Title : Thermodynamics of AB2 type hydrogen storage alloys with different composition
Abstract:
Hydrogen is increasingly recognized as one of the most promising substitutes for fossil fuels because of its clean combustion, negligible greenhouse gas emissions, and high energy output per unit mass. However, a major barrier to widespread adoption of hydrogen technology is safe and efficient storage. Conventional techniques, such as cryogenic tanks and high-pressure cylinders, are hindered by cost, safety, and energy efficiency concerns. In contrast, metal hydrides offer a compact, secure, and reversible storage approach by absorbing hydrogen into solid materials. Among them, Ti-based AB₂ alloys are particularly attractive due to their large volumetric capacity, moderate plateau pressures, and ability to form stable hydrides.
The performance of Ti-Fe based AB₂ alloys can be tuned by incorporating alloying elements such as chromium (Cr) and manganese (Mn). Mn generally improves the kinetics of hydrogen absorption, while Cr stabilizes phases and adjusts plateau pressures. However, optimizing these substitutions remains challenging since each element alters the thermodynamic characteristics differently. The combined effects of Cr and Mn on cycling durability, hysteresis, and plateau pressure are not yet fully understood.
In this study, three alloys—TiCr₁.₁Mn₀.₃Fe₀.₆, TiCr₁.₃Mn₀.₁Fe₀.₆, and TiCr₁.₅Mn₀.₁Fe₀.₄—were synthesized by arc melting followed by long-term annealing to promote a homogeneous C14 Laves phase structure. Hydrogen absorption–desorption behavior was evaluated using pressure–composition–temperature (PCT) analysis, along with activation tests under controlled hydrogen pressures and temperatures. Stability was assessed over 25 hydrogenation–dehydrogenation cycles. Microstructural and phase changes were examined using X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Thermodynamic parameters, including enthalpy and entropy, were derived from Van’t Hoff analysis.
The results revealed that TiCr₁.₅Mn₀.₁Fe₀.₄ exhibited the highest hydrogen storage capacity (≈0.75–0.78 wt%) with stable plateau pressures, making it the most balanced composition thermodynamically. TiCr₁.₃Mn₀.₁Fe₀.₆ showed slightly reduced capacity due to higher Cr and Fe content, but all alloys demonstrated good activation and rapid hydrogen uptake. Notably, none of the alloys displayed significant phase separation or particle coarsening after 25 cycles, indicating strong structural integrity and retention of storage capacity. Compression cycling experiments further confirmed their resilience: TiCr₁.₅Mn₀.₁Fe₀.₄ required 245 °C to reach 12 MPa hydrogen pressure, while TiCr₁.₁Mn₀.₃Fe₀.₆ and TiCr₁.₃Mn₀.₁Fe₀.₆ required 260 °C and 305 °C, respectively.
Overall, these findings highlight that careful adjustment of Cr, Mn, and Fe contents in Ti-Fe based AB₂ alloys can significantly enhance hydrogen absorption capacity, activation, and cycling stability. The studied alloys, particularly TiCr₁.₅Mn₀.₁Fe₀.₄, show strong potential for application in future solid-state hydrogen storage and thermochemical compression systems, advancing the practical use of hydrogen as a clean energy carrier.
