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

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-onset, 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 TiVCrNbH8 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. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.