Hunting Hydrogen : Structure-property relations in High Entropy Alloy-based metal hydrides

Metal hydrides have many uses when switching the energy system from fossil fuels to renewable sources, such as rechargeable batteries, hydrogen storage, hydrogen compression and thermal storage. State of the art materials for these applications such as LaNi5 and TiFe, however, suffer certain limitations such as degradation during repeated hydrogen cycling and harsh activation conditions for initial hydrogen uptake, promoting the need for novel materials. One class of materials that are interesting options are High Entropy Alloys (HEA), which are solid solutions where typically four or more different elements occupy a single crystallographic site in a simple structure such as body centered cubic (bcc) or cubic close packed (ccp). Due to the random distribution of the elements, there is a large variety of local environments for hydrogen, potentially unlocking sites that are unavailable in conventional transition metal hydrides. There is also the possibility of vast chemical tunability when using this many principal elements. It is therefore imperative to establish design rules to enable tuning of the hydrogen sorption properties of these materials by changing the composition. The effect of having many differently sized metals on the crystal structure is also not fully understood, and is believed to have a high impact on the bulk properties such as hydrogen sorption in these materials. This thesis covers the experimental synthesis of a wide range of HEAs and subsequent evaluation of their structural and hydrogen sorption properties. Several new design rules have been established, such as that the atomic size mismatch between the constituent metals has no effect on the maximum hydrogen capacity, that the addition of large elements like Zr leads to phase separation and that controlling the valence electron concentration, VEC, destabilizes the HEA-based metal hydrides. Based on these findings, the material TiVCrNbH8 has been identified as a candidate with properties rivaling that of TiFeH2