Compositional effects on the hydrogen storage properties in a series of refractory high entropy alloys

The possible combinations in the multidimensional space of high entropy alloys are extremely broad, which makes the incremental experimental research limited. As a result, establishing trends with well-known empirical parameters (lattice distortion, valence electron concentration etc.) and predicting effects of the chemical composition change are vital to guide future research in the field of materials science. In this context, we propose a strategy to rationalize the effect of chemical composition change on the hydrogen sorption properties in a series of high entropy alloys: Ti0.30V0.25Zr0.10Nb0.25 M (0.10) with M = Mg, Al, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Ta and null (corresponding quaternary alloy). All materials are bcc alloys and absorb hydrogen at room temperature forming fcc or pseudo-fcc dihydride phases. The maximum hydrogen storage capacity at room temperature strongly depends on the valence electron concentration (VEC) of the alloys: the capacity is high (1.5-2.0 H/M) for low values of VEC (< 4.9) whereas, a drastic fading is observed for VEC & GE;4.9 which is the case for alloys with M being a late 3d transition metal. The structural analysis suggests that steric effects might not be responsible for this trend and electronic reasons may be invoked. Increasing the VEC by alloying with late 3d transition metals will fill the unoccupied valence states and the electrons from interstitial hydrogens can no longer be accommodated, which is unfavorable for hydrogen storage. Moreover, the onset temperature of desorption increases almost linearly with VEC for this composition series. These findings suggest that alloys with low VEC are more likely to become promising candidates for hydrogen storage