Nanoscale composition modulations in MgyTi1−yHx thin film alloys for hydrogen storage

Abstract

IntroductionThe Mg-Ti-H system is currently attracting a lot of attention, with potential applications in very different fields. Niessen et al. [1] proposed the use of Mg-Ti thin films as high-capacity hydrogen storage materials for batteries. By means of electrochemical loading of Mg 0.8 Ti 0.2 thin films they found a gravimetrical storage capacity of 6.53 wt% H: w4 times higher than the commercially available NiMH batteries. In our group we demonstrated the possibility of gas loading of Pdcapped Mg y Ti 1Ày thin films. These films, when exposed to hydrogen gas exhibit fast and reversible transitions from the metallic to the hydrogenated state. The Ti doping of Mg greatly enhances the kinetics of hydrogen uptake and release. A 65 nm thick film of Mg 0.7 Ti 0.3 , exposed to 5% H 2 in Ar at room temperature, hydrogenates completely in w10 s; if it is then exposed to 20% O 2 in Ar, it fully desorbs in less than 3 min, returning to its original metallic state [2]. The role of Ti is to favor the formation of a face-centered cubic hydride phase of Mg y Ti 1Ày H x (for y < 0.87 [3]) instead of the tetragonal MgH 2 -like phase.The hydrogenation of Mg y Ti 1Ày thin films is also accompanied by dramatic optical changes, which make them suitable for application as hydrogen sensors [4] and smart coating for solar collectors [5]: when exposed to hydrogen gas, they exhibit fast and reversible (>100 cycles) optical transitions

Divacancies and the hydrogenation of Mg-Ti films with short range chemical order

“…Instead, a partial chemical segregation into Mg and Ti domains on a length scale of the order of less than 10 nm was suggested. 4 The stabilization of the cubic fluorite phase is thought to occur via elastic coupling of MgH 2 to the nanoscale TiH 2 domains. This coupling remains effective during the full ͑de-͒hydrogenation cycle because of the similarity of the unit cell volumes of cubic TiH 2 and hexagonal closed packed Mg in the intermediate state.…”

“…This coupling remains effective during the full ͑de-͒hydrogenation cycle because of the similarity of the unit cell volumes of cubic TiH 2 and hexagonal closed packed Mg in the intermediate state. 4 A direct observation of these phase segregated domains, however, is currently lacking.Furthermore, the fast hydrogen kinetics of these types of films-especially in the cubic hydride phase formed at Ti concentrations larger than 15%-is not well understood. The metal hydride lattice contracts upon increased incorporation of Ti, leading to reduced space for hydrogen mobility.…”

“…The metal hydride lattice contracts upon increased incorporation of Ti, leading to reduced space for hydrogen mobility. 3,4 It was suggested that the unoccupied octahedral positions in the fluorite MgH 2 structure could aid the hydrogen mobility. 6 Ab initio studies indicate that vacancies form a key factor in the hydrogen diffusion of both the rutile and fluorite MgH 2 phases.…”

“…1 The Mg-Ti͑-H͒ films have an intriguing microstructure. While Ti and Mg are immiscible metals on a macroscopic scale, codeposition of Mg and Ti by magnetron sputtering leads to Mg-Ti films with a coherent structure, as indicated by x-ray and electron diffraction studies, [3][4][5] and a very high degree of intermixing of the Mg and Ti. Extended x-ray absorption fine structure ͑EXAFS͒ studies, in contrast, indicated that atomically mixed alloys are not formed.…”

“…This coupling remains effective during the full ͑de-͒hydrogenation cycle because of the similarity of the unit cell volumes of cubic TiH 2 and hexagonal closed packed Mg in the intermediate state. 4 A direct observation of these phase segregated domains, however, is currently lacking.…”