What Is a Hydrogen Storage Alloy?
Hydrogen storage alloys are alloy materials that can store hydrogen.
They utilize the mechanism whereby certain metals combine with hydrogen to form hydrides. Magnesium (Mg) is one of the many metallic elements that are relatively easy to hydride.
Hydrogen storage alloys can be obtained by mixing metals that easily turn into hydrides in specific proportions to form an alloy.
Uses of Hydrogen Storage Alloys
Hydrogen storage alloys are used in fuel tanks for hydrogen and fuel cell vehicles, and as anode materials for nickel-hydrogen rechargeable batteries. Hydrogen storage alloys can store more than 1,000 times more hydrogen than the volume of the hydrogen storage alloy itself, so they are used in applications that can take advantage of this feature.
The most famous application of hydrogen storage alloys is the anode material of nickel-metal hydride rechargeable batteries. Dry cell type nickel-metal hydride rechargeable batteries have been commercialized and are still sold today. However, they are not widely used due to their small battery capacity compared to lithium-ion rechargeable batteries.
Current hydrogen storage alloys are expensive to produce because they often use rare metals (rare metals). If technological innovations that can solve the disadvantage of high cost can be developed, there will be an opportunity to promote the widespread use of rechargeable batteries that use hydrogen storage alloys.
Properties of Hydrogen Storage Alloys
Hydrogen storage alloys are characterized not only by their ability to store hydrogen but also by their ability to release the stored hydrogen. Under normal conditions, the element hydrogen (H) is very diluted in the form of hydrogen gas (H2). Hydrogen storage alloys can store hydrogen at a higher packing density than in the hydrogen gas (H2) state. Hydrogen storage alloys generate heat when storing hydrogen, while releasing hydrogen when heated.
Hydrogen is one of the promising future energy sources for replacing fossil fuels. However, the energy density of hydrogen gas in its gaseous state is low, so transporting hydrogen gas as it is quite inefficient. Hydrogen storage alloys can store hydrogen in a high-energy-density state and may be a potential solution to future energy problems.
Hydrogen storage alloys that can store enough hydrogen for practical use have conventionally used alloys made from rare earth elements (rare earths), which are rare and expensive. However, the development of new alloy materials that can sufficiently store hydrogen without the use of rare earth elements (rare earths), which are expensive and rare, is in progress.
Types of Hydrogen Storage Alloys
Hydrogen storage alloys can be broadly classified into five types in terms of composition. The classification name and crystal structure differ depending on the combination ratio and the type of metal used.
1. AB5 Type
This type is made mainly from rare earth elements. It also contains transition elements, such as nickel (Ni). Because it can absorb a relatively large amount of hydrogen, it is used as the anode material for nickel-hydrogen rechargeable batteries.
2. AB2 Type
Type AB2 is mainly made from transition elements, such as manganese, titanium, nickel, and zirconium. Compared to the AB5 type above, this alloy material has a higher hydrogen storage capacity and is the subject of active research and development.
3. Type AB
This alloy is made mainly from titanium and iron, and is sometimes abbreviated as a Ti-Fe type. It can be produced without using rare earth elements, so the production cost is relatively low.
4. Type A2B
This is a Mg alloy made mainly from magnesium. It is characterized by its high hydrogen storage capacity because magnesium can absorb a large amount of hydrogen.
5. Solid Solution Type BCC Alloy
This alloy is made mainly from vanadium and chromium, and is sometimes referred to as Ti-V or Ti-Cr.
Other Information on Hydrogen Storage Alloys
1. Applications of Hydrogen Storage Alloys
Applications of hydrogen storage alloys include heat pumps and compressors. Other applications include hydrogen storage tank materials that combine hydrogen storage alloys and high-pressure vessels for use in hydrogen-powered vehicles.
2. Future of Hydrogen Storage Alloys
Hydrogen storage alloys are materials that can store and release hydrogen. Hydrogen is expected to replace fossil fuels as a clean energy source.
If hydrogen storage alloys can be developed that can store and release large amounts of hydrogen and can be manufactured at low cost, hydrogen storage alloys will be a promising material for the future.