Hydrogen storage behaviours of high entropy alloys: A Review

High entropy alloys (HEAs) formed by multi-principal elements show promising hydrogen storage performance. However, many aspects of their behaviours remain poorly understood. Properties of the HEAs are related to their chemical composition and the nature of their constituent elements, including electronegativity, atomic radii, and valence electron concentration (VEC). Unfortunately, many of the related studies do not adopt unambiguous meaning of these fundamental properties, consequently causing uncertainties. The present review quoting 177 reference publications, aims at clarification of these features by performing systematic analysis of the available experimental data for the multi-component hydrogen storage alloys crystallizing as BCC solid solutions and Laves phase intermetallics. The correlations between the hydrogen sorption performance and the composition-related features including atomic size mismatch, VEC and, most importantly, mixing entropy and enthalpy, have been analysed. We show that VEC plays the most significant role in tuning the hydrogen storage performance of the HEAs. At the same time, no clear correlations of the hydrogen sorption capacity or the enthalpy of hydride formation with other key properties of the HEAs were observed. The correlations were however established when simultaneously accounting the effects of several HEA properties on their hydrogen sorption performance. Based on our observations, we conclude that hydrogen storage performance of the medium- and high-entropy alloys is rather similar, and thus hydride-forming HEAs might be considered as a group of conventional hydrogen storage alloys rather than their separate class.