Hydrogen in Metals

Application of machine learning to thermal management of solid-state hydrogen storage: A comprehensive review

HyCARE project, supported by the Clean Hydrogen Partnership of the European Union, deals with a prototype hydrogen storage tank using a solid-state hydrogen carrier. Up to 40 kilograms of hydrogen are stored in 12 tanks at less than 50 barg and less than 100°C. The innovative design is based on a standard 20-foot container including 12 TiFe-based metal hydride (MH) hydrogen storage tanks, coupled with a thermal energy storage in phase change materials (PCM). This article aims at showing the main risks related to hydrogen storage in a MH system and the safety barriers considered, based on HyCARE’s specific risk analysis. Regarding the TiFe MH material used to store hydrogen, experimental tests showed that the exposure of the MH to air or water did not cause spontaneous ignition. Furthermore, an explosion within the solid MH cannot propagate due to internal pore size. Additionally, in case of leakage, the speed of hydrogen desorption from the MH is self-limited, which is an important safety characteristic since it reduces the potential consequences from the hydrogen release. Regarding the integrated system, the critical scenarios identified during the risk analysis were explosion due to release of hydrogen inside or outside the container, internal explosion inside MH tanks due to accidental mix of hydrogen and air and asphyxiation due to inert gas accumulation in the container. The identification phase of risk analysis identified the most relevant safety barriers already in place and recommended additional ones, if needed, which were later implemented to further reduce the risk. The main safety barriers identified were material and component selection (including the MH selected), safety interlocks, safety valves, ventilation, gas detection and safety distances. The risk management process, based on risk identification and assessment, contributed to coherently integrate inherently safe design features and safety barriers.

A basic physical model has been proposed for the statistical thermodynamics of the interstitial solid solution of hydrogen in metals where the behavior of not only the solute hydrogen but also of the parent metal is considered. The solid phase was represented as a superposition of the metal and hydrogen assemblies. The interaction energy between both the assemblies, which is partitioned into each assembly in an amount just large enough to cancel each other according to the rule of action and reaction, was expressed as a function of the hydrogen content with the aid of the theory of elasticity. The absolute activity for each component was derived from the respective grand partition function. The energy part in the state equation obtained was given in a quadratic expression with respect to the hydrogen content, unlike a linear expression in Lacher’s equation. It has been established that the energy associated with the parent lattice expansion is sufficient to cause the occurrence of two solid phases which coexist in a region of the hydrogen content, as suggested by various investigators. The critical condition, and the relation between the parent lattice expansion coefficient and the interaction energy among nearest-neighbor hydrogen atoms were determined. For simplicity, Hooke’s equation was used to calculate the strain energy, and the results obtained are subject to the limitations of validity of this constitutive equation.

The nature and properties of transition metal hydrides

One of the limitations surrounding the exploitation of hydrogen as a fuel is the difficulty in storing it economically and conveniently. At present, hydrogen is typically stored as a compressed gas or as a cryogenic liquid. However, neither method is very appealing. Gaseous storage requires a large volume and/or weight of storage vessels, while liquid hydrogen requires a rather expensive liquification process and some energy as well. Another possibility for storage that is currently being studied and utilized is metal hydrides. The hydrogen reacts with the metals and forms metal hydrides. Storing hydrogen as a metal hydride has many advantages. Firstly, hydrogen can be stored in metal hydrides more efficiently (in terms of volume) than as liquid, or even solid, hydrogen. Secondly, the formation reaction is easily reversible. The formation of the hydride is an exothermic and usually spontaneous reaction, but the hydrogen can easily be recovered by heating the hydride. Therefore, the metal hydride can be classified as a safe method of storing hydrogen since the hydrides are generally stable below their dissociation temperatures. Moreover, the self-cooling effect will slow down and eventually stop any loss of hydrogen if a leak is developed in the storage system. Several metal hydrides have been developed, but none of them satisfies all these requirements. Some of the more common drawbacks of the currently available materials are that (a) hydrogen-metal ratios are too low, (b) metals involved are too costly, (c) the absorption or release of hydrogen is difficult, slow or sensitive to poisoning phenomena and (d) significant hysteresis is exhibited. The available materials for hydrogen storage are briefly reviewed and a more detailed overview is given of two classes of materials that are being developed in the author's laboratory, namely zirconium-based Laves phase intermetallic compound and body-centred cubic solid solutions based on niobium.

Because of its low density, storage of hydrogen in the gaseous and liquids states possess technical and economic challenges. One practical solution for utilizing hydrogen in vehicles with proton-exchange fuel cells membranes is storing hydrogen in metal hydrides. Magnesium hydride (MgH2) remains the best hydrogen storage material due to its high hydrogen capacity and low cost of production. Due to its high activation energy and poor hydrogen sorption/desorption kinetics at moderate temperatures, the pure form of MgH2 is usually mechanically treated by high-energy ball mills and catalyzed with different types of catalysts. These steps are necessary for destabilizing MgH2 to enhance its kinetics behaviors. In the present work, we used a small mole fractions (5 wt.%) of metallic glassy of Zr70Ni20Pd10 powders as a new enhancement agent to improve its hydrogenation/dehydrogenation behaviors of MgH2. This short-range ordered material led to lower the decomposition temperature of MgH2 and its activation energy by about 121 °C and 51 kJ/mol, respectively. Complete hydrogenation/dehydrogenation processes were successfully achieved to charge/discharge about 6 wt.%H2 at 100 °C/200 °C within 1.18 min/3.8 min, respectively. In addition, this new nanocomposite system shows high performance of achieving continuous 100 hydrogen charging/discharging cycles without degradation.

It has been observed that hydrogen either in solid solution or precipitated under the form of hydrides has an impact on the viscoplasticity of CWSR Zircaloy-4 cladding tubes, increasing significantly the creep resistance. The use of TEM on the structurally complex CWSR material being unlikely to identify the deformation mechanisms, it has been decided to complete this R&D program on recrystallized material. A study has been carried out on fully annealed unirradiated Zircaloy-4 sheet used for the manufacturing of the fuel subassembly grids. Mechanical tests were performed for large ranges of temperatures (300 to 400°C), stresses (120 to 250 MPa), and strain rates (2 ∙ 10-7 to 2 ∙ 10-3 s-1) on as-received and hydrided specimen. The results emphasize: • Hydrogen in solid solution induces a softening of the material. The TEM observations have revealed identical structure of dislocations for both as-received and hydrided specimens. The softening has been particularly observed when dynamic strain aging is activated. It is assumed that atomic hydrogen decreases the dislocation pinning caused by interstitial and/or enhances the intrinsic mobility of the dislocations. With respect to abinitio calculation, atomic hydrogen might be trapped easily by the core of the dislocation, this phenomenon contributing to decrease the lattice friction and to enhance planar glide. • Precipitated hydrides induce a hardening of the material as observed for CWSR Zircaloy 4. The magnitude of the phenomenon depends upon temperature and stress. An analysis of the unload sequences for tension tests and of the secondary strain rates for creep tests leads to the conclusion that hydrides change the kinematics hardening by increasing the internal stress with respect to the as-received material. TEM observation combined with this viscoplasticity approach has revealed that: first, as long as the internal stress is increasing versus plastic strain, hydride are obstacles to the dislocation glide; second, once the internal stress reaches saturation, the hydrides can be jumped over by the dislocations.

Heat and mass transfer during the storage of hydrogen in LaNi5-based metal hydride: 2D simulation results for a large scale, multi-pipes fixed-bed reactor

Summary Zirconium and its alloys are key structural materials used in the nuclear power industry. In service, these metals are susceptible to a slow corrosion process that leads to a gradual pickup of hydrogen impurities from the environment. It is well known that hydrogen impurity will be attracted to stress concentrators such as notch and crack tips. At a certain hydrogen level, a complicated pattern of hydride precipitates can develop around these stress concentrators. Because of the brittleness of these hydrides, the original strength of the alloys can be reduced by orders of magnitude, and the fracture through these hydrides may occur. It is believed that critical conditions for the initiation of fractures at hydrides are controlled by the morphology and microstructure of hydride precipitates. In recent years by using a linear elastic approach, the author’s research team has simulated the morphological and microstructural evolution of hydride precipitation in single and polycrystalline zirconium under uniform and non-uniform stress fields. The methodology was based on the principles of the Ginzburg-Laudau kinetic phase-field theory coupled with Cahn-Hilliard diffusion dynamics, the method that has enjoyed much success in recent years. However, it is well known that plastic deformation will occur during hydride formation in zirconium alloys, and that plasticity around flaw tips may also play an important role during hydride fracture. The objective of this work is to incorporate plastic deformation into the phase field model and to develop computational methodologies to predict the realistic 3-D morphological evolution of hydride precipitates around notch and crack tips, hydride blister formation, and ultimately to predict fracture initiation at hydrides in zirconium.

Solid-state metal hydrides display hydrogen densities close to that of liquid hydrogen and thus provide a safe and efficient way of storing hydrogen. As a result of recent neutron and synchrotron diffraction work some novel metal hydrides have been characterized that shed new light on the nature of metal-hydrogen interactions. While hydrogen appears as an anion surrounded by a large inventory of cation configurations in ionic hydrides such as Ca4Mg3H14, Ca19Mg8H54, Eu2MgD6, Eu6Mg7D26 and Eu2Mg3D10, it acts as a terminal ligand in covalently bonded hydride complexes based on p-elements such as [BH4]– and d-elements such as [IrH5]4– and [IrH4]5– in the complex hydrides LiBH4 and Mg6Ir2H11, respectively. Surprisingly, hydride complexes and hydride anions can also be discerned in typically metallic (interstitial) hydrides such as NdMgNi4H4 (= Nd3+Mg+2·[Ni4H4]5–) and LaMg2NiD7 (= La3+Mg+2 2·[NiH4]4–·3H–). Some hydrides also reveal other interesting features such as a hydrogenation induced Ce4+ → Ce3+ valence change in CeMn1.8Al0.2H4.4 at room temperature that is accompanied by a Mn/Al metal atom exchange over distances of ~2.6 Å, and a hydrogen induced metal-to-nonmetal transition near ambient conditions that leads from the metallic compound Mg3Ir to the red colored hydride Mg6Ir2H11. In this article recent work and some methodological aspects are highlighted.

In-situ quantitative measurement of phase-sensitive hydrogen diffusion in metals

An analytic relation for the terminal solid solubility of hydrogen in a stressed metal is derived, based on finite deformation theory. Phase transformation is assumed to be a reversible process, which occurs under local chemical equi- librium among hydride, metal and hydrogen in solid solution. Hydrogen terminal solid solubility depends on stress due to the interaction of the applied stress field with the field of the expanding hydride as well as due to the reduction of hydro- gen chemical potential in solid solution, caused by hydrostatic stress. The present analysis is complementary to hydride- induced embrittlement studies in metals, under conditions, which require the use of finite deformations.

Formation of metal hydrides by mechanical alloying

Research on metal hydrides indicates that surface chemical effects and dynamic thermal properties will play major roles in the use of these materials as hydrogen fuel storage media. Various techniques have been developed to optimize the formation of metal hydride films, and near-surface analytical techniques including ion microprobe mass analysis, Auger electron spectroscopy, and proton backscattering have been used to elucidate the surface chemistry of these materials. Thermal diffusivities have been measured for hydrides of Sc, Y, Zr, and Er.

Interface effects in NaAlH4–carbon nanocomposites for hydrogen storage