Session D36: Focus Session: Hydrogen Storage I: Chemical Hydride and Complex Metal Hydride Materials I

Nanoscale Hydrides in Porous Carbon Scaffolds

Light element and complex anion hydrides (such as LiH, MgH$_{2}$, NaAlH$_{4}$, and LiBH$_{4})$ are being studied intensely as reversible hydrogen storage materials for fuel cell powered vehicles because they have high gravimetric and volumetric capacities. However, the \textit{rates} of dehydrogenation and rehydrogenation of these hydrides are typically much too slow to be compatible with proton exchange membrane fuel cell temperatures. The slow rates originate, at least in part, from the high activation energies for diffusion associated with the ionic and covalent bonds found in these materials. In contrast to metallic systems, ionic and covalent bonds are directional. Thus, the transition states for atomic rearrangement occur in particularly unfavorable bonding configurations. This increases the activation energies for diffusion and results in slow phase transformation and hydrogen sorption kinetics. Overcoming these kinetic limitations has become a critical element in the development of light-metal complex hydrides for practical storage applications. Small quantities of catalytic additives have been shown to greatly improve the rates of hydrogen exchange in MgH$_{2}$ and NaAlH$_{4}$. Another approach involves formation of nanoscale hydrides by incorporation into nanoporous scaffolds. The small pores of the scaffold limit the sizes of the hydrogenated and dehydrogenated phases and thus the diffusion lengths to nanoscale dimensions. These relatively short diffusion lengths reduce diffusion times and therefore, increase overall rates of hydrogen sorption. The size limitations also result in increased interfacial area between reacting phases, which improves hydrogen capacity retention during cycling. This talk will focus on the preparation, characterization, and hydrogen sorption behavior of LiBH$_{4}$, MgH$_{2}$ and NaAlH$_{4}$ incorporated into nanoporous carbon aerogels.   

A Study of the Structural and Dynamical Properties of Lithium Borohydride Confined within Nanoporous Framework Structures using Neutron Scattering Investigations

Lithium borohydride, LiBH$_{4}$, is a complex metal hydride that shows great promise as a hydrogen storage medium with a volumetric hydrogen density of 122 kg H/m$^{3}$ and a gravimetric hydrogen density of 18.5 wt. {\%}. We have previously reported on the structural and dynamical properties of neat $^{7}$Li$^{11}$BH$_{4}$ as determined by neutron powder diffraction, neutron vibrational spectroscopy, and quasielastic neutron scattering. Here we report on recent measurements undertaken to investigate the changes in the structural and dynamical properties that are observed when this material is confined within nanoporous structures with pore sizes ranging from 4 nm to 25 nm. These materials exhibit a reduction in the structural transition and melting temperatures, which we associate with a marked decrease in the activation energy for reorientational motions of the [BH$_{4}$]$^{-}$ tetrahedra.   

Density Functional Screening of Metal Hydride Reactions

The on-board storage of hydrogen is one of the most vexing problems associated with the development of viable fuel cell vehicles. Hydrides of period 2 or 3 metals can store hydrogen at high gravimetric and volumetric densities. However, existing hydrides either have unacceptable thermodynamics or kinetics. New materials for hydrogen storage are therefore needed. We demonstrate how first principles density functional theory (DFT) can be used to screen potential candidate materials for hydrogen storage. We have used DFT calculations in conjunction with a free energy analysis to screen over a million reactions involving 212 known compounds. This approach has identified several interesting reaction schemes that have not yet been explored experimentally. We have computed the phonon density of states and used this information to predict the van't Hoff plots for some of the most promising candidate reactions identified though our modeling. We have also examined the thermodynamics of thin films and nanoparticles for selected metal hydrides by accounting for the surface energies of the films or nanoparticles.   

Reversible Hydrogen Storage in the Lithium Borohydride -- Calcium Hydride Coupled System

We report large reversible hydrogen storage in a new coupled system, LiBH$_{4}$/CaH$_{2}$, via the reaction 6 LiBH$_{4}$ + CaH$_{2}$ $\leftrightarrow $ 6 LiH + CaB$_{6}$ + 10 H$_{2}$ having a theoretical hydrogen capacity of 11.7 wt{\%} and an estimated reaction enthalpy of $\Delta $H = 59 kJ/mole H$_{2}$. Samples that include 0.25 mole (18.2 wt{\%}) TiCl$_{3}$ reproducibly store 9.1 wt{\%} hydrogen, corresponding to 95{\%} of the available hydrogen. H$_{2}$ is the only evolved gas detected by mass spectrometry. X-ray diffraction confirms that the sample cycles between LiBH$_{4}$ and CaH$_{2}$ in the hydrogenated state and LiH and CaB$_{6}$ in the dehydrogenated state.   

Structures and Bonding of Li-B-N-H Quaternary Hydrides

Complex metal hydrides are of great interest for hydrogen-storage applications due to their potential high hydrogen capacity. Intense efforts has been made on the Li-B-N-H system, with the discovery of several novel quaternary phases. There have been prior studies investigating the structure of one of these new phases (Li$_{4}$BN$_{3}$H$_{10})$. However, all these studies were undertaken on hydrides without any isotope enrichment, thus resulted in diffraction data of limited quality and yielded structures with questionable bond lengths and uncharacteristically deformed anion groups. So far, no studies have been reported on the isotopically labeled samples, which are necessary to determine correct structures for these hydrides. We report for the first time the crystal structures of Li$_{2}$BNH$_{6}$ and Li$_{4}$BN$_{3}$H$_{10}$ derived from high-resolution neutron diffraction data on samples labeled with $^{7}$Li, $^{11}$B, and D. Our refined structures clarify the prevailing structural discrepancies. We also report corresponding neutron vibrational spectra combined with first-principles calculations to gain more insight between structure and bonding. The configurations of both BH$_{4}^{-}$ and NH$_{2}^{-}$ anions and the structural variations upon compositional changes will be discussed. Our study provides implications to the mechanisms of hydrogen absorption/desorption in these complex hydrides.   

Thermodynamic guidelines for the prediction of hydrogen storage reactions and their application to destabilized hydride mixtures

We propose a set of thermodynamic guidelines aimed at facilitating more robust screening of hydrogen storage reactions. The utility of the guidelines is illustrated by reassessing the validity of reactions recently proposed in the literature and through vetting a list of more than 20 candidate reactions based on destabilized LiBH$_4$ and Ca(BH$_4$)$_2$ borohydrides. Our analysis reveals several reactions having both favorable thermodynamics and relatively high hydrogen densities (ranging from 5--9 wt \% H$_2$ and 85--100 g H$_2 $/l), and demonstrates that chemical intuition alone is not sufficient to identify valid reaction pathways.   

Crystal Chemistry of the Perovskite-type Hydride NaMgH$_{3}$

The crystal structure, lattice dynamics, and local metal-H bonding configuration of the perovskite hydride NaMgH$_{3}$ were investigated using combined neutron powder diffraction, neutron vibrational spectroscopy, and DFT calculations. NaMgH$_{3}$ crystallizes in the orthorhombic GdFeO$_{3}$-type perovskite structure (\textit{Pnma}) with $a^{-}b^{+}a^{-}$ octahedral tilting in the temperature range of 4-370~K. In contrast with previous structure studies, the refined Mg-H lengths and H-Mg-H angles indicate that the MgH$_{6}$ octahedra maintain a near ideal configuration, which is corroborated by bond valence methods and our DFT calculations, and is consistent with perovskite oxides with similar tolerance factor values. The temperature dependences of the lattice distortion, octahedral tilting angle, and atomic displacement of H are consistent with the recently observed high H mobility at elevated temperature. The stability and dynamics of NaMgH$_{3}$ are discussed and rationalized in terms of lattice distortion, cation octahedra tilting, and local bonding configurations in the observed perovskite structure. Further experiments reveal that its perovskite crystal structure can be used to improve the slow hydrogenation kinetics of some strongly bound light-metal-hydride systems such as MgH$_{2}$ and to design new alloy hydrides with desirable hydrogen-storage properties.   

Bonding changes in compressed NaBH$_{4}$ probed by inelastic X-ray scattering

Hydrogen storage for commercial applications is an ongoing challenge in materials science research in recent years. Complex borohydrides are technologically promising materials due to their light weight and high gravimetric and volumetric hydrogen density. So far knowledge of the structural and bonding changes in these systems is elusive due to low z elements and lack of in-situ experimental probes. Here we present the first experimental results of boron K-edge inelastic X-ray scattering performed on NaBH$_{4}$ revealing the nature of bonding changes during compression up to 12 GPa. NaBH$_{4}$ undergoes structural phase transition from cubic (Fm-3m) to tetragonal (P421/c) above 6 GPa and to orthorhombic (Pnma) above 8.3 GPa. The high pressure tetragonal and orthorhombic phases show weakening of B-H bonding during phase transition. Further, NaBH$_{4}$ may be considered as a representative example for isostructural systems since a similar structural sequence is also observed in KBH$_{4}$ on compression. The experimental details and the inelastic x-ray scattering results will be presented.   

On the Formation of LaFe$_{5}$H$_{n}$

Formation of a LaFe$_{5}$H$_{n}$ hydride is explored by means of density functional theory. Enthalpies of formation $\Delta$H with respect to the elemental metals and H$_{2}$ are calculated for various hydrogen configurations in four prototype crystal structures. We find $\Delta$H $<$ 0 in many cases, suggesting the existence of LaFe$_{5}$H$_{n}$, as does Miedema's semi-empirical model. $\Delta$H is a minimum for the LaFe$_{5}$H$_{n}$ stoichiometry with hydrogen occupying the 4e, 8g, and 16m sites in the orthorhombic Cccm structure. Phonon dispersion relations and elastic constants computed for that structure exhibit no anomalies, demonstrating vibrational stability. Similar results for LaFe$_{5}$ indicate that compound may form under pressure.