Using first principles calculations to identify new destabilized metal hydride reactions for reversible hydrogen storage

Hydrides of period 2 and 3 elements are promising candidates for hydrogen storage, but typically have heats of reaction that are too high to be of use for fuel cell vehicles. Recent experimental work has focused on destabilizing metal hydrides through mixing metal hydrides with other compounds. A very large number of possible destabilized metal hydride reaction schemes exist, but the thermodynamic data required to assess the enthalpies of these reactions are not available in many cases. We have used density functional theory calculations to predict the reaction enthalpies for more than 300 destabilization reactions that have not previously been reported. The large majority of these reactions are predicted not to be useful for reversible hydrogen storage, having calculated reaction enthalpies that are either too high or too low, and hence these reactions need not be investigated experimentally. Our calculations also identify multiple promising reactions that have large enough hydrogen storage capacities to be useful in practical applications and have reaction thermodynamics that appear to be suitable for use in fuel cell vehicles and are therefore promising candidates for experimental work.     

Light metal hydrides and complex hydrides for hydrogen storage

The availability of feasible methods for hydrogen storage is one of the key-maybe the key-requirements for the large scale application of PEM fuel cells in cars. There are in principle four different approaches, i.e. cryostorage in liquid form, high pressure storage, storage in the form of a chemical compound which is converted to hydrogen by on-board reforming, or reversible chemical storage in different kinds of storage materials. New developments in the field of chemical storage make such systems attractive compared to the other options. This review will discuss the different possibilities for chemical storage of hydrogen and the focus on the presently most advanced system with respect to storage capacity and kinetics, i.e. catalyzed alanates, especially NaAlH(4).     

Large-scale screening of metal hydrides for hydrogen storage from first-principles calculations based on equilibrium reaction thermodynamics

Systematic thermodynamics calculations based on density functional theory-calculated energies for crystalline solids have been a useful complement to experimental studies of hydrogen storage in metal hydrides. We report the most comprehensive set of thermodynamics calculations for mixtures of light metal hydrides to date by performing grand canonical linear programming screening on a database of 359 compounds, including 147 compounds not previously examined by us. This database is used to categorize the reaction thermodynamics of all mixtures containing any four non-H elements among Al, B, C, Ca, K, Li, Mg, N, Na, Sc, Si, Ti, and V. Reactions are categorized according to the amount of H(2) that is released and the reaction's enthalpy. This approach identifies 74 distinct single step reactions having that a storage capacity >6 wt.% and zero temperature heats of reaction 15 ≤ΔU(0)≤ 75 kJ mol(-1) H(2). Many of these reactions, however, are likely to be problematic experimentally because of the role of refractory compounds, B(12)H(12)-containing compounds, or carbon. The single most promising reaction identified in this way involves LiNH(2)/LiH/KBH(4), storing 7.48 wt.% H(2) and having ΔU(0) = 43.6 kJ mol(-1) H(2). We also examined the complete range of reaction mixtures to identify multi-step reactions with useful properties; this yielded 23 multi-step reactions of potential interest.     

Reversible storage of hydrogen in destabilized LiBH4

Destabilization of LiBH4 for reversible hydrogen storage has been studied using MgH2 as a destabilizing additive. Mechanically milled mixtures of LiBH4 + (1/2)MgH2 or LiH + (1/2)MgB2 including 2-3 mol % TiCl3 are shown to reversibly store 8-10 wt % hydrogen. Variation of the equilibrium pressure obtained from isotherms measured at 315-400 degrees C indicate that addition of MgH2 lowers the hydrogenation/dehydrogenation enthalpy by 25 kJ/(mol of H2) compared with pure LiBH4. Formation of MgB2 upon dehydrogenation stabilizes the dehydrogenated state and, thereby, destabilizes the LiBH4. Extrapolation of the isotherm data yields a predicted equilibrium pressure of 1 bar at approximately 225 degrees C. However, the kinetics were too slow for direct measurements at these temperatures.     

Problem of hydrogen storage and prospective uses of hydrides for hydrogen accumulation

Merits and demerits of existing methods of hydrogen storage are discussed. Special attention is given a metal hydride technology based on the ability of metals, intermetallic compounds, and alloys for reversible reaction with hydrogen. It is noted that the basic advantages of metal hydrides are a high volumetric hydrogen content, operational safety, technological flexibility, and low power inputs on hydrogen absorption and desorption.     

Superhalogens as building blocks of complex hydrides for hydrogen storage

Superhalogens are species whose electron affinity (EA) or vertical detachment energy (VDE) exceeds those of halogens. These species typically consist of a central electropositive atom with electronegative ligands. The EA or VDE of species can be further increased by using superhalogens as ligands, which are termed as hyperhalogens. Having established BH₄⁻ as a superhalogen, we have studied BH_4 − x(BH₄)_x⁻ (x = 1–4) hyperhalogen anions and their Li-complexes LiBH_4 − x(BH₄)_x using density functional theory. The VDE of these anions is larger than that of BH₄⁻, which increases with the increase in number of peripheral BH₄ moieties (x). The hydrogen storage capacity of LiBH_4 − x(BH₄)_x complexes is higher but binding energy is smaller than that of LiBH₄, a typical complex hydride. The linear correlation between the dehydrogenation energy of LiBH_4 − x(BH₄)_x complexes and the VDE of BH_4 − x(BH₄)_x⁻ anions is established. These complexes are found to be thermodynamically stable against dissociation into LiBH₄ and borane. This study demonstrates the role of superhalogens in designing new materials for hydrogen storage and should also motivate experimentalists to synthesize LiBH_4 − x(BH₄)_x (x = 1–4) complexes.     

Structural and dynamical stability of cadmium nitride using first principles calculations

We report the results of a theoretical study on the behavior of the structural parameters, electronic band structure, vibrational and thermodynamical properties of transition metal nitride, CdN in the rocksalt (RS), NiAs (P63/mmc) and CuS (B₁₈) phases at ambient pressure. The calculations are based on the ab-initio plane-wave pseudopotential density functional theory (DFT), within the generalized gradient approximations (GGA) for the exchange and correlation functional. The calculated values of lattice parameters, bulk modulus and its first order pressure derivative are in good agreement with other reports. A linear response approach to the density functional theory is used to derive the phonon frequencies, phonon densities of states and thermodynamical properties. We discuss the contribution of the phonons in the dynamical stability of CdN and detailed analysis of thermodynamical properties of specific heat and Debye temperature for CdN in all considered structures.     

Formation region and hydrogen storage abilities of perovskite-type hydrides

Formation region and hydrogen desorption and absorption properties of the perovskite-type structure in Li_xNa_1−xMgH₃ with x = 0, 0.17, 0.33, 0.50 and 1.00 were studied in the present work. The experimental results are reasonably explained from the viewpoint of the geometric restrictions of ions that are described by so-called Goldschmidt tolerance factors. In NaMgH₃ (x = 0), two plateau pressures of about 0.040 and 0.15 MPa were clearly detected by hydrogen pressure–composition (p–c) isotherm measurement at 673 K. Moreover, NaMgH₃ can be reversibly formed in 1.0 MPa of hydrogen at 673 K, from the decomposed phase of elemental Na and Mg.     

Dynamics of photochemical reactions: Simulation by quantum calculations for transition metal hydrides

The photodissociation dynamics of some organometallic molecules in the lowest repulsive electronically states are reported for the following concurrent primary reactions: (i) the homolysis of a metal–hydrogen bond vs. the heterolytic loss of a carbonyl ligand in HCo(CO)₄; (ii) the photoinduced elimination of molecular hydrogen vs. the loss of a carbonyl ligand in H₂Fe(CO)₄; and (iii) the photoinduced elimination of molecular hydrogen vs. the loss of a mesithylene ligand in H₂Os(CO)Mes (Mes = C₆H₃(CH₃))₃. The dynamics are simulated quantum mechanically using a time-dependent wavepacket propagation technique on potential energy surfaces obtained from CASSCF /CCI calculations for HCo(CO)₄ and H₂Fe(CO)₄ and from SCF -INO /MRCI calculations for H₂Os(CO)Mes. This approach gives a rather detailed view of some important elementary processes that contribute to the photochemistry of these complexes. The nature of the photoactive excited states is determined without ambiguity, as well as the time scales, the branching ratio of the different primary dissociation pathways, and some features of the absorption spectra. © 1994 John Wiley & Sons, Inc.     

含金金属纳米颗粒用于液相化学氢化物催化制氢

液相化学氢化物以化学键的形式储存氢能,被认为是一类很有前景的化学储氢材料。液相化学氢化物的大规模应用很大程度上依赖于高效催化系统的开发。含金金属纳米颗粒在用于液相化学氢化物催化制氢中表现出优异的催化性能。本文综述了金纳米颗粒和含金异金属纳米颗粒用于液相氢化物催化制氢的最新研究进展。     

Probing the surface structure of hydroxyapatite using NMR spectroscopy and first principles calculations

The surface characteristics of hydroxyapatite (HA) are probed using a combination of NMR spectroscopy and first principles calculations. The NMR spectrum is taken from a bone sample and the first principles calculations are performed using a plane-wave density functional approach within the pseudopotential approximation. The computational work focuses on the (100) and (200) surfaces, which exhibit a representative range of phosphate, hydroxyl and cation bonding geometries. The shielding tensors for the 31P, 1H and 17O nuclei are calculated from the relaxed surface structures using an extension of the projector augmented-wave method. The calculated 31P chemical shifts for the surface slab are found to be significantly different from the bulk crystal and are consistent with the NMR data from bone and also synthetically prepared nanocrystalline samples of HA. Rotational relaxations of the surface phosphate ions and the sub-surface displacement of other nearby ions are identified as causing the main differences. The investigation points to further calculations of other crystallographic surfaces and highlights the potential of using NMR with ab initio modelling to fully describe the surface structure and chemistry of HA, which is essential for understanding its reactivity with the surrounding organic matrix.     

Electronic structure of alkaline metal hydrides according to MO LCAO-SCF-CNDO cluster calculations

This paper presents the results of a quantum chemical study of compounds MH (M = Li, Na, K, Rb, and Cs) in a cluster approximation. The calculations were performed by the MO LCAO-SCF-CNDO method (special variation which proved to be effective for studying model systems of high-temperature superconductors). The calculation reproduces the expected electron density distribution on the hydrogen and metal atoms in the hydrides as well as the energy characteristics: M-H and M-M bond energ.es and the binding energies of compounds. The latter qualitatively correlate with the bond energies in the series of compounds LiH-CsH. The calculated Fermi energies and forbidden gaps at the Fermi level suggest that in the series being investigated a perfect crystal of lithium hydride will have the highest electric resistance. It is established that the quantum chemical characteristics of the electronic structure of MH change nonmonotonically from Li to Cs. Translated fromZhurnal Strukturnoi Khimii, Vol. 38, No. 3, pp. 431–437, May–June, 1997.     

Crystal structure evolution of complex metal aluminum hydrides upon hydrogen release

Complex aluminum hydrides have been widely studied as potential hydrogen storage materials but also,for some time now, for electrochemical applications. This review summarizes the crystal structures of alkali and alkaline earth aluminum hydrides and correlates structure properties with physical and chemical properties of the hydride compounds. The crystal structures of the alkali metal aluminum hydrides change significantly during the stepwise dehydrogenation. The general pathway follows a transformation of structures built of isolated [AlH4]~- tetrahedra to structures built of isolated [Al H6]~(3-) octahedra.The crystal structure relations in the group of alkaline earth metal aluminum hydrides are much more complicated than those of the alkali metal aluminum hydrides. The structures of the alkaline earth metal aluminum hydrides consist of isolated tetrahedra but the intermediate structures exhibit chains of cornershared octahedra. The coordination numbers within the alkali metal group increase with cation sizes which goes along with an increase of the decomposition temperatures of the primary hydrides. Alkaline earth metal hydrides have higher coordination numbers but decompose at slightly lower temperatures than their alkali metal counterparts. The decomposition pathways of alkaline metal aluminum hydrides have not been studied in all cases and require future research.     

Matrix infrared spectra and density functional calculations of transition metal hydrides and dihydrogen complexes

Metal hydrides are of considerable importance in chemical synthesis as intermediates in catalytic hydrogenation reactions. Transition metal atoms react with dihydrogen to produce metal dihydrides or dihydrogen complexes and these may be trapped in solid matrix samples for infrared spectroscopic study. The MH(2) or M(H(2)) molecules so formed react further to form higher MH(4), (H(2))MH(2), or M(H(2))(2), and MH(6), (H(2))(2)MH(2), or M(H(2))(3) hydrides or complexes depending on the metal. In this critical review these transition metal and dihydrogen reaction products are surveyed for Groups 3 though 12 and the contrasting behaviour in Groups 6 and 10 is discussed. Minimum energy structures and vibrational frequencies predicted by Density Functional Theory agree with the experimental results, strongly supporting the identification of novel binary transition metal hydride species, which the matrix-isolation method is well-suited to investigate. 104 references are cited.     

Optical properties of alkaline-earth metal oxides from first principles

This study reports the results of an ab initio electronic and optical calculation of alkaline-earth metal oxides (MgO, CaO, SrO and BaO) in the NaCl crystal structure using the full potential linearized augmented plane wave (FP-LAPW) method within the density functional theory. The exchange-correlation potential is treated by the generalized gradient approximation within the Perdew et al scheme. Moreover, the Engel–Vosko GGA formalism is applied so as to optimize the corresponding potential for band structure calculations. The real and imaginary parts of the dielectric function ?(ω), the optical absorption coefficient I(ω), the reflectivity R(ω) and the energy loss function are calculated by random phase approximation (RPA). The calculated results show a qualitative agreement with the available experimental results in the sense that we can recognize some peaks qualitatively, those due to single particle transitions. Furthermore the interband transitions responsible for the structures in the spectra are specified. It is shown that the oxygen 2p states and metal d states play the major role in optical transitions as initial and final states respectively. The effect of the spin–orbit coupling on the optical properties is also investigated and found to be quite small, especially in the low energy region. The dielectric constants are calculated and compared with the available theoretical and experimental results.     

Pseudopotential calculations using the FSGO Method: Application to first-row hydrides

Various pseudopotential schemes are examined in floating spherical Gaussian orbital (FSGO) calculations on the first row hydrides to help determine which pseudopotential scheme, if any, would be most useful in the FSGO method.     

High pressure reactivity of propene by first principles molecular dynamics calculations

The reactivity of propene under high pressure has been investigated in the framework of Car-Parrinello molecular dynamics. Changes in structural and electronic properties due to pressure have been analyzed in systems with a density ranging from 0.855 to 2.151 g/cm(3). A ionic collective mechanism which leads to the formation of oligomers has been found by both spin restricted and spin polarized formalism. The maximally localized Wannier centers analysis has allowed us to characterize the addition scheme and to identify a Wannier center with a high spread value involved in the formation of the principal reaction products.