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Symposium S: Materials and Technology for Hydrogen Storage

SYMPOSIUM S

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S: Materials and Technology for Hydrogen Storage

November 26 - 28, 2007

Chairs

Gholam-Abbas Nazri General Motors R&D Centre MC 480-102-RCEL 30500 Mound Rd. Warren, MI 48090-9055 586-986-0737		Chen Ping Physics Dept. National University of Singapore 10 Kent Ridge Crescent Singapore, 119260 Singapore 65-874-2982
Aline Rougier Laboratoire de Reactivite et Chimie des Solides 33 Rus St. Leu Amiens, F-80039 France 33-3-2282-7604		Azarnoush Hosseinmardi Liebenauer Hauptstr. 317 Magna Steyr Graz, A-8041 Austria 43-316-404-4219

Proceedings to be published online
(see Proceedings Library at www.mrs.org/publications_library)
as volume 1042E
of the Materials Research Society
Symposium Proceedings Series.
This volume may be published in print format after the meeting.

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* Invited paper

SESSION S1: Metal Hydride Hydrogen Storage I
Chairs: Gholam-Abbas Nazri and Aline Rougier
Monday Morning, November 26, 2007
Room 309 (Hynes)
8:30 AM *S1.1
Crystal Structures and Hydrogenation Behaviors of the RM_n (3≤n<5) ''Superlattice” Alloys. Etsuo Akiba, Y. J Chai, Jin Nakamura, Hirotoshi Enoki, Kouji Sakaki, Kohta Asano and Yumiko Nakamura; ETRI, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, Japan.

RM_n (3≤n<5) (R= rare earth, M=transition metal) type intermetallic compounds with various stacking structures were reported by Kadir et al. [1]. Kohno et al. reported that (La, Mg)(Ni, Co)_n (n=3.0-4.0) alloys have higher discharge capacity and significant rate capability than conventional AB5-type alloys [2]. This type of intermetallic compounds consists of [AB5] and [A2B4] layers. Both AB5 and AB2 Laves phase alloys are the most popular hydrogen absorbing alloys. Today, this type of alloy that was named as the “superlattice alloy” by SANYO is used for the negative electrode of commercially available Ni-hydrogen batteries. In this study, the crystal structures of La₄MgNi₁₉ and its full hydride were refined using powder neutron diffraction. The metallic sublattice kept the same structure with hydrogenation and dehydrogenation. Mg selectively occupied the La site in the [A2B4] layer same as we have already reported using single crystal analysis [3]. The structural change for La_0.7Mg)_0.3Ni_2.8Co_0.5 with hydrogenation and dehydrogenation was measured using in-situ X-ray diffraction. The alloy is composed of more than 80 % of Ce₂Ni₇-type La₃Mg(Ni,Co)₁₄ and other minor phases of PuNi₃-type La₂Mg(Ni,Co)₉, Pr₅Co₁₉-type La₄Mg(Ni,Co)₁₉ and LaNi₅-type phase. Metal sublattice of the Ce₂Ni₇-type phase was stable with hydrogenation and dehydrogenation up to H/M = 1.0. The lattice expansion was isotropic and volumetric expansion with hydrogenation was 22.7%. [1] K. Kadir et al. J. Alloys Comp., 257, 115 (1997). [2] T. Kohno et al., J. Alloys Comp., 311, L5 (2000). [3] H. Hayakawa et al., Meter. Trans., 46, 1393 (2005).


9:00 AM S1.2
Phase Transformation and Structural Properties of La(Ni_5-xCo_x) Hydrides. Yumiko Nakamura and Etsuo Akiba; ETRI, AIST, Tsukuba, Ibaraki, Japan.

La(Ni_5-xCo_x) have a CaCu₅ structure for any ratio of Ni and Co, while Ni-rich (x ≦ 1) and Co-rich (x ≧ 2) compositions take different phase transformation upon hydrogenation [1]. In particular, the alloys for x = 2 and 3 form four hydrides subsequently: α (hexagonal), β (orthorhombic), γ (orthorhombic), and δ (hexagonal). The first three are similar to hydrides of LaCo₅. We found the δ phase unique to x = 2, 3 using in situ X-ray diffraction [1]. In this study, the crystal structure of δ-La(Co₃Ni₂)D₆ was investigated using powder neutron diffraction. The data was collected under 3 MPa of D₂ on the 90-deg banks of Sirius at KENS (Tsukuba, Japan). Hydrogen occupation and lattice expansion are discussed in comparison with other hexagonal La(Ni, M)₅ hydrides. [1] Y. Nakamura, T. Nomiyama, E. Akiba, J. Alloys Compd., 413 (2006) 54.


9:15 AM S1.3
A Numerical Analysis of the Influence of the Effective Thermal Conductivity on the Cooling Mechanisms of Ti-Cr-V(-Fe) Solid Solution Metal Hydride Beds in Hydrogen Storage Vessels. Sang-kun Oh¹, Kyung-Woo Yi¹ and Sung-Wook Choi²; ¹Materials Science and Engineering, Seoul National University, Seoul, South Korea; ²Minerals & Materials Processing Research, Korea Institute of Geoscience & Mineral Resources, Daejeon, South Korea.

Storage of hydrogen in metal hydride alloys is a promising technology which offers high potential storage density in a safe solid state. A major factor to be considered for the use of these alloys in storage applications is the significant amount of heat which is released during the exothermic hydrogen absorption process, as well as the consequent need for effective cooling methods. Various difficulties are involved in large-scale experimentation of these phenomena in storage vessels. Therefore numerical analysis of simulation models is a more practical and effective approach. The thermal conductivity of a metal hydride bed has a major impact on its thermal profile during the hydrogen absorption process. Because of the complex nature of the metal hydride powders, an accurate simulation model requires the use of an effective thermal conductivity value (K_ef) which reflects the conductivity of the bed as a whole. This value takes into account both the metal hydride material itself and the gas occupying the spaces between the powder particles. In this study, we incorporate experimentally obtained K_ef values for Ti-Cr-V and Ti-Cr-V-Fe solid solutions into numerical model simulations to predict the heating mechanisms and temperature profiles for various bed formations of these alloys. The results are then used as a reference to define appropriate parameters for simulation models which take the hydride powder and gas elements into account separately, in order to treat the metal hydride beds more accurately as porous media. Based on the resulting thermal profiles, a number of possible cooling facilitation methods and their effects on the alloy bed are investigated through additional numerical simulations. We were able to conclude through these simulations that higher effective conductivities result in significantly enhanced cooling behavior for most configurations of Ti-Cr-V and Ti-Cr-V-Fe beds. Consequently, a number of methods are proposed to increase the effective thermal conductivity K_ef, and thereby facilitate the cooling of the beds.


9:30 AM S1.4
Direct Production of the Ni-Ti-Zr Icosahedral Phase for Hydrogen-storage Applications by Rapid Quenching from the Melt. Andraz Kocjan, Paul McGuiness, Aleksander Recnik and Spomenka Kobe; Department for nanostructured materials, Joseph Stefan Institute, Ljubljana, Slovenia.

Our study focused on the formation of Ti40Zr40Ni20 icosahedral (i-phase) quasicrystals directly from the melt and their subsequent characterization and high-pressure hydrogenation. The samples were produced in an inert-gas melt-spinning device from a series of arc-melted precursor buttons of approximately 9 grams. The buttons were melted at 1400°C in a boron-nitride crucible with a 1.5-mm nozzle so as to produce broad melt-spun ribbons at various wheel speeds. By varying the wheel speeds we were able to produce a range of crystallographic structures, from amorphous, through quasicrystalline, to crystalline. The structures were crystalline at 12 m/s, quasicrystalline at 22 m/s and amorphous above 28 m/s. Initially, we studied the dependence of ribbon thickness (d) and saturation magnetization (Ms) on the wheel speed. The ribbon thickness showed the expected exponential reduction with wheel speed, from 120 microns to 30 microns, while the Ms exhibited a surprisingly modest change with wheel speed; varying between 0.03 and 0.02 emu/g. The XRD analysis showed that depending on the cooling rate (controlled by the wheel speed) it was possible to freeze the icosahedral phase directly from the melt, without the need for any subsequent heat-treatment steps, at 22 m/s. We believe this represents the first report of such direct formation of the icosahedral phase for this material. Using the same procedure to test the range of formation wheel speeds for a comparable system we also produced the samples containing up to 3 atomic % copper. Using transmission electron microscopy we have confirmed that the ribbons contain nanosized particles of Ti40Zr40Ni20 icosahedral phase imbedded in an amorphous matrix with same composition. The average particle size of the i-phase was approximately 20 nm. The 5-fold symmetry was confirmed by selected-area electron diffraction and high-resolution TEM having the crystallite oriented close to the symmetry axis. Both our XRD measurements and the TEM observations have provided direct evidence for the quasicrystalline ordering of Ti40Zr40Ni20 by rapid quenching from melt. To test the hydrogen-absorption properties of the icosahedral phase we crushed ribbons into finer particles to provide fresh, new surfaces to aid hydrogen dissociation at the metal surface. The uptake of hydrogen was found to be critically dependent on the surface state of the Ni-Ti-Zr ribbons; even modest exposure to the atmosphere produced a protective layer of oxide on the surface that practically prevented any hydrogen being taken up by the icosahedral phase. However, when the ribbons were crushed in a protective argon atmosphere it became possible to 95% charge the phase with hydrogen. The extent of hydrogen charging was determined from the shift in the XRD peaks, and we observed a near-linear dependence between the [H]/[M] ratio and the quasi-lattice constant aq. The calculated expansion of aq was 6% and the corresponding [H]/[M] value was 1.5.


9:45 AM S1.5
Improving the Packing Density of Adsorbed Hydrogen and a Novel Configuration of Hydrogen Molecules in MOF-74. Yun Liu^1,2, Houria Kabbour³, Craig M. Brown^1,4, Dan A. Neumann¹ and Channing C. Ahn³; ¹Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland; ²Department of Materials Science and Engineering, University of Maryland, College Park, Maryland; ³Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California; ⁴Indiana University Cyclotron Facility, Indiana University, Bloomington, Indiana.

Metal-organic frameworks (MOFs) are promising hydrogen storage candidates due to their high surface areas. To achieve technologically relevant levels, the hydrogen packing density as well as the total available surface area are critical metrics. MOF-74 is used as a model material showing significant packing density. The hydrogen adsorption sites in MOF-74 are identified for the first time using neutron powder diffraction and a shorter H2-H2 interaction distance is observed than is expected given the nearest neighbor distance of solid H2. Of significance is the presence of coordinatively unsaturated metal centers. Inelastic neutron scattering spectra measured at different temperatures reveal large binding energy differences between some adsorption sites. Moreover, the adsorbed hydrogen molecules form a one-dimensional nanotube-like structure. These results extend our insights into physisorbed hydrogen molecules in micro-porous media including MOFs, carbon nanotubes/nanohorns and amorphous carbons.


10:30 AM S1.6
Abstract Withdrawn

10:45 AM S1.7
Positron Lifetime Dtudy of the Lattice Defect Formation by Hydrogenation in Ti-based BCC Alloys. Kouji Sakaki¹, Kenji Iwase¹, Yumiko Nakamura¹, Yasuharu Shirai² and Etsuo Akiba¹; ¹Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan; ²Department of Materials Science and Engineering, Osaka University, Suita, Japan.

Ti-based BCC alloys developed by Iba and Akiba in 1995 [1] have twice larger gravimetric hydrogen capacity than that in conventional hydrogen storage alloys such as AB5 and AB2 type alloys. Ti-based BCC alloys are one of the candidates for hydrogen storage media in fuel cell vehicles. Although these have higher gravimetric density of hydrogen, the decrease of capacity with cycles should be improved for the practical use. Our previous results for AB5 alloys suggested that the concentration of the lattice defects introduced by hydrogenation is close related with the cyclability [2]. In this study, we investigated the introduction of the lattice defects in Ti45Cr55-xMox (x=15, 30) alloys and their thermal stability by the positron lifetime measurement. In-situ positron lifetime measurement showed that lattice defects started to be introduced even in the hydrogen solid solution region and they remained after dehydrogenation. Further hydrogenation to hydride-phase region increased the concentration of lattice defect. The annealing experiments showed that introduced lattice defects were vacancy and dislocation. The onset temperatures of their migration were 573 and 1173 K, respectively. They were completely recovered around 773 and 1573K, respectively. The release behavior of the residual hydrogen that is settled down in the activated sample even after evacuation was investigated by TG-DTA measurement in Ti45Cr25Mo30. The release of the residual hydrogen started around 550 K and then was completed below 773 K. This temperature region well agrees with the vacancy migration temperature. It suggests that the vacancy introduced by hydrogenation is coupled with the residual hydrogen. Similar result has been reported in LaNi5 alloy [3]. We found the reversible phenomenon of vacancy introduction and recovery during hydrogenation and dehydrogenation in LaNi4.93Sn0.27 [2]. If we appended this reversible property into Ti-based BCC alloys, we could dramatically improve the hydrogen storage capacity and degradation behaviors of Ti-based BCC alloys. [1] H. Iba and E. Akiba: J. Alloys Compd. 231 (1995) 508. [2] Kouji Sakaki, Ryosuke Date, Masataka Mizuno, Hideki Araki, Yumiko Nakamura, Yasuharu Shirai, Robert C. Bowman, Jr. and Etsuo Akiba: to be published. [3] K. Sakaki, H. Araki and Y. Shirai: Mater. Trans. 43 (2002) 1494.


11:00 AM S1.8
Effects of Carbon Nanostructures and AB2 Alloy Additives on Hydrogen Storage Properties of Magnesium. Sundara Ramaprabhu and Srinivas G Srinivas; Indian Institute of Technology Madras, Chennai, India.

Mg-based hydrogen storage alloys have attracted great attention as potential hydrogen storage materials for fuel cell vehicles and metal hydride (MH) electrode materials for Ni/MH battery because of their extremely high hydrogen storage capacity, high abundance and low cost. However, the high operating temperature and sluggish hydrogen absorption and desorption kinetics still remains an obstacle to practical use. There have been extensive efforts aiming at overcoming these drawbacks and developing magnesium as a viable hydrogen storage medium. A commonly used method is to prepare composite materials by mechanically milling host Mg with various alloys and/or elements with potential catalytic function. Recently, Mg/Carbon allotrope composite system has drawn considerable interests following the observation that the hydrogen storage performance of Mg could be improved by adding graphite [1-7]. In this work, the hydrogen storage performance of ball-milled Mg with different carbon nanostructures and Zr based AB2 alloy as additives have been investigated. Magnesium powder milled with three types of carbon nanostructures: single, multi-walled carbon nanotubes and carbon nanocoils respectively as well as AB2 alloy under hydrogen atmosphere. X-ray diffraction (XRD) and scanning electron microscopy (SEM) has been used for structural and morphological studies of these composites. The changes in the nature of carbon-carbon bonding have been observed using Raman spectroscopy. The hydrogen absorption-desorption storage capacity and kinetic properties have been carried out using Sieverts-type apparatus. The improvement in the hydrogen absorption-desorption kinetics of magnesium by these co-additives and the reduction in the absorption-desorption operating temperature have been discussed. References 1. G. Liang, J. Huot, S. Boily, A. Van Neste, R. Schulz, J. Alloys Compd., 292 (1999) 247. 2. A. L. Yonkeu, I. P. Swainson, J. Dufour and J. Huot A. L. J. Alloys Compd., In Press. 3. L. Xie, Y. Liu, Y.T. Wang, J. Zheng and X.G. Li, Acta Materialia, In Press. 4. Ye Luo, Ping Wang, Lai-Peng Ma and Hui-Ming Cheng, Scripta Materialia, 56 (2007) 765. 5. C.Z. Wu, P. Wang, X. Yao, C. Liu, D.M. Chen, G.Q. Lu and H.M. Cheng, J. Alloys Compounds, 414 (2006) 259. 6. Z.G. Huang, Z.P. Guo, A. Calka, D. Wexler, J. Wu, P.H.L. Notten and H.K. Liu, Materials Science and Engineering: A, 447 (2007) 180. 7. Z.G. Huang, Z.P. Guo, A. Calka, D. Wexler and H.K. Liu, J. Alloys Compounds, 427 (2007) 94.


11:15 AM S1.9
Carbon Nanotube/Zr-based Alloy Nanocomposites as Electrode Material for Nickel-metal Hydride Batteries. Sundara Ramaprabhu and G. Srinivas Srinivas; Indian Institute of Technology Madras, Chennai, India.

Nickel-metal hydride (Ni-MH) batteries using hydrogen storage alloys as the negative electrode have been developed and commercialized to meet strong market demand for a power source with high energy rate capability, long cycle life and better environmental compatibility. Zr-based Laves phase hydrogen-storage alloys are promising electrode materials for Ni-MH batteries due to their much higher discharge capacities [1]. However, most of these alloys have low surface-reaction kinetics. The surface catalytic activity of the Zr-based alloys has been improved by chemical modification of the surface, such as the F-treatment and the ball milling process [2,3]. The surface modification by ball milling with different additives was found to be an effective method for improving the electrode properties of Laves phase AB2-type alloys [4,5]. In this work, to develop a hydrogen storage alloy with higher discharge capacity, rate capability and long cycle life, nanocrystalline Zr based AB2 alloy surfaces has been modified with carbon nanotubes (CNTs) by ball milling. The nanocomposites were prepared by ball milling the mixture of powders of ZrCrFe0.5Co0.5 alloy and CNTs. The effects of addition of CNTs on the discharge capacity and cycle life of these electrodes have been studied and discussed based on the higher specific surface area of CNTs and reduction in the degradation of alloy. References 1. D. M. Kim, S. M. Lee, J. H. Jung, K. J. Jang and J. Y. Lee, J. Electrochem. Soc., 145 (1998) 93. 2. X. P. Gao, W. Zhang, H. B. Yang, D. Y. Song, Y. S. Zhang, Z. X. Zhou and P. W. Shen, J. Alloy Compd., 235, (1996) 225. 3. D. Sun, M. Latroche and A. Percheron-Guegan, J. Alloy Compd., 257 (1997) 302. 4. X. B. Yu, T. Dou, Z. Wu, B. J. Xia and J. Shen, Nanotechnology, 17 (2006) 268. 5. S. Bouaricha, J. P. Dodelet, D. Guay, J. Huot and R. Schulz, J. Alloy Compd., 325 (2001) 245.


11:30 AM S1.10
Abstract Withdrawn


11:45 AM S1.11
Investigation of Hydrogen Storage Using Combinatorial Thin Films and IR Imaging. Hiroyuki Oguchi¹, Ichiro Takeuchi¹, Daniel Josell², Edwin J. Heilweil² and Leonid A. Bendersky²; ¹Department of Materials Science and Engineering, University of Maryland, College Park, Maryland; ²National Institute of Standards and Technology, Gaithersburg, Maryland.

Hydrogen storage is one of the stumbling blocks for the practical use of hydrogen as a new source of energy for transportation. Absorption and desorption of hydrogen by a storage material depends on the materials precise composition and microstructural state. Combinatorial thin films with continuously changing composition provide an opportunity for studying a wide range of compositions and microstructures (amorphous, nanocrystalline, single crystal, multiphases) on a single substrate. Here we report preparation, characterization and hydrogenation of MgxNi1-x thin films with a Pd overlayer. Mg-rich MgxNi1-x, has good gravimetric density of hydrogen. Capping the film with a Pd layer is known to both prevent oxidization and catalyze hydrogenation reaction. These 200 nm-thick, square-shape films have compositional variation in one in-plane direction and variation in Pd thickness (from 0 to 20 nm) in the other direction. Therefore rapid study of hydrogen absorption/desorption properties of various compositions with various thicknesses of Pd catalytic layer is possible in just one experiment. Structures of the films were characterized by scanning x-ray and cross-sectional TEM. Hydrogen absorption/desorption of the films was monitored with an infrared (IR) CCD camera that could image the full area of the substrate. The observed changes in infrared intensity were attributed to changing electronic states of the MgxNi1-x thin films upon accommodation of hydrogen. In the study reaction temperature and kinetics dependence on the alloy composition and Pd layer thickness were obtained.


SESSION S2: Metal Hydride Hydrogen Storage II
Chairs: Ping Chen and Azarnoush Hoseinmardi
Monday Afternoon, November 26, 2007
Room 309 (Hynes)
1:30 PM S2.1
The Effect of Specific Surface Area and Chemistry of Inco Nano Ni on the Improvement of Hydrogen Storage Properties of MgH2. Robert A. Varin^1,2,3, Tom Czujko^1,2,3, Zbigniew S. Wronski^1,2,3 and Eric B. Wasmund^1,2,3; ¹Mechanical and Mechatronics Engineering, University of Waterloo, Waterloo, Ontario, Canada; ²CANMET Energy Technology Centre, Natural Resources Canada, Ottawa, Ontario, Canada; ³INCO Special Products, Mississauga, Ontario, Canada.

It is now well established that nanostructuring of magnesium hydride (MgH2) usually achieved by mechanical (ball) milling improves its hydrogen absorption/desorption properties. Further improvement of hydrogen storage properties of MgH2 can be obtained by catalytic additives one of them being elemental nickel (Ni). In particular, catalytic additives which have nanometric sizes are the most effective for the improvement of hydrogen storage properties of various hydrides. Inco Special Products has synthesized a number of experimental batches of nanometric Ni (nano Ni) powder using a modified nickel carbonyl decomposition process. The obtained nano Ni powders are characterized by increasing Specific Surface Area (SSA in m2/g) and varying concentrations of carbon and oxygen. The objective of this work was to examine in a systematic manner the catalytic effect of Inco nano Ni additive with increasing SSA and various concentrations of carbon and oxygen on the hydrogen desorption properties of nanostructured MgH2 synthesized by ball milling in a magneto-mill. The investigated Ni powders had SSA ranging from 0.7 to 85 m2/g and concentrations of carbon and oxygen within the range of 0.34-2.97 and 0.051-11.7wt.%, respectively. The morphology of powders could be divided into two groups: spherical (discreet) and filamentary. A mixture of MgH2 powder (98% purity; the remaining Mg) with 5wt.% of Ni additive was milled 15 min in the magneto-mill Uni-Ball-Mill 5 under hydrogen gas atmosphere. High Resolution Scanning Electron Microscopy of milled mixtures showed at 20,000x magnification, a very uniform distribution of nano-Ni particles while at 100,000x magnification, the particles are observed to be locally agglomerated at the contact points between MgH2 particles whose average size is reduced from the as received 40μm to about 1μm after 15min of milling. XRD shows the presence of nanosized β-MgH2, γ-MgH2 and Ni, having grain sizes on the order of 30-40, 20-30 and 10-30nm, respectively. No alloying of nano Ni with Mg to form Mg2Ni is observed. Differential Scanning Calorimetry (DSC) shows a dramatic decrease of onset and peak temperatures of hydrogen desorption from MgH2 by nano Ni with SSA up to about 14-18m2/g. Further increase of SSA of nano Ni does not affect the hydrogen desorption temperature of MgH2 any longer. The rate constant, k, in the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation, which characterizes the kinetics of hydrogen desorption, dramatically increases with SSA up to about 14-18m2/g and then saturates becoming practically independent of increasing SSA of nano Ni catalyst. No apparent dependence of hydrogen desorption temperature and kinetics on the morphology of nano Ni and the concentration of carbon and oxygen has been observed. The hydrogen storage properties of MgH2 doped with Inco nano Ni are discussed in relation to its microstructure obtained by very short high-energy ball milling.


1:45 PM S2.2
Hydrogen Storage in Mg-2 wt.% Multiwall Carbon Nanotubes Composite Processed by Equal Channel Angular Pressing. Eugen Rabkin¹, Vladimir Skripnyuk¹, Yuri Estrin^2,3, Leonid Bendersky⁴, Arnaud Magrez⁵ and Efrain Carreno-Morelli⁶; ¹Technion, Haifa, Israel; ²Monash University, Clayton, Victoria, Australia; ³CSIRO Division of Manufacturing and Materials Technology, Clayton, Victoria, Australia; ⁴National Institute of Standards and Technology, Gaithersburg, Maryland; ⁵Swiss Federal Institute of Technology, Lausanne, Switzerland; ⁶University of Applied Sciences of Western Switzerland, Sion, Switzerland.

A Mg-based composite material containing 2 wt.% of multiwall carbon nanotubes (MWCNT) was synthesized by hot isostatic pressing of a mixture of Mg powder with MWCNT. In addition, the as-sintered composite was processed by one or two passes of equal channel angular pressing (ECAP). The hydrogen storage properties of the prepared materials were determined by volumetric method and compared with those of pure Mg. It was found that addition of MWCNT to Mg eliminates the pressure hysteresis and increases the slope of the pressure plateau in the "pressure-composition" isotherms measured at 573 K. The kinetics of hydrogen desorption is also significantly enhanced. The hydrogen desorption pressure in the middle of plateau region for the as-sintered composite is by about 50% higher than that for pure Mg. Surprisingly, ECAP processing of the as-sintered composite lowers hydrogen desorption pressure, although it still remains higher than for pure Mg. This is in contrast with the results of previous studies in which it was shown that ECAP processing of Mg-based alloys increases hydrogen desorption pressure. Transmission electron microscopy observations of the ECAP processed composite indicate that the changes in morphology of Mg-MWCNT interface upon ECAP may be responsible for the deterioration of hydrogenation properties.


2:00 PM S2.3
MgH2 - Aerogel Composite for Hydrogen Storage. Yanjia Zuo¹, Chunwei Wu¹, Sam Mao² and Taofang Zeng¹; ¹Mechanical Engineering, North Carolina State Univ., Raleigh, North Carolina; ²Berkeley National Laboratory, Berkeley, California.

We have been developing a method for hydrogen storage by combining chemisorption and physorption in nanocomposite materials. Magnesium hydrides have been successfully incorporated in silica aerogels, which has high specific surface areas (600-1000 m2/g), large pore volumes (1-4 cm3/g), and nano-pore radius (around 10 nm). In this study, we further incorporate magnesium hydrides into other nanoporous materials with doped catalysts. BET shows that MgH2/aerogel still has a surface area of 877.9m2/g and pore size of 6.36 nm. Low decomposition temperature (260C0) and promising capacity of hydrogen were also confirmed from XRD and DSC.


2:15 PM S2.4
Insights from First Principles Simulation into the Mechanisms of Catalysed Hydrogen Release from MgH₂ Surfaces via Nickel Doping and Ammonia-Borane Adsorption. Stephen Shevlin and Zheng Xiao Guo; Materials, Queen Mary, University of London, London, United Kingdom.

Energy consumption intimately linked to CO₂ emission, a major human contributor to climate change and thus currently of major worldwide concern. In order to combat this a switchover to cleaner fuels, of which hydrogen is a prime alternative, must be made. However in order to store hydrogen safely solid-state materials must be used. Magnesium hydride has long being considered as a prospective hydrogen storage material due to it’s cost, gravimetric, and volumetric properties, but to date applications have been limited because of poor thermodynamic properties leading to high temperature hydrogen release. Catalysis can aid this. In this talk we will present the results of first principles ab initio simulations on two different types of catalyst acting on magnesium hydride: transition metal dopants and molecules that possess hydrogen species that are electron acceptors. Transition metal dopants are well known to catalyse hydrogen release, while molecules that possess hydrogen electron acceptors should dehydrogenate metal hydrides that store hydrogen in electron donor form. The energetics of nickel substitution into bulk MgH₂ and onto different surfaces of MgH₂, and the effects of these dopants on hydrogen removal energies and the dehydrogenated structure will be presented. NiH_x cluster formation is observed directly following Ni substitution, suggesting that in experimental systems solid solution (Ni, Mg)H_x is formed. The diffusion rates of hydrogen diffusion throughout bulk MgH₂ and Ni-doped MgH₂ will be calculated and compared. Additionally we present results on the chemisorption of the prospective hydrogen storage material ammonia-borane (H₃BNH₃) onto the (100) and (110) surfaces of MgH₂. We find that the chemisorption of this molecule on the (100) surface is weak whereas on the (110) surface it is strong. Additionally this molecule is unstable with respect to dissociation into BH₃ and NH₃ components, which are thermodynamically preferred over dissociation into H₂BNH₂+H₂ form. These separate molecular components then attack the MgH₂ surface, strongly reducing the removal energy for hydrogen with respect to the removal energy for ideal MgH₂ and allowing hydrogen release at lower temperatures. The effects of finite temperature and the rate-limiting step for these dissociation reactions will be discussed.


2:30 PM S2.5
Composition Dependence of Electrical Resistivity of Magnesium-cobalt Films During Hydridation and Dehydridation. Yiu Bun Chan and Chung Wo Ong; Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic University, Hong Kong, China.

Magnesium-transition metal alloys are interested because of their ability of storing and releasing hydrogen. The reactions involve reversible transitions between a metallic state and a hydride state, which are accompanied by drastic changes in electrical conductivity and optical transmission. Thin films of these materials are further investigated because of the potential of being used in switchable mirrors, hydrogen sensors, and many other conceived applications. In this study, we focused on the Mg-cobalt (Co) thin film system. Film samples were prepared by co-sputtering a Mg target and a Co target. The ratio of the sputtering powers was set such that films of various compositions covering a broad range were fabricated. Each film was covered with a palladium (Pd) overcoat with a thickness less than 10 nm to protect the film from oxidation. The film structure was investigated by using X-ray diffraction (XRD). Except the films containing predominantly one metal, the diffraction patterns of Mg-Co films with other compositions in between only showed broad halos. This suggests that the structure of Mg-Co films is highly disordered and not isostructural to any known stochiometric Mg-Co compounds. X-ray photoelectron spectroscopy (XPS) analysis provided information on the depth profiles of elemental composition. Pd was only detected at shallow depths. Conspicuous amount of oxygen was found in the Pd layer, and started to drop from the Pd/Mg-Co interface with increasing depth. Mg showed a peak value near the Pd/Mg-Co interface and a coherent drop with oxygen content along depth. This leads one to suggest that oxygen atoms can penetrate through the Pd layer and react with Mg, inducing more Mg atoms to diffuse towards the top surface. At deeper regions, the Mg:Co ratio approached some stable value in accordance with the power ratio used to prepare the sample. Prolonged exposure to air resulted in significant oxidation, as reflected by XPS data and the appearance of pits on the film surface. The change of electrical conductivity (σ) of a sample in a small chamber was continuously monitored during hydridation and dehydridation at room temperature. Each cycle consisted of exposure to 15% hydrogen in argon at 10⁵ Pa (10 min), followed by evacuation to rough vacuum and then exposure to air (10 min). In general, the value of σ of a film with a higher Mg content showed a stronger drop in the hydridation process, but the response times for both hydridation and dehydridation processes were longer, such that the range of the change in σ in the subsequent cycles appeared to be smaller.


3:30 PM *S2.7
A Study of Hydrogen Absorption in Magnesium. Fereshteh Ebrahimi¹, Mahesh Tanniru¹, Ki-Joon Jeon² and Chang-Yu Wu²; ¹Materials Science and Engineering, University of Florida, Gainesville, Florida; ²Department of Environmental Engineering Sciences, University of Florida, Gainesville, Florida.

The kinetics of hydride formation during the hydrogenation of a Mg powder coated with nano-size nickel catalyst was investigated. The caoting was performed using a dry system (Theta Composer, Tokuju Corp.). Hydrogen absorption experiments were conducted using a custom-made hydrogenation system. TEM analysis of a sample prepared using FIB technique revealed that the hydride formation took place in a zone near the surface of the magnesium particle. The large volume expansion due to the hydride formation caused microcracking along the grain and twin boundaries of the magnesium powder and hence revealed the reacted layer. While XRD and TGA tests confirmed the presence of magnesium hydride, none was found in the thin foil. However, nanocrystalline magnesium was observed in the reacted zone. Consistent with previously reported studies, it is believed that the instability of hydride under the electron beam caused the release of hydrogen and resulted in the development of an extremely fine microstructure. The analysis of hydrogenation results revealed two distinct stages. In this presentation, the processes of nucleation and growth of magnesium hydride and their correlation with the kinetics of hydrogen absorption will be discussed.


S2.8
Abstract Withdrawn


4:00 PM S2.9
Hydrogen Sorption Properties and the Microstructure of Mg-Al Alloys and of MgH2 - Carbon Nanotube Composites. David Mitlin¹, Julian Haagsma¹, Mohsen Danaie¹, Babak Shalchi¹, Erik Luber¹, Colin Ophus¹, Helmut Fritzsche³, Velimir Radmilovic² and Ulrich Dahmen²; ¹Chemical And Materials Engineering, University of Alberta and NINT, Edmonton, Alberta, Canada; ²NCEM, Lawrence Berkeley National Laboratory, University of California, Berkeley, California; ³Canadian Neutron Beam Centre, Chalk River Laboratories, Chalk River, Ontario, Canada.

This study focuses on Mg-Al alloy thin films, and on high-energy ball milled magnesium hydride powder-carbon nanotube (CNT) composites, as potential candidates for low temperature hydrogen storage. To characterize the sorption properties of these materials we employed gravimetric methods, combined with differential scanning calorimetry and neutron reflectometry. In parallel, we explored the materials’ microstructure in detail, through the use of transmission electron microscopy (TEM) and x-ray diffraction (XRD). We found that we can create metastable Mg-Al alloys that are either supersaturated solid solutions, or amorphous-nanocrystalline composites by room temperature co-sputtering. These materials show tremendously improved absorption and desorption, with appreciable (4.5 - 6wt.%) hydrogen capacity that can be readily charged and discharged at temperatures as low as 150°C. Neutron reflectometry demonstrated that after sorption, the hydrogen is uniformly dispersed throughout the films, and does not segregate to either the film-substrate nor the film-Pd overlayer interface. We also explored the microstructural stability of these alloys during sorption cycling. When testing the hydride powders, we observed that the desorption rate was significantly enhanced by the addition of mixed carbon nanotubes with the catalytic nanoparticles still attached. We also tested amorphous carbon and catalyst particles mixed with the magnesium hydride in the same weight ratio. This addition caused very little improvement in the desorption kinetics, indicating that there is something unique about the CNT-catalyst combination. Combined TEM and XRD analysis of the milled hydride powders indicated that much of the enhanced kinetics due to milling treatment was caused by the introduction of defects, rather than the reduction in the particle or the grain size.


4:15 PM S2.10
Hydrogen Desorption from Pd Capped Mg Based Switchable Mirrors. Erdni Batyrev, Ruud Westerwaal, Martin Slaman, Bernard Dam and Ronald Griessen; Department of Physics and Astronomy, Condensed Matter Physics, Free University Amsterdam, Amsterdam, Netherlands.

The hydrogen adsorption properties of Pd are very well-studied. This inspired the use of Pd caplayers to enhance the reversible absorption of hydrogen by an underlying transition metal hydride. The hydrogen desorption from a metal hydride through Pd caplayer appears to be more complex than expected. At room temperature, desorption is severely hampered in UHV, while a bit of oxygen triggers desorption immediately. We measured the optical reflection change of Pd-capped Mg based films to probe the kinetics of the hydrogen absorption and desorption through the Pd caplayers [1]. To measure the optical properties in-situ from the substrate side we deposited the films on the freshly cleaved surface of multimode optical microfiber in a metal MBE-system, at a background pressure of 3×10-8 mbar. The light from a halogen source was reflected from the backside side of the deposited metallic film and guided through the optical fiber to the Ocean Optics USB 2000 CCD spectrometer. The hydrogenation was performed at 1 bar of 5 N purity hydrogen while for dehydrogenation the chamber was pumped to below 10-8 mbar. A huge difference between absorption and desorption kinetics of Pd coated Mg-based films was found. The 100 nm Mg2Ni/10 nm Pd film was loaded in 15 s while the unloading took 32000 s. This is in contrast to a single Pd film of 100 nm where the hydrogen absorption and the desorption kinetics are equally fast in 15 s. Depositing a fresh Pd-layer of only a few nanometers on top of the blocked Pd layer improves the dehydrogenation kinetics by a factor 100 and the film unloads in 674 s. Similarly, a short Ar sputtering of the Pd caplayer removes the blocking mechanism and results in an increased hydrogen desorption. The possible mechanisms of the hydrogen desorption behaviour will be discussed in light of surface/subsurface hydrogen [2], differences in expansion of Pd and Mg2Ni lattices upon hydrogenation and the effect of surface impurities. Additionally, the role of the driving force for hydrogen diffusion through an intermediate layer will be discussed [3]. These findings offer interesting perspectives for the development of hydrogen storage/sensor technologies. [1] R. Westerwaal et al.: Catalysis of hydrogenation of Pd capped Mg based switchable mirrors, to be published [2] D. Farias et al.: Helium Diffraction Investigations of the Transition of Chemisorbed Hydrogen into Subsurface Sites on Palladium Surfaces, Phys. Stat. Sol. (a) 159, 1997, 255 [3] M. Pasturel et al.: Influence of the chemical potential on the hydrogen sorption kinetics of Mg2Ni/TM/Pd (TM = transition metal) trilayers, Chem. Mater. 19, 2007, 624


4:30 PM S2.11
Methane Decomposition over Defective Carbonaceous Materials for Hydrogen Generation. Liping Huang^1,2,3, Erik E. Santiso^1,3, Keith E. Gubbins^1,3 and Marco Buongiorno Nardelli^2,3; ¹Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, North Carolina; ²Department of Physics, North Carolina State University, Raleigh, North Carolina; ³Center for High Performance Simulation (CHiPS), North Carolina State University, Raleigh, North Carolina.

The decomposition of methane is an environmentally attractive approach to CO and CO2-free hydrogen production. In this work, we studied the decomposition of methane over defective carbonaceous materials for hydrogen generation by using Density Functional Theory (DFT) and Statistical Thermodynamics. Our results indicate that defective carbonaceous materials can be used as catalysts for methane decomposition, without the need for other catalysts like transition metals or oxides. The active sites can be continually regenerated by the deposition of carbon from methane, to make the hydrogen generation a continuous process. Additionally, since no other gases are produced in the process, the costs of CO2 sequestration and hydrogen purification from CO contamination will be dramatically reduced. We will also discuss the conditions at which this process is thermodynamically favorable.


4:45 PM S2.12
Electron Beam Coated Pt-Sn and Pt Thin Film/MWNT Electrocatalysts for Direct Ethanol Fuel Cell. Imran Jafri, Leela Mohana Reddy and Sundara Ramaprabhu; Indian Institute of Technology Madras, Chennai, India.

Recently there has been an increasing interest in the development of direct alcohol proton exchange membrane fuel cells (PEMFC) due to the ease in handling the fuel and flexibility in the cell operations. Ethanol offers an attractive alternative as a fuel for PEMFC because it can be produced in large quantities from agricultural products and it is the major renewable biofuel from the fermentation of biomass [1]. The electro-oxidation of ethanol is more complicated than that of methanol and imposes the necessity to develop more selective and active anode catalysts [2-5]. It was found that better performance was achieved when Pt-Sn was used instead of pure Pt [2,6,7,8]. In the present work multi-walled carbon nanotubes (MWNT) have been used as catalyst support for both ethanol oxidation and oxygen reduction reactions due to its high surface area and good electrical conductivity. Pt-Sn/MWNT is used for ethanol oxidation (anode) and Pt/MWNT is used for oxygen reduction (cathode). To get a uniform deposition of electrocatalysts, electron beam evaporation technique is used to prepare Pt-Sn and Pt thin films. The electrocatalysts Pt-Sn and Pt thin films have been deposited over the gas diffusion layer coated with MWNT catalysts support. Three sets of electrodes having different Pt-Sn and Pt loadings have been prepared by varying the deposition time. Single cell assembly of direct ethanol fuel cell (DEFC) has been constructed using above prepared electrodes with Nafion-1110 membrane as electrolyte and their performances have been studied at different temperatures. The present work highlights the effective utilization of electrocatalysts and catalysts support material for better performance of DEFC. Keywords: Carbon nanotubes; catalyst support; Electrocatalyst; Direct Ethanol Fuel Cell; Electron beam evaporation References [1] Almir Oliveira Neto, Richardo R. Dias and Marcelo M. Tusi, J. Power Sources 166 (2007) 87-91. [2] P. E. Tsiakaras in press J. Power Sources. [3] W.Y. Yu, W.X. Tu, H. F. Liu, Langmuir 15 (1999) 6. [4] Huanqiao Song, Xinping Qiu, Fushen Li, Wentao Zhu, Liquan Chen, Electrchem. Comm. 9 (2007) 1416-1421. [5] Zhaolin Liu, Xing Yi Ling, Xiaodi Su, Jim Yang Lee, Leong Ming Gan, J. Power Sources 149 (2005) 1-7. [6] C. Lamy, E.M. Belgsir and J.M. Leger, J. Applied Electrochemistry 31 (2001) 799-809. [7] W.X. Tu, H.F. Liu, Chem. Mater. 12 (2000) 564. [8] F.C. Simoes, D.M. dos Anjos, F. Vigier, J.M. Leger, F. Hahn, C. Coutanceau, E.R. Gonzalez, G. Tremiliosi-Filho, A.R. de Andrade, P. Olivi, K.B. Kokoh, J. Power Sources 167 (2007) 1-10.


SESSION S3: Poster Session: Hydrogen Storage Materials and Technology
Chairs: Ping Chen, Azarnoush Hoseinmardi, Gholam-Abbas Nazri and Aline Rougier
Monday Evening, November 26, 2007
8:00 PM
Exhibition Hall D (Hynes)
S3.1
Thermodynamic Studies of the Palladium Hydrogen System. Paul Nevitt, Andrew Bailey, Amanda Hynes, Malcolm Pizey and Linda Bulmer; AWE, Berkshire, United Kingdom.

A baseline thermodynamic characterization of palladium hydride and deuteride is currently being undertaken. The thermodynamic work was instigated to be both investigative and/or confirmatory for a range of basic properties. Despite the enormous amount of work in the study of PdH(D)x there has not been (to our knowledge) a systematic study of several beta phase properties. This work aims to address some of these areas. Using differential scanning calorimetry (DSC), simultaneous thermal analysis (STA) and "photographic flash tube" thermal diffusivity analysis a number of properties have been studied. These include heat capacity and thermal diffusivity and their behaviour with respect to stoichiometry, temperature, and pressure. In this report values for heat capacity, heat of formation and thermal diffusivity/conductivity are presented, and their relationship with stoichiometry discussed.


S3.2
Porous Nanocomposites for Making Hydrogen Gas Via Water Photolysis. S. M. Sarif Masud¹ and Geoffrey B. Saupe²; ¹Materials Science and Engineering, University of Texas at El Paso, El Paso, Texas; ²Department of Chemistry, University of Texas at El Paso, El Paso, Texas.

We are developing dye-sensitized metal oxides for photochemical energy conversion. We would like to understand the basic structural and chemical characteristics that control electron conduction and reactivity in particle-based porous photocatalysts. These issues are fundamental to the design of efficient high surface area heterogeneous photocatalysts. With the information gained from these experiments, we can increase the yields of sun driven direct water photolysis to provide a cost-effective source of hydrogen gas. We have synthesized highly porous metal oxides made of wide bandgap semiconductors. These new materials work well as electron-hole producing catalysts under UV light. However, we are interested in using them in conjunction with visible light sensitizers to develop visible light photocatalysts for a direct water splitting process. Here we have tested the un-sensitized catalyst for its ability to reduce gold chloride ions in water to their metallic state. The metallic gold is deposited on the surface of the catalyst at locations that are most catalytically active. This provides some insight on the best catalyst structure for future experiments.


S3.3
Ultra-high Sensitive pH Sensors Based on Pd Patterned Structures. Young Tack Lee¹, Eun Song Yi Lee², Min Hong Juen², Ju Hoon Kang², Kye Jin Joen¹ and Wooyoung Lee^1,2; ¹NCRC (Nanomedical National Core Research Center), Yonsei University, Seoul, South Korea; ²Department of Materials Science and Engineering, Yonsei University, Seoul, South Korea.

In the past decade, significant efforts have been focused on the development of pH sensors due to high possibility for environmental, biological, and biomedical applications. In general, there are two conventional types of pH sensors based on electromotive force (EMF) and ion sensitive FET (ISFET). However, the EMF pH sensors have critical disadvantages of bulky size and requirement of calibration for each sensing time. ISFET pH sensors are well-known to have problems on selectivity, precise detection and continuous measurements. In the present work, we report a novel method to fabricate highly sensitive pH sensors based on Pd structures, patterned by electron-beam lithography from sputtered Pd thin films. Pd thin films were deposited on a thermally oxidized Si(100) substrate in a dc magnetron sputtering system with a base pressure of 4 × 10^-8 Torr. A combination of electron beam lithography and a lift-off process has been utilized to fabricate Pd mesowires (w = 0.3 - 5 μm, l = 10 μm) from continuous Pd films with t = 60 - 400 nm. A SiO2 was deposited on the Pd structures as an electrical passivation layer except for sensing area of a circlet window with d = 1 μm, where Pd structures can absorb hydrogen ions in a buffer solution. Real-time detection of electrical resistance of the devices were carried out by 4-probe measurements. A sensing current of 10 μA was applied for pH sensing by measuring electrical resistance. The initial resistance of a lithographically patterned Pd structure with w = 5 μm was 72 Ω when deionized water was dropped. After dropping a buffer solution of pH4, the electrical resistance was found to increase up to 88 Ω. The sensitivity is defined S = (R-R0)/R0×100%, where R and R0 are the resistances in the presence of the buffer solution and deionized water, respectively. The sensitivity of the Pd structure was found to be as high as 18 %. The variation of the resistance in the Pd structure was observed to be reproducible before and after dropping the buffer solution. The pH sensing mechanism of the Pd structure is believed to be as follows. When a pH buffer solution is dropped on the Pd structure, (1) hydrogen ions are absorbed on the Pd surface (hydrogen atoms of α phase) (2) absorbed hydrogen ions diffuse into the Pd interstitial sites, and (3) react with Pd atoms to form Pd-hydride (PdHx, β phase). The resistivity of PdHx is higher then pure Pd, since absorbed hydrogen ions play a role as additional scattering sources. Our results demonstrate the possibility of implementing pH sensors based on the Pd meso-structures. The effect of diameters of Pd mesowires on the pH sensing performance for a variety of buffer solutions will be discussed.


S3.4
High Hydrogen Permeability of Nb₄₉Zr₁₅Ni₃₆ Alloy Enhanced by Pd Addition. Huixiang Tang, Kazuhiro Ishikawa and Kiyoshi Aoki; Materials Science, Kitami Institute of Technology, Kitami, Japan.

Hydrogen permeability of the as-cast Nb₄₀Zr₃₀Ni₃₀ alloy is relatively high, which suggests that Nb-Zr-Ni alloys are promising for hydrogen purification. Pd is utilized to enhance the dissociation and recombination of hydrogen molecules for the non-Pd based hydrogen permeation membranes. Therefore, effect of Pd addition on hydrogen permeability and the microstructure of the as-cast Nb₄₉Zr₁₅Ni₃₆(mol%) alloy prepared by arc melting has been investigated. Hydrogen permeability of Nb₄₉Zr₁₅Ni₃₆ and Nb₄₉Zr₁₅Ni₃₀Pd₆ alloys at 673K is 3.79 and 6.58 × 10^-8 [mol H ₂ m^-1 s^-1 Pa^-0.5], respectively. These values are much higher than that of pure Pd and the Pd-Ag alloys. The Nb₄₉Zr₁₅Ni₃₆ alloy consists of the primary body-centered-cubic (bcc), rod-like and eutectic phases. On the contrary, the Nb₄₉Zr₁₅Ni₃₀Pd₆ alloy is composed mainly of the primary body-centered-cubic (bcc), plate-like (bcc) and ZrNi phases. In the latter alloy, the primary phase is connected by the plate-like phase, which is helpful to enhance hydrogen diffusion. Activation energy Ea for hydrogen permeation of the Nb₄₉Zr₁₅Ni₃₆ alloy is 23.01 kJ mol-1 and deceases to 11.80 kJ mol^-1 with 6 mol % Pd addition. The latter is close to that of pure Pd (13.72 kJ mol^-1). The changes in the microstructure and activation energy due to Pd addition may be the main causes for increment of hydrogen permeability, which make hydrogen atoms diffuse more easily in this metal membrane. It is expected that a small amount of Pd addition is effective to tune Nb-Zr-Ni alloys properties.


S3.5
The Peculiarities of Formation of Alloys Structure in the System Ti-Zr-Hf-H. Veniamin Sholomovich Shekhtman¹, Seda Dolukhanyan^1,2, Anahit Aleksanyan^1,2, David Mayilyan^1,2, Ofelia Ter-Galstyan^1,2, Mikhail Sakharov¹ and Salavat Khasanov¹; ¹Lab. of struct. investigations, Institute of Solid State Physics RAS, Chernogolovka, Moscow district, Russian Federation; ²Institute of Chemical Physycs NAN RA, Yerevan, Armenia.

In the Laboratory of High Temterature Synthesis of IChPh of Armenian NAS, a principally new technique is elaborated for receiving the alloys of refractory metals. In the present work, the results of investigations of crystal structures of alloys obtained in the systems Ti-Zr; Ti-Hf and Ti-Zr-Hf are presented. The performed experiments had shown that the compacted mixture of metal hydrides' powders after removing hydrogen at heating temperature a little higher than the temperature of hydrides dissociation transform to alloys of these metals. So, in this regime, two- and three-component alloys of metals are formed stably and reproducibly from the compositions of hydrides of this elements. Uncommonness of this result is in essential remoteness of the process from the standard regimes of fusion and diffusion sintering on temperature (lower by 500-800 grad C). At this, the total time of thermal treatment appeared also too short: does not exceed 60-90 min. According X-ray structure analysis, a number of alloys of α- and ω-modifications were received. The experimental results evidence that the structures of obtained alloys peculiarly depend on the ratio of hydrides quantity in the initial mixture. At special ratio of hydrides of titanium\zirconium\hafnium, the vacuum annealing results in formation of alloys with ω-phase structure unusual for state diagrams of Ti-Zr and Ti-Hf at atmosphere pressure. In accordance with known reference data the structures of titanium and zirconium ω-phases are of hexagonal aluminum diboride, crystallographic family, hP3 structural type, P6/mmm space group. In this discussion, it is important, that an essential difference of ω-phase from alfa- and beta -modifications of titanium (zirconium, hafnium) is the arrangement of one element atoms in two symmetrical-nonequivalent positions, i.e., they are specifically ordered. The hydrogen removing from them resulted again in formation of initial α- and ω-phases. The performed cycle of experiments allowed us to manifest a specific trend of compositions in the range of hydrides' ratio toward ω-phase formation. The next sequence of structural transformations in the presence of hydrogen was stated: the synthesis of hydrides of superior compositions. The fact of formation of ω-modification alloys at atmosphere pressure is worthy of attention. The work is performed at financial support of ISTC Grant A-1249, and Ministry of Education and Science of Armenia, theme 0567.


S3.6
Hydrogenation-Induced Quantum Criticality in U₂Co₂In. Ladislav Havela¹, Khrystyna Miliyanchuk^1,2 and Laura C.J. Pereira³; ¹Department of Electronic Structures, Charles University, Faculty of Mathematics and Physics, Prague 2, Czech Republic; ²Department of Inorganic Chemistry, Faculty of Chemistry, Ivan Franko University, Lviv, Ukraine; ³Departamento do Quimica, Instituto Technologico e Nuclear, Sacavem, Portugal.

High hydrogen absorption can be found in several types of uranium intermetallics. Due to the crucial importance of inter-U spacing for magnetic and other electronic properties, hydrogenation can be used for tuning the ground state, leading typically to stronger magnetic features. An interesting case among the U₂T₂X compounds, which absorb up to 2 H atoms per f.u., is U₂Co₂In-H. U₂Co₂In is a weak Pauli paramagnet. The lattice expansion by 8.4% in U₂Co₂InH_1.9 brings the compound to the verge of magnetism, to the vicinity of the quantum critical point, where the dominance of quantum fluctuations induces non-Fermi liquid phenomena. This hydride is a band metamagnet, i.e. magnetic order is induced only if magnetic field exceeds the critical value of 2.2 T. In lower fields, the specific heat in the C/T vs. T representation exhibits a pronounced low-temperature upturn, which can be described down to T = 0.4 K by additional non-Fermi liquid -T^1/2 term. The γ coefficient of the specific heat (244 mJ/mol f.u. K²) proves that the hydride belongs to heavy fermion materials.


S3.7
Hydrogen Storage Properties and Phase Transition of Mg/Pd Laminate Composites. Nobuhiko Takeichi¹, Koji Tanaka¹, Hideaki Tanaka¹, Nobuhiro Kuriyama², Tamotsu Tohmas Ueda², Hiroshi Miyamura³ and Shiomi Kikuchi³; ¹Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, OSAKA, Japan; ²IMRA Material R&D Co.Ltd., Kariya, Aichi, Japan; ³The University of Shiga Prefecture, Hikone, Shiga, Japan.

Magnesium is a one of promising materials for hydrogen storage media because it can absorb a large amount of hydrogen as MgH2. However, However, the hydrogen absorption/desorption kinetics is too slow for practical use and needs high temperatures such as 573K. To improve the reaction kinetics and diffusion properties, a reduction of the grain size and an addition of various catalysis have been investigated. Recently, co-author Ueda et al. reported Mg-based laminate composites, prepared by a repetitive-rolling method, can reversible hydrogenation and dehydrogenation at 473K. Pd actively reacts with hydrogen. Therefore, adding Pd to Mg is expected to improve the activation properties and kinetics for hydrogen reaction. In this study, we investigated hydrogen storage properties of Mg/Pd laminate composites and phase transition during hydrogenation/dehydrogenation. The Mg/Pd laminate composites with (Mg/Pd)=6,4,3 and 2.5, where (Mg/Pd) means the ratio of the numbers of Mg to Pd atoms, were prepared by a repetitive fold and roll method using conventional two-high rolling mill. The PC-isotherms were measured with a Sieverts’ apparatus. The phase transformation of Mg/Pd laminate composites during hydrogen absorption and desorption was analyzed by in-situ XRD measurement. The Mg/Pd laminate composites can reversibly absorb and desorb a large amount of hydrogen, up to 1.46~0.9 H/M, at 573K. Except Mg/Pd laminate composites with (Mg/Pd)=2.5, PC-isotherms of the Mg/Pd laminate composites show two plateaux, P_L and P_H. In case of Mg/Pd laminate composite with (Mg/Pd)=6, P_L and P_Hat 573 K were 0.02 and 2 MPa, respectively. PC-isotherms for the Mg/Pd laminate composite with (Mg/Pd)=2.5 at 573K show single plateau at 2 MPa. During the initial activation process, intermetallic compounds, Mg₆Pd, Mg₄Pd Mg₃Pd and Mg₅Pd₂, are formed from Mg/Pd laminate composites with (Mg/Pd)=6, 4, 3 and 2.5, respectively. In the lower plateau pressure region, P_L, those intermetallic compounds, expect Mg₅Pd₂, decomposed to Mg₅Pd₂ and MgPd. In case of Mg₆Pd, the reaction equation is expressed as follows: Mg₆Pd + (7/2)H₂ ↔ (1/2)Mg₅Pd₂ + (7/2)MgH₂. This amount of hydrogen, 1 H/M, corresponds to large low plateau of PC-isotherms of Mg₆Pd. In high plateau pressure region, P_H, Mg₅Pd₂ decomposed to MgH₂ and MgPd. In case of Mg₆Pd, the reaction equation is expressed as follows: (1/2)Mg₅Pd₂ + (7/2)MgH₂ + (3/2)H₂ ↔ MgPd + 5MgH₂. These reaction equations agree with the PC-isotherms of Mg/Pd laminate composites. From those results, Mg/Pd laminate composites can absorb and desorb hydrogen reversible through complex multistage disproportionation reaction and recombination of Mg and Pd. This work has been supported by New Energy and Industrial Technology Development Organization (NEDO) under “Development for Safety Use and Infrastructure of Hydrogen Program”.


S3.8
The Effect of Initial Activation on Microstructures of Mg/Cu Super-laminates and Hydrogen Absorption Properties. Koji Tanaka¹, Nobuhiko Takeichi¹, Hideaki Tanaka¹, Nobuhiro Kuriyam¹, Tamotsu Tohmas Ueda², Makoto Tsukahara², Hiroshi Miyamura³ and Shiomi Kikuchi³; ¹Res. Inst. for Ubiquitous Enegy Devices, Natl. Inst. of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka, Japan; ²IMRA Material R&D Co.Ltd., Kariya, Aichi, Japan; ³The University of Shiga Prefecture, Hikone, Shiga, Japan.

Hydrogen storage materials have attracted more and more attention with the advance of R & D activities of fuel cell vehicles. Magnesium is expected as one of hydrogen storage media because it can store a large amount of hydrogen up to 7.6 mass%, as MgH2. However, MgH2 is too stable to release hydrogen smoothly; a practical decomposition rate is given at the temperatures above 673K, which is too high for practical applications. Thus, various Mg-based alloys and compounds have been investigated to improve the rate and lower the temperature of dehydrogenation. Recently, co-authors Ueda et al. reported that Mg/Cu super-laminates showed reversible hydrogenation and dehydrogenation at 473K. The Mg/Cu super-laminates were prepared by a repetitive fold and roll method using conventional two-high rolling mill. Initial activation at 573 K leads the super-laminates to absorb hydrogen at 473K. In order to clarify the role of initial activation at 573K, we performed TEM observations of nano/micro-structures of the super-laminates that were initial-activated and heat-treated in vacuum at 573K, and compared hydrogen storage properties of those. As-rolled Mg/Cu super-laminates started to absorb hydrogen immediately and the reaction was very fast although initial activated Mg/Cu super-laminates started to absorb immediately and the reaction was much faster. On the other hand, heat-treated Mg/Cu super-laminates started to absorb hydrogen immediately and the reaction was slower than as-rolled Mg/Cu super-laminates although heat-treated Mg/Cu super-laminates for the second hydrogenation started to absorb immediately and the reaction was fast as same as initial activated Mg/Cu super-laminates. And then, heat-treated Mg/Cu super-laminates for the first and the second hydrogenation reached equilibrium state faster than as-rolled and initial-activated Mg/Cu super-laminates, however, the hydrogen content H/M is lower than those. The differences of hydrogen storage properties are caused by the differences of nano/microstructures. Heat-treated Mg/Cu super-laminates have more uniform structures than initial-activated Mg/Cu super-laminates, however, include more MgCu2 phase that is irresponsible to a reaction with hydrogen. Uniform nano/micro structures, and more Mg2Cu and less MgCu2 phases are the key to the improvement of hydrogen storage properties.


S3.9
Quantitative Analysis of Ammonia Emission During the Cycle-life Measurement of Amide-based Hydrogen Storage Materials. Shingo Ikeda¹, Kosei Nakamura^2,1, Hiroyuki T Takeshita³, Tetsu Kiyobayashi¹, Kazuhiko Tokoyoda⁴, Toyoyuki Kubokawa⁴ and Nobuhiro Kuriyama¹; ¹Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology, Ikeda, Japan; ²Graduate School of Engineering, Kansai University, Suita, Japan; ³Faculty of Chemistry, Materials and Bioengineering, Kansai University, Suita, Japan; ⁴R&D Center, Taiheiyo Cement Corporation, Sakura, Japan.

Amide-based hydrogen storage materials, such as the LiH-LiNH₂ mixture [1] (termed ‘Li-N system’ hereafter) or LiH-Mg(NH₂)₂ mixture [2] (Li/Mg-N system), have received much attention as a promising candidate for a hydrogen storage medium due to the high gravimetric hydrogen capacity in view of applying to the fuel cell vehicles. A big problem in the amide-based system is that a certain amount of ammonia is emitted as a byproduct of hydrogen desorption [3,4]. Ammonia emission affects not only the performance of fuel cells but also the hydrogen capacity because the loss of nitrogen amounts to the loss of hydrogen storage material. Therefore, it is practically important to simultaneously investigate the quantitative relationship between the cyclic properties and the amount of ammonia emission all through the cycle-life measurement. In the present study, we investigated the cyclic properties of the Li-N and Li/Mg-N systems as well as the amount of ammonia emission during the cycle life tests. The Li-N system is prepared by ball-milling the mixture of LiNH₂:LiH:TiCl₃= 1.0:1.1:0.02 (molar ratio) in which TiCl₃ works as a catalyst. The Li/Mg-N system, prepared in Taiheiyo Cement Co Ltd., is of the ball-milled mixture of Mg(NH₂)₂:LiH= 3:8. The cyclic properties are evaluated with using the Sieverts’ type apparatus. The ammonia gas emitted during the cycles is accumulated in a cold-trap at liquid nitrogen temperature for a couple of cycles and is subsequently quantified with using a mass spectrometer which is calibrated with using three materials as a reference, namely, diluted ammonia gas and thermal decomposition of NH₄HCO₃ and LiNH₂. In the case of the Li-N system, the initial hydrogen capacity of ca. 5mass% decreases exponentially by half after 100 cycles at 300 °C. The concentration of ammonia in the desorbed hydrogen becomes stable at 0.4±0.2 mol% after initial few tens of cycle. (More precise value of ammonia concentration will be presented at the conference, as the final calibration is now under way.) On the other hand, Li/Mg-N system shows good stability through 100 cycles at 200 °C: The initial hydrogen capacity of ca. 4mass% shows a barely appreciable degradation. The ammonia concentration in hydrogen is 0.08±0.04 mol%. The better stability and less ammonia emission of Li/Mg-N system than Li-N system can be attributed to the lower cycling temperature and/or to the difference in their innate properties. In both systems the degree of degradation in hydrogen capacity seems to be explained mainly by the loss of constituent nitrogen due to the ammonia emission. Acknowledgements This work was financially supported by the New Energy and Industrial Technology Development Organization (NEDO), Japan. References [1] P. Chen et al., Nature, 420 (2002) 302-304. [2] W. Luo, J. Alloys Compd., 381 (2004) 284-287. [3] S. Hino et al., Chem. Commun., (2005) 3038-3040. [4] W. Luo et al., J. Alloys Compd., in press.


S3.10
Ab initio Calculations on Al and Si-substituted Magnesium Hydride. Tuhina Kelkar¹, Sourav Pal¹ and Dilip Kanhere²; ¹Physical Chemistry Division, National Chemical Laboratory, Pune, Maharashtra, India; ²Department of Physics and Center for Modeling and Simulations, University of Pune, Pune, Maharashtra, India.

The crystal structure, iso-surface charge density, density of states (DOS), and thermodynamics of X = Al or Si substituted magnesium hydride MgH₂ was investigated by ab initio projected augmented wave (PAW) pseudopotential-plane wave method based on density functional theory (DFT). The substituted MgH₂ structures resembled the pure MgH₂ with respect to arrangement of the atoms but with smaller cell volumes. The presence of covalent bonding between X and H atoms which weakens the Mg-H bonds was exhibited by the directional charge density along X-H bonds. On replacement of Mg by a fraction of X additional states appeared. From DOS calculations, it was found that the occupied Al or Si 3s states form the new fermi level Efermi. The new conduction band was formed from the unoccupied Al or Si 3p states and the Mg 3s and 3p states. The resulting zero band gap DOS calculations implied a shift from insulating to conducting nature of MgH₂ on addition of Al or Si. The removal of the band gap resulted in a reduced stability which was reflected in the much lower heat of formation (△H_f) for the substituted MgH₂ as opposed to its pure form. The △H_f values showed that the Si causes a greater lowering of the dehydrogenation energy and hence the dehydrogenation temperature than Al. The calculated mixing energies were positive implying that Al and Si substitution of Mg in MgH₂ will be thermodynamically unfavorable. These results suggest exploring application of Al and Si in the form of catalysts as a potential route for improving the hydriding/dehydriding kinetics of magnesium hydride for the purpose of hydrogen storage.


S3.11
Hydrogen Degradation Property of Electrochemically Charged Aluminum. Hiroshi Suzuki, Daisuke Kobayashi, Kenichi Takai and Yukito Hagihara; Mechanical Engineering, Sophia University, Tokyo, Japan.

Degradation property of aluminum due to hydrogen is studied. A safe and efficient method to introduce hydrogen to aluminum is needed to access the material in the application. In this study, we chose the electrolytic charging method instead of high pressure gas to introduce hydrogen. Annealed and polished pure aluminum (99%) is used as a testing material. Hydrogen is introduced by electrolytic charge in aqueous solution. The value of electronic potential and the pH of the solution are chosen from -2.5 to 1.0 V and 2 to 10 so that corrosive, passive and stable state on the Pourbaix diagram is attained. The amount of hydrogen and its existing state in the material is analyzed by hydrogen desorption curves measured by the thermal desorption spectroscopy. Tensile properties are obtained to determine degradation property of the material due to hydrogen. The maximum amount of hydrogen introduced to pure aluminum is 18 mass ppm when charged in passivity region with addition of 0.1 mass % NH₄SCN as a hydrogen recombination poison. The hydrogen desorption curves of the charged aluminum showed two peaks, one at less than 100 and the other around 400 degrees centigrade. The existing state of hydrogen corresponding to each peaks are identified as weakly trapped solute hydrogen and free hydrogen molecule located in uniformly distributed blisters of several microns of diameter. The fracture strain of tensile deformation decreased from 0.5 to less than 0.2 due to hydrogen charge. Hydrogen corresponding to the lower peak of desorption curve is released by heating up to 200 degrees, causing slight recovery of the fracture strain. Almost all hydrogen is released by heating up to 500 degrees. But the fracture strain remains the same as that of 200 degrees heated specimen. This indicates that solute hydrogen and blisters affects ductility of aluminum, whereas hydrogen molecule in blisters has no effect. The fracture strain of charged aluminum increases with faster strain rate during tensile deformation, showing inverse tendency compared with uncharged material. The fracture strain becomes constant when tested with strain rate faster than 10^-3 s^-1, where interaction between solute hydrogen and mobile dislocation becomes small due to large difference between rate of diffusion of hydrogen in the matrix and the average velocity of moving dislocations. Therefore the effect of solute hydrogen on ductility of aluminum comes from its interaction with mobile dislocations. The method to introduce hydrogen and analyze its existing state in aluminum developed in this study is applicable to obtain aluminum hydride that can be used as hydrogen storage material.


S3.12
Synthesis, Properties and Applications of Porous Metal-Organic Framework Materials. Gary DiFilippo, Kunhao Li, Wenhua Bi and Jing Li; Chemistry and Chemical Biology, Rutgers University, Piscataway, New Jersey.

Metal-Organic Framework (MOF) materials have become a hot topic in recent years due to the myriad of applications they might find uses in, such as gas storage and separation. The draw of these hybrid, porous materials is that they combine the variety found in organic ligands with the properties of metal based compounds. For example, by employing a multitude of different building units, porous MOF materials with specific pore size, shape, and properties for specific application may be synthesized as desired. To increase the diversity and to modify the structures and properties of MOF materials different types of organic linkers are used. In this presentation, I will discuss our use of multiple ligands in structure construction of a new group of MOFs. Typically a dicarboxylate will act as a “linker” to bind with the metal center to form 1D chains or 2D layers, and another type of ligands, such as a dicarboxylate, amine or solvent molecule, will act as a “pillar” to form a three dimensional framework. The synthesis, structure, and properties of some of these materials will be displayed.


S3.13
Enhancing Photoelectrochemical Hydrogen Production by Using TiO₂-MgO Core-Shell-Structured Nanoparticles. Shin-Tae Bae¹, Jin Young Kim¹, Sangwook Lee¹, Hyun Suk Jung², Taehoon Noh¹ and Kug Sun Hong¹; ¹Seoul National University, Seoul, South Korea; ²Kookmin University, Seoul, South Korea.

TiO₂-MgO core-shell-structured nanoparticles were prepared, and their photoelectrochemical characteristics for hydrogen production were investigated. The coating of a MgO layer onto TiO₂ nanoparticles by the thermal topotactic decomposition of the Mg(OH)₂ gel was clearly exhibited by high resolution transmission electron microscopy (HRTEM) and the MgO-modified TiO₂ films showed maximum hydrogen production at 0.03 wt % addition of MgO. These photoelectrochemical characteristics with the amount of MgO addition were discussed in terms of competition of two factors such as the blocking effect and the resistivity of MgO coating layer. The MgO layer increased the water photolysis efficiency by retarding the electron-hole recombination, which was explained by the “blocking effect” of MgO coating layer with high band gap energy. On the other hand, the decrease of hydrogen production resulting from large amount of MgO addition was explained by increase in resistivity of MgO coating layer.


S3.14
Catalyst Addition for Reduced Hydrogen Desorption Temperature of Ball-Milled MgH2. Aline Rougier¹, Nadir Recham¹, Manickam Kandavel¹, Vinay V. Bhat¹, Luc Aymard¹, Gholam Abbas Nazri² and Jean Marie Tarascon¹; ¹LRCS, Amiens, France; ²GM, Warren, Michigan.

Magnesium is among the most promising materials for hydrogen storage applications since it forms a dihydride providing 7.6 wt.% of hydrogen. Despite its high hydrogen content, the hydride formation is very slow and occurs at very high temperature (above 300 °C). Among the ways to improve its sorption properties, i.e. faster kinetics and reduced desorption temperature, the catalyst addition remains one of the most promising. As a matter of fact, we recently reported the desorption of 4.5 wt%. and 3 wt%. of H2 at temperatures as low as 200 °C and 150 °C for ball-milled MgH2 catalyzed with Nb2O5 [1] and NbF5 [2], respectively. Investigation of other catalysts reveals that oxy-fluorine compounds are also active. In this presentation, the role of the catalyst on improving the sorption properties of ball-milled MgH2 will be discussed in relation with its chemical and physical nature. [1] V.V. Bhat, A. Rougier, L. Aymard, G.A. Nazri and,J-M. Tarascon J. of Alloys and Compounds, V. V. Bhat, A. Rougier, L. Aymard, G. Nazri, J.-M. Tarascon, J. of Alloys and Compds., doi:10.1016/j.jallcom.2007.05.084. [2] N. Recham, V.V. Bhat, M. Kandavel, L. Aymard, J-M. Tarascon, and A. Rougier, J. of Alloys and Compounds, submitted.


S3.15
Carbide-Based Fuel System for Solid Oxide Fuel Cells. A. Alan Burke and Louis G. Carreiro; Code 8231, Naval Undersea Warfare Center, Newport, Rhode Island.

Despite wide-ranging efforts to replace batteries with fuel cells in niche military applications, the transition is proceeding slowly, due in part, to the unique fuel storage requirements associated with fuel cell systems. In the case of underwater applications, such as unmanned undersea vehicle (UUV) propulsion, mass and volume constraints often dictate system energy density and specific energy, which must exceed 300 W-hr/L and 300 W-hr/kg, respectively, in order to compete with state-of-the-art battery technologies. To address this need, a novel carbide-based fuel system (CFS) intended for use with a solid oxide fuel cell (SOFC) is under development that is capable of achieving these energy metrics as well as sequestering carbon dioxide. The proposed CFS consists of a composite of calcium carbide and calcium hydride that when combined with water generates acetylene and hydrogen as the fuel and calcium hydroxide as a carbon dioxide scrubber. The acetylene is hydrogenated to ethane and then further processed by steam reforming to syngas (carbon monoxide and hydrogen) before being utilized by the SOFC. Carbon dioxide effluent from the SOFC is reacted with the calcium hydroxide to produce a storable solid, calcium carbonate, thus eliminating gas evolution from the UUV. In this paper, a system configuration is proposed and discussion follows concerning energy storage metrics, operational parameters and preliminary safety analysis.


S3.16
First-Principles Study of a Hydrogen Molecule Interaction with Tin (N=3-8 and 13 Atoms) Clusters. Jorge Alejandro Medina-Garcia¹, G. Canto² and R. de Coss³; ¹Nanostructures Department, CICESE-UNAM, Ensenada, Baja California, Mexico; ²Nanostructures Department, CCMC-UNAM, Ensenada, Baja California, Mexico; ³Department of Applied Physics, CINVESTAV, Merida, Baja California, Mexico.

There are theoretical works that report the use of Titanium atoms in order to increase the Hydrogen storage capacity of nanostructures. Also, some experimental works have been found that using clusters of Ti can be increasing its Hydrogen absorption capacity. Here, we show the energetic and structural properties of bare and Hydrogen decorated Titanium clusters (Tin clusters for n=3-8 and 13). We have performed density functional theory calculations with the SIESTA code. The wave functions are expanded on a Linear Combination of Atomic Orbitals (LCAO). We are employed the Generalized-Gradient Approximation (GGA) for the exchange-correlation. Our results for the bare clusters are in good agreement with works previously published. We consider several positions to hydrogen adsorption on the decorated clusters. We found a dissociative adsorption at the interstitial positions as the most probable in all clusters considered. Also, we show the structural results for the saturation of the interstitial sites on Ti13. Formation energies are showed.


S3.17
Abstract Withdrawn


S3.18
Ti-coated Single-walled Carbon Nanotubes for Hydrogen Storage: A Spectroscopic and Microscopic Study. Ich C Tran¹, Roberto Felix¹, Lothar Weinhardt¹, Marcus Baer¹, Clemens Heske¹, Oliver Fuchs², Monika Blum² and Jonathan Denlinger³; ¹Department of Chemistry, University of Nevada Las Vegas, Las Vegas, Nevada; ²Experimentelle Physik II, Universität Würzburg, Würzburg, Germany; ³Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California.

In recent years, carbon nanomaterials have drawn considerable attention as possible candidates for solid state hydrogen storage. In order to enhance their hydrogen storage capabilities, several theoretical publications have proposed models of transition metal atoms bound to carbon nanomaterials, for example Ti on single-walled carbon nanotubes (SWCNT) [1]. In this contribution we will present an experimental investigation of the interaction between Ti and SWCNT. Thin layers of Ti were deposited onto SWCNT films under ultra-high vacuum conditions and investigated in-situ using photoelectron spectroscopy (PES) as well as scanning tunneling microscopy (STM) and spectroscopy (STS). In addition, synchrotron-based x-ray emission (XES) and x-ray absorption (XAS) experiments were performed ex-situ. The combination of laterally integrating techniques (PES, XES, and XAS) with microscopic approaches (STM and STS) results in a powerful tool chest to characterize the electronic and chemical structure, both at the surface and in the near-surface bulk. After Ti deposition, we observe a dramatic change of the C 1s line in PES, which indicates a strong interaction between Ti and C. In addition, the deposited Ti is found to act as a host for various adsorbates. In particular the formation of TiO and TiO2 can be concluded from the Ti 2p spectra, as well as the presence of carbon-containing adsorbates from the C 1s spectra. These findings, combined with results from XES, XAS, and STM/STS, exemplarily show the challenges associated with using Ti-decorated nanomaterials as a material for hydrogen storage, as will be discussed in this presentation. [1] T. Yildirim, S. Ciraci, Phys. Rev. Lett. 94, 175501 (2005)


S3.19
Experimental Study on Feasible Metal Hydrides for Destabilization of Lithium Borohydrides. Hongwei Yang and Andrew Goudy; Department of Chemistry, Delaware State University, Dover, Delaware.

Destabilization of lithium borohydrides for reversible hydrogen storage materials has been studied by using different metal hydrides as a destabilizer. First principles calculation predicts that CaH₂ may be a potential candidate as destabilizing metal hydrides. In our experimental work, mechanically milled mixtures of LiBH₄ and CaH₂ are shown to store about 13 wt% hydrogen. The equilibrium pressures of absorption and desorption at the different temperature are measured and a van’s Hoff diagram is plotted, which are compared to those for pure LiBH₄. In addition, the kinetics of absorption and desorption of destabilized LiBH₄ will be discussed.


S3.20
Pt-coated Multiwalled Carbon Nanotube : Promising Hydrogen Storage in Fuel Cells. A. Halder and Ravishankar Narayanan; Materials Research Centre, Indian Institute of Science, Bangalore, India.

Hydrogen storage is critical for developing energy sources based on fuel cells. Carbon nanotube is a potential medium for hydrogen storage and has unique properties such as light mass density, high surface to volume ratio and high degree of reactivity with hydrogen. But there are certain issues regarding the storage capacity of carbon nanotubes. Transition metals like platinum increase the storage capacity of carbon nanotubes by dissolving hydrogen. Here, we report a novel route to deposit nanoporous platinum on multi-walled carbon nanotube that combines the benefits of both porous platinum and carbon nanotube towards the hydrogen storage. This method relies on the synthesis of nanoporous Pt by fast reduction in a toluene medium containing functionalized carbon nanotubes. Microstructural studies have been carried out using high resolution transmission electron microscopy (TEM). Cyclic voltammetry(CV) studies have been carried out to verify the enhanced hydrogen storage capacity.


S3.21
Hydrogen Permeability and Microstructures of Melt-Spun and Annealed Nb₄₀Ti₃₀Ni₃₀ Ribbons. Yuta Seki¹, Koichi Kita², Kazuhiro Ishikawa¹ and Kiyoshi Aoki¹; ¹Department of materials Science, Kitami Institute of Technology, Kitami, Hokkaido, Japan; ²Mithubishi Materials Corporation, Kitamoto, saitama, Japan.

Pd-Ag based hydrogen permeation alloys are mainly used for separation and purification of hydrogen gas. However, since Pd is too expensive and a rare metal, it is strongly desired to develop non-Pd based alloys. Because hydrogen flux (J) passing through the alloy membrane is inversely proportional to its thickness, the preparation technique of alloy membranes is also important for the development of non-Pd based alloys. The Nb-TiNi alloys consisting of the bcc-(Nb, Ti) and the B2-TiNi phase show high hydrogen permeability equivalent to that of pure Pd. However, its membrane is prepared by means of complex processes such as cold rolling and intermediate annealing. On the other hand, it is well known that alloy ribbons can at a stroke be obtained by a melt-spinning technique. In the present work, hydrogen permeability, crystal structures and microstructures of melt spun Nb-TiNi alloy ribbons before and after annealing treatments are investigated in order to develop the preparation method of alloy membrane. The Nb₄₀Ti₃₀Ni₃₀ (mol%) alloy ribbons were prepared by a melt-spinning technique. Its thickness was about 40 μm. The ribbon samples were annealed at various temperatures and periods. Microstructural observation and phase identification were carried out using a scanning electron microscope (SEM) and an X-ray diffractometer (XRD). Chemical composition of the ribbon was measured by an electron dispersion X-ray spectroscope (EDS). Both surfaces of the samples were coated with 190 nm Pd using the DC sputtering machine in order to prevent oxidation and enhance a dissociation and recombination of hydrogen. Hydrogen permeability was measured by using a mass-flow meter in the temperature range of 573 to 673 K at 0.4 MPa H₂. The as-spun Nb₄₀Ti₃₀Ni₃₀ ribbon consisted of the bcc-(Nb, Ti) and B2-TiNi phases, but it was too brittle to measure its hydrogen permeability. On the contrary, it showed ductility and hydrogen permeability equivalent to that of pure Pd at 673 K by the heat treatment above 1173 K, This alloy consisted of fine granule (Nb, Ti) and TiNi phases. The grain size and hydrogen permeability of these samples increased with increasing temperature and time. However, the hydrogen embrittlement occurred when its grain size was too large, because the TiNi phase, which mainly contributed resistance against the hydrogen embrittlement, might not absorb the large volume expansion of the (Nb, Ti) phase. From the present work, it was concluded that melt-spinning technique is effective for the preparation of the Nb-TiNi hydrogen permeation alloy membrane.


S3.22
Development of SEM Metallography for the Study of the Mg-MgH2 Phase Transformation. Nadica Abazovic, Annalisa Aurora, Daniele Mirabile Gattia, Amelia Montone, Luciano Pilloni and Marco Vittori Antisari; FIM, ENEA, Rome, Italy.

Mg is a suitable material for hydrogen storage, owing to several features among which the gravimetric storage capacity is particularly favourable. However the phase transformation from MgH2 to metallic Mg is particularly sluggish and gives rise to decomposition temperatures too high for most applications. To find the way of fastening this reaction represents an active field of research and several methods, ranging from the nanostructuring by ball milling to the addition of suitable catalytic particles, have been explored in order to assist the decomposition reaction of the MgH2 phase. The analysis of the reaction kinetics shows how the above treatments can facilitate most of the steps of the reaction so that the rate limiting step changes as a consequence of the material processing. However despite the abundant literature, the kinetics analysis is seldom supported by some microstructural evidences, deriving from the high resolution observation of the samples under investigation. In this work we present an analysis of the experimental conditions allowing to image different contrast for each different phase present in a typical sample, i.e. metallic Mg, MgH2 and the catalyzing particles often constituted by 3d transition elements. Optical microscopy represents a suitable method since the phases are well differentiated by their natural contrast even if the limited spatial resolution often does not allow to define the microstructural details of the reaction. On the other hand, TEM is practically useless in this case owing to the fast decomposition of the hydride phase under the effect of the high energy electron beam. By mean of a SEM, the backscattered electron (BSE) signal, being sensitive to the average atomic number, was used in this work to show the phase distribution in sectioned samples. The problem is to find experimental conditions allowing to differentiate the contrast between two light phases having a small difference in their intrinsic BSE emission (Mg and MgH2) even in presence of a heavier phase constituted by the 3d catalysing particles. The study is based on a systematic use of the MonteCarlo simulation of electron trajectories in the sample compared with experimental data obtained on samples constituted by partially decomposed MgH2 processed in different conditions. To this purpose the powders embedded in epoxy resin have been polished in order to expose the internal microstructure. Observations have been carried out both at high accelerating voltage (20 kV) on samples coated with a vacuum deposited carbon film and at low accelerating voltage (0.3-2 kV) on samples with a free surface. In the first case the purpose was to find the optimum positioning of the BSE detector, while in the second case the purpose was to explore the possibility of using the complementarity between secondary and BS electron yields which is supposed to be setup below a critical accelerating voltage.


S3.23
Structure and Conductivity of Sodium Hydride Doped Calcium Nitride Hydride. Maarten Christiaan Verbraeken and John T.S. Irvine; School of Chemistry, University of St Andrews, St Andrews, Fife, United Kingdom.

Samples of calcium nitride hydride (Ca2NH) and its solid solution with NaH have been synthesized. The structure and conductivity of these phases have been determined. A synthesis route has been used that has not been reported in literature, previously, resulting in a new Ca2NH phase. This phase is believed to be a conductor of hydride ions, which is a chemically interesting species. The material has potential to be used as an electrolyte in solid state electrolyser cells or hydrogen sensors. Experimental: Ca2NH was synthesised by firing CaH2 in a 5% H2/1% N2/94% Argon atmosphere at 780°C. The samples doped with NaH were obtained by dry mixing powders of CaH2 and NaH followed by firing pressed pellets at 330°C for 16 hours, then at 780°C for 4 hours in the same atmosphere. The samples with 0%, 5%, 10% and 20% NaH doping were analysed with XRD and AC impedance spectroscopy. Results: Ca2-xNaxNH1-x crystallises in the FM3M spacegroup, with a = 5.0788Å for undoped Ca2NH and 5.0718Å for Ca1.60Na0.40NH0.60. The lattice parameter changes linearly with dopant concentration. The H- conductivity of undoped Ca2NH is ~4.0E-5 S/cm at 600°C. The activation energy below 400°C is 87 kJ/mol and decreases to 12 kJ/mol above that temperature.


S3.24
High Pressure Hydrogen Clathrates Examined under Raman Spectroscopy, X-ray diffraction, and Inelastic Neutron Scattering. Timothy Jenkins, Russell Hemley, Ho-kwang Mao and Viktor Struzhkin; Carnegie Institution of Washington, Washington, District of Columbia.

We have investigate the hydrogen storage character of tetrahydrofuran clathrate as a promoter molecule in the formation of structure II clathrate hydrates for hydrogen storage using inelastic neutron scattering. The low frequency scattering of hydrogen shows the rotational interactions of the hydrogen to the clathrate cage environment and the lattice vibrations of the clathrate cage. With increasing pressure, hydrogen clathrates form higher pressure C1 and C2 clathrate phases (Vos, et al., Phys. Rev. Lett., 1993). The low temperature phase diagram has been investigated to examine the phase stability with temperature upon the release of pressure. Analysis of these model compounds will assist in future investigations to additional clathrate compounds. W. Mao, H. Mao (2004). Proceedings of the National Academy of Sciences, 101(3), 708-710. W. Vos, L. Finger, R. Hemley, H. Mao (1993). Physical Review Letters, 71, 3150-3153.


S3.25
Evaluation of Hydrogen Absorption/desorption Characteristics of Mg-Al alloys. Mahesh Tanniru¹, Sankara Sarma V Tatiparti¹, Nathan Hicks¹, Fereshteh Ebrahimi¹ and Darlene K Slattery²; ¹Materials Science and Engineering, University of Florida, Gainesville, Florida; ²Florida Solar Energy Center, Cocoa, Florida.

Hydrogen absorption of light weight metals is being improved by addition of catalysts as well as alloying elements. For example, in case of magnesium different transition metals such as Ni, V, Ti etc. are used to enhance hydrogen uptake kinetics. In this study the effects of Al addition as an alloying element and Ni as a catalyst, on the hydrogen uptake and release behaviors of magnesium are evaluated. We have produced supersaturated solid solutions of Mg-Al alloy powders with 5 to 10% Al using electrodeposition from organometallic based electrolytes. These powders are fine in size and porous in nature. Furthermore, they consist of nanocrystalline grains. These qualities promote fast absorption of hydrogen on the surface, facilitate hydride nucleation and increase diffusivity. The hydrogenation kinetics of these powders were investigated in the temperature range of 50-200°C with and without the presence of nickel coating. A sievert’s type apparatus was employed for the hydrogenation experiments. Due to the thermal instability of supersaturated Mg-Al solid solutions, parallel annealing tests on the as deposited powders were conducted in an inert atmosphere to understand the effect of intermetallic phase formation on the hydrogenation process. The powders were characterized for their composition, morphology, phases and grain size. In this presentation the effect of Ni catalyst on kinetics and Al addition on hydride formation will be discussed. This project is supported by a grant from NSF (DMR-065406).


S3.26
Solubility and Diffusivity of H in the bcc Alloys Ti35Cr 65-x Vx. Giovanni Mazzolai, Dept. of Physics, University of Perugia, Perugia, Italy.

The solubility of H has been investigated at low H contents in alloys of the series Ti35Cr65-xVx (x=18, 22). The enthalpy and entropy for H solution have been determined from the Sieverts law, which has been found to be valid at high temperatures ( T>800 K) and low H contents (nH=H/Me<0.07). Also the kinetics of H absorption/desorption in these alloys has been investigated together with the H diffusivity. It has been found that H can be absorbed/desorbed only at temperatures higher than about 600 K. Below this temperature, oxide films, which form on the external surface of the sample even under vacuum, prevent H uptake and release. The diffusion coefficient D=Doexp(-W/kT) obeys an Arrhenius-type of law and is about the same in the two alloys. The diffusion parameters are: W=0.31 eV; D0= 4x10-7m2/s


S3.27
Microwave Assisted Desorption of Hydrogen from Sodium Alanate (NaAlH4) for Improved Energy Efficiency in the Retrieval of Stored Hydrogen. Rahul Krishnan¹, Larry Hurtt², Dinesh Agrawal³ and Tabbetha Dobbins^1,4; ¹Institute for Micromanufacturing, Louisiana Tech University, Ruston, Louisiana; ²SYNO-Therm Co, Changsha City, Hunan Province, China; ³Materials Research Institute, Pennsylvania State University, University Park, Pennsylvania; ⁴Department of Physics, Grambling State University, Grambling, Louisiana.

Hydrogen has been desorbed from Sodium Alanate (NaAlH4) using microwave irradiation. Gravimetric analysis shows weight loss from the sample during microwave exposure. Samples subjected to microwave treatment were analyzed for phase composition using powder X-ray diffraction (XRD). The gas evolved upon microwave treatment is confirmed to be hydrogen by mass spectrometry analysis. Microwave heating method is well recognized for its energy efficiency in general. It is expected that in the desorption of H2 from NaAlH4, less energy will be used than in a conventional process. Temperature profiles show that NaAlH4 heats with increasing rates as the input power to the microwave furnace is increased. The particle morphology changes with microwave treatment have been characterized using Ultra-Small Angle X-ray Scattering (USAXS). Additionally, our work shows that adding TiCl2 as a dopant improves the desorption kinetics of NaAlH4 with microwave heating.


S3.28
The Energetics of Hydrogen Adsorbed in Nanoporous Carbon. An ab initio Simulational Study. R. M. Valladares¹, Alexander Valladares¹, A. G. Calles¹ and Ariel Alberto Valladares²; ¹Physics Department, Fac. de Ciencias, Universidad Nacional Autónoma de México, Mexico, D.F., Mexico; ²Condensed Matter Department, Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Mexico, D.F., Mexico.

Porous carbon has consistently been considered a promising material to store hydrogen. In powder form it has been used for a long time as a reactive cleaning agent to get rid of unwanted byproducts in catalytic processes. Carbon displays a rich variety of bond types that makes it very versatile; these bonds lead to structures of molecules or solids not encountered in other elements belonging to group IV of the Periodic Table. Nanoporous carbon should then manifest some of this richness in structure; in particular the internal surfaces of the pores may show a different density of dangling bonds than those of nanoporous silicon [1] since bond rearrangement in carbon due to hybridization of the sp² and sp¹ types may play a role in the total number of dangling bonds existing on these surfaces. A priori we expect nanoporous carbon to have a lower density of dangling bonds than nanoporous silicon. In this work we report studies of a porous atomic structure of carbon with 50 % porosity that, due to the size of the supercell falls within the regime of nanoporous carbon. This structure is generated using a novel ab initio molecular dynamics procedure that leads to more realistic materials [1]. As with silicon [2] our carbon sample has been hydrogenated both by attaching hydrogens to the dangling bonds and relaxing it, and by placing hydrogen within the cavity of the pore and applying a molecular dynamics process at 300 K to see if the hydrogen is either physisorbed or chemisorbed. The total energy of the supercell was obtained before and after the hydrogen incorporation. From these values the average energy per hydrogen atom was then deduced. We compare our results to CH bond energies and hydrogen chemisorption in silicon [2] and carbon [3]; conclusions are drawn. 1. A. A. Valladares et al. Accepted for publication MRS Proceedings Symposium QQ, MRS, Fall Meeting, 2006. 2. A. A. Valladares et al. Accepted for publication MRS Proceedings Symposium ZZ, MRS, Fall Meeting, 2006. 3. E.R.L. Loustau et al., J. Non-Cryst. Solids 352 1332 (2006).


S3.29
Activation Energies of the Thermal Decomposition of LiNH₂ under NH₃ Atmosphere. Kosei Nakamura^1,3, Hiroyuki T. Takeshita², Nobuhiro Kuriyama³, Shingo Ikeda³ and Tetsu Kiyobayashi³; ¹Graduate School of Engineering, Kansai University, Suita, Osaka, Japan; ²Materials and Bioengineering, Kansai University, Suita, Osaka, Japan; ³Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Industrial Science and Technology, Ikeda, Osaka, Japan.

After Chen et al. suggested Li₃N^[1] as a hydrogen storage material (Reaction (1)), LiH-LiNH₂ mixture^[2]-[6] has been studied because Reaction (2) takes place at lower temperatures than Reaction (1) for application. The peak top temperature of the thermal desorption spectrum (TDS) of Reaction (2) is around 500K. LiNH₂ +2LiH ↔ Li₂NH + LiH + H₂ ↔ Li₃N + 2H₂ (10.4mass%H₂) (1) LiNH₂ + LiH ↔ Li₂NH + H₂ (6.5mass%H₂) (2) Although the thermal decomposition of LiH-LiNH₂ mixture releases H₂ as a major product, a small amount of NH₃ is released as a byproduct according to the thermal decomposition of LiNH₂ alone (Reaction (3)), whose peak top temperature of TDS is 600-700K. 2LiNH₂→Li₂NH+ NH₃(3) The mechanism of NH₃-release must be elucidated to prevent the NH₃-release because NH₃ in the H₂ poisons the catalyst of polymer electrolyte membrane fuel cells. The NH₃-release also amounts to the irreversible loss of constituent nitrogen element from the mixture which eventually leads to the loss in hydrogen capacity. As for the H₂ desorption mechanism of the LiH-LiNH₂ mixture, Hu et al.^[2] and Ichikawa et al.^[7] suggest that the quick reaction of LiH with NH₃ (Reaction (4)) promotes the thermal decomposition of LiNH₂ in the mixture, making the H₂ release 100-200 K lower than the decomposition of LiNH₂ (Reaction (3)). LiH+NH₃ →LiNH₂+H₂(4) We have observed the TDS spectrum of the decomposition of LiNH₂ under He atmosphere shifted to lower temperature than in case of He+NH₃ atmosphere. This observation together with Hu and Ichikawa’s studies implies that the promotion effect of LiH to the decomposition of LiNH₂ is brought by the reduction of NH₃ atmosphere around LiNH₂ in the mixture. In present study we investigate the influence of NH₃ product gas on the activation energies of the thermal decomposition of LiNH₂ by means of the thermogravimetry, differential thermal analysis and mass spectrometry. NH₃ from the decomposition of LiNH₂ under He+NH₃ shift to higher temperature than under He. Increase in the pressure of NH₃ increases the activation energies of Reaction (3) by 40-130kJ/mol. Acknowledgment This study is financially supported by New Energy and Industrial Technology Development Organization (NEDO), Japan. References [1] P. Chen, Z. Xiong, J. Luo, K. Ten, Nature, 420 (2002), 302-304 [2] Y. H. Hu., E. Ruckenstein, J. Phys. Chem A, 107 (2003) 9737-9739 [3] T. Ichikawa, N. Hanada, S. Isobe, H. Fujii, J. Alloys Compd., 365 (2004), 271-276 [4] Y. Nakamori, Y. Yokoyama, S. Orimo, J. Alloys Compd. 377 (2004) L1. [5] Y. Kojima, Y. Kawai, Chem. Commun. (2004) 2210. [6] G.P. Meisner, F.E. Pinkerton, M.S. Meyer, M.P. Balogh, M.D. Kundrat, J. Alloys Compd. 24 (2005) 404-406. [7] T. Ichikawa, H. Fujii, N. Hanada, S. Isobe, H. Leng, J. Phys. Chem B, 108 (2004) 7887-7892.


S3.30
Abstract Withdrawn

S3.31
Abstract Withdrawn


S3.32
NMR Investigations of Carbon Aerogels for Hydrogen Storage. Julie Lynn Herberg¹, Robert S. Maxwell¹, Ted Baumann¹ and Joe Satcher¹; ¹Chemistry and Material Science, Lawrence Livermore National Laboratory, Livermore, California; ²Physics, University of North Carolina, Chapel Hill, North Carolina.

A basic understanding of hydrogen sorption behavior in materials will be critical to the transportation sector needs of high gravimetric and volumetric density. An important criterion for effective physisorption is a high surface area that exposes a large number of sorption sites to ad-atom or ad-molecule interaction. Previous work with carbon-based sorbants has shown that the amount of surface excess hydrogen adsorbed at 77 K varies linearly with surface area (1 wt% H2 per 500 m^2/g of surface area)(1). Recent progress has been made in the design of high surface area sorbants, such as activated carbons, carbon aerogels, and metal-organic frameworks (MOFs), that show high gravimetric density of adsorbed hydrogen (1,2). For example, activated carbon aerogels prepared at LLNL have shown a hydrogen uptake as high as 5.3 wt% at 77 K (1). Despite this progress, these materials still fall short of the gravimetric and volumetric storage targets set for the transportation sector. In addition, significant work is required to increase the hydrogen sorption enthalpies if such materials are to be used at practical temperatures. To address these issues, a fundamental understanding of the interaction between molecular hydrogen and the surface of the sorbant is required. We will present 11B NMR and Xenon NMR data to provide insight into understanding the surface of the carbon adsorbents and the interaction between molecular hydrogen and the carbon-supported adsorbent. 1. H. Kabbour, T.F. Baumann, J.H. Satcher et al., The Journal of Chemical Physics in press (2006). 2. J.L. Rowsell, E.C. Spencer, J. Eckert et al., Science 309, 1350 (2005). This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory, under contract # W-7405-ENG-48.


S3.33
Hydrogen Permeability Measurements of Pure Nb and NbTi Solid Solution Alloys Prepared by Arc Melting. Naoyoshi Ota, Kazuhiro Ishikawa and Kiyoshi Aoki; Department of Materials science, Kitami Institute of Technology, Kitami, Hokkaido, Japan.

In recent years, non-palladium based hydrogen permeation alloys have actively been investigated by several research groups. Group 5 metals such as V, Nb and Ta showing large hydrogen solubility and high hydrogen diffusivity are promising for hydrogen permeation membranes, because hydrogen permeability is the product of hydrogen solubility and hydrogen diffusivity. However, it is recognized that these metals suffer severe hydrogen brittleness and are pulverized spontaneously during hydrogenation. Then, they are unusable as a hydrogen permeation alloy. However, it is still uncertain why they are easily broken in a hydrogen atmosphere. In other words, there is some possibility that these metals are used as hydrogen permeation alloys. Among 5 metals, Nb shows the highest hydrogen permeability, and is the most cheap and abundant metal. In the present work, pure Nb and Nb_1-xTi_x alloys were prepared by arc melting in an arc melting and their Φ were measured by the gas flow meter. Pure Nb and Nb_1-xTi_x alloys ingots were prepared by arc melting in an Ar atmosphere. Disk samples of 12mm in diameter and 0.7mm in thickness were cut from the ingots by spark erosion method. Both sides of the disk surface were polished with an abrasive paper and a buff, and coated with 190nm thickness Pd by the DC magnetron sputtering machine to avoid oxidation and enhance hydrogen dissociation and recombination. Hydrogen permeation tests were carried out using a mass-flow meter. Microstructural observation and phase identification of the samples were carried out with a scanning electron microscope (SEM) and an X-ray diffractometer (XRD), respectively. Hydrogen permeability of as-cast Nb and Nb_1-xTi_x (x = 0.1, 0.2, 0.3, 0.4 and 0.5) alloys prepared by arc melting were successfully measured using a conventional gas permeation method. Hydrogen permeability at 673K of pure as-cast Nb was 8.3×10^-8(mol H₂m^-1s^-1Pa^-0.5). Hydrogen permeability at 673 K increased with increasing Ti content at once and decreased, i.e. it showed the maximum value of 20×10^-8(mol H₂m^-1s^-1Pa^-0.5) for Nb_0.7Ti_0.3 alloy. We discuss why hydrogen permeability of the as-cast pure Nb and the NbTi alloy is measurable on the basis of the microstructural observation.


S3.34
Hydrogen Permeation in Nb-TiNi Alloy with Anisotropic Microstructures. Kazuhiro Ishikawa¹, Sho Tokui² and Kiyoshi Aoki¹; ¹Department of Materials Science, Kitami Institute of Technology, Kitami, Hokkaido, Japan; ²Mitsubishi Materials Corporation, Okegawa, Saitama, Japan.

Pd alloys are commercially used for a separation and purification of hydrogen, but Pd is expensive and rare in resources. Although V, Nb or Ta has 10-100 times higher hydrogen permeability than that of pure Pd, they show severe the hydrogen embrittlement in a hydrogen atmosphere. Recently, we have found out that coexistence of the B2-TiNi phase with Nb suppress the hydrogen embrittlement. For example, the Nb₄₀Ti₃₀Ni₃₀ (mol %) alloy, consisting of the (Nb, Ti) solid solution and the TiNi phases, shows the higher hydrogen permeability than that of pure Pd without the hydrogen embrittlement. This alloy is a duplex phase one, so that its hydrogen permeability may depend strongly on the microstructure. In other words, its hydrogen permeation performance must drastically be improved by the controlling of the microstructure. In the present work, the relation between the microstructure formed by rolling and subsequent annealing and hydrogen permeability is investigated for developing the high performance hydrogen permeation alloy. The large size Nb-TiNi alloys were prepared using an induction melting under an Ar atmosphere. The alloy ingot was forged and rolled, then annealed at various temperatures for distinct periods for microstructural controlling. The disk sample of 12 mm in diameter was cut from the sheet sample using a spark erosion cutting machine. Microstructural observation and phase identification were carried out using a scanning electron microscope (SEM) and an X-ray diffractometer (XRD). After both surfaces of the alloy disk were coated by Pd using a sputtering machine, the hydrogen permeability measurement was carried out using a mass-flow meter. After forging and rolling, an anisotropic microstructure was formed in this alloy. That is, the (Nb, Ti) phase was strongly elongated along the rolling direction and compressed to the vertical one. The value of hydrogen permeability along the rolling direction was 30 times higher than that of along vertical one. In this case, hydrogen atoms could diffuse for long distance in the (Nb, Ti) phase. On the contrary, hydrogen atoms must pass frequently the (Nb, Ti)/TiNi phase boundary. Resulting from further annealing, hydrogen permeability along the rolling direction was reduced. The elongated (Nb, Ti) phase was cut into pieces and changed to spherical morphology, which implied that the continuity of this phase is reduced along the rolling direction. It was concluded that maximum hydrogen permeability can be obtained in the structure in which the (Nb, Ti) phase, which contributes the hydrogen permeability, was elongated along the direction for hydrogen diffusion.


S3.35
Microstructures and Hydrogen Permeability of Nb-TiNi Alloys with Various Ti/Ni Ratios. Tetsuya Kato, Kazuhiro Ishikawa and Kiyoshi Aoki; Department of Materials Science, Kitami Institute of Technology, Kitami, Hokkaido, Japan.

Hydrogen produced by steam reforming includes impurities such as CO and CO₂, so that it must be purified by some methods. Pd-based alloys are used for purification of hydrogen gas, but Pd is too expensive and rare in resources, so that non-Pd based alloys are strongly desired. Recently, we have demonstrated that the Nb-TiNi alloys having the Ti/Ni ratio =1.0 show high hydrogen permeability (Φ) and large resistance against the hydrogen embrittlement. However, their performance is insufficient for industrial applications. The value of Φ increases with increasing Nb content in the Nb-TiNi alloys with Ti/Ni ratio=1.0, but the higher Nb content alloys suffer from the hydrogen embrittlement. In the present work, the effect of Ti/Ni ratio on the microstructures, crystal structures and Φ of Nb-TiNi alloys is investigated and discussed on the basis of the experimental data. The Nb-TiNi alloy ingots were prepared by arc melting in an Ar atmosphere. Disk samples of 12mm in diameter and 0.7mm in thickness were cut from the ingots by spark erosion method. Both sides of the disk surface were polished with an abrasive paper and a buff, and coated with 190nm thickness Pd by the DC magnetron sputtering machine to avoid oxidation and enhance hydrogen dissociation and recombination. Hydrogen permeation tests were carried out using a mass-flow meter. Microstructural observation and phase identification of the samples were carried out with a scanning electron microscope (SEM) and an X-ray diffractometer (XRD), respectively. Chemical composition of the samples was determined by an electron dispersion X-ray spectroscope (EDS). In Nb₄₀Ti_30+xNi_30-x alloys, a brittle NbNi compound is formed in the side of the Ti/Ni ratio <1.0, which reduces both Φ and resistance against the hydrogen embrittlement. On the contrary, the alloys with the Ti/Ni ratios between 1.0 and 1.3 have higher Φ values. The SEM observation indicates that the volume fraction of the (Nb, Ti) phase, which contributes mainly hydrogen permeability, increases with increasing Ti/Ni ratio. This microstructural change can be explained by the phase equilibrium between (Nb, Ti) and TiNi phases, i.e. “lever rule”. Solubility of Nb in the TiNi phase decreases with increasing Ti/Ni ratio. On the other hand, that of Ti or Ni in Nb is almost constant independent of the Ti/Ni ratio. This behavior causes the change of the volume fraction of the (Nb, Ti) phase, which increases However, a brittle Ti₂Ni intermetallic compound is also formed in the alloy when the Ti/Ni ratio is above 1.3, which reduces both Φ and ductility of the alloys. Thus, Φ in Nb-TiNi alloys can be controlled by the Ti/Ni ratio at the constant Nb content.


S3.36
Effect of the Excess Volume of Lattice Defects on the Enthalpy of Formation and Desorption Temperature of Metal Hydrides. Vincent Berube¹, Gregg Radtke², Gang Chen² and Millie Dresselhaus¹; ¹Physics, MIT, Cambridge, Massachusetts; ²Mechanical Engineering, MIT, Cambridge, Massachusetts.

Metal and complex hydrides offer very promising prospects for hydrogen storage that reach the DOE targets for 2015. However, slow sorption kinetics and high release temperature must be addressed to make automotive applications feasible. Reducing the enthalpy of formation by destabilizing the hydride reduces the heat released during the hydrogenation phase and conversely allows desorption at a lower temperature. High-energy ball milling has been shown to decrease the release temperature, increase reaction kinetics and lower the enthalpy of formation in certain cases. Increased surface and grain boundary energy could play a role in reducing the enthalpy of formation, but the predicted magnitude is too small to account for experimental observations. As the particle and grain sizes are reduced considerably under high-energy treatments, structural defects and deformations are introduced. These regions can be characterized by an excess volume due to deformations in the lattice structure, and have a significant effect on the material properties of the hydride. We propose the use of two thermodynamic models to characterize the excess energy present in the deformed regions. The equations of state (EOS) provided by the models are used to explain the change in physical properties of metal hydrides. The EOSs are compared to density functional theory calculations at zero temperature for Mg and MgH2 to determine the range of excess volume over which they accurately predict the energy change. Also, an experimental investigation using the TEM to study the effect of lattice deformations and other nanostructures on the desorption process is underway.


SESSION S4: Metal Hydride Hydrogen Storage III
Chairs: Gholam-Abbas Nazri and Aline Rougier
Tuesday Morning, November 27, 2007
Room 309 (Hynes)
8:30 AM *S4.1
Reactivity of Metal Hydrides toward Lithium Storage - A Bridge between NiMeH and Li-ion Technologies. Yassine Oumellal¹, Aline Rougier¹, Jean-marie Tarascon¹, Gholam-abbas Nazri² and Luc Aymard¹; ¹LRCS, Amiens, France; ²GM, Warren, Michigan.

In conjunction with the challenge of this decade devoted to the replacement of fossil energy, especially for mobile applications, improvements in terms of storage capacities, kinetics, and energy density must be achieved. In this context, new concepts must merge to succeed such goal. The use of metal hydride toward Li-ion technology coupling the advantages of high gravimetric and volumetric energy density of metal hydride anodes for application in Li-ion batteries is proposed as a promising new opportunity. This scenario is supported by recent experimental investigations showing, for the first time, the reversible electrochemical reactions of metal hydrides during charge and discharge. MgH2 with a hydrogen weight capacity of 7,6 wt.%, abundance, low cost and environmental acceptance has been chosen as one of the best candidates for such study. Improvement of the reversible electrochemical reaction of metal hydrides in lithium cell has been achieved through mechanochemical formation of an hydride-carbon composite. The reversible electrochemical Li-driven conversion-reconstruction reaction has been evidenced by x-ray diffraction analysis of the metal hydride-carbon composite at various states of charge and discharge. Finally, the reaction of conversion can be applied to other metals and intermetallics hydrides, opening new routes of preparing tailor made materials for energy storage applications.


9:00 AM S4.2
Preparation and Hydrogen Storage Properties of Nanostructured Mg50Co50 BCC Alloys. Huaiyu Shao, Kohta Asano, Hirotoshi Enoki and Etsuo Akiba; National Institute of Advanced Industrial Science and Technology, Tsukuba, Japan.

Much less research work has been done on Mg-Co system as hydrogen storage materials than that on Mg-Ni system though Mg2CoH5 (4.5 mass%) has higher hydrogen capacity than Mg2NiH4 (3.6 mass%). Body centered cubic (BCC) structure metals/alloys have been widely studied for hydrogen storage materials such as V, Ti-V-based or Ti-Cr-based, Mg-V-Tm, Mg-Ca-V et al, which show excellent hydrogen storage properties. Compared with face centered cubic (FCC) and hexagonal close packing (HCP) lattice, BCC lattice has lower packing efficiency and more octahedral and tetrahedral sites per atom, which means more interstitial sites for hydrogen occupancy are available in BCC structure materials. Here we present the fabrication and hydrogen storage property study of nanostructured Mg50Co50 BCC alloys prepared by improved MA method. The Mg50Co50 BCC alloy could absorb hydrogen with a fast rate at low temperatures (below 273 K), which is great improvement for the properties of Mg-based alloys. The obtained Mg50Co50 BCC alloy with nanostructure can absorb hydrogen with a capacity of 2.67 mass% at 258 K with the hydrogen pressure up to 8 MPa. After the hydrogenation, the BCC structure is kept in the formed hydrides. The Mg50Co50 BCC alloy shows a lattice parameter of 0.3000(1) nm and it is interesting that we find all of the current BCC structure systems such as V, Ti-Cr-V, Mg-V-Tm and Mg-Ca-V, which show good hydrogen storage properties at room temperature, show lattice parameters in the range of 0.300~0.305 nm. There seems to be an optimum lattice parameter at about 0.30 nm and tetrahedral hole radius of about 38 pm for BCC lattice to absorb hydrogen at ambient temperature and pressure due to certain geometrical effect. [1] J.J. Reilly and R.H. Wiswall: Inorganic Chemistry Vol. 7 (1968), p. 2254 [2] E.M. Cramer, H.P. Nielsen and F.W. Schonfeld: Light Metal Age Vol. 5 (1947), p. 6


9:15 AM S4.3
Fabrication and Characterization of Nanoscale MgH2 for Hydrogen Storage Applications. Adam F Gross, Sky L Skeith, Ping Liu and John J Vajo; HRL Laboratories, Malibu, California.

Nanoscale materials are promising for reversible hydrogen storage applications because their small particle size and high surface area provide a means to potentially overcome barriers to hydrogen dissociation and diffusion that limit the sorption kinetics in many bulk metal hydrides. A porous host material may be used as a template to create nanoscale metal hydrides and will prevent sintering of the hydride upon thermal cycling. We create a nanostructured hydride by filling a porous carbon aerogel with Mg through a two-stage process with Ni as wetting agent. First, Ni (~1.5 wt%) is incorporated into the aerogel by reducing a dried impregnated solution of Ni(NO3)2 in acetone (0.2 M) with forming gas at 500 °C for 2 hr. Next, Mg is incorporated by heating the aerogel with powdered Mg in a sealed tube at 900 °C for >60 hr. With this process we have incorporated up to 21 wt% Mg into an aerogel with a pore size distribution peak at 25 nm and a total pore volume of 1.3 cm3/g. Additionally, we have incorporated up to 12 wt% Mg into an aerogel with a pore size distribution peak of 13 nm and a total pore volume of 0.8 cm3/g. XRD patterns of both of these materials show broad peaks indicating nanometer domains of Mg. Complete hydrogenation of the Mg filled 25 nm aerogel occurred during heating in our Sieverts apparatus at 2 °C/min in 100 bar H2. The midpoint of the hydrogenation reaction occurred at 70 °C. After hydrogenation of the Mg was complete and the sample was cooled, complete dehydrogenation was observed as a result of heating the sample to 300 °C. The equilibrium pressure during dehydrogenation at 250 °C (and ~ 3 wt% desorbed relative to the MgH2) was measured to be 0.36 bar which is close to the accepted value of 0.41 bar for bulk Mg. Thus confinement of Mg in this aerogel does not change the thermodynamics of hydrogenation. However, isothermal measurements at 250 °C with P(H2) < ~0.05 bar give a dehydrogenation rate of ~20 wt%/hr which is comparable to Ti catalyzed ball milled MgH2 and faster than bulk MgH2. Although much higher Mg loadings are required for practical applications, this work along with our previous work on LiBH4 filled aerogels indicates improved kinetics from nanoscale confinement occurs for multiple metal hydrides.


9:30 AM S4.4
Quantitative Analysis of the Effects of Cycling and Alloy Additions on Material Microstructure and Reaction Kinetics in Thin Film Mg/MgH₂ Systems. Stephen T. Kelly and B. M. Clemens; Materials Science and Engineering, Stanford University, Stanford, California.

Magnesium is an attractive material for hydrogen storage because it stores an appreciable amount of hydrogen (7.6 wt.%) as MgH₂, is abundant in the earth's crust and is relatively inexpensive. Understanding of the structural changes and associated kinetics for the magnesium/magnesium hydride phase transition is crucial to engineering practical metal hydride hydrogen storage materials involving magnesium. Using UHV sputter deposited thin films we are able to precisely control the composition and microstructure of our samples through the use of co-sputtering to deposit alloy films and appropriately chosen substrates for epitaxial growth. Using these highly controlled samples, we examined the structural changes and associated kinetic mechanisms present in the magnesium/magnesium hydride phase transition on nearly the atomic scale. By utilizing in-situ and ex-situ XRD with point and 2D image plate detectors we quantified the effects of alloy additions and hydriding/dehydriding cycling on both material microstructure and reaction kinetics. We saw evidence for solid phase epitaxial regrowth mechanisms as well as textural degradation as the samples were cycled, depending on the charging and discharging conditions and the presence of alloy additions.


9:45 AM S4.5
Crystal Shapes of MgH₂ Fiber Prepared by Vapor Deposition in Pressurized Hydrogen. Itoko Saita¹, Tomohiro Akiyama² and Etsuo Akiba¹; ¹Energy technology Research Institute, AIST, Tsukuba, Ibaraki, Japan; ²CAREM, Hokkaido University, Sapporo, Japan.

It is difficult to prepare pure MgH₂ by the conventional solid-gas reaction between magnesium and hydrogen because hydrogen diffusivity in Mg is low. We proposed the novel method of MgH₂ synthesis depositing magnesium vapor in an atmosphere of pressurized hydrogen. The deposit consists of fibrous MgH₂ single crystal. The diameter of the fiber is about 500 nm and the length is over 100 μm. The fibrous shape is the unique feature of the MgH₂ prepared by vapor deposition. In this study, we examined the effects of the deposition conditions on the MgH₂ morphology. We prepared a P_H2-T diagram of MgH₂ crystal shapes and found that the conditions corresponded to the van’t Hoff plot of solid-gas reaction between Mg and H₂: Around the van’t Hoff plot, the MgH₂ deposits had two shapes of curved fibers and straight fibers. Those were involved by two crystal growth mechanisms; epitaxial and non-epitaxial crystal growth. On the other hand, the deposits were straight fibers when the hydrogen pressure was significantly higher than the equilibrium pressure of the given temperature. We conclude that the experimental conditions controls both the crystal growth mechanism and the crystal morphology.


10:30 AM S4.6
Evolution of Crystal Orientation in Obliquely Deposited Mg Nanorod Arrays for Hydrogen Storage Applications. Mehmet Fatih Cansizoglu and Tansel Karabacak; Applied Science, University of Arkansas at Little Rock, Little Rock, Arkansas.

Crystal orientation (texture) is an important parameter in the hydrogen absorption and desorption properties of various materials. In this study, we investigate the formation of magnesium nanorod arrays with crystal orientations that are not normally observed in conventional Mg thin films. Mg nanorods are produced using an oblique angle deposition technique through a physical self-assembly process. In this study sputtering and thermal evaporation systems are utilized for the growth of Mg nanorods and thin films on silicon wafer pieces. We present the effects of deposition angle, incident particle energy, and substrate rotation speed on the evolution of different crystal orientations through a detailed X-ray diffraction analysis. It is discussed that under oblique incidence, evolution of crystal orientations with lower adatom mobility are promoted due to the shadowing effect.


10:45 AM S4.7
Comparisons between MgH₂ - and LiH-Containing Systems for Hydrogen Storage Applications. Tippawan Markmaitree, William Osborn and Leon L. Shaw; Chemical, Materials & Biomolecular Engineering Department, University of Connecticut, Storrs, Connecticut.

The present study compares the dehydrogenation kinetics of (2LiNH₂ + MgH₂ and (LiNH₂ + LiH) systems and their vulnerabilities to the NH₃ emission problem. (2LiNH₂ + MgH₂) and (LiNH₂ + LiH) mixtures with different degrees of mechanical activation are investigated in order to evaluate the effect of mechanical activation on the dehydrogenation kinetics and NH₃ emission rate. The activation energy for dehydrogenation, the phase changes at different stages of dehydrogenation, and the level of NH₃ emission during the dehydrogenation process are studied. It is found that the (2LiNH₂ + MgH₂) mixture has a higher rate for hydrogen release, slower rate for approaching a certain percentage of its equilibrium pressure, higher activation energy, and more NH₃ emission than the (LiNH₂ + LiH) mixture. On the base of the phenomena observed, the reaction mechanism for the dehydrogenation of the (2LiNH₂ + MgH₂) system has been proposed for the first time. Approaches for further improving the hydrogen storage behavior of the (2LiNH₂ + MgH₂) system are discussed in the light of the newly-proposed reaction mechanism.


11:00 AM S4.8
MgH2 by Gas Phase Condensation : Nanostructure Morphology and Hydrogen Sorption Behaviour. Ennio Bonetti¹, Elsa Callini¹, Amelia Montone², Luca Pasquini¹, Emanuela Piscopiello³ and Marco Vittori Antisari²; ¹Dept. Physics, University of Bologna, Bologna, Italy; ²FIM Dept., ENEA Casaccia, Rome, Italy; ³FIM Dept., ENEA, Brindisi, Italy.

Several issues have been extensively explored in last years in order to improve the hydrogen sorption properties of magnesium. These include both alloying with transition metals to lower the high thermodynamic stability and intermixing with catalysing particles to improve the sorption kinetics. A synergic route towards improvement of the Mg sorption behaviour consists in a careful nanostructuring and control of defects morphology achieved by employing tailored synthesis procedures. Mechanical milling in inert or reactive atmosphere also including the addition of transition metal oxides has been usually employed being relatively simple and easily scaled up for industrial production. A crucial aspect of this procedure in order to reduce the desorption temperature by reducing the kinetic constraints is connected to the possibility of obtaining a very fine dispersion of nanometer sized particles/grains avoiding the cold welding effects and the structural coarsening by thermal cycling, both leading unavoidably to strong degradation of sorption properties. The gas phase condensation technique extensively employed to prepare loose nanoparticles agglomerates of pure metals or clusters of nanoparticles, also with a core shell morphology, was systematically employed in this work to prepare clusters of Mg and MgH2 nanoparticles . The nanoparticles morphology, clustering degree and structure stability was investigated by XRD and electron microscopy. To this purpose the as prepared clusters dispersed on holey carbon grids were observed by TEM. High resolution and spectrally resolved images, together with X-ray and energy loss spectroscopy, were used to characterize the particle structure, the particle morphology and the elemental distribution, with particular attention to the presence of poisoning elements like oxygen. Ab- de-sorption runs employing high pressure differential scanning calorimetry were employed to investigate thermodynamic properties of the Mg and MgH2 nanoparticle clusters. The structural and functional properties are compared systematically with those of the same powders prepared by ball milling and reactive ball milling. Some specific features of the morphology of the particles prepared by gas condensation which seem particularly appealing to obtain improved sorption kinetics are presented.


11:15 AM S4.9
Hydrogen Sensors Based on PdAg Alloy Nanoparticles. Manika Chawla Khanuja¹, Bodh Raj Mehta¹ and S. M. Shivaprasad²; ¹PHYSICS, IIT DELHI, New Delhi, Delhi, India; ²2Surface Physics and Nanostructures, National Physical laboratory, New Delhi, Delhi, India.

In the search for materials that enhance catalytic interaction with hydrogen, PdAg bimetallic alloy is highly potent due to its large sticking coefficient, low activation barrier to adsorption and high diffusion rate for hydrogen. Also the problems that exist in pure Pd-H system like α→b phase transition, which is accompanied by a volume expansion of 12%, resulting in a large compressive stress in the Pd thin film and deactivation of Pd in the presence of H2S and H2O can be overcome by using PdAg alloy. In recent years, “nanoparticle route” has emerged as a promising way of tailoring the material properties due to enhanced surface area and quantum confinement effects. In the present study, hydrogen-sensing response of PdAg nanoparticles formed by inert gas evaporation has been studied in terms of change in electrical resistance during hydrogen loading - deloading cycles. The studies of electronic and geometrical structures of bimetallic AgPd alloy nanoparticles have been done by x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD) and atomic force microscopy (AFM) technique. XPS studies reveal that as the size is changed, density of states near the Fermi level, binding energy of the core-levels and position of the d-band centroid also change that influence the PdAg interaction with hydrogen. XRD studies reveal increase in lattice constant in PdAg as compared to Pd that can be responsible for the enhanced hydrogen adsorption. In situ H-loading has been done on different sized PdAg nanoparticle samples and experimental results demonstrate that hydrogen sensors based on PdAg alloy nanoparticles exhibit high sensitivity, fast response and excellent reversibility.


11:30 AM S4.10
Study of H-induced Structural Changes in Metal Membranes for Hydrogen Purification. Diana Elena Nanu¹, Amarante J. Boettger¹, Wim G. Haije², Jaap F. Vente², Bas B. vanAken² and Matt G. Tucker³; ¹Materials Science and Engineering, Delft University of Technology, Delft, Netherlands; ²Energy research Centre of the Netherlands, Petten, Netherlands; ³ISIS Facility, Rutherford Appleton Laboratory, Chilton, United Kingdom.

One of the critical technical challenges for the transition to a hydrogen based economy is the materials development for hydrogen production, purification, and storage. Major hurdles occur in the currently envisioned potential materials for hydrogen separation and storage, e.g. low long term performance stability under relevant conditions, limited stability during numerous hydrogen absorption/desorption cycles, low or insufficient hydrogen capacity and/or poor hydrogen desorption kinetics. The design of materials that surpass these problems requires knowledge of the stability of phases and structural changes that occur during H absorption/desorption. These aspects have a great impact on the H absorption and desorption characteristics and are responsible for the short life time of existing materials. Material degradation begins with changes in the structure at (sub)micron-level. For noble metals, one of the main failure mechanisms is deformation and fracture caused by the large specific volume increase during the hydride formation and decomposition. The phase stability range and the volume differences of the phases formed during H absorption/desorption can be modified by alloying. Metal alloys with desirable properties can be designed by combining computational methods and experimental techniques. Recently, we have developed a computational method that describes the possible structural changes during H absorption/desorption in noble metal alloys. The method takes into account the possible ordering of H atoms and/or the correlations between the atomic positions in the metal lattice and the occupation of interstitial sites with H atoms. The method is based on the cluster variation method (CVM) and allows the prediction of the phase stability as function of composition and temperature. Thus multicomponent alloys with specific phases and structures suitable for predetermined process conditions can be designed. This paper concerns in particular the design of metal alloys that are suitable membrane materials for producing high-purity hydrogen. The model and experimental results on the stability of phases and the H-induced structural changes in Pd-alloys are discussed. Alloy systems expected to have various order-disorder transitions on both metal and interstitial sublattices were chosen for this purpose. Neutron diffraction measurements were performed to determine the H (D)-site occupancies and the correlations with the occupancies of the metal atoms. Rietveld refinement and Reverse Monte Carlo methods were used for the analysis of long- and short-range order, respectively. The atomic configurations determined from experimental information are discussed in comparison with those assessed by our CVM approach and by first principles calculations.


11:45 AM S4.11
Ab initio Calculation of H in Pd Dislocation Core. Dallas R Trinkle, Materials Science and Engineering, Univ. Illinois, Urbana-Champaign, Urbana, Illinois.

Pd has a high H solubility for a metal, and a high diffusivity due to low binding energy in the bulk. However, experiments have shown that additional binding sites are available in single-crystal Pd with much higher binding energy, effectively storing residual H in the crystal after removal from high pressure H. The source of the traps are believed to be dislocation cores, acting as nanoscale H traps. Electronic-structure calculations of an isolated Pd dislocation core using flexible boundary conditions--to accurately couple to the long-range elasticity solution--determine the binding energy of H to a dislocation core, the changes in local geometry and electronic structure. These calculations help elucidate the physical ingredients to provide more energetically favorable hydrogen storage traps in materials.


SESSION S5: Complex Chemical Hydrides for Hydrogen Storage
Chairs: Ming Au and Ping Chen
Tuesday Afternoon, November 27, 2007
Room 309 (Hynes)
1:30 PM *S5.1
Phase Boundaries and Reversibility of LiBH₄ / MgH₂ Hydrogen Storage Material. Frederick E. Pinkerton¹, Martin S. Meyer¹, Gregory P. Meisner¹, Michael P. Balogh² and John J. Vajo³; ¹Materials and Processes Lab, General Motors R&D Center, Warren, Michigan; ²Chemical and Environmental Sciences Lab, General Motors R&D Center, Warren, Michigan; ³HRL Laboratories, Malibu, California.

The coupled system LiBH₄ + ½ MgH₂ ↔ LiH + ½ MgB₂ + 2 H₂ demonstrates improved hydrogen cycling thermodynamics compared to either LiBH₄ or MgH₂ alone; in effect, formation of MgB₂ “destabilizes” the decomposition of LiBH₄. Here we establish the thermodynamically and kinetically stable region of the H₂ pressure-temperature phase diagram for reversible hydrogen storage in TiCl₃-catalyzed LiBH₄ + ½ MgH₂. Although MgB₂ formation was thermodynamically favored at elevated temperature, it was kinetically more favorable for MgH₂ and LiBH₄ to decompose independently in a two-step dehydrogenation starting with MgH₂ ↔ Mg + H₂. At high temperature and low H₂ pressure, direct LiBH₄ decomposition is both thermodynamically allowed and kinetically favored, thus the second dehydrogenation step from LiBH₄ produced LiH and amorphous boron along with the Mg metal from the first step. From this state, recombination of LiH with amorphous boron had very poor kinetics, and the system did not fully rehydrogenate. Applying an H₂ gas overpressure of at least 3 bar during dehydrogenation, however, suppressed direct decomposition of LiBH₄ and reaction of Mg with LiBH₄ produced LiH and MgB₂, which was fully reversible. Our results emphasize the importance of understanding the interaction between the equilibrium thermodynamic phase diagram and the reaction kinetics in destabilized complex hydrides. Only by doing so can the most favorable conditions for destabilization be chosen. The best case is to tune the reaction kinetics, for example by finding appropriate catalysts, so that the transformation occurs within the desired thermodynamically stable region. Even when optimal kinetics are difficult to obtain, however, it is still possible, by tuning a suitable thermodynamic variable such as H₂ pressure, to practically navigate the phase diagram and avoid detrimental reaction regions. Finally, we note that our data for direct LiBH₄ decomposition to LiH and B are inconsistent with thermodynamic calculations using database values of the enthalpy of formation ΔH and entropy S of the reactant and product phases, suggesting that the database values of ΔH and S for LiBH₄ are not appropriate for describing its high temperature decomposition.


2:00 PM S5.2
Complex Borohydrides for On-board Hydrogen Storage Applications. Sesha S. Srinivasan¹, Nicole Ramos¹, Michael Jurczyk^1,2, Ashok Kumar^1,2 and Elias Stefanakos^1,3; ¹Clean Energy Research Center, College of Engineering, University of South Florida, Tampa, Florida; ²Mechanical Engineering Department, University of South Florida, Tampa, Florida; ³Electrical Engineering Department, University of South Florida, Tampa, Florida.

Metal/complex hydrides for on-board hydrogen storage applications are in great demand to develop holy grail storage systems that can meet the 2010 DOE and FreedomCAR targets. Some of the well known complex hydride families such as alanates, amides, borohydrides and combination mixtures are mainly based on light weight alkali or alkaline elements (Li, Na, Mg etc.) and possesses high theoretical hydrogen storage capacity. Since NaBH4 and LiBH4 are highly stable systems, it is necessary to explore less stable materials or develop destabilization mechanism for hydrogen storage in complex borohydrides. The present study aims on developing the new transition metal based complex borohydrides such as Zn(BH4)2, Zr(BH4)4, Mn(BH4)2 which are having promising capacities at low decomposition temperatures. Solid state synthesis employing mechano-chemical route is followed. Extensive characterization techniques such as XRD, FTIR, DSC, TGA, TPD/TPR have been carried out. The volumetric hydrogen sorption to understand the kinetics and cyclic stability is performed through high pressure Sievert’s type set up. Gas analysis during thermal decomposition has been performed using GC-MS.


2:15 PM S5.3
The Potential of Binary Lithium Magnesium Nitride - LiMgN for Hydrogen Storage Application. Jun Lu, Zhigang Zak Fang, Youngjoon Choi and Hong Yong Sohn; Metallurgical Engineering, University of Utah, Salt Lake City, Utah.

Metal hydrides and amides are potential candidate materials for hydrogen storage. Lithium- and magnesium-based material systems are among the most promising materials owing to their high hydrogen contents. In the present work, we investigated hydrogenation/dehydrogenation reactions of a binary nitride, LiMgN. LiMgN can be formed by a reaction of MgH2 with LiNH2 in 1:1 ratio. The reaction also releases approximately ~ 8.1 wt% H2 (theoretical value is 8.2 wt%) between 160 and 220 C. The reaction product LiMgN can be rehydrogenated by reacting with H2 under 2000 psi of hydrogen pressure and 160 C with small amount of TiCl3 doping. TGA results showed that about 8.0 wt% of hydrogen was stored in TiCl3-doped LiMgN during the hydrogenation process. The reversible hydrogenation and dehydrogenation mechanisms involving LiMgN and H2 are discussed.


2:30 PM S5.4
Low Temperature Milling of the Lithium Hydride and Amide Hydrogen Storage System. Will Osborn¹, Tippawan Markmaitree¹, Leon Shaw¹, Jainzhi Hu² and Zheng Guo Yang²; ¹Chemical, Materials, and Biomolecular Engineering, University of Connecticut, Storrs, Connecticut; ²Pacific Northwest National Laboratory, Richland, Washington.

Ball milling is a common production route for generating solid-state hydrogen storage materials. In this work, high energy ball milling of a 1:1.1 molar ratio of LiNH₂:LiH was performed at 20°C, -40°C, and -196°C. Specific surface area (SSA), x-ray diffraction (XRD), thermogravemetric analysis (TGA), nuclear magnetic resonance (NMR), and isothermal hydriding/dehydriding cycles were used to evaluate the effect of reducing the temperature of the milling environment. The SSA and crystalline sizes of the liquid nitrogen (LN₂) milled samples showed a 34% increase and 27% decrease, respectively, in comparison with those of the room-temperature milled samples. Although this result is typically indicative of a loss in milling effectiveness, the performance of the LN₂ milled sample showed a 22% gain in the utilization of the theoretical storage capacity in isothermal cycling studies. This increase is attributed to the increased kinetic performance of the LN₂ milled samples. Furthermore, the increased performance of the LN₂ milled system is stable throughout 35 hours of cycling at 285°C. Powders produced by milling at -40°C did show small improvements in the milling efficiency indicators; however, the hydrogen storage characteristics remained consistent with that of the room-temperature milled samples.


2:45 PM S5.5
The Stability and Reversibility of Metallic Borohydrides. Ming Au, Savannah River National Laboratory, Aiken, South Carolina.

In effort to develop reversible metallic borohydrides with high hydrogen storage capacity and low dehydriding temperature, several new materials have been synthesized by modifying LiBH4 with various metal halides and hydrides. The investigation shows that the halide modification effectively reduced the dehydriding temperature through ion exchange interaction. The material LiBH4+xTiF3 desorbs 3.5wt% and 8.5wt% hydrogen at 150oC and 450oC respectively. It also re-absorbed 6wt% hydrogen at 500oC and 70 bar after dehydrogenation. The XRD of the rehydrided samples confirmed the formation of LiBH4. It indicates that the materials are partially reversible at the conditions given. However, a number of other halides did not reduce dehydriding temperature of LiBH4 significantly. TGA-RGA analysis indicated that some halide modified lithium borohydrides evolved diborane during dehydrogenation, but others did not. The formation of diborane caused unrecoverable capacity loss resulting in irreversibility. It is suggested that the lithium borohydrides modified by the halides containing the metals that can not form metal borides with boron are likely to evolve diborane during dehydriding. It was discovered that halide modification reduces sensitivity of LiBH4. Some of materials can be handled in open air without visible reaction.


3:30 PM S5.6
Amide-hydride Combinations for Hydrogen Storage. Ping Chen, Physics, National University of Singapore, Singapore, Singapore.

Strong interactions between amides and hydrides result in hydrogen desorption, which enable these substances potential materials for hydrogen storage. In the previous investigations, ~ 10.5 wt.% and 5.5 wt.% of hydrogen storage capacities have been achieved in lithium amide-lithium hydride (LiNH2-2LiH) and magnesium amide-lithium hydride (Mg(NH2)2-2LiH) systems, respectively. However, the dehydrogenation temperatures (180 - 300°C) are somehow higher than the operation temperature of PEMFC (80°C) due to thermodynamic and/or kinetic reasons. Through compositional and structural alterations, the thermodynamics and kinetics of subject material can be improved. Successful attempts have been achieved to alkali-B-N-H system, in which more than 10 wt% of hydrogen can be desorbed at temperatures around 90°C. A few new structures have been developed and characterized. Part of the research results are derived from an IPHE project.


3:45 PM S5.7
The Direct and Reversible Synthesis of the AlH₃ Adduct of Triethylenediamine (TEDA) Starting with Activated Al and Hydrogen. James Joseph Reilly, Jason Graetz, James Wegryzn, Yusuf Celibi, John Johnson and Wei-Min Zhou; Energy Science and Technology, Brookhaven National Laboratory, Upton, New York.

Aluminum hydride, AlH₃, is the most well known alane. It is a covalent, binary hydride that has been known for more than 60 years. Though thermodynamically unstable under ambient conditions it can undergo a controlled decomposition to produce H₂ and Al. Thus, it is a very attractive compound for on-board automotive, hydrogen storage, since it contains 10.1 weight % hydrogen and has a density of 1.48 g/ml. Graetz and Reilly (J. Alloys and Comp. 424, 262-264, 2006) determined ΔH and ΔG_f298 for the decomposition of α-AlH₃ to be 9.9 and 48.5 kJ/mol AlH₃ respectively. The latter value yields an equilibrium hydrogen fugacity of 5x10^5, bar at 298K. Thus the direct regeneration of AlH₃ from spent Al with gaseous H₂ is deemed impractical. This paper will describe a novel approach to the regeneration of AlH₃ via a reversible reaction using Ti doped Al powder, hydrogen and triethylenediamine. The reaction takes place in a slurry of Al powder and a solution of TEDA in THF at room temperature. The adduct is insoluble and immediately precipitates from solution. The direction of the reaction depends on the departure of the partial pressure of H₂ from the equilibrium pressure. The theoretical, reversible H storage capacity is 2.1 wt %. Pressure composition isotherms in the range 70-90^oC will be presented from which the pertinent thermodynamics have been calculated. It may be noted that the authors are not aware of any other alane reaction that is readily reversible at convenient temperatures and pressures and its implications will be discussed.


4:00 PM S5.8
Correlated Rotational Motions of Ammonia Borane. Vencislav M. Parvanov¹, Nancy Hess¹, Gregory Schenter¹, Luke Daemen², Monika Hartl², Thomas Proffen², Craig Brown³, Shaw Wendy¹, Herman Cho¹, Donald Camaioni¹ and Tom Autrey¹; ¹Fundamental Science Division, Pacific Northwest National Laboratory, Richland, Washington; ²Los Alamos National Laboratory, Los Alamos, New Mexico; ³NIST, Washington, District of Columbia.

Hydrogen rich ammonia borane is attractive for research because of its potential use in energy storage, but a description of its structure has always been a challenge. Ammonia borane is a solid powder and has a structure with NH---HB dihydrogen bonds forming a network near perpendicular to the BN dative bond. Because of this specific molecular arrangement, early studies suggested rotational dynamic behavior for such molecular crystals. Later experimental studies considered independent rotations of ammonia and borane parts of the molecule in the crystal. Measured activation energies for these motions show much higher barrier for BH3 rotation of 21-25 kJ/mol than NH3 rotation of 8-13 kJ/mol. We will present the structure from Neutron Diffraction measurements determined from a Pair Distribution refinement technique, as well as results for rotational activation energies from fitting of Quasi-Elastic Neutron Scattering and 2H NMR data. These energies agree with previously published barriers and confirm the higher barrier of BH3 compared to NH3 rotation. We used electronic structure methods to simulate the rotations and calculate the barriers in cluster models of the structure refined at 175 K. These calculations clearly show the different barriers for ammonia and borane rotations about the BN bond. Furthermore, the model reveals correlation in these motions and suggests that the barrier measured for BH3 does not represent independent rotation. We will conclude with an explanation of the difference in barrier heights based on an energy decomposition analysis of the intra and intermolecular interactions at energy maxima for BH3 and NH3 rotations. This work was supported by the Office of Basic Energy Sciences of the Department of Energy. The Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.


4:15 PM S5.9
Electronic and Vibrational Properties of γ-AlH3. Yan Wang and Mei-Yin Chou; School of Physics, Georgia Institute of Technology, Atlanta, Georgia.

Aluminum hydride (alane) AlH3 is an important material in hydrogen storage applications. It is known that AlH3 exists in multiply forms of polymorphs, where α-AlH3 is found to be the most stable with a hexagonal structure. Recent experimental studies on γ-AlH3 reported an orthorhombic structure with a unique double-bridge bond between certain Al and H atoms. This was not found in α-AlH3 or other polymorphs. Using density functional theory, we have investigated the energetics, and the structural, electronic, and phonon vibrational properties for the newly reported γ-AlH3 structure. The current calculation concludes that γ-AlH3 is less stable than α-AlH3 by 2.1 KJ/mol. Interesting binding features associated with unique geometry of γ-AlH3 are discussed from the calculated electronic properties and phonon vibrational modes. The binding of H-s with higher energy Al-p and -d orbitals is enhanced within the double-bridge arrangement, giving rise to a higher electronic energy for the system. Distinguishable new features in the vibrational spectrum of γ-AlH3 were attributed to the double-bridge and hexagon-ring structures.


4:30 PM S5.10
Novel Combinations of High Density Hydrogen Storage Materials. Andrea Sudik¹, Jun Yang¹, Don Siegel¹ and Christopher Wolverton²; ¹Hybrid & Fuel Cell Vehicle Research, Ford Motor Company, Dearborn, Michigan; ²Materials Science and Engineering Department, Northwestern University, Evanston, Illinois.

One of the most promising classes for reversible hydrogen storage is represented by complex hydrides. In particular elements such as Li, Na, Mg, B, Al, and N form a large number of metal-hydrogen complexes. Unfortunately, the majority of these materials have been deemed practically inadequate for on-board storage applications due to: a) irreversibility b) the high temperatures required for hydrogen release (stemming from poor thermodynamics and/or kinetics) and/or c) liberation of undesirable byproducts which are either hazardous or act as fuel cell poisons. To address these drawbacks, mixtures of complex hydrides such as LiBH₄/MgH₂, LiNH₂/LiBH₄, and LiNH₂/MgH₂ have presented a renewed area of focus for achieving improved hydrogen storage properties. Therefore, recent efforts have focused on incorporation of additives (for example, metal halides, hydrides, amides, etc.) to high-density hydrides (for example, to metal borohydrides, alanates, or ammonia borane, etc.) to realize thermodynamic and/or kinetic enhancements. Here, we present a combined experimental-computational approach for the identification of new combinations of high-density hydrogen storage materials. Experimental results for these mixtures include temperature-programmed desorption mass spectrometry (TPD-MS), powder X-ray diffraction, infrared spectroscopy, kinetic, and/or equilibrium desorption data. We demonstrate reactions with improved hydrogen storage properties in terms of reversibility, kinetics, thermodynamics, and/or hydrogen purity, while preserving high hydrogen-density. Additionally, we show that our experimental results can be corroborated, clarified, and even guided using computational means.


4:45 PM S5.11
Metal Hydride Alloys for Electrochemical Energy Sources Applications. Stoyan Bliznakov¹, Nikolay Dimitrov¹, Tony Spassov^3,2 and Alexander Popov²; ¹Department of Chemistry, SUNY at Binghamton, Binghamton, New York; ²Institute of Electrochemistry and Energy Systems -Bulgarian Academy of Sciences, Sofia, Bulgaria; ³Department of Chemistry, University of Sofia “St. Kl. Ohridski”, Sofia, Bulgaria.

There are continuous efforts to develop high energy and power density and low-cost batteries to meet the ever increasing needs for applications ranging from emergency, spacecraft, defence, communication to electric vehicles, computers, camcorders, cellular phones, power tools and other home appliances. Although in recent years design and packaging of conventional storage batteries such as nickel cadmium, lead acid, nickel metal hydride and lithium batteries have been further improved, there is still great demand for enhanced performance power sources. The innate toxicity of cadmium and lead has also came under scrutiny. The interest to metal hydride (MH) alloys has been growing for the last four decades because of their unique property for reversible high density hydriding-dehydriding, in some cases higher than that of liquid hydrogen. The possibility for hydrogen absorption/desorption, either from gas phase or electrochemically, motivates the MH application as active materials in hydrogen storage units and in electrochemical energy sources. The electrochemical systems that currently use the metal hydrides as anode materials are commercialized rechargeable Ni-MH batteries. Future trends in increasing the energy density of secondary cells using MH anodes and further cost reducing perspectives lay in the possible replacement of the positive nickel electrode by a lighter air gas-diffusion electrode. In this work, high-capacity conventional and advanced multicomponent MH alloys are synthesized by two different methods. A set of AB5 -type intermetallic compounds, with different Al content, are produced by high-frequency vacuum induction melting method. Also, mechanochemical high-energy ball milling in a planetary type mill is employed for synthesis of AB, A2B and mixed (AB5+Mg) -types composite nanocrystalline-amorphous alloys. The alloys are characterized structurally by XRD and morphologically by SEM. A thermodynamical study is performed by measuring the alloys PCT isotherms at various temperatures and by analyzing the van’t Hoff’s plots derived from those isotherms. Different optimized techniques for model electrodes preparation from selected MH alloys are applied. The electrodes are assessed by electrochemical charging-discharging cycles in concentrated potassium hydroxide solution. Summarized and compared in this report are both values for the electrochemical maximum capacity and cycle-life performance of the electrodes prepared from the alloys of interest. Future developments of state-of-the-art high-capacity anodes for advanced batteries and alkaline fuel cells applications are also discussed.


SESSION S6: Hydrogen Storage in Chemical Hydrides and Porous Materials
Chairs: Azarnoush Hoseinmardi and Gholam-Abbas Nazri
Wednesday Morning, November 28, 2007
Room 309 (Hynes)
8:30 AM S6.1
A New Concept of Hydrogen Storage Using Lithium Hydride and Ammonia. Yoshitsugu Kojima¹, Satoshi Hino², Kyoichi Tange² and Takayuki Ichikawa¹; ¹Institute for Advanced Materials Research, Hiroshima University, Higashi-Hiroshima, Japan; ²Department of Quantum Matter, Hiroshima University, Higashi-Hiroshima, Japan.

Ammonia NH3 has a high hydrogen H2 storage capacity of 18 mass% and a similar standard enthalpy change (heat of formation) of -31 kJ/molH2 compared with hydrogen-absorbing alloy (Ti-Cr-V: -34kJ/molH2, LaNi5: -31kJ/molH2). However, the high decomposition temperature (~1000 K) due to mainly slow reaction kinetics limits the practical application of NH3 as a hydrogen storage material. In this study, lithium hydride LiH was mechanically milled in the NH3 atmosphere of 0.9 MPa for 24 h at room temperature. H2 was generated from a reaction between NH3 and LiH at room temperature by the mechanochemical reaction due to the exothermic reaction (ΔH:-40 kJ/molH2). X-ray diffraction and thermogravimetry indicated that LiNH2 was formed by the gas-solid reaction. After this H2 ‘desorption’, the generated LiNH2 solid phase was able to react with H2 gas at 573 K under the high pressure H2 flow of 0.5 MPa to form NH3 and LiH, while the reaction between LiNH2 and H2 could not proceed in a closed vessel at the pressure and the temperature due to likely the low equilibrium NH3 pressure. Thus, we found that the H2 ‘absorption’ and ‘desorption’ of the LiH-NH3 and LiNH2-H2 systems took the following reaction path. LiH + NH3 <-> LiNH2 + H2 (1) In principle, H2 of 8.1 mass% [H2/LiH+NH3] can be reversibly stored in this reaction. Particularly, the formation enthalpy of the H2 absorption was endothrtmic (positive) and quite different from the conventional hydrogen storage materials. Furthermore, we noticed that a threshold pressure of the H2 flow existed to make a progress of this H2 ‘absorption’. It can be understood that the Gibbs free energy difference was changed from positive to negative with the pressure of H2 flow.


8:45 AM S6.2
The Reaction Pathway and Rate-Limiting Step of Dehydrogenation of the LiHN2 + LiH Mixture. Leon L Shaw, William Osborn, Tippawan Markmaitree and Xuefei Wan; University of Connecticut, Storrs, Connecticut.

Hydriding and dehydriding behaviors of the lithium amide (LiNH2) and lithium hydride (LiH) mixture have been investigated extensively since 2002 as potential hydrogen storage media for fuel-cell powered automobile applications. However, the pathway and rate-limiting step of hydriding and dehydriding reactions have not been investigated so far. This study is conducted as the first attempt to investigate the reaction pathway and rate-limiting step of dehydrogenation of the LiNH2 + LiH mixture. Studies of the dehydriding process indicate that dehydrogenation of the LiNH2 + LiH mixture is diffusion controlled. This phenomenon is explained based on a proposed model describing the elemental steps of the dehydriding reaction of the mixture, and related to the evidence obtained from X-ray diffraction and specific surface area measurements of the mixture before and after multiple isothermal hydriding/dehydriding cycles. The implications of this study in how to increase the hydriding and dehydriding rates of the LiNH2 + LiH mixture and other solid-state hydrogen storage materials are discussed.


9:00 AM S6.3
Discovering New Mixed Cation and Anion Borohydrides/Alanates Using Crystal Structure Prediction. Eric Majzoub¹ and Vidvuds Ozolins²; ¹Physics, University of Missouri, St. Louis, St. Louis, Missouri; ²Engineering and Applied Science, University of California, Los Angeles, Los Angeles, California.

Complex ionic hydrides represent the current state of the art in hydrogen storage materials. These compounds consist of the alanates, borohydrides, and nitrogen containing materials, examples include LiAlH4, LiBH4, and Li2NH, respectively. We present a global optimization Monte Carlo algorithm for minimization of the total energy of a crystal structure, utilizing a Hamiltonian which includes electrostatic and soft sphere repulsive energies. Potential energy smoothing is performed using the distance scaling method (DSM). Our DSM-MC optimization correctly predicts the ground states of many known complex ionic hydrides. In addition, we find a structure for LiAlH4 which has lower total energy (DFT) at T=0K than the room temperature observed structure. We show that the total free energy of the LiAlH4 observed crystal structure is lowered below that of the T=0K ground state structure by phonon mode contributions. We address crystal stability and approximate enthalpy estimates for several unreported compounds including Ca(AlH4)(BH4), Li2(AlH4)(BH4), and LiNa(AlH4)(BH4).


9:15 AM S6.4
Destabilization of a LiBH₄ / LiNH₂ Hydrogen Storage System Using MgH₂ and Catalyst Combinations. Michael Ulrich Jurczyk¹, Sesha Srinivasan², Ashok Kumar¹, Elias Stefanakos² and Matt Smith²; ¹Mechanical Engineering, University of South Florida, Tampa, Florida; ²Clean Energy Research Center, University of South Florida, Tampa, Florida.

The present work addresses the grand challenge of hydrogen storage by mechano-chemcially milling LiBH₄ with LiNH₂ to produce a new complex material. While LiBH₄ and LiNH₂ are able to store 18.5 wt% and 8.77 wt% hydrogen, respectively, the temperature required to release the hydrogen is too high for practical applications. Different molar mixtures of LiBH₄ and LiNH₂ are prepared by high energy milling under inert ambient. Various catalysts and dopants such as Ti-compounds are investigated to lower the hydrogenation temperature, increase kinetics and tailor the plateau pressure for hydrogen release. The as-synthesized materials are characterized using XRD, FTIR, DSC, TGA and TPD/TPR. The hydrogen sorption behavior of these materials is measured using a Sievert’s type apparatus. XRD analysis reveals the formation of a quatenary phase (Li-B-N-H), dependant upon ball-milling time and molar ratio of parent compounds. FTIR profiles suggest the BH₄^- and NH₂^- anions remain around 2370 cm^-1 and 3250 cm^-1 while confirming the formation of a new quaternary structure. This quaternary structure exhibits non-reversible hydrogen storage properties with a capacity as high as 8 wt% and desorption temperature as low as 175 ^oC with the addition of catalyst materials in optimized concentrations. The addition of MgH₂ destabilizes the structure and allows for reversible hydrogen storage at temperatures as low as 125 ^oC.


9:30 AM S6.5
Abstract Withdrawn


10:15 AM S6.6
Evaluation of Conductive Polymers’ Hydrogen Storage Capability Using Conductometric Gas Sensors: Effect of Thermal Pre-Treatments on HCl-Doped Polyaniline Nanofibers. Yaping Dan¹, Pen-Cheng Wang², Stephane Evoy³, Alan G. MacDiarmid² and Alan T. Charlie Johnson^4,1; ¹Electrical Engineering, University of Pennsylvania, Philadelphia, Pennsylvania; ²Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania; ³Department of Electrical and Computer Engineering and National Institute of Nanotechnology, University of Alberta, T6G 2V4, Alberta, Canada; ⁴Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, Pennsylvania.

Among the candidate materials capable of hydrogen storage, conducting polymers were reported to be promising materials for this application. However, another later study showed that no hydrogen storage was observed in this material. It is believed that the above controversy could be related to the ambiguous and somewhat inconsistent materials processing procedures, as the properties of conducting polymers can be affected by many subtle processing variables. Thus, it is of particular interest to develop a convenient and easy method to screen conducting polymers subject to various processing conditions for the identification of the optimized species capable of hydrogen storage. Recently, a study on the interactions between polyaniline and H2 using conductometric sensors showed that the transduction response of PANI to H2 in electrical resistance is closely related to the H2 uptake in PANI, which suggests that the H2 storage capability of the conductive polymers subject to different experimental conditions could be evaluated by monitoring their transduction behaviors in response to H2. Since controversy in hydrogen storage mentioned earlier in this abstract might be caused by the different thermal pre-treatments on HCl-treated polyaniline, in this paper we present the effect of thermal pre-treatments on the transduction behaviors of HCl-doped polyaniline nanofibers integrated in conductometric devices upon exposure to 1% H2 at 25C. In general, after drying in N2 at 25C for 12 hours, devices showed a ~10% decrease in electrical resistance upon exposure to 1% H2. However, devices subject to thermal pre-treatments in N2 at 100C, 164C or 200C for 30 min showed different transduction behaviors. Specifically, devices subject to thermal pre-treatments at 100C and 164C showed a decrease in electrical resistance by ~11% and <0.5%, respectively. More interestingly, the device subject to thermal pre-treatment at 200C showed a transduction behavior with opposite polarity, i.e. a ~5% increase in electrical resistance upon exposure to 1% H2. In addition, FTIR and TGA were employed to investigate the effect of thermal treatments on the chemical characteristics of HCl-doped polyaniline nanofibers. The results indicated that the change in the devices’ interesting transduction behaviors might be related to the thermal treatment effects on the HCl-doped polyaniline nanofibers in (i) removal of adsorbed water that could block the H2-binding sites, and (ii) crosslinking of polymer backbones. We believe the above thermally-induced events could be respectively correlated to (i) the enhancement of HCl-doped polyaniline nanofibers’ H2 uptake capability, and (ii) the loss of the H2-binding sites in the polymers. In conclusion, our study suggests that the gentle thermal pre-treatment in N2 on the HCl-doped PANI might enhance the hydrogen storage capability by removing adsorbed water. Deterioration of the HCl-doped PANI will occur at even higher temperatures.


10:30 AM S6.7
¹H NMR Measurements of Hydrogen Adsorption in Modified Carbon Materials as a Function of Temperature. Alfred Kleinhammes, B. J. Anderson, Qiang Chen and Yue Wu; Department of Physics and Astronomy, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina.

Large surface areas in carbon based materials - aerogels and activated carbons are examples of such materials - offer an abundance of adsorption sites for hydrogen. However, at room temperature the adsorption energy of H₂ on carbon surfaces is too low to facilitate adequate hydrogen storage. Carbon materials need to be modified to enhance the binding energy while maintaining a sufficiently large surface area. Theory predicts that boron doped into carbon nanoclusters will enhance the binding energy for H₂ which forms a Kubas type compound with boron. NMR studies at room temperature indeed show that H₂ binds stronger - by a factor of three - to boron doped graphitic material than to pure carbon. However, the measured weight percentages are small because the original material lacks porosity and the boron content is low. Ongoing NMR studies are performed on new materials designed to increase boron content and porosity. Various approaches in material design are compared with respect to their ability to improve hydrogen storage: boron doped graphitic materials, aerogels and nanoporous carbon. In addition Pt decorated carbon nano horns are investigated. All measurements are done in situ with hydrogen loading to 100 atm. and at temperatures ranging from 77 K to room temperature. ¹¹B NMR is used to elucidate the local structure and symmetry around the doped boron in the carbon matrix. ---------- Funding provided by the US Department of Energy’s Office of Energy Efficiency and Renewable Energy within the Hydrogen Sorption Center of Excellence as part of DOE’s National Hydrogen Storage Grand Challenge


10:45 AM S6.8
Car-Parrinello Molecular Dynamics Simulation to Determine the Effect of Palladium-Doping on the Chemical Structure of Pitch-Based Carbon Fibers. Samir Mushrif and Alejandro Rey; Chemical Engineering, McGill University, Montreal, Quebec, Canada.

Effective and safe storage of hydrogen and its delivery is one of the crucial issues in hydrogen economy. Palladium loaded pitch-based activated carbon fibers have demonstrated a hydrogen storage capacity that is an order of magnitude greater than that of other carbonaceous materials. This research aims to develop computational modeling of structure and performance of these fibers with application to hydrogen storage. The present work uses Car-Parrinello Molecular Dynamics (CPMD), an ab-initio molecular dynamics approach to model the chemical structure of the fibers. CPMD performs the electronic structure calculations using a planewave-pseudopotential implementation of density functional theory. The first step of fiber formation is the mixing of Pd-acetylacetonate with the isotropic pitch. Then the fibers are spun and they undergo stabilization, carbonization and activation. Experimental analysis showed that the concentration of polyaromatic hydrocarbons in the palladium salt - containing pitch is lower than that in the pure pitch and hence; formation of pitch-metal complex is speculated. CPMD simulations are performed to investigate the effect of palladium doping on the hydrocarbon structure during the mixing stage of fiber preparation.


11:00 AM S6.9
Hydrogen Storage in Micro- and Mesoporous Materials under High Pressure. Michelle B. Weinberger, Maddury Somayazulu and Russell J. Hemley; Geophysical Lab, Carnegie Institution of Washington, Washington, District of Columbia.

To date, the materials considered to be the best candidates for hydrogen storage fuel cells include activated carbon and metal organic frameworks. Both very high surface area activated carbon and MOF-5 have been shown to adsorb around 4.5 wt % of hydrogen gas at 78 K. We have investigated the fundamental structural response of these materials to high pressure, as well as their behavior at high pressure when packed with dense hydrogen. Further investigation of these materials at low temperatures while still at elevated pressures may in fact provide a route for recovery of these hydrogen-packed materials to near ambient conditions. Covalent organic frameworks offer the potential for even better hydrogen storage capacity. These materials have significantly lower densities than the MOF materials and offer a significantly larger number of adsorption sites. Diamond anvil cells are uniquely suited for the study of these materials, allowing in situ measurements at high pressure as well as at low temperatures. Using X-ray diffraction and Raman spectroscopy we probe the behavior of the hydrogen confined in these porous materials at high pressure by tracking shifts in the hydrogen vibron peaks as well as studying the C-H bond behavior.


11:15 AM S6.10
SWNTs Bundle Dispersion Study and Analysis of its Potential as a Hydrogen Tank. Kyoichi Tange^1,2, Katsuhiko Hirose¹, Katsutoshi Noda¹, Rana Mohtadi³ and Yoshitsugu Kojima²; ¹Material Engineering Div.3, TOYOTA MOTOR CORPORATION, Susono, Japan; ²Institute for Advanvced material Research, Hiroshima University, Higashi-Hiroshima, Japan; ³Materials Research Development, Toyota Motor Engineering and Manufacturing North America, Ann Arbor, Michigan.

Hybrid hydrogen tank utilizing high pressure storage combined with high purity and high surface area physi-sorption SWNTs is considered to be one of the most promising hydrogen tank systems. However, the gravimetric and volumetric potential capacity of SWNTs is not well known, mainly due to experimental difficulties in the measurement of the uptake amount at high pressure. The results we report were obtained utilizing a highly accurate hydrogen uptake measurement technique developed for measurements conducted at room temperature up to 70MPa pressure. Using our technique, the error in the gas uptake measured was less than 10 Nml for a total gas volume including compressed gas of 10,000 Nml. This means that for a 1 gram sample, an error of only less than 0.1mass% is introduced. We found that the hydrogen uptake amount at saturation was strongly dependent on the SWNTs caps' openness, purity and bundle dispersion. While the surface area measured for a typical 'as received' SWNTs samples ranged s between 400-800 m2/g, the measured surface area for the purified and cap opened SWNTs with bundle formation was 1000- 1300m2/g. Therefore, the lower surface area for the 'as received' SWNTs was attributed to caps closure,low purity,and poor bundle dispersion. Here we also studied the effects of heat treatment on the bundle dispersion and the surface area. The TEM results showed changes in the cross sectional structures of the tubes and bundles dispersion when heated in vacuum at 1970-2070K. As a result of the heat treatment, the BET surface area measured was further enhanced and values ranging between 1800-2000m2/g were obtained. Based on these results, SWNTs with diameters larger than the 'as received',cap opened,and bundle dispersed ones by 0.5-2nm, were prepared. The heat treated SWNTs comprised 10-30wt% graphite impurities, which were generated during the high temperature vacuum treatment. We expect to increase the surface area up to 2700-2800 m2/g by reducing the generated graphite impurities. Saturated hydrogen uptake amounts were measured for each sample. As a result, we figured out that the saturated hydrogen uptake amount and the surface area have a good proportional relationship. Depending on the uptake measurements, 2.4-2.7 mass% hydrogen uptake amount is expected to allow for 6-6.5 mass% storage at 35MPa and room temperature. In addition, we also evaluated the molding effects on the volumetric hydrogen storage density. For SWNTs with 1950m2/g surface area, a 0.8 g/ml bulk density was obtained. The bulk density value is much higher than that found for regular activated carbon and micro porous materials. We observed no changes the surface area before and after the molding process. Finally, we will also discuss the analysis results of the hybrid physi-sorption hydrogen tank where the calculated tank weight and volume will be compared with other candidate materials.


SESSION S7: Hydrogen Storage in Carbon and Porous Materials
Chairs: Azarnoush Hoseinmardi and Gholam-Abbas Nazri
Wednesday Afternoon, November 28, 2007
Room 309 (Hynes)

1:30 PM *S7.1
Modified Carbon Cryogel-Hydride Nanocomposites for H2 Storage. Saghar Sepehri, Betzaida Bettalla and Guozhong Cao; Material Science and Engineering, University of Washington, Seattle, Washington.

Among the porous carbon materials, carbon aerogels with a range of properties, such as controllable mass densities, continuous porosities, and high surface areas has been received considerable attention for numerous applications. Recently, we have reported H2 storage properties of carbon cryogel-AB nanocomposites, made by impregnating carbon cryogel with ammonia borane (AB). This method showed up to 9 wt% hydrogen release at a much lower temperature (90 C), compared to AB decomposition temperature (105 C), and suppression of borazine which is a harmful byproduct of AB decomposition [1]. Here, we evaluate the effects of chemical modification on the structure of carbon cryogels and the kinetics of hydrogen release. Carbon cryrogels are derived from resorcinol-formaldehyde hydrogels, following the previously reported procedure [2], modified samples are made by adding ammonia borane to the gels during the first step of solvent exchange process. For H2 release experiments, both the original and modified samples were loaded with ammonia borane by soaking them in ammonia borane - THF solution. Nitrogen sorption analysis, X-ray diffraction, differential scanning calorimetry, differential thermal/thermal gravimetric analyses, mass spectrometry, scanning electron microscopy, Fourier transform infrared spectroscopy, and impedance spectroscopy are used to characterize the nanocomposites structure and investigate the hydrogen release kinetics. Our results show that different structural changes happen during the pyrolysis step. In the pyrolyzed modified samples, contrary to the original ones, cumulative surface area, mesopores volume and total pore radius, are all increased. Moreover, SEM and porosity observations show a highly ordered structure, with a narrow mesopore size distribution for modified samples in comparison to the original carbon cryogels. Also, DSC exotherm for AB loaded carbon cryogels shows a narrower peak for the modified samples. These results suggest that modified carbon cryogels can provide a very promising tunable scaffold for various hydrogen-rich materials while improving the hydrogen release kinetics. References: [1] A. Feaver, S. Sepehri, P. Shamberger, A. Stowe, T. Autrey, G. Z. Cao, J. Phys. Chem. B. 2007, 10.1021/jp072448t [2] A. Feaver, G. Z. Cao, Carbon. 2006, 44, 590.


2:00 PM S7.2
Cryogenic Hydrogen Storage Capacity of Nanoporous Carbon Materials Synthesized using an Aerosol-Assisted Approach. Qingyuan Hu¹, Yunfeng Lu^2,3 and Gregory P Meisner⁴; ¹Materials Science & Engineering, Purdue University, West Lafayette, Indiana; ²Chemical and Biomolecular Engineering, Tulane Unversity, New Orleans, Louisiana; ³Chemical and Biomolecular Engineering, University of California, Los Angeles, Los Angeles, California; ⁴Materials & Processes Lab, General Motors Research & Development Center, Warren, Michigan.

Spherical nanoporous carbon particles were synthesized from carbon precursor solutions of sucrose with either silica sols, colloidal silica particles, or both, in a direct one-step aerosol-assisted process, followed by carbonization and then removal of the silica template. The resulting particles show very high porosity with narrow pore size distributions, surface areas up to 2000 m2/g, and pore volumes up to 4.0 cm3/g. Three different kinds of spherical nanoporous carbon particles were prepared: (1) unimodal nanoporous particles using tetraethyl orthosilicate (TEOS) as the only silica source for the template, (2) bimodal nanoporous particles using both TEOS and colloidal silica nanoparticles as a composite template, and (3) foam-like highly porous particles using only colloidal silica for the template. The porosity and pore sizes depend on the type and amount of silica template precursor added to the sucrose precursor solutions. The carbon particles were characterized by transmission electron microscopy, field emission scanning electron microscopy, and nitrogen sorption surface area measurements. Hydrogen adsorption was measured at various temperatures between 77 K and room temperature and at pressures up to 50 bars. The maximum hydrogen uptake of up to 4.0 wt% at 77 K and >20 bar was found for nanoporous carbon particles made using the silica sol template.


2:15 PM S7.3
Enhanced Hydrogen Adsorption on Palladium-doped Nanoporous Carbon Fibers. Vinay V. Bhat¹, Nidia C Gallego¹, Cristian I. Contescu¹, E. A. Payzant¹, A. J. Rondinone¹, Halil Tekinalp², Dan D. Edie²; ¹Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee; ²Center for Advanced Engineering Fibers and Films, Clemson University, Clemson, South Carolina.

Efficient storage of hydrogen for use in fuel cell-powered vehicles is a challenge that is being addressed in different ways, including adsorptive, compressive, and liquid storage approaches. In this paper we report on adsorptive storage in nanoporous carbon fibers in which palladium is incorporated prior to spinning and carbonization/activation of the fibers. Nanoparticles of Pd, when dispersed in activated carbon fibers (ACF), enhance the hydrogen storage capacity of ACF. The adsorption capacity of Pd-ACF increases with increasing temperature below 0.4 bar, and the trend reverses when the pressure increases. To understand the cause for such behavior, hydrogen uptake properties of Pd with different degrees of Pd-carbon contact (Pd deposited on carbon surface and Pd embedded in carbon matrix) are compared with Pd-sponge using in situ XRD under various hydrogen partial pressures (<10 bar).

Rietveld refinement and profile analysis of diffraction patterns does not show any significant changes in carbon structure even under 10 bar H₂. Pd forms β PdH_0.67 under 10 bar H₂, which transforms to α PdH_0.02 as the hydrogen partial pressure is decreased. However, the equilibrium pressure of transition (corresponding to a 1:1 ratio of α and β phases) increases with increasing the extent of Pd-carbon contact. This pressure is higher for Pd embedded in carbon than for Pd deposited on carbon surface. Both these Pd-carbon materials have higher H₂ desorption pressure than pure Pd, indicating that carbon “pumps out” hydrogen from PdHx and the pumping power depends on the extent of Pd-carbon contact. These results support the spillover mechanism (dissociative adsorption of H₂ followed by surface diffusion of atomic H).

This research is sponsored by the Division of Materials Sciences and Engineering, U.S. Department of Energy under contract with UT-Battelle, LLC. A portion of this research was conducted at the Center for Nanophase Materials Science at Oak Ridge National Laboratory, sponsored by the Division of Scientific User Facilities of U.S. Department of Energy. VVB acknowledges additional support from ORAU/ORISE.

2:30 PM S7.4
Enhanced Room Temperature Hydrogen Storage of Metallic Carbon Nanotubes via Titanium Doping. Stephen Shevlin and Zheng Xiao Guo; Materials, Queen Mary, University of London, London, United Kingdom.

The storage of hydrogen in solid-state materials is an important and essential step that must be resolved in order to achieve the transition from a polluting fossil-fuel economy to a clean hydrogen-based economy. Carbon-based materials have been at the forefront of research into hydrogen storage materials, however without substantial improvements in hydrogen binding strength they will not meet the DOE targets for hydrogen storage. Recent work however indicates that transition metal atom doping can boost the gravimetric and thermodynamics of hydrogen storage in these materials, albeit at the expense of these dopants being unstable with respect to coalescence into nanoparticles. In this talk we present the results of first principles ab initio simulations on the adsorption and hydrogen storage properties of titanium atom dopants on the native point defects of an (8,0) carbon nanotube. Atomic adsorption on a vacancy strongly binds the titanium (-1.61 eV), whereas the binding to an undefective nanotube or a Stone-Wales defect is weaker (3.36 and 3.68 eV respectively). Crucially the binding energy with respect to the reference state of pure titanium is positive for the latter two defects but negative for the vacancy system, meaning that only the vacancy adsorbed Ti atom is stable with respect to metallic particle coalescence. This defect was found to adsorb 5H₂ molecules, with all the H₂ binding energies found to be in the range of −0.2 to −0.7 eV/H₂, desirable for technological applications. Examination of the electronic structure reveals that the hydrogen binding is initiated by H₂ σ-bond electron donation into the empty d-states of the Ti atom, however due to the interaction between the Ti atom and the carbons of the CNT this binding does not obey the 18-electron rule. This strong interaction (caused by electron donation from the C atoms of the vacancy to the Ti atom) also renders the dopant less susceptible to oxidation. Calculation of the free energy reveals that full hydrogenation occurs at a pressure of 63.4 atmospheres with almost complete dehydrogenation occurring over a pressure range of 12.4 atmospheres, it is thus very controllable. Molecular dynamics simulations at finite temperature show that this complex is stable at room temperature, while large scale simulation of a C₁₁₂Ti₁₆H₁₆₀ unit cell shows that a structure with a reversible gravimetric storage capacity of 7.1 wt% is feasible. This particular system is thus of considerable interest for hydrogen storage applications.


2:45 PM S7.5
Hydrogen Storage in Carbon Nanoscrolls. Scheila F. Braga¹, Vitor R. Coluci¹, Ray H. Baughman² and Douglas S Galvao¹; ¹Applied Physics, State University of Campinas, Campinas, Brazil; ²University of Texas at Dallas, Dallas, Texas.

Recently, a low-temperature synthesis method was developed to produce a new and different type of carbon nanostructure, the so called carbon nanoscroll (CNS). This structure is formed by wrapped graphite sheets into papyrus-like form. In contrast to carbon nanotubes, CNSs provide inter-layer galleries that can be intercalated with different agents, and the CNS diameter can expand to accommodate the volume of the intercalant. This feature is potentially important for hydrogen storage. Hydrogen (H2) has been recently object of intense research as an alternative energy source. Among the problems that have to be solved for the wide use of hydrogen energy, how to store H2 easily and cheaply represent a great technological challenge. In this work we have used molecular dynamics simulations to investigate the hydrogen uptake mechanism and the gas sorption/desorption cycle. Grand-canonical Monte Carlo simulations were also carried out to estimate the storage capacity of CNSs. The scroll adsorption capacity was explored by changing the space between scroll layers. At equilibrium geometries interlayer spacing about 3.4 Angstroms the scroll storage capacity can reach 2.5 wt% which is higher than the values obtained for crystallographically packed carbon nanotubes at 150 K and 1 MPa. Higher capacities were reached when interlayer spacings were increased. The value of 5.5 wt% hydrogen was obtained when the interlayer spacing was 6.4 Angstroms. Our Monte Carlo simulations were performed at low pressure (1 MPa) and relatively high temperatures (150-350 K), we expect an increase in the storage capacity of CNSs for pressures available in experimental setups (~10 MPa). The sorption/desorption for an isolated CNS was investigated through a temperature-controlled cycle. After the scroll had been loaded reaching about 4 wt%, the desorption process was carried out by heating up the system. During this process, the absorbed H2 amount decreased to 1 wt% when almost all the H2 molecules were released. A recharging process was then performed through cooling, that can lead to even higher wt% values than the ones obtained before the desorption process. Our simulations suggest that CNSs can be very promising systems for hydrogen storage especially when intercalants are present.


3:30 PM S7.6
H2 Adsorption in Pristine and Li-Doped Carbon Replicas of Zeolites. Thomas Roussel, Roland J.-M. Pellenq and Christophe Bichara; Centre de Recherches en Matiere Condensee et Nanosciences, CNRS, Marseille, France.

The first step of this work was to generate, using atomistic simulation, a porous carbon material with an ordered pore network using the faujasite Y zeolite as a templating matrix. For this purpose, we used the Grand Canonical Monte-Carlo (GCMC) technique in which the carbon-carbon interactions were described with reactive potentials (quantum Tight Binding and bond order) assuming the carbon-zeolite interactions to be relevant to physisorption. The intrinsic stability of the nanoporous carbon was then investigated. We also performed a structural analysis of the resulting carbon porous structure including the determination of the pore size distribution. Such a new carbon form has a very ordered porous structure that can be used as a model adsorbent to validate adsorption theory and characterization methods. At a second stage, for these pristine structures, we calculated hydrogen adsorption isotherms at different temperatures with the GCMC technique using ab initio adsorbate-carbon interaction potentials. Although our carbon replicas have an optimum pore size for H2 docking, we demonstrated that they cannot be used as for an efficient storage at room temperature under 300 bars. By contrast, when doped with lithium (LiC6), we found, that room temperature H2 storing become possible: we obtain 4% in weight (37 kg/m3) under 300 bars. The origin of these good storing performances is to be found at the atomistic level thanks to the Li to C electron transfer giving rise to an attractive polarization term in the H2-doped-host hamiltonian.


3:45 PM S7.7
Single-walled Carbon Nanohorns: Tunable Media for Hydrogen Storage and Metal Catalyst Supports. Hui Hu^1,2, Bin Zhao², Alex Puretzky^1,2, David Styers-Barnett², Christopher Rouleau^1,2, Mina Yoon¹, David Geohegan^1,2, Yun Liu³, Craig Brown³, Houria Kabbour⁴, Alfred Kleinhammes⁵, Jeffrey Blackburn⁶, Lin Simpson⁶, Wei Zhou³, Chaiwat Engtrakul⁶, John Zielinski⁷, Dan Neumann³, Channing Ahn⁴, Yue Wu⁵, Anne Dillon⁶, Michael Heben⁶, Charles Coe⁷ and Alan Cooper⁷; ¹Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee; ²Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee; ³National Institute of Standard Technology, Gaithersburg, Maryland; ⁴Department of Materials Science, California Institute of Technology, Pasadena, California; ⁵Department of Physics and Astronomy, University of North Carolina at Chapel, Chapel Hill, North Carolina; ⁶National Renewable Energy Laboratory, Golden, Colorado; ⁷Air Products and Chemicals Inc., Allentown, Pennsylvania.

Single-walled carbon nanohorns (SWNHs) are ball-shaped agglomerates (50-100 nm in diameter) of individual single-walled closed-shell carbon structures. They are ideal structures to explore mechanisms of supercritical hydrogen adsorption at 77K and higher temperatures, and they provide possible vessels to confine and store hydrogen in cluster form within the SWNH nanopores [1, 2]. Previous studies have shown that hydrogen interacts much more strongly with SWNHs structures than it does to carbon nanotubes, suggesting that nanohorns and related nanostructures may be promising materials for hydrogen storage [3, 4]. Here we report the effects of tailoring the morphology and metal loading of SWNH composites for hydrogen storage, through adjustment of the shape and size of the individual nanohorn units during synthesis, and the surface area and pore size distributions by controllable post-processing treatments [5, 6]. The SWNHs used here were synthesized by laser ablation of carbon targets using a high-power (600 W) industrial Nd:YAG laser with adjustable pulse-widths and energies. Oxidative methods to produce high surface area nanohorn supports (1900 m2/g) with variable pore sizes were developed. Methods to controllably decorate the SWNHs with 1-5 nm particles of Pt, Pd, and other materials while maintaining high surface area were also investigated to probe the spillover mechanism and enhance the binding energy for stored hydrogen. Evidence for spillover in both Pt- and Pd-decorated SWNHs was observed by neutron scattering monitoring of free H2. The onset temperature for catalyst-assisted hydrogen storage was determined to be between 150K < T < 298K. Nuclear magnetic resonance measurements showed possible spillover-related room-temperature storage in Pt-decorated SWNHs. Moreover, Pt-decorated SWNHs exhibited enhanced binding energies as measured by both TPD (36 ± 2 kJ/mol) and NMR (7.1 kJ/mol). Hydrogen storage measurements demonstrated hydrogen uptakes of SWNHs with 0.2-0.8 wt% at room temperature and 1-3.5% at 77K. The effects of compression and thermal treatments to further vary the pore sizes and the graphitic structure of SWNHs will be described. These effects will be compared to theory and simulation of doping and decoration for enhanced hydrogen binding energy and storage. Research supported by the U. S. Department of Energy (EERE) through the Hydrogen Sorption Center of Excellence. Characterization measurements at the Center for Nanophase Materials Sciences, Oak Ridge National Laboratory funded by the Division of Scientific User Facilities, DOE-BES. Reference: [1]C. Yang, et al, Adv. Mater. 2005, 17, 866. [2]E. Bekyarova, et al, J. Phys. Chem. B 2005, 109, 3711. [3]H. Tanaka, et al, J. Am. Chem. Soc. 2005, 127, 7511. [4]F. Fernandez-Alonso, et al, PRL 2007, 98, 215503. [5]M. Cheng, et al., Nanotechnology 2007,18,185604. [6]H. Hu, et al, Carbon 2007 Conference Proceedings.


4:00 PM S7.8
Physical and Chemical Properties of Hydrogen Clathrate Hydrate: Theoretical Aspects of Energy Storage Application. Rodion Belosludov¹, Oleg Subbotin², Hiroshi Mizuseki¹, Vladimir Belosludov² and Yoshiyuki Kawazoe¹; ¹Institute for Materials Research, Tohoku University, Sendai, Miyagi, Japan; ²Nikolaev Institute of Inorganic Chemistry, SB RAS, Novosibirsk, Russian Federation.

Interest in hydrogen clathrate hydrates as potential hydrogen storage materials has risen recently after a report that the clathrate hydrate of structure CS-II can store around 4.96 weight% of hydrogen at 220 MPa and 234 K [1]. However, the extreme pressure required to stabilize this material makes it impractical. It is well known that there are several types of gas hydrate structures with different cage shapes, and some of these hydrate structures can hypothetically store more hydrogen than the hydrate of structure CS-II. Therefore, for practical application of gas clathrates as hydrogen storage materials, it is important to know the region of stability of these compounds as well as the hydrogen concentration at various pressures and temperatures. In order to accurately estimate the thermodynamic properties of hydrogen hydrates, we developed a method based on the solid solution theory of van der Waals and Platteeuw with some modifications that include multiple occupancies, host relaxation, and the description of the quantum nature of hydrogen behavior in the cavities. In this approach we used a combination of quasiharmonic lattice dynamics and a first-principles calculation to estimate the free energies, equations of state, and chemical potentials of hydrogen hydrates. The validity of the proposed approach was checked for argon, methane, and xenon hydrates [2]. The results were in agreement with known experimental data. The phase diagram (P,T) of hydrogen clathrate of structure CS-II has been constructed in agreement with the recent experimental diagram [3]. The H₂-tetrahydrofuran(THF)-H₂O and H₂-Propane-H₂O systems (hydrate CS-II structure) have been investigated with different THF, propane, and H₂ concentration. The calculations showed that the formation pressure of hydrogen hydrates was drastically reduced in the presence of propane and THF. However, in the case of propane there is no possibility of increasing the amount of hydrogen stored (around 1 weight% of hydrogen) because propane molecules fill all large cages at all pressures. In contrast, using THF as help gas, there is a possibility of hydrogen occupancy of the large cage at low pressure due to the fact that THF molecules fill all large cages only at higher pressure. The charge density distribution shows that there exists a weak interaction between hydrogen molecules and water cages. Interaction of the THF molecule with the host is larger than the interaction of hydrogen with the host, meaning that THF plays a more significant role in the stabilization of hydrogen hydrate. The possibility of the formation of hydrogen hydrate with other structures has also been discussed based on a theoretically constructed phase diagram. REFERENCES [1] W.L. Mao et al. Science 297 (2002) 2247-2249. [2] V. R. Belosludov et al. Mater. Trans. 29 (2007) 704-710. [3] K. A. Lokshin and Y. Zhao, Appl. Phys. Lett. 88 (2006) 131909.


4:15 PM S7.9
Microporous Organic Polymers for Gas Storage: Simulation, Synthesis, and Characterization. Abbie Trewin¹, Colin Wood¹, Bien Tan¹, Jia-Xing Jiang¹, Darren Bradshaw¹, Matthew Rosseinsky¹, Neil Campbell², Ev Stoeckel¹, Yaroslav Khimyak¹, Fabing Su¹ and Andrew Cooper^1,2; ¹Department of Chemistry, University of Liverpool, Liverpool, United Kingdom; ²Centre for Materials Discovery, University of Liverpool, Liverpool, United Kingdom.

The widespread use of hydrogen as a fuel is limited presently by the lack of a convenient, safe, and cost-effective method of H₂ storage. The 2010 Department of Energy (DOE) gravimetric and volumetric storage targets for H₂ are 6.0 wt. % and 45 g H₂/L, respectively. These targets are very challenging because they are system targets; that is, calculations of storage capacity should include pressure containment, valves, cooling systems, etc. As such, gravimetric capacities significantly greater than 6.0 wt. % based solely on the storage material would likely be required. The use of elevated pressures (and the necessary containment technology) or low temperatures (with the associated cooling systems) will generally increase system weights and exacerbate this problem. Porous organic polymers possess a number of potential advantages as physisorptive sorbents for H₂ storage. First, polymers can be composed solely of light elements and may be thermally and chemically robust. Second, there exist a large number of synthetic routes by which a wide range of functionalities can be introduced - for example, moieties that could enhance H₂ binding affinities. Third, polymers are a scalable technology and there are already examples of systems (e.g., macroporous polymer resins for separations) which are produced commercially on a large scale. A less obvious advantage is that porous organic polymers can be produced in a molded "monolithic" form. This may avoid volumetric storage issues relating to the packing of porous particulate materials. A significant challenge is the very limited number of routes to produce ultramicroporous organic polymers with high specific surface areas (>1000 m²/g). Hypercrosslinked polymers represent a unique class of microporous organic materials which can exhibit apparent BET surface areas of at least 2000 m²/g. In the first section of this paper we will describe hypercrosslinked polymers^1,2 which reversibly adsorb up to around 4 wt. % H₂ at 77.3 K - the highest gravimetric storage yet observed for an organic polymer. We will also discuss for the first time the volumetric storage advantages of molded monolithic formats, as well as the CH₄ storage properties of such sorbents.³ A combination of solid-state NMR and detailed gas sorption measurements was used to rationalize structure, surface area, pore size, and H₂ sorption properties of the materials. The bulk of the paper will focus on atomistic simulation approaches to describe the micropore structure and gas sorption properties for these amorphous materials.^2,4 In particular, we will outline simulation methods which can estimate the isosteric heat of sorption for microporous materials⁴ and hence aid in the search for systems which physisorb H₂ at more practicable temperatures. (1) Lee et al., Chem. Commun., 2006, 2670 (2) Wood et al., Chem. Mater., 2007, 19, 2034 (3) Wood et al., unpublished results (4) Trewin et al., unpublished results


4:30 PM S7.10
Probing the Sorption Capability of Polymer Coordination Networks. Jason M. Simmons^1,2, Yun Liu^1,3, Craig M. Brown^1,4, Taner Yildirim^1,2, Shengqian Ma⁵, Xi-Sen Wang⁵ and Hong-Cai Zhou⁵; ¹NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland; ²Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania; ³Materials Science and Engineering, University of Maryland, College Park, Maryland; ⁴Indiana University Cylcotron Facility, Indiana University, Bloomington, Indiana; ⁵Department of Chemistry and Biochemistry, Miami University, Oxford, Ohio.

Polymer Coordination Networks (PCNs), a class of microporous organic-inorganic compounds, are actively studied as host systems for storage and separation of technologically important gases, such as hydrogen and methane. We use a combination of high-pressure adsorption isotherms, first-principles calculations, and neutron scattering measurements to probe the structure and dynamics of adsorption in PCNs over a large temperature and pressure range. In particular, PCN-10 (Cu-stilbene tetracarboxylate) has an appreciable hydrogen saturation capacity (5.5wt.%) and initial heat of adsorption (~7kJ/mol) at 77K. In addition, PCN-10 exceeds DOE targets for useful methane storage at room temperature. Adsorption isotherms are complemented by neutron scattering measurements and first-principles calculations to determine the location, binding energy, and filling of the adsorption sites. These investigations are used to understand the role of the metal-oxide cluster and organic linker in the adsorption of hydrogen and methane.


4:45 PM S7.11
Mechanisms for the Variation of Electrical Conductivity of Palladium Films in Hydridation-dehydridation Processes. Chung Wo Ong, Yu Ming Tang and Yiu Bun Chan; Applied Physics, The Hong Kong Polytechnic University, Hong Kong, China.

Two models are proposed in literature to explain different trends of electrical conductivity (σ) observed during hydridation-dehydridation of palladium (Pd). One involves reversible hydrogen dissolution, and phase transitions between a low hydrogen-containing phase and a high hydrogen-containing phase (referred to as phase transition mechanism). Under this mechanism, σ drops with increasing hydrogen content. For a defective Pd specimen with nano-sized clusters, volume expansion occurs during hydridation (referred to as hydrogen-induced lattice expansion: HILE), giving rise to electrical shorting of the clusters and subsequent rise in σ. We examined these two models with Pd films prepared at different conditions. Four Pd films were prepared by sputtering at argon ambient pressure (P) ranging from 2 to 15 mTorr at room temperature, and one Pd film was deposited at P = 10 mTorr and 200^oC. One expects that at low P condition, the average mean free path in vacuum is longer. Hence, the substrate is subjected to more severe particle bombardment during deposition. The film structure is denser to favor the phase transition mechanism during hydridation. High substrate temperature condition also results in similar effect. At high P condition, scattering of sputtered species in vacuum is more severe. The film structure thus produced is more defective and favors the HILE mechanism during hydridation. X-ray diffraction spectra of all the films contain the (111) and (200) peaks of crystalline Pd. Atomic force microscopy (AFM) analysis was conducted to observe the roughness of the film surface. The change of σ during hydridation-dehydridation were investigated in a chamber, with each cycle consisting of exposure to 15% hydrogen in Ar at 10⁵ Pa (10 min.), followed by subsequent evacuation, and finally exposure to air (10 min.). Results show that a film deposited at low P condition contains larger grain size and a smoother surface, supporting the assertion that the condition is favorable for the formation of a denser film structure. σ was found to decrease during hydridation, consistent with the hypothesis that the phase transition mechanism should dominate for such a sample. The film deposited at a higher substrate temperature, e.g. 200^oC, was found to have a smoother film, and so the phase transition mechanism also prevails. For films deposited at higher P and room temperature conditions, the average grain size became smaller, while the film surface was rougher. Correspondingly, the value of σ was found to increase during hydridation, indicating that the HILE mechanism should dominate for this group of samples.