On the interplay of tetrahedral and octahedral hydrogen absorption sites in the Ti28V20Cr52 alloy: First principles study

Titanium–Vanadium–Chromium alloys have enormous potential for stationary hydrogen storage applications. However, they are also known to experience a loss of nearly half of their theoretical hydrogen storage capacity (~4 wt%) after the first charge-discharge cycle. Despite several approaches to overcome this handicap by eliminating defects or suppressing the intermediate BCT phase, there is still a lack of theoretical understanding of what causes hydrogen to remain trapped inside the alloy. In this computational work, we use first principles calculations to reproduce the absorption process of the Ti28V20Cr52 alloy. Our results show that hydrogen absorbed in octahedral positions in FCC phases induces the formation of two types of hydrogen, one tightly bonded in octahedral positions, and the other one, weakly bonded at tetrahedral sites and thus easily releasable. Additionally, internal octahedral to tetrahedral hydrogen migrations can lead to a quick degradation of the storage capacity of the alloy by increasing the overall strength of the metal-hydrogen bond.