Activation of Ti-Fe-Cr alloys containing identical AB(2) fractions

Abstract

TiFe-based alloys are solid-state hydrogen storage materials operated at room temperature (RT). The current study presents a systematic approach for solving the activation issue (difficulty in the first hydrogenation), one of the major obstacles to the practical application of TiFe alloys, via the use of a secondary AB(2) phase. Based on the Ti-Fe-Cr ternary phase diagram, Ti-Fe-Cr alloys containing 80 at% Ti(Fe,Cr) (AB phase) and 20 at% Ti(Fe,Cr)(2) (AB(2) phase) were designed; the Cr concentrations in the AB and AB(2) phase were systematically varied while maintaining fixed phase fractions. Activation at RT was achieved when the overall Cr concentration was higher than 9.7 at%. Analysis of the activation characteristics of the individual phases revealed that the AB(2) phase readily absorbed hydrogen, thereby initiating activation of AB + AB(2) alloys. Notably, higher Cr concentrations enable the AB phase to absorb hydrogen at RT during the activation process, although the kinetics are much slower than that of the co-existing AB(2) phase. The equilibrium hydrogen pressures from the pressure-composition isotherms decrease as the Cr concentration increases, indicating that Cr stabilizes hydrides. Increased hydride stability may also promote the kinetics of the initial hydride formation in both the AB and AB(2) phases. An optimal composition for Ti-Fe-Cr alloys can be designed given the conditions of easy activation at RT and maximum reversible capacity within an operating pressure range. (C) 2021 Published by Elsevier B.V.