For the operation of vehicles, either conventional fuel is used. This is considered harmful to the environment. Or electricity can be stored in large batteries. This is expensive, and the batteries do not have sufficient capacity for longer journeys despite their high weight. Or hydrogen is used to generate electricity via fuel cells. Advantage: In contrast to conventional fuels, no harmful carbon dioxide or soot particles are released during its later use. But hydrogen today still has one major disadvantage: for mobile use, hydrogen is usually stored in compressed gas containers, which have a relatively large volume and take up a correspondingly large amount of installation space in the vehicles. High pressures of up to 900 bar are also required for charging. In order to withstand these stresses, special containers made of high-quality, non-recyclable fibre-reinforced polymer materials must be used.
Scientists at the Helmholtz Centre Geesthacht near Hamburg (Germany) have been working since the late 1990s on the development of so-called solids storage facilities. “We have been investigating the possibility of using complex light metal hydrides as storage media for many years,” explains Dr. Claudio Pistidda, materials researcher at the Helmholtz Centre Geesthacht and one of the authors of the current publication.
These systems can store more hydrogen in less space than high-pressure tanks. Apart from this, the significantly lower charging pressures required for solid storage allow the use of cheaper and more environmentally friendly tank containers in automobiles. “But one of the big problems with the search for the right material was that the storage capacity decreased with every store,” says Pistidda.
A similar problem is known with rechargeable batteries: as the number of cycles increases, they continuously lose their capacity due to mechanical voltages and undesirable reactions at the electrodes, and therefore have a limited service life.
After many years, a system has now been developed in Geesthacht that can possibly solve these problems. For the first time, Pistidda and his colleagues in the laboratory were able to demonstrate that calcium borohydride can not only release the absorbed hydrogen by adding magnesium-nickel hydride, but that the system also returns to its original chemical structure during reloading with hydrogen and is thus available as a fully reversible long-term storage facility.
“We have thus achieved a real breakthrough on the way to developing new types of hydrogen storage materials for mobile and stationary applications,” Pistidda is pleased to report. “The discovered reaction path avoids undesired side reactions, which would otherwise hinder recharging with hydrogen. The new material thus opens up excellent prospects for long-term usable energy storage”.
Participating research institutions were the Helmholtz Centre for Materials and Coastal Research, University of Pavia, University of Helsinki, Rutherford Appleton Laboratory, Universitat Autònoma de Barcelona (UAB), University of Turin and the Helmut Schmidt University of the armed forces in Hamburg were involved in this research project.