Evaluating some design criteria for TiFe-based ternary hydrogen storage alloys

Elemental substitution is widely practised to fabricate TiFe-based alloys with enhanced hydrogen storage characteristics. While some hydrogen storage properties can be improved others are usually adversely affected by this method. Hence, establishing sensible alloying design criteria is important for further developing Ti-Fe-based alloys. Here, the effects of substitutional elements in Ti-Fe-based ternary alloys are assessed based on several factors such as hydrogen bond dissociation energy, lattice parameters, interstitial site size, and intermetallic phase formation. Our calculations show that all the substitutional elements studied in this work (except Ni and Co) increase the lattice parameter compared to the TiFe alloy, while they have a diverse effect on the interstitial site sizes. The thermodynamic stability of the alloys could not be adequately described based on the lattice parameter and factors such as interstitial site sizes along the local atomic environment should be also considered. Hydrogen storage capacity of the Ti-Fe-based ternary alloys also appears to be influenced by the metal-hydrogen bond dissociation energy of the substitutional elements and their capability to form intermetallic phases with Ti. TiFe-based ternary alloys were modified by V and Zr substitutions according to the above criteria. The modified alloys were mostly composed of the B2 TiFe phase, offering significantly easier activation and a higher reversible hydrogen storage capacity compared to the binary alloy.