ABSTRACT
Guillaume Petitpas and Salvador Aceves discussed in detail in Chapter 4 the physical limits to the hydrogen storage density achievable using compressed hydrogen gas at various temperatures as well as LH2. Considering only the hydrogen itself (and ignoring the tankage and balance of plant), one can compress gaseous hydrogen at 300 K to a density of 39 g/ L using 700-bar pressures. If one cools the gas to 100 K, the gas intrinsically becomes denser, and it requires only 300-bar pressure to densify it to 50 g/ L. Liquid hydrogen LH2 at 20 K has a density of 70.7 g/ L. Isentropically compressing LH2 at 3 bar can increase the density into the solid hydrogen range of about 90 g/ L. More details can be found in Chapter 4. So, one can see that the low-density nature of hydrogen, in all of its pure states, allows
storage density to at most 90 g/ L and considerably less when one takes into account the necessary tankage. The cryogenic options offer considerably more density than the room temperature compression of hydrogen gas. However, LH2 has, up to now, been nowhere near as available as hydrogen from compressed gas hydrogen stations and merchant compressed gas cylinders. As a result, there has historically been interest in finding another means of storing hydrogen that combines the near-ambient-temperature character of compressed gas storage with the higher density and lower-pressure attributes of LH2 and cryocompressed storage. This third option is storing hydrogen in a chemical compound that ideally releases
and reabsorbs hydrogen like a sponge, with little inducement for either process. This was alluded to in Chapter 3, where it was noted that the mass density of hydrogen in water (H2O) is 111 g/ L at room temperature and ambient pressure. It turns out water is a lousy sponge for hydrogen because the H-O bond is sufficiently strong (~426 kJ/mol) that water does not readily release H atoms that would eventually form molecular H2. However, there exist a number of compounds with even higher volumetric hydrogen density than water
CONTENTS
Introduction ................................................................................................................................. 109 Reversible Metal Hydride Materials ........................................................................................ 110 AB-Type Intermetallic Interstitial Compounds ...................................................................... 113 AB5-Type Intermetallic Compounds ........................................................................................ 115 AB2-Type Intermetallic Compounds ........................................................................................ 117 V-Ti-Cr-Based Solid Solution BCC Alloys ............................................................................... 123 Applications of Interstitial Metal Hydrides ............................................................................ 126 References ..................................................................................................................................... 128
that do act as excellent sponges for hydrogen. Storing hydrogen in the solid-state hydride form not only holds a volumetric advantage over compressed and liquid hydrogen states but also can potentially offer several additional key features. These features include lowpressure operation, compactness, safety, full reversibility, tailorable delivery pressure, excellent absorption/ desorption kinetics, modular design for easy scalability, and long cycle life. As given by Figure 3.1 in Chapter 3, research on solid-phase hydrogen storage systems has
focused on “on-board-reversible materials,” by which the spent material remains on-board the vehicle and is refueled with molecular hydrogen, and “off-board-reversible materials,” for which the rehydrogenation requires removal of the material off the vehicle followed by industrial processing. The on-board-reversible materials include interstitial metal hydrides, complex hydrides, and sorption materials. The interstitial and complex hydrides involve chemical bonding between the hydrogen and elements in the storage material, whereas “sorption materials” involve hydrogen physically absorbed on materials with high surface densities, such as various forms of carbon and the metal organic frameworks (MOFs) and their derivatives. Activated carbon is a good example of a sorption material in which molecular hydrogen
is adsorbed on the carbon surface by the weakly bonded van der Waals force. The storage capacity of physical adsorption is dramatically increased with reduced temperature and increased pressure, a regime called cryoadsorption in which the hydrogen gas forms a condensed form on the substrate at the temperature of liquid nitrogen (77 K) [1]. The development of sorption materials for hydrogen storage is comprehensively examined by Channing Ahn and Justin Purewal in Chapter 7. Metal hydrides, in which atomic hydrogen is chemically bonded to the host elements, is
an example of chemical hydrogen storage. Depending on the nature of chemical bonding and its bond strength, the hydride formation process can be either reversible or irreversible. The nature of the metal-hydrogen bonds can be classified into three types: metallic, ionic, and covalent. Listing in order of decreasing level of reversibility, we can order the types of bonding as metallic > ionic > covalent in terms of reversibility of hydride formation. For example, the metallic-bonded TiFeH2, LaNi5H6, and TiMn2H3 are all reversible. Similarly, the ionic-bonded lithium, calcium, and magnesium hydrides (LiH, CaH2, and MgH2) are also reversible but with more difficulty. However, the covalent bonds formed between hydrogen and metals and metalloids of elements in group IB to VB of the periodic table lead to chemical compounds that are irreversible, such as CH4 (methane) and C8H18 (octane). A special class of metal hydrides, called complex metal hydrides, consists of compounds
with a mixed ionic and covalent character. For example, the material LiAlH4 consists of Li+ cations bound to (AlH4)– anions, yet within the (AlH4)– moiety there is substantial covalency. As a class, these complex metal hydrides often have reversibility problems introduced by their partial covalent nature. The complex metal hydrides are addressed by Vitalie Stavila, Lennie Klebanoff, John Vajo, and Ping Chen in Chapter 6. We continue here with a discussion of the interstitial metal hydrides.
