Compositional effects on the hydrogen cycling stability of multicomponent Ti-Mn based alloys

Abstract

Intermetallic alloys such as AB, AB2, and AB5 type have been studied due to their capability to reversibly store hydrogen. These alloys exhibit varying hydrogen storage properties depending on the crystal structure and composition. Compositional modification is commonly known as an effective method to modify the alloys thermodynamic and kinetics for various applications such as metal hydride batteries, metal hydrides hydrogen storage and compression. However, the effects of the compositional modification on the cyclic stability of these alloys are not usually well studied.Here, the hydrogen cycling stabilities of Ti-Mn based alloys with C14 type structure are studied. Hyper-stoichiometry, stoichiometry and hypo-stoichiometry alloys were prepared accordingly: Ti30.6V16.4Mn48.7 (Zr0.7Cr0.8Fe2.8) (B/A = 2.19), Ti32.8V15.1Mn47.1 (Zr0.9Cr1.2Fe2.9) (B/A = 1.97) and Ti34.5V15.4Mn44.7 (Zr0.9Cr1.3Fe3.2) (B/A = 1.87). Whilst the hyper-stoichiometry alloy showed almost a stable (about 9% capacity reduction) hydrogen capacity after 1000 cycles of hydrogenation and dehydrogenation, the stoichiometry and hypo-stoichiometry alloys failed to hydrogenate after about 950 and 500 cycles respectively. A limited reduction in the calculated crystalline size of the alloys was observed before and after the hydrogen cycling, denoting that pulverisation plays a less significant role on the observed hydrogen capacity loss. In addition, a reduction in the B/A ratio from 2.19 to 1.82 (hyper to hypo-stoichiometry) encouraged the formation of more stable hydride and a higher level of heterogeneous lattice strain. Whilst a small loss of hydrogen capacity (9%) in the hyper-stoichiometry alloy was attributed to the trapped hydrogen, the complete loss of hydrogen capacity in the stoichiometry and hypo-stoichiometry alloys seemed to originate from the formation of stable hydride and the lattice distortion.