Alane hydrogen storage for automotive fuel cells - Off-board regeneration processes and efficiencies

Alane is considered an attractive carrier of hydrogen for on-board light-duty vehicle hydrogen storage systems because of its high intrinsic capacity (10.1 wt% H₂), small heat of formation (∼7 kJ/mol H₂), and fast apparent decomposition kinetics. Regeneration of spent Al by direct hydrogenation is impractical due to the extremely high hydrogen equilibrium pressure required (∼7000 bar). This paper examines the off-board regeneration of alane using a three-step organometallic process. In the first step, a relatively stable adduct of a tertiary amine and alane is formed from elemental aluminum and hydrogen gas under moderate conditions of temperature and pressure. The second step involves transamination of the adduct by a second tertiary amine to form a secondary tertiary amine-alane adduct that is less stable than the first adduct. This secondary amine alane adduct is thermally decomposed in the final step to yield alane and the secondary amine for reuse in the process. All reagents, except aluminum and hydrogen, are recovered and recycled. Two process flowsheets have been constructed, and energy consumption in each step of the regeneration process has been calculated. Additionally, total energy requirements across the entire chain of production, delivery, storage, recovery, and regeneration has been evaluated to determine the overall well-to-tank efficiency and greenhouse gas emissions. In one flowsheet, the well-to-tank efficiency is ∼24.2% which improves to ∼42.1% if waste heat is freely available from industrial sources. The estimated greenhouse gas emissions are 31.6 kg CO₂ (eq) per kg H₂ delivered to the vehicle and reduce to 20.6 kg/kg-H₂ if free waste heat is readily available.     

Design tool for estimating chemical hydrogen storage system characteristics for light-duty fuel cell vehicles

The U.S. Department of Energy (DOE) developed a vehicle Framework model to simulate fuel cell-based light-duty vehicle operation for various hydrogen storage systems. This transient model simulates the performance of the storage system, fuel cell, and vehicle for comparison to Technical Targets established by DOE for four drive cycles/profiles. Chemical hydrogen storage models have been developed for the Framework for both exothermic and endothermic materials. Despite the utility of such models, they require that material researchers input system design specifications that cannot be estimated easily. To address this challenge, a design tool has been developed that allows researchers to directly enter kinetic and thermodynamic chemical hydrogen storage material properties into a simple sizing module that then estimates system parameters required to run the storage system model. Additionally, the design tool can be used as a standalone executable file to estimate the storage system mass and volume outside of the Framework model. These models will be explained and exercised with the representative hydrogen storage materials exothermic ammonia borane (NH₃BH₃) and endothermic alane (AlH₃).     

Automotive storage of hydrogen in alane

Although alane (AlH₃) has many interesting properties as a hydrogen storage material, it cannot be regenerated on-board a vehicle. One way of overcoming this limitation is to formulate an alane slurry that can be easily loaded into a fuel tank and removed for off-board regeneration. In this paper, we analyze the performance of an on-board hydrogen storage system that uses alane slurry as the hydrogen carrier. A model for the on-board storage system was developed to analyze the AlH₃ decomposition kinetics, heat transfer requirements, stability, startup energy and time, H₂ buffer requirements, storage efficiency, and hydrogen storage capacities. The results from the model indicate that reactor temperatures higher than 200 °C are needed to decompose alane at reasonable liquid hourly space velocities, i.e., > 60 h⁻¹. At the system level, a gravimetric capacity of 4.2 wt% usable hydrogen and a volumetric capacity of 50 g H₂/L may be achievable with a 70% solids slurry. Under optimum conditions, 80% of the H₂ stored in the slurry may be available for the fuel cell engine. The model indicates that H₂ loss is limited by the decomposition kinetics rather than by the rate of heat transfer from the ambient to the slurry tank.     

Using metal hydride H2 storage in mobile fuel cell equipment: Design and predicted performance of a metal hydride fuel cell mobile light

This study examines the practical prospects and benefits for using interstitial metal hydride hydrogen storage in “unsupported” fuel cell mobile construction equipment and aviation GSE applications. An engineering design and performance study is reported of a fuel cell mobile light tower that incorporates a 5 kW Altergy Systems fuel cell, Grote Trilliant LED lighting and storage of hydrogen in the Ovonic interstitial metal hydride alloy OV679. The metal hydride hydrogen light tower (mhH₂LT) system is compared directly to its analog employing high-pressure hydrogen storage (H₂LT) and to a comparable diesel-fueled light tower with regard to size, performance, delivered energy density and emissions. Our analysis indicates that the 5 kW proton-exchange-membrane (PEM) fuel cell provides sufficient waste heat to supply the desorption enthalpy needed for the hydride material to release the required hydrogen. Hydrogen refueling of the mhH₂LT is possible even without external sources of cooling water by making use of thermal management hardware already installed on the PEM fuel cell. In such “unsupported” cases, refueling times of ∼3–8 h can be achieved, depending on the temperature of the ambient air. Shorter refueling times (∼20 min) are possible if an external source of chilled water is available for metal hydride bed cooling during rapid hydrogen refueling. Overall, the analysis shows that it is technically feasible and in some aspects beneficial to use metal hydride hydrogen storage in portable fuel cell mobile lighting equipment deployed in remote areas. The cost of the metal hydride storage technology needs to be reduced if it is to be commercially viable in the replacement of common construction equipment or mobile generators with fuel cells.     

High and rapid hydrogen release from thermolysis of ammonia borane near PEM fuel cell operating temperatures

Ammonia borane (NH₃BH₃, AB), containing 19.6 wt % hydrogen, is a promising hydrogen storage material for use in proton exchange membrane fuel cell (PEM FC) powered vehicles. Our experiments demonstrate the highest H₂ yield (∼14 wt %, 2.15 H₂ equivalent) values obtained by neat AB thermolysis near PEM FC operating temperatures, along with rapid kinetics, without the use of either catalyst or additives. It was also found that only trace amount of ammonia (<10 ppm) is produced during dehydrogenation reaction and spent AB products are polyborazylene-like species, which can be efficiently regenerated using currently demonstrated methods. The results indicate that our proposed method is the most promising one available in the literature to-date for hydrogen storage, and could be used in PEM FC based vehicle applications.     

Thermal decomposition of alkaline-earth metal hydride and ammonia borane composites

We demonstrate a method to improve the promising hydrogen storage capabilities of ammonia borane by making composites with alkaline-earth metal hydrides using ball-milling technique. The ball-milling for the mixtures of alkaline-earth metal hydride (MgH₂ or CaH₂) and ammonia borane (AB) yields a destabilization compared with the ingredient of the mixture, showing the hydrogen capacity of 8.7 and 5.8 mass% at easily accessible dehydrogenation peak temperatures of 78 and 72 °C, respectively, without the unwanted by-product borazine. Through detailed analyses on the dehydrogenation performance of the composite at various ratios in the hydride and AB, we proposed a different chemical activation mechanism from that in the LiH/AB and NaH/AB systems reported in a previous literature.     

Excellent catalytic effect of LaNi5 on hydrogen storage properties for aluminium hydride at mild temperature

The catalytic effect of rare-earth hydrogen storage alloy is investigated for dehydrogenation of alane, which shows a significantly reduced onset dehydrogenation temperature (86 °C) with a high-purity hydrogen storage capacity of 8.6 wt% and an improved dehydrogenation kinetics property (6.3 wt% of dehydrogenation at 100 °C within 60 min). The related mechanism is that the catalytic sites on the surface of the hydrogen storage alloy and the hydrogen storage sites of the entire bulk phase of the hydrogen storage reduce the dehydrogenation temperature of AlH₃ and improve the dehydrogenation kinetic performance of AlH₃. This facile and effective method significantly improves the dehydrogenation of AlH₃ and provides a promising strategy for metal hydride modification.     

Hydrogen sorption in orthorhombic Mg hydride at ultra-low temperature

Mg can store up to ∼7 wt.% hydrogen and has great potential as light-weight and low cost hydrogen storage materials. However hydrogen sorption in Mg typically requires ∼573 K, whereas the target operation temperature of fuel cells in automobiles is ∼373 K or less. Here we demonstrate that stress-induced orthorhombic Mg hydride (O-MgH₂) is thermodynamically destabilized at ∼ 373 K or lower. Such drastic destabilization arises from large tensile stress in single layer O-MgH₂ bonded to rigid substrate, or compressive stress due to large volume change incompatibility in Mg/Nb multilayers. Hydrogen (H₂) desorption occurred at room temperature in O-MgH₂ 10 nm/O-NbH 10 nm multilayers. Ab initio calculations show that constraints imposed by the thin-film environment can significantly reduce hydride formation enthalpy, verifying the experimental observations. These studies provide key insight on the mechanisms that can significantly destabilize Mg hydride and other type of metal hydrides.     

Interface effects in NaAlH4–carbon nanocomposites for hydrogen storage

For practical solid-state hydrogen storage, reversibility under mild conditions is crucial. Complex metal hydrides such as NaAlH₄ and LiBH₄ have attractive hydrogen contents. However, hydrogen release and especially uptake after desorption are sluggish and require high temperatures and pressures. Kinetics can be greatly enhanced by nanostructuring, for instance by confining metal hydrides in a porous carbon scaffold. We present for a detailed study of the impact of the nature of the carbon–metal hydride interface on the hydrogen storage properties. Nanostructures were prepared by melt infiltration of either NaAlH₄ or LiBH₄ into a carbon scaffold, of which the surface had been modified, varying from H-terminated to oxidized (up to 4.4 O/nm²). It has been suggested that the chemical and electronic properties of the carbon/metal hydride interface can have a large influence on hydrogen storage properties. However, no significant impact on the first H₂ release temperatures was found. In contrast, the surface properties of the carbon played a major role in determining the reversible hydrogen storage capacity. Only a part of the oxygen-containing groups reacted with hydrides during melt infiltration, but further reaction during cycling led to significant losses, with reversible hydrogen storage capacity loss up to 40% for surface oxidized carbon. However, if the carbon surface had been hydrogen terminated, ∼6 wt% with respect to the NaAlH₄ weight was released in the second cycle, corresponding to 95% reversibility. This clearly shows that control over the nature and amount of surface groups offers a strategy to achieve fully reversible hydrogen storage in complex metal hydride-carbon nanocomposites.     

Improved low-temperature hydrogen generation from NH3BH3 and TiO2 composites pretreated with water

Ammonia borane (AB, NH₃BH₃) is nontoxic easily transportable solid hydride with high stability in air. In this work we demonstrate that simple mixing of AB with TiO₂ (anatase) allows for hydrogen gas to be generated at temperatures as low as 80 °C. No losses of hydrogen have been observed during preparation of hydride-containing composites. It was shown that the adsorption of water vapor on TiO₂ and the increase of TiO₂ loading considerably accelerated the rate of AB decomposition. The experimentally observed formation of B–O chemical bonds and the elevated heat emission suggest strong interaction of AB with the adsorbed water species on TiO₂ surface. It has been found that this interaction proceeds at a higher rate comparing with binary AB/H₂O systems. The heat being released in the process is thought to contribute to overcoming the activation barrier in the dehydrogenation of ammonia borane to produce hydrogen gas.     

High and rapid hydrogen release from thermolysis of ammonia borane near PEM fuel cell operating temperatures: Effect of quartz wool

Ammonia borane (NH₃BH₃, AB), containing 19.6 wt% hydrogen, is a promising hydrogen storage material for use in proton exchange membrane fuel cell (PEM FC) powered vehicles. We recently demonstrated that using quartz wool, the highest H₂ yield (2.1–2.3H₂ equivalent) values were obtained by neat AB thermolysis near PEM FC operating temperatures, along with rapid kinetics, without the use of either catalyst or chemical additives. It was found that quartz wool minimizes sample expansion and facilitates the production of diamoniate of diborane (DADB), which is a key intermediate for the release of hydrogen from AB. It was also found that only trace amount of ammonia (<10 ppm) is produced during dehydrogenation reaction and spent AB products are found to be polyborazylene-like species, which can be efficiently regenerated using currently demonstrated methods. The results indicate that our proposed method is the most promising one available in the literature to-date for hydrogen storage, and could be used in PEM FC based vehicle applications.     

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 (AlH₃) coated (5, 5) single-walled carbon nanotube (SWCNT). Our study indicates that a SWCNT coated with Aluminum hydride (Alane – AlH₃) can bind up to four hydrogen molecules. At half coverage of AlH₃, 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 H₂ is weak. All the H₂ 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.     

Noncatalytic hydrothermolysis of ammonia borane

Hydrolysis of ammonia borane (AB) is attractive as a chemical method for hydrogen storage. The use of catalysts is, however, usually required. In the present paper, two new methods for releasing hydrogen from AB and water are investigated which do not involve any catalyst. One method is based on combustion of AB mixtures with nanoscale aluminum powder and gelled water. It is shown experimentally that these mixtures, upon ignition, exhibit self-sustained combustion with hydrogen release from both AB and water. The other method involves external heating of aqueous AB solutions to temperatures 120 °C or higher, under argon pressure to avoid water boiling. To clarify the reaction mechanism, isotopic experiments using D₂O instead of H₂O were conducted. It is shown that heating AB/D₂O solution to temperatures 117–170 °C releases 3 equiv. of hydrogen per mole AB, where 2–2.1 equiv. originate from AB and 0.9–1 equiv. from water. The prospects of both methods for hydrogen storage are discussed.     

AB5/ABS composite material for hydrogen storage

AB5 metal hydride (MH) particles were polymer dispersed in order to entrap the micro and nanoparticles produced by repeated fragmentations of the metal phase during the hydrogen charging/discharging cycles. Acrylonitrile-butadiene-styrene copolymer (ABS) was selected as a matrix on the basis of its physical and chemical properties. AB5/ABS composite pellets were obtained by using a dry mechanical particle coating approach in a tumbling-mill apparatus and successive consolidation by uniaxial hot pressing. A number of characterization techniques were used to assess the morphological, chemical and structural properties of the composites. High pressure DSC measurements, conducted at different pressure values, were used to assess the H₂ absorption properties and profile the Van't Hoff plots of the material. The overall results indicated that the AB5/ABS composite well tolerated the hydriding effects on metal particles, with no losses in hydriding kinetics. The material characteristics were found to be compatible with its application in developing MH-based H₂ storage devices.     

Metal hydride hydrogen storage tank for fuel cell utility vehicles

The “low-temperature” intermetallic hydrides with hydrogen storage capacities below 2 wt% can provide compact H₂ storage simultaneously serving as a ballast. Thus, their low weight capacity, which is usually considered as a major disadvantage to their use in vehicular H₂ storage applications, is an advantage for the heavy duty utility vehicles. Here, we present new engineering solutions of a MH hydrogen storage tank for fuel cell utility vehicles which combines compactness, adjustable high weight, as well as good dynamics of hydrogen charge/discharge. The tank is an assembly of several MH cassettes each comprising several MH containers made of stainless steel tube with embedded (pressed-in) perforated copper fins and filled with a powder of a composite MH material which contains AB₂- and AB₅-type hydride forming alloys and expanded natural graphite. The assembly of the MH containers staggered together with heating/cooling tubes in the cassette is encased in molten lead followed by the solidification of the latter. The tank can provide >2 h long H₂ supply to the fuel cell stack operated at 11 kWe (H₂ flow rate of 120 NL/min). The refuelling time of the MH tank (T = 15–20 °C, P(H₂) = 100–150 bar) is about 15–20 min.     

Destabilization of LiAlH4 by nanocrystalline MgH2

In this work, we report the synthesis, characterization and destabilization of lithium aluminum hydride by ad-mixing nanocrystalline magnesium hydride (e.g. LiAlH₄ + nanoMgH₂). A new nanoparticulate complex hydride mixture (Li–nMg–Al–H) was obtained by solid-state mechano-chemical milling of the parent compounds at ambient temperature. Nanosized MgH₂ is shown to have greater and improved hydrogen performance in terms of storage capacity, kinetics, and initial temperature of decomposition, over the commercial MgH₂. The pressure–composition isotherms (PCI) reveal that the destabilized LiAlH₄ + nanoMgH₂ possess ∼5.0 wt.% H₂ reversible capacity at T ≤ 350 °C. Van't Hoff calculations demonstrate that the destabilized (LiAlH₄ + nanoMgH₂) complex materials have comparable enthalpy of hydrogen release (∼85 kJ/mole H₂) to their pristine counterparts, LiAlH₄ and MgH₂. However, these new destabilized complex hydrides exhibit reversible hydrogen sorption behavior with fast kinetics.     

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.     

Theoretical calculation of hydrogen desorption energies of calcium hydride clusters

Recently calcium hydride has attracted attention as a possible component in ternary complex hydrides such as Ca(AlH₄)₂, Ca₂SiH_x and quaternary complex hydrides of the type Li–B–Ca–H. Although in bulk form CaH₂ decomposes reversibly above 600° centigrade we were motivated to see whether calcium hydride in cluster form has properties suitable for hydrogen storage. We report here the results of DFT calculations using VASP^® package for clusters Ca_nH_2n with n = 1, 2, 3, 4, 6, 8, 10, 12, 14, 16, 20 to get the ground state geometries, energies, bond lengths, and desorption energies, after molecular dynamics optimization. The desorption energy vs. cluster size n curve showed that the desorption energy goes up sharply to ∼1.4 eV per H₂ for n up to 4, followed by a broad maximum of ∼1.8 eV per H₂ around n = 12–14, and then tapers off to a nearly constant value of 1.6 eV per H₂ approximating bulk behavior, which compares favorably with previously reported results. Comparison of these results with those of Mg_nH_2n shows that Ca_nH_2n has a lesser potential as a hydrogen storage medium.     

Effect of boric acid on thermal dehydrogenation of ammonia borane: Mechanistic studies

Ammonia borane (AB, NH₃BH₃) is a promising hydrogen storage material for use in proton exchange membrane (PEM) fuel cell applications. In this study, the effect of boric acid on AB dehydrogenation was investigated. Our study shows that boric acid is a promising additive to decrease onset temperature as well as to enhance hydrogen release kinetics for AB thermolysis. With heating, boric acid forms tetrahydroxyborate ion along with some water released from boric acid itself. It is believed that this ion serves as Lewis acid which catalyzes AB dehydrogenation. Using boric acid, we obtained high H₂ yield (11.5 wt% overall H₂ yield, 2.23 H₂ equivalent) at 85 °C, PEM fuel cell operating temperatures, along with rapid kinetics. In addition, only trace amount of NH₃ (20–30 ppm) was detected in the gaseous product. The spent AB solid product was found to be polyborazylene-like species. The results suggest that the addition of boric acid to AB is promising for hydrogen storage, and could be used in PEM fuel cell based vehicles.     

Size and stress dependent hydrogen desorption in metastable Mg hydride films

Mg is a promising light-weight material that has superior hydrogen storage capacity. However H₂ storage in Mg typically requires high temperature, ∼500–600 K. Furthermore it has been shown that there is a peculiar film thickness effect on H₂ sorption in Mg films, that is thinner Mg films desorb H₂ at higher temperature [1]. In this study we show that the morphology of DC magnetron sputtered Mg thin films on rigid SiO₂ substrate varied from a continuous dense morphology to porous columnar structure when they grew thicker. Sputtered Mg films absorbed H₂ at 373 K and evolved into a metastable orthorhombic Mg hydride phase. Thermal desorption spectroscopy studies show that thinner dense MgH₂ films desorb H₂ at lower temperature than thicker porous MgH₂ films. Meanwhile MgH₂ pillars with greater porosity have degraded hydrogen sorption performance contradictory to general wisdom. The influences of stress on formation of metastable MgH₂ phase and consequent reduction of H₂ sorption temperature are discussed.