| dc.description.abstract | Hydrogen energy is considered as the most promising clean and renewable alternative energy source, owing to its high energy density, low carbon emissions, and abundance. However, one of the challenges in the realization of a hydrogen economy lies upon the effective storage of hydrogen. As compared with the pressurized gaseous hydrogen and liquid hydrogen storage techniques, solid-state hydrogen storage offers advantages such as relatively higher hydrogen storage capacity and energy efficiency, lower costs, and increased safety. The emergence of high-entropy alloys (HEAs) in the past two decades has provided a promising means to store hydrogen effectively, owing to their severe lattice distortion and cocktail effects. In particular, the unlimited possibilities of the alloy and composition designs, as well as microstructure engineering, offer very attractive approaches for them to be valuable candidates for hydrogen storage in different applications. The concept of using novel high-entropy alloys for hydrogen storage is explored in this study.
The main focus of this study has been to investigate the low-pressure hydrogen storage properties of Ti42Zr35Ta3Si5Co12.5Sn2.5 HEA fabricated via atomization process and vacuum arc melting process. Additionally, it aims to optimize hydrogen storage performance by utilizing Ti-V as a base to readjust the composition of this HEA. Initially, HEA design was carried out based on thermodynamic considerations, such as mixing enthalpy (ΔHmix), mixing entropy (ΔSmix), and the degree of lattice distortion (δ), valence electron concentration (VEC), as well as the affinity of the elements with hydrogen. The surface element distribution and surface morphology were confirmed using Electron Probe Microanalyzer (EPMA). Subsequently, X-Ray Diffraction (XRD) and Transmission Electron Microscope (TEM) were employed to examine the crystalline structures and phase transformations of the HEAs before and after hydrogen absorption. Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC) were utilized to determine the dehydrogenation temperature and melting point of the materials. Finally, the hydrogen storage behavior of the two types of HEAs, including Pressure-Composition Temperature (PCT) curves and kinetic curves, was investigated using a Sieverts apparatus. Furthermore, High Resolution Neutron Powder Diffraction (HRNPD) testing at the Australian Nuclear Science and Technology Organization (ANSTO) was performed to study the hydrogen storage positions and lattice changes.
It was found here that the multiphase Ti42Zr35Ta3Si5Co12.5Sn2.5 HEA achieved a hydrogen storage capacity of 0.6 wt.% at 400°C and 0.00001 bar of hydrogen charging pressure, reaching a maximum of 2.15 wt.% at 45 bar. However, its complete dehydrogenation temperature was as high as 800°C, rendering it unsuitable for normal operation conditions. By introducing V to reduce hydrogen absorption temperature and utilizing Mo and Cr as BCC stabilizers, a single-phase BCC Ti36V11Ta16Mo21Cr16 HEA has been successfully designed. The modification effectively lowers both hydrogen absorption temperature and pressure. The Ti36V11Ta16Mo21Cr16 exhibited a hydrogen storage capacity of 0.7 wt.% at room temperature, under a pressure of 0.00001 bar; with a maximum of 1.9 wt.% of hydrogen uptake achieved at only 36 bar, and its complete dehydrogenation temperature is only 500°C. Both materials can reach maximum hydrogen storage capacity within 10 minutes, with material costs only about a quarter of those reported for the HEA like TiZrVNbHf in the literature. This study has confirmed that the BCC HEA structure is effective for hydrogen storage and has identified a low-cost HEA for this purpose. The results demonstrate that alloying design can enhance hydrogen storage properties, corroborating other research that hydrogen can be absorbed by HEAs at very low pressures, thus promoting a solid-state hydrogen storage material which serves as one of the sustainable green energy solutions to replace fossil fuels and potentially reduce global warming in the future. | en_US |