Renewable energy is vital to achieving net zero and, as production and demand are not always concurrent, storage is an issue. Although batteries are the best-known and most advanced form of energy storage to date, hydrogen storage (where excess energy is used to generate hydrogen that is stored for later use) is gaining traction.
Storage requirements depend on the end use of the hydrogen. For example, hydrogen for the national grid can be held in large containers in the same way as natural gas; size and weight are not particular considerations. Hydrogen tanks in vehicles, however, need to be small and very light. Hydrogen cars currently use cylinders that require gas compression, which takes a lot of energy. This reduces efficiency and limits the number of vehicles that can be served at hydrogen ‘petrol’ stations. Reusable chemical compounds that can take on or release hydrogen at close to ambient conditions are a promising alternative.
The Hereon team is researching one such potential compound for use in vehicles. As it needs to be light, the choice of elements is restricted; the team has therefore chosen to investigate Mg(NH2)2:LiH, which forms LiNH2 as an intermediate product. To optimise the material properties of the compound, the team added LiBH4 as a catalyst to accelerate the uptake and release of hydrogen. LiBH4 has previously been tested as a hydrogen storage material in its own right but was not performant enough for applications. However, adding a specific amount of LiBH4 to LiNH2 forms a eutectic (i.e., a mixture that melts as a single phase at a much lower temperature than its individual components). The team observed faster hydrogen uptake and release in a sample that contained liquid LiBH4:LiNH2 than in previous samples containing solid LiNH2 without an LiBH4 catalyst; they attribute this to improved hydrogen mobility in the molten eutectic mixture. To corroborate their theory, they came to SANDALS to analyse the atomic structure of the liquid mixture.
Neutron scattering characterization methods are particularly well-suited to investigating systems composed of light elements that do not provide significant signatures with X-rays; hydrogen is the most extreme case. Moreover, the use of neutrons allows different elements to be highlighted through isotopic substitution. As SANDALS was purpose-built for analysing the structure of samples such as LiNH2:LiBH4, it was optimal for the team’s experiment. Their results could open a road to understanding how the mechanism behind the combination of an LiNH2 compound with an LiBH4 catalyst enhances its properties. The team hopes that this knowledge will improve hydrogen storage and consequently lead to the widespread use of hydrogen as a vehicle fuel.
