MATERIALS TRANSACTIONS 46 (1) 1 - 14 1345-9678 2005/01 [Not refereed][Not invited]
     
    In this paper, we review our recent experimental results on hydrogen storage properties of light elements Li, C and Mg based nanocomposite materials. The results are summarized as follows: In the Li-N-H system, such as the ball milled 1: 1 mixture of Li amide and Li hydride containing a small amount of TiCl3 (1 mol%), a large amount of hydrogen (similar to 6 mass%) is absorbed and desorbed in the temperature range from 150 to 250 degrees C with good reversibility and high reaction rate. Furthermore, in the ball milled mixture of 3Mg(NH2)(2) and 8LiH, similar to 7 mass% of hydrogen is reversibly stored in the temperature from 140 to 220 degrees C, indicating one of the suitable hydrogen storage materials. In graphite containing a small amount of nanometer sized Fe (similar to 2at.%), a large amount of hydrogen (similar to 7 mass%) is chemisorbed by ball milling for 80 h under less than 1 MPa of H-gas pressure. However, the chemisorbed hydrogen capacity decreases with increase in the milling pressure for the 80 h ball milled graphite (down to similar to 4.1 mass% at 6 MPa), while the physisorbed hydrogen capacity in graphite increases with increase in the milling pressure, reaching up to 0.5 similar to 1.0 mass% at 6 MPa. Unfortunately, the desorption temperature of chemisorbed hydrogen is higher than 300 degrees C. Therefore, some break-through is necessary for the development of carbon-based materials as one of the hydrogen storage systems'. On the other hand, some nano-composite Mg catalyzed by Ni nano-particle or Nb oxide reveals superior reversible hydrogen storage properties: similar to 6.5 mass% of hydrogen is reversibly stored in the temperature range from 150 to 250 degrees C. Especially, the Nb metals uniformly dispersed in nanometer scale on the surface of MgH2, which was produced by reduction of Nb2O5, is the best catalyst we have studied so far. Thus, it seems that some Mg nano-composites catalyzed by nano-particles of d-electron transition metals is acceptable for practical applications.
  • J. Alloys and Compd. J. Alloys and Compd. 353, 310-314, 2003
    Ichikawa, Takayuki, Hanada, Nobuko, Isobe, Shigehito, LENG, Haiyan, FUJII, Hironobu, Nakamori, Yuko, Li, Haiwen, Miwa, Kazutoshi, TOWATA, Shin-ichi, ORIMO, Shin-ichi
    Materials transactions 46 (1) 1 - 14 2005
  • Honda R & D Technical Review Honda R & D Technical Review 14, 91, 2002
    Ichikawa, Takayuki, Hanada, Nobuko, Isobe, Shigehito, LENG, Haiyan, FUJII, Hironobu
    Materials transactions 46 (1) 1 - 14 2005
  • T Ichikawa, N Hanada, S Isobe, HY Leng, H Fujii
    JOURNAL OF PHYSICAL CHEMISTRY B 108 (23) 7887 - 7892 1520-6106 2004/06 [Refereed][Not invited]
     
    The mechanism of the hydrogen desorption (HD) reaction from the 1:1 mixture of lithium amide (LiNH2) and lithium hydride (LiH) to lithium imide (Li2NH) and hydrogen (H-2) has been proposed on the basis of our experimental results in this paper. The proposed model is constituted by 2 kinds of elementary reactions: the one is that 2LiNH(2) decomposes to Li2NH and ammonia (NH3) the other is that the emitted NH3 reacts with LiH and transforms into LiNH2 and H-2. Since the former and the latter reactions are, respectively, endothermic and exothermic, the HD reaction corresponding to the latter reaction occurs as soon as LiNH2 has decomposed into Li2NH and NH3. Therefore, the HD reaction can be understood by the following processes: at the first step, LiNH2 decomposes into Li2NH/(2) + NH3/2, and then the emitted NH3/2 quickly reacts with LiH/2, transforming into LiNH2/2 + H-2/2; at the second one, the produced LiNH2/2 decomposes to Li2NH/4 + NH3/4, and then NH3/4 + LiH/4 transform to LiNH2/4 + H-2/4, and such successive steps continue until LiNH2 and LiH completely transform into Li2NH and H-2, even at low temperatures, by the catalytic effect of TiCl3.(.)
  • T Ichikawa, DM Chen, S Isobe, E Gomibuchi, H Fujii
    MATERIALS SCIENCE AND ENGINEERING B-SOLID STATE MATERIALS FOR ADVANCED TECHNOLOGY 108 (1-2) 138 - 142 0921-5107 2004/04 [Not refereed][Not invited]
     
    We investigated hydrogen absorption/desorption and structural properties in mechanically milled graphite under hydrogen pressures up to 6 MPa to clarify catalytic and hydrogen pressure effects in the milling. The results indicate that a small amount of iron contamination during milling plays a quite important role as a catalyst for hydrogen absorption/desorption properties in graphite. Two-peak structure for hydrogen desorption in the TDS profile is due to existence of two different occupation sites for hydrogen, which is caused by the existence of high dispersing iron on graphite. From the experiment of high pressure milling, we clarified that the number of defects in graphite decreases with increasing the atmospheric hydrogen pressure during milling, leading to the decrease in the absorbed hydrogen content and the stabilization of hydrogen on the occupation sites. In addition, we confirmed the existence of physisorption-like reversible hydrogen at room temperature in the graphite prepared by milling under high pressure hydrogen atmosphere above 3 MPa. This unstable hydrogen might strongly be correlated with the existence of the lamella structure in nanometer scale, which only leaves in graphite prepared by high pressure milling. (C) 2003 Elsevier B.V. All rights reserved.
  • T Ichikawa, S Isobe, N Hanada, H Fujii
    JOURNAL OF ALLOYS AND COMPOUNDS 365 (1-2) 271 - 276 0925-8388 2004/02 [Not refereed][Not invited]
     
    In this paper, we examined the basic properties in the 1:1 mixture of lithium amide LiNH2 and lithium hydride LiH as a candidate of reversible hydrogen storage materials. The thermal desorption mass spectra of the ball milled mixture without any catalysts indicated that hydrogen H-2 is released in the temperature range from 180 to 400degreesC while emitting a considerable amount of ammonia NH3. On the other hand, the ball milled mixture containing a small amount of TiCl3 as a catalyst showed the most superior hydrogen storage properties among the 1: 1 mixtures with a small amount of catalysts, Ni, Fe, Co metals and TiCl3 (I mol.%). That is, the product desorbs a large amount of hydrogen (similar to5.5 wt.%) in the temperature from 150 to 250degreesC under the condition of a heating rate of 5 degreesC/min, but it does not desorb ammonia at all within our experimental accuracy. In addition, we confirmed that the product shows an excellent cycle retention with an effective hydrogen capacity of more than 5 wt.% and a high reaction rate until at least 3 cycles. (C) 2003 Elsevier B.V. All rights reserved.
  • S Isobe, T Ichikawa, JI Gottwald, E Gomibuchi, H Fujii
    JOURNAL OF PHYSICS AND CHEMISTRY OF SOLIDS 65 (2-3) 535 - 539 0022-3697 2004/02 [Refereed][Not invited]
     
    In this paper, we examined the catalytic effect of 3d transition metals on hydrogen storage properties in nanostructural graphite prepared by ball milling under hydrogen atmosphere. The Fe-doped nanostructured graphite shows the most marked hydrogen storage properties among the Fe-, Co-, Ni- and Cu-catalyzed graphite systems. The absorbed hydrogen concentration reaches up to similar to4 wt% by mechanically milling for 32 h (similar to7 wt% for 80 h), and two peaks of hydrogen (mass number = 2) around 730 and 1050 K were observed in the thermal desorption mass spectra (TDS). The starting temperature for hydrogen desorption was similar to600 K. On the other hand, the Co-doped graphite indicates that absorbed hydrogen concentrations reaches up to similar to2 wt% by mechanically milling for 32 h. The TDS spectrum showed only a broad peak around 1100 K, but the starting point for hydrogen desorption lowered down to similar to500 K. The Ni- and Cu-doped graphites did not show any significant improvement for hydrogen storage. These results suggest that the catalytic effect on hydrogen storage properties strongly depends on the affinity of graphite and doped metals. (C) 2003 Elsevier Ltd. All rights reserved.

MISC

Presentations

Research Projects

  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research
    Date (from‐to) : 2024/04 -2029/03 
    Author : 宮岡 裕樹, 浅野 耕太, 礒部 繁人, 圓谷 貴夫
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research on Innovative Areas (Research in a proposed research area)
    Date (from‐to) : 2021/04 -2023/03 
    Author : 礒部 繁人
     
    本研究の目的は，１族・２族元素をメインとするハイエントロピー合金を創製することである。固溶体を形成しにくい金属元素こそ，いわゆるハイエントロピー合金の概念「配置エントロピー増大によるギブスエネルギー減少化」で，合金化したいと考えた。応用先としては，次世代の軽金属材料や水素吸蔵材料などの機能性材料が考えられる。試料作製にはボールミリング法を用いた。先ず，LiMg0.5AlTi1.5に対し回転速度を400 rpmに固定し，種々のミリング時間(0～40 h)で試料を作製した。ボール対試料質量比96:1とした。また，基本組成の組成比を変化させた4元系，元素を追加した5元系の試料を，ミリング時間20 h，回転速度400 rpmで作製した。試料組成はLi，Mg，Al，Ti，Mを等原子比で混合したLiMgAlTiMとし，添加元素MにはV，Cr，Mn，Fe，Co，Ni，Cu，Zn，Ga，Ge，Y，Zrを使用した。試料作製はAr雰囲気下のグローブボックス内で行った。作製した試料に対してXRD及びSEMによる結晶構造解析及び微細組織観察を行った。また，バルク体の作製条件を検討する目的で，熱処理による相変化の有無を調査した。 4元系合金LiMgAlTiについて，ボールミリング法によりHCP単相が作製できた。5元系LiMgAlTiM，（M＝V，Cr，Mn，Fe，Co，Ni，Cu，Zn，Ga，Ge，Y，Zr，Nb）については，ボールミリング法による単相固溶体の作製には至らなかったが，Li-Mg-Al-Ti-V系合金およびLi-Mg-Al-Ti-Nb系合金ではBCC相が主相となることが判明した。
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)
    Date (from‐to) : 2020/04 -2023/03 
    Author : 宮岡 裕樹, 圓谷 貴夫, 礒部 繁人
     
    アンモニア(NH3)や窒化ガリウム(GaN)に代表される機能性窒化物は，安定な窒素分子(N2)を解離するため高温高圧条件が必要とされるのが一般的であるが，より低温低圧での合成が可能となれば，エネルギー消費を抑え，且つ低コストに窒化物を製造することができる。リチウム(Li)合金は，窒素分子を室温でも解離しうるが，その詳細については理解されていない。本研究では，このLi合金を用いた「活性窒素」生成/利用技術について，特に，窒素分子解離能発現メカニズム，及び原子拡散のダイナミクスを実験的及び理論的アプローチにより理解することを目的する。 Liと14族元素の合金について，メカノケミカル法及び熱化学法を用いて粉末及びバルク単相試料を合成した。得られたLi合金について，熱重量分析及び質量数分析装置を用いて，窒化，アンモニア合成，再生反応を実施し，各反応の特性評価を行った。生成物については，X線回折及び電子顕微鏡を用いたキャラクタリゼーションを行った。また，比較試料として，Liと同族であるNaと14族元素の合金も作製し，そのキャラクタリゼーションを行った。 Li-Sn二元合金であるLi13Sn5及びLi17Sn4について，Sn原子周りの局所構造が相安定性に与える影響に着目し，第一原理計算による研究を進めた。得られた最安定構造（原子配置）を基に，合金表面における窒素分子の解離機構とリチウム原子が脱離しLi3Nが生成される原子ダイナミクスを分子動力学で計算する手法の構築を進めた。 窒化，アンモニア合成，再生反応後のL-Sn合金バルク試料を作製し，FIB(Focused ion beam)加工を用いた断面電子顕微鏡観察及びエネルギー分散型X線分光(EDS)を用いた元素分布の解析を行った。この結果を基に，アンモニア合成プロセスにおけるLi原子の拡散ダイナミクスについて議論した。
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (B)
    Date (from‐to) : 2017/04 -2020/03 
    Author : Miyaoka Hiroki
     
    In this work, physical properties, reactivity, and reaction mechanism of lithium alloys (Li-M) are investigated to establish “active nitrogen” generation and utilization techniques. As a result, ammonia can be produced below 400℃ under ambient pressure by heat treatment of the Li-Sn alloy in order of nitrogen, hydrogen, inert gas (Ar) flow conditions. In addition, the Li-Sn alloy is also available for ammonia synthesis by conventional catalytic process using H2-N2 (3:1 molar ratio) mixed gas flow conditions. From the characterization of reaction process by microscopes, it is indicated that the reactions proceed with extraction and insertion of Li as reaction medium. Therefore, it is concluded that the Li alloy systems are recognized as a novel nitride synthesis process by the reactions based on high N2 dissociation and diffusion properties of Li.
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Young Scientists (B)
    Date (from‐to) : 2017/04 -2020/03 
    Author : ISOBE Shigehito
     
    In this study, we developed hydrogen storage materials with large capacity represented by inorganic hydrides such as Li-amide / imide. As a result, it was found that the addition of lithium titanate to Li amide / imide system improved the rate of hydrogen release reaction, and this improvement was strongly correlated with the improvement in Li ion conductivity. On the other hand, by adding lithium titanate to the alanate-based material, the hydrogen release reaction rate could be improved, but almost no correlation was found with the improvement in lithium ion conductivity. That is, it has been found that the rate of the hydrogen release reaction of the inorganic hydride is different from each other, and that the addition of lithium titanate is effective when the diffusion of lithium ions is rate-limiting.
  • Japan Society for the Promotion of Science：Grants-in-Aid for Scientific Research Grant-in-Aid for Young Scientists (B)
    Date (from‐to) : 2015/04 -2017/03 
    Author : Miyaoka Hiroki, YAMAGUCHI SHOTARO, ISOBE SHIGEHITO, ICHIKAWA TAKAYUKI, KOJIMA YOSHITSUGU
     
    In this work, novel technique of reversible nitrogen dissociation and recombination by reactions using compounds of lithium and 14 group elements is proposed. The reaction properties and mechanism are experimentally investigated. The synthesized compounds reacted gaseous nitrogen under ambient pressure below 500 ℃, especially the nitrogenation of some alloys can proceed even at room temperature. Thus, the above lithium compounds possess high reactivity for the nitrogen dissociation. It was indicated that nano-sized lithium nitride was formed as the product after the reaction with nitrogen. By using the active nitride generated due to the nitrogenation, ammonia can be synthesized below 300 ℃ under 0.5 MPa.
  • 日本学術振興会：科学研究費助成事業 特別研究員奨励費
    Date (from‐to) : 2006 -2007 
    Author : 礒部 繁人
     
    水素エネルギー社会の実現を目指して、燃料電池用の水素貯蔵物質の研究が急がれている。数ある貯蔵物質の候補のうち、現在最も有望視されている物質としてM-N-H系物質(M=Li, Na, Mg, Ca)がある。この物質群は、左記アルカリ金属およびアルカリ土類金属のアミドと水素化物の複合体により構成されており、金属原子(M)、窒素原子(N)および水素原子(H)にて系が特徴付けられるために、M-N-H系物質と総称されている。この系は、その水素吸放出反応機構が明らかにされていない。その水素吸放出反応機構を理解するために,触媒作用に関する調査を行った。前年度,高輝度放射光(@SPring8)のXAS(X線吸収分光)法でLi-N-H系水素貯蔵物質中の添加物の化学状態分析を行い,触媒作用を示すTiの電子状態が一意的であるということ(XANES:X線吸収端近傍構造)、その一意性が局所構造には見られないこと(EXAFS:広域X線吸収微細構造)、が知見として得られた。本年度,その一意的な電子状態が塩化チタンアンミン錯体(Ti(NH_3)_5Cl_3)のそれと非常によく一致することが明らかになった。Ti(NH_3)_5Cl_3はアンモニアを容易に吸放出する性質を有する。これは,この反応機構においてアンモニアが仲介的な役割として存在することを裏付ける。この知見は,触媒機構解明に繋がるだけでなく,本系の反応機構解明に向けた大きな成果である。

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