Al-Containing High-Entropy Alloys and High-Pressure Sievert’s Apparatus for Hydrogen Storage and Compression

To develop more efficient hydrogen storage and compression solutions using metal hydrides, incorporating light elements in the material’s composition is essential for increasing gravimetric capacity. Additionally, hydrogen storage in hybrid tanks (a combination of metal hydrides with pressurized hydrogen) and compression applications demand metal hydrides that can operate under high-pressure conditions. In this context, high-entropy alloys (HEAs) offer a promising approach, as they make it possible to add light elements in a high range of concentrations and, at the same time, fine-tuning the thermodynamic properties of hydride formation to be compatible with high-pressure regimes. To fully explore the extensive compositional possibilities of HEAs, effective strategies and tools for alloy design must be developed, optimized, and employed. Additionally, while studying metal-hydrogen systems at high pressures (above 200 bar) holds great potential for advancing hybrid hydrogen storage and compression technologies using metal hydrides, significant challenges remain due to the limited availability of analytical techniques that can operate in this pressure range. In this thesis, it was investigated the phase formation and hydrogen storage properties of AB2-type Al-containing HEAs from the C14 Laves-forming alloy systems TiZrAlCrMn, TiZrAlCrFe, and TiZrAlMnFe. Initially, two sets of alloys from each alloy system were analyzed: equiatomic alloys, which contained the highest concentrations of all elements in the composition, and non-equiatomic alloys, which had reduced concentrations of Zr and Al. The non-equiatomic alloys presented a more homogeneous microstructure after synthesis and absorbed higher amounts of hydrogen due to the less pronounced slopes in the pressure-composition-isotherms. The difference in the hydrogen sorption properties was initially attributed to the difference in microstructural homogeneity. The second part of the analysis was concerned with investigating the influence of Al on hydrogen storage properties in more detail. By systematically substituting Mn for Al in three alloy compositions (Ti0.29Zr0.05Al0.05Cr0.26Mn0.35, Ti0.29Zr0.05Al0.10Cr0.26Mn0.30, Ti0.29Zr0.05Al0.15Cr0.26Mn0.25), it was observed that higher aluminum content led to steeper isotherm slopes, which significantly reduced the hydrogen sorption capacity. From the alloys investigated, the composition Ti0.29Zr0.05Al0.05Cr0.26Mn0.35 (with an average molar mass of 52.54 g mol-1) absorbed the highest amount of hydrogen (H/M = 0.94, 1.76 wt.% H2) with thermodynamic properties that make this alloys a promising candidate for further hydrogen storage and compression studies. The presented analysis indicates aluminum can increase gravimetric capacity in this kind of alloy; however, high concentrations of this element lead to the opposite effect. Based on the experimental results of Al-containing HEAs investigated in this thesis, optimization was performed on a thermodynamic model for calculating isotherms of C14 Laves-forming alloys that absorb hydrogen via solid solution. After optimization, the model showed good predictive capability despite some limitations concerning temperature variation. The optimized thermodynamic model is suggested to guide the HEA design process for hydrogen storage and compression. In this thesis, the design, construction, and validation of a high-pressure Sievert’s apparatus that can be used to perform hydrogen absorption and desorption experiments up to 400 bar of hydrogen pressure was presented. Being able to measure at pressures up to 400 bar, along with the heating and cooling system integrated into the apparatus, enables the measurement of a wide range of metal-hydrogen systems.