Eric H. Majzoub

First-Principles Prediction of Thermodynamically Reversible Hydrogen Storage Reactions in the Li-Mg-Ca-B-H System

Introduction of economically viable hydrogen cars is hindered by the need to store large amounts of hydrogen. Metal borohydrides [LiBH(4), Mg(BH(4))(2), Ca(BH(4))(2)] are attractive candidates for onboard storage because they contain high densities of hydrogen by weight and by volume. Using a set of recently developed theoretical first-principles methods, we predict currently unknown crystal structures and hydrogen storage reactions in the Li-Mg-Ca-B-H system. Hydrogen release from LiBH(4) and Mg(BH(4))(2) is predicted to proceed via intermediate Li(2)B(12)H(12) and MgB(12)H(12) phases, while for Ca borohydride two competing reaction pathways (into CaB(6) and CaH(2), and into CaB(12)H(12) and CaH(2)) are found to have nearly equal free energies. We predict two new hydrogen storage reactions that are some of the most attractive among the presently known ones. They combine high gravimetric densities (8.4 and 7.7 wt % H(2)) with low enthalpies [approximately 25 kJ/(mol H(2))] and are thermodynamically reversible at low pressures due to low vibrational entropies of the product phases containing the [B(12)H(12)](2-) anion.

Titanium-halide catalyst-precursors in sodium aluminum hydrides

Abstract The kinetics of absorption and desorption of hydrogen from NaAlH 4 have previously been shown to improve upon the addition of a catalyst-precursor such as TiCl 3 . In this paper we demonstrate that TiCl 2 , TiF 3 , and TiBr 4 all effectively improve sorption kinetics. Arrhenius data indicate that the catalyst precursors behave in essentially the same manner. Evidently the valency of Ti in the catalyst-precursor is inconsequential to the role of Ti in altering the kinetic mechanism. The formation of TiAl 3 on doping with TiCl 3 has been observed. The presence of TiAl 3 appears to contribute in part to the enhanced kinetics in these systems.

The effects of titanium precursors on hydriding properties of alanates

An overview is presented of recent advances in the development of new and improved alanates for applications and in the fundamental understanding of how Ti-doping enhances hydrogen absorption. Sample materials were produced using approaches based on direct-synthesis and dry Ti-doping methods. It is desirable to introduce Ti through non-reactive processes to avoid the hydrogen capacity loss that occurs through the formation of inactive byproducts (for example Na–halide from the decomposition of Ti–halides and Na–oxides from the decomposition of Ti–alkoxides). We show, for the first time, that alanates can be Ti-doped using TiH2 or through indirect-doping by pre-reacting TiCl2 with LiH. Both methods result in enhanced kinetics. However, improved rates were achieved only after a prolonged activation period of about a 10 cycles, suggesting that cycling leads to Ti diffusion and substitution into the alanate lattice which provides the mechanism through which Ti-doping enhances kinetics. Thus, the reactive decomposition of Ti–halide and alkoxide precursors in the doping process serves an important but not necessarily required function.

Ternary phase diagram studies in Ti-Zr-Ni alloys

Abstract Equilibrium phase formation is reported for ternary Ti–Zr–Ni alloys near the i-phase-forming composition, for temperature slices between 500 and 700°C. Selected microstructural results in as-cast and annealed samples are discussed. Dominant equilibrium phases identified are the C 14 -like Laves phase, a Ti 2 Ni-like phase, α(Ti/Zr), the b.c.c. 1/1 W-phase and the i-phase. The i-phase forms over a small compositional range from a high-temperature equilibrium phase mixture of the Laves and α(Ti/Zr) solid solution phases. Lower annealing treatments have not been found to transform the i-phase, suggesting that in this alloy, the quasicrystal phase might be the ground state phase. Additions of small amounts of Pb and Pd, 1–2 at.%, are demonstrated to substantially effect the equilibrium phase formation for the i-phase and the W-phase. The addition of 1 at.% of Pb significantly orders the i-phase structure.

Cluster structure and hydrogen in Ti–Zr–Ni quasicrystals and approximants

Abstract Elastic neutron diffraction data from icosahedral Ti 45 Zr 38 Ni 17 are presented and analyzed using information from the 1/1 approximant Ti 50 Zr 35 Ni 15 . These data indicate that similar clusters exist in the approximant and the i-phase. This is shown to be consistent with simulated diffraction from an icosahedral glass model of the quasicrystal, placing a Bergman cluster on the glass sites. An electrochemical method was used to hydrogenate Ti-based quasicrystals and their crystal approximants. This technique gives a consistently high hydrogen to metal atom ratio of 1.9, without crystal hydride formation in the quasicrystal.

Probing the unusual anion mobility of LiBH4 confined in highly ordered nanoporous carbon frameworks via solid state NMR and quasielastic neutron scattering

Particle size and particle–framework interactions have profound effects on the kinetics, reaction pathways, and even thermodynamics of complex hydrides incorporated in frameworks possessing nanoscale features. Tuning these properties may hold the key to the utilization of complex hydrides in practical applications for hydrogen storage. Using carefully synthesized, highly-ordered, nanoporous carbons (NPCs), we have previously shown quantitative differences in the kinetics and reaction pathways of LiBH4 when incorporated into the frameworks. In this paper, we probe the anion mobility of LiBH4 confined in NPC frameworks by a combination of solid state NMR and quasielastic neutron scattering (QENS) and present some new insights into the nanoconfinement effect. NMR and QENS spectra of LiBH4 confined in a 4 nm pore NPC suggest that the BH4− anions nearer the LiBH4–carbon pore interface exhibit much more rapid translational and reorientational motions compared to those in the LiBH4 interior. Moreover, an overly broadened BH4− torsional vibration band reveals a disorder-induced array of BH4− rotational potentials. XRD results are consistent with a lack of LiBH4 long-range order in the pores. Consistent with differential scanning calorimetry measurements, neither NMR nor QENS detects a clear solid–solid phase transition as observed in the bulk, indicating that borohydride–framework interactions and/or nanosize effects have large roles in confined LiBH4.

Structural modelling of the Ti–Zr–Ni quasicrystal

Abstract The atomic structure of the icosahedral Ti–Zr–Ni quasicrystal is studied using a canonical cell tiling approach. An investigation of the previously proposed structure for W-Ti–Zr–Ni, the 1/1 crystal approximant to the Ti–Zr–Ni icosahedral quasicrystal, has revealed improbable occupation of sites in the region between the Bergman clusters and at the cluster centre. The 1/1 structure is refined for a better-ordered sample of the approximant, using X-ray and neutron diffraction measurements and ab initio relaxation studies. Based on this refined structure, an atomic decoration model for large canonical cell tilings is developed. Calculated diffraction patterns from the generated structures are in reasonable agreement with those measured for the icosahedral quasicrystal.

Tuning metal hydride thermodynamics via size and composition: Li-H, Mg-H, Al-H, and Mg-Al-H nanoclusters for hydrogen storage.

Nanoscale Li and intermetallic Al-Mg metal hydride clusters are investigated as a possible hydrogen storage material using the high-level quantum Monte Carlo computational method. Lower level methods such as density functional theory are qualitatively, not quantitatively accurate for the calculation of the enthalpy of absorption of H(2). At sizes around 1 nm, it is predicted that Al/Mg alloyed nanoparticles are stable relative to the pure compositions and the metal composition can be tuned in tandem with the size to tune the hydrogen absorption energy, making this a promising route to a rechargeable hydrogen storage material.

Local structure in hydrogenated Ti–Zr–Ni quasicrystals and approximants

Abstract We have investigated the influence of hydrogen on the local structure of Ti–Zr–Ni alloys, icosahedral quasicrystals or crystalline approximants, using extended X-ray absorption fine structure (EXAFS). With an increasing hydrogen-to-metal ratio from 0 to 1.7, a general increase of all the mean first distances was found except for the Zr–Ni (Ni–Zr) ones. The perturbation of the (quasi)lattice, induced by hydrogenation, is a maximum around the Ti and Zr atoms, which suggests that hydrogen atoms sit preferentially near titanium and zirconium atoms.

Crystal structure and local structure of Mg(2-x)Pr(x)Ni4 (x = 0.6 and 1.0) deuteride using in situ neutron total scattering.

We studied crystal structure and local structure of Mg(2-x)Pr(x)Ni4 (x = 0.6 and 1.0) and their deuterides using in situ neutron total scattering and first-principles calculations. The total scattering data were analyzed using Rietveld refinement and pair distribution function analysis (PDF). The crystal structure of Mg(2-x)Pr(x)Ni4 before deuterium absorption was C15b in space group F43m. No difference between the crystal and local (PDF) structures was observed. The crystal structure of Mg1.0Pr1.0Ni4D(∼4) was found to be orthorhombic in space group Pmn2(1), with three deuterium occupation sites: PrNi3 and two types of bipyramidal Pr2MgNi2 that have a plane of symmetry composed of MgNi2. There is no significant difference between the crystal structure and the local structure of Mg1.0Pr1.0Ni4D(∼4). On the other hand, the average crystal structure of the Mg-rich Mg1.4Pr0.6Ni4D(∼3.6) was C15b with two deuterium occupation sites: PrNi3 and MgPrNi2 suggesting that the deuterium occupation shifts away from the Pr2MgNi2 bipyramid. First-principles relaxed structures also showed the shift of the hydrogen occupation site toward the Pr atom of the bipyramid, when induced by Mg substitution for the opposing Pr, resulting in hydrogen occupation in the MgPrNi2 tetrahedral site. The PDF pattern of Mg1.4Pr0.6Ni4D(∼3.6) cannot be refined below 7.2 Å in atomic distances using the C15b structure which was obtained from Rietveld refinement but can be done using an orthorhombic structure. It suggests that Mg1.4Pr0.6Ni4D(∼3.6) was locally distorted to the orthorhombic.