Design of TiVNb-(Cr, Ni or Co) multicomponent alloys with the same valence electron concentration for hydrogen storage

Abstract

Recently, it has been hypothesized that the hydrogen sorption properties of body-centered cubic (BCC) multicomponent alloys might be related to the alloy’s valence electron concentration (VEC). In this work, we employed CALPHAD method to design three BCC alloys with the same VEC to evaluate this hypothesis. The (TiVNb)85Cr15, (TiVNb)95.3Co4.7 and (TiVNb)96.2Ni3.8 multicomponent alloys with VEC = 4.87 were produced by arc melting. Although some amount of segregation could not be avoided, the as-cast microstructures were composed of a BCC phase with highly homogeneous chemical composition. The alloys quickly absorb hydrogen at room temperature and formed FCC hydrides with high capacities of 2 H/M (3.1–3.2 wt%). For the three alloys, it was observed a two-step hydrogenation sequence indicating the formation of an intermediate BCC monohydride prior to the formation of the FCC dihydride. Moreover, the three alloys presented very similar values of enthalpy and entropy of hydrogenation, showing that the initial hypothesis seems to hold for the thermodynamic properties. The alloys hydrogenation/dehydrogenation cycling behavior were also evaluated. (TiVNb)85Cr15 had a small and continuous drop in capacity over cycling that was addressed to incomplete transformation of the BCC monohydride to the FCC dihydride, whereas (TiVNb)95.3Co4.7 and (TiVNb)96.2Ni3.8 showed a more stable behavior achieving a reversible capacity of 1.76 H/M (2.77 wt%) after 20 cycles. The desorption is strongly affected by absorption/desorption cycling for all alloys, as revealed by thermo desorption spectroscopy (TDS). The onset temperature of desorption depends on the alloys’ composition as well as the number of hydrogenation/dehydrogenation cycles. The results suggest that cycling and desorption properties of the alloys are less related to their VEC.