In general, MgH2 phase transformation is assumed to be a hydrogen diffusion-controlled process e.g. in thin film system; however, in our work, through STEM-EELS and a cross-sectional view, we have shown that nucleation and growth-dependent phase transformation is indeed possible in these systems in addition to hydrogen diffusion controlled transformation, and one must consider this to understand the hydrogenation kinetics in the Mg thin film system and thereby to improve for its application. Moreover, we investigate this in detail, as it can affect the growth and propagation of MgH2 during hydrogenation. Additionally, we demonstrate that the Mg phase transformation with nucleation can occur at any interface within the Mg thin film system, regardless of the direction by which hydrogen enters the system. We also demonstrate that even in an Mg system with low H2 solubility, an oversaturation occurs for a nucleation and growth process. In contrast, dehydrogenation follows an H2 diffusion-controlled process without nucleation, and Mg formation always happens near the Pd interface, as Pd acts as a catalyst for dehydrogenation and a pathway for hydrogen release. In conclusion, both (de)hydrogenation follow an asymmetric pathway for phase transformation. Moreover, hydrogenation leads to the collapse of the columnar structure of the Mg thin film into a nanoparticulate structure. This is due to the 30% volume expansion of the material, resulting in stress buildup from hydrogen incorporation and a transformation of the crystal structure from hexagonal close-packed (HCP) to tetragonal. Moreover, the structure collapses after dehydrogenation into a porous/void structure. We also inferred that we do not always observe the same exact hydrogenation mechanisms in in-situ and ex-situ analyses. It shows that any alterations in the annealing and conditions of the samples, which are influenced by both in situ and ex-situ setups, modify the sample boundary conditions. This affects its boundary conditions, which plays a critical role in the mechanism.
In the MgTi system, we observed that the MgTi hydride phase transformation is significantly faster than that of Mg, due to improved kinetics. However, it was unclear why the MgTi system exhibited greater cyclability and why its structure and morphology remained unaffected by the transformation accompanying ~30% volume expansion, which can lead to stress within the system. To understand this, we used TEM to hydrogenate in in-situ conditions and observe the morphology, hydride transformation, nucleation, and growth using plasmon peak shift in EELS. Unfortunately, since the process of hydride formation in MgTiHx is very rapid, we were unable to observe nucleation and growth, as the hydride phase transforms immediately throughout the sample once hydriding begins. We observed that MgTi, in can be (re) hydrided multiple times without any morphological changes, and the initial columnar structure of MgTi remains intact, unlike that of Mg. Most importantly, we observed deformation twinning during hydrogenation in MgTi, which was absent in pure Mg. This phenomenon involves volume expansion during hydride formation, which maintains the structural morphology, making it stable for the next cycle. This indicates a crucial reason for the stability and cyclability of MgTi compared to Mg. The improvement in kinetics is associated with the metal hydride fluorite-based crystal structure observed during in situ hydrogenation in STEM, as previously indicated in the literature. We also incorporated in-situ 4DSTEM to understand the structural transformation that occurs before and after hydrogenation, which is currently being evaluated to determine the effect of grain orientation on phase transformation. The dehydrogenation is similar to Mg and starts from Pd interface. However, we have to analyze the repeatability of the same to confirm.
The results were shared through poster and oral presentations at key stages of the project, primarily at three types of conferences: 1. Materials Science (e.g. MRS, USA),2. Electron Microscopy (e.g. EMS, SCANDEM, and MRS EM symposia), and 3. Hydrogen-focused (e.g. Gordon Research Conference), as well as at the host institute’s user meeting. The sub-project with similar objectives was also added to student project banks to inspire thesis work, and additional presentations were given at KIT during group meetings. While there is no current IP or commercial output, the experience is being used to develop a hydrogen-focused research proposal in collaboration with industry, aiming at TRL development and joint Denmark–India funding opportunities.
