Hydrogen Storage Alloys Increasingly Higher Capacity and Lower Cost

Hydrogen Storage Alloys Increasingly Higher Capacity and Lower Cost
Hydrogen storage alloys were first discovered in the late 1960s by Philips Corporation of the Netherlands and Brooklyn Haven National Laboratory of the United States, who discovered nickel-lanthanum alloys and titanium-iron alloys, respectively. It has long been known that metals react with hydrogen to form metal hydrides, but since the amount of hydrogen absorbed is low and hydrogen is only released at high temperatures, a second metal was added to the alloy to improve the alloy so that hydrogen can be released even at low temperatures.

Types of hydrogen storage alloys

There are three main types of hydrogen storage alloys

Alloys of magnesium and nickel
Nickel alloys with rare earth metals such as lanthanum
Alloys of iron and titanium
About 500 types of hydrogen storage alloys have been discovered so far, and since each composition has different hydrogen storage capacity and absorption/release conditions, research has been conducted on alloys that are easy to handle. Alloys using mishmetals, which are mixtures of rare metals, and lanthanum-rich mesh metals, which contain large amounts of lanthanum, were also developed in an effort to reduce costs.

Challenges in Hydrogen Storage Alloys

Conventional hydrogen storage alloys can only store about 2% of hydrogen by weight, and to supply a sufficient amount of hydrogen, a large amount of hydrogen storage alloys are required, resulting in large facilities and high costs. The International Energy Agency (IEA) is also aiming to achieve a hydrogen absorption/release ratio of 6% by weight. However, many performance issues remain, such as the high temperature of 400°C or higher for hydrogen release conditions and the short absorption/release cycle.

Hydrogen Fluoride Storage Alloys

A research group led by Professor Seijiro Suda of Kogakuin University has developed a technology to fluoridate hydrogen storage alloys to form a fluoride film on the surface to prevent impurities from entering the alloy and lowering its efficiency. Since the fluoride film only allows hydrogen to pass through, only high-purity hydrogen can be absorbed. Using this technology, a new alloy with higher hydrogen storage capacity and lower temperature during absorption than conventional products has been developed.

There are two types of new alloys: one based on nickel and aluminum, and the other based on nickel-based alloys with magnesium added. These alloys do not contain rare metals, thus reducing costs. Fluorine treatment makes alloys with calcium added 1.4 times more capable than rare metal-based alloys, and alloys with magnesium added about 6 times more capable, with hydrogen absorption conditions as low as 40 degrees C and 10 atmospheres or less.

New alloys with multilayer nanostructure

Mazda, Hiroshima University, and the Hiroshima Prefectural Western Industrial Technology Center have developed a new alloy that absorbs and releases two to three times more hydrogen than conventional hydrogen storage alloys, based on a different principle. The new alloy has a multilayered nanostructure consisting of layers of palladium and magnesium thin films, using the sputtering method used to manufacture semiconductors. The active introduction of defects into the nanostructured metal improves its ability to absorb and release hydrogen.

In the three-layer structure, it was confirmed that at 100 degree C and 0.1 megapascal of hydrogen pressure, 5.6% of hydrogen by weight is absorbed and released below 100 degree C. Further multilayering did not change the amount of hydrogen absorbed, but lowered the temperature of hydrogen release, reaching below 90°C in the seven-layer structure.

Carbon Materials

Carbon nanotubes, in which carbon atoms are connected in a tubular shape with a diameter of one millionth of a millimeter, are one of the materials that have recently been attracting attention. Like storage alloys, they absorb and desorb hydrogen by adjusting the temperature, and Professor Sumio Iijima of Meijo University estimates that they can absorb a maximum of 2 to 3% of hydrogen by weight by devising the structure of the tube.

New Applications for Hydrogen Storage Alloys

Hydrogen storage alloys are being used in heat pumps as well as in rechargeable batteries for portable devices and automobiles. Hydrogen storage alloys generate heat when they absorb hydrogen and absorb heat when they release it, and attempts are being made to utilize this property to efficiently use energy.

Thus, the technological development of hydrogen storage alloys will contribute to improved energy efficiency and lower costs, and may lead to innovations in energy supply in the future.