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Magnesium-based hydrides are considered as promising candidates for solid-state hydrogen storage and thermal energy storage, due to their high hydrogen capacity, reversibility, and elemental abundance of Mg. To improve the sluggish kinetics of MgH2, catalytic doping using Ti-based catalysts is regarded as an effective approach to enhance Mg-based materials.
- magnesium hydride
- titanium-based hydride
- catalysis
- hydrogen storage properties
1. Introduction
Depletion of fossil fuels and changes in the global climate urge people to seek green, sustainable energy resources and high-efficiency energy systems. Hydrogen is one of the secondary energy solutions with high gravimetric energy density, high efficiency, and zero carbon emission [1]. However, the hydrogen economy relies on safe and mature technology to store hydrogen, which remains a great challenge [2]. Solid-state hydrogen storage using metal hydrides is considered to be a safe and efficient method in comparison to other storage technologies, such as compressed hydrogen gas or liquid hydrogen.
Among various solid-state hydrogen storage materials, magnesium hydride (MgH2) is one of the metal hydrides that has been considered to be promising, due to its high storage capacity, abundant resources, and relative safety. MgH2 was first prepared in 1912 [3], and was proposed that can be used as energy storage media since the 1960s [4]. MgH2 is known for its high hydrogen storage content, up to 7.76 wt%. More importantly, Mg has a single and flat pressure plateau under desorption/absorption, and is an abundant resource in the crust, which makes it one of the most promising hydrogen storage materials comparing to others. Thus, Mg-based hydride is expected to play important roles in future hydrogen storage techniques. In past decades, research efforts have made significant progress on improving Mg-based hydrides in terms of thermodynamics, kinetics, and reversibility. The utilization of MgH2 for “energy storage” relates to two aspects, namely, hydrogen storage (HS) [5] and thermal energy storage (TES) [6]. Despite the difference in material-level for HS and TES, both applications require Mg-based hydride with fast hydrogen absorption and desorption rates. This leads to a large demand for studying catalysis in the Mg-H2 system.
Due to the extensive research activities on Mg-based hydrides, a series of review papers have been published [7][8][9][10][11][12][13][14][7,8,9,10,11,12,13,14]. A comprehensive review by Yartys et al. [15] provides a historical overview as well as future perspectives. Recent reviews have covered various directions for Mg-based hydrogen storage, such as downsizing (nanostructuring) [7][10][7,10], catalysis and kinetics [7][16][17][7,16,17], and destabilization [18][19][18,19].
2. Fundamentals of the Mg-H2 System
2.1. Crystal Structure
MgH2 is a stoichiometric compound with a H/Mg atomic ratio of 1.99 ± 0.01 [20]. The Mg-H bond is an ionic type that is similar to alkali and alkaline earth metal hydrides [21]. MgH2 with different types of structures can be synthesized by the reaction of magnesium with hydrogen under different conditions. β-MgH2, which is stable at ambient pressure (1 bar) and room temperature, has a tetragonal TiO2-rutile-type structure with space group P42/mnm [22]. β-MgH2 can be formed under moderate conditions during reversible hydrogen cycling. Nevertheless, MgH2 has at least four high-pressure forms, and the corresponding crystal structure parameters are tabulated in Table 1. At high applied pressures exceeding 0.387GPa or milled under high energy, β-MgH2 transforms into the orthorhombic γ-MgH2 form with α-PbO2-type structure [23]. Additionally, a subsequent phase transition from γ-MgH2 to a modified-CaF2-type structure was observed experimentally using in situ synchrotron diffraction when hydrogen pressure is above 3.84 GPa [24]. According to Varin et al., high energy ball-milling of MgH2 produced γ-MgH2 coexisted with nanocrystalline β-MgH2. They suggested that the presence of the γ-MgH2 phase contributed to reducing the hydrogen desorption temperature of MgH2 [25].
Table 1. Optimized structural parameters, bulk modulus (B0), and pressure derivative of bulk modulus (B’0) for MgH2 in ambient and high-pressure phases. (Reprinted with permission from ref. [22]. Copyright 2006 American Physical Society).
2.2. Thermodynamics of the Mg-H2 System
The first experimental evaluation of the thermodynamics of the Mg-H2 system was reported by Stampfer et al., showing the enthalpy of formation of MgH2 to be −74.5 kJ/mol·H2, and the entropy of formation is 136 J/K·mol·H2 [20]. The thermodynamics parameters of the Mg-H2 system have been reported, see Table 2. The pressure-composition-isotherm (PCI) method is commonly used to determine the enthalpy (ΔH) and entropy (ΔS) of the Mg-H2 system. By measuring a series of equilibrium pressures at various temperatures, ΔH and ΔS can be derived by Van’t Hoff relation.
|
Thermodynamic Parameters |
Values |
|---|---|
|
Formation enthalpy, kJ/(mol·H2) |
Table 3. Dissociation energy of hydrogen molecule on the surface of Mg. (Reproduced with permission from ref. [49]. Copyright 2008 AIP Publishing).
|
Metal |
Dissociation Energy (eV) |
|---|---|
−74.5 | |
|
Pure Mg |
0.87, 0.40, 0.50, 1.15, 1.05, 0.95, 1.00 |
|
Ti-doped Mg |
Null, negligible |
|
Ni-doped Mg |
0.06 |
|
V-doped Mg |
Null |
|
Cu-doped Mg |
0.56 |
|
Pd-doped Mg |
0.39 |
|
Fe-doped Mg |
0.03 |
|
Ag-doped Mg |
1.18 |
Catalytic doping and nanosizing of Mg-based systems have been considered as important methods to improve their kinetics. In general, the catalyst is defined as an agent which reduces the activation barrier without participating in the chemical reaction, as illustrated in Figure 5. A common consensus is that transition metals (TM) and their compounds are effective catalysts. These catalysts can be doped into Mg/MgH2 material by different synthetic approaches. Most TM catalysts are effective in both hydrogenation and dehydrogenation reactions. The roles of different Ti-based catalysts and the underlying mechanism will be reviewed in the following section.
Figure 5. Representation of the kinetic barrier of the reaction and lowering the activation energy (Ea) using a catalyst. (Reprinted from ref. [52]. Copyright 2018 MDPI).
Downsizing MgH2 to nano-scale is also shown to be effective to improve the kinetics. It is believed that nano-sizing can enhance kinetics by the creation of a large amount of fresh surface, shortening hydrogen diffusion, and promoting nucleation of the hydride/metal phase [12]. It is noteworthy that a combination of nanosizing and catalytic doping is usually realized during synthesis. For example, using a high-energy ball milling technique, co-milling MgH2 with transition metal powder could produce a nanocomposite with nano-size microstructure and homogeneously doped catalyst particles.
3. Catalytic Effects
3.1. Transition Metals Catalysts
Among various additives for improving Mg-based materials, TM catalysts have been intensively investigated. Interestingly, most of the transition metals and their compounds are found to be effective as both hydrogenation and dehydrogenation catalysts. In general, 1–5 at.% addition of TM catalyst leads to dramatic improvement while the hydrogen storage capacity is not sacrificed significantly. Research efforts have been directed to investigate the effectiveness of various TM-based catalysts. Table 4 compiles the reported results from Ti-based additive-enhanced MgH2 systems as well as corresponding synthetic approaches and kinetic behaviors.
Table 4. Hydrogen storage properties of Mg with various types of Ti-based catalysts.
For on-board solid-state hydrogen storage, a thermodynamic window in the range of approximately 25–45 kJ/mol·H2 is recognized for suitable metal hydride material [27]. Therefore, efforts have been directed to destabilize the MgH2, or in other words, reducing the ΔH of MgH2. It is expected that reducing ΔH can lower the working temperature for Mg-based hydride, which is crucial for on-board applications. Three typical approaches were proposed to destabilize MgH2, namely, alloying, downsizing, and stress effect.
The alloying method refers to alloying other elements with Mg to form a new alloy or hydride compound with lower stability of its hydride. So far, alloying systems have been reported including Mg2NiH4 [28], Mg2FeH6, Mg2CoH5, Mg2Cu [29], Mg(Al) [30], Mg51Zn20 [31], Mg2Si [32], Mg(In) [33], Mg(Sn) [21], Mg(AgIn) [34], MgReNi [35], Mg2M-xMxHy (M = Fe, Co, Ni), and so on. The principle is using a less-stable hydriding element A to form an Mg-A alloy. The energy diagram of the alloying method is illustrated in Figure 1. Since Mg-Ti is an immiscible system, Mg and Ti do not form an alloy. However, metastable Mg-Ti-H compounds have been reported. Kohta et al. [36][37][38][39][36,37,38,39] successfully synthesized MgxTi100−x (35 ≤ x ≤ 80) alloys with hexagonal close-packed (HCP), face-centered cubic (FCC), and body-centered cubic (BCC) structures by ball milling. Vermeulen et al. [40] reported that Mg-Ti-H system has a very low plateau pressure (≈10−6 bar at room temperature). Additionally, it will have a higher plateau pressure and a reversible hydrogen storage capacity of more than 6 wt%, when forming ternary compositions with Al or Si.
Figure 1. Schematic of destabilization process of a hydride MH using third element A. (Reproduced with permission from ref. [41]. Copyright 2016 Elsevier).
Nano-sizing of Mg-based materials is not only a strategy to enhance kinetics, but also considered as an approach to destabilize MgH2. It has attracted a great deal of effort in the past decades, despite its effectiveness and feasibility remaining controversial. The influence of nano-sizing on pressure-temperature dependence as well as ΔH is given in Figure 2. Theoretically, nanosizing to hydrides introduces excessive free energy to bulk or coarse particles. The excessive free energy may originate from lattice distortion [42]. Sadhasivam et al. [8] summarized the dimensional effects of nanostructured Mg/MgH2 materials. They reported that Mg/MgH2 with a particle size <5 nm has improved hydrogen storage properties. However, a great challenge remains in synthesizing such fine particles as well as maintaining the nano-size after thermal cycling for Mg-based materials. According to [8], the 1-dimensional Mg nanowire shows a promising hydrogen storage property. However, the nanowire structure would collapse into nanoparticles after a few cycles. Additionally, it is reported that reducing magnesium hydride structure to nanosize induces the stress/strain effect, which has been reviewed by Zhang et al. [43] It was pointed out that the stress/strain applied on MgH2 leads to lattice deformation and volume change, which endows the extra strain energy for MgH2. The research of Berube et al. [44] supported this claim. They reported that a 15% reduction of the formation enthalpy of nanostructured MgH2 can be achieved by the introduction of surfaces, grain boundaries, as well as the presence of γ-MgH2. Recent reviews [8][45][46][8,45,46], have provided thoughtful introduction and discussion into the thermodynamic aspects.
Figure 2. (a)Temperature dependence of the dissociation pressure of MgH2 and associated evolution of such a dissociation pressure for various approaches investigated, and (b) evolution of ΔH and ΔS as a function of the particle size of Mg. (Reproduced with permission from ref. [47]. Copyright 2018 Elsevier).
2.3. Kinetics
Kinetics for hydrogen storage materials is generally defined as the dynamic rate where hydrogenation and dehydrogenation take place in time. Kinetics measurements provide critical information on the rates of hydrogen uptake or release from Mg-based materials. It is necessary to be rather explicit when investigating hydrogenation and dehydrogenation kinetics. For pure Mg and MgH2 in the conventional form of coarse powders, they demonstrate very sluggish kinetics for hydrogen absorption and release, usually requiring over 400 °C for the reverse reactions. The slow hydrogenation rate of Mg, as well as dehydrogenation rate of MgH2, can be attributed to several intrinsic factors: dissociation of the hydrogen molecule, penetration of hydrogen through the surface, diffusion of hydrogen in the matrix, in addition to possible contamination in the sample environment.
For hydrogenation of Mg, dissociation of the hydrogen molecule on the Mg surface is often considered as a rate-limiting step. Table 3 summarizes the energies for hydrogen molecule dissociation on Mg and modified Mg surfaces. The reported values of hydrogen dissociation energy on the Mg surface are in the range of 0.4–1.15 eV (38.59–110.96 kJ/mol), which is higher than most transition metals, such as Ti, V, Ni, and Fe [48]. This means that a large energy barrier needs to be overcome for dissociation of H2 on pure Mg (0001) surfaces [49]. Another intrinsic issue is the slow hydrogen diffusion rate in MgH2. Figure 3 shows the geometry model of the reaction for an Mg/MgH2 particle. Based on the model, the hydride layer formed on the particle surface becomes the major barrier during hydrogenation, since the hydrogen atom diffusion rate in the hydride phase is much slower than that in the metallic phase. According to Spatz et al., the hydrogen diffusion coefficient (DH) of MgH2 is quite low (1.1 × 10−20 m2/s at 305 K) [50]. Figure 4 shows that the DH of MgH2 is at magnitudes lower than the DH of the Mg metal phase. It is also evident in this figure from the diffusion coefficient plots that most transition metals and their hydrides have DH several magnitudes higher than the DH of MgH2.
Figure 3. Schematic of the hydrogen absorption/desorption process in the MgH2/Mg. (Reproduced with permission from ref. [47]. Copyright 2018 Elsevier).
Figure 4. Hydrogen diffusion coefficients in different metals and hydrides. (Reprinted from ref. [51]. Copyright 2015 Chengshang Zhou).
Early work by Liang et al. [57][53] evaluated the catalytic effects of 3d-TM elements (Ti, V, Mn, Fe, and Ni) on the reaction kinetics of ball-milled catalyzed MgH2 (see Figure 6). The MgH2-Ti composite showed superior hydrogen desorption/absorption kinetics, exhibiting the best desorption kinetics at 573 K, followed in order by V, Fe, Ni, and Mn. The activation energies (Ea) of MgH2-Ti, MgH2-V, MgH2-Mn, MgH2-Fe, and MgH2-Ni are calculated to be 71.1 kJ/mol, 62.3 kJ/mol, 104.6 kJ/mol, 67.6 kJ/mol, and 88.1 kJ/mol, respectively, which are significantly reduced compared to that of the ball-milled pure MgH2 (120 kJ/mol). It was indicated that the TM catalysts could drastically improve the kinetic properties of MgH2, among which Ti-catalyzed MgH2 shows superior performance. Rizo-Acosta et al. [58][54] compared hydrogenation properties of MgH2 with the addition of early transition metals (Sc, Y, Ti, Zr, V, and Nb). As shown in Figure 7a,b, their results indicated that full reactions finished within less than 120 min in all cases and the hydrogen absorption rate increased along the sequence Y < V < Ti < Nb < Sc < Zr. However, an apparent degradation was observed when the cycling number increases. Interestingly, this evolution is less pronounced in the Ti-doped system, as shown in Figure 7c, which was attributed to the lattice mismatch between Mg and TiH2 hydride that limits Mg grain growth. Among all cases, MgH2-TiH2 nanocomposite presented the best cycling properties with a reversible capacity of 4.8 wt% after 20 cycles and the reaction time arbitrarily limited to 15 min.
Figure 7. (a) Hydrogen uptake curves of 95Mg-5ETM powder mixtures during reactive ball milling synthesis; (b) the corresponding absorption rates (derivative curves of a); and (c) hydrogen sorption curves at 573 K of MgH2-ETMHx NCs for different sorption sweeps. (Reproduced from ref. [58][54]. Copyright 2019 RSC).
Zhou et al. [90] prepared 49 additive-doped MgH2 samples by ultra-high-energy-high-pressure ball milling, in order to conduct a comprehensive survey on a wide range of additives and corresponding dehydrogenation temperatures of the catalyzed MgH2. The plot of the Thermogravimetric Analysis (TGA) dehydrogenation temperatures is shown in Figure 8, indicating that the additives containing the IV-B and V-B group elements are the most effective catalysts while the VII-B (Mn), VIII-B (Fe, Co, and Ni) groups show moderate catalytic effects. Besides, Ti and its compounds are more effective compared to those catalysts based on heavier elements (Zr, ZrH2, ZrO2, and Ta) in the same periodic group.
Figure 8. Effect of various additives on dehydrogenation temperatures of MgH2. (Reprinted with permission from ref. [90]. Copyright 2015 Elsevier).
Cui et al. [91] synthesized micro-sized Mg particles coated with nano-sized TM catalyst, showing that the nano-coating of TM on the Mg/MgH2 surface is more effective than co-ball-milling of Mg with TMs. The authors also suggested that the catalytic improvement on dehydrogenation kinetics can be ranked as Mg-Ti, Mg-Nb, Mg-Ni, Mg-V, Mg-Co, and Mg-Mo, and the hydrogenation kinetics is in a sequence of Mg-Ni, Mg-Nb, Mg-Ti, Mg-V, Mg-Co, and Mg-Mo.
It has been recognized that early transition metals (ETM) belong to the group of most effective catalysts. Despite some discrepancies in reported data, Ti-based catalysts, involving not only elemental Ti but also Ti hydrides, oxides, halides, and intermetallic compounds have shown great benefits in improving the hydrogen storage properties of MgH2. In-depth investigations of Ti-based catalysts are also beneficial for understanding the catalysis mechanism for the Mg-H2 system.
3.2. Catalytic Effects of Ti-Based Compounds
A large number of Ti-based catalysts have been explored for enhancing the hydrogen storage properties of MgH2. Early attempts using elemental Ti powder to ball-mill with MgH2 received encouraging results [57][53]. Soon, researchers found that TiH2 powder additive is very effective as well. Lu et al. [92] reported exceptional room temperature hydrogenation properties of MgH2-0.1TiH2 material prepared by ultra-high-energy-high-pressure (UHEHP) ball milling. Liu et al. [72] studied the effects of two different Ti hydrides (TiH1.971 and TiH1.5) on the hydrogenation kinetics of Mg. It pointed out an important fact that elemental Ti can easily react with hydrogen to form various Ti hydrides under certain temperatures and hydrogen pressures. During the reverse hydrogen reaction, the following equations can be summarized:
According to the Mg-Ti phase diagram, neither Ti nor Ti hydrides are immiscible with Mg or MgH2 phases. Furthermore, no ternary Mg-Ti hydride exists in the phase diagram. However, under a metastable condition, it is possible for Ti to dissolve into Mg and form a solid solution. Ponthieu et al. [93] reported Ti solubility in β-MgD2 up to 7 at.%, and Mg solubility in TiD2 up to 8%, which suggested shortened D-diffusion path due to the introduction of TiD2. An Nuclear Magnetic Resonance (NMR) study of MgD2/TiD2 composite found lattice coherent fluorite (fcc) structured TiD2 and MgD2, which is expected to be a fast H-diffusion pathway to accelerate the kinetics [94].
Another focus is discovering a novel metastable Mg-Ti-H hydride with a new structure. Kyoi et al. [95] synthesized Mg7-Ti-H FCC hydride using a high-pressure anvil cell. Asano and Akiba reported the ball-milling synthesis of a series of Hexagonal Closest Packed (HCP), Face-centered Cubic (FCC), and Body-centered Cubic (BCC) MgxTi100−x alloys, and Mg-Ti-H FCC hydride phases with chemical formulae of Mg40Ti60H113 and Mg29Ti71H57. These ternary hydrides had lower stabilities in comparison to MgH2 and thus show lower desorption temperatures.
TiO2 was considered an effective catalyst. Wang et al. [75] prepared ball-milled Mg-TiO2 and showed good hydrogenation and dehydrogenation kinetics. For the past two decades, however, the investigation of oxide catalysts paid more attention to Nb2O5, since it seems to be more efficient among transition metal oxides [96]. Actually, doping of TiO2 would present a similar effect comparing to the Nb2O5 catalyst. As suggested by Pukazhselvan et al. [97], TiO2 can be partially reduced to a lower 3+/2+ state (TiO and Ti2O3). The presence of MgxTiyOx + y oxide was also suspected, but no direct support was seen by X-ray Diffraction (XRD) results. More recently, Zhang et al. [98] showed good catalytic activity of carbon-supported nanocrystalline TiO2 (TiO2@C). It was reported that the dehydrogenation temperature of MgH2-10 wt%TiO2@C can be lowered to 205 °C and hydrogen uptake took place at room temperature. Berezovets et al. [99] reported that the Mg-5 mol% Ti4Fe2Ox was able to absorb hydrogen even at room temperature after hydrogen desorption at 300–350 °C and its cycling stability could be substantially improved by introduction of 3 wt% graphite into the composite.
Ti halides have been reported to offer a positive effect on the kinetics of MgH2. TM fluorides usually present superior catalytic effects and satisfactory kinetics. Malka et al. [80] reported the catalytic effects of a group of TM fluorides (FeF2, NiF2, TiF3, NbF5, VF4, ZrF4, CrF2, CuF2, CeF3, and YF3) on the kinetics of MgH2. The best catalysts for magnesium hydride decomposition were selected to be ZrF4, TaF5, NbF5, VCl3, and TiCl3. In another investigation by Jin et al. [100], it was suggested that TiF3 and NbF5 showed better effects over other TM fluorides. It was found that the hydride, for example, TiH2, formed after co-milling MgH2 with the fluorides, with an in situ reaction described as follows:
Moreover, Wang et al. [101] conducted a comparison study on the elemental Ti, TiO2, TiN, and TiF3 catalyzed MgH2 materials, showing that TiF3 had the strongest catalytic effect among them.
Ti-based intermetallics as catalysts have been receiving active attention in recent years. Early researchers used TiFe [102], (Fe0.8Mn0.2)Ti [103], Ti2Ni [104], and TiMn1.5 [105] additives to improve hydrogen storage properties of MgH2, showing that all these intermetallics were effective catalysts. Interestingly, some Ti-based intermetallics themselves, including TiFe and TiMn1.5, are known as hydrogen storage alloys. Zhou et al. [56][58] conducted a systematic investigation focusing on a series of Ti-based intermetallic catalysts (i.e., TiAl, Ti3Al, TiNi, TiFe, TiNb, TiMn2, and TiVMn). The results found that TiMn2-doped Mg demonstrated extraordinary hydrogen absorption capability at room temperature and 1-bar hydrogen pressure while its apparent activation energy is 20.59 kJ/mol·H2. The strong catalytic effect of TiMn2 is also confirmed by another experimental work by El-Eskandarany et al. [106][107][106,107] and first principles calculation by Dai et al. [108].
4. Synthetic Approaches
The synthesis methods of Mg-based hydrides have a great impact on their hydrogen storage properties. With expanding research scope of hydrogen storage materials, there are emerging preparation methods in recent years. Many hydrogen storage alloys can be prepared by physical methods, including ball milling [109], induction melting [110], arc melting [111], et cetera. Complex hydrides are usually prepared by chemical methods, such as organic synthesis, hydrothermal method, and solvothermal method [112]. However, conventional high-temperature preparations such as sintering or melting have been largely restricted due to the low melting temperature and high vapor pressure of magnesium [15]. Widely-used methods for Mg-based hydride preparation include ball milling, thin film deposition, and chemical methods.
Understanding the catalysis is critical to improving hydrogen absorption and desorption kinetics for Mg-based systems. Based on the understanding of the hydrogen reaction in the metal-hydrogen system [113][140], the hydrogenation of metal should go through the following five steps: (1) Physisorption of the H2 molecule, (2) dissociation of the H2 molecule, (3) surface penetration of H atoms, (4) diffusion of H atoms in the host lattice, and (5) hydride formation at metal/hydride interface, as shown in Figure 9. For the dehydrogenation reaction, a hydride particle could go through the following steps: (1) Hydride decomposition, (2) diffusion of hydrogen atom, (3) surface penetration, (4) recombination to hydrogen molecule, and (5) desorption to the gas phase. Either hydrogen absorption or desorption should be controlled by a rate-limiting step while other steps are likely in equilibrium.
However, the rate-controlling mechanisms in hydrogenation and dehydrogenation may not necessarily be the same. The physisorption of a H2 molecule on a metal surface needs a very low activation energy, so it is generally not considered a limiting step. The rest of the steps can be rate-limiting which is worthy of discussion. For dehydrogenation, steps 1, 2, and 3 (illustrated in Figure 9) can be considered as possible rate-limiting steps. Note that the hydrogen atoms should diffuse across the metal phase, in which the diffusion coefficient is much higher compared to that in the hydride phase. Moreover, the dehydrogenation has a H2 recombination step instead of dissociation. The recombination of H atoms into a molecule does not have an energy barrier to overcome [114][141]. From these aspects, it seems reasonable that the kinetic barrier of dehydrogenation could be lower than that of hydrogenation. However, dehydrogenation is an endothermic reaction whereas hydrogenation is exothermic, which means the hydrogenation of Mg is favored in respect of thermodynamics. These fundamental differences may change the activation barrier and lead to different reaction behaviors.
5. Mechanisms of Catalysis