Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D5: The Grand Challenge of Hydrogen Storage |
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Sponsoring Units: FIAP Chair: Sunita Satyapal, U.S. DOE and Frederick E. Pinkerton, GM R&D Center Room: LACC 502B |
Monday, March 21, 2005 2:30PM - 3:06PM |
D5.00001: Energetics of Hydrogen Storage Reactions: The Power of DFT Invited Speaker: Jan Herbst Calculations of hydrogen site energetics in LaNi$_{5}$H$_{n}$ (hexagonal P6$_{3}$mc crystal structure) and LaCo$_{5}$H$_{n}$ (orthorhombic Cmmm structure) have been performed within density functional theory (DFT). In each case DFT correctly identifies the most stable hydrogen site configuration, yields an accurate value for the enthalpy of hydride formation, and predicts hydrogen-richer hydrides. The novel hydrogen storage reaction LiNH$_{2}$ + LiH $\leftrightarrow $ Li$_{2}$NH + H$_{2}$ has also been investigated, with the inclusion of zero point energies and finite temperature corrections. The generalized gradient approximation for the exchange-correlation energy functional $\mu _{xc}$ provides much better agreement with experiment than the local density approximation for the structural parameters as well as for the enthalpy of formation of LiNH$_{2}$, LiH, and the reaction enthalpy. While the choice of $\mu _{xc}$ may have substantial impact on results, it is indisputably clear that DFT is a powerful tool for understanding hydrogen storage energetics. [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:42PM |
D5.00002: Novel Nanostructured Materials for Hydrogen Storage Invited Speaker: The United States Department of Energy's (DOE's) Office of Energy Efficiency and Renewable Energy and the Office of Basic Sciences have concluded that hydrogen storage is a cornerstone technology for implementing a hydrogen energy economy. However, significant scientific advancement is still required if a viable on-board storage technology is to be developed. For example, an adsorption process for on-board vehicular storage will require a hydrogen binding energy between $\sim $20-60 kJ/mol to allow for near-room temperature operation at reasonable pressures. Typically, non-dissociative physisorption due purely to van der Waals forces involves a binding energy of only $\sim $ 4 kJ/mol, whereas a chemical bond is $\sim $ 400 kJ/mol. The desired binding energy range for vehicular hydrogen storage therefore dictates that molecular H$_{2}$ be stabilized in an unusual manor. Hydrogen adsorption has been observed with a binding energy of $\sim $ 50 kJ /mol on carbon multi-wall nanotubes (MWNTs) containing iron nanoparticles at their tips. However, hydrogen adsorption at near ambient conditions is neither anticipated nor observed on either purified MWNTs or iron nanoparticles by themselves. Recent theoretical studies have shown that an iron adatom forms a complex with a C$_{36}$ fullerene and shares charge with four carbon atoms of a bent five-membered ring in the C$_{36}$ molecule. Three H$_{2}$ ligands then also coordinate with the iron forming a stable 18-electron organo-metallic complex. Here the binding energy of the molecular hydrogen ligands is $\sim $ 43 kJ /mol. It is believed that a similar interaction may be occurring for MWNTs containing iron nanoparticles. However, a more optimized material must be produced in order to increase the hydrogen capacity. Iron has also been predicted to complex with all twelve of the five-membered rings in C$_{60}$ with a binding energy of $\sim $42 kJ/mol and an H$_{2}$ capacity of 4.9 wt.{\%}. Further, Scandium has been shown to complex with the twelve five-membered rings in C$_{60}$ with a binding energy of $\sim $42 kJ/mol and an H$_{2}$ capacity of 8.7 wt.{\%}. These theoretical findings as well as experimental efforts to synthesize organo-metallic fullerene complexes for vehicular hydrogen storage applications will be discussed in detail. [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 4:18PM |
D5.00003: Invited Speaker: |
Monday, March 21, 2005 4:18PM - 4:54PM |
D5.00004: Destabilization of light element hydrides with high hydrogen capacities: metal imides/nitrides Invited Speaker: High hydrogen capacity materials are highly desirable for hydrogen storage for on-board applications. Some light elements form hydrides with high hydrogen capacities, such as LiH (12.7 wt{\%}) and MgH$_{2}$ (7.6 wt{\%}). These hydrides, however, are very stable, releasing hydrogen only at very high temperature, above 600$^{o}$C and 350$^{o}$C, respectively, with poor kinetics. These hydrogen storage features are unsatisfactory for on-board application. Chen et al [1] reported the hydrogen storage properties of lithium nitride/imide. According to their results lithium imide can absorb hydrogen at 1 bar at 285$^{o}$C reversibly with hydrogen capacity of 6.5wt{\%}. Lithium nitride, on the other hand, can absorb 5wt{\%} more hydrogen, however, it is much more stable compared with lithium imide. \begin{center} Li$_{3}$N + 2H$_{2}$ (Li$_{2}$NH + LiH + H$_{2}$ ( LiNH$_{2}$ + LiH \end{center} This indicates that it is an effective method to destabilize lithium hydride by converting hydride to nitrogen-containing material, such as lithium imide/nitride. Here we report a new approach to further de-stabilize lithium imide by partial substitution of lithium by magnesium in this system. This Mg-substituted material releases hydrogen of significant higher pressure at much lower temperature than those for lithium imide, with minimal capacity reduction [2]. One of the examples is the mixture of (LiNH$_{2}$-MgH$_{2})$, which can release hydrogen of approximately 30 bar at 200$^{o}$C reversibly, with hydrogen capacity of 5 wt{\%}. This material has the potential to deliver hydrogen of 3 bar at 100$^{o}$C. It may be further dehydrogenated to nitride with total hydrogen capacity of approximately 9wt{\%}. The destabilization mechanism for this system will be discussed since this may provide clue in the future searching for high capacity hydrogen storage materials [1] P. Chen, Z. Xiong, J. Luo, J. Lin, L Tan, Nature Vol. 420 (2002) 302-304. [2] W. Luo, J. Alloys and Compounds, 381 (2004) 284-287. [Preview Abstract] |
Monday, March 21, 2005 4:54PM - 5:30PM |
D5.00005: Controlled Hydrogen Release From Ammonia Borane Using Mesoporous Scaffolds Invited Speaker: Hydrogen storage on chemical hydrogen storage materials may provide an attractive new opportunity to meet and exceed the goals of the recent DOE Grand Challenge in Hydrogen Storage for on-board fuel cell applications. We have been investigating the feasibility of using ammonia borane (NH$_{3}$BH$_{3})$, and polyammonia borane (-NH$_{2}$BH$_{2}$-)$_{n }$as reversible hydrogen storage materials. This family of molecules is promising given capacity for high volumetric storage densities, ca. $>$12 wt {\%} hydrogen, and recent computational results that suggest hydrogen uptake and release is near thermoneutral. Ammonia borane (AB) is a stable solid at room temperature that requires heating to release the H$_{2}$. AB decomposes upon melting at 114 $^{o}$C with the vigorous bubbling of H$_{2}$ gas. Alternatively the hydrogen from AB can be released from the solid material at temperatures below 100 $^{o}$C, albeit at significantly lower rates. Thermal decomposition of NH$_{3}$BH$_{3}$ at temperatures below 100 $^{o}$C yields H$_{2}$ and a complex polyaminoborane-like --(NH$_{2}$BH$_{2})_{n}$-- material (PAB). The solid phase thermal reaction involves a bimolecular dehydrocoupling reaction to yield a new B-N bond, i.e., HNB-H --- HNBH to yield HNB-NBH in contrast to our observations of the catalytic pathway involves the intramolecular abstraction of H-H from a single H-NB-H molecule to yield N=B intermediate. At temperatures above 150 $^{o}$C the PAB decomposes to yield a second equivalent of H$_{2}$, concurrent with formation of a polyiminoborane-like --(NHBH)$_{n}$-- material (PIB) and borazine $c-$(NHBH)$_{3}$. The latter is a volatile inorganic analog of benzene, which is highly undesirable in the H$_{2}$ feed. While AB exceeds volumetric and gravimetric density targets for a hydrogen storage material, three additional physical obstacles must be overcome: (i) increasing the rates of H$_{2}$ release at temperatures below 80 $^{o}$C, (ii) preventing borazine formation and (iii) demonstrating the potential for reversibility. There are reports that nano-phase metal hydrides show enhanced kinetics for reversible hydrogen storage relative to the bulk materials. However, after a few hydriding/dehydriding cycles the kinetic enhancement is diminished for some materials as they lose nano-phase structure. We suggest that a rigid nano-phase scaffold loaded with a hydrogen-rich material, may provide an attractive option to preserve the nano-scale dimensions through several hydriding/dehydriding cycles. To demonstrate the effect of a nano-phase scaffold on hydrogen release we use a high-surface area mesoporous silica, loaded with AB as a model system. The work presented in this symposium will highlight our success in lower the temperature of hydrogen release from ammonia borane ($<$80 $^{o}$C) and to minimize the formation of borazine from polyammonia borane decomposition using mesoporous silica templates (SBA-15). Three notable observations are described in this work: (i) increased rates of H$_{2}$ release, (ii) modifications of the non-volatile polymeric products that change the thermodynamics of hydrogen release and (iii) minimized formation of borazine. [Preview Abstract] |
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