Application of thin coatings of Zr and Ti on AlO3 granules using magnetron sputtering for metal hydride formation, accompanied by a numerical assessment of the hydrogenation process

Abstract

Hydrogen is an ideal energy carrier for future transportation, including automobile use. In this perspective, hydrogen storage represents an important challenge in advancing a hydrogen economy. The more sophisticated storage techniques, such as high-pressure gas or liquid, are inadequate for future storage objectives. The chemical or physical incorporation of hydrogen within alternative materials presents possible benefits compared to existing storage techniques. In this context, metal hydrides have garnered heightened interest as hydrogen storage materials because of their ability to achieve large volumetric energy densities, hence alleviating costs and safety issues. The significant differences in thermodynamic properties among various material classes make them highly versatile. Three-dimensional spherical metal hydrides offer a great opportunity due to their unique characteristics, particularly their high surface area and volume-to-surface ratio. The attributes make spherical structures highly appropriate for compact and reversible hydrogen storage, establishing them as prominent candidates for applications like fuel cells, portable energy devices, and renewable energy storage. [1,2]. This study examines the structural and chemical alterations in titanium (Ti) and zirconium (Zr) thin coatings on aluminium hydroxide (AlO3) granules before and after hydrogenation. The materials are subjected to hydrogenation at 400 °C and 5 atm of hydrogen pressure for 2 hours, with a hydrogen flow rate of 0.8 L/min. SEM analyses demonstrated significant morphological changes, including surface roughening, grain boundary separation, and microcrack formation, indicating the formation of metal hydrides. EDS analysis revealed decreased Ti and Zr concentrations during hydrogenation, likely due to hydride formation. XRD analyses confirmed the presence of hydride phases, with shifts in diffraction peaks indicating structural changes caused by hydrogen absorption. FTIR analysis revealed dihydroxylation, characterised by removing surface hydroxyl groups and forming new metal–hydride bonds, thereby confirming the structural alterations. The appearance of supplementary peaks in the 1100–1200 cm-1 range signifies the formation of metal hydrides, implying the inclusion of hydrogen. Mathematical modelling employing experimental values was performed to evaluate hydride production and hydrogen diffusion rates. The hydride conversion rates for Ti- and Zr-coated AlO3 granules were 3.5% and 1.6%, respectively. After 2 hours, the hydrogen penetration depths for Ti- and Zr-coated AlO3 granules were approximately 1200 nm and 850 nm, respectively. These results are in agreement with the experimental data. This study elucidates the effects of hydrogenation on the microstructure and chemical composition of Ti- and Zr-coated AlO3, providing insights into potential applications in hydrogen storage and related fields.