Hydride System Research Articles

Complex transition metal hydrides incorporating ionic hydrogen: Synthesis and characterization of Na2Mg2FeH8 and Na2Mg2RuH8

A new class of quaternary complex transition metal hydrides (Na2Mg2TH8 (T=Fe, Ru)) have been synthesized and their structures determined by combined synchrotron radiation X-ray and powder neutron diffraction. The compounds can be considered as a link between ionic and complex hydrides in terms of incorporating independently coordinated ionic and covalent hydrogen. These novel isostructural complex transition metal hydrides crystallize in the orthorhombic space group Pbam, where the octahedral complex hydride anion is surrounded by a cubic array of four Mg2+ and four Na+ cations, forming distinct two-dimensional layers. An intriguing feature of these materials is the distorted octahedral coordination of the isolated H− anions by four Na+ and two Mg2+ cations, which form layers between the transition metal containing layers. The vibrational modes of the H− anions and complex hydride anion are independently observed for the first time in a quaternary complex transition metal hydride system by Raman and IR spectroscopy.

Numerical study of hydrogen purification using metal hydride reactor with aluminium foam

The present paper describes a numerical study of heat and mass transfer processes in a metal hydride reactor for hydrogen purification with aluminium foam. The mathematical model, which was used previously for numerical simulation of heat and mass transfer processes in different types of metal hydride reactors, was modified for a reactor with aluminium foam. To validate the thermal equilibrium between the aluminium foam and metal hydride, pore-scale modelling was performed. In the case of pure hydrogen sorption, the model was validated by a comparison with experimental data; good agreement for sorption dynamics was obtained. The effects of various parameters of the pressure swing absorption method on the effectiveness of metal hydride systems for hydrogen purification were studied. The obtained results show that the use of aluminium foam enhances intensity of heat and mass transfer and consequently decreases the time required for hydrogen purification (up to two times) compared to a case without aluminium foam, while the volumetric capacity of the system is reduced slightly (9%). It is shown that pressure swing absorption method with fixed time interval between purges and the method with fixed interval of the fraction changing of the transformed material between purges provide a similar performance; in the second case the hydrogen recovery ratio remains constant during the hydrogen purification process.

Unexpected Dehydrogenation Behaviors of the 2LiBH4–MgH2 Composite Confined in a Mesoporous Carbon Scaffold

Nanoconfinement has been widely employed as a promising strategy to improve dehydrogenation kinetics, reversibility, and equilibrium pressure of complex metal hydrides. In this paper, we report a careful study of the influence of nanoconfinement on the reversible dehydrogenation property and reaction mechanism pathway of the 2LiBH4–MgH2 composite system. Compared to the bulk 2LiBH4–MgH2 composite, the 2LiBH4–MgH2 confined in the mesoporous carbon (CMK-3) scaffold host exhibits the significantly enhanced dehydrogenation kinetics but meanwhile shows the serious loss of hydrogen capacity upon cycling, particularly in the first two cycles. Moreover, the observed dehydrogenation property is independent of the hydrogen back pressure. The combination analyses of XRD, FTIR, and NMR definitely detected the dominant Mg and B phases in the dehydrogenation products, suggesting the mainly individual desorption of MgH2 and LiBH4 in the confined 2LiBH4–MgH2 system. This unfavorable change of the dehydrogenation reaction pathway would result in the poor reversibility, which is not expected for the combined hydride systems. These findings might provide renewed insight into the nanoconfinement effect on the hydrogen storage property for multiple phase combined systems.

A case study of a hydride container performance applying non dimensional parameters

Many efforts have been done so far to understand sorption dynamics of hydride containers for hydrogen storage. Particularly, there are many articles in literature where experimental results for different hydride systems and container set-ups were successfully simulated using basically the same group of models. This fact is the base of a previous work where we defined a series of non dimensional parameters which may be used to estimate absorption time of hydride containers.In this work we compare estimated absorption times with experimental outcomes for a prototype hydride container. We performed non dimensional analysis of our finned container prototype at two scales, i.e.: overall or macroscopic container scale and pore or microscopic scale. We discuss about this simplified model approach that allows estimating, with few parameters, the reaction time of a complex-geometry prototype.The prototype container was designed according to the results of a numerical optimization that maximized the amount of hydrogen absorbed for a 3 min charging period. Experimental results indicate good agreement between estimated and experimental absorption time, making the non dimensional method a useful tool at preliminary stages of hydride container design.

Effect of gas back pressure on hydrogen storage properties and crystal structures of Li2Mg(NH)2

The ternary imide Li2Mg(NH)2 is considered to be one of the most promising on-board hydrogen storage materials due to its high reversible hydrogen capacity of 5.86 wt%, favorable thermodynamic properties and good cycling stability. In this work, Li2Mg(NH)2 was synthesized by dynamically dehydrogenating a mixture of Mg(NH2)2–2LiH up to 280 °C under different gas (Ar and H2) and pressures (0–9.0 bar). The crystal structure of Li2Mg(NH)2 was found to depend on the gas back pressure in the dehydrogenation process. The crystal structure of Li2Mg(NH)2 and the dehydrogenation/rehydrogenation properties of the Mg(NH2)2–2LiH system strongly depend on the gas back pressure in the dehydrogenation process due to the effect of the pressure on the dehydrogenation kinetics. This study provides a new approach for improving the hydrogen storage properties of the amide–hydride systems.

Stereoselective Hydrodehalogenation via a Radical‐Based Mechanism Involving T‐Shaped Chiral Nickel(I) Pincer Complexes

We herein report the catalytic enantioselective hydrodehalogenation based on the interplay of a chiral molecular nickel(I)/nickel(II)hydride system. Prochiral geminal dihalogenides are dehalogenated via a secondary configurationally unstable, potentially metal-stabilized radical intermediate. In a subsequent step, the liberated radical is then trapped by the nickel(II) hydrido complex, present in a large excess under the catalytic conditions, which in turn induces the enantioselectivity during the hydrogen atom transfer onto the radical intermediate. These new chiral nickel(I) complexes were found to catalyze the asymmetric hydrodehalogenation of geminal dihalogenides with moderate to good enantiomeric excess values using LiEt3 BH as reductant. The main side product generally observed is the dehalogenated alkene, whereas the hydrodehalogenation of the chiral monohalogen compound occurred much more slowly despite the large excess of reductant.

Powered by DFT: Screening methods that accelerate materials development for hydrogen in metals applications.

CONSPECTUS: Not only is hydrogen critical for current chemical and refining processes, it is also projected to be an important energy carrier for future green energy systems such as fuel cell vehicles. Scientists have examined light metal hydrides for this purpose, which need to have both good thermodynamic properties and fast charging/discharging kinetics. The properties of hydrogen in metals are also important in the development of membranes for hydrogen purification. In this Account, we highlight our recent work aimed at the large scale screening of metal-based systems with either favorable hydrogen capacities and thermodynamics for hydrogen storage in metal hydrides for use in onboard fuel cell vehicles or promising hydrogen permeabilities relative to pure Pd for hydrogen separation from high temperature mixed gas streams using dense metal membranes. Previously, chemists have found that the metal hydrides need to hit a stability sweet spot: if the compound is too stable, it will not release enough hydrogen under low temperatures; if the compound is too unstable, the reaction may not be reversible under practical conditions. Fortunately, we can use DFT-based methods to assess this stability via prediction of thermodynamic properties, equilibrium reaction pathways, and phase diagrams for candidate metal hydride systems with reasonable accuracy using only proposed crystal structures and compositions as inputs. We have efficiently screened millions of mixtures of pure metals, metal hydrides, and alloys to identify promising reaction schemes via the grand canonical linear programming method. Pure Pd and Pd-based membranes have ideal hydrogen selectivities over other gases but suffer shortcomings such as sensitivity to sulfur poisoning and hydrogen embrittlement. Using a combination of detailed DFT, Monte Carlo techniques, and simplified models, we are able to accurately predict hydrogen permeabilities of metal membranes and screen large libraries of candidate alloys, selections of which are described in this Account. To further increase the number of membrane materials that can be studied with DFT, computational costs need to be reduced either through methods development to break bottlenecks in the performance prediction algorithm, particularly related to transition state identification, or through screening techniques that take advantage of correlations to bypass constraints.

ChemInform Abstract: Study of New Reactions Catalyzed by Copper(I) Complexes: Investigation of Domino Catalysis‐ and Dual Catalysis‐Type Reactions

In this thesis we are going to first discuss the application of a copper hydride system, which was previously developed in our lab, to new Michael acceptors bearing a nitrogen group on the double bond. We are equally going to illustrate the development of a new cooperative dual catalysis strategy based on a CuI/Pd0 system. This strategy allowed us to successfully access α-allylated ketones diastereoselectively as well as enantioselectively. In a later point, we are also going to examine the development of two synthetic pathways to generate unprecedented (NHC)CuI bifluoride complexes. We tested the activity of these complexes in several known CuI-catalyzed reactions, and consequently established the first CuI-catalyzed diastereoselective allylation of (R)-N-tert-butanesulfinyl aldimines. The method enables efficient, simple and general synthesis of enantiomerically enriched homoallylic amines at room temperature in high yields.

Mechanisms of Zr surface corrosion determined via molecular dynamics simulations with charge-optimized many-body (COMB) potentials

A charge-optimized many-body (COMB) potential is proposed for the zirconium–zirconium oxide–zirconium hydride system. This potential is developed to describe the energetics of the interactions of oxygen and hydrogen with zirconium metal. We perform classical molecular dynamics simulations showing the initial corrosion behavior of three low-index zirconium surfaces via the deposition of O2 and H2O molecules. The basal (0001) surface shows greater resistance to oxygen diffusion than the prism (101¯0) and (112¯0) surfaces. We suggest ways in which the surface structure has a unique role in the experimentally observed enhanced corrosion of the prism surfaces.

Seaweeds as bioindicators of heavy metals off a hot spot area on the Egyptian Mediterranean Coast during 2008-2010.

Concentrations of Cu, Zn, Cd, Pb, Ni, Co, Fe, Mn, and Hg were measured successively in water, sediments, and six macroalgal species belonging to three algal classes during 3 years (2008-2010) from Abu Qir Bay, Alexandria, Egypt: Chlorophyceae (Enteromorpha compressa, Ulva fasciata), Phaeophyceae (Padina boryana), and Rhodophyceae (Jania rubens, Hypnea musciformis, Pterocladia capillacea). The study aimed to assess the bioaccumulation potential of the seaweeds, as well as to evaluate the extent of heavy metal contamination in the selected study site. Metals were analyzed using atomic absorption spectrophotometry coupled with MH-10 hydride system. The obtained data showed that the highest mean concentrations of Cu, Zn, Fe, and Mn were recorded in E. compressa; Cd, Ni, and Hg exhibited their highest mean concentrations in P. boryana, while Pb and Co were found in J. rubens. Abundance of the heavy metals in the algal species was as follow: Fe > Mn > Zn > Pb > Ni > Co > Cu > Cd > Hg. E. compressa showed the maximum metal pollution index (MPI) which was 11.55. Bioconcentration factor (BCF) for the metals in algae was relatively high with a maximum value for Mn. The Tomlinson pollution load index (PLI) values for the recorded algal species were low, which ranged between 1.00 in P. boryana and 2.72 in E. compressa. Enrichment factors for sediments were low fluctuating between 0.43 for Hg to 2.33 for Mn. Accordingly, the green alga E. compressa, brown alga P. boryana, and red alga J. rubens can be nominated as bioindicators. Based on MPI and PLI indices, Abu Qir Bay in the present study is considered as low-contaminated area.

Study of the thermodynamic properties of a hydrogen-absorbing alloy LaFe0.1Mn0.3Ni4.8 for systems for hydrogen storage and purification

The article is devoted to methodological problems of investigation of the pressure-concentration-temperature (PCT) phase diagrams of hydrogen-absorbing materials that are used in metal hydride systems for storage and purification of hydrogen based on intermetallic compounds of the AB5-type. The problem is discussed with an example of the PCT-diagram of the LaFe0.1Mn0.3Ni4.8 alloy, which is promising for creation of metal hydride systems for storage and purification of hydrogen integrated with power plants (with a power of up to 5 kW) based on fuel cells with a solid-polymer electrolyte.

ChemInform Abstract: Nanoconfined Light Metal Hydrides for Reversible Hydrogen Storage

AbstractReview: 58 refs.

Development and evaluation of portable and wearable fuel cells for soldier use

A number of fuel cell systems have been recently developed to meet the U.S. Army's soldier power requirements. The operation and performance of these systems are discussed based on laboratory results and limited soldier evaluation. The systems reviewed are primarily intended for soldier use in an austere environment with minimum access to resupply and vehicular transportation. These applications require high power and energy density sources that are portable (300 W) and wearable (20 W) to minimize the soldier's load burden. Based on soldier field evaluations of portable fuel cell systems, improvements in power density and compatibility with logistical fuels are required to be successfully deployed. For soldier worn applications, a novel chemical hydride system has shown significant advances in power and energy density while maintaining a small form factor. The use of a high energy dense fuel cartridge (800 Wh kg−1) based on AlH3 (Alane) thermolysis, allows a power density of (28 W kg−1) which offers promising weight savings compared to the standard military batteries.

Improved de/hydrogenation properties and favorable reaction mechanism of CeH2 + KH co-doped sodium aluminum hydride

Sodium aluminum hydride (NaAlH4) was directly synthesized by ball milling NaH/Al co-doped with CeCl3 + KH under a hydrogen pressure of 3 MPa at room temperature. Out of various samples corresponding to xNaH/Al + 0.02CeCl3 + yKH (x + y = 1; y = 0, 0.02, 0.04 mol%) composites, the composite with y = 0.02 exhibits the optimum de/hydrogenation properties. It shows that the addition of KH can effectively improve the dehydrogenation properties of second step reaction of NaAIH4 system. The composite with y = 0.02 starts to release hydrogen from 87 °C and completes dehydrogenation within 20 min at 170 °C, with good cycling de/hydrogenation kinetics at relatively lower temperature (100–140 °C). After ball milling, the CeCl3 precursor can be changed into CeH2 catalytic active component in the first several de/hydrogenation cycles. Apparent activation energy of the second decomposition step of NaAIH4 system can be effectively decreased by addition of KH, resulting in the decrease of desorption temperatures. Based on the microstructure analyses combined with hydrogen storage performances, the improved dehydrogenation properties of sodium aluminum hydride system are ascribed to the lattice volume expansion of Na3AlH6 during the dehydrogenation process resulted from the addition of KH. Moreover, by analyzing the reaction kinetics of CeCl3 + KH co-doped sample, both of the decomposition steps of composite with y = 0.02 were conformed to the two-dimension phase-boundary growth mechanism. The mechanistic investigations gained here could help to understand the de/rehydrogenation behaviors of catalyzed complex metal hydride systems.

Large Entropic Effects on the Thermochemistry of Silicon Nanodusty Plasma Constituents

Determination of the thermodynamic properties of reactor constituents is the first step in designing control strategies for plasma-mediated deposition processes and is also a key fundamental issue in physical chemistry. In this work, a recently proposed multistructural statistical thermodynamic method is used to show the importance of multiple structures and torsional anharmonicity in determining the thermodynamic properties of silicon hydride clusters, which are important both in plasmas and in thermally driven systems. It includes five different categories of silicon hydride clusters and radicals, including silanes, silyl radicals, and silenes. We employed a statistical mechanical approach, namely the recently developed multistructural (MS) anharmonicity method, in combination with density functional theory to calculate the partition functions, which in turn are used to estimate thermodynamic quantities, namely Gibbs free energy, enthalpy, entropy, and heat capacity, for all of the systems considered. The calculations are performed using all of the conformational structures of each molecule or radical by employing the multistructural quasiharmonic approximation (MS-QH) and also by including torsional potential anharmonicity (MS-T). For those cases where group additivity (GA) results are available, the thermodynamic quantities obtained from our MS-T calculations differ considerably due to the fact that the GA method is based on single-structure data for isomers of each stoichiometry, and hence lack multistructural effects; whereas we find that multistructural effects are very important in silicon hydride systems. Our results also indicate that the entropic effect on the thermochemistry is huge and is dominated by multistructural effects. The entropic effect of multiple structures is also expected to be important for other kinds of chain molecules, and its effect on nucleation kinetics is expected to be large.

Formation kinetics and microstructure of Mg–Ti hydrides made by reactive ball milling

In this work we report the formation kinetics and microstructural properties of Mg1−xTix powder metal hydride systems (x = 0.1, 0.2, 0.3, 0.4, 0.5), obtained by reactive ball milling (RBM) via three different ways. The resulting MgH2–TiH2 nanocomposites are characterized and an optimal composition is found. Differences among the preparation routes are analyzed.

Kinetic behaviour of a metal-polymer composite suitable for hydrogen storage applications

Hydrogen storage in composite materials is a feasible way to overcome the main drawbacks of the metal hydride systems. In the present paper, we report on the hydrogenation properties of three polydimethylsiloxane (PDMS)–palladium composite samples with different content of metallic fraction (5, 15, 50 in wt.%). Hydrogenation tests in different conditions of temperature and pressure were performed using a Sievert's type apparatus, while the microstructure of the samples was characterised by means of scanning electron microscopy observations and X-ray diffraction measurements. Results show that the hydrogen storage capacity is inversely proportional to the metallic content in the composite samples.

Enhanced hydrogen desorption of an ammonia borane and lithium hydride system through synthesised intermediate compounds

Low energy mixing and higher LiH content effectively destabilize AB for enhanced hydrogen release at a desirable temperature.

Fundamental environmental reactivity testing and analysis of the hydrogen storage material 2LiBH4·MgH2

While the storage of hydrogen for portable and stationary applications is regarded as critical in bringing PEM fuel cells to commercial acceptance, little is known of the environmental exposure risks posed in utilizing condensed phase chemical storage options as in complex hydrides. It is thus important to understand the effect of environmental exposure of metal hydrides in the case of accident scenarios. Simulated tests were performed following the United Nations standards to test for flammability and water reactivity in air for a destabilized lithium borohydride and magnesium hydride system in a 2 to 1 molar ratio respectively. It was determined that the mixture acted similarly to the parent, lithium borohydride, but at slower rate of reaction seen in magnesium hydride. To quantify environmental exposure kinetics, isothermal calorimetry was utilized to measure the enthalpy of reaction as a function of exposure time to dry and humid air, and liquid water. The reaction with liquid water was found to increase the heat flow significantly during exposure compared to exposure in dry or humid air environments. Calorimetric results showed the maximum normalized heat flow of the fully charged material was 6 mW/mg under liquid phase hydrolysis; and 14 mW/mg for the fully discharged material also occurring under liquid phase hydrolysis conditions.

Human brain mercury levels related to exposure to amalgam fillings

The safety of dental amalgam as the primary material in dental restoration treatments has been debated since its introduction. It is widely accepted that amalgam restorations continuously release elemental mercury (Hg) vapor, which is inhaled and absorbed by the body and distributed to tissues, including the brain. The aim of the present study was to investigate whether the presence of amalgam fillings is correlated with brain Hg level. The Hg levels in the parietal lobes of the brains of 32 cadavers were analyzed with an atomic absorption spectrometer with the mercury hydride system. A total of 32 brain samples were tested; of these, 10 were from cadavers with amalgam fillings, while 22 of them were amalgam free. Hg was detected in 60.0% (6 of 10) of the samples in the amalgam group and in 36.3% (8 of 22) in the amalgam-free group. The average Hg level of the amalgam group was 0.97 ± 0.83 µg/g (minimum: 0.3 µg/g and maximum: 2.34 µg/g), and in the amalgam-free group, it was 1.06 ± 0.57 µg/g (minimum: 0.17 µg/g and maximum: 1.76 µg/g). The results of the present study showed no correlation between the presence of amalgam fillings and brain Hg level.