The hydrogen economy – using molecular hydrogen as a cleaner fuel than hydrocarbons for motor vehicles, power stations, etc. – is still in need of materials that can store hydrogen cheaply. Hydrogen has a very high energy density by weight but low by volume, so researchers are always trying to find better materials to store hydrogen tightly together. To this end, new research appearing in Chemistry of Materials describes a new crystal structure made of lithium, nitrogen, boron, and hydrogen that has great potential as a hydrogen storage material.Here’s a view of the crystal structure, which actually consists of molecular units of LiN2H3BH3 spaced in an orderly pattern (the order is what makes it a crystal – all crystals have long range order). This particular structure came about because researchers had discovered that hydrazine borane (N2H4BH3) has a fairly high hydrogen gravimetric capacity of 15.3% wt. H, good enough to have potential use in hydrogen storage applications. There was a problem, though. These storage materials are heated up to to remove the hydrogen to be used as fuel, but at higher temperatures, both pure hydrogen and nitrogen- and boron-based molecules (such as ammonia – NH3) are also released as gaseous byproducts. Hydrogen fuels for applications such as fuel cells need to be extremely pure – sometimes as high as 99.99% pure H2 – so these byproducts limit the usefulness of N2H4BH4 as a storage material.
Enter lithium! The researchers found that by combining hydrazine borane with LiH (through a process called ball milling), the new material LiN2H3BH3 release 9.3 % wt H over one hour at 130 degrees C without the additional creation of nitrogen- or boron-based byproducts. The inclusion of Li makes the parent compound less stable and thus more reactive at higher temperatures, leading to the release of hydrogen. The low-temperature, orthorhombic phase of the material shows especially robust hydrogen storage up to 400 degrees Celsius, up to which point it converts to a less effective, high temperature phase (which is not discussed in detail in the paper). Regardless, this paper provides another potential material to explore further for hydrogen storage.
Moury, R., Demirci, U., Ban, V., Filinchuk, Y., Ichikawa, T., Zeng, L., Goshome, K., & Miele, P. (2014). Lithium Hydrazinidoborane: A Polymorphic Material with Potential for Chemical Hydrogen Storage Chemistry of Materials, 26 (10), 3249-3255 DOI: 10.1021/cm500980b