Counting electrons - a new approach to tailor the hydrogen sorption properties of high-entropy alloys Magnus Moe Nyg˚ ard a,∗ , Gustav Ek b , Dennis Karlsson b , Magnus H. Sørby a , Martin Sahlberg b , Bjørn C. Hauback a a Institute for Energy Technology, Department of Neutron Materials Characterization, P.O. Box 40, NO-2027 Kjeller, Norway b Department of Chemistry - ˚ Angstr¨om Laboratory, Uppsala University, Box 523, SE-75120 Uppsala, Sweden 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 cu- bic (fcc) metal hydrides with hydrogen-to-metal ratios close to 2 by hydro- genation. The onset temperature for hydrogen desorption, T onset , decreases linearly with the valence-electron concentration, VEC. Moreover, the volu- metric 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 ∗ Corresponding author: magnus.moe.nygaard@ife.no Preprint submitted to Acta Materialia May 27, 2019