It’s easy to be optimistic about hydrogen as an ideal fuel. It is much more difficult to solve an absolutely fundamental problem: How to store this fuel efficiently? A joint effort between Swiss and Polish experimental physicists and theorists has revealed why previous attempts to use magnesium hydride to store hydrogen failed to meet expectations and why future attempts may be successful.

Hydrogen has long been seen as the energy carrier of the future. However, in order for this to become a reality in the energy sector, effective storage methods must first be developed. The optimum solution is materials selected in such a way that hydrogen, at low energy costs, can first be injected into them and then regenerated as needed, preferably under conditions similar to those typical of our everyday environment. Magnesium is a promising candidate for hydrogen storage.

However, its conversion to magnesium hydride requires a suitable, effective catalyst, which has not yet been found. A group of scientists from the Swiss Federal Laboratory for Materials Science and Technology in Dübendorf, the Faculty of Chemistry of the University of Zurich and the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) Empa in Krakow have found that the cause of the failure, which lasted for many years, was due to the formation of magnesium during hydrogen injection. It showed an incomplete understanding of the events.

Theoretical and experimental ideas

The biggest obstacle to using hydrogen as an energy source is the difficulty of storing it. In cars that still run on rare hydrogen, hydrogen is stored in compressed form under approximately 700 atmospheres of pressure. This is not the cheapest or safest method and has little to do with efficiency: one cubic meter contains only 45 kg of hydrogen.

The same volume can hold 70 kg of hydrogen if pre-condensed. Unfortunately, the liquefaction process requires a lot of energy and an extremely low temperature of around 20 Kelvin must be maintained during storage. An alternative could be suitable materials such as magnesium hydride, which can hold up to 106 kg of hydrogen per cubic metre.

Magnesium hydride is one of the simplest materials tested for its ability to store hydrogen. Its content here can reach 7.6% (by mass). Therefore, magnesium hydride devices are quite heavy, so they are mostly suitable for stationary applications. However, it is important to remember that magnesium hydride is a very safe substance and can be stored without risk, for example, in the basement, and magnesium itself is an easily available and inexpensive metal.

An idea about the limitations of magnesium hydride

“Research on the incorporation of hydrogen into magnesium has been ongoing for decades, but has not led to solutions that could be expected to be widely used,” says theoretical physicist Professor Zbigniew Lodziana (IFJ PAN), one of the study’s co-authors. paper. inside Advanced Science where the latest discovery is presented.

“One source of the problems is hydrogen itself. This element can effectively penetrate the crystal structure of magnesium, but only when present as individual atoms. To obtain this from typical molecular hydrogen, a sufficiently effective catalyst would be required to make the hydrogen transition process in the material rapid and energetically feasible.” “Therefore, everyone was looking for a catalyst that met the above conditions, but unfortunately it was not very successful. Today, we finally know why these attempts were doomed to failure.”

Professor Lodziana developed a new model of the thermodynamic and electronic processes that occur in magnesium in contact with hydrogen atoms. The model assumes that local, thermodynamically stable magnesium hydride clusters are formed during the migration of hydrogen atoms in the material. Changes in the electronic structure of the material occur at the interface between metallic magnesium and hydride and play an important role in reducing the mobility of hydrogen ions. In other words, the kinetics of magnesium hydride formation is mainly determined by events at the boundary with magnesium. This effect has not yet been taken into account in the search for effective catalysts.

Professor Lodziana’s theoretical work complements experiments carried out in the Swiss laboratory in Dubendorf. Here, the migration of atomic hydrogen in a layer of pure magnesium sputtered onto palladium in an ultrahigh vacuum chamber was investigated. The measuring device was able to record changes in the state of several outer atomic layers of the sample under study, caused by the formation of a new chemical compound and the corresponding transformations in the electronic structure of the material. The model proposed by the IFJ PAN researcher allows us to fully understand the results of the experiment.

The achievements of the Swiss-Polish group of physicists not only pave the way for new searches for the most suitable catalyst for magnesium hydride, but also explain why some previously discovered catalysts showed higher efficiency than expected.

“There is much to suggest that the lack of significant progress in storing hydrogen in magnesium and its compounds is due to an incomplete understanding of the processes involved in hydrogen transport in these materials. We have all been looking for better catalysts for decades, but we can’t seem to find where we need to look. New theoretical and experimental results now show that hydrogen can be converted to magnesium.” It allows us to rethink with optimism about further improvements in addition methods,” concludes Professor Lodziana.