Bulk Mg has poor hydrogen absorption/desorption activity and usually requires temperature up to 620 K to release hydrogen. The Mg film is composed of nanocrystals and amorphous crystals, in which the diffusion rate of hydrogen is fast, thereby accelerating the hydrogen absorption/desorption rate. The microstructure and hydrogen storage properties of Mg film are controlled by deposition parameters, such as gas pressure (the total pressure and hydrogen partial pressure of the Ar–H₂ gas mixture for reactive sputtering), the sputtering rate of Mg, the substrate temperature, and the deposition process model.

Typical features of films prepared by PVD are nanocrystals and columnar structures, which are associated with deposition rate of Mg film and the substrate temperature. In the early stage of deposition, the Mg particles reach the surface of substrate, the temperature of which is relatively low but the temperature of Mg particles is so high that the cooling rate is extremely fast, resulting in a weak mobility of Mg particles with a high nucleation rate. As a result, a large amount of dispersive tiny grains are dispersed on the surface of the substrate until the grains cover the entire substrate surface to form a continuous dense morphology. After that, the Mg particles reaching the substrate will be deposited on the tiny Mg grains with higher temperature, thereby the cooling rate is reduced and the diffusion time of the atoms is increased. Under this condition, the grains have an anisotropic growth tendency, which possibly leads to a deposition process with a certain preferred growth orientation, thereby forming a columnar structure.^[38,59–61] Studies show that compared with continuous dense morphology, columnar structure is detrimental to hydrogen storage capacity.^[62] Moreover, as the thickness of the Mg film increases, the porous columnar structure increases, which results in a raise of hydrogen desorption temperature.^[63] Another point to note is that the preferred orientation of columnar structure also exerts an influence on hydrogen storage properties of Mg film. Leon et al.^[64] reported that for Mg, which has columnar structure highly oriented along the [002] direction, the kinetics of hydrogen absorption/desorption are faster compared with that of Mg film with dispersive columnar structure in different directions. The type of substrate exerts little effect on the microstructure of the Mg film, but it was observed that new phases generated by the reaction of Mg and substrate drastically destabilized MgH₂.^[38,62,65] In addition, the surface properties of substrate significantly influence the hydrogen absorption/desorption properties of pure Mg film. Lelis et al.^[66] reported that plasma treated Si substrate seriously inhibited the formation of magnesium hydride phase. This is mainly attributed to that the plasma treatment leads to an increase of the substrate temperature, thereby increasing the size of columnar structure. This phenomenon is consistent with the influence of columnar structure on hydrogen storage capacity reported by the reference.^[62]

In addition to the microstructure of pure Mg film, different preparation methods as well have various effects on the hydrogen storage properties of pure Mg film. Unlike traditional methods of preparing Mg-based hydrogen storage materials, in pure Mg film prepared by magnetron sputtering as a result of plasma based in situ hydrogenation, MgH₂ phase is formed near substrate-film region instead of on the surface of film.^[54,67,68] Besides, compared with Mg film prepared by thermal evaporation, the Mg film prepared by magnetron sputtering is more easily oxidized in the air so that it is difficult to be hydrogenated using plasma-based hydrogenation approach after being exposed to the air.^[54,60]

Although due to the size effect, the hydrogen absorption/desorption properties of pure Mg film have been improved compared with bulk Mg, both the complete absorption and desorption of hydrogen require a temperature of at least 573 K and 673 K, respectively. Moreover, the high hydrogen storage capacity observed in the fist hydrogen absorption decreases in the subsequent cycles of hydrogen absorption/desorption.^[60,69] Pd has a good hydrogen absorption/desorption activity ( ) but a low hydrogen storage capacity, which is only 0.6 mass%; on the contrary, the metal Mg has a high hydrogen storage capacity of up to 7.6 mass% but poor kinetics properties. In order to make up for respective shortcomings of Mg and Pd, a Pd-capped Mg thin film with better hydrogen storage capacity and hydrogen desorption activity was obtained. In Pd-capped Mg thin film, Pd can act not only as a protective layer to prevent oxidation of Mg in the air and interaction of Mg with the substrate, but also as a catalyst in enhancing the rate-limiting dissociation of hydrogen molecules at the sample surface, thereby effectively reducing the hydrogen absorption/desorption temperature, allowing possibility of hydrogen absorption below 373 K and hydrogen desorption below 473 K.^[70,71]

Gautam et al.^[72] prepared Pd-capped Mg thin films using DC magnetron sputtering and found that hydrogen absorption stars at 423 K and reached a maximum at 523 K. However, it is worth noting that unlike pure Mg film in Pd-capped Mg thin films, the MgH₂ phase forms epitaxially relative to Mg/Pd interface, which results in an anomalous phenomenon along with hydrogen absorption/desorption in Mg/Pd thin film, i.e., the reduction of hydrogen absorption rate with the increase of hydrogen pressure^[73,74] due to the “blocking effect”,^[75] and suggests the hydrogen diffusion process, thereby the nucleation and growth of hydride phase is the rate determining step in Mg/Pd films in low temperature.^[76] Reddy et al.^[77,78] reported that Mg/Pd film can form Mg–Pd intermetallic compounds in high temperature resulting from reactive diffusion between Mg and Pd, which benefits the cyclic stability of film but has a negative impact on the kinetics. They attributed this kind of influence to the fact that intermetallic compounds occupy grain boundaries of Mg, which are channels for hydrogen diffusion. The Mg–Pd intermetallic compounds in Mg/Pd films were also observed by other groups.^[78–81] Unlike Reddy et al., Ares et al.^[57] used an interface mechanism to explain the influence of Mg–Pd intermetallic compounds on kinetics. They believed that it was the Mg₆Pd/MgH₂ interface having a higher activation energy than that of Mg/MgH₂ that limits the interface migration rate, thereby limiting the kinetics of Mg/Pd film.

Gharavi et al.^[82] investigated the influence of the thickness of Mg film on hydrogen absorption/desorption properties of Mg/Pd film. They found that decreasing the thickness of the Mg film leaded to a lower hydrogen absorption/desorption temperature as well as a fuller hydrogenation/dehydrogenation. Under the condition that the thickness of the Mg film was reduced to 200 nm, a full hydrogenation/dehydrogenation of Mg/Pd film was achieved in low temperatures below 423 K.

Singh et al.^[53] found columnar structures with different orientations in Mg/Pd film like that in pure Mg film, and they contributed this small variation in orientation of the columns to rotations of the columns during the growth of the films due to the presence of small open space between the columns, resulting in the porous structure of columns.

On basis of Mg/Pd film, Higuchi et al.^[83] synthesized 3-layer Pd/Mg/Pd hydrogen storage composite film, which absorbed up to 5.5 mass% H₂ under 0.1 MPa in 24 h at 373 K. By comparing the hydrogen storage capacity of the 2-layer and 3-layer composite films, it was confirmed that the H/Mg ratio in the 3-layer Pd/Mg (800 nm)/Pd film was approximately twice that in the Pd/Mg (800 nm) film, which suggested that coating both sides of Mg film with Pd allows further improving its kinetics. It is believed that the improved hydrogen absorption/desorption properties of the film as a result of double-layer are due to the cooperative effect, the essence of which is elastic interactions between nanostructured Mg and Pd layers: (i) in Pd layer, hydrogen becomes unstable as a result of the catalytic effect of Pd and thereby can be easily released from the upper or lower layer containing at relatively low temperature; (ii) the upper and lower Pd layers exert external pressure on the middle Mg layer, which obviously affects the properties of Mg. Thus, the hydrogen in the Mg layer behaves as in the Pd layer being released below 370 K. This cooperative effect was also observed by other groups.^[84] Not only Pd catalyst layer but also the microstructure of Mg layer can improve the kinetics of Pd/Mg/Pd film. Higuchi et al.^[83] reported that columnar grains in the Mg layer of the Pd/Mg/Pd film were narrower and denser compared with that of in the Mg layer of the Mg/Pd film. Another interesting phenomenon in Pd/Mg/Pd film reported by Higuchi et al. was that unlike the Mg/Pd film, the hydrogen desorption temperature of the Pd/Mg/Pd film decreased as the thickness of Mg increased.^[83] It is a pity that Higuchi et al. did not make an explanation of this phenomenon. In contrast to findings of Higuchi et al. about the thickness effect of the Mg layer of the Pd/Mg/Pd thin film on hydrogen storage properties, Qu et al.^[85] reported that at room temperature, the absorption/desorption kinetics of the Pd/Mg/Pd thin film was improved by reducing the thickness of the Mg layer. As shown in Fig. 1, both hydrogenation and dehydrogenation rates apparently increased when the Mg layer was thinner than 60 nm. Qu et al. attributed the thickness effect to the reduction of hydrogen desorption activation energies and the increase of diffusion path for H atoms as well as the negligible blockage of the hydride layer with decrease of the Mg layer thickness down to 60 nm. We tentatively put forward that the difference between research results of Higuchi et al. and Qu et al. may be due to: (i) in Mg/Pd film, Mg is directly deposited on the substrate of a low temperature, therefore a dense nanocrystal or amorphous structure is formed on the side near the substrate, while a less dense columnar structure is formed on the side away from the substrate. With this in mind, for Mg/Pd film, the thicker the Mg layer, the denser the microstructure of the Mg layer, and then the lower the hydrogen desorption temperature; (ii) the deposition process was continuous in the experiments of Higuchi et al., as a result, the deposition of Mg layer stated before the temperature of the deposited Pd layer was reduced down to the temperature of substrate. As a result, the deposition rate of Mg layer was relatively slow, which leaded to the formation of uniform columnar structure of constant density. Thus, as the thickness of Mg layer increased, the length of the columnar crystal increased, and the grain boundary increased accordingly, which promoted the hydrogen desorption process. It is worth emphasizing that the thickness effect on the microstructure and density of the Mg layer strongly depend on the sputtering parameters, therefore, we believe that the kinetics of the Pd/Mg/Pd film or the Mg/Pd film depending on the thickness of the Mg layer is not unique. However, it can be sure that the kinetics of the Pd/Mg/Pd film is much better than that of the Mg/Pd film.

Fig. 1.

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	Fig. 1. (a) The relative resistance changes of all the hydride films during dehydrogenation in air at 298 K. Rmax is the initial resistance of hydride films. (b) The time-dependent volume fractions of Mg, fMg, of all the hydride films during dehydrogenation in air at 298 K.[85]

Compared to bulk Mg, for Pd/Mg/Pd, the enthalpy of formation of MgH₂ is reduced. The reason for the reduction of the enthalpy of formation varies, depending on the particle size of Mg: when the particle size is on the order of 7–9 nm, the grain boundary effects play a leading role, while if the particle size is bigger than the order of 7–9 nm, the effects of excess volume make a main difference.^[86]

The Mg deposition method and parameters have a great impact on the hydrogen storage properties of two or more layered Mg/Pd films.^[87–92] For example, it was observed that increasing pressures of He effectively allowed better dehydriding properties and the mechanical stability upon cycling. This can be explained that the increasing pressure of He allows on the one hand reduced crystallinity of the Mg layer^[92] as well as probably enhanced cooperation effect;^[93] on the other hand, it would induce an extension of the outer Mg/Pd interface region between the Pd and Mg layers, thereby essentially locating hydrogen in Mg phase.^[90–94] The extension of the outer Pd/Mg interface region was related to alloying between Pd and Mg layer and/or to the penetration of Pd atoms in the pores of the Mg sublayer.^[92]

Some studies have also shown that hydrogenation properties of the Pd/Mg/Pd system can be significantly enhanced by adding interlayer.^[95–98] According to literature reports, there are two ways for adding interlayer into the Pd/Mg/Pd system: (i) interlayer located between the Mg layer and the Pd layer has a catalytic effect as well as a function preventing the formation of Mg_xPd_y intermetallics during the thermal treatments, and allows the hydrogen storage capacity to be increased by inducing the formation of additional hydrogen absorption phases; (ii) when interlayer is located between two Mg layers, only a catalytic effect can be observed. Al is a widely used element for interlayer owing to its ability to improve the thermodynamics of the desorption process.^[99] Jain et al.^[100] prepared Pd (20 nm)/Al (50 nm)/Mg (100 nm)/Pd (20 nm) using vapor deposition method and found that the introduction of Al layer promoted the formation of complex hydride phase Mg(AlH₄)₂ along with magnesium hydride, which is responsible for the increase in hydrogen content from 4.34 ×10¹⁷ atoms/cm² to 6.01 ×10¹⁸ atoms/cm². In addition, they also proved that the introduction of Al layer can also effectively suppress the formation of Mg_xPd_y intermetallic phase, which exerts negative effect on hydrogenation. Dom’enech-Ferrer et al.^[101] prepared Pd/Mg/Al/Mg/Pd multilayer film using electron beam physical vapor deposition. They found that the addition of the appropriate amount of Al can promote the hydrogenation of the Mg layer as an activator, thereby improving the hydrogen storage capacity. But they also found that the addition of Al did not significantly reduce the desorption temperature compared to Pd/Mg/Pd films, with a desorption temperature around 403 K–413 K. Jain et al.^[102] prepared Pd/Mg/Ni/Pd and Pd/Mg/Mg₂Ni/Pd films with the Pd/Mg/Pd base system using vapor deposition and found that Pd/Mg/Ni/Pd and Pd/Mg/Mg₂Ni/Pd films absorb 7.08×10¹⁷ and 1.68×10¹⁸ hydrogen atoms/cm², respectively, in comparison to 4 ×10¹⁷ atoms/cm² absorbed by the base system. They contributed the high hydrogen content to the formation of additional hydrogen absorption phase in interlayers of Ni and Mg₂Ni. After adding the interlayer between Mg and Pd layers, Mg₅Pd₂ phase was not generated during hydrogen absorption, which is consistent with the research results of Jain et al.^[100]

Tan et al.^[103] investigated the influence of various interlayers (7.5 nm) located between Mg and Pd layers on hydrogen absorption/desorption properties and cycle stability of the Pd (7.5 nm)/Mg ( )/Pd (7.5 nm) thin film system at 523 K. It was observed that from the third cycle, the catalytic effect of Pd/Nb and Pd/Ti on both absorption and desorption begins to stabilize. In spite of a two times higher value of the steady desorption time in Pd/Ta than in Pd/Nb and Pd/Ti, it is worth noting that samples with the Pd/Ta catalytic layers showed a more steady absorption behavior with the number of cycles increasing. Despite having very rapid initial sorption rates, the composite with the Pd/Fe bi-layer cannot be cycled beyond the first adsorption/desorption. No confirmed explanation of the effect of different interlayers on the hydrogen absorption/desorption properties was not offered by Tan et al.

Like in the Pd/Mg/Pd sandwich film, adding interlayer into Mg/Pd multilayer film as well allows for greatly improved hydrogen absorption/desorption properties of the original system. Jung et al.^[104] investigated the microstructure and hydrogen storage properties of a 60-layer film of Pd (40 nm)/Ti(x nm)/Mg (360 nm)/Ti (x nm) (x = 10, 20, 30, 40, and 50) using an ultra-high vacuum DC magnetron sputtering system. The highest hydrogen absorption capacity of the Pd/Ti/Mg/Ti film was 4.46 mass% for x = 50. The hydrogen absorption rate accelerated remarkably with increased thickness of the Ti layer, from 209 min for x = 10 to 39 min for x = 40. These results demonstrate that the Ti layer in the Pd/Ti/Mg/Ti films serves as a blocking layer, preventing the formation of Mg–Pd intermetallic phases and provides additional diffusion paths for hydrogen atoms, which leads to enhanced hydrogen storage properties. The positive effect of Ti interlayer was also confirmed by Jung et al.^[105] and Reddy et al.^[106] Fry et al.^[107] compared 150-layer Pd/Mg/Pd film with interlayers of Cr and V respectively. It was found that the chromium-catalyzed coating had the most favorable hydrogen storage kinetics with an activation energy for the dehydrogenation reaction of , which was reduced by compared with Pd/Mg/Pd multilayer film without transition metal interlayers. Besides, 6.1 ± 0.3 mass% was achieved at around 573 K. Furthermore, Fry et al.,^[108] changing the pure transition metal interlayer into a mixture of transition metals (TM) rich in Ni (target purity 99.99%) with additions of Fe and Cr, obtained Pd/Mg/Pd multilayer with hydrogen storage properties similar to Ref. [107]. It was observed that hydrogenation of the thin film took place within minutes at 563 K with a reversible capacity of 4.6 mass% and the activation energy of the dehydrogenation reaction was calculated to be .

The hydrogen storage properties of Mg/Pd film added with Fe interlayer^[109] and Nb interlayer^[110] have also been reported in literatures and have been improved to varying degrees in different aspects.

In general, the Mg/Pd film has better hydrogen absorption/desorption kinetics, but the price of Pd is too high that it is difficult to realize large-scale applications of Mg/Pd film. Therefore, the research on non-palladium capped Mg film appears. The studies of Ouyang et al.^{[39,111–113]} are representative in terms of Mg/(rare earth alloy) multilayer films. They reported that for nine-layer Mg/Mm–Ni (Mm denotes La rich rare earth alloy) multilayer film, the hydriding and dehydriding temperatures of Mg layers in the Mg/Mm-Ni multilayer film were remarkably decreased that the hydrogen absorption and desorption started at 373 K and 473 K, respectively. Ouyang et al. attributed the improvement of the hydrogen storage properties of Mg to the catalytic roles of the Nd(La)Ni₃ existing in the Mm–Ni layer and Mg₂Ni phases existing in the interfacial regions between the Mg and Mm–Ni layers.^[111] They also reported that in MmM₅/Mg (here, Mm denotes mischmetal, MmM₅ denotes MmNi_3.5(CoAlMn)_1.5) multilayer thin films, the absorbed hydrogen can be fully released at 523 K. In addition, they explained the improvement of hydrogen absorption/desorption properties of the film system in terms of the microstructural characteristics of MmM₅ and Mg layers.^[39,112] The MmM₅ layers consisted of two kinds of structures: an amorphous structure with a thickness of 4 nm grown at the bottom of the layers, on which a randomly orientated nanocrystallite structure was formed. Unlike the MmM₅ layers, the Mg layers were composed of columnar crystallite with [001] direction nearly parallel to the growth direction. The reason for the formation of amorphous, nanocrystalline, and columnar structures can be referred to Section 3.1.1 or literature,^[113] in which Ouyang et al. gave a very detailed explanation. When the film starts to absorb hydrogen, the MmM₅ layer on the surface of the film easily absorbs hydrogen due to that the hydrogen absorption kinetics of MmM₅ is splendid. Then, the hydrogenation of Mg, which is adjacent to the MmM₅ layer, is able to be easily completed due to the catalytic effect of the MmM₅ phase on the hydrogenation process of Mg^[114] or “cooperative phenomena”.^[83,84,115] Furthermore, a fast diffusion of hydrogen into the interior of the Mg layer can be achieved owing to many hydrogen diffusion channels provided by the boundaries distributed between the columnar crystallites of Mg. It is worth emphasizing that the hydrogen diffusion through the closest packed plane of Mg could be avoided due to the gateway, which is provided by the diffusion channels and through which hydrogen could diffuse along the substrate plane of Mg, close packed (001) plane. Therefore, the improvement of hydrogenation kinetics of Mg could be achieved. The explanation of improving diffusion and reaction kinetics given above is valid as well as for the dehydrogenation reaction since dehydrogenation is the reverse process of hydrogenation. It is noteworthy that unlike Mg/Pd film, in MmM₅/Mg film, interface phases are not observed in all interfaces between different layers.

Mn-capped,^[116] Pt-capped,^[117] and Cu-capped^[118] Mg films have also been reported in literatures. Especially in the Mg/Cu multilayer film prepared by Akyildiz et al.,^[118] the Mg phase could be converted totally into MgH₂ at temperatures not greater than 473 K; however, when hydrogenation occurred above 473 K, some Cu still diffused into the Mg layer to form Mg₂Cu intermetallic, which inhibited hydrogenation of Mg. In recent years, a new type of Mg thin film system appears, i.e., the Mg/polymer thin film. Studies have shown that a polymer coating at the surface of Mg film facilitated the uptake and release of hydrogen in Mg at low temperatures below 423 K (at high temperatures, the polymer will decompose) as well as being an effective way to protect the underneath Mg film against oxidation owing to the low oxygen and water permeability, and no significant oxidation was observed after exposing film to air for one week.^[119] In addition, polymer layer significantly improved the possibility of the Mg layer remaining hydrogen in the film exposed to air that the hydrogen storage capacity could be remained after a few hours,^[120] even after one week of exposure to air.^[119] However, it is worth emphasizing that polymer may greatly reduce the hydrogen storage capacity of the film.^[121]