## 1 Introduction  
Metal hydrides are materials which can chemically react with hydrogen to form a compound including hydrogen and at least one metal. A well-known group of metal hydrides are intermetallic compounds which are obtained by combining a stable-hydride-forming element and an unstable-hydride-forming element. They demonstrate good hydrogen storage properties in low pressures, while their volumetric densities are comparable with liquid hydrogen. Research on applications of metal hydrides has been in process for many years and includes investigating simple metal hydrides, such as magnesium hydride, and metal-hydride alloys, such as titanium-iron hydride. A useful property of metal hydrides is their ability to absorb and desorb hydrogen at a constant pressure [1]. They also offer the advantage of hydrogen storage under moderate temperatures while safely performing as high volume efficient storage [2]. Another advantage of metal hydrides is that they can be used to store hydrogen and heat simultaneously. Their gravimetric and volumetric energy density is usually higher than other means of energy storage like Li-ion batteries while energy is stored at ambient temperature [3]. Metal hydride storage systems (like storage in LaNi\({}_{5}\), for instance) usually have higher volumetric efficiency as compared to compressed gas technology, which is due to their near ambient operational pressures. A comparison of different hydrogen storage technologies is presented in Figure 1.  
The performance of a metal hydride system depends on the properties of the selected metal hydride. Some of the desired properties for a metal hydride are: highly specific gravimetric energy density, compact in design, long life and low performance degradation, and, of course, being economical. For a storage system, properties such as high hydrogen absorption capacity, high thermal conductivity, fast reaction kinetics, favorable equilibrium pressure, a simple activation process, and minimum degradation after cyclic operation are desired as well [4; 5; 6; 7; 8]. Some of the hydrogen release reaction characteristics are its thermochemistry, kinetics, phase changes, reagent requirements, catalyst morphology and amount, and species and levels of gaseous byproducts relative to hydrogen [9; 10; 11; 12]. Metal hydrides can be categorized into different types including AB\({}_{5}\) (LaNi\({}_{5}\)), AB\({}_{2}\) (ZrV\({}_{2}\)), AB (FeTi), and A\({}_{2}\)B (Mg\({}_{2}\)Ni) based on their chemical composition. Among all these types, AB\({}_{2}\) and AB\({}_{5}\) are the best known metal hydrides to absorb hydrogen. The AB\({}_{2}\) family of metal hydrides have an excellent hydrogen capacity (up to over 7 wt %) but have unacceptably slow kinetics of hydrogenation and dehydrogenation, even after extensive activation at 673 K (400 \({}^{\circ}\)C). The AB\({}_{5}\) group has an excellent hydrogenation performance at ambient temperatures but poor hydrogen capacity typically in the range of 1-1.5 wt % [13]. Metal hydrides useful for on-board hydrogen storage and can be broadly categorized as: (1) complex metal hydrides (e.g., NaAlH\({}_{4}\)) that have low hydriding pressures (20-150 bar), relatively high hydriding temperatures (200-300 \({}^{\circ}\)C), and high volumetric capacities; and (2) high pressure metal hydrides (HPMH) that have high hydriding pressures and hydriding temperatures lower than 100 \({}^{\circ}\)C but have a low volumetric capacity (2-3 wt %) [14; 15; 16]. Both these types of metal hydrides require additional mechanisms such as external cooling, high pressure cylinders, etc., that increase the overall system weight and hence the reduced system capacity. In addition, carbon-based physisorption materials such as CNTs and MOFs show very high material storage capacities at low temperatures (77 K and 50-100 bar), and their hydrogen absorption capacity greatly decreases with an increase in temperatures [17; 18].