Research and Development of New High-Entropy Alloys for Hydrogen Storage †

: Hydrogen is a key element in the changing energy sector and presents an accessible alternative to conventional fossil fuel sources. In this work, a system of ten high-entropy alloys was prepared based on the Hume-Rothery rules. One of the biggest advantages of these alloys is their storage capacity, which reaches the highest value among all known alloys intended for hydrogen storage. Alloys based on Al-Ti-Nb-Zr elements with different atomic fractions show interesting accumulation capabilities with fast absorption kinetics and low specific gravity. Each alloy in this study underwent high-pressure gravimetric absorption and desorption tests. The main goals of this work were to prepare alloys with the lowest-possible specific gravity and the highest-possible storage capacity. One alloy from our system shows storage capacity values similar to commercial alloys, without any rare-earth elements.


Introduction
Hydrogen is currently the major topic worldwide because it can produce electricity very efficiently and without emissions.Therefore, it is ideal choice for powering vehicles such as cars and buses, as well as for various other modes of transportation, including shipping, aviation, and space transport.It has great potential for use as an alternative fuel, but only if it can be stored safely and efficiently.Agreements such as Clean Europe or Net Zero Emission by 2050 Scenario support actions to reduce CO 2 and increase carbon neutrality [1,2].An "excellent" step forward in this direction is represented by the publication of M. Sahlberg et al. entitled "Superior hydrogen storage in high entropy alloys" [3].In this study, the authors investigated the hydrogenation of a high-entropy alloy in a TiVZrNbHf solid solution with a BCC structure and found that it is possible to absorb an extremely large amount of hydrogen (2.7 wt.% hydrogen) this way.The amount of hydrogen corresponded to an H/M ratio of 2.5 and produced a world-record volumetric energy density of 219 kg H/m 3 [3].High-entropy alloys are utilized for hydrogen storage due to their low specific weight, making them well-suited for applications in the automotive industry.High-entropy alloys are defined as a mixture of three to five elements with an atomic ratio ranging from 5 to 35%.Newer definitions also accept alloys with less than three main elements and an atomic ratio higher than 35%.It is evident from existing publications that high-entropy alloys can store a higher amount of hydrogen compared to known metal hydrides.

Materials and Methods
Alloys were prepared based on a predictive model by arc melting (Mini Arc Melting System MAM-1) from highly pure elements (>99.9 at.%) in a protective gas atmosphere Eng.Proc.2024, 64, 9. https://doi.org/10.3390/engproc2024064009https://www.mdpi.com/journal/engproc(Ar with purity 99.999%).Each sample was remelted five times to ensure homogeneity.Hardness analyses were performed on these alloy samples using a Wilson-Wolper Tukon 1102 Vickers indenter with a load of 0.3 kgHV 0.3 .Subsequently, the samples underwent vibration grinding, resulting in a grain fraction below 45 µm.An EDX analysis was conducted on the prepared powder sample using a Tescan Vega-3LMU, Bruker Nano XFlash detector 410 m (Tescan, Brno, The Czech Republic) in secondary mode at an accelerating voltage of 20 kV.X-ray diffraction was performed using a Philips X Pert Pro (Malvern Panalytical, Almelo, The Netherlands), and the specific gravity was determined using the pycometric method (AccuPyc II 1345, (Micromeritics, Norcross, GA, USA)).The powder material, thus prepared and characterized, underwent high-pressure gravimetric analysis to assess its hydrogen absorption and desorption at the Institute of Nonclassical Chemistry, Leipzig.The sorption experiment consisted of four steps: The first step involved activation the sample by heating it to 350 • C for two hours in a vacuum.This was followed by the first cycle of absorption under conditions of 200 • C and a pressure 2 × 10 6 Pa of hydrogen.The next step was desorption at 370 • C in a vacuum.The final step involved a second absorption cycle under the same conditions as the first cycle

Results
Figure 1 shows a prediction map of the prepared alloys created based on Hume-Rothery's rules.The map is divided into two regions, namely the region of high-entropy phases and the region of amorphous phases.The alloys do not fit into the region of highentropic phases but exhibit high-entropy properties.All of the produced alloys have a single-phase BCC structure.

Materials and Methods
Alloys were prepared based on a predictive model by arc melting (Mini Arc Melting System MAM-1) from highly pure elements (>99.9 at.%) in a protective gas atmosphere (Ar with purity 99.999%).Each sample was remelted five times to ensure homogeneity.Hardness analyses were performed on these alloy samples using a Wilson-Wolper Tukon 1102 Vickers indenter with a load of 0.3 kgHV0.3.Subsequently, the samples underwent vibration grinding, resulting in a grain fraction below 45 µm.An EDX analysis was conducted on the prepared powder sample using a Tescan Vega-3LMU, Bruker Nano XFlash detector 410 m (Tescan, Brno, The Czech Republic) in secondary mode at an accelerating voltage of 20 kV.X-ray diffraction was performed using a Philips X Pert Pro (Malvern Panalytical, Almelo, The Netherlands), and the specific gravity was determined using the pycometric method (AccuPyc II 1345, (Micromeritics, Norcross, GA, USA)).The powder material, thus prepared and characterized, underwent high-pressure gravimetric analysis to assess its hydrogen absorption and desorption at the Institute of Nonclassical Chemistry, Leipzig.The sorption experiment consisted of four steps: The first step involved activation the sample by heating it to 350 °C for two hours in a vacuum.This was followed by the first cycle of absorption under conditions of 200 °C and a pressure 2 × 10 6 Pa of hydrogen.The next step was desorption at 370 °C in a vacuum.The final step involved a second absorption cycle under the same conditions as the first cycle

Results
Figure 1 shows a prediction map of the prepared alloys created based on Hume-Rothery's rules.The map is divided into two regions, namely the region of high-entropy phases and the region of amorphous phases.The alloys do not fit into the region of highentropic phases but exhibit high-entropy properties.All of the produced alloys have a single-phase BCC structure.Table 1 describes theoretical high-entropy properties that define the properties of the alloy and its storage capacity, where ΔHmix a ΔSmix are mixing enthalpy and enthropy, δr is a parameter of the difference in the size of the atomic diameters, and VEC is valence electron concentration [4,5], which were calculated in a program created by us in the Matlab programming language [6].Table 2 describes the basic characteristics of the alloys, such as the results of the EDX analysis, microhardness, and specific gravity.One of the main goals of this work is to prepare a material with the lowest-possible specific gravity with Table 1 describes theoretical high-entropy properties that define the properties of the alloy and its storage capacity, where ∆H mix a ∆S mix are mixing enthalpy and enthropy, δr is a parameter of the difference in the size of the atomic diameters, and VEC is valence electron concentration [4,5], which were calculated in a program created by us in the Matlab programming language [6].Table 2 describes the basic characteristics of the alloys, such as the results of the EDX analysis, microhardness, and specific gravity.One of the main goals of this work is to prepare a material with the lowest-possible specific gravity with regard to future applications.These alloys achieve significantly lower specific densities compared to commercial alloys such as LaNi 5 [7].The alternative alloy names describe the precise atomic ratios after arc melting and EDX analysis.The measurement scheme using high-pressure thermogravimetric analysis is shown in Figure 2. The results of the high-pressure analysis of hydrogen absorption and desorption on the Al 15 Ti 38 Nb 23 Zr 24 alloy are divided into four graphs, which are shown in Figure 3.The first deals with the change in corrected weight and temperature during individual measurement steps.The following graph describes the actual absorption or desorption of hydrogen during the measurement of how many milligrams of hydrogen were stored in one gram of the sample.The third graph describes the change in the hydrogen-to-metal atom ratio (H/M ratio).The last graph shows the absorption kinetics.The yellow area shows unbalanced conditions, such as a change in pressure or temperature.
Table 3 describes the values of absorbed, residual, and desorbed hydrogen, with the last column indicating the efficiency of the cycle.The highest storage capacity was achieved by the Al 15 Ti 38 Nb 23 Zr 24 sample, with a value of 1.61 wt.% and an H/M ratio of 1.05.Table 3 describes the values of absorbed, residual, and desorbed hydrogen, last column indicating the efficiency of the cycle.The highest storage capac achieved by the Al15Ti38Nb23Zr24 sample, with a value of 1.61 wt.% and an H/M 1.05.Table 3 describes the values of absorbed, residual, and desorbed hydrogen, with the last column indicating the efficiency of the cycle.The highest storage capacity was achieved by the Al15Ti38Nb23Zr24 sample, with a value of 1.61 wt.% and an H/M ratio of 1.05.

Conclusions
Based on the prediction theory compiled according to the Hume-Rothery rules, the emergence of high-entropy phases was assumed.Most of the alloys show the one BCC phase, which was partially confirmed via X-ray diffraction.Samples with the elemental composition Al-Ti-Nb-Zr align with our goal of reducing specific weight, with the highest specific weight being recorded for the Al 20 Ti 25 Nb 25 Zr 30 sample at 6.15 g.cm −3 .All samples underwent high-pressure gravimetric analysis for hydrogen absorption and desorption in two cycles at 200 • C and a hydrogen pressure of 2.10 6 Pa.No rare-earth elements were used in the production of the samples, which also reduced the cost of manufacturing the alloys.The Al 15 Ti 38 Nb 23 Zr 24 alloy reached a storage capacity of 1.61 wt.% and a density of 5.87 g.cm −3 , comparable to the commercial material LaNi 5 , which reaches a storage capacity value of 1.4 wt.% and a density of 7.95 g.cm −3 [7].

Figure 1 .
Figure 1.Prediction map of the prepared alloys.

Figure 1 .
Figure 1.Prediction map of the prepared alloys.

Figure 3 .
Figure 3. Progress of the high-pressure analysis of hydrogen absorption and desorptio Al15Ti38Nb23Zr24 alloy.

Figure 3 .
Figure 3. Progress of the high-pressure analysis of hydrogen absorption and desorption for the Al15Ti38Nb23Zr24 alloy.

Figure 3 .
Figure 3. Progress of the high-pressure analysis of hydrogen absorption and desorption for the Al 15 Ti 38 Nb 23 Zr 24 alloy.

Table 1 .
Overview table of the high-entropy properties of the prepared materials with different Al-Ti-Nb-Zr compositions.

Table 3 .
Sorption properties of the alloys.

Table 3 .
Sorption properties of the alloys.

Table 3 .
Sorption properties of the alloys.