*4.2. AB*<sup>3</sup> *and A*2*B*<sup>7</sup> *Hydrogen Storage Alloy*

Kadir et al. [100] reported the study of a new type of ternary alloy, the general formula of which is RMg2Ni9 (R = RE, Ca, Y), PuNi3-type structure. It has been found that some ternary alloys based on R–Mg–Ni can reversibly absorb/desorb H2 at 1.8–1.87 wt.% and are therefore considered potential candidates for H2 storage alloys [100]. However, its cycle stability and overall performance must be further improved.

Many scholars study high-capacity alloys with AB3 components (A: RE metals, Mg, Ca, etc.; B: transition metals) [101–103]. Most scholars synthesize AB3 alloy by melting method, but MA also has unique advantages in synthesizing AB3 alloy. For example, Hassen Jaafar et al. [104] successfully synthesized the AB3 alloy LaMg2Ni5Al4 using MA.

Mouna Elghali et al. [105] produced AB3 alloy according to the equation *AB*<sup>5</sup> + 2*AB*<sup>2</sup> = 3*AB*3. They produced LaZr2Mn4Ni5. Due to the immiscibility of La and Zr elements, this compound cannot be obtained by melting methods. Therefore, mechanical alloying methods are used. It is found that the cell volume is increased compared with the LaMg2Ni9 alloy [105]. They also used the above methods to produce alloys such as LaTi2Cr4Ni5 [106], LaZr2Cr4Ni5 [107], and CeTi2Cr4Ni5 [108], with the results showing that these alloys have good reversibility, high discharge capacity, and a good cycle stability.

In the AB3 alloy, Ca and Mg can be used instead of A. Such an alloy has a H2 storage capacity of about 1.9 wt.% and a discharge capacity of about 370 mAh/g [109]. LaCaMgNi9 compounds are always produced by smelting or sintering method. However, it is difficult to produce stoichiometric LaCaMgNi9 using melting technology due to the vapor pressures of Ca and Mg being very high, and it is very easy to evaporate during the melting process. Another difficulty is that Ca and Mg are easily contaminated by oxygen at high temperatures [110]. The use of mechanical alloying (MA) can avoid these difficulties. S. Chebab et al. [111] used MA to treat pure La, Ni, Ca, and Mg powders for 30 hours, and the weight percent of this phase reached 67 %. This alloy can absorb 6 H/f.u. of H2.

In the past few years, people have studied the possibility of La2Ni7 alloy as a H2 storage alloy because of its good H absorption capacity. However, the La2Ni7 phase is hindered by its poor cycle stability because of poor corrosion resistance [112]. Different RE elements are usually used to replace La, while Co, Mn, Al, and some other elements are used to replace Ni to improve the electrochemical properties of such alloy electrodes [113].

In the work of M. Balcerzak et al. [114], MA technology was used to manufacture La2Ni7 alloy, and then, Mg element was also incorporated to produce the ternary alloy La2−*x*Mg*x*Ni7 (*x* = 0–1). It was found that the electrochemical and thermodynamic properties of this alloy increased with the rise content of Mg, and the alloy with the best performance was La1.5Mg0.5Ni7. Ni element forms a film on the surface of the alloy particles, and the film is very dense, which can effectively protect the material from corrosion by strong alkaline solutions.

In their another work [115], they also synthesized La1.5−*x*Pr*x*Mg0.5Ni7 and La1.5−*x*Nd*<sup>x</sup>* Mg0.5Ni7 alloys (*x* = 0, 0.25, 0.5, 1) with MA. It was found that replacing La with Pr or Nd elements resulted in increased cycle stability of the alloy and optimized H2 absorption kinetics.

Martyna Dymek et al. [116] also doped the above materials. La is partially replaced by Mg (5.6 at.%) and Ni is partially replaced by Co. The doped material shows easy activation characteristics (maximum capacity is reached after the second cycle) and excellent H2 capacity (14% larger than Co-free materials). The exchange current density of the Comodified La1.5Mg0.5Ni7 alloy is increased by 15%, and the H2 diffusivity is also improved.

Marek Nowak et al. [117] used MA to synthesize La1.5Mg0.5Ni7 alloy and doped Al or Mn into it. It is found that both elements can improve its H2 adsorption. The time required for the third cycle to reach 95% of the maximum H2 capacity was respectively reduced to 5 and 6 minutes and enhanced the stability of the discharge capacity.

### **5. Body-Centered Cubic (BCC) Alloys**

In addition to the abovementioned H2 storage alloys that have been extensively studied, MA can also be used in the synthesis and performance optimization of BCC structure H2 storage alloys.

Y.Q. Hu et al. [118] used mechanical alloying to synthesize TiCr2 with BCC structure and compared the performance with the alloy of the same composition produced by mechanical grinding. They found that the overall performance of the alloy produced by mechanical alloying is better than that of the mechanically crushed alloy. The H2 absorption capacity of the MA sample is 1.0 wt.% (52 ◦C, 2.5 MPa), and the desorption capacity is 0.6 wt.%. Nobuhiko Takeichi et al. [119] studied the effect of different Cr content on the performance of TiCr2−*x*(*x* = 0, 0.2 and 0.5). The results show that the sample can react with H2 under the conditions of 5 MPa and 250 ◦C. TiCr1.5 has the highest H2 content, reaching 0.47 H/M (40 ◦C, 8 MPa)

Compared with other intermetallic compounds, the V-rich solid solution with BCC structure has attracted great attention because of its relatively high H2 storage capacity (up to 4 wt.%) [120]. The volumetric H2 storage capacity of the BCC phase (VH2 is 0.16 g/cm3) exceeds that of liquid H2 (0.07 *g*/*cm*3) [121]. In addition, due to their high reactivity, V-BCC alloys can absorb H2 at relatively low temperatures without catalysts.

M. Balcerzak et al. [120] synthesized the V-based BCC solid solution Ti0.5V1.4−*x*Ni0.1Cr*<sup>x</sup>* (*x* = 0, 0.1, 0.2, 0.3) using mechanical alloying, studied its structure and electronic properties, and studied the addition of Cr atom pairs The effect of vanadium-rich body core-alloy on H2 storage performance. X-ray photoelectron spectroscopy measurements show that the addition of Cr has a significant impact on the oxidation resistance of V-BCC alloys. The cyclic charge and discharge method proves that the Cr-doped V-BCC alloy significantly improves the cycle life of the material stability.

They also synthesized Ti0.5V1.5−*x*Mn*<sup>x</sup>* (*x* = 0, 0.1, 0.2, 0.3) [120], Ti0.5V1.5−*x*Ni*<sup>x</sup>* (*x* = 0, 0.1, 0.2, 0.3) [122] by mechanical alloying.

Ti0.5V1.5−*x*Co*<sup>x</sup>* and Ti0.5V1.4−*x*Ni0.1Co*x*(*x* = 0, 0.1, 0.2, 0.3) [123] solid solutions synthesized by mechanical alloying can absorb H2 with no activation. Their H2 storage capacities decrease as Co atoms number increases. However, Co raises the hydrogenation kinetics, lowers the hysteresis, and improves the reversibility of the H2 adsorption.

Toshihiko Kondo et al. [124] mechanically alloyed CaMg2 with V and synthesized Mg2CaV3 ternary BCC alloy. This alloy is activated by graphite grinding and then can reach a H2 storage capacity of 3.3 wt.% (10 h) at 25 ◦C. Unlike the classic V-based BCC alloy, the alloy still maintains the BCC structure after hydrogenation. The desorption process starts at 270 ◦C and 0.1 MPa argon atmosphere.

Huaiyu Shao et al. [125] synthesized the Mg-based BCC alloy Mg60Ni5Co*m*X35−*m*(X = Co, B, Al, Cr, V, Pd, and Cu) using the mechanical alloying method and studied the relationship between its lattice parameters and H2 absorption performance. The results show that the alloys with lattice parameters in the range of 0.300–0.308 nm absorb more H2 while the alloys with lattice parameters greater than 0.313 nm have difficulty in absorbing H2. The geometric effect is one of the main influencing factors.

The details of the H2 storage performance of the intermetallic compounds synthesized by mechanical alloying for H2 storage in the past five years are listed in Table 1. The literature [126] shows the thermodynamic data of some materials.



#### *Energies* **2021**, *14*, 5758


**Table 1.** *Cont*.

### **6. Conclusions**

As a pure solid-state synthesis method, mechanical alloying has many applications in the synthesis and optimization of H2 storage materials. For different alloy systems, the specific effects of mechanical alloying are different. However, in general, mechanical alloying can reduce the particle size (even down to the nanometer scale), can increase the specific surface area, and can reduce the diffusion distance of H2. For some elements that cannot form a solid solution or alloy for various reasons, mechanical alloying makes the alloying of these elements possible because it is a pure solid-state method. This is not only a unique advantage compared to traditional melting but also an important basis for the synthesis of new H2 storage alloys and the addition of various additives to optimize performance. However, many scholars have also proposed mechanical alloying defects, the most important of which is the consolidation of the powder on the inner wall of the container and the surface of the ball. This causes an uneven composition, waste of raw materials, etc., and more importantly, it is difficult to finely control the composition of the material. Several methods have been proposed, such as adding grinding aids (carbon material, etc.) and grinding aid liquids (such as alcohol, etc.), but they still cannot be completely resolved. Mechanical alloying is also a problem without uniform parameters for each alloy system. Before performing mechanical alloying, various scholars or organizations usually conduct a series of experiments to determine the optimal parameters, but this not only consumes a lot of time and materials but may only obtained the best within the set parameter range.

In summary, for solid H2 storage materials, mechanical alloying is an extremely attractive synthesis method. Many scholars have used this method to synthesize H2 storage materials with great potential. However, for possible large-scale commercial applications in the future, further optimization of the technology still needs to be considered.

**Author Contributions:** Supervision, O.E., D.C. and Y.L.; Validation, O.E. and D.C.; Writing—Original draft, Y.L., D.C. and O.E.; Writing—Review and editing, Y.L., D.C. and O.E. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors of the paper would like to thank the China Scholarship Council(CSC).

**Conflicts of Interest:** The authors declare no conflict of interest.
