Hydrogen storage was identified by the US Department of Energy as a “grand challenge” for the implementation of hydrogen-powered fuel cell vehicles for reduced CO2 emissions from transportation vehicles. None of the hydrogen storage options currently developed can satisfy the high gravimetric, volumetric and system design requirements. Intermetallic compounds with Laves structures in the formula of AB2 have long been known to store hydrogen in their interstitial sites to serve as reversible hydrogen storage materials (A and B are metallic elements). They have the potential to be hydrided to a maximum of ~ AB2H6 due to the impeding H-H interactions at neighboring interstitial sites. To achieve the highest weight percent of hydrogen storage in AB2H6, the lowest combined atomic weight of AB2 is required. The CaLi2 compound is the lightest known Laves phase, but it could not maintain its Laves structure when it was hydrided. Existing work of Akiba’s group showed that a ternary Laves phase CaLi1.8Mg0.2 could be hydrided to form a hydrogenated Laves phase, but the absorbed hydrogen could not be released for reversible storage. Substitutions (Ca,X)Li1.8Mg0.2 are explored in the present study to see whether heavier elements [X = Sr, Ba and Ce] in small quantities can make the lightweight Laves compounds reversibly store hydrogen.
Induction melting was successful in obtaining the desired Laves phases. The base system, CaLi1.8Mg0.2, formed a single phase, consistent with the literature result. Both Ca0.9Ba0.1Li1.8Mg0.2 and Ca0.9Ce0.1Li1.8Mg0.2 also formed a single-phase C14 Laves, whereas both Ca0.9Sr0.1Li1.8Mg0.2 and Ca0.8Sr0.2Li1.8Mg0.2 formed two seperature Laves phases with the same crystal structure, indicating a phase separation. The Ca0.8Ba0.2Li1.8Mg0.2 composition completely lost the Laves-phase structure, forming CaLi2, CaMg2, BaLi4 and Ca.
All compounds tested at temperatures from 25 ¿¿C to 150 ¿¿C show the characteristic “plateau” behavior in the pressure-composition isotherms and the “plateau” pressure decreases with increasing temperature of the adsorption scans, which is opposite to all reversible hydrides indicating non-reversibility and different hydriding mechanisms. When Sr or Ba was added, the plateau pressure decreased with Ba being more potent in the reduction. Cerium (Ce) substitution, on the other hand, increases the plateau pressure. None of the synthesized and tested samples reversibly stored hydrogen.
Post hydriding structure analysis showed that CaH2 was the primary phase observed in the XRD patterns of all compounds, instead of forming the desired hydrogenated Laves phases. When CaLi1.8Mg0.2 was charged with hydrogen, it did not maintain its C14 Laves structure, but forming CaH2 (and very likely also amorphous LiH). This result is contradictory to what was reported in the literature. For Ca0.9Ba0.1Li1.8Mg0.2, only a small amount of the Laves structure remained with a slighted expaned lattice parameters together with the formation of CaH2 and BaH2. The XRD pattern of hydrogenated Ca0.9Ba0.1Li1.8Mg0.2 sample also showed the presence of LiH. Therefore, none of the materials has formed the desired Laves hydride in significant quantities. Future work, if any, is suggested to perform some B site substitutions with a metal (such as Al) with less affinity with hydrogen.