The role of Al addition on the hydrogen sorption properties of TiVZrNbHf high entropy alloy

Refractory BCC high entropy alloy TiVZrNbHf is a promising material for solid-state hydrogen storage with high hydrogen sorption capacity but unfavourable thermodynamics of hydride phase, i.e. too stable hydride that need high temperature to reversibly recover the absorbed hydrogen. As an attempt to destabilize the hydride phase, this study reports on the effect of Al addition (limited concentrations: 5 and 10 at.%) into this alloy on the physicochemical and hydrogen sorption properties. Despite traces of a V-Al secondary phase, the BCC (TiVZrNbHf)1-xAlx alloys are random solid solutions which form high-capacity FCC hydride phases under hydrogen atmosphere at room temperature, as proven by synchrotron and neutron diffraction. Although Al decreases the hydrogen sorption capacity, the presence of a p element destabilizes the FCC hydride phase. A comparison with previous literature data helps understanding the role of Al which strongly depends on the chemical composition of the initial alloys. XANES studies allowed access to details of the electronic structure of the unoccupied levels complemented by density functional theory calculations. Moreover, the addition of Al favours the formation of larger open volume defects during hydride formation than the initial Al-free alloy which might explain the faster absorption kinetics in Al-containing alloys.