Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S14: Focus Session: Hydrogen Storage II: Measurements |
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Sponsoring Units: FIAP Chair: Frederick Pinkerton, General Motors Room: LACC 403B |
Wednesday, March 23, 2005 2:30PM - 2:42PM |
S14.00001: Anelastic Spectroscopic Studies of Point Defect Dynamics and Evolution of Chemical Reactions in Alanates Craig Jensen, Rosario Cantelli, Oriele Palumbo, Annalisa Paolone, Sesha Srinivasan, Martin Sulic As part of our effort to characterize the active species in Ti-doped NaAlH$_{4}$ and elucidate its mechanism of action, we have carried out the first measurements of elastic modulus and energy dissipation in Ti-doped and undoped sodium aluminium hydride. We have found that the dehydrogenation of the hydride can be monitored through its effects on the elastic constants. After a well-defined thermal treatment, a relaxation process appears at 70 K in the kHz range, denoting the existence of a new species, likely involving hydrogen, that has a very high mobility. The species is estimated to ``jump'' at rate of 10$^{3}$ s$^{-1}$ at the peak temperature corresponding to a relaxation rate of about 10$^{11}$ s$^{-1}$ at room temperature. The activation energy of the process is 0.126 eV and the pre-exponential factor 7$\cdot $10$^{-14}$ s, which is typical of point defect relaxation. The peak is very broad with respect to a single Debye process, indicating strong interaction or/and multiple jumping type of the mobile entity. The results of these studies will be presented and discussed in terms of their relationship to the mechanism of reversible elimination of hydrogen from the doped hydride. [Preview Abstract] |
Wednesday, March 23, 2005 2:42PM - 2:54PM |
S14.00002: Electron Paramagnetic Resonance and X-ray Absorption Studies of Fluctuating Titanium Species During the Reversible Dehydrogenation of Ti-Doped Sodium Alanate Meredith Kuba, Craig Jensen, Sandra Eaton, Job Rijssenbeck, Yan Gao As part of our effort to characterize the active species in Ti-doped NaAlH$_{4}$ and elucidate its mechanism of action, we have carried out tandem electron paramagnetic resonance and X-ray absorption studies. We find that upon mechanical milling NaAlH$_{4}$ with 2 mol {\%} TiF$_{3}$, the majority of the titanium is present as a Ti(III) species. However, following a few cycles of dehydrogenation/re-hydrogenation, the majority of the titanium is converted to a Ti(0) species, \textbf{A} that is subsequently replaced by a different Ti(0) species,\textbf{ B} upon further hydrogen cycling. Hydride milled with TiCl$_{3}$, was found to contain mainly the Ti(0) species \textbf{A} and only a minor amount of a Ti(III) component. However, a parallel is seen with the TiF$_{3}$ doped hydride as after 10 cycles tthe Ti(0) species \textbf{A} is seen to completely convert to Ti(0) species \textbf{B}. These results will be presented and discussed in terms of their relationship to the mechanism of reversible elimination of hydrogen from the doped hydride. [Preview Abstract] |
Wednesday, March 23, 2005 2:54PM - 3:06PM |
S14.00003: NMR studies of hydrogen storage materials: TiCl3-doped NaAlH4 Sean Barrett, Anatoly Dementyev, Dale Li, Rona Ramos, Yanquan Dong An exciting development in the field of hydrogen storage materials was the 1997 discovery that a small amount of Titanium doping can significantly improve the hydrogen discharging/recharging characteristics of sodium alanate (NaAlH$_{4})$. Understanding the dopant action in this ``model'' compound may translate into the rational design of improved storage materials. We report static NMR measurements of both TiCl$_{3}$-doped and undoped NaAlH$_{4}$, including our detection of the Ti-NMR signal. Future directions will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S14.00004: Lattice dynamics of NaAlH$_4$ from high-temperature single-crystal Raman scattering: Evidence of highly stable AlH$_4^-$ anions Eric Majzoub, Kevin McCarty, Vidvuds Ozolins Polarized Raman scattering on single crystals of NaAlH$_4$ has been used to determine the symmetry properties and frequencies of the Raman-active vibrational modes over the temperature range from 300 to 425~K, i.e., up to the melting point $T_{\mathrm{melt}}$. Significant softening (by up to 6\,\%) is observed in the modes involving rigid translations of Na$^{+}$ cations and translations and librations of AlH$_4^{-}$. Surprisingly, the data indicate mode softening of less than 1.5\,\% for the Al-H stretching and Al-H bending modes of the AlH$_4^{-}$ anion. These results show that the AlH$_4^{-}$ anion remains a stable structural entity even near the melting point. The enhanced kinetics of absorption and desorption in Ti-doped NaAlH$_4$ powders is attributed to the effectiveness of Ti in promoting the break-up of the AlH$_4^{-}$ anions. [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S14.00005: NMR studies of the metal-hydrogen system ZrNi(H/D)$_{x}$ Caleb Browning, Timothy Ivancic, Robert Bowman, Jr., Mark Conradi Relaxation studies of the intermetallic ZrNiH$_{x}$ and ZrNiD$_{x}$ were performed using hydrogen and deuterium NMR. Correlation times for atomic diffusion were determined based on the temperature dependence of spin-lattice and spin-spin relaxation times. The motion is shown to be thermally activated over the temperature range 200~-~575~K, and the activation energies for diffusion are determined. The deuterium NMR spectra exhibit comparatively little line narrowing with temperature, indicating that the average electric field gradient is not zero, averaged over the deuterium atom sites of these non-cubic cells. Furthermore, the spectrum of ZrNiD$_{1.87}$ reveals a coexistence of two phases, in agreement with the phase diagram. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S14.00006: Hydrogen clathrate hydrate - novel hydrogen storage material: crystal structure, kinetics, and phase diagram Konstantin Lokshin, Yusheng Zhao The detailed crystal structure information for the hydrogen clathrate hydrate was determined by neutron diffraction as a function of temperature (10-300 K) and pressure (1-2000 bar) for the first time. We found that hydrogen occupancy in the (32+X)H$_{2}$*136H$_{2}$O, x=0-16 clathrate can be reversibly varied by changing the large (hexakaidecahedral) cage occupancy between 2 and 4 molecules, but keeping single occupancy of the small (dodecahedral) cage in the sII structure. Above 130-160K the guest hydrogen molecules were found in the delocalized state, rotating around the centres of the cages. Decrease of temperature results in the rotation freezing followed by a complete localization below 50 K. We have discovered an extremely fast method of the clathrate synthesis, which allows the complete hydrogen hydrate formation in minutes. The influence of substitutions of different entities for hydrogen on the clathrate structure and stability was studied. High hydrogen capacity (up to 3.77 mass {\%} at ambient pressure), fast kinetics, and readily accessible $P-T$ range are the features that make hydrogen clathrate an excellent candidate for a hydrogen storage material. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 3:54PM |
S14.00007: Catalytic role of defective carbon substrates in the dissociation of small molecules Milen Kostov, Aaron George, Erik Santiso, Keith Gubbins, Marco Buongiorno Nardelli A necessary step towards the achievement of a hydrogen economy is the development of a production process that is able to drastically revamp the energetic cost while leaving, at the same time, a smaller environmental footprint than the current industry standards. Chemical reactions are often carried out in nano-structured media, where the reaction mechanism can be dramatically changed due to the interactions of reacting species with the substrate. One point of interest is the recent experimental evidence for stable defects in graphene layers such as vacancies. Our aim is to report that physical and chemical properties of such defects can have an astounding effect on certain chemical reactions. Using state of the art first principles modeling techniques, we have explored the potential of nano-structured carbon materials to lower the activation energy barrier of dissociation reactions for small molecules. Using water as a prototypical example, we will show how the carbonaceous environment and the defects present in it, can aid in lowering the activation energy barrier of adsorption and dissociation reactions. Finally we will discuss this exploration in the context of a complete cycle of energy storage and release through the production of hydrogen in defective carbon substrates. [Preview Abstract] |
Wednesday, March 23, 2005 3:54PM - 4:06PM |
S14.00008: Doping of AlH$_3$ with alkali metal hydrides for enhanced decomposition kinetics Gary Sandrock, James Reilly, Jason Graetz, Wei-Min Zhou, John Johnson, James Wegrzyn Aluminum hydride, AlH$_{3}$, has inherently high gravimetric and volumetric properties for onboard vehiclular hydrogen storage (10 wt{\%} H$_{2}$ and 0.148 kg H$_{2}$/L). Yet it has been widely neglected because of its kinetic limitations for low-temperature H$_{2}$ desorption and the thermodynamic difficulties associated with recharging. This paper considers a scenario whereby doped AlH$_{3}$ is decomposed onboard and recharged offboard. In particular, we show that particle size control and doping with small levels of alkali metal hydrides (e.g., LiH) results in accelerated H$_{2}$ desorption rates nearly high enough to supply fuel-cell and ICE vehicles. The mechanism of enhanced H$_{2}$ desorption is associated with the formation of alanate windows (e.g., LiAlH$_{4})$ between the AlH$_{3}$ particles and the external gas phase. These alanate windows can be doped with Ti to further enhance transparency, even to the point of accomplishing slow decomposition of AlH$_{3}$ at room temperature. It is highly likely 2010 gravimetric and volumetric vehicular system targets (6 wt{\%} H$_{2}$ and 0.045 kg/L) can be met with AlH$_{3}$. But a new, low-cost method of offboard regeneration of spent Al back to AlH$_{3}$ is yet needed. [Preview Abstract] |
Wednesday, March 23, 2005 4:06PM - 4:18PM |
S14.00009: Ti-based catalytic effect on hydrogen desorption in crystalline NaBH4: an ab initio investigation C. Moys\'es Ara\'ujo, Rajeev Ahuja, Puru Jena The application of hydrogen fuel cell technology in portable electronic devices and transportation vehicles has led to a great deal of interest in the study of complex alkali hydrides (MXH$_{4}$ with M=Na, Li and X=Al,B) primarily due to their high gravimetric hydrogen density (eg.18.5{\%} in LiBH$_{4})$. In particular, NaBH$_{4}$ slurry has been suggested as the most promising system for applications in fuel cell technology (1) as it provides one of the simplest ways of generating hydrogen. Additionally, the NaBH$_{4}$ itself is also a promising hydrogen storage material since it has one of the highest gravimetric hydrogen density (13.0 wt{\%}) among the alkali metal hydrides. However, its irreversibility with respect to hydrogen absorpton/desorption cycle limits its practical application for hydrogen storage. To overcome this limitation we have explored the role of Ti on the electronic and crystalline structures of NaBH$_{4}$. Using density functional calculations we show that Ti prefers to occupy the Na site in sodium borohydride. In addition, Ti weakens the strength of the covalent bond between B and H atoms and the hydrogen removal energy is reduced from 5.64 eV in pure sodium borohydride to 4.70 eV when doped with Ti. Thus, Ti might work as a catalytic agent allowing hydrogen to desorb at a lower temperature. Calculations are underway to examine if other dopants may be even better candidates for hydrogen desorption from sodium borohydride. 1. Z. P. Li, B. H. Liu, K. Arai, K. Asaba and S. Suda \textit{Journal of Power Sources}\textbf{\textit{ }}\textbf{126}, 28 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S14.00010: A role of Ti dopants in catalyzing NaAlH$_4$ from x-ray absorption studies and first-principal density functional calculations. A.Yu. Ignatov, T.A. Tyson, J. Graetz, J.J. Reilly, J. Johnson We have performed Ti $K$-edge XAFS measurements on 2 and 4 mol$\% $ TiCl$_3$ doped sodium alanates. Ti does not enter substitutially or interstitially into the perfect NaAlH$_4$ lattice. A substance formed as a result of multiple hydrogen cycling is of close resemblance of an amorphous TiAl$_3$ alloy with local structure about the Ti atom given by a cluster expansion of Ti-H$_x$-Al$_{10}$-Ti$_2$-... Interatomic distances and Debye-Waller factors are determined for several structural models. These results are elaborated by Ti $K$-edge XANES measurements which are interpreted in terms of single-electron multiple scattering calculations. Main features of the absorption edge are reproduced reasonably well assuming that either 3-5 hydrogen atoms enter the tetrahedron positions of the bulk $I4/mmm$ phase or a few monolayer thick TiAl$_3$ clusters are formed. Structural properties and phase stability of hydrided Ti-Al alloys, NaAlH$_4$, and Na$_3$AlH$_6$, as well as several products of the decomposition reaction were determined at zero temperature within LDA approximation to DFT using LAPW method. The calculations reveal that partial decomposition of NaAlH$_4$ accompanied by formation of TiAl$_3$ alloy is preferred to Ti substitution for Na, in good agreement with our XAFS finding. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S14.00011: Volumetric Analyses of Sieverts Apparatus Data Anne Dailly, Channing Ahn, John Vajo, Robert Bowman, Jr. Sieverts measurements of volumetric excess physisorption adsorption/desorption data at temperatures other than ambient require assumptions related to temperature gradients within the system. In addition, assumptions related to the sample volume need to be considered if this data is not available {\it a priori}. We consider two approaches to Sieverts analysis for data obtained at 77K. In the first approach, we assume that we have three distinct volumes that consist of the reactor, the manifold, and a transition volume between the manifold and the reactor. In this case, the reactor temperature and the manifold temperature are known and measured with thermocouples. The transition volume temperature is assumed to be at a value half way between that of the reactor and the manifold. In the second approach, we make no assumptions about the transition volume temperature, but use a known blank volume within the reactor to produce an ``instrument response" for an assumed sample volume. This data is normalized for the actual volume and used to generate an isotherm. The results of these analyses show that the two approaches yield almost identical isotherm results for hydrogen sorption in a high surface area activated carbon. We will also present data from metal-organic framework (MOF-5 or IRMOF-1) and alkali metal modified MOF structures. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S14.00012: Theoretical characterization of the NHxBHx compounds Maciej Gutowski The NH$_{x}$BH$_{x}$ (x=1-4) compounds display favorable gravimetric and volumetric properties of hydrogen storage. Molecular species have been characterized using highly correlated electronic structure methods. Extended systems (polymers, solids) have been characterized at the density functional level of theory with a Perdew-Wang exchange correlation functional. The results demonstrate unique cohesive properties resulting from dihydrogen intermolecular bonds between protic and hydridic hydrogens of NH and BH, respectively. The kinetic and thermodynamic parameters for hydrogen release and uptake will be discussed. [Preview Abstract] |
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