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

Abstract

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.

1. 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 CO₂ 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/m³ [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.

2. 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 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 × 10⁶ 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

3. 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 high-entropic phases but exhibit high-entropy properties. All of the produced alloys have a single-phase BCC structure.

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

Alloy	ΔHmix [kJ/mol]	δ× 100	ΔSmix	VEC
Al30Ti35Nb15Zr20	−25.50	4.39	11.10	3.85
Al23Ti25Nb30Zr22	−19.11	4.64	11.46	4.07
Al30Ti35Nb20Zr15	−23.80	3.99	11.10	3.90
Al25Ti25Nb20Zr30	−22.94	5.03	11.44	3.95
Al15Ti38Nb23Zr24	−14.08	4.61	11.08	4.08
Al35Ti20Nb25Zr20	−25.82	4.54	11.29	3.90
Al30Ti40Nb15Zr15	−24.72	3.95	10.78	3.85
Al20Ti25Nb25Zr30	−18.46	5.04	11.44	4.05
Al20Ti40Nb15Zr25	−19.48	4.62	10.97	3.95
Al15Ti40Nb30Zr15	−12.72	3.96	10.78	4.15

describes theoretical high-entropy properties that define the properties of the alloy and its storage capacity, where ΔH_mix a ΔS_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. Overview table of the material properties of the prepared materials with different Al-Ti-Nb-Zr compositions.

Alloy	EDX	Microhardness [HV0.3]	Density [g.cm−3]
Al30Ti35Nb15Zr20	Al30Ti34Nb14Zr22	532 ± 93.41	5.311 ± 0.009
Al23Ti25Nb30Zr22	Al23Ti27Nb28Zr22	453 ± 43.79	6.011 ± 0.006
Al30Ti35Nb20Zr15	Al31Ti36Nb20Zr13	542 ± 20.81	5.52 ± 0.14
Al25Ti25Nb20Zr30	Al24Ti26Nb21Zr30	444 ± 7.04	5.83 ± 0.02
Al15Ti38Nb23Zr24	Al16Ti38Nb23Zr23	376 ± 9.039	5.87 ± 0.012
Al35Ti20Nb25Zr20	Al33Ti23Nb24 Zr20	747 ± 31.15	5.811 ± 0.008
Al30Ti40Nb15Zr15	Al28Ti38Nb16Zr18	587 ± 18.5	5.461 ± 0.017
Al20Ti25Nb25Zr30	Al21Ti26Nb26Zr27	470 ± 141.3	6.151 ± 0.019
Al20Ti40Nb15Zr25	Al19Ti41Nb15Zr25	353 ± 10.6	5.641 ± 0.004
Al15Ti40Nb30Zr15	Al14Ti39Nb29 Zr18	398 ± 20.9	6.11 ± 0.02

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 LaNi₅ [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 Al₁₅Ti₃₈Nb₂₃Zr₂₄ 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. Sorption properties of the alloys.

Alloy	Absorption H [wt.%/H/M]	Residual H [wt.%/H/M]	Desorption H [wt.%/H/M]	Cycle Efficiency [%]
Al30Ti35Nb15Zr20	1.28/0.74	0.46/0.26	0.82/0.48	64.86
Al23Ti25Nb30Zr22	1.06/0.69	0.25/0.16	0.81/0.53	76.81
Al30Ti35Nb20Zr15	0.98/0.54	0.27/0.15	0.71/0.39	72.22
Al25Ti25Nb20Zr30	1.23/0.82	0.41/0.27	0.82/0.55	67.07
Al15Ti38Nb23Zr24	1.61/1.05	0.62/0.40	0.99/0.65	61.90
Al35Ti20Nb25Zr20	0.79/0.48	0.18/0.10	0.61/0.38	79.16
Al30Ti40Nb15Zr15	1.10/0.63	0.37/0.22	0.73/0.41	65.08
Al20Ti25Nb25Zr30	1.28/0.86	0.43/0.29	0.85/0.57	66.28
Al20Ti40Nb15Zr25	1.28/0.79	0.54/0.33	0.74/0.46	58.23
Al15Ti40Nb30Zr15	1.33/0.88	0.30/0.20	1.03/0.68	77.27

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.

4. 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 Al₂₀Ti₂₅Nb₂₅Zr₃₀ 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 2.10⁶ Pa. No rare-earth elements were used in the production of the samples, which also reduced the cost of manufacturing the alloys. The Al₁₅Ti₃₈Nb₂₃Zr₂₄ alloy reached a storage capacity of 1.61 wt.% and a density of 5.87 g.cm⁻³, comparable to the commercial material LaNi₅, which reaches a storage capacity value of 1.4 wt.% and a density of 7.95 g.cm⁻³ [7].