Hydride System Research Articles

Nanosizing and Nanoconfinement: New Strategies Towards Meeting Hydrogen Storage Goals

Hydrogen is expected to play an important role as an energy carrier in a future, more sustainable society. However, its compact, efficient, and safe storage is an unresolved issue. One of the main options is solid-state storage in hydrides. Unfortunately, no binary metal hydride satisfies all requirements regarding storage density and hydrogen release and uptake. Increasingly complex hydride systems are investigated, but high thermodynamic stabilities as well as slow kinetics and poor reversibility are important barriers for practical application. Nanostructuring by ball-milling is an established method to reduce crystallite sizes and increase reaction rates. Since five years attention has also turned to alternative preparation techniques that enable particle sizes below 10 nanometers and are often used in conjunction with porous supports or scaffolds. In this Review we discuss the large impact of nanosizing and -confinement on the hydrogen sorption properties of metal hydrides. We illustrate possible preparation strategies, provide insight into the reasons for changes in kinetics, reversibility and thermodynamics, and highlight important progress in this field. All in all we provide the reader with a clear view of how nanosizing and -confinement can beneficially affect the hydrogen sorption properties of the most prominent materials that are currently considered for solid-state hydrogen storage.

Computational Study of Silica-Supported Transition Metal Fragments for Kubas-type Hydrogen Storage

To verify the role of the Kubas interaction in transition metal grafted mesoporous silicas, and to rationalize unusual rising enthalpy trends with surface coverage by hydrogen in these systems, computational studies have been performed. Thus, the interaction of H2 with the titanium centers in molecular models for experimentally characterized mesoporous silica-based H2 absorption materials has been studied quantum chemically using gradient corrected density functional theory. The interaction between the titanium and the H2 molecules is found to be of a synergic, Kubas type, and a maximum of four H2 molecules can be bound to each titanium, in good agreement with previous experiments. The average Ti-H2 interaction energies in molecules incorporating benzyl ancillary ligands (models of the experimental systems) increase as the number of bound H2 units increases from two to four, in agreement with the experimental observation that the H2 adsorption enthalpy increases as the number of adsorbed H2 molecules increases. The Ti-H2 interaction is shown to be greater when the titanium is bound to ancillary ligands, which are poor π-acceptors, and when the ancillary ligand causes the least steric hindrance to the metal. Extension of the target systems to vanadium and chromium shows that, for molecules containing hydride ancillary ligands, a good relationship is found between the energies of the frontier molecular orbitals of the molecular fragments, which interact with incoming H2 molecules, and the strength of the M-H2 interaction. For the benzyl systems, both the differences in M-H2 interaction energies and the energy differences in frontier orbital energies are smaller than those in the hydrides, such that conclusions based on frontier orbital energies are less robust than for the hydride systems. Because of the high enthalpies predicted for organometallic fragments containing hydride ligands, and the low affinity of Cr(III) for hydrogen in this study, these features may not be ideal for a practical hydrogen storage system.

Scattering law of a magnesium hydride moderator

Metal hydrides have long been considered possible moderator and pre-moderator materials for neutron sources. These materials have hydrogen density comparable to liquid hydrogen or light water. They usually do not undergo phase transitions in the desired operating range of 0–300 K, and display reasonable resistance to radiation damage. Magnesium hydride is such a simple, robust hydride system. To assess its neutronic usefulness as a moderator material, we determined experimentally the total scattering cross-section of the material. We compared our theoretical results to the experimental total neutron cross-section and to the data from quasi-elastic neutron scattering experiments, and produced a scattering kernel suitable for neutron transport calculations.

Nanoconfined Magnesium Borohydride for Hydrogen Storage Applications Investigated by SANS and SAXS

Nanoscale hydride systems have recently gained increased attention for their possible energy storage applications. In the present work, nanoscale particles of Mg(11BD4)2 infiltrated into an activated carbon scaffold were studied by small-angle scattering techniques, and their behavior was compared with that of the bulk powders. Upon heating to 400 °C, under dynamic vacuum, the nanoconfined particles maintain their size distribution, and the decomposition affects only the morphology of the particle surface. On the contrary, the bulk powders showed a significant modification of both particle size and surface morphology under the same conditions. The carbon scaffold therefore serves to ensure both the desired nanoscale organization of the magnesium borohydride and stabilization in size of the incorporated material.

Performance of W4 theory for spectroscopic constants and electrical properties of small molecules

Accurate spectroscopic constants and electrical properties of small molecules are determined by means of W4 and post-W4 theories. For a set of 28 first- and second-row diatomic molecules for which very accurate experimental spectroscopic constants are available, W4 theory affords near-spectroscopic or better predictions. Specifically, the root-mean-square deviations (RMSDs) from experiment are 0.04 pm for the equilibrium bond distances (r(e)), 1.03 cm(-1) for the harmonic frequencies (ω(e)), 0.20 cm(-1) for the first anharmonicity constants (ω(e)x(e)), 0.10 cm(-1) for the second anharmonicity constants (ω(e)y(e)), and 0.001 cm(-1) for the vibration-rotation coupling constants (α(e)). These RMSDs imply 95% confidence intervals of about 0.1 pm for r(e), 2.0 cm(-1) for ω(e), 0.4 cm(-1) for ω(e)x(e), and 0.2 cm(-1) for ω(e)y(e). We find that post-CCSD(T) contributions are essential to achieve such narrow confidence intervals for r(e) and ω(e), but have little effect on ω(e)x(e) and α(e), and virtually none on ω(e)y(e). Higher-order connected triples T(3)-(T) improve the agreement with experiment for the hydride systems, but their inclusion (in the absence of T(4)) tends to worsen the agreement with experiment for the nonhydride systems. Connected quadruple excitations T(4) have significant and systematic effects on r(e), ω(e), and ω(e)x(e), in particular they universally increase r(e) (by up to 0.5 pm), universally reduce ω(e) (by up to 32 cm(-1)), and universally increase ω(e)x(e) (by up to 1 cm(-1)). Connected quintuple excitations T(5) are spectroscopically significant for ω(e) of the nonhydride systems, affecting ω(e) by up to 4 cm(-1). Diagonal Born-Oppenheimer corrections have systematic and spectroscopically significant effects on r(e) and ω(e) of the hydride systems, universally increasing r(e) by 0.01-0.06 pm and decreasing ω(e) by 0.3-2.1 cm(-1). Obtaining r(e) and ω(e) of the pathologically multireference BN and BeO systems with near-spectroscopic accuracy requires large basis sets in the core-valence CCSD(T) step and augmented basis sets in the valence post-CCSD(T) steps in W4 theory. The triatomic molecules H(2)O, CO(2), and O(3) are also considered. The equilibrium geometries and harmonic frequencies (with the exception of the asymmetric stretch of O(3)) are obtained with near-spectroscopic accuracy at the W4 level. The asymmetric stretch of ozone represents a severe challenge to W4 theory, in particular the connected quadruple contribution converges very slowly with the basis set size. Finally, the importance of post-CCSD(T) correlation effects for electrical properties, namely, dipole moments (μ), polarizabilities (α), and first hyperpolarizabilities (β), is evaluated.

Development of a Sievert apparatus for characterization of high pressure hydrogen sorption materials

A volumetric gas absorption (Sievert) apparatus has been developed to measure hydrogen absorption and desorption at pressures up to 700 bar and temperatures between 240 K and 320 K. The apparatus is designed to reduce uncertainty for high pressure measurements while maintaining proper temperature control in the sample. Pressure-composition isotherms (PCI) and kinetics measurements of a well-studied material, LaNi 5 have been obtained for validation of the apparatus. Measurements of both absorption and desorption PCI curves as well as full absorption kinetics data have been obtained for TiCrMn to examine the performance at high pressures, as well as to examine the thermodynamic hysteresis effect in TiCrMn for applications in metal hydride system design. Due to this hysteresis, the thermodynamics of the absorption reaction differ significantly from those of the desorption reaction, which must be accounted for when considering thermal design of a metal hydride reactor and the suitability of the metal hydride for energy storage applications.

Raising the cycling stability of aqueous lithium-ion batteries by eliminating oxygen in the electrolyte

Aqueous lithium-ion batteries may solve the safety problem associated with lithium-ion batteries that use highly toxic and flammable organic solvents, and the poor cycling life associated with commercialized aqueous rechargeable batteries such as lead-acid and nickel-metal hydride systems. But all reported aqueous lithium-ion battery systems have shown poor stability: the capacity retention is typically less than 50% after 100 cycles. Here, the stability of electrode materials in an aqueous electrolyte was extensively analysed. The negative electrodes of aqueous lithium-ion batteries in a discharged state can react with water and oxygen, resulting in capacity fading upon cycling. By eliminating oxygen, adjusting the pH values of the electrolyte and using carbon-coated electrode materials, LiTi(2)(PO(4))(3)/Li(2)SO(4)/LiFePO(4) aqueous lithium-ion batteries exhibited excellent stability with capacity retention over 90% after 1,000 cycles when being fully charged/discharged in 10 minutes and 85% after 50 cycles even at a very low current rate of 8 hours for a full charge/discharge offering an energy storage system with high safety, low cost, long cycling life and appropriate energy density.

Research and Development of Nano-Composite Materials for Hydrogen Storage

Hydrides with light elements such as MgH2, LiH, NH3 and NH3BH3 are known as high hydrogen containing materials. However, the high work temperature and the slow reaction rate limit the practical application of hydride systems. Those properties can be improved by the nano-composite materials. The nano-composite materials for hydrogen storage encompass a catalyst and composite hydrides at the nanometer scale. The catalyst increases reaction rate. The thermodynamic stability of the nano-composite materials can be controlled by the composite hydrides. In addition, the hydrogen absorption kinetics is accelerated by the nano-size materials and they may change the thermodynamic stability of the materials. In this study, we reviewed our experimental results on hydrogen storage properties of light weight nano-composite materials. The Mg-based nano-composite material with Nb2O5 showed excellent kinetics as compared with that of Mg. The Li-Mg-N-H system absorbed and desorbed above 5.5 mass % of H2 at 423K (8LiH ＋ 3Mg(NH2)2 3Li2.667MgN2H1.333＋8H2). We found that the H2 absorption and desorption of the MH-NH3 (M: Li, Na, K) system takes the following reaction path, MH + NH3  MNH2 + H2.

A NOVEL METHOD OF THE HYDRIDE SEPARATION FOR THE DETERMINATION OF ARSENIC AND ANTIMONY BY AAS

A novel method is proposed for the hydride separation when determinining of arsenic and antimony by AAS. A chromatomembrane cell was used as preconcentration-, extraction- and separation-manifold instead of the U-tube phase separator, which is normally fitted in continuous flow vapour systems generating conventionaly the hydrides. The absorbances of the hydrides produced were measured by an atomic absorption spectrophotometer at 193.7 nm and 217.6 nm. Under optimized analytical conditions, the calibration plot for arsenic was linear from 50 to 500 ng.mL-1 (r2 = 0.9982). The precision for three subsequent measurements of 500 ng.mL-1 arsenic gave rise to a relative standard deviation of 0.4%. The detection limit was 15 ng.mL-1, which is much lower compared with that of the conventional hydride system (2000 ng.mL-1). A similar result was observed in case of antimony: the detection limit was 8 ng.mL-1 when the proposed method was applied. Consequently, the sensitivity of the novel method surpasses systems with conventional hydride generation, i.e. the precision and the acuracy increase whereas the standard deviation and the detection limit decrease. The proposed method was applied in pharmacheutial analysis and the certified As-content of a commercial product was very sufficiently confirmed. Keywords: Chromatomembrane Cell, Hydride separation, Arsenic detection, Antimony detection, AAS

Reversible Hydrogen Storage in Destabilized LiAlH4−MgH2−LiBH4 Ternary-Hydride System Doped with TiF3

This paper reports the hydrogen storage properties of a ternary hydride system, LiAlH4−MgH2−LiBH4 (molar ratio 1:1:1), both undoped and doped with TiF3 addition. It was found that there is a mutual destabilization among the three hydrides. This new ternary system possesses superior hydrogen desorption properties compared with the unary components (LiAlH4, MgH2, and LiBH4) or binary mixtures of those components (LiAlH4−MgH2, LiAlH4−LiBH4, and MgH2−LiBH4). On doping with TiF3, the system starts to release hydrogen at 60 °C and completes dehydrogenation below 400 °C. Three major dehydrogenation steps were observed in the undoped and TiF3-doped systems, which corresponds to the decomposition of LiAlH4, MgH2, and LiBH4, respectively. X-ray diffraction (XRD) measurements on the as-dehydrogenated samples were executed to identify the dehydrogenation pathway. The third step decomposition enthalpy of the doped system was determined by pressure−composition−temperature (PCT) measurements and the van’t Hoff equation ...

Anisotropic magnetoresistance and planar hall effect in GaAs structure with Mn-delta-doped layer

Anisotropic magnetoresistance and the planar Hall effect are discovered and studied in a GaAs structure containing a single layer delta-doped with manganese to its total content equivalent to 0.18 monolayer. The Mn-delta-doped structures were obtained by combining the methods of metalorganic chemical vapor deposition (MOCVD) in a hydride system and laser ablation of solid targets in a common technological cycle. The character of the dependence of the Hall resistance on the magnetic field is indicative of the presence of a planar cubic magnetic anisotropy.

Ag enhances optical and switching properties of gadolinium hydride films

An improvement of the optical properties of switchable mirrors is obtained by incorporating of silver (Ag) into the palladium (Pd) cap layer of nanocrystalline gadolinium hydride system Gd/GdH 3. Two methods for modification of Pd layer with Ag are employed. The first method is the forming of an AgPd binary alloy. The second method is the forming of Ag/Pd bilayer. In both cases the modification of the catalytic top layer of Pd with Ag gives higher transparency and better switching times. The optimal Ag layer thickness for the Gd/GdH 3 system is determined.

For high-pressure experiments using total scattering spectrometer NOVA at J-PARC

To reveal hydrogen-metal interaction in metal hydride systems with high hydrogen density, we have constructed the high-pressure device for neutron diffraction at total scattering spectrometer NOVA in J-PARC. The device consists of the Paris-Edinburgh press, the alignment stages, the automatically regulated oil pump system and the temperature control system. By using this device, diffraction data are expected to be taken at pressures above 20 GPa over wide Q-range of 1-40A−1 with the resolution of ΔQ/Q~0.6 %, which enables us to determine hydrogen positions in high hydrogen-density materials in the real-space resolution of 0.16A.

Active cooling of a metal hydride system for hydrogen storage

Reversible high-pressure metal hydrides offer excellent cold start capability and potentially increased hydrogen storage capacity. However, efficient thermal management is a critical issue in this kind of systems. In the present work, hydriding and dehydriding tests have been conducted with 100 g of Ti 1.1MnCr metal hydride at pressures up to 330 bar. Thermal characteristics of the Ti 1.1MnCr system have been quantified from the experimental data and simulated using a basic numerical model. By varying the coolant flow rates, a hydrogen filling time of 12 min has been achieved.

Predicting impurity gases and phases during hydrogen evolution from complex metal hydrides using free energy minimization enabled by first-principles calculations

First-principles calculations represent a potent tool for screening metal hydride mixtures that can reversibly store hydrogen. A number of promising new hydride systems with high hydrogen capacity and favorable thermodynamics have been predicted this way. An important limitation of these studies, however, is the assumption that H(2) is the only gas-phase product of the reaction, which is not always the case. This paper summarizes new theoretical and numerical approaches that can be used to predict thermodynamic equilibria in complex metal hydride systems with competing reaction pathways. We report thermochemical equilibrium calculations using data obtained from density functional theory (DFT) computations to describe the possible occurrence of gas-phase products other than H(2) in three complex hydrides, LiNH(2), LiBH(4), and Mg(BH(4))(2), and mixtures of these with the destabilizing compounds LiH, MgH(2), and C. The systems under investigation contain N, C, and/or B and thus have the potential to evolve N(2), NH(3), hydrocarbons, and/or boranes as well as H(2). Equilibria as a function of both temperature and total pressure are predicted. The results indicate that significant amounts of these species can form under some conditions. In particular, the thermodynamic model predicts formation of N(2) and NH(3) as products of LiNH(2) decomposition. Comparison with published experimental data indicates that N(2) formation must be kinetically limited. Our examination of C-containing systems indicates that methane is the stable gas-phase species at low temperatures, not H(2). On the other hand, very low amounts of boranes (primarily BH(3)) are predicted to form in B-containing systems.

Synthesis and thermolytic behavior of tin(iv) formates: in search of recyclable metal–hydride systems

The synthesis and characterization of the series of organotin formates together with their thermolytic behavior are described. The diformate Bu(n)(2)Sn{OC(O)H}(2) (1) was synthesized by the reaction of Bu(n)(2)SnH(2) with formic acid. The triorganotin monoformate compounds R(3)SnOC(O)H (R = Cy (cyclohexyl)) 3, Mes, (mesityl, 2,4,6-trimethylphenyl) 4, and Dmp (2,6-dimethylphenyl 5) were obtained by the reaction of R(3)SnOH with formic acid. Their X-ray crystal structures along with that of the previously reported formate (PhCH(2))(3)SnOC(O)H (2) were determined. The diformate 1 exhibits an extended two-dimensional polymeric structure in which six-coordinate tin centers are linked by formate bridges. The tribenzyltin formate (2) possesses a chain structure in which the five-coordinate Sn(CH(2)Ph)(3) units are bridged by formate ions. The cyclohexyl derivative 3 was observed to have a similar structure. In contrast, the Mes and Dmp derivatives 4 and 5 support monomeric structures in which the four-coordinate tin atom is bound to an oxygen of the formate ligand. Heating the compounds in various high boiling solvents produced no decomposition up to 120 °C in the case of 1 and refluxing a solution of 2 or 3 in mesitylene or diglyme left the starting material mostly unchanged, although 3 decomposed to an insoluble orange solid in refluxing decalin. In contrast, the heating of 4 and 5 in refluxing mesitylene led to elimination of CO to give the tin hydroxides. The results are in contrast to the known thermolytic behavior of R(3)SnOC(O)H (R = Pr(n) or Bu(n)) complexes, which eliminate CO(2) to generate R(3)SnH. Compounds 3-5 are rare examples of structurally characterized tin formates.

Thermodynamic and kinetic destabilization in LiBH4/Mg2NiH4: promise for borohydride-based hydrogen storage

The reversible LiBH(4)/Mg(2)NiH(4) combined hydride system for hydrogen storage displays a concerted dehydrogenation reaction beginning at 250 degrees C with significantly low enthalpy (DeltaH = 15.4 +/- 2 kJ mol(-1) H(2)) and entropy (DeltaS = 62.2 +/- 3 J K(-1) mol(-1) H(2)).

Valence band of catalyst doped sodium alanate by X-ray photoelectron spectroscopy using synchrotron radiation

In this work, a study is presented that shows the evolution of the valence band analysis of NaAlH 4 doped with 1 mol% of Ce- and 3 mol% of Ti/Zr-hydride using X-ray photoelectron spectroscopy (XPS). The two NaAlH 4 + metal hydride systems contained ball-milled, dehydrogenated, and rehydrogenated samples. The density of states of NaAlH 4, Al, and Na 3AlH 6 were calculated with the generalized gradient approximation (GGA). In both of the two-system samples, during the dehydrogenation and rehydrogenation, most electrons in the p orbital disappeared, and the electrons in the s orbital increased. The catalyst Ce-hydride activated the electrons in the s orbital, whereas Ti/Zr-hydride affected those of the p orbital. Thus, the dehydrogenation and rehydrogenation might activate the more electrons in the Al s and p orbitals.

The effects of ball milling and molar ratio of LiH on the hydrogen storage properties of nanocrystalline lithium amide and lithium hydride (LiNH2+LiH) system

Abstract High-energy ball milling was applied to the mixtures of LiNH 2 and LiH having the molar ratio 1:1, 1:1.2 and 1:1.4LiH. During a high-energy ball milling of the 1:1 molar ratio mixture the grain (crystallite) size of LiNH 2 and LiH constituent decreases monotonically with increasing milling time while the specific surface area (SSA) of powder increases up to 25 h of milling duration and then decreases after milling for 100 h due to the excessive agglomeration of powder particles into lumps. A single-phase LiNH 2 decomposes through melting and the release of ammonia (NH 3 ). A just mixed LiNH 2 + LiH mixture still mostly decomposes through the melting of LiNH 2 and release of NH 3 . For the hydrogen to be effectively released from the mixture of (LiNH 2 + LiH) a high-energy ball milling is necessary which makes an intimate contact between both constituents. The activation energy for hydrogen desorption from the ball milled mixture of (LiNH 2 + LiH) decreases with increasing SSA of powders up to ∼26 m 2 /g and then levels off with further increase of SSA. For the ball milled mixture of LiNH 2 :LiH the lowest activation energy is observed for the molar ratio of 1:1.2LiH. The hydrolysis/oxidation of the fraction of LiH into LiOH in the mixture makes a fraction of LiH inactive in the intermediate reaction NH 3 + LiH → LiNH 2 + H 2 and creates the major obstacle to the hydrogen desorption from the ball milled mixture of LiNH 2 + LiH. At the molar ratio 1:1.2 of LiNH 2 :LiH the mass of the active LiH is the largest one which leads to the largest quantity of desorbed hydrogen (∼5 wt.%). The amount of hydrogen desorbed from LiNH 2 + LiH slightly decreases with increasing milling time from 5 to 100 h due to the reduction in grain (crystallite) size of LiH which renders it more sensitive to hydrolysis and the formation of LiOH.

Hydrogenography of Mg yNi 1− yH x gradient thin films: Interplay between the thermodynamics and kinetics of hydrogenation

We monitor the hydrogenation of PTFE/Pd-capped Mg y Ni 1− y H x gradient thin films using the change in optical transmission as a function of time, temperature and hydrogen pressure, to study the relation between kinetics and thermodynamics of hydrogenation of this multiphase hydride system. The interplay between kinetics and thermodynamics is used to extrapolate the hydrogenation equilibrium pressures via the H-absorption rate. Pressure–optical-transmission–isotherms determined independently at different temperatures provide a cross-check for the equilibrium pressure and the enthalpy of H-absorption. We find that the hydrogenation reaction is destabilized with respect to the Mg 2Ni → Mg 2NiH 4 reaction for Mg fractions close to the Mg 2Ni–Mg eutectic point. From a comparison with calculated enthalpies obtained from density functional theory, we conclude that the experimentally observed destabilization originates from the well-mixed microstructure around the eutectic point.