Light Metal Hydrides Research Articles

Characterization of Hydrogen Storage Materials and Systems with Photons and Neutrons

AbstractComplex hydrides are very promising candidates for future light‐weight solid state hydrogen storage materials. The present work illustrates detailed characterization of such novel hydride materials on different size scales by the use of synchrotron radiation and neutrons. The comprehensive analysis of such data leads to a deep understanding of the ongoing processes and mechanisms. The reaction pathways during hydrogen desorption and absorption are identified by in situ X‐ray diffraction (XRD). Function and size of additive phases are estimated using X‐ray absorption spectroscopy (XAS) and anomalous small‐angle X‐ray scattering (ASAXS). The structure of the metal hydride matrix is characterized using (ultra) small‐angle neutron scattering (SANS/USANS). The hydrogen distribution in tanks filled with metal hydride material is studied with neutron computerized tomography (NCT). The results obtained by the different analysis methods are summarized in a final structural model. The complementary information obtained by these different methods is essential for the understanding of the various sorption processes in light metal hydrides and hydrogen storage tanks.

The role of Ni in increasing the reversibility of the hydrogen release from nanoconfined LiBH4

Nanoconfinement and the use of catalysts are promising strategies to enhance the reversibility of hydrogen storage in light metal hydrides. We combined nanoconfinement of LiBH4 in nanoporous carbon with the addition of Ni. Samples were prepared by deposition of 5-6 nm Ni nanoparticles inside the porous carbon, followed by melt infiltration with LiBH4. The Ni addition has only a slight influence on the LiBH4 hydrogen desorption, but significantly enhances the subsequent uptake of hydrogen under mild conditions. Reversible, but limited, intercalation of Li is observed during hydrogen cycling. X-ray diffraction shows that the initial crystalline 5-6 nm Ni nanoparticles are not present anymore after melt infiltration with LiBH4. However, transmission electron microscopy showed Ni-containing nanoparticles in the samples. Extended X-ray absorption fine structure spectroscopy proved the presence of Ni(x)B phases with the Ni-B coordination numbers changing reversibly with dehydrogenation and rehydrogenation of the sample. Ni(x)B can act as a hydrogenation catalyst, but solid-state 11B NMR proved that the addition of Ni also enhanced the reversibility of the system by influencing the microstructure of the nanoconfined LiBH4 upon cycling.

The impact of carbon materials on the hydrogen storage properties of light metal hydrides

The safe and efficient storage of hydrogen is still one of the remaining challenges towards fuel cell powered cars. Metal hydrides are a promising class of materials as they allow the storage of large amounts of hydrogen in a small volume at room temperature and low pressures. However, usually the kinetics of hydrogen release and uptake and the thermodynamic properties do not satisfy the requirements for practical applications. Therefore current research focuses on catalysis and the thermodynamic tailoring of metal hydride systems. Surprisingly, carbon materials used as additive or support are very effective to improve the hydrogen storage properties of metal hydrides allowing fast kinetics and even a change in the thermodynamic properties. Even though the underlying mechanisms are not always well understood, the beneficial effect is probably related to the peculiar structure of the carbon materials. This feature article gives an introduction to the different carbon materials, an overview of the preparation strategies to synthesize carbon/hydride nanocomposites, and highlights the beneficial effect of carbon by discussing two important hydrides: MgH2 and NaAlH4.

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.

Reaction Intermediates during the Dehydrogenation of Metal Borohydrides: A Cluster Perspective

Complex light metal hydrides such as alanates, amides, and borohydrides have among the highest hydrogen storage capacities of any materials investigated thus far. However, their use in the transportation industry is plagued by poor thermodynamics and kinetics as well as lack of reversibility at ambient pressure and temperature. To achieve a fundamental understanding of the properties of these materials, considerable efforts have been made to study the reaction intermediates as hydrogen continues to desorb. Recent experiments(1) have confirmed the formation of [B12H12]2− complexes during the dehydrogenation of metal tetrahydroborates. We show that the existence of these complexes can be understood by studying the stability of borane clusters and their interaction with metal atoms. We further examine the possibility that other reaction intermediates with different B and H stoichiometry may be present during the dehydrogenation process. Study of the interaction of boron−hydrogen complexes with various metal ...

Towards understanding a mechanism for reversible hydrogen storage: theoretical study of transition metal catalysed dehydrogenation of sodium alanate

On the basis of density functional theory and coupled-cluster CCSD(T) calculations we propose a mechanism of the dehydrogenation of transition metal doped sodium alanate. Insertion of two early 3d-transition metals, scandium and titanium, both of which are promising catalysts for reversible hydrogen storage in light metal hydrides, is compared. The mechanism is deduced from studies on the decomposition of a model system consisting of one transition metal atom and two NaAlH(4) units. Subsequently, the significance of such minimal cluster model systems to the real materials is tested by embedding the systems into the surface of the NaAlH(4) crystal. It is found that the dehydrogenation proceeds via breaking of the bridge H-Al bond and consequent formation of intermediate coordination compounds in which the H(2) molecule is side-on (eta(2)-) bonded to the transition metal centre. The total barrier to the H(2) release is thus dependent upon both the strength of the Al-H bond to be broken and the depth of the coordinative potential. The analogous mechanism applies for the recognized three successive dehydrogenation steps. The gas-phase model structures embedded into the surface of the NaAlH(4) crystal exhibit an unambiguous kinetic stability and their general geometric features remain largely unchanged.

Spatially resolved hydrogen desorption from aluminum hydride observed by magnetic resonance imaging

Light metal hydrides represent a promising class of materials for hydrogen storage. However, there are several technical challenges to overcome before their potential can be realized. Key among these is the often adverse absorption and desorption kinetics, which are a function of intrinsic reaction rates and practical operating temperatures. Modifying and controlling these kinetics require a thorough understanding of the hydrogen absorption and desorption processes. In this study, we have investigated the thermal decomposition of aluminum hydride, AlH 3, with Magnetic Resonance Imaging in order to visualize spatially the progress and extent of the reaction.

Hydrogen release properties of lithium alanate for application to fuel cell propulsion systems

Abstract In this paper the results of an experimental study on LiAlH 4 (lithium alanate) as hydrogen source for fuel cell propulsion systems are reported. The compound examined in this work was selected as reference material for light metal hydrides, because of its high hydrogen content (10.5 wt.%) and interesting desorption kinetic properties at moderate temperatures. Thermal dynamic and kinetic of hydrogen release from this hydride were investigated using a fixed bed reactor to evaluate the effect of heating procedure, carrier gas flow rate and sample form. The aim of this study was to characterize the lithium alanate decomposition through the reaction steps leading to the formation of Li 3 AlH 6 and LiH. A hydrogen tank was designed and realized to contain pellets of lithium alanate as feeding for a fuel cell propulsion system based on a 2-kW Polymeric Electrolyte Fuel Cell (PEFC) stack. The fuel cell system was integrated into the power train comprising DC–DC converter, energy storage systems and electric drive for moped applications (3 kW). The experiments on the power train were conducted on a test bench able to simulate the vehicle behaviour and road characteristics on specific driving cycles. In particular the efficiencies of individual components and overall power train were analyzed evidencing the energy requirements of the hydrogen storage material.

Hydrogen diffusion in MgH2 and NaMgH3 via concerted motions of charged defects

The transport properties of hydrogen in metal hydrides are crucial to the kinetics of H2 storage in these materials. We use first principles calculations to identify the defects that are relevant for H transport in MgH2 and NaMgH3. In both materials, the physically relevant defects are charged and H diffusion is dominated by mobility of negatively charged interstitial H. Moreover, the diffusion of these species occurs via concerted “interstitialcy” mechanisms with low energy barriers. Our results indicate that any examination of H diffusion in light metal hydrides must consider the concerted diffusion of charged defects.

Aluminum hydride coated single-walled carbon nanotube as a hydrogen storage medium

We report a first principle study on the hydrogen storage in Aluminum hydride (AlH 3) coated (5, 5) single-walled carbon nanotube (SWCNT). Our study indicates that a SWCNT coated with Aluminum hydride (Alane – AlH 3) can bind up to four hydrogen molecules. At half coverage of AlH 3, the hydrogen storage capacity of the SWCNT is 8.3 wt%. The system with full coverage is also studied and it is found that, even though the hydrogen storage capacity increases, the binding of H 2 is weak. All the H 2 adsorption is molecular with H–H bond length of 0.756 Å. Our result on a full molecular adsorption of hydrogen via light metal hydride is new and it leads to a practically viable storage process.

Electronic structure and optical properties of LiXH3 and XLiH3 (X = Be, B or C)

The equilibrium lattice constant, the cohesive energy and the electronic properties of light metal hydrides Li XH3and XLiH3 (X = Be, B or C) with perovskite lattice structures have been investigated by using the pseudopotential plane-wave method. Large energy gap of LiBeH3 indicates that it is insulating, but other investigated hydrides are metallic. The pressure-induced metallization of LiBeH3 is found at about 120 GPa, which is attributed to the increase of Be-p electrons with pressure. The electronegativity of the p electrons of X atom is responsible for the metallicity of the investigated Li XH3 hydrides, but the electronegativity of the s electrons of X atom plays an important role in the metallicity of the investigated XLiH3 hydrides. In order to deeply understand the investigated hydrides, their optical properties have also been investigated. The optical absorption of either LiBeH3 or BeLiH3 has a strong peak at about 5eV, showing that their optical responses are qualitatively similar. It is also found that the optical responses of other investigated hydrides are stronger than those of LiBeH3 and BeLiH3 in lower energy ranges, especially in the case of CLiH3.

Sodium Alanate Nanoparticles − Linking Size to Hydrogen Storage Properties

Important limitations in the application of light metal hydrides for hydrogen storage are slow kinetics and poor reversibility. To alleviate these problems doping and ball-milling are commonly applied, for NaAlH 4 leading to particle sizes down to 150 nm. By wet-chemical synthesis we have prepared carbon nanofiber-supported NaAlH 4 with discrete particle size ranges of 1-10 microm, 19-30 nm, and 2-10 nm. The hydrogen desorption temperatures and activation energies decreased from 186 degrees C and 116 kJ.mol (-1) for the largest particles to 70 degrees C and 58 kJ.mol (-1) for the smallest particles. In addition, decreasing particle sizes lowered the pressures needed for reloading. This reported size-performance correlation for NaAlH 4 may guide hydrogen storage research for a wide range of nanostructured light (metal) hydrides.

Hydrogen-Induced Phase Transformations in Mg-Ni Alloys

Hydrogen is suggested as a promising fuel of the near future for the utilization in automotive and mobile applications. Therefore, safe and effective hydrogen storage systems need to be developed. One of the possibilities, suitable especially for mobile applications, is the storage of hydrogen in the form of light-metal hydrides. In this work we studied microstructure and hydrogen absorption and desorption kinetics in selected Mg-Ni alloys. Hydrogen saturation was carried out by the cathodic polarization in alkaline water-based solution. It was confirmed that hydrogen could be stored in the Mg2Ni intermetallic phase forming Mg2NiH0.3 phase using this technology. MgH2 hydride is also formed when the temperature of 90 °C is applied. The total content of hydrogen in the material after saturation is approx. 0.7 wt. % according to the thermogravimetry analysis. This low value is caused probably by the surface oxidation, blocking further hydrogen diffusion. Thermal hydrogen desorption tests showed that the Mg2NiH0.3 phase is able to release hydrogen even at temperatures lower than 100 °C.

Structure and Properties of Magnesium-Based Hydrogen Storage Alloys

Hydrogen is the promising pollutant-free fuel of the near future. For various hydrogen applications, suitable storage systems have to be developed. One of the safe ways is the reversible storage of hydrogen in the form of light metal (lithium or magnesium) hydrides. MgH2 magnesium hydride shows very high storage capacity (approx. 7 wt. %), but its problem is high thermodynamic stability. Therefore, high temperature (over 400°C) is necessary for MgH2 to decompose producing hydrogen. The solution of this problem can be the utilization of the complex magnesium hydrides containing nickel, copper or other transition metals. In this work, the microstructure and hydrogen storage properties of the various magnesium alloys (Mg-Ni, Mg-Zn, Mg-Cu and Mg-Cu-Al) are described. The aim was to find suitable hydrogen storage system with good storage capacity and sufficient rate of formation and decomposition of hydrides. Microstructure, chemical and phase composition of the alloys were determined by the light and scanning electron microscopy, EDS and XRD. Hydrogen saturation was carried out by cathodic polarization in the alkaline solution. Hydrogen content in the material was estimated by XRD from the shift of the diffraction lines of present phases.

Hydrogen sorption properties of MgH 2–LiBH 4 composites

A detailed analysis of the reaction mechanism of the reactive hydride composite (RHC) MgH 2 + 2LiBH 4 ↔ MgB 2 + 2LiH + 4H 2 was performed using high-pressure differential scanning calorimetry (HP-DSC) measurements and in situ synchrotron powder X-ray diffraction (XRD) measurements along with kinetic investigations using a Sievert-type apparatus. For the desorption the following two-step reaction has been observed: MgH 2 + 2LiBH 4 ↔ Mg + 2LiBH 4 + H 2 ↔ MgB 2 + 2LiH + 4H 2. However, this reaction is kinetically restricted and proceeds only at elevated temperatures. In contrast to the desorption reaction, LiBH 4 and MgH 2 are found to form simultaneously under fairly moderate conditions of 50 bar hydrogen pressure in the temperature range of 250–300°C. As found in pure light metal hydrides, significant improvement of sorption kinetics is possible if suitable additives are used.

Sorting Storage Options

CHEMISTRY In the search for a practical mode of hydrogen storage for vehicle applications, light metal hydride compounds such as LiBH4 are appealing because of their high weight percentage of hydrogen. However, many such compounds are thermodynamically quite stable, and so require excessive heating

Thermodynamic destabilization and reaction kinetics in light metal hydride systems

Experimental approaches for altering the thermodynamics and kinetics of light element hydride systems are discussed. Equilibrium hydrogen pressures and reaction enthalpies can be varied with additives that form new alloy or compound phases upon dehydrogenation. The formation of new phases lowers the dehydrogenated state enthalpy and effectively destabilizes the component hydrides. This strategy is illustrated for LiBH 4 destabilized by MgH 2, MgF 2, MgS, and MgSe. The slow rates of hydrogen exchange in light element hydrides can be improved with catalysts and by reducing diffusion distances to the nanometer scale. The catalytic effects of a variety of transition metal sources on hydrogen exchange in the LiBH 4/MgH 2 system are described. The effects of reduced diffusion distances are illustrated using LiBH 4 incorporated into a nanoporous carbon aerogel.

Industrial production of light metal hydrides for hydrogen storage

Light metal hydrides show a high potential for reversible hydrogen storage applications. In view of the potential future storage of large amounts of hydrogen, an economic tonnage scale production will be required. This viewpoint set introduces production methods and discusses the potential for simplifying processing routes and reducing costs in view of an industrial mass production. For this purpose, sodium alanate, for which cost-competitive large-scale production is already considered feasible, is used as an example for future promising hydrides, like complex hydrides or reactive hydride composites.

Hydrogen storage in destabilized chemical systems

We describe approaches for modifying the thermodynamics and kinetics of hydrogen sorption reactions in light-metal hydrides. Thermodynamic destabilization is achieved using additives that form new compounds during dehydrogenation. This lowers the enthalpy and increases the equilibrium pressure. We review current research in this area, emphasizing destabilization reactions involving LiBH 4. We also describe the use of nano-engineering approaches for reducing diffusion distances and increasing surface area, thereby increasing the hydrogen exchange rate. We focus on improving reaction rates by incorporating metal hydrides into nanoporous scaffolds.

Predicting Reaction Equilibria for Destabilized Metal Hydride Decomposition Reactions for Reversible Hydrogen Storage

Reversible storage of hydrogen still remains one of the biggest challenges for widespread use of hydrogen as a fuel. Light metal hydrides have high hydrogen content but are typically too thermodynamically stable. Destabilization of metal hydrides is an effective way to improve their thermodynamics. First principles calculations have proven to be effective for screening potential destabilized reactions, but these calculations have previously been limited to examining approximations for reaction enthalpies. We have used density functional theory calculations to calculate the reaction free energy and van't Hoff plots for a variety of potential destabilized metal hydride reactions. Our calculations suggest a multistage approach for efficiently screening new classes of metal hydrides prior to experimental studies.