A metal hydride is a compound formed between a metal and hydrogen, in which the hydrogen atoms are bonded to the metal atoms through chemical bonds. Metal hydrides have a wide range of applications as energy storage materials, catalysts, and structural materials.
1. Introduction
A metal hydride is a compound formed between a metal and hydrogen, in which the hydrogen atoms are bonded to the metal atoms through chemical bonds. Metal hydrides have a wide range of applications as energy storage materials, catalysts, and structural materials. There are various types of metal hydrides, each with their own unique properties and challenges. These include interstitial, substitutional, and complex types. Interstitial hydrides are those in which the hydrogen atoms are located between the metal atoms in the crystal lattice, while substitutional hydrides are those in which the hydrogen atoms replace metal atoms in the crystal lattice. In complex hydrides, the hydrogen atoms form covalent bonds with the metal atoms, resulting in a compound with a more complex chemical structure. Some of the most well-known metal hydrides include sodium aluminum hydride (NaAlH4), magnesium hydride (MgH2), and titanium hydride (TiH2). These hydrides can store and release large amounts of hydrogen in a relatively safe and controlled manner, making them ideal for energy storage applications.
One of their main characteristics is the ability to absorb and release hydrogen gas through processes known as hydriding and dehydriding, respectively
[1]. This property makes them attractive for use as hydrogen storage materials, as they can store large amounts of hydrogen at relatively low pressures
[2]. Metal hydrides can also be used as catalysts in chemical reactions and as structural materials due to their high strength and low density.
There are several advantages to using metal hydrides for hydrogen storage:
High capacity: Metal hydrides have a high hydrogen storage capacity, meaning that they can store large amounts of hydrogen in a relatively small volume. This makes them a compact and efficient storage option.
Safe: Metal hydrides are generally considered to be a safe storage option because they do not release hydrogen gas unless they are subjected to specific conditions, such as high temperatures or pressures. This reduces the risk of explosions or fires.
Stable: Metal hydrides are stable and do not react with other materials, making them a safe and reliable storage option.
Reusable: Metal hydrides can be used to store and release hydrogen multiple times, making them a reusable and environmentally friendly storage option.
Lightweight: Metal hydrides are typically lightweight, making them a suitable storage option for applications where weight is a concern, such as in vehicles.
Some of the applications of metal hydrides are provided in
Table 1.
Table 1.
Some applications where metal hydrides are used.
Overall, metal hydrides offer several advantages over traditional hydrogen storage methods and may be a more efficient and safe option in certain applications. In
Figure 1, hydrogenation enthalpies and entropies of various hydride materials and their suitable applications are provided.
Figure 1. Hydrogenation enthalpies and entropies of various hydride materials and their suitable applications: (
A) heat pumps, (
B) heat storage, (
C) hydrogen storage, (
D) hydrogen compression
[24].
Overall, metal hydrides have the potential to play a significant role in a variety of applications, and their properties and behavior are an active area of research in the field of materials science.
Type AB
5 metal hydrides (for which LaNi
5 serves as the paradigm) and type AB
2 metal hydrides are the most well known for hydrogen absorption (such as Mn
2Zn). The AB
5 group has excellent hydrogenation ability at ambient temperature. However, its hydrogen capacity is typically in the range of 1 to 1.5 wt%. Metal hydrides based on magnesium (Mg and Mg
2Ni) display unacceptably slow rates of hydrogenation and dehydrogenation even after significant activation at 673 K (400 °C)
[25].