Light Metal Hydrides Research Articles

A DFT study on the hydrogen desorption from the lithium borohydride and aluminohydride upon the addition of nanostructured carbon catalyzing agent

Here we study in van der Waals-corrected DFT calculations the dehydrogenation mechanism of the light metal hydrides LiBH4 and LiAlH4 by focusing on the effect of the addition of the carbon fullerene C60 as catalyzing agent. The results show a rather significant gain in the energy cost for H desorption in the presence of the catalyst, which is substantially even more pronounced when considering boron-doping the fullerene. In the source of this effect is the disturb introduced in the distribution of bonding charge upon the hybridization of states in the interplay cluster-fullerene with a consequent weakening of the hydrogen bonds, leading therein to an enhanced kinetics for the hydrogen release.

Aluminium complexes of B- and N-based hydrides: Synthesis, structures and hydrogen storage properties

The storage of hydrogen in a solid state is one of the main challenges for stationary and mobile applications. Light metal hydrides have attracted significant attention as potential candidates for energy storage. Remarkably, Al-containing hydrides, namely AlH3 and M(AlH4)n, are among the most fascinating classes of materials, able to cycle up to 5.5 wt% of hydrogen at moderate temperatures. This review covers the recent research on the families of Al-based complex hydrides involving other light elements such as B and N. They were classified according to the charge of the Al-based complexes, as anionic, molecular, cationic or “autoionized” where Al is centering both the cation and the anion. The factors influencing the stability and the hydrogen purity of the series of anionic aluminium amides M[Al(NH2)4]n, borohydrides M[Al(BH4)4] and amidoboranes M[Al(NH2BH3)4], as well as molecular [Al(L)(BH4)3] (L = molecular ligands) and cation [Al(NH3)6]3+-based complexes are discussed. In particular, the ability of the strong Lewis acid Al3+ to coordinate both the initial hydrogenated species as well as their dehydrogenation products makes it a good template for chemical transformations involving light chemical and complex hydrides.

First principles study of hydrogen storage material NaBH4 and LiAlH4 compounds: electronic structure and optical properties

A comprehensive study of structure, phase stability, optical and electronic properties of LiAlH4 and NaBH4 light-metal hydrides is presented. The calculations are carried out within density functional theory using the full potential linear augmented plane wave method. The exchange-correlation potential is treated within the local density approximation and the generalized gradient approximation (GGA) to calculate the total energy. Furthermore, the Engel–Vosko GGA approach is employed to compute electronic and optical properties such as reflectivity spectra. The phases α, β and γ of LiAlH4 and NaBH4 hydrides are investigated, the phase transition from the β to the high-pressure γ phase is determined for NaBH4 and is accompanied by a 1% volume decrease. For LiAlH4, no phase transition is detected. The materials under consideration are classified as wide band gap compounds. From the analysis of the structures at different phases, it is deduced that the hydrides show strong covalent interaction between B (Al) and H in the [BH4]− ([AlH4]−) anions and ionic bonding character between [BH4]− and Na+ for NaBH4, and [AlH4]− and Li+ for LiAlH4. The complex dielectric function, absorption coefficient and the reflectivity spectra are also computed and analyzed in different phases.

Hydrogen Storage in Rippled Graphene: Perspectives from Multi-Scale Simulations

Exploring new perspectives for green technologies is one of the challenges of the third millennium, in which the need for non-polluting and renewable powering has become primary. In this context, the use of hydrogen as a fuel is promising, since the energy released in its oxidation per unit mass (~142 MJ/kg) is three times that released, on average, by hydrocarbons, and the combustion product is water (Ramage, 1983). Being hydrogen a vector of chemical energy, efficient conservation, and non-dispersive transportation are the main goals. Three issues must be considered to this respect: (i) storage capacity, (ii) storage stability, and (iii) kinetics of loading/release. Commercial technologies are currently based on cryo-compression or liquefaction of H2 in tanks. These ensure quite a high gravimetric density [GD, point (i)], namely 8–13% in weight of stored hydrogen, and a relatively low cost (Zuttel, 2003). However, concerning points (ii) and (iii), these technologies pose problems of safety, mainly due to explosive flammability of hydrogen, and consequent unpractical conditions for transportation and use (Mori and Hirose, 2009). Therefore, research efforts are directed toward solidstate based storage systems (energy.gov, Bonaccorso et al., 2015). Interactions of hydrogen with materials are classified as physisorption, occurring with H2 by means of van der Waals (vdW) forces, or chemisorption, i.e., chemical binding of H leading to the formation of hydrides (Mori and Hirose, 2009), requiring dissociative(associative) chemi(de)sorption of H2. Intermediate nature interactions, sometimes called “phenisorption,” can also occur between hydrogen electrons and the electrons of external orbital of metals. Indeed, stable and robust (light) metal hydrides (Sakintuna et al., 2007; Harder et al., 2011) are currently considered an alternative to tanks. Their main drawback is their high chemisorption and chemidesorption barrier, both many times the typical thermal energy, implying slow operational kinetics, which becomes acceptable only at very high temperature. Physisorption, conversely, generally results in barrierless and weak binding. It was considered as a storage mechanism in layered (Zhirko et al., 2007) or porous (Sastre, 2010) materials, and shown to be effective at low temperatures and/or high pressure. Therefore, it generally seems that if storage stability (ii) is improved then the loading/release kinetics (iii) is worsened. Graphene shows good potential to be an efficient hydrogen-storage medium (Tozzini and Pellegrini, 2013): carbon is among the lightest elements forming layered and porous structures, and graphene is probably the material with the largest surface to mass ratio. These two conditions are in principle optimal to produce high GD [point (i)]. In addition, the chemical versatility of carbon allows it to interact with hydrogen both by physisorption (in sp2 hybridization) and chemisorption (Goler et al., 2013a) (in sp3 hybridization). [“Phenisorption is also obtained in graphene by functionalization with metals (Mashoff et al., 2013)]. On the other hand, concerning points (ii) and (iii), pure graphene does not perform dramatically better than other materials. H2 easily physisorbs onto graphene layers or within multilayers, but it was theoretically shown (Patchkovskiim et al., 2005) that large GD (6–8%) are reached within multi-layered graphene at cryogenic temperatures, while the room temperature value is at best ~2–3%. This was confirmed by measurements (Klechikov et al., private communication), which also indicate that graphene does not perform better than other carbon based bulk materials, such as nanoporous carbon or carbon nanotubes. In all cases, a key parameter determining GD is the specific surface to volume ratio. Theoretical works also show that stability can be improved (and GD optimized) at specific interlayer spacing (~7–8 A), due to a cooperative effect of vdW forces (Patchkovskiim et al., 2005). A similar effect is responsible for the accumulation of physisorbed hydrogen within graphene troughs at low temperatures (~100 K) observed in simulations (Tozzini and Pellegrini, 2011). On the other hand, hydrogen chemisorption on graphene produces graphane (Sofo et al., 2007), its completely hydrated alkane counterpart, stable at room temperature [point (ii)] and with 8.2% GD [point (i)]. Graphane, however, shares with other hydrides high chemi(de)sorption barrier (~1.5 eV/atom). As in other materials, physisorption has good kinetics (iii), and

Interaction of electrons with light metal hydrides in the transmission electron microscope.

Transmission electron microscope (TEM) observation of light metal hydrides is complicated by the instability of these materials under electron irradiation. In this study, the electron kinetic energy dependences of the interactions of incident electrons with lithium, sodium and magnesium hydrides, as well as the constituting element effect on the interactions, were theoretically discussed, and electron irradiation damage to these hydrides was examined using in situ TEM. The results indicate that high incident electron kinetic energy helps alleviate the irradiation damage resulting from inelastic or elastic scattering of the incident electrons in the TEM. Therefore, observations and characterizations of these materials would benefit from increased, instead decreased, TEM operating voltage.

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: Nanoconfined Light Metal Hydrides for Reversible Hydrogen Storage

AbstractReview: 58 refs.

H2 Desorption from MgH2 Surfaces with Steps and Catalyst-Dopants

Light-metal hydrides, like MgH2, remain under scrutiny as prototypes for reversible H-storage materials. For MgH2, we assess hydrogen desorption/adsorption properties (enthalpy and kinetic barriers) for stepped, catalyst-doped surfaces occurring, e.g., from ball-milling in real samples. Employing density functional theory and simulated annealing in a slab model, we studied initial H2 desorption from stepped surfaces with(out) titanium (Ti) catalytic dopant. Extensive simulated annealing studies were performed to find the dopant’s site preferences. For the most stable initial and final (possibly magnetic) states, nudged elastic band (NEB) calculations were performed to determine the H2-desorption activation energy. We used a moment-transition NEB method to account for the dopant’s transition to the lowest-energy magnetic state at each image along the band. We identify a dopant-related surface-desorption mechanism that reloads via bulk H diffusion. While reproducing the observed bulk enthalpy of desorption, we find a decrease of 0.24 eV (a 14% reduction) in the activation energy on doped stepped surface; together with a 22% reduction on a doped flat surface, this brackets the assessed 18% reduction in kinetic barrier for ball-milled MgH2 samples with low concentration of Ti from experiment.

Desorption of hydrogen from light metal hydrides: concerted electronic rearrangement and role of H⋯H interactions

A theoretical study of the desorption of hydrogen from rhombic Group 1 metal hydride dimers reveals a concerted reorganisation of the electron density for the M-H and H-H moieties as the reaction coordinate is traversed and a closed-shell H···H interaction evolves into a covalent H2 bond. The central role played by homopolar dihydrogen bonding in this process is revealed and analysed.

Well‐Defined, Nanometer‐Sized LiH Cluster Compounds Stabilized by Pyrazolate Ligands

The assembly of well-defined large cluster compounds of ionic light metal hydrides is a synthetic challenge and of importance for synthesis, catalysis, and hydrogen storage. The synthesis and characterization of a series of neutral and anionic pyrazolate-stabilized lithium hydride clusters with inorganic cores in the nanometer region is now reported. These complexes were prepared in a bottom-up approach using alkyl lithium and lithium pyrazolate mixtures with silanes in hydrocarbon solutions. Structural characterization using synchrotron radiation revealed isolated cubic clusters that contain up to 37 Li(+) cations and 26 H(-) ions. Substituted pyrazolate ligands were found to occupy all corners and some edges for the anionic positions.

First principle investigations of the physical properties of hydrogen-rich MgH2

Hydrogen being a cleaner energy carrier has increased the importance of hydrogen-containing light metal hydrides, in particular those with large gravimetric hydrogen density like magnesium hydride (MgH2). In this study, density functional and density functional perturbation theories are combined to investigate the structural, elastic, thermodynamic, electronic and optical properties of MgH2. Our structural parameters calculated with those proposed by Perdew, Burke and Ernzerof generalized gradient approximation (PBE-GGA) and Wu–Cohen GGA (WC-GGA) are in agreement with experimental measurements, however the underestimated band gap values calculated using PBE-GGA and WC-GGA were greatly improved with the GGA suggested by Engle and Vosko and the modified Becke–Johnson exchange correlation potential by Trans and Blaha. As for the thermodynamic properties the specific heat values at low temperatures were found to obey the T3 rule and at higher temperatures Dulong and Petit's law. Our analysis of the optical properties of MgH2 also points to its potential application in optoelectronics.

Unusual oxidation behavior of light metal hydride by tetrahydrofuran solvent molecules confined in ordered mesoporous carbon

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Nanoconfined light metal hydrides for reversible hydrogen storage

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Reducing the dehydrogenation temperature of lithium hydride through alloying with germanium

LiH is a highly stable light metal hydride with a hydrogen capacity of 12.5 wt%. However, having a dehydrogenation enthalpy, ΔH(dehy), of 181.2 kJ mol(-1)(H2) and a resultant T(1 bar) of 944 °C, it is not a practical hydride for most hydrogen storage applications. In the work presented here, germanium has been found to dramatically reduce the dehydrogenation temperature for LiH down to just 270 °C. The enthalpy of dehydrogenation was reduced through the formation of lithium germanides. The reaction pathway was identified in this study using in situ powder neutron diffraction, showing the successive formation of more Li-rich germanides, following the series: LiGe, Li4Ge2H, Li9Ge4, and Li7Ge2. The enthalpy of formation for these germanides provides the thermodynamic tuning to reduce the ΔH(dehy) for the system. The 3LiH-Ge system investigated is found to be reversible with a maximum capacity of 3.0 ± 0.1 wt%.

Surface and Particle-Size Effects on Hydrogen Desorption from Catalyst-Doped MgH2

With their high capacity, light-metal hydrides like MgH2 remain under scrutiny as reversible H-storage materials, especially to develop control of H-desorption properties by decreasing size (ball-milling) and/or adding catalysts. By employing density functional theory and simulated annealing, we study initial H2 desorption from semi-infinite stepped rutile (110) surface and Mg31H62 nanoclusters, with(out) transition-metal catalyst dopants (Ti or Fe). While Mg31H62 structures are disordered (amorphous), the semi-infinite surfaces and nanoclusters have similar single, double, and triple H-to-metal bond configurations that yield similar H-desorption energies. Hence, there is no size effect on desorption energetics with reduction in sample size, but dopants do reduce the H-desorption energy. All desorption energies are endothermic, in contrast to a recent report.

Ab Initio Study of Magnesium and Magnesium Hydride Nanoclusters and Nanocrystals: Examining Optimal Structures and Compositions for Efficient Hydrogen Storage

On the basis of the attractive possibility of efficient hydrogen storage in light metal hydrides, we have examined a large variety of Mg(n)H(m) nanoclusters and (MgH(2))(n) nanocrystals (n = 2-216, m = 2-436) using high level coupled cluster, CCSD(T), ab initio methods, and judicially chosen density functional calculations of comparable quality and (near chemical) accuracy. Our calculated desorption energies as a function of size and percentage of hydrogen have pinpointed optimal regions of sizes and concentrations of hydrogen which are in full agreement with recent experimental findings. Furthermore, our results reproduce the experimental desorption energy of 75.5 kJ/mol for the infinite system with remarkable accuracy (76.5 ± 1.5 kJ/mol).

Excellent Catalytic Effects of Graphene Nanofibers on Hydrogen Release of Sodium alanate

One of the most technically challenging barriers to the widespread commercialization of hydrogen-fueled devices and vehicles remains hydrogen storage. More environmentally friendly and effective nonmetal catalysts are required to improve hydrogen sorption. In this paper, through a combination of experiment and theory, we evaluate and explore the catalytic effects of layered graphene nanofibers toward hydrogen release of light metal hydrides such as sodium alanate. Graphene nanofibers, especially the helical kind, are found to considerably improve hydrogen release from NaAlH4, which is of significance for the further enhancement of this practical material for environmentally friendly and effective hydrogen storage applications. Using density functional theory, we find that carbon sheet edges, regardless of whether they are of zigzag or armchair type, can weaken Al–H bonds in sodium alanate, which is believed to be due to a combination of NaAlH4 destabilization and dissociation product stabilization. The helical form of graphene nanofibers, with larger surface area and curved configuration, appears to benefit the functionalization of carbon sheet edges. We believe that our combined experimental and theoretical study will stimulate more explorations of other microporous or mesoporous nanomaterials with an abundance of exposed carbon edges in the application of practical complex light metal hydride systems.

Effect of TiH2–Induced Strain on Thermodynamics of Hydrogen Release from MgH2

MgH2 is a prototypical light metal hydride, but it is too stable for use in practical hydrogen storage applications. Experiments with ball-milled MgH2–0.1TiH2 have shown that the enthalpy of reaction of MgH2 in this mixture is lower than for bulk MgH2 (Lu, J. et al. J. Am. Chem. Soc.2009,131, 15843). We have explored the mechanism for this phenomenon using first-principles density functional theory calculations of MgH2/TiH2 interfaces. We found that epitaxial contact between MgH2 and TiH2 can be formed with strongly favorable interface formation energies. Similar epitaxial interfaces also exist for Mg and TiH2. The strain induced by TiH2(111) on MgH2 and Mg in these interfaces can reduce the enthalpy for H2 release from MgH2.

Irreversible High-Temperature Hydrogen Interaction with the Metal Organic Framework Cu3(BTC)2

Unsaturated metal coordination sites in metal organic frameworks (MOFs) offer preferential hydrogen adsorption sites. In addition, MOFs can be applied as scaffolds for nano metal hydrides operating at elevated temperatures. For these reasons, the thermal stability of the framework with unsaturated Cu2+ coordination sites in a hydrogen atmosphere has been investigated in Cu3(BTC)2. It is found that Cu3(BTC)2 irreversibly binds 1.1 wt % of hydrogen at the temperatures between 323 and 423 K and 2 bar H2 pressure, which corresponds to three hydrogen atoms per BTC ligand. In accordance, FT-IR and NMR spectroscopy show that the BTC linkers are converted to their acid form, whereas XRD reveals the presence of reduced Cu0 metal particles and decomposition of the framework. When designing more strongly interacting MOFs for H2 storage applications or as template for light metal hydrides, one has to take possible reduction of the metal centers and the stability of the lattice into account.

Theoretical study of C60 as catalyst for dehydrogenation inLiBH4

Complex light metal hydrides possess many properties which make them attractive as astorage medium for hydrogen, but typically catalysts are required to lower the hydrogendesorption temperature and to facilitate hydrogen uptake in the form of a reversiblereaction. The overwhelming focus in the search for catalysing agents has beenon compounds containing titanium, but the precise mechanism of their actionsremains somewhat obscure. A recent experiment has now shown that fullerenes(C60) can also act as catalysts for both hydrogen uptake and release in lithium borohydride(LiBH4). In an effort to understand the involved mechanism, we have employed density functionaltheory to carry out a detailed study of the interaction between this complex metal hydrideand the carbon nanomaterial. Considering a stepwise reduction of the hydrogen content inLiBH4, we find thatthe presence of C60 can lead to a substantial reduction of the involved H-removal energies.This effect is explained as a consequence of the interaction between theBHx − complexand the C60 entity.