High- and Low-Density High Entropy Alloys for Hydrogen Storage

Hydrogen is considered a secondary energy carrier and is expected to contribute to the transition from a fossil fuel to a renewable energy system. Under normal conditions, hydrogen is a colorless and odorless gas, and above 4 vol.% in the presence of oxygen results in an explosive atmosphere. These properties raise safety issues and complicate its storage concerning its low volumetric energy density as a gas. Therefore, the development of technologically viable storage possibilities is essential. A high potential exists in metal hydrides, which can be used for rechargeable batteries, hydrogen storage, hydrogen compression, and as thermal storage. The storage within the metal hydrides is based on chemical nature, in which the hydrogen is stored in the crystal lattice of the metal and can be released when required. However, a disadvantage of metal hydrides is that mostly heavy elements are used as storage materials, which reduces the gravimetric storage capacity or requires very high temperatures to recover the hydrogen. These properties are challenging for certain applications, such as in the transportation sector or for energy-efficient storage. To increase gravimetric storage capacity, lighter elements in the way of alloys can be used. One class of alloys that represent an interesting option is high entropy alloys. In contrast to bimetallic alloys, high entropy alloys consist of at least four to five elements in equiatomic or nearly equiatomic proportions. Mixing multiple elements in near-equiatomic ratios increases the configuration entropy to overcome the enthalpies of compound formation (intermetallic phase). Accordingly, the formation of a concentrated disordered solid solution consisting of the elements used would be favored and the presence of multiple phases could be suppressed. Considering that the hydrogen storage properties of metal hydrides strongly depend on the phase and the chemical composition, high entropy alloys with their high number of compositions open the way for chemical tunability of hydrogen storage properties.  Consequently, this work focuses on the experimental synthesis of different high entropy alloys, specifically using light metals such as lithium, aluminum, and magnesium, to pursue increasing the gravimetric storage capacity. For this purpose, a detailed evaluation of the structural and hydrogen storage properties was carried out.

In the first part of the work, the new material composition AlLiMgTiZr-Cx was synthesized by mechanical alloying. The first experiments have shown that process control agents such as toluene, n-hexane, and ethanol are necessary for powder processing to suppress an enhanced cold welding effect. Based on time-dependent experiments, it was found that the hydrocarbons decompose, resulting in the formation of a rocksalt-like crystal structure of a high entropy carbide, with zirconium hydride as the intermediate phase. Attempts to suppress the decomposition of the process control agents by reducing the grinding speed or using ZrH2 as the reactant were unsuccessful. The reduction of the milling speed from 1200 to 800/600 rpm showed only a change in the conversion rate, and further reduction to 400 rpm did not provide enough energy for the alloying process. The attempt to suppress the decomposition by using ZrH2 as a reactant also ended in a rocksalt-like fcc structure, with an initial hydrogen content of 1.56 wt.% in the synthesized carbide detected via thermal gravimetry. Subsequent absorption experiments showed that the nanocrystalline (~3 nm) metal carbide can reversibly store up to 0.66 wt.% hydrogen at 150 °C and exhibits high phase stability during the first three cycles.

The work's second part focuses on the systematic investigation of Ti0.325V0.275Nb0.275Zr0.125 compounds, where aluminum and lithium have been incorporated explicitly into MTi0.3V0.25Nb0.25Zr0.1 alloys with M = Li0.01, Al0.01 or Al0.05Li0.05. It was found that lithium (Li0.1Ti0.3V0.25Nb0.25Zr0.1) positively affects the gravimetric storage capacity (~ 2.6 wt.%), but the solid solution phase is destabilized, and disproportionation of the initial phase to the multiphase material occurs over several cycles of hydrogen storage. Aluminum in Al0.1Ti0.3V0.25Nb0.25Zr0.1 was found to destabilize the metal hydride phase by reducing hydrogen capacity (~2.0 wt.%). The combination of aluminum and lithium in Al0.05Li0.05Ti0.3V0.25Nb0.25Zr0.1 led to the element's influence's compensation, so the properties appeared comparable to the quaternary compound studied. In addition, it was identified that reactive milling under a hydrogen atmosphere resulted in smaller particles than mechanical alloying due to the more brittle properties of the hydride phases and consequently reduced the dehydrogenation temperature (onset ~150 °C). Moreover, compared to the mechanically alloyed samples, the storage capacity was increased by reactive milling, which was observed with an increased phase transformation from bcc to fcc due to hydrogen uptake.


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