Hydrogenation properties of the high-entropy alloy series (TiVCrNb)0.95M0.05

The multi-principal element alloy TiVCrNb is considered particularly suitable for hydrogen storage thanks to its high gravimetric capacity (3.23 wt%) but it suffers from unfavourable thermodynamics. To remedy this, the alloy series (TiVCrNb) 0.95 M 0.05 (M = Al, Ti, V, Cr, Mn, Fe, Zr, Nb, Mo, Hf, & Ta) is reported here to assess the variation of structural and hydrogen-storage properties with the addition of only 5 at.% of various dopant elements, M. Addition of further elements M = Fe, Zr, and Hf yields multiphase materials, i.e. a majority BCC solid-solution phase coexists with a minor C14 Laves phase. The alloys with M = Al, Ti, V, Cr, Mn, Nb, Mo, and Ta form single-phase BCC solid solutions and rapidly absorb hydrogen at 25 °C forming an FCC dihydride phase. The maximum capacity of the (TiVCrNb) 0.95 M 0.05 alloys varies between 1.80 - 1.98 H/M (2.74 - 3.23 wt%). Pressure-composition isotherms were used to determine the thermodynamics of absorption and desorption of the dihydride phase. A destabilisation of the dihydride was found for most of the alloys and steric and electronegativity effects can be invoked to explain this trend. Furthermore, the hysteresis between absorption and desorption was found to decrease with increasing the valence-electron concentration in this series of alloys, which may aid future design and development of multi-component materials for hydrogen storage.