Structural stability of metal hydrides,alanates and borohydrides of alkali and alkali- earth elements: A review

Alkali and alkali– earth metal hydrides have high hydrogen storage capacity, but high operation temperature hinders their use. The alanates and borohydrides of alkali and alkali– earth metals are widely studied because of their light weight and high hydrogen content. Borohydrides are highly stable and decompose only at elevated temperatures while alanates decompose in two steps. A detailed study of the properties of these hydrides is required for further understanding of their stability. A lot of thermodynamic information can be derived from investigation of materials under pressure or temperature. Structural measurements on the hydride compounds at ambient and high P–T conditions result in a better understanding of the stability of the hydride structures and will assist us in the design of suitable storage materials with desired thermodynamic properties. The structural data are potential source of information regarding inter-atomic forces which determine pressure/temperature induced changes. During the past few years, structural stability of hydrides is widely investigated under pressure and temperature, both experimentally and theoretically. In this review we discuss structural phase transition and decomposition behavior of light metal hydrides, borohydrides and alanates of the elements that belong to first and second group in the periodic table.     

Modeling and simulation of absorption–desorption cyclic processes for hydrogen storage-compression using metal hydrides

This work is aimed to develop and analyze reduced and simplified lumped models of cyclic processes for hydrogen storage and thermal compression using metal hydrides. Rigorous models involve several thousands of variables whereas reduced models we are interested in involve only several tens of variables. The models here presented reproduce the main dynamic behavior of rigorous models and experimental data found in the literature. Furthermore, the main tradeoffs arisen in process design are well described with these models, which is always an objective of optimal process design.In the first part of the work, a simplified lumped model is developed and validated by comparing the simulations outcome with numerical results and experimental measurements obtained from the literature for absorption and desorption individual processes. Our model is then used to simulate the process behavior using real parameters and constraints required by continuous recovery and compression systems such as those found in the metal treatment industry. The simulation results are used to improve the process performance by adjusting some key parameters of the system. These results are also used to perform a sensitivity analysis, i.e. evaluate the storage/compression system behavior when introducing variations to parameters such as operating conditions, reactor design, and material properties.Finally, we further reduce the model by considering that the inlet and outlet hydrogen flow is approximately constant. This particular specification is usually required by continuous processes in the metal treatment industry where hydrogen flow must remain constant. This requirement allows considering reaction rate as a constant. The constant reaction rate constraint allows integrating the ordinary differential equations; hence the system no longer has differential and algebraic equations but just algebraic equations. As a consequence of the simplification, the number of equations to be solved is reduced from over 15,000 to less than 50, maintaining an excellent match in the results.     

Characterization of metal hydrides using pneumato-chemical impedance spectroscopy

Pneumato-chemical impedance spectroscopy (PIS) is a tool derived from electrochemical impedance spectroscopy (EIS). PIS can be used to analyze the kinetics of various solid-gas reactions, such as the hydriding kinetics of metals in the presence of hysteresis. Pneumato-chemical impedance diagrams are obtained from simple gas transfer experiments, using non-harmonic pressure perturbations. In single-phase domains, the global sorption mechanism consists of mainly two steps, a surface chemisorption step and a bulk hydrogen transport step, controlled by diffusion. In two-phase domains, an additional phase transformation step must be considered. Model impedances are obtained by interconnecting microscopic impedances associated with each reaction step. By fitting model impedances to the experimental ones, microscopic rate parameters such as the surface dissociation resistance, the bulk hydrogen diffusion coefficient and the phase transformation resistance can be obtained. Different results obtained on palladium, palladium–silver and LaNi₅ samples (foils and powder) are presented to illustrate the potentialities of this spectroscopy.     

The effect of hydrogen on the electronic,mechanical and phonon properties of LaMgNi4 and its hydrides for hydrogen storage applications

Density functional theory calculations are used herein to explore the effect of hydrogen on the electronic, mechanical and phonon properties of LaMgNi₄ and its hydrides. The polycrystalline elastic moduli, Poisson's ratios and Debye temperatures are calculated based on the single-crystal elastic constants and Voigt-Reuss-Hill approximations. It is also found that all these materials are metallic behavior, ductile and anisotropic in nature. The mechanical anisotropy is discussed via several anisotropy indices and three-dimensional (3D) surface constructions. The effect of high temperature on the free energy, entropy, and heat capacity are also studied by using the quasi-harmonic Debye model. LaMgNi₄ and its hydrides are found to be energetically, mechanically and dynamically stable. Also, they are thermodynamically stable and the order of phase stability is LaMgNi₄H₇ > LaMgNi₄H₄ > LaMgNi₄H > LaMgNi₄. In addition, the highest gravimetric hydrogen storage capacity is found to be 1.74 wt% for LaMgNi4H₇.     

A feasibility analysis of hydrogen delivery system using liquid organic hydrides

The paper discusses the techno-economic feasibility of a hydrogen storage and delivery system using liquid organic hydrides (LOH). Wherein, LOH (particularly cycloalkanes) are used for transporting the hydrogen in chemical bonded form at ambient temperature and pressure. The hydrogen is delivered through a catalytic dehydrogenation process. The aromatics formed in the process are used for carrying more hydrogen by a subsequent hydrogenation reaction. Cost economics were performed on a system which produces 10 kg/h of hydrogen using methylcyclohexane as a carrier. With proprietary catalysts we have demonstrated the possibility of hydrogen storage of 6.8 wt% and 60 kg/m³ of hydrogen on volume basis. The energy balance calculation reveals the ratio of energy transported to energy consumed is about 3.9. Moreover, total carbon footprint calculation for the process of hydrogen delivery including transportation of LOH is also reported. The process can facilitate a saving of 345 tons/year of carbon dioxide emissions per delivery station by replacing gasoline with hydrogen for passenger cars. There is an immense techno-economic potential for the process.     

Acoustic emission characteristics of used 70 MPa type IV hydrogen storage tanks during hydrostatic burst tests

Currently, the periodic inspection of composite tanks is typically achieved via hydrostatic test combined with internal and external visual inspections. Acoustic emission (AE) technology demonstrates a promising nondestructive testing method for damage mode identification and damage assessment. This study focuses on AE signals characteristics and evolution behaviors for used 70 MPa Type IV hydrogen storage tanks during hydrostatic burst tests. AE-based tensile tests for epoxy resin specimen and carbon fiber tow were implemented to obtain characteristics of matrix cracking and fiber breakage. Then, broad-band AE sensors were used to capture AE signals during multi-step loading tests and hydrostatic burst tests. K-means ++ algorithm and wavelet packet transform are performed to cluster AE signals and verify the validity. Combining with tensile tests, three clusters are manifested via matrix cracking, fiber/matrix debonding and fiber breakage according to amplitude, duration, counts and absolute energy. The number of three clustering signals increases with the increase of pressure, showing accumulated and aggravated damage. The sudden appearance of a large number of fiber breakage signals during hydrostatic burst tests suggests that the composite tank structure is becoming mechanically unstable, namely the impending burst failure of the tank.     

Simulation and burst validation of 70 MPa type IV hydrogen storage vessel with dome reinforcement

The dome reinforcement (DR) technology was studied to reduce the amount of carbon fiber of the type IV hydrogen storage vessel in this paper. Firstly, the influence of the angle and thickness of the dome reinforcement part on the stress distribution of the dome section is studied by finite element analysis. Secondly, the weight reduction of carbon fiber composite layer is studied based on the dome reinforcement model. The strain-based Hashin progressive damage model is used to predict the burst pressure and burst mode with user-defined material subroutine (UMAT) of ABAQUS. Finally, the dome reinforcement technology is further verified in comparison with non-dome reinforcement by burst tests. The results show that the progressive damage model can effectively represent matrix cracking and fiber fracture, and the predicted burst pressure and mode is consistent with the test results. The fiber stress near the equator of the dome section affects the burst mode, and the smaller the angle of dome reinforcement parts, the better the reinforcing effect, and the dome reinforcement technology can help to improve the fiber damage state at the dome, transfer the maximum stress to the cylinder section of the vessel, and ensure the burst mode to be a safe mode. Also, it can help to reduce the consumption of carbon fiber by up to 5.5% in composite material.     

Characterization of metal hydrides by in-situ XRD

In-situ synchrotron radiation powder X-ray diffraction (SR-PXD) technique is a powerful tool to gain a deeper understanding of reaction mechanisms in crystalline materials. In this paper, the implementation of a new in-situ SR-PXD cell for solid–gas reactions is described in detail. The cell allows performing measurements in a range of pressure which goes from light vacuum (10⁻² bar) up to 200 bar and temperatures from room temperature up to 550 °C. The high precision, with which pressure and temperature are measured, enables to estimate the thermodynamic properties of the observed changes in the crystal structure and phase transformations.     

Monitoring and control of a hydrogen production and storage system consisting of water electrolysis and metal hydrides

Renewable energy sources such as wind turbines and solar photovoltaic are energy sources that cannot generate continuous electric power. The seasonal storage of solar or wind energy in the form of hydrogen can provide the basis for a completely renewable energy system. In this way, water electrolysis is a convenient method for converting electrical energy into a chemical form. The power required for hydrogen generation can be supplied through a photovoltaic array. Hydrogen can be stored as metal hydrides and can be converted back into electricity using a fuel cell. The elements of these systems, i.e. the photovoltaic array, electrolyzer, fuel cell and hydrogen storage system in the form of metal hydrides, need a control and monitoring system for optimal operation. This work has been performed within a Research and Development contract on Hydrogen Production granted by Solar Iniciativas Tecnológicas, S.L. (SITEC), to the Politechnic University of Valencia and to the AIJU, and deals with the development of a system to control and monitor the operation parameters of an electrolyzer and a metal hydride storage system that allow to get a continuous production of hydrogen.     

Dehydriding and re-hydriding properties of high-energy ball milled LiBH4 + MgH2 mixtures

Here we report the first investigation of the dehydriding and re-hydriding properties of 2LiBH₄ + MgH₂ mixtures in the solid state. Such a study is made possible by high-energy ball milling of 2LiBH₄ + MgH₂ mixtures at liquid nitrogen temperature with the addition of graphite. The 2LiBH₄ + MgH₂ mixture ball milled under this condition exhibits a 5-fold increase in the released hydrogen at 265 °C when compared with ineffectively ball milled counterparts. Furthermore, both LiBH₄ and MgH₂ contribute to hydrogen release in the solid state. The isothermal dehydriding/re-hydriding cycles at 265 °C reveal that re-hydriding is dominated by re-hydriding of Mg. These unusual phenomena are explained based on the formation of nanocrystalline and amorphous phases, the increased defect concentration in crystalline compounds, and possible catalytic effects of Mg, MgH₂ and LiBH₄ on their dehydriding and re-hydriding properties.     

CFD analysis of fast filling scenarios for 70 MPa hydrogen type IV tanks

High injection pressure is combined with high refueling rate for vehicles storing pressurized gaseous hydrogen onboard. As a drawback, high temperatures are developed inside the tank, which can jeopardize the structural integrity of the storage system. Computational Fluid Dynamics (CFD) codes already proved to be a valuable tool for predicting the temperature distribution within the tank during fast refueling. Results of hydrogen fast filling CFD simulations for a type IV tank, filled to 70 MPa at different working conditions are presented as follow up of the CFD model validation performed against experimental data. Alternative rates of pressure rise, adiabatic and cold filling are investigated to evaluate the effect on maximum hydrogen temperatures inside the tank. Results confirmed that the developed CFD model could be a suitable tool for investigating fast filling scenarios when experimental data are not yet available or of difficult realization.     

Phase field modeling of hydrogen transport and reaction in metal hydrides

The reaction of hydrogen with metals to form metal hydrides has numerous potential energy storage and management applications. The metal hydrogen system has a high volumetric energy density and is often reversible with a high cycle life. However, improving the often poor gravimetric performance of such systems through the use of lightweight metals usually comes at the cost of reduced reaction rates or the requirement of pressure and temperature conditions far from the desired operating conditions. Most studies of reaction kinetics of such systems focus on fitting low-dimensional kinetic models to measured rates and inferring the rate-limiting process based on the quality of the fit.     

Integrating hydrogen generation and storage in a novel compact electrochemical system based on metal hydrides

The development of efficient and reliable energy storage systems based on hydrogen technology represents a challenge to seasonal storage based on renewable hydrogen. State of the art renewable energy generation systems include separate units such as electrolyzer, hydrogen storage vessel and a fuel cell system for the conversion of H₂ back into electricity, when required. In this work, a novel electrochemical system has been developed which integrates hydrogen production, storage and compression in only one device, at relatively low cost and high efficiency. The developed prototype comprises a six-electrode cell assembly using an AB₅-type metal hydride and Ni plates as counter electrodes, in a 35-wt% KOH solution. Metal hydride electrodes with chemical composition LaNi_4.3Co_0.4Al_0.3 were prepared by high frequency vacuum melting followed by high temperature annealing. X-ray phase analysis showed typical hexagonal structure and no traces of other intermetallic compounds belonging to the La–Ni phase diagram. Thermodynamic study has been performed in a Sieverts type of apparatus produced by Labtech Int. During cycling, the charging/discharging process was studied in situ using a gas chromatograph from Agilent. It is anticipated that the device will be integrated as a combined hydrogen generator and storage unit in a stand-alone system associated to a 1-kW fuel cell.     

Water resistant hydrogen storage materials comprising encapsulated metal hydrides

Water resistance was imparted to _ZrMn2${\mathrm{ZrMn}}_2$, Mg-10%Ni and Ti–Zr–Fe–Mn alloys by encapsulating hydrided powders with ceramic overlayers derived from alcoxides. The treated powders readily absorbed hydrogen after immersed in water or showed essentially the same hydrogen capacity under a humidified hydrogen atmosphere (partial pressure of water: 0.6 kPa) as that under pure hydrogen. Loss of hydrogen capacity on the encapsulating treatment and/or poor kinetics of absorption and desorption due to coverage of surface are to be resolved prior to practical use of these composite materials.     

A novel batch-type hydrogen transmitting system using metal hydrides

In the case of transporting hydrogen by means of metal hydrides, a key problem is to reduce the weight of the portable container filled with metal hydrides. The paper describes a novel batch-type hydrogen transmitting system characterized by a portable light container filled with metal hydrides, which is not pressure-proof but only mechanically durable. Hydriding is performed by setting the portable light container in a fixed pressure-proof vessel and admitting hydrogen and nitrogen inside and outside the portable container, respectively, while adjusting the pressure difference between both gases to be zero. Using this system, 2.9 Nm³ of hydrogen can be stored in 14.3 kg of the total mass of the solid constituents including 3.5 kg of Mg-10% Ni alloy. The portable container contains twice as much hydrogen per unit weight and volume as a conventional compressed gas cylinder. Due to the advanced design of this portable container, the optimum hydrogen content could be around 5 wt % based upon the total mass of the container.     

Nanoscopic Al1−xCex phases in the NaH + Al + 0.02CeCl3 system

The NaH + Al + 0.02CeCl₃ system has been studied by high-resolution X-ray synchrotron diffraction and transmission electron microscopy (TEM), after planetary milling under hydrogen and hydrogen (H) cycling. Isothermal absorption kinetics were determined at 150 °C, and compared with the NaH + Al + 0.02TiCl₃ system, indicating that CeCl₃ and TiCl₃ are equally effective additives, with CeCl₃ preferred on the basis of hydrogen storage capacity. After milling, AlCe contains 100% of the Ce. After the first H absorption, we observe two Al_1−xCe_x phases. The first, AlCe, contains ca. 60% of the originally added Ce atoms. The AlCe phase observed after milling and H cycling is chemically disordered, with complete exchange between the Al and Ce sublattices occurring, yielding zero intensity in ordering reflections such as (100). In the absorbed state after H cycling, the remaining 40% of Ce atoms are contained in a cubic Al_1−xCe_x phase not previously observed in the Al-Ce phase diagram. Indexing yields a primitive cubic unit cell of dimension 7.7111 Å, in space group P23. Lineshape analysis indicates the AlCe and unknown cubic Al_1−xCe_x phases are ca. 35 nm and 30 nm in dimension respectively. High resolution TEM imaging confirms that both Al_1−xCe_x phases are embedded on the NaAlH₄ surface, and localised energy dispersive X-ray spectroscopy (EDS) indicates a ca. 2:1 Al:Ce ratio for the unknown cubic Al_1−xCe_x phase.     

Numerical study on fast filling of 70 MPa type III cylinder for hydrogen vehicle

There will be significant temperature rise within hydrogen vehicle cylinder during the fast filling process. The temperature rise should be controlled under the temperature limit (85 °C) of the structure material (set by ISO/TS 15869), because it may lead to the failure of the structure. In this paper, a 2-dimensional axisymmetric computational fluid dynamics (CFD) model for fast filling of 70 MPa hydrogen vehicle cylinder is presented. The numerical simulations are based on the modified standard k − ? turbulence model. In addition, both the equation of state for hydrogen gas and the thermodynamic properties are calculated by National Institute of Standards and Technology (NIST) database: REFPROP 7.0. The thermodynamic responses of fast filling with different pressure-rise patterns and filling times within type III cylinder have been analyzed in detail.     

Novel hydrogen generator/storage based on metal hydrides

A novel electrochemical system has been developed which integrates hydrogen production, storage and compression in only one device, at relatively low cost and higher efficiency than a classical electrolyser. The prototype comprises a six-electrode cell assembly using an AB₅ type metal hydride and Ni plates as counter electrodes, in a KOH solution. Metal hydride electrodes with chemical composition LaNi₄.₃Co_0.4Al₀.₃ have been prepared by high frequency vacuum melting followed by high temperature annealing. X-ray phase analysis showed typical hexagonal structure and no traces of other intermetallic compounds belonging to the La–Ni phase diagram. Thermodynamic study of the alloy has been performed in a Sievert-type apparatus produced by Labtech Ltd. In the present prototype during charging, hydrogen is absorbed in the metal hydride and corresponding oxygen is conveyed out of the system. Conversely, in the case of discharging the hydrogen stored in the metal hydride it is released to an external H₂ storage. Released hydrogen is delivered into the hydrogen storage up to a pressure of 15 bar. It is anticipated that the device will be integrated as a combined hydrogen generator in a stand-alone system associated to a 1 kW fuel cell.     

Influence of intrinsic hydrogenation/dehydrogenation kinetics on the dynamic behaviour of metal hydrides: A semi-empirical model and its verification

Metal hydrides can store hydrogen at low pressures and with high volumetric capacity. For the possible application as storage medium in hydrogen stand-alone power systems, large metal hydride hydrogen storage units are usually required. A reliable and verified kinetic correlation is an important tool in the designing process of a larger storage unit. This paper describes kinetic investigation of a AB₅-type alloy and its corresponding hydride, with the purpose of finding a semi-empirical correlation suitable for use in heat and mass transfer modelling and engineering design of metal hydride storage units.     

Enhanced dehydrogenation/hydrogenation kinetics of the Mg(NH2)2-2LiH system with NaOH additive

The effects of NaOH addition on hydrogen absorption/desorption properties of the Mg(NH₂)₂-2LiH system were investigated systematically by means of dehydrogenation/hydrogenation measurements and structural analyses. It is found that the NaOH-added Mg(NH₂)₂-2LiH samples exhibit an enhanced dehydrogenation/hydrogenation kinetics. In particular, a ∼36 °C reduction in the peak temperature for dehydrogenation is achieved for the Mg(NH₂)₂-2LiH-0.5NaOH sample with respect to the pristine sample. Structural examinations reveal that NaOH reacts with Mg(NH₂)₂ and LiH to convert to NaH, LiNH₂ and MgO during ball milling. Then, their co-catalytic effects result in a significant improvement in the dehydrogenation/hydrogenation kinetics of the Mg(NH₂)₂-2LiH system. This finding will help in designing and optimizing the novel high-performance catalysts to further improve hydrogen storage in the amide-hydride combined systems.