Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session L30: Focus Session: Hydrogen Storage II -- Complex Hydrides |
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Sponsoring Units: DMP Chair: Jan Herbst, General Motors Room: D139 |
Tuesday, March 16, 2010 2:30PM - 3:06PM |
L30.00001: Suppressing the formation of stable intermediates in hydride destabilization reactions Invited Speaker: Comprehensive computational screening efforts have identified numerous candidates for hydride destabilization reactions with reaction enthalpies that fall in the range of interest for engineering requirements (20 to 40 kJ/mol H$_2$). Less certain are the actual kinetic pathways of the solid--state reactions under consideration. Of particular interest are LiBH$_4$--based reactions. We have studied a number of destabilization reactions based on LiBH$_4$ and note that the B$_{12}$H$_{12}$ closo--borane is a commonly formed intermediate. Moreover, formation of M$_{2/n}$B$_{12}$H$_{12}$ phases as major intermediate species is observed in hydrogen desorption reaction of various M(BH$_4$)$_n$ systems before further conversion to either MB$_n$ or B+MH$_n$. We have also examined a number of possible reactions that might destabilize B$_{12}$H$_{12}$ phases using reactions suggested in the recent literature but even under severe processing conditions, this closo--borane phase remains. Our inability to destabilize the closo--borane is particularly puzzling given that the initial work of Vajo on the LiBH$_4$--MgH$_2$ with dehydrogenation into several bar pressure showed complete reversibility. Our work however has generally been done under static vacuum. We have re-examined the reactions in the LiBH$_4$--MgH$_2$ and through the use of $^{11}$B NMR, note that when dehydrogenated under 4 bar hydrogen pressure, no amorphous boron or B$_{12}$H$_{12}$ can be detected. When dehydrogenated into pressures below 1.5 bar hydrogen however, the closo--borane structure is formed. We will discuss these results and consider other possible reaction pathways that dictate the way in which these solid--state reaction experiments are conducted. [Preview Abstract] |
Tuesday, March 16, 2010 3:06PM - 3:18PM |
L30.00002: Theoretical prediction of decomposition paths for Ca(BH$_4$)$_2$ and Mg(BH$_4$)$_2$ Yongsheng Zhang, Chris Wolverton, Eric Majzoub, Vidvuds Ozolins Experimental and theoretical studies on Ca(BH$_4$)$_2$ indicate that the decomposition pathway of this compound is not a simple one-step reaction to final products, but instead may be a multi-step decomposition path (though the reaction pathway is currently unknown). We have studied the decomposition pathways of both Ca- and Mg-borohydride using Density Functional Theory (DFT) as well as a Monte Carlo-based crystal structure prediction method called PEGS. We find that a recently experimentally proposed CaB$_2$H$_2$ intermediate is extremely high in energy and hence very unlikely to form. We then symmetrically studied the low-energy structures of CaB$_2$H$_4$ and CaB$_2$H$_6$ stoichiometries by PEGS+DFT simulations. Based on all these possible reaction paths, the Ca(BH$_4$)$_2$ decomposition convex hull using our calculated reaction enthalpies finds a new compound, CaB$_2$H$_6$, which forms a nearly degenerate pathway to the previously-proposed CaB$_{12}$H$_{12}$ phase. Similar calculations for the Mg system show that the MgB$_2$H$_6$ predicted structure does not form a stable intermediate in the decomposition of Mg(BH$_4$)$_2$. [Preview Abstract] |
Tuesday, March 16, 2010 3:18PM - 3:30PM |
L30.00003: Theoretical investigation of intermediate phases between Li$_2$NH and LiNH$_2$ Feng Zhang, Yan Wang, Mei-Yin Chou The cycling between Li imide (Li$_2$NH) and Li amide (LiNH$_2$) represents the key reactions in the Li-N-H hydrogen storage system. It is important to know whether there exist intermediate phases between these two stable compounds in order to fully understand the mechanism of these reactions. We investigate from first principles possible intermediate compounds Li$_{2-x}$NH$_{1+x}$ and Li$_{1+x}$NH$_{2-x}$ with $x$ equal to 1/8 and 1/4. Li$_{2-x}$NH$_{1+x}$ is created by replacing a certain amount of NH$^{2-}$ with NH$_2^-$ in pure Li imide and removing a proper amount of Li$^+$ to satisfy charge neutrality. Similarly, Li$_{1+x}$NH$_{2-x}$ is created by replacing a certain amount of NH$_2^-$ with NH$^{2-}$ in Li amide and adding a suitable amount of Li$^+$. At $T=0$ K, Li$_{2-x}$NH$_{1+x}$ is energetically favorable with respect to phase separation into pure Li$_2$NH and LiNH$_2$. On the amide side, Li$_{1+x}$NH$_{2-x}$ is only slightly less stable than the phase-separated mixture of amide and imide. These findings suggest that the intermediate phases may appear during the cycling reactions at finite temperatures. Electronic signatures for the intermediate phases resulting from the coexistence of NH$_2^-$ and NH$^{2-}$ anions will also be discussed. [Preview Abstract] |
Tuesday, March 16, 2010 3:30PM - 3:42PM |
L30.00004: Predicting new multicomponent materials for hydrogen storage using first-principles calculations Dilpuneet Aidhy, Chris Wolverton Wide research has unraveled some very promising hydrogen storage materials such as metal borohydrides, amides and alanates. However, all of these materials are limited either thermodynamically or kinetically. The recent observation of mixing in these systems (e.g., borohydride-amide mixing in Li$_{4}$(BH$_{4})$(NH$_{2})_{3}$ [1] and metal mixing in NaZn$_{2}$(BH$_{4})_{3})$ [2] has demonstrated the possibility of forming new multicomponent ordered compounds that may have desirable hydrogen storage properties. However, these multicomponent systems are largely unexplored. Here, we use density functional theory (DFT) along with Monte Carlo-based crystal structure prediction methods to search for new multicomponent hydrides. We find evidence for stable compounds in the Mg(BH$_{4})_{2}$/Mg(NH$_{2})_{2}$ system, which have not yet been observed. In addition, we also study a wide range of mixed metal borohydride systems, and find evidence of ordered stable structures such as Li$_{2}$Na(BH$_{4})_{3}$. 1. F. E. Pinkerton, M. S. Meyer, G. P. Meisner and M. P. Balogh, J. Phys. Chem. B 110, 7967 (2006). 2. D. Ravnsbeak, Y. Filinchuk, Y. Cerenius, H. J. Jakobsen, F. Besenbacher, J. Skibsted and T. R. Jensen, Angew. Chem. 48, 6659 (2009). [Preview Abstract] |
Tuesday, March 16, 2010 3:42PM - 3:54PM |
L30.00005: Characterization of Carbon Aerogels as Scaffolds for Hydrogen Storage Materials. Nina Verdal, Terrence Udovic, Michael Hartman, Margaret Bacon, John Vajo, Adam Gross, Theodore Baumann, John Rush Alkali borohydrides are strong candidates for on-board hydrogen storage. Nanoconfinement of these materials in carbon aerogels improves the kinetics for the dehydrogenation reaction. Efforts have been made to understand the mechanisms behind this improvement. Prompt gamma activation analysis shows that the amount of hydrogen bound to the bare aerogel from the synthesis is dependent on pyrolysis temperature. Neutron vibrational spectra show that these hydrogen atoms are primarily sp$^{2}$ bound to the carbon scaffold. Small angle neutron scattering (SANS) data have been collected for the bare aerogel and the aerogel partly and entirely filled with LiBH$_{4}$ providing information about the pore filling and morphology. [Preview Abstract] |
Tuesday, March 16, 2010 3:54PM - 4:06PM |
L30.00006: Kinetic Enhancement of Hydrogen Cycling in NaAlH$_{4 }$ by Melt Infusion into Nanoporous Carbon Aerogel Frederick Pinkerton, Robert Stephens, Adam Gross, Sky van Atta, John Vajo Enhanced kinetic performance and reversibility have been achieved with uncatalyzed NaAlH$_{4}$ by incorporation into nanoporous carbon aerogel. Aerogel with a pore size distribution peaked at 13 nm and a pore volume of 0.8 cm$^{3}$/g was filled with NaAlH$_{4}$ to 94{\%} capacity by melt-infusion at 189 C under 183 bar H$_{2}$ gas overpressure. Dehydrogenation to NaH + Al with reasonable kinetics was accomplished at 150 C, well below the NaAlH$_{4}$ melting temperature (183 C), compared to hydrogen release above 230 C for bulk uncatalyzed NaAlH$_{4}$. Uncatalyzed bulk samples did not rehydrogenate under laboratory conditions, whereas NaAlH$_{4}$ in a carbon aerogel host was readily rehydrogenated at $\sim $160 C and 100 bar H$_{2}$ to $\sim $85{\%} of its initial capacity. Ball-milled NaAlH$_{4}$ catalyzed with 4 mol{\%} TiCl$_{3}$ showed somewhat better kinetics compared to the infused aerogel; nevertheless, the large kinetic enhancement obtained by incorporation into carbon aerogel, even in the absence of a catalyst, demonstrates the substantial benefit of confining the NaAlH$_{4}$ to nanoscale dimensions. [Preview Abstract] |
Tuesday, March 16, 2010 4:06PM - 4:18PM |
L30.00007: Reaction Paths, Transition States and Catalysis in Li$_{4}$BN$_{3}$H$_{10}$ from First Principles David Farrell, Chris Wolverton Thermodynamic analyses of the complex hydride Li$_{4}$BN$_{3}$H$_{10}$ ($>$10 wt.\% H$_{2}$) predicted favorable hydrogen desorption reactions in the solid, whereas experiments found temperatures above melting were needed before appreciable H$_{2}$ desorption was observed, and the material released NH$_{3}$ at approximately the same temperature. More recent experimental studies successfully catalyzed H$_{2}$ desorption using CoCl$_{2}$ and NiCl$_{2}$, decreasing the H$_{2}$ release temperature. To elucidate the catalytic and decomposition mechanisms that resulted in the measured changes, we have applied \textit{Ab Initio} Molecular Dynamics and Transition State Theory methods to find reaction pathways and determine the rate limiting steps in pristine and catalyzed Li$_{4}$BN$_{3}$H$_{10}$. We observed the formation of several important reaction intermediates, as well as free H$_{2}$ and NH$_{3}$ in the bulk liquid. Finally, We studied the formation of vacancies and interstitials that are promising candidates for rate-limiting steps in the desorption reactions and determined energy barriers for each reaction step. [Preview Abstract] |
Tuesday, March 16, 2010 4:18PM - 4:30PM |
L30.00008: Role of native defects in the Li amide/Li imide hydrogen storage reaction Khang Hoang, Chris G. Van de Walle Reversible reaction involving Li amide/Li imide (LiNH$_{2}$ + LiH $\leftrightarrow $ Li$_{2}$NH + H$_{2})$ has been shown to be a potential mechanism for hydrogen storage [1]. Recent synchrotron x-ray diffraction refinement suggests that the transformation between LiNH$_{2}$ and Li$_{2}$NH is a bulk reaction that occurs through non-stoichiometric processes [2]. To build a deeper understanding of these processes, we have carried out first-principles studies based on density functional theory of native point defects and defect complexes in LiNH$_{2}$ and Li$_{2}$NH. Among the native defects, we find that positively and negatively charged Li and H interstitials and vacancies have the lowest formation energies. Some of the Li-related defects are found to be very mobile, and should be the dominant migratory species in the systems. Our first-principles results suggest specific mechanisms for the role of native defects in the Li amide/Li imide reaction. [1] P. Chen \textit{et al.}, Nature \textbf{420}, 302 (2002). [2] W. I. F. David \textit{et al.}, J. Am. Chem. Soc. \textbf{129}, 1594 (2007). [Preview Abstract] |
Tuesday, March 16, 2010 4:30PM - 4:42PM |
L30.00009: Fullerene mediated hydrogen release in lithium borohydride Ralph Scheicher, Sa Li, Puru Jena Complex metal hydrides possess many properties which make them attractive as a storage medium for hydrogen, but typically, catalysts are required to lower the hydrogen desorption temperature and to facilitate hydrogen uptake in the form of a reversible reaction. The overwhelming focus in the search for catalyzing agents has been on compounds containing titanium, but the precise mechanism of their actions remains somewhat obscure. A recent experiment has now shown that fullerene (C60) can also act as catalysts for the hydrogen uptake and release in lithium borohydride (LiBH$_{4})$. In an effort to understand the involved mechanism, we have employed density functional theory to carry out a detailed study of the interaction between this complex metal hydride and the carbon nanomaterial. Considering a step-wise reduction of the hydrogen content in LiBH$_{4}$, we find that the presence of C60 can lead to a substantial reduction of the involved H-removal energies. This catalyzing effect is explained by us as a consequence of the interaction between the BH$_{x}^{-}$ part and the C60 entity. [Preview Abstract] |
Tuesday, March 16, 2010 4:42PM - 4:54PM |
L30.00010: Prediction of a Stable Solid Boron Hydride for Onboard Hydrogen Storage Tesfaye Abtew, Peihong Zhang We present a first principles study of a new solid boron hydride structure that contains more than 9 wt. percentage of hydrogen for its potential for hydrogen storage. The structural, electronic and dynamic properties are calculated using density functional theory based electronic structure methods. In addition to its high gravimetric capacity, the structure has several desired properties for onboard hydrogen storage: (1) The bonding between boron and hydrogen is relatively weak, which can be weakened further upon charge doping; (2) A stable backbone boron network is favorable for reversible hydrogenation and dehydrogenation. The proposed synthetic route of this solid boron hydride as well as the hydrogen release kinetics will also be discussed. [Preview Abstract] |
Tuesday, March 16, 2010 4:54PM - 5:06PM |
L30.00011: ABSTRACT WITHDRAWN |
Tuesday, March 16, 2010 5:06PM - 5:18PM |
L30.00012: Determination of structure and phase transition of nanophase NH$_{3}$BH$_{3}$ embedded in MCM-41 mesoporous silica Hyunjeong Kim, Abhi Karkamkar, Thomas Autrey, Peter Chupas, Thomas Proffen Nanocomposition of ammonia borane (AB), NH$_{3}$BH$_{3}$, by loading AB in a mesoporous silica has shown great improvement in the hydrogen storage properties [1]; faster hydrogen desorption was observed at reduced temperature and the formation of borazine, by-products that affects hydrogen purity, was significantly suppressed. Even though an improvement was striking, its lack of long-range structural order and relatively light composed elements hinder conventional structural analyses. We have employed the atomic pair distribution function (PDF) analysis to investigate the nanophase AB residing in mesoporous channels of MCM-41 [2]. Temperature dependent x-ray PDF study shows that the AB confined in pores does not undergo the orthorhombic to tetragonal phase transition at 225 K that was observed in the bulk molecular crystal. Instead, it stays in the high temperature tetragonal phase over a temperature range of 110-240 K and becomes amorphous above 240 K. [1] A. Gutowska \textit{et al}., \textit{Angew. Chem. Int. Ed}., \textbf{44}, 3578-3582 (2005). [2] H. J. Kim \textit{et al}., \textit{J. Am. Chem. Soc}., \textbf{131}, 13749-13755 (2009). [Preview Abstract] |
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