Counting electrons - A new approach to tailor the hydrogen sorption properties of high-entropy alloys

Abstract We have investigated the structure and hydrogen storage properties of a series of quaternary and quintary high-entropy alloys related to the ternary system TiVNb with powder X-ray diffraction (PXD), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and manometric measurements in a Sieverts apparatus. The alloys have body-centred cubic (bcc) crystal structures and form face-centred cubic (fcc) metal hydrides with hydrogen-to-metal ratios close to 2 by hydrogenation. The onset temperature for hydrogen desorption, T o n s e t , decreases linearly with the valence-electron concentration, VEC . Moreover, the volumetric expansion per metal atom from the bcc alloys to the fcc hydrides, [ ( V / Z ) fcc − ( V / Z ) bcc ] / ( V / Z ) bcc , increases linearly with the VEC . Therefore, it seems that a larger expansion of the lattice destabilizes the metal hydrides and that this effect can be tuned by altering the VEC . Kissinger analyses performed on the DSC measurements indicate that the destabilization is a thermodynamic rather than kinetic effect. Based upon these insights we have identified TiVCrNbH 8 as a material with suitable thermodynamics for hydrogen storage in the solid state. This HEA-based hydride has a reversible hydrogen storage capacity of 1.96 wt% H at room temperature and moderate H 2 -pressures. Moreover, it is not dependent on any elaborate activation procedure to absorb hydrogen.