Fuhu
Yin†
a,
Yu
Chang†
b,
Tingzhi
Si
*a,
Jing
Chen
b,
Hai-Wen
Li
c,
Yongtao
Li
*ad and
Qingan
Zhang
a
aSchool of Materials Science and Engineering, Anhui University of Technology, Maanshan 243002, China. E-mail: liyongtao@ahut.edu.cn; tzsiahut@163.com; Tel: +86-555-2311 570
bInstitute of Nuclear and New Energy Technology, Tsinghua University, Beijing, 100084, China
cHefei General Machinery Research Institute, Hefei 230031, China
dKey Laboratory of Green Fabrication and Surface Technology of Advanced Metal Materials of Ministry of Education, Anhui University of Technology, Maanshan 243002, China
First published on 19th July 2023
Exploration of high-entropy alloys for hydrogen storage has recently attracted attention owing to their serious defects, high-entropy induced reduction of thermodynamic stability and abundant raw metal elements. In this study, new Zr-based high-entropy alloys with Laves phases were designed where their structure and hydrogen storage properties were adjusted by introducing Mg element. The results show that the addition of Mg element makes the crystal structure of Zr2MgV2−xFexCrNi (x = 0 and 1) alloys change from AB2 type to A3B4 type structure, which further leads to the improvement of their hydrogen storage capacity and hydrogen sorption kinetics. The Zr2MgV2CrNi and Zr2MgVFeCrNi high-entropy alloys can rapidly absorb 0.8 wt% (0.43 H/M) and 0.9 wt% (0.48 H/M) hydrogen at room temperature, respectively, three times higher than those of the Zr-based alloys without Mg-substitution. And more importantly, the absorbed hydrogen of Zr2MgV2CrNi and Zr2MgVFeCrNi high-entropy alloys can be partially released at room temperature with distinct desorption processes. These capacity and kinetic enhancements should be related to the higher lattice parameters, lower electron concentration, severe lattice distortion and smaller average valence electron concentration (VEC) of those alloys. The development of Mg-containing Zr-based high-entropy alloys with Laves phases provides a new idea for the design of new hydrogen storage materials.
Recently, high-entropy alloys (HEAs), which contain five or more elements in relatively high concentrations (5–35 at%) and have a random state of coordination entropy greater than 1.5 R, have received significant attention for hydrogen storage applications.16–18 This is because under the influence of the cocktail effect, all the principal components will affect the performance of the alloy. Due to the diversity of element selection and the controllability of element ratio, it is possible to prepare high-entropy alloys with good properties for hydrogen storage applications.18–20
Owing to the above-mentioned advantages, some high-entropy alloys for hydrogen storage have been designed. Among them, the intermetallic high-entropy alloys containing Laves phases have attracted much attention owing to their high hydrogen storage capacity and good low-temperature reversible hydrogen storage performance. In recent years, several Laves phase high-entropy alloys have been studied but their reversibility is poor. For example, the hydrogen absorption capacity of the CoFeMnTiVZr high-entropy alloy is 1.4 wt% but its reversible hydrogen storage capacity is only 0.7 wt%.21 The hydrogen absorption capacity of the LaNiFeVMn high-entropy alloy is 0.82 wt%, but its reversible hydrogen storage capacity is only 0.2 wt%.22 In this regard, Edalati et al.23 designed a TiZrCrMnFeNi high-entropy alloy with C14 Laves phase. They show that a valence electron concentration (VEC) of 6.4 is associated with a fast reversible hydrogen capacity of 1.7 wt% at room temperature without the activation process. Unfortunately, the cycling performance of the alloy has not been tested. On this basis, Mohammadi et al.24 used first-principles calculations to design AB2-type Laves phase TixZr2−xCrMnFeNi (x = 0.4–1.6) alloys with a valence electron concentration of 6.4 and low hydrogen binding energies (negative values close to −0.1 eV), which reversibly absorb and desorb hydrogen at room temperature (298 K) with fast kinetics. Among them, the hydrogen storage capacity of the Ti0.4Zr1.6CrMnFeNi alloy is almost unchanged at 1.6 wt% after 1000 cycles at 3.7 MPa and 298 K. This not only shows that the alloy has excellent cycling properties, but also implies that the concept of binding energy engineering can be used to adjust the temperature and pressure of hydrogen storage in high-entropy alloys to adapt to environmental conditions. After that, Chen et al.25 designed a TiZrFeMnCrV high-entropy alloy with single C14 Laves phase, and its capacity remained stable at about 1.76 wt% after 50 cycles.
However, these alloys do not show better hydrogen storage performance than traditional intermetallic compounds, which may be related to the large number of transition metal elements in the alloys and the composition system and crystal structure of high-entropy alloys. In this regard, Floriano et al.26 reported the A3B2-type TiZrNbFeNi and AB-type Ti20Zr20Nb5Fe40Ni15 high-entropy alloys, and found that the non-equiatomic alloys have a larger reversible hydrogen storage capacity, which is related to the larger valence electron concentration. After that, the research group27 reported an A3B2 type TiZrNbCrFe high-entropy alloy that can reversibly absorb and release 1.9 wt% hydrogen at 473 K. Recently, Andrade et al.28 also reported an AB-type TiZrNbFeCrNi high-entropy alloy that can reversibly absorb 1.1 wt% hydrogen at 473 K. Therefore, to have higher H/M values at room or moderate temperatures, new systems with different A/B ratios should be designed. In addition, the use of light-weight alloying elements is also the key to efficient hydrogen storage. In this regard, Ma et al.29 reported ZrTiVAl1−xFex high-entropy alloys. Due to the severe lattice distortion and friable HCP interdendritic phase, these alloys exhibited rapid hydrogen chemisorption kinetics even at room temperature. In addition, the study also shows that the increase of VEC will lead to the decrease of the stability of alloy hydrides. However, although the addition of the light element Mg to the intermetallic high-entropy alloy can also reduce the density of the alloy and has the possibility of further improving the hydrogen storage capacity, there are few studies on magnesium-containing Lave-phase high-entropy alloys for hydrogen storage. This may be related to the preparation of Mg Laves phase hydrogen storage high entropy alloys: due to the large difference in melting and boiling points between Mg and transition metal elements such as Zr and V, this makes it impossible to accurately prepare Mg Laves phase hydrogen storage high entropy alloys using conventional melting methods, and the production is unsafe. In addition, the use of mechanical alloying to prepare Mg-containing high-entropy alloys not only requires a long ball milling time, which leads to the risk of contamination and oxidation of the alloy, but also makes it difficult to prepare high-purity Laves-phase Mg-hydrogen storage high-entropy alloys, which leads to deterioration of the material properties.
In this study, Zr2MgV2−xFexCrNi (x = 0 and 1) was first prepared by a combination of melting, ball milling and sintering. The effects of Mg addition on the microstructure and hydrogen storage properties of Zr-based Laves phase high-entropy alloys were studied in detail, and the hydrogen storage mechanism of these two Mg-containing Laves phase high-entropy alloys was also revealed.
Fig. 1 Rietveld refinements of the XRD patterns for (a) Zr2V2CrNi, (b) Zr2VFeCrNi, (c) Zr2MgV2CrNi and (d) Zr2MgVFeCrNi high-entropy alloys. |
Sample | Phase | Lattice parameters (Å) | Abundance (wt%) | |
---|---|---|---|---|
a | c | |||
Zr2V2CrNi (RWP = 6.94%; S = 3.17) | C14 Laves | 5.0765(2) | 8.2744(2) | 90 |
C15 Laves | 6.9256(4) | 10 | ||
Zr2VFeCrNi (RWP = 9.12%; S = 3.73) | C14 Laves | 4.9987(2) | 8.1744(1) | 100 |
Zr2MgV2CrNi (RWP = 5.38%; S = 2.80) | C14 Laves | 5.0802(7) | 8.2967(1) | 88 |
C15 Laves | 6.9340(5) | 12 | ||
Zr2MgVFeCrNi (RWP = 5.25%; S = 2.73) | C14 Laves | 5.0031(1) | 8.1748(1) | 100 |
The microstructural features of Zr2MgV2CrNi and Zr2MgVFeCrNi high-entropy alloys are further studied by using the SEM technique and the corresponding mapping images are also presented in Fig. 2 and 3. It can be found that all the particles of the high-entropy alloys are lumpy-like with a size of about 10–100 μm. In addition, the elemental mapping shows that the elements Zr, Mg, V, Cr and Ni in the Zr2MgV2CrNi alloy and Zr, Mg, V, Fe, Cr and Ni in the Zr2MgVFeCrNi alloy are uniformly distributed within the alloys.
In order to further reveal the microstructure of the Zr2MgV2CrNi high-entropy alloy, TEM and HRTEM techniques were employed. Fig. 4(a) shows the TEM image of the Zr2MgV2CrNi sample. The SAED of the corresponding particle (see Fig. 4(b)) displays a set of ordered arrays of diffraction points and diffraction rings, which can be identified as C14 Laves phase and C15 Laves phase, agreeing well with the XRD results (Fig. 1(c)). The HRTEM image in Fig. 4(c) further confirms the existence of these two phases, where the d-spacing of 0.300 and 0.204 nm is observed, corresponding to the (102) and (222) planes for C14 and C15 Laves phases, respectively.
Fig. 5 Hydrogen absorption kinetics curves of Zr2MgxV2−yFeyCrNi (x = 0 and 1, and y = 0 and 1) high-entropy alloys at 303 K. |
Fig. 6 shows hydrogen desorption kinetic behaviours of Zr2MgxV2−yFeyCrNi (x = 0 and 1, and y = 0 and 1) high-entropy alloys at different temperatures. It can be seen that all high-entropy alloys can dehydrogenate at room temperature, and more hydrogen is released with the increasing desorption temperature. In addition, Zr2V2CrNi and Zr2VFeCrNi alloys can be completely dehydrogenated at 373 K, while Zr2MgV2CrNi and Zr2MgVFeCrNi alloys can be completely dehydrogenated only at 523 K, indicating that the addition of Mg has an adverse effect on the stability of the alloy hydride.
Fig. 6 Hydrogen desorption kinetic curves of (a) Zr2V2CrNi, (b) Zr2VFeCrNi, (c) Zr2MgV2CrNi and (d) Zr2MgVFeCrNi high-entropy alloys at different temperatures. |
In addition, it can be seen from Fig. 7 that the C14 Laves phase of the Zr2MgVFeCrNi high-entropy alloy shifted toward the lower-angle during hydrogen absorption, and began to dehydrogenate at about 30 °C, and completely desorbed at about 282 °C. At this time, the C14 Laves phase returned to its original position, indicating that the alloy can completely reversibly absorb and desorb hydrogen, and the maximum hydrogen storage capacity is about 0.9 wt%. In summary, the final hydrogen release temperature of both high-entropy alloys is around 280 °C. This indicates that the substitution of Fe element basically does not improve the thermodynamic properties of the alloy, which is related to the fact that both alloys contain a large amount of V element and the structure of the alloy, because too much V element is not conducive to the hydrogen release of the alloy, and there is an excess of A-side atoms in this alloy, which makes some A-side atoms occupy the position of the B-side atoms, thus leading to an increase in the cell volume of the alloy. Eventually, the pressure of the hydrogen release plateau of these alloys decreases, which is not favorable for the release of hydrogen.22,28
(1) |
(2) |
VEC = ΣciVECi | (3) |
e/a = Z1C1 + Z2C2 + …+ ZmZm | (4) |
Alloy | δ r/% | ΔχAllen/% | VEC | e/a |
---|---|---|---|---|
Zr2V2CrNi | 10.77 | 15.17 | 5.7 | 4 |
Zr2VFeCrNi | 11.18 | 13.80 | 6.2 | 3.2 |
Zr2MgV2CrNi | 10.96 | 14.89 | 5.1 | 3.7 |
Zr2MgVFeCrNi | 11.48 | 14.78 | 5.6 | 3 |
(i) In the previous study,33 the criteria for alloys containing the Laves phase were obtained as δr > 5.0% and ΔχAllen > 7.0%, which is consistent with the results of this study.
(ii) The addition of Mg to the Zr-based Laves phase high-entropy alloy will lead to the decrease of the VEC value, and a larger VEC destabilizes the hydrides,34 which makes the dehydrogenation temperature of Zr2MgV2−xFexCrNi hydride higher than that of Zr2V2−xFexCrNi hydride. However, the final hydrogen release temperatures of these four high-entropy alloys are still high, which may be related to factors such as the VEC parameter in the alloys not yet reaching 6.4 and the large amount of elemental V in the alloys.23,24
(iii) In general, the larger the lattice parameters of the alloy, the higher the hydrogen storage capacity. However, in this study, the Zr2MgVFeCrNi high-entropy alloy with smaller lattice parameters has the highest hydrogen storage capacity, which is related to the VEC, e/a and alloy structure. Compared with the Zr2MgV2CrNi high-entropy alloy, the Zr2MgVFeCrNi high-entropy alloy contains only C14 Laves phase with relatively high hydrogen storage capacity, and its electron concentration is relatively low. According to Wagner's36 theory, the higher electron concentration would result in the lower hydrogen absorption capacity, because more outer electrons of the alloy would exert stronger repulsive interaction to the hydrogen atoms, thus hindering the hydrogen atoms into the lattice, so the hydrogen storage capacity of the Zr2MgVFeCrNi high-entropy alloy is relatively high. Compared with the Zr2VFeCrNi high-entropy alloy, the Zr2MgV2CrNi high-entropy alloy not only has larger lattice parameters and smaller VEC and e/a, but also contains a light element Mg. Since a larger average valence electron concentration destabilizes the hydride, eventually this destabilization leads to a reduction in the maximum hydrogen storage capacity.34 In addition, the addition of Mg not only reduces the density of the alloy, but also makes the alloy change from AB2 type structure to A3B4 type structure, which leads to more defects in the alloy, and makes the position occupied by the B-side atoms in the alloy appear vacant. At the same time, some A-side atoms occupy the position of the B-side atoms, which leads to the increase of the cell volume of the alloy, and finally improves the hydrogen storage capacity and hydrogen absorption kinetics of the alloy.37,38 Therefore, among the four alloys, the Zr2MgVFeCrNi high-entropy alloy has the highest hydrogen storage capacity. However, the hydrogen storage capacity of these four high-entropy alloys is low, which may be related to factors such as the structure of the alloys, the high content of transition group metal elements, and the larger e/a and VEC in the alloys.34,36
(iv) In this study, the four high-entropy alloys have good hydrogen absorption and desorption kinetics, which is related to lattice parameters, defects, electron concentration and alloy structure. Because the high-entropy alloy is composed of a variety of elements, the difference in atomic radius and electronegativity makes the alloy have more defects such as lattice distortion, dislocation and strain, and weaken the corresponding bond energy, which is more conducive to the diffusion, dissociation and recombination of hydrogen atoms.25,29 Compared with the Zr2V2−xFexCrNi high-entropy alloy, the Zr2MgV2−xFexCrNi high-entropy alloy has better kinetic properties, which can be attributed to the fact that both alloys are under-stoichiometric alloys, and have higher lattice constants and smaller electron concentration, resulting in a larger tetrahedral gap of the alloy, which is more conducive to hydrogen absorption and desorption of the alloy.35–38
Table 3 compares the high entropy alloys in this study with some high entropy alloys. Although the hydrogen storage capacity of the high-entropy alloys in this study is lower than that of some Laves-phase high-entropy alloys,25,39 the successful introduction of Mg element in this study resulted in the transformation of the high-entropy alloys from AB2-type to A3B4-type structures, as well as the reduction of the VEC of the alloys, which improved the hydrogen storage capacity and kinetic properties of the alloys and demonstrated that A3B4-type Laves-phase high-entropy alloys can be designed to achieve high reversible hydrogen storage at moderate temperature. This provides insight into the design and performance improvement of new high-entropy alloys. Furthermore, the Mg-containing HEAs studied by predecessors are mainly composed of BCC/FCC phases, and generally have a high dehydrogenation temperature.40,41 However, the high-entropy alloy in this study has a pure Laves phase and outstanding reversible hydrogen storage at room temperature, which indicates that the presence of the Laves phase is beneficial for the development of Mg-containing high-entropy alloys with excellent low-temperature reversibility.
HEAs | Phase | Storage capacity (wt%) | Start/end dehydrogenation temperature (K) |
---|---|---|---|
TiZrFeMnCrV | C14 Laves | 1.8 | 323/773 |
TiVZrNbFe | C14 Laves (major) | 1.65 | 303/641 |
MgAlTiFeNi | BCC | 1.0 | 558/598 |
Mg12Al11Ti33-Mn11Nb33 | BCC | 1.75 | 573/793 |
Zr2MgV2CrNi | C14 Laves (major) | 0.8 | 303/558 |
Zr2MgVFeCrNi | C14 Laves | 0.9 | 303/555 |
Footnote |
† These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2023 |