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. • The authors have successfully designed and synthesised a new multi-principal element alloy with a controlled small amount of dopants of M = Al, Ti, V, Cr, Mn, Fe, Zr, Nb, Mo, Hf, & Ta, in order to modulate the high-performance hydride’s desorption temperature. • Most novel alloys adopt a BCC solid-solution (SS) structure and reversibly absorbs high quantities of hydrogen (1.80 - 1.98 H/M, or 2.74 - 3.23 wt%). • However, Fe 5 , Zr 5 , and Hf 5 alloys formed a secondary C14 phase due to the simultaneously high δ and %χ A . • The SS BCC alloys form an FCC dihydride phase on hydrogenation, which is fully reversible, and single-phase BCC alloys retain a high capacity after cycling with a slight a reduction in capacity after 5 cycles. Nevertheless, Synchrotron X-ray PDF analysis after the 1 st and 5 th cycles shows that the overall structural correlation is reduced, with the Q broad increase after cycling implying an increase in the number of crystalline defects. • Δ H abs of the alloys depends on M, with the most stable being Ti 5 and the least stable being Al 5 . The change in Δ H abs is substantial with as little as 5 at.% of variation in composition, without a significant decrease in the hydrogen capacity. This is a useful finding to design and produce materials with milder desorption requirements, improving the applicability of the alloys as hydrogen-storage materials. • Finally, the variation in M was shown to be a potential method to tune the hysteresis between absorption and desorption, Δ G hys , as this parameter linearly decreases with increasing VEC. This observation may be key for further developments in performance-tuning of solid-state hydrogen-storage materials, however, it needs further attention.