1. Introduction

Currently, as fossil energy is on the verge of disappearing [1] and pollution caused by fossil fuels is becoming more serious [2] , it is urgent to develop clean energy. As an important type of clean energy, hydrogen (H 2) energy has the advantages of being non-polluting, being easy to be produced, and having extremely high energy density. Compared with other clean energy sources, such as geothermal energy, wind energy, and tidal energy, it is the best choice. The utilization of H 2 energy involves many aspects, such as the production, transportation, storage, and utilization of H 2 energy. Among them, what restricts the use of H 2 energy is the storage technology of H 2 energy. The storage of H 2 energy can be roughly divided into two types: physical storage methods and chemical storage methods.

Some factors may restrict the hydrogenation kinetics of Mg and Mg alloys. These factors include (i) the formation of oxide layers on the surface that inhibits the penetration of H 2 into the alloys ^[3][14], (ii) the slow dissociation rate of H 2 molecules on the Mg surface, (iii) the low speed of the MgH 2/Mg interface ^[4][15], and (iv) the slow diffusion of H 2 through magnesium hydride ^[4][15]. Most of these factors can be solved by nanostructured Mg using mechanical alloying with the existing catalytic additives.

Extensive research on H 2 storage alloy LaNi 5 is to partially replace La and Ni with other elements to reduce the volume expansion ratio of the alloy to its hydride ^[5][30]. The equilibrium pressure of the alloy can be reduced by introducing many elements: Cr, Co, Cu, Al, and Mn, etc. ^[6][31].

This review focuses on the application of mechanical alloying in the synthesis of intermetallic compounds as solid-state H 2 storage materials.

2. Application of Mechanical Alloying in Mg-Based Hydrogen Storage Alloys

L. Zaluski et al. ^[7][43] prepared a nano-sized Mg 2Ni alloy using the mechanical alloying method under the conditions of a ball-to-alloy ratio of 5 and a grinding time of 60 h. He found that the nanocrystalline Mg 2Ni alloy synthesized by mechanical alloying showed better H 2 adsorption performance than the alloy prepared by conventional methods and that the produced powder can easily absorb H 2 without activation because the mechanically alloyed nanocrystalline produces many very active fresh surfaces in the ball milling process. Conventional polycrystalline Mg 2Ni can react with H 2 at a temperature higher than 250 °C, while nanocrystalline Mg 2Ni can also absorb H 2 at a lower temperature (for example, at 200 °C, which is below the structural transition temperature of Mg 2NiH 4 hydride). No activation is required. Pd can catalyze the H 2 absorption kinetics of nanocrystalline Mg 2Ni at 200 °C. Nanocrystalline Mg 2Ni with a small amount of Pd can absorb H 2 even at room temperature, does not require activation, and has good kinetics.

The nanocrystalline and amorphous MgNi alloy prepared by mechanical alloying is also a potential alloy as a negative electrode for NiMH batteries. For example, after 10 h of ball milling, the MgNi alloy had an initial discharge capacity of 522 mAh/g ^[8][52]. In addition, unlike traditional AB 5 and AB 2 H 2 storage materials, this MgNi alloy does not require any activation and absorbs H 2 directly. They also have the advantages of being almost nontoxic and low cost. However, their H 2 absorption and desorption kinetics still needs to be further improved, and the actual discharge capacity is difficult to reach its theoretical value. In addition, from the point of view of commercial applications, this alloy has poor cycle stability as a hydride electrode. For example, after only 20 charge and discharge cycles, the discharge capacity of the MgNi electrode decays by more than 70%. Such cycle stability prevents MgNi from being used as a battery electrode, so it must be improved by other methods. The decrease in capacity is related to the irreversible corrosion of the alloy electrode by the KOH in the battery. This reaction forms a Mg(OH) 2 layer ^[9][10][11][53,54,55] on the surface of particles. This not only consumes the alloy itself but also greatly increases the charge transfer resistance at the alloy/electrolyte interface and may hinder the diffusion of H 2 into and out of the alloy body ^[12][56]. The pulverization of the alloy during the H 2 absorption and desorption cycle exacerbates this harmful phenomenon because the pulverization produces a new active surface and thus forms new additional Mg(OH) 2 layers after making contact with the electrolyte.

Mustafa Anik et al. ^[13][57] studied the electrochemical properties of Mg 2Ni and MgNi synthesized by Mechanical Alloying (MA). The results showed that the charge and discharge capacity of Mg 2Ni alloy increased sharply with the increase in grinding time within 40 h. The capacity of the alloy for which the grinding time exceeds 40 h no longer increases. They also found that the electrochemical performance of MgNi is much better than that of Mg 2Ni, and the charge–discharge reversibility of the Mg 2Ni alloy is very poor. The lower initial discharge capacity and cycle stability of the Mg 2Ni alloy are not only due to the blocking effect of the Mg(OH) 2 layer but also maybe owing to the highly irreversible reaction of the alloy. The authors believes that the existence of free electrocatalytically active Ni particles on the surface of the MgNi particles is the main factor that promotes the H 2 transfer reaction on the surface of the alloy.

Chiaki Iwakura et al. ^[14][58] dissolved Ti and V into MgNi alloy by mechanical alloying. They found that the two-element solid-solution amorphous Mg 0.9 Ti 0.06 V 0.04 Ni alloy prepared from MA is better than the single-element solid-solution Mg 0.9 Ti 0.1 Ni or Mg 0.9 V 0.1 Ni alloys show better cycle performance. The AES depth distribution shows that, after the charge–discharge cycle in an alkaline solution, the oxide layer on the surface of the Mg 0.9 Ti 0.06 V 0.04 Ni alloy is thinner than the surface of the Mg 0.9 Ti 0.1 Ni or Mg 0.9 V 0.1 Ni alloy. The XRD data show that a composite oxide layer composed of Ti and V species precipitates on the surface of the alloy particles, which may be the reason for the synergistic effect of the solid solution of the two elements to promote the charge–discharge cycle performance.

3. RE-Based Hydrogen Storage Alloys

Kadir et al. ^[15][100] reported the study of a new type of ternary alloy, the general formula of which is RMg 2Ni 9 (R = RE, Ca, Y), PuNi 3-type structure. It has been found that some ternary alloys based on R–Mg–Ni can reversibly absorb/desorb H 2 at 1.8–1.87 wt.% and are therefore considered potential candidates for H 2 storage alloys ^[15][100]. However, its cycle stability and overall performance must be further improved.

In the past few years, people have studied the possibility of La 2Ni 7 alloy as a H 2 storage alloy because of its good H absorption capacity. However, the La 2Ni 7 phase is hindered by its poor cycle stability because of poor corrosion resistance ^[16][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 ^[17][113].

In the work of M. Balcerzak et al. ^[18][114], MA technology was used to manufacture La 2Ni 7 alloy, and then, Mg element was also incorporated to produce the ternary alloy La 2− x Mg x Ni 7 ( 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 La 1.5 Mg 0.5 Ni 7. 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 ^[19][115], they also synthesized La 1.5 − x Pr x Mg 0.5 Ni 7 and La 1.5 − x Nd x Mg 0.5 Ni 7 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 H 2 absorption kinetics.

4. Body-Centered Cubic (BCC) Alloys

In addition to the abovementioned H 2 storage alloys that have been extensively studied, MA can also be used in the synthesis and performance optimization of BCC structure H 2 storage alloys.

Y.Q. Hu et al. ^[20][118] used mechanical alloying to synthesize TiCr 2 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 H 2 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. ^[21][119] studied the effect of different Cr content on the performance of TiCr 2− x ( x = 0, 0.2 and 0.5). The results show that the sample can react with H 2 under the conditions of 5 MPa and 250 °C. TiCr 1.5 has the highest H 2 content, reaching 0.47 H/M (40 °C, 8 MPa)

Ti 0.5 V 1.5 − x Co x and Ti 0.5 V 1.4 − x Ni 0.1 Co x ( x = 0, 0.1, 0.2, 0.3) ^[22][123] solid solutions synthesized by mechanical alloying can absorb H 2 with no activation. Their H 2 storage capacities decrease as Co atoms number increases. However, Co raises the hydrogenation kinetics, lowers the hysteresis, and improves the reversibility of the H 2 adsorption.

The details of the H 2 storage performance of the intermetallic compounds synthesized by mechanical alloying for H 2 storage in the past five years are listed in Table 1 . The literature ^[23][126] shows the thermodynamic data of some materials.