Evaluation of Hydrogen Storage Performance of Ti0.25Zr0.25V0.15Nb0.15Ta0.2 High-Entropy Alloy Using Calorimetric Technique

The features of phase transformations during interaction of BCC-type Ti0.25Zr0.25V0.15Nb0.15Ta0.2 high-entropy alloy (HEA) with hydrogen and the accompanying thermal effects have been examined using the Tian-Calvet calorimetric technique. The alloy was produced by the pendent drop melt extraction method in the form of microfibers and then coated with a thin layer of palladium to eliminate the preliminary high-temperature activation and enable first hydrogenation at room temperature. Two reaction stages were distinguished on the measured heat emission curves. Using XRD data, these stages were attributed to the formation of a BCC-type hydrogen solid solution and a phase transition to FCC-type dihydride phase. Recorded values of the reaction enthalpy decrease in absolute terms from 145 kJ/mol H2 (dilute solid solution) to 73-77 kJ/mol H2 (BCC -> FCC) and to 20 kJ/mol H2 (H solution in FCC dihydride). The overall heat emission of the entire process as a whole is 110 kJ/ mol H2. The course of the dehydrogenation process depends strongly on the applied temperature and at 493 K results in the formation of a BCT-type monohydride, which was not detected during hydrogenation. The detailed calorimetric data obtained here significantly change the previously existing ideas about thermal effects during this type HEAs hydrogenation, based on calculations using the Van't-Hoff equation. The reported findings are of special importance for various hydrogen related applications of HEAs