While the overwhelming number of papers on multi-principal-element alloys (MPEAs) focus on the mechanical and microstructural properties, there has been growing interest in these alloys as solid-state hydrogen stores. We report here the synthesis optimization, the physicochemical and the hydrogen sorption properties of Ti_0.325V_0.275Zr_0.125Nb_0.275. This alloy was prepared by two methods, high temperature arc melting and ball milling under Ar, and crystallizes into a single-phase bcc structure. This MPEA shows a single transition from the initial bcc phase to a final bct dihydride and a maximum uptake of 1.7 H/M (2.5 wt%). Interestingly, the bct dihydride phase can be directly obtained by reactive ball milling under hydrogen pressure. The hydrogen desorption properties of the hydrides obtained by hydrogenation of the alloy prepared by arc melting or ball milling and by reactive ball milling have been compared. The best hydrogen sorption properties are shown by the material prepared by reactive ball milling. Despite a fading of the capacity for the first cycles, the reversible capacity of the latter material stabilizes around 2 wt%. To complement the experimental approach, a theoretical investigation combining a random distribution technique and first principle calculation was done to estimate the stability of the hydride.

A number of high-entropy alloys (HEAs) in the TiVZrNbHf system have been synthesized by arc melting and systematically evaluated for their hydrogen sorption characteristics. A total of 21 alloys with varying elemental compositions were investigated, and 17 of them form body-centered-cubic (bcc) solid solutions in the as-cast state. A total of 15 alloys form either face-centered-cubic (fcc) or body-centered-tetragonal (bct) hydrides after exposure to gaseous hydrogen with hydrogen per metal ratios (H/M) as high as 2.0. Linear trends are observed between the volumetric expansion per metal atom [(V/Z)_fcc/bct – (V/Z)_bcc/hcp]/(V/Z)_bcc/hcp with the valence electron concentration and average Pauling electronegativity (χ_p) of the alloys. However, no correlation was observed between the atomic size mismatch, δ, and any investigated hydrogen sorption property such as the maximum storage capacity or onset temperature for hydrogen release.

Al_0.10Ti_0.30V_0.25Zr_0.10Nb_0.25 was prepared to evaluate the effect of 10% aluminum into the previously reported quaternary alloy, Ti_0.325V_0.275Zr_0.125Nb_0.275. The as-cast quinary alloy formed a single-phase body centered cubic solid solution and transformed into a body centered tetragonal after hydrogenation. The alloy had a storage capacity of 1.6 H/M (2.6 wt.%) with fast absorption kinetics at room temperature, reaching full capacity within the first 10 min. The major improvements of Al addition (10%) were related to the desorption and cycling properties of the material. The temperature for hydrogen release was significantly decreased by around 100 °C, and the quinary alloy showed superior cycling stability and higher reversible storage capacity than its quaternary counterpart, 94% and 85% of their respective initial capacity, after 20 hydrogenation cycles without phase decomposition.

TTE (1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether) shows better performance than BTFE (bis(2,2,2-trifluoroethyl)ether as diluent in a localized highly concentrated electrolyte based on lithium bis(fluorosulfonyl)imide in triethylposphate.