1 Introduction
The development and utilization of renewable energy is important for carbon neutrality, and carbon-free fuel, which replaces the conventional ones, could promote the development in the field of transportation where petroleum production occupies a great proportion [1]. Hydrogen is considered as the most promising clean energy source owing to its capability to generate energy without any negative effects to the environment [2,3]. The gravimetric density of hydrogen energy is generally about seven times higher than the density of fossil fuels.
Hydrogen storage and transportation are essential in the industrial chain of hydrogen energy [4,5]. Pure hydrogen should be stored stably and able to output a steady hydrogen flow with the required state, pressure, and temperature, which is important for both mobile vehicles and hydrogen fueling stations. Today, most commercial hydrogen storage systems have a high pressure of over 700 bar in the vessel made of metals and carbon fiber reinforced plastic composite materials to avoid the risk of embrittlement and explosion [6,7]. Liquid hydrogen has a higher storage density than gas hydrogen, but an extra temperature control system that maintains the liquefaction temperature of 20 K leads to huge energy consumption [8]. As high-pressure hydrogen vessels and cryogenic liquefaction are far from the targets of performance and cost [9–11], the solid sorption technology for hydrogen storage, which is a modification method that decreases the absolute pressure and the additional energy consumption, has been focused on by the academics world nowadays [4,12].
Solid sorption can be divided into two main categories, physisorption and chemisorption. Physisorption is developed with novel porous materials which have a large specific surface area, such as activated carbon, graphene oxide, carbon nanotube, metal-organic frameworks, covalent-organic frameworks, and polymer [13–18]. However, the ultralow temperature of 77 K, which is the boiling point of the liquid nitrogen [19], is the commonly best temperature with the highest sorption capacity for physisorption. Such a low temperature causes huge additional energy consumption by the liquid nitrogen supplement.
Compared with physisorption, chemisorption which is based on the reversible reaction between the material and hydrogen could enhance the working temperature and consequently decrease the additional energy consumption. The chemisorption materials include alkaline-earth metal, rare-earth metal, and many kinds of alloys. Of all kinds of reactive metal elements, magnesium (Mg) is considered as the most promising candidate for solid hydrogen storage with its quite high hydrogen storage capacity, abundant resources, and relatively low cost. However, it has the disadvantages of high operation temperature and excess energy consumption [20,21]. Considering that adequate working temperature is indispensable to energy saving, researchers have studied many kinds of materials with chemisorption such as alkaline-earth metals [22,23], rare-earth metals [24,25], titanium-based (Ti-based) composites [26,27], aluminum-based (Al-based) composites [28,29], and other alloys with different components [30] and structures [31]. Among these materials, rare-earth metal-based alloy, which works at ambient temperature, has been extensively studied for its excellent performance with a prominent reversibility, stable cyclic ability, low equilibrium pressure, fast kinetics, and strong resistance to impurity. However, due to the heavy molecular weight of the alloy and the CaCu5-type hexagonal lattice structure of the intermetallic hydrides, the gravimetric hydrogen storage capacity of rare-earth metal-based alloy is rather low, which is less than w(hydrogen) = 1.5% (w is mass fraction) [32]. Calcium (Ca) hydride can be hydrolyzed to release hydrogen [33] and synthesized as a typical intermetallic compound with the capacity of w(hydrogen) = 1.9% and hexagonal lattice structure of AB5 hydride [23], leading to two approaches to the hydrogen storage system with the ambient decomposition temperature. However, the sorption temperature of the Ca hydride is 200–300°C which needs an additional energy consumption, and the disproportionation of the Ca-based alloy is prone to happen during the reversible reaction, leading to the irreversible reduction of the hydrogen storage capacity [34]. Beryllium (Be) is a light reactive metal with a high hydrogen storage density, but Be-based material has the obstacles of a worse stability and a hard synthetic route toward hydride. TiFe is a kind of cheap alloy with a capacity of w(hydrogen) = 1.9% and a good reaction performance. It has the advantage of a wide reactive temperature zone of 78–600 K, but the excitation process needs high pressure, and it is also easily infected by gas impurity, resulting in a poor capacity and stability [35]. The Al-based hydride has a low desorption temperature and a fast desorption kinetics. It has similar problems with Ca-based ones that the sorption pressure is impractically high.
The latest hydrogen storage target of the US Department of Energy (DOE) is w(hydrogen) = 6.5% for gravimetric density and 30 g/L for volumetric density respectively at ambient temperature and appropriate pressure [36]. In summary, for this target, the temperature of physisorption is too low, causing high energy consumption. The ideal chemisorption material having a high hydrogen storage capacity, a strong stability, and an excellent reaction kinetics is expected to achieve energy saving by working at near ambient temperature. But up to the present, few studies have reviewed the literature in this aspect yet.
In this review, the emphasis is focused on the filtration and summary of the solid chemisorption hydrogen storage materials available at the near ambient temperature defined from −20 to 100°C. Hydrogen storage materials are discussed either individually or combined as composites. Toward the mechanism of chemisorption, the principles of solid hydrogen sorption are studied and summarized from the lattice structure, phase equilibrium, thermodynamic process, and mechanical deformation. Based on the classification of the metal hydride, each kind of metal has its characteristics and shortcoming, and the specific optimization methodologies toward targeted temperature and capacity are obtained. Mechanical and chemical synthetic routes are analyzed, and molecular simulation and quantum simulation are summarized for screening the best material. This review gives a list of hydrogen storage candidates with chemisorption at near ambient temperature and technologies to the target temperature, and it provides guidance for the further development of hydrogen energy.
2 Mechanisms of chemisorption hydrogen storage
Hydrogen has the lightest density of known gases, which is only 1/14 of air. Its extremely small diameter and molecular weight allow hydrogen to enter the crystal lattice of many metals and materials [37,38]. The mechanism of chemisorption is distinguished from that of physisorption or conventional physical methods.
2.1 Lattice structures of chemisorption
The metal hydride is a kind of representative hydrogen storage material that can also be called interstitial hydrides since hydrogen occupies interstitial sites in the metal host lattice. In the hydrides, the microstructure of the metal lattice of the hydride is frequently different from elements and compositions. The typical lattice structures [39] are face-centered-cubic (FCC), hexagonal-close-packed (HCP), and body-centered-cubic (BCC), as demonstrated in Fig.1. Many types of interstitial sites depend on the host lattice. The tetrahedral sites where hydrogen is located inside a tetrahedron formed by metal atoms and octahedral sites where six hydrogen atoms enter the lattice forming an octahedron are the most common. The hydrogen atoms can fit into octahedral or tetrahedral interstitial lattice sites in the metal lattice, or a combination of both two types. In addition, there exist square pyramidal, triangular, or triangular bi-pyramidal sites to be occupied in some intermetallic hydrides. It is worth noting that sites are gradually filled with the increased hydrogen content until reaching the maximum capacity, and each lattice structure has its intrinsic site number which is the limit of chemisorption.
The metallic hydrides of intermetallic compounds can be represented as the ternary system ABxHn, and the variation of the elements makes it possible to tailor the properties of the hydrides. The most important types of intermetallic compounds forming hydrides including the prototype and the structure are listed in Tab.1. Most lattice structures are the three basic structures shown in Fig.1, while different intermetallic compounds may have the same structure due to the size and properties of components. The A element is usually rare earth or an alkaline earth metal and tends to form a stable hydride. The B element is often a transition metal and forms only unstable hydrides. Some well-defined ratios of B to A in the intermetallic compound (x = 0.5, 1, 2, 5) have been found to form hydrides with a hydrogen-to-metal ratio of up to 2. Generally, the hydride phase determines the limit of the hydrogen storage capacity of the corresponding material, while the sites among the lattice have been fully occupied by hydrogen atoms.
Tab.1 Most important families of intermetallic compounds forming hydrides including the prototype and the structure [40] |
| Intermetallic compound | Prototype | Hydrides | Structure |
|---|---|---|---|
| AB5 | LaNi5 | LaNi5H6 | Haucke phases, hexagonal |
| AB2 | ZrV2, ZrMn2, TiMn2 | ZrV2H5.5 | Laves phase, hexagonal or cubic |
| AB3 | CeNi3, YFe3 | CeNi3H4 | Hexagonal, PuNi3 type |
| A2B7 | Y2Ni7, Th2Fe7 | Y2Ni7H3 | Hexagonal, Ce2Ni7 type |
| A6B23 | Y6Fe23 | Ho6Fe23H12 | Cubic, Th6Mn23 type |
| AB | TiFe, ZrNi | TiFeH2 | Cubic, CsCl or CrB type |
| A2B | Mg2Ni, Ti2Ni | Mg2NiH4 | Cubic, MoSi2 or Ti2Ni type |
Laves phase alloy is a promising material with a higher hydrogen storage capacity. It has a stoichiometry of AB2 and can be formed when the atomic size ratio is between 1.05 and 1.67. Laves phase alloys can be easily prepared by arc melting, induction melting, or levitation melting of pure constituent metals in an inert atmosphere. There are three classes of Laves phases, i.e., cubic MgCu2 (C15), hexagonal MgZn2 (C14), and hexagonal MgNi2 (C36), as depicted in Fig.2. The C14, C15, and C36 lattice structures differ only by the particular stacking of the same two-layered structural units, which allows these structures to transform and twin by synchro shear. The stability of the three crystal structures is controlled by both the atomic size ratio of the A atoms and B atoms and by the valence electron concentration of the Laves phase. In these compounds, the A atoms occupy ordered positions as in diamond, hexagonal diamond, or related structures while the B atoms occupy tetrahedral positions around A atoms. Plenty of Laves phase hydrides are renowned for sorption of a considerable amount of hydrogen up to compositions of AB2H7.
2.2 Hydrogen-solid phase equilibrium of chemisorption
During the hydrogenation/dehydrogenation process, there is a phase equilibrium of different components, resulting in good stability and performance of the hydrogen storage [43–45]. The metal-hydrogen phase equilibrium diagram is manifested in Fig.3 [33], in which the abscissa is the atomic ratio of hydrogen to metal, and the ordinate is the desorption hydrogen pressure. Each curve is the ideal isotherm of the metal hydride formation process at different temperatures. The solid dissolution of hydrogen in metal will produce two phases which are the solid solution phase and the metal hydride phase. At the left of point A in Fig.3, when the hydrogen content is low, the solid solubility is proportional to the square root of the equilibrium hydrogen pressure of the solid solution, i.e., where is the equilibrium hydrogen pressure, H is the hydrogen atom, and M is the metal element. As the temperature is constant and the hydrogen pressure gradually increases, the hydrogen is dissolved in the metal to obtain the component of the solid-dissolve hydrogen metal phase called the α phase (O–A in Fig.3). For the process of A–B, the temperature and pressure of a certain interval will not change with the increasing hydrogen content, and the reaction is shown in Eq. (1). The α phase reaches saturation point and continues to react with hydrogen to form the metal hydride phase called the β phase (B–C in Fig.3). The interval between the two points is the two-phase mixture zone called the plateau zone (A–B in Fig.3), where the equilibrium hydrogen pressure remains unchanged under the condition of constant temperature. At the right of point B in Fig.3, the mixture is completely transformed into hydride, which means that the composition of the β phase gradually approaches the stoichiometric composition of the metal hydride. If hydrogen pressure continues to rise, the hydrogen content increases slowly, which is not efficient to be saturated in actual operating conditions.
2.3 Thermodynamic process of chemisorption
The advantage of hydrogen storage at near ambient temperature is an excellent way for energy conservation, and the thermodynamic process of chemisorption is, to a great extent, the key property of the hydrogen storage material [46–49]. The absorption of hydrogen is an exothermic reaction, while desorption is an endothermic process. The reaction can be written as [33]
where M is the metal element, H is the hydrogen atom, x and y are the hydrogen number of the compounds, and Q is the reaction heat.
Chemisorption can be regarded as the binding of the molecules or atoms of the gas and the molecules on the surface of the material by exchanging and sharing the electrons, which are called ionic bonds and covalent bonds respectively. According to the potential-energy curve, there is a distance where the total external force of the molecule is equal to zero, which is usually called physisorption. If the hydrogen molecule possesses a sufficient energy greater than the activation energy, the bonding process between hydrogen molecules and the material surface occurs and finally keeps stable at the distance where the total energy of the system is the lowest, which has a shorter distance between the molecules and stabler properties than physisorption.
Indeed, the most significant property of a metal hydride is its reversible hydrogen storage capacity, which enables it to be a solid hydrogen storage material and requires both excellent sorption and desorption performances. The hydrogen-metal interaction principles are almost the same for all metal hydride systems as shown in Fig.4. The features of the van’t Hoff diagram are obtained as expressed in Eqs. (2)–(5) [38]. Because the sorption process is generally exothermic and the desorption process is reversibly endothermic, the formula of the reaction at the plateau can be represented by the variation of standard enthalpy, standard entropy, Gibbs free energy, and other properties.
where ΔH and ΔS represent the enthalpy and the entropy change, respectively; R, the gas constant; ΔG, the change of Gibbs free energy; pdecomposition and phydrogeneration, the pressure of hydrogeneration and decomposition; and T, the absolute temperature.
From Fig.4 and Eq. (5), it is obvious observed that lnp is strictly proportional to 1/T over a large temperature zone. ΔH and ΔS can easily be inferred by the slope and the intercept of the fitting van’t Hoff diagram from experimental data. ΔH is a measure of the strength of the H-metal bond, which varies from metal to metal; ΔS reflects the configurational entropy loss of hydrogen from the gaseous to the solid state. The van’t Hoff diagram of different kinds of hydrides is exhibited in Fig.4 where the boxed area represents the optimal working temperature and pressure zone, among which ambient temperature and atmospheric pressure are the targets for sorption and desorption to realize energy-saving and reliable operation.
Generally, there exists hysteresis between desorption and sorption [51]. In brief, the desorption pressure, which is the dotted line in Fig.4, is always lower than the sorption pressure in a hydrogen storage cycle at the same temperature. Toward each isotherm, the driving force is hydrogen pressure, resulting in two branches of the reaction. Under the sorption condition, small hydrogen atoms enter the metal lattice through the gap among the metal atoms first and gradually form chemical bonds and hydride. According to the kinetics and the principle of chemical potential, the slightly high pressure accelerates the reaction, pushing the one-way reaction. By comparison, the desorption needs a slightly lower pressure as a driving force and chemical potential to move the component point from B to A in Fig.3. Hysteresis can be understood as the large stresses associated with the metal to hydride transformation which give rise to internal defects such as dislocations and stacking faults. In other words, the variation of pressure assists to open the lattice and the high pressure lets hydrogen atoms enter the metal for sorption while the low pressure lets atoms out of the hydride for desorption. Hysteresis decreases with increasing temperature as thermally activated stress relaxation processes set in. Generally, it is important to eliminate or at least minimize hysteresis for most applications.
2.4 Relation between mechanical deformation and chemisorption
For many storage scenarios, the mechanical deformation caused by hydrogen embrittlement during sorption leads to great risks. The atoms in the metal are arranged periodically before the hydrogenation. After hydrogen atoms are dissolved in the metal, they are generally in the gap of the atomic lattice. The places in the lattice where the atoms are dislocated are called dislocations, and hydrogen atoms tend to gather near the dislocations. Hydrogen atoms and other interstitial atoms can change the lattice distance and the initial stable lattice, and weaken the bonding. When an external force is applied to the metal, the stress distribution inside the material is uneven, resulting in the stress concentration in the transition region of the shape or at the internal defects and micro-cracks of the material. The hydrogen atom which has a strong diffusive ability, selectively appears in a specific position, especially at a tensioned position. With the stress gradient, hydrogen atoms diffuse along with the dislocation to the stress concentration region. Due to the interaction between hydrogen and metal atoms, the bonding force between the metal atoms is weakened, so that the high hydrogen region sprouts and propagates the cracks, causing the brittle fracture. The enrichment of hydrogen in the stress concentration region promotes the plastic deformation in this region, resulting in cracks and defects in the crystal. When hydrogen is concentrated on the cracks, it is sorbed on the surface of the cracks, reducing the surface energy, so that the cracks are easy to grow. The volume expansion after sorption and the pulverization after desorption prove the effect of the mass transfer of hydrogen atoms. The hydrogen molecules spontaneously move toward these positions together and cause the material to break, which is the reason why hydrogen is more deadly and risky. Heubner et al. [52] focused on the in-operando characterization of the volume expansion of metal hydride composites that could trigger mechanical stresses acting on the walls and internal assemblies of the storage container. The metal hydride composites were axially and radially confined for in situ stress analysis. It contained nearly equal porosities of φ = (28.2 ± 0.9) % (φ is volume fraction) in the as-compacted state. This enabled the volume of the hydrogen-sorbing metal particles to increase at the expense of the residual porosity without causing any macroscopic volume change of the metal hydride composites. The stresses during hydrogenation of confined composites could be more than 330% of the applied gas pressure. Radial stresses were higher than axial stresses. Goto et al. [53] investigated the effect of the stress and the diffusion coefficient variation with the concentration on hydrogen diffusion at low concentration. The concentration increase caused the lattice expansion which subjected the vicinity of the outer surface to the larger compressive stress of about two times of initial stress.
Chemisorption could cause mechanical deformation, whereas appropriate mechanical deformation could also improve the sorption performance. From the viewpoint of internal energy, the free energy of hydride is higher than the total amount of metal and hydrogen, thus the metal lattice itself does not form metal hydride at atmosphere hydrogen pressure. When the metal is subjected to external stress, it is equivalent to adding an extra free energy to the solid solution, which promotes the formation of metal hydrides. With this principle, the mechanical process is a common synthesis method to obtain the hydride with high capacity and stability [54–57]. The high-energy ball-milling technique is a delegate for the preparation and modification of the material, and it can simplify and achieve macroscopic preparation as well as reducing the whole cost, which is especially suitable for the preparation of hydrogen storage materials. Hydride-powder-sintering was applied by Chen et al. [58] in V40Ti26Cr26Fe8 hydrogen storage alloys. The sintering temperature ranged from 1623 to 1673 K, and the sintering temperature ranged from 4 to 8 h. The hydrogen desorption plateau of the alloy had been significantly improved with the sintering temperature increasing from 1623 to 1673 K. Furthermore, the hydrogen desorption plateau was further improved when continuing to increase the sintering time up to 6 h with the sintering temperature of 1673 K. From the scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) analysis, alloys with better performances contained less oxide phase and more BCC and C14 Laves phase.
3 Chemisorption materials for hydrogen storage
In this section, chemisorption materials including alkaline-earth metal clusters, rare-earth metal clusters, Ti-based materials, Al-based materials, and other alloys with different components and structures are discussed. The performance parameters at near-ambient-temperature are compared and analyzed with the reaction mechanism. Moreover, the results of some simulation research are included in material design and synthesis.
3.1 Alkaline-earth metal cluster
Alkaline-earth metal cluster, by definition, is the combination of elements of group IIA in the periodic table of elements, including Be, Mg, Ca, strontium (Sr), barium (Ba), and radium (Ra) [59,60]. It maintains the same covalent electron configuration which is ns2, leading to the loss of electrons easily in chemical reactions and forming positive divalent cations, and showing a strong reducibility. Therefore, Alkaline-earth metal cluster usually exists in the form of ions. Ca, Mg, and Ba are abundant in the earth’s crust, and their elemental substances and compounds are widely used.
Alkaline-earth metal clusters can react with most non-metal elements, resulting in the synthesis of different kinds of compounds such as oxide and hydride [60,61]. This property can be taken advantage of to be utilized in the field of solid sorption hydrogen storage. Compared with alkali metal hydride [62], alkaline-earth metal hydride can decompose at near-ambient-temperature and have a higher sorption capacity. A conventional method is synthesizing the alloy with exothermic metal and endothermic metal while hydrogen dissolves in the solid. The Alkaline-earth metal cluster works as the exothermic metal and its alloy leads to a better performance.
In this section, the three most common and promising alkaline-earth metal-based (Mg-based, Ca-based, and Be-based) compounds and corresponding hydrides are introduced and reviewed, and many optimized methodologies including modification and simulation are applied to adjust the sorption/desorption temperature, accelerate the reaction rate, increase sorption capacity, and predict the theoretical results.
3.1.1 Mg-based material
Mg-based materials usually have an extremely high operating temperature which is unacceptable to fuel cell vehicles and any other devices. The main purpose of this section is to summarize the method to decline the sorption/desorption temperature while maintaining a high capacity. Zhang et al. [63] reported the non-confined ultrafine Mg hydrides with w(hydrogen) = 6.7% reversible storage of hydrogen at ambient temperature. A novel strategy to synthesize nanoscale MgH2 in an organic solvent with the assistance of ultrasound was developed. It took advantage of the thermodynamically favored reaction between MgCl2 and LiH, and the ultrafine MgH2 nanoparticles predominantly of around 4–5 nm in size without scaffolds or supports were successfully obtained. A best reversible hydrogen storage capacity of w(hydrogen) = 6.7% at 30°C was measured by the thermodynamic destabilization and decreased kinetic barriers. Compared with other Mg hydrides with conventional preparation, the unique ultrasound-driven liquid-solid phase metathesis strategy was the key point of the Mg-based material to match the near-ambient zone and realize energy conservation in the system in the future.
Doping catalysts is a method for chemical reaction and material modification [64,65]. Generally, it can effectively improve reaction kinetics and the threshold value toward the target condition. Hanada et al. [22] reported the catalytic effect of nanoparticle 3d-transition metals, including nanoparticles Fe, Co, Ni, and Cu, with mechanical milling. The desorption temperature of MgH2 decreased to about 200°C successfully. The ball milling method proved to be an effective preparing technology which was regarded as a new technology called mechanochemical synthesis [66]. All ingredients were prepared by ball-milling for different milling times, and a longer milling period led to finer particle sizes and a relatively low hydrogen capacity. It was proved that MgH2 doped with x(Ni) = 2% (x is mole percent) could decompose and release at least w(hydrogen) = 6.5% hydrogen from 150 to 250°C, and the reversibility attenuated at the lower operating temperature of 150°C with the Mg and Ni ensemble than that at 200°C with the growth of the MgNi2 phase. Zhang et al. [67] synthesized a novel complex transition metal oxide, TiVO3.5, by using a solid-solution (Ti0.5V0.5)3C2 as a precursor, and studied the catalytic activity for the hydrogen storage reaction of MgH2. The sample prepared by oxidation at 300°C exhibited the optimal catalytic performance. The addition of w(catalysts) = 10% catalysts induced a 76°C reduction in the dehydrogenation temperature of MgH2 from 267 to 197°C. Approximately w(hydrogen) = 5% of the hydrogen was desorbed in 10 min at 250°C, the dehydrogenated sample could sorb approximately w(hydrogen) = 3.9% hydrogen in 5 s at 100°C. The addition of TiVO3.5 remarkably reduced the desorption activation energy from 153.5 to 62.4 kJ/mol, which was the most important reason for the reduced dehydrogenation temperature. They [68] also studied the catalytic activity of carbon-supported nanocrystalline TiO2 in the hydrogen storage reaction of MgH2. A 95°C reduction in the dehydrogenation temperature was obtained. Zhou et al. [69] also researched the desorption of MgH2 doped with a series of Ti-based intermetallic alloys. Most catalysts could adjust the temperature of desorption and the kinetics of both desorption and sorption of hydrogen. Fig.5 and Fig.5 indicate that the desorption temperature after ultra-high-energy-high-pressure planetary milling decreases from 414 to 333°C compared with the as-milled undoped MgH2. The Ti-based intermetallic dopants all show varying degrees of effectiveness in reducing the desorption temperature of MgH2. Meanwhile, the equilibrium pressures and the van’t Hoff diagram revealed that the thermodynamics of original reaction remained the same after the additions of the Ti-based intermetallic alloys [69]. The results shown in Fig.5 and Fig.5, clearly demonstrate that a relatively large hydrogen storage capacity could be sorbed by Mg at near-ambient-temperature and atmosphere hydrogen pressure, suggesting the strong effect of Ti-dopants that accelerate the sorption process. Among all systematic investigations, MgH2 doped with TiMn2 showed an extraordinary decomposition capacity of w(hydrogen) = 5.1% at ambient temperature and 1 bar hydrogen pressure and did participate in both sorption and desorption reactions. Overall, doping metal catalysts by mechanical methods such as ball milling and cold rolling can provide a better kinetic parameter and meet the requirements of lower operation temperature of Mg-based materials. At present, transition metals are widely applied to hydrogen storage alloy. However, generated unexpected alloy lattice structures during the process may attenuate the hydrogen storage capacity and the reversibility due to the decline of available sites of hydrogen in the materials. Catalyst type, proportion, and process are three keys to the optimization of Mg-based hydrogen storage materials.
The first principles calculation and Monte-Carlo simulation can calculate the molecular movement, bonding and bond fracture, and various atomic and electronic information [70–72]. It was reported by Bahou et al. [73] that Mg vacancies and hydrogen doping in MgH2 assisted to improve the hydrogen sorption capacity and decrease the desorption temperature. With the first principles calculation, the crystal structure (shown in Fig.6), the total energy, the gravimetric hydrogen capacity, and the relationship between desorption temperature and concentration were contained and the optimization scheme was proposed. As shown in Fig.6 and Fig.6, the decomposition temperature could be adjusted by controlling the concentrations of Mg vacancies and hydrogen dopant atoms from 4.2% to 5.8% to reach the optimum zone from 289 to 393 K respectively for the actual application of fuel cell vehicles, and the sorption capacity increased from w(hydrogen) = 7.658% to w(hydrogen) = 9.816% which exceeded the theoretical value. However, Mg vacancies are meant to restructure the Mg hydride. It needed much more effort and innovation process of the material except for the hydrogen storage performance. Based on the density functional theory and Monte-Carlo simulation, the sorption/desorption performance of Mg doped with different metals were studied, which set 400 K as its reference operation temperature [74]. The results demonstrated that Mg doped with nickel with a certain ratio which formed the hydride called Mg0.9375Ni0.0625H2 was able to reach the minimum desorption temperature of 326 K, satisfying fuel cell vehicles. Fig.6 and Fig.6 reflected the reaction kinetic of each composite, and a capacity of nearly w(hydrogen) = 7% was achieved, while the cyclic effective capacity varied widely and the Ni-doped one was the best. Following this regularity, a cluster of Mg-based alloys can be designed and synthesized to implement the targeted situation. Edalati et al. [75] reported the design and fabrication of the Mg4NiPd alloy which could store hydrogen at ambient temperature due to its low hydrogen binding energy. Although only a hydrogen storage capacity of w(hydrogen) = 0.7% of Mg4NiPd was feasible which was far lower than the hydrogen storage capacity requirement by the DOE and lower than that of LaNi5 with a capacity of w(hydrogen) = 1.4%, it was doubtless a Mg-based alloy operated at near ambient temperature. Simulation results can predict the hydrogen storage performance under a certain condition, and the optimization proportion and structure can also be carried out. To make sense, the experiment result is a significant proof, while some of the predicted materials are still hard to be synthesized.
Generally, the utilization of Mg-based alloy is meaningful despite of the low hydrogen storage capacity at the ambient temperature zone in consideration of the low cost and abundant resources. Compared with some conventional alloys, Mg has a lighter element weight and better hydride formation. As shown in Tab.7, efforts have been made to decline the operating temperature and enhance the hydrogen storage capacity. However, it is still hard to realize ambient temperature operation due to the relatively high activation energy and low capacity at near ambient temperature [76]. To fit the ideal and targeted operation condition, further development should concentrate on combining various methodologies and exploiting new kinds of reaction mechanisms.
Tab.2 Mg-based materials and the properties |
| Based material | Additive material | Method | Capacity /wt.% | Pressure /bar | Tsorption /°C | Tdesorption /°C | Ref. |
|---|---|---|---|---|---|---|---|
| MgH2 | – | Ultrasound-driven liquid-solid phase metathesis strategy | 6.7 | 30 | 30 | 30 | [63] |
| MgH2 | Nanoparticle Fe, Co, Ni, and Cu | Doping catalysts | 6.5 | 30 | – | 150–250 | [22] |
| MgH2 | Ti intermetallic | Doping catalysts | 3.3/5.1 | 1 | 25 | −3 | [69] |
| MgH2 | Mg vacancies and hydrogen doping | First principles calculations | 9.816 | – | – | 16–120 | [73] |
| MgH2 | Metals | First principles calculations, Monte Carlo simulations | 7.081–7.660 | 10 | 360 | 53–176 | [74] |
| MgH2 | Metals | Alloying | 0.7 | 100 | 32 | 32 | [75] |
3.1.2 Ca-based material
Ca is a kind of active metal, whose hydride CaH2 hydrolyzes and releases hydrogen after contacting water [33]. CaH2 is stable which provides a promising approach to applying Ca as the basic material of hydrogen storage. Given a lower reaction temperature than Mg, Ca-based hydride has more potential to be utilized in solid sorption hydrogen storage devices.
The pure Ca hydride can be synthesized by highly purified Ca and hydrogen at 200–300°C for about 2 h, and the hydride can decompose at about 600°C, which is even higher than that of Mg hydride and adverse to the hydrogen storage. As shown in Fig.7, Kong et al. [77] developed a new hydrogen supplication system that is a cylinder reactor with water input and hydrogen output and investigated the feasibility of using Ca hydride as a hydrogen storage system for fuel cell vehicles. To satisfy the required hydrogen generation rate of the fuel cell, the reactor, pipeline, and experimental procedure were well-designed. The key parameters were the water vapor partial pressure and the nature of the hydride form, and the stable hydrogen generation rate was feasible at the stable water vapor partial pressure. In other words, the hydrogen generation rate was proportional to the water vapor partial pressure. Xiao et al. [78] used Ca hydride as the additive of Mg17Al12 hydride to improve the reaction mechanism. The sample was treated with ball milling and put into pure water for hydrolysis. Compared with the rapidly interrupted reaction by the passive film, the composite showed a continuous hydrogen generation ability. The active metal hydride worked as a kind of catalyst to prevent the formation of the film over the hydride and enhance the hydrolysis activity [79]. In this kind of hydrogen production system, the Ca hydride is disposable, thus the recycling preparation is necessary for further development.
CaNi5 is a typical intermetallic compound with the hexagonal lattice structure of AB5 hydride [23]. With the addition of nickel, this kind of Ca-based material shows a good hydrogen storage capacity of w(hydrogen) = 1.9% and acceptable pressure plateau at ambient temperature. Sandrock [23] reported a hydrogen sorption pressure of about 0.2–5 bar at the temperature zone of 10–80°C, which formed the intermediate product called CaNi5H5, and the second plateau for the last hydrogen atom was over 25 bar. The appropriate working temperature makes low-pressure hydrogen storage at ambient temperature possible for fuel cell vehicles and other fields. However, the disproportionation phenomenon of Ca-based hydrides occurs at any temperature zone while decomposing, resulting in the formation of Ca2Ni7Hx and a decline in the capacity of hydrogen storage [34]. The solution to this problem is annealing the sample at a high temperature for recovering the capacity and the thermodynamic characteristics. The cyclic stability and activity of Ca-based binary alloy need to be improved. Considering the superiorities of low cost, appropriate temperature, and relatively low-pressure plateau [34], the multicomponent alloy should be a good development direction [80]. Chumphongphan et al. [81] used a synthesis method in which other metals like Zirconium and Chromium were added to the intermediate products of CaNi5 during the annealing process and occupied the sites of Ca and Ni. The mass proportion of each metal was designed for the target alloy and different products were studied for screening the suitable product for actual applications. Although the capacity decreased a bit, CaNi4.9Cr0.1 showed the lowest desorption plateau pressure at 25°C. Integrating Mg into CaNi5 is a direct way to improve the capacity, and for this purpose, Liang and Schulz [82] investigated the properties of Ca-Mg-Ni alloys prepared by mechanical alloying. Although the performance was improved, results showed a higher pressure plateau and restrained the generation of Ca2Ni7, and other impurities such as CaNi2 and CaNi3 formed and needed to be annealed. Similar results were reported by Si et al. [83] that Ca3−xMgxNi9 had a higher pressure plateau and lower capacity due to the transformation of structure which needed improvements. From Fig.8, measured pressure-concentration-temperature (P-C-T) curves of Ca-based composites (Ca-Ni, Ca-Ni-Zr, Ca-Ni-Cr, Ca-Ni-Zr-Cr, Ca-Ni-Mg) under various conditions were obtained. Moreover, some researchers reported that palladium treatment by mechanical milling could decrease the pressure plateau of CaNi5, increase the reaction rate, and improve the resistance to being poisoned in the air [84–86].
The active property of Ca makes decoration an effective method to improve the driving force of materials with physisorption. Despite the distinction of basic principles, Ca decoration plays a chemical part in the total sorption process. Ma et al. [87] investigated the hydrogen sorption capacity and atomic structure of Ca-decorated defective boron nitride nanosheets (BNNS) by first principles calculation based on the density functional theory. Ca could be strongly bonded with the defects in the BNNS and maintain a stable state, resulting in 3 kinds of sorption modes including physisorption, chemisorption, and quasi-molecular binding mode. Obviously, much more hydrogen was adsorbed around the Ca than in other spaces in the BNNS due to the strong activity. A supercell of Boron nitride nanotube (BNNT) with three unit cells was used to simulate materials with 8 boron monovacancies or boron nitride monovacancies defects. Then, the 8 Ca atoms were sorbed separately on the 8 vacancy sites (labeled as 8Ca/BNNT-VB and 8Ca/BNNT-VBN, VB is boron monovacancy, VBN is boron nitride monovacancy), and the optimized atomic geometries are shown in Fig.9 and Fig.9. Bader analysis confirmed that each Ca atom donated 1.36 e and 1.35 e to the defective BNNT in 8Ca/BNNT-VB and 8Ca/BNNT-VBN substrates, respectively. Then, the 6 hydrogen molecules were sorbed on each Ca atom and the optimized structures were presented in Fig.9 and Fig.9 at ambient temperature and 70 bar. The maximum theoretical hydrogen sorption capacity could reach w(hydrogen) = 6.4% and w(hydrogen) = 6.9% in two kinds of Ca-defect pairs, respectively. Mao et al. filtrated four kinds of metal atoms coated B40 fullerene composites and recommended that 6Ca@B40 was the best candidate for hydrogen storage with w(hydrogen) = 6.66%. Compared to the weak physisorption in the pure B40, the density functional theory (DFT) calculations showed that Ca atoms were bonded strongly to the surface of B40 and prevented from clustering, while hydrogen bonded around the Ca atoms and the surface of the sphere structure. Moreover, Ca decorated graphene-based nanostructure for hydrogen storage could improve the sorption performance effectively [89–92].
Due to the limited gravimetric capacity and poor reversibility, Ca-based solid sorption materials have not been the hotspot so far. In Tab.3 and Tab.4, Ca has served as the decoration and catalyzer with its active properties, especially in the field of the physisorption of nanomaterials. Although the ambient temperature working zone of Ca alloys has the best advantage, the worse cyclic stability is the main obstacle because of the film caused by the active properties of Ca and CaH2 during the cycle.
Tab.3 Ca-based materials and the properties |
| Based material | Additive material | Method | Capacity/(wt. %) | Pressure/bar | Tsorption/°C | Tdesorption/°C | Ref. |
|---|---|---|---|---|---|---|---|
| Ca | Ni | Alloying | 1.9 | 25 | 25 | 25 | [23] |
| Ca | Ni | Alloying | 1.48 | 0.1–0.5 | 10–80 | 10–80 | [34] |
| CaNi5 | Zr, Cr | Metal Substitute | 1.04/1.05/0.85 | 0.42–0.72 | 25 | 25 | [81] |
| Ca | Mg, Ni | Alloying | 0.85–1.56 | 0.4–12 | 30 | 30 | [82] |
| Ca | Mg, Ni | Alloying | 1.71/1.23 | 30 | 25 | 25 | [83] |
| CaNi5 | Pd | Alloying | 0.8 | 1 | 25 | 25 | [84–86] |
| CaH2 | Mg17Al12 | Inducing and hydrolysis | 13.6 | 40–50 | − | 70 | [78] |
3.1.3 Be-based
Be hydride is always regarded as a high-density hydrogen generator and is applied in the solid propellant rocket engine. Be is the first element of group two, resulting in the lowest molecular weight and highest weight capacity of its hydride, thus Be has been a potential metal that receives attention from researchers.
A common synthesis method of Be hydride is the reduction reaction between LiAlH4 and BeCl2 [93,94]. It is worth noting that Al is also a hydrogen storage material candidate, making the mixed products a potential material directly. Hosseinabadi [95] reported the characteristics of Be-doped hydrogen storage alanate nanopowders which utilized the properties of both Be and Al. The results showed that different ratios of two main ingredients led to different crystal systems and capacities, but it was still close to the theoretical value by linear interpolation. In the near-ambient temperature zone, the 12-sorption/desorption cycle of particles of nano-size was tested and proved to be stable, and the pressure plateau increased with the temperature. Bruzzone et al. [96] investigated the Be-doped TiFe alloy, in which Be worked as a decoration. This kind of ternary alloy had a better performance at ambient temperature than the prototype due to the modification of equilibrium pressure. According to the temperature-driven target, a decreasing amplitude of about 70°C shifted the initial working temperature to ambient temperature. Lithium-Be hydrides were studied by Zaluska et al. [59], proving the reversibility, safety, and near-ambient operation temperature. Isothermal curves of different LiH-Be ratios exhibited a relatively low-pressure plateau of 1 bar, and characterization analysis exhibited multiple compounds and molecular structures. A capacity of w(hydrogen) = 8.7% was tested when the ratio was m(LiH): m(Be) = 3:2 (m is mass) at 300°C which was the optimal condition. Usually, Be is hard to serve as the main hydride but as the decoration to improve the overall performance. It is also one of the most hazardous elements for humans in the form of powders, which carries the challenge during cycling with hydrogen.
First-principles calculation helps to filtrate and verify candidate Be materials and structure, respectively [97–100]. Ghosh and Padmanabhan [101] reported the Be-doped single-walled carbon nanotubes, resulting in a significant impact on the storage capacities of the octagonal rings of the Stone-Wales defects at 298 K and 140 bar. Both Wang et al. [99] and Li et al. [97] did the research on boron and Be doped nanostructure material with a capacity of about w(hydrogen) = 12%–25%. Beheshtian and Ravaei [102] designed a BeO nano-cage with a stereochemical structure and optimized the structure stability and binding energies. In the past few years, efforts have been made to predict wonderful Be-based materials for both chemisorption and physisorption. Be-based hydrogen storage materials have disadvantages of high price and meandering path to synthesize and storage. The synthesis of the target microstructure is still hard to be realized. In addition, risk control and safeguard procedures are also necessary toward the energy system design.
In Tab.5, Be decoration in the nanomaterials exhibits a high hydrogen storage performance theoretically but experimental results at near-ambient temperature are still needed. The actual synthesis and system design still have great challenges.
Tab.5 Be-based materials and properties |
| Based material | Additive material | Method | Capacity /(wt. %) | Pressure/bar | Tsorption/°C | Tdesorption/°C | Ref. |
|---|---|---|---|---|---|---|---|
| Alanate | Be | Doping element | 19 | 50 | – | 250 | [95] |
| TiFe | Be | Alloying | 1.4 | 10–20 | 50 | 10–20 | [96] |
| LiH | Be | Alloying | 8.7 | 40 | 270-300 | 150 | [59] |
| Single-walled carbon nanotubes | Be | First principles/molecular dynamics calculations | 16 | 267 | 25 | 25 | [101] |
| Be | B | First principles calculations | 25.3/21.1 | – | 27 | 27 | [99] |
| Graphene | B, Be | First principles calculations | 12.4 | – | 27 | 27 | [97] |
| BeO | Nano cage | First principles calculations | 7.64 | – | – | – | [102] |
3.2 Rare-earth metal cluster
The most common Rare-earth metal hydrogen storage material is LaNi5. To improve the comprehensive performance, the trinary alloy is produced. It can be represented by LaNi5 −xMx and keep the hexagonal lattice of alloy and hydride, in which M is the Ni substitute and x is the atomic proportion of the alloy. Cr, Fe, Co, Cu, Ag, Pd, Al, and Mn are the common substitute metals and the main differences caused by substitute metals are the reaction pressure plateau, reaction enthalpy, and the unit lattice volume. At present, rare-earth metal mixtures are also utilized to substitute expensive lanthanum, forming multi-alloy with Mm (mischmetal). Zhu et al. [24] investigated the hydrogen storage capacities and kinetics characteristics of the AB5-type LaNi5−xCox alloy materials with various proportions. Co-substitution increased the cell parameters from P-C-T curves, decreasing the plateau pressure and increasing the hydride stability. The alloys had good kinetics characteristics with a full sorption period of 300 s, and a maximum improvement of 30% was proved when x was equal to 0.5 in Fig.10(a), 10(b), and 10(c). The target temperature zone was 70 to 110°C which was close to the ambient temperature and feasible to be applied to the proton exchange membrane fuel cells. A cyclic stability of 1000 times was effectively improved by the Co addition. The maximum hydrogen storage capacity was w(hydrogen) = 0.1% higher than that of pure LaNi5 in Fig.10 and Fig.10. The SEM images (Fig.10 and Fig.10) showed the anti-crash ability of the Co-doped sample, as the average grain sizes were almost the same. Similar efforts were also made to study the performance and property of LaNi5−xAlx [103]. The larger volume of Al detained atomic immigration during the reaction, thus stabilizing the crystal structure and reliving the capacity degradation, resulting in a maximum increase of 1000 cycles from 89.2% (LaNi5) to 98.2% (LaNi4.5Al0.5).
MmNi5 alloy has the same structure and mechanism as LaNi5 with a lower reaction enthalpy and activation energy, but the reaction pressure plateau is about 7 times higher than that of LaNi5. MmCo5 has an appropriate pressure plateau, but its capacity is only half that of LaNi5. Srivastava and Panwar [25] synthesized the MmNi5 alloy with a stoichiometric ratio of Mm (x(La/Mm) = 22%, x(Ce/Mm) = 52%, x(Nd/Mm) = 15%, x(Pr/Mm) = 11%) and nickel by melting and mechanical ball milling. The maximum hydrogen storage capacity was w(hydrogen) = 1.5% with fast activation and fast reaction kinetics (2 min and 5 min for 90% sorption/desorption) in comparison to LaNi5 alloy. Molinas et al. [104] modified two kinds of MmNi5−xAlx alloys and reported their feasibility to be applied on a polymeric membrane fuel cell installed onboard. At a sorption temperature of 30°C and desorption temperature of 0°C, an acceptable pressure of 30 and 3 bar were both available. The maximum capacity was maintained between w(hydrogen) = 1.3% and w(hydrogen) = 1.5% in the ambient temperature zone. Meanwhile, the sample kept a good hydrogen sorption capacity and only lost a relative capacity of about 5% after 180 cycles. All progresses are concentrated on the kinetics and operation condition, while the all kinds of hydrides can be represented as the La1−xMmxNi5−yMyH6 with a certain stoichiometry and lattice structure. The optimal gravimetric hydrogen storage capacity can be easily obtained by the hydride in Tab.6. The substitution of light metal elements increases the capacity, and the overall performance and the mechanism change with the property of the additive such as atom size and electronegativity.
Tab.6 Rare-earth metal and properties |
| Based material | Additive material | Method | Capacity/(wt. %) | Pressure/bar | Tsorption/°C | Tdesorption/°C | Ref. |
|---|---|---|---|---|---|---|---|
| LaNi5 | Co | Alloying | 1.561 | 30 | 70–110 | 70–110 | [24] |
| LaNi5 | Al | Alloying | 1.4 | 65 | 70 | 70 | [103] |
| MmNi5 | w(La) = 22%, w(Ce) = 52%, w(Nd) = 15%, w(Pr) = 11% | Mechanical modification | 1.5 | 90 | 27 | 27 | [25] |
| MmNi5 | Al | Alloying | 1.3/1.5 | 25–35 | 30 | 0 | [104] |
A series of simulations and calculations have been developed for rare-earth metal materials due to the potential for actual application in consideration of their excellent performance. To predict the system efficiency and performance, kinetic equations of materials and transfer equations were required. Researchers made a series of simplifications on the experimental platform and used the equations of numerical heat transfer as well as sorption models fitted from the experimental data. Many types of reaction vessels have been designed to get different heat and mass transfer conditions. On this basis, multi-dimensional simulation has been conducted. At present, researchers have published mathematical models of metal hydride reactors with reasonable assumptions and simplifications, variable controlling, equation selection, boundary conditions, initial conditions, control equations, and iterative solutions. For a single material, the most important experimental parameters are the geometry of the reactor and the thermal management mode, which means that the heat transfer performance of the material itself and the heat transfer capacity of the designed vessel determine the performance of the hydrogen storage material under actual working conditions. Mohammadshahi et al. [105] provided a mathematical model of LaNi5-H2 pair (schematic of the metal-hydride tank in Fig.11(a)). The model followed the former work [106,107] and was modified to study the sorption and desorption processes inside an axially symmetric cylindrical pressure vessel. A simplified computation region of the tank in polar coordinates was chosen as half of the space between two fins, starting from the center of one fin, over half of the internal diameter of the tank (the meshed area in Fig.11). These equations matched the initial conditions of the experimental condition and modified the equilibrium pressure and effective thermal conducting equation based on experimental data. P-C-T curves with reaction fraction that was the relative capacity of both sorption and desorption were obtained in Fig.11 and Fig.11. As can be seen in Fig.11, the plateau slope of experimental data at 100°C increased a bit compared with the two other isotherms at 40°C and 80°C, due to the occurrence of LaNi5H3 in the phase transformation from α to β above 80°C. These theoretical isotherms matched the experimental data well and were available for system simulation. Chandra et al. [108] modeled and numerically simulated a 5 kg LaNi5-based hydrogen storage reactor. Under the operation condition, 290 s and 375 s were necessary for reaching a relative hydrogen capacity of w(hydrogen) = 80% and w(hydrogen) = 90%, respectively.
The reaction enthalpy of hydrogen would impede the reaction due to the negative feedback and thermal inertia, and designed fins could accelerate the releasing process of the reaction enthalpy by effective heat transfer performance. To deal with the poor heat transfer capacity of LaNi5 powder material, composite material and modified reactor were studied. Oi et al. [109] described a total of six modes of heat transfer within the bulk metal hydride packed particle structure, which was heat conduction at the particle contacts, heat conduction through a thin hydrogen film, heat radiation between the particles, heat conduction through the particles, heat conduction through hydrogen in larger void spaces, and heat radiation between vacant spaces. Afzal et al. [110] reviewed the heat transfer techniques in a metal hydride hydrogen storage system, whose proportion of fields was shown inFig.12. Graphite compacts, mesh, fins, cooling fluid jackets, cooling tubes, and other non-conventional methods were the popular solutions to this problem [111–116]. Initiative methods included cooling fluid jackets and cooling tubes aimed to increase the temperature gradient and the mass flow rate to offset the low heat transfer coefficient. On the other hand, in Fig.13, passive enhancements including graphite compacts, mesh and fins were aimed to improve the intrinsic heat transfer coefficient of either the material or the reactor [117,118]. Nguyen et al. [119] emphasized the thermal management of sorption storage systems. The requirements of the effective thermal conductivity of the material and heat transfer coefficient with heating/cooling media were worked out, which were 2 W/(mK) and 1000–1200 W/(m2K) for use in fuel cell systems. Both heat transfer enhancement of metal hydride and self-sufficient thermal management solutions were reviewed with specific designs of hydrogen storage and fuel cell system.
3.3 Ti-based material
Ti is reactive with hydrogen. TiFe has inferiorities in the sorption process with high temperature and pressure and poor anti-impurity ability [35]. To overcome these drawbacks and develop a more practical alloy, other metal additives such as cobalt, chromium, and manganese are studied [120–124]. TiFe and TiFe2 are 2 kinds of table phases of intermetallic compounds. Only TiFe has sorption ability in the temperature zone of −196 to 300°C at 65 bar. Its hydride releases hydrogen under the atmosphere condition, thus repeated excitation process is required. In a study, different methods including heat treatment, dry milling, wet milling with hexane, and ethanol were adopted to activate TiFe alloy for sorption [26]. The sample milled with ethanol proved the best performance, owing to the higher polarity and high reusing ability. Through the milling process, hydrogen could spread more easily in the TiFe alloy and the oxidation layer was easily reduced. The TiFe-based multicomponent alloy replaced or added by transition metal was studied and regarded as an important method to improve the performance of TiFe. Dematteis et al. [125] investigated the chemical and microstructural characteristics of nine different compositions in the Ti-Fe-Mn system in Fig.14. The Ti and Mn substitution in TiFe based alloy are in the range from y(Ti) = 48.8 % to y(Ti) = 54.1% (y is atom percent) and from y(Mn) = 0 to y(Mn) = 5.3 %. The formation of secondary phases enhanced the activation properties and lowered the environment requirement of first hydrogenation. According to the experiments, a high reversible capacity of w(hydrogen) = 1.63% at 25 °C between 0.3 and 25 bar. Yang et al. [126] synthesized and studied the effect of chromium, manganese, and yttrium on the microstructure and hydrogen storage properties of TiFe-based alloy. The sample was prepared by arc melting in an argon atmosphere using pure metal as raw materials with the target proportion, and the powder sample was crushed mechanically. The results exhibited sloped and lower equilibrium plateau regions in Cr substituted alloys, and flatter and higher equilibrium plateaus in Mn substituted alloys. Yttrium addition could decrease the equilibrium plateaus of Mn substituted alloys. Zhou et al. [27] developed Ti-Zr-Mn-Cr-V alloys and studied the effect of partial substitution of Zr for Ti and Cr for Mn on microstructures and hydrogen storage performances. The hydrogenation process happened at a pressure plateau of 26.4 bar and 30°C with a maximum hydrogen storage capacity of w(hydrogen) = 1.78 %; the desorption process happened at a pressure plateau of 93.7 bar and 90°C with an extremely low hysteresis. The microstructure kept intact after at least 100 effective cycles. Nayebossadri and Book [127] focused on the selection and development of high-pressure Ti-Mn based alloy for compressing hydrogen from 15 bar to over 350 bar with a maximum operating temperature of 130°C. Fourteen kinds of samples with different additive metals and proportions were prepared by arc melting on a water-cooled copper crucible in an argon atmosphere and pressed into pellets. At ambient temperature, Ti30V15.8Mn49.4 (Zr0.5Cr1.1Fe2.9) was selected as the optimum composite for high pressure with the best overall performance. In addition, the cyclic properties of the modified alloy were proved relatively stable with a reduction of only 9% in hydrogen capacity for more than 1000 cycles of sorption and desorption at 70 and 0.5 bar respectively. Compositional analysis by EDS showed that the microstructure of alloys maintained the TiMn2 C14 Laves phase.
Ti can also work as a decorated element in other hydrogen storage materials. Particularly, decorating Ti into the materials of physisorption modifies the pattern of hydrogen storage to the combination of chemisorption and physisorption, by which the hydrogen storage capacity is greatly improved. Sathe et al. [128] used the density functional theory to study the hydrogen storage capacity in Jahn-Teller distorted fullerene functionalized with Ti. The practical hydrogen storage capacity and the van’t Hoff desorption temperature revealed that hydrogen is reversibly stored in the proper temperature and pressure zone with a capacity ofw(hydrogen) = 10.5%. The desorption temperature zone was from −28 to 253°C, and the sorption condition was between −173 and 27°C and pressure of 0–50 bar. According to the fact that the borophene had been proved to have the most stable structure of the boron monoatomic layer [129,130], Wen et al. [131] theoretically calculated the pattern and capacity of Ti-decorated borophene. Meanwhile, Ti atoms were also decorated to enhance the performance of hydrogen storage to a maximum 15.065%. The reason why Ti-decorated borophene can improve the capacity of hydrogen storage was that the charge transfer occurred between Ti and borophene, resulting in a built-in electric field. Lebon et al. [132] investigated the sorption of hydrogen on Ti-doped zigzag graphene nanoribbons by a nonlocal van der Waals functional. The simulation results exhibited a capacity of over w(hydrogen) = 6% of model systems by considering the graphene nanoribbons (GNR) decorated with Ti atoms at both sides of the ribbon.
Ti is usually an excellent metal basis for hydrogen storage due to its active property and light mass. Tab.7 shows that the gravimetric storage capacity is influenced by the pair of metal elements and the proportions. Meanwhile, Ti can improve the hydrogen storage performance of nanomaterials with a relatively higher capacity than the alloy, but the actual synthesis is still under study.
Tab.7 Ti-based materials and the properties |
| Based material | Additive material | Method | Capacity /(wt. %) | Pressure /bar | Tsorption /°C | Tdesorption /°C | Ref. |
|---|---|---|---|---|---|---|---|
| TiFe | Hexane (C6H14) and ethanol (CH3CH2OH) | Mechanical modification | 1.2 | 30 | 25 | 25 | [26] |
| TiFe | Mn | Alloying | 1.63 | 50 | 25 | 25 | [125] |
| TiFe | Cr, Mn, Y | Alloying | 1.4 | 30 | 10–50 | 10–50 | [126] |
| Ti | Mn, Cr, Zr, V | Alloying | 1.88 | 26.4–93.7 | 30–90 | 30–90 | [27] |
| Ti, Mn | V, Zr, Cr, Fe | Alloying | 1.7 | 15–350 | 27–50 | 27–50 | [127] |
| C24 fullerene | Ti | First principles calculations | 10.5 | 0–50 | −173 to 27 | −28 to 253 | [128] |
| borophene χ3 | Ti | First principles calculations | 15.065 | — | — | — | [131] |
| Zigzag graphene nanoribbons | Ti | First principles calculations | 6.4 | — | 25 | 25 | [132] |
3.4 Al-based material
Al is an abundant and frequently-used metal. AlH3 is a fascinating metal hydride that contains a tremendous volumetric and gravimetric energy and mass density. Its low desorption temperature and fast hydrogen desorption kinetics make Al a promising hydrogen storage media. AlH3 is kinetically stable and the desorption to Al and H2 is kinetically restricted due to the activation barriers such as particle sizes, surface oxide layers, or coatings. Therefore, it can be tailored to various performances based on different application scenarios.
Graetz & Reilly [ 28] and Ahluwalia et al. [29] designed the solids AlH3 mixed with light mineral oil as a hydrogen carrier, studied the hydrogen storage performance of the corresponding hydrogen storage system shown in Fig.15, and developed a detailed model assembled with desorption kinetics, heat transfer requirements, startup energy and time, stability, hydrogen buffer requirements, storage efficiency, and hydrogen storage capacity. The simulation results indicated that alane needed a reactor temperature higher than 200°C to decompose at reasonable velocities. At the system level, compared with the overall capacity of w(hydrogen) = 10.1%, a capacity of w(hydrogen) = 4.2% usable hydrogen was achievable with configured alane slurry. Under optimum conditions, a maximum usable capacity of w(hydrogen) = 8% as available. The desorption kinetics was the main limitation of hydrogen loss rather than any other factors from the simulation model. Similarly, Grew et al. [133] built a model to evaluate the feasibility of applying AlH3 as hydrogen storage media for fuel cell vehicles. About 0.4 kg AlH3 supported 30 W power output in a row for 25 h, providing a specific energy of 600 Wh/kg, and all values of output would continuously decrease with the decline of the hydrogen density. Because it was hard to be regenerated, solid AlH3 could not be applied for long-term utilization. In terms of practical experiments, Wang et al. [134] developed a kind of nanosized Al hydride that was confined within the mesopores of a high surface area graphite. At 150°C, it showed a reversible hydrogen capacity of w(hydrogen) = 0.25%. The minimum desorption temperature started at about 60°C. The nanostructure cannot improve the sorption capacity effectively but enhance the mechanism and kinetics of the sorption/desorption cycle. Liang et al. [135] reported the synergistic catalytic effects of TiO2 and Pr6O11 on superior dehydrogenation performances of α-AlH3. It reduced the initial dehydrogenation temperature from 140 to 43°C accompanied by a hydrogen storage capacity of w(hydrogen) = 8.3% and the apparent activation energy of the dehydrogenation reaction of only 56 kJ/mol. The reason for this was that the multiple valence state conversions of two additives promoted the electron transfer of hydrogen atoms in AlH3, and a novel dehydrogenation pathway of PrH2.37 formed simultaneously, which could accelerate the breakage of Al-H bonds. Theoretical calculations further confirmed that there were fewer electrons transferred around the hydrogen atoms in AlH3 with additives.
Al-based alloy composite hydride can effectively overcome the shortcomings of Al hydride. Ianni et al. [136] successfully synthesized Al porous scaffolds by sintering a pelletized NaAlH4 and x(TiCl3) = 2% mixture and melt-infiltrated with high loadings of NaAlH4. With the temperature-programmed desorption (TPD) test, the NaAlH4 loading affected hydrogen release, resulting in a desorption temperature rising with the increasing scaffold loading, which was about 100°C lower than that of the NaAlH4-bulk standard sample. The shift in the desorption temperature of the infiltrated NaAlH4 showed that the material was in intimate contact with the Ti additive in the Al scaffold. Assuming a consistent porosity across all non-infiltrated scaffolds, increasing the loading of infiltrated NaAlH4 would decrease the specific surface area of contact between NaAlH4 and Al scaffold, leading to a change in behavior toward bulk NaAlH4. Nanoconfined NaAlH4 had a higher relative hydrogen release in the temperature range of 148–220°C. Urbanczyk et al. [137] built and studied a hydrogen storage tank operated with Na3AlH6, getting the stable performance and proving the feasibility for applications. Hexahydride Na3AlH6 had a theoretical hydrogen storage capacity of w(hydrogen) = 3% and a lower pressure plateau compared to NaAlH4. The material was prepared from TiCl3 doped NaAlH4 by the addition of NaH in the ball milling process. At least 45 cycles were available under the condition of 177°C and 25 bar with a cyclic hydrogen storage capacity of w(hydrogen) = 1.7%. To improve the sluggish kinetics and high dehydrogenation peak temperature of NaAlH4, Huang et al. [138] employed the bottom-up strategy to confine NaAlH4 between graphene nanosheets with a millefeuille-liked multi-layer morphology. A dehydrogenation temperature decline of 55.7°C was observed with a high graphene layer loading of up to 90%. Compared with the pure NaAlH4 sample, the hand-milled NaAlH4 with a graphene nanosheets mixture of 10% also showed an enhanced kinetic performance, because the integration of graphene nanosheets accelerated the nucleation rate in dehydrogenation, and improved the kinetic.
Compared with other materials, Al-based composites have a near-ambient desorption temperature and reasonable pressure plateau for application in Tab.8, while the hydrogenation process with high pressure and risk has become the main obstacle for researchers and engineers. The onboard system with a long-term cycle ability is the research direction. Additionally, the reaction mechanism and kinetics should be further explored.
Tab.8 Al-based materials and properties |
| Based material | Additive material | Method | Capacity/(wt. %) | Pressure/bar | Tsorption/°C | Tdesorption/°C | Ref. |
|---|---|---|---|---|---|---|---|
| Alane | – | Simulation | 4.2 | – | 200 | 200 | [29] |
| Alane | – | Nano-confining | 0.25 | 60 | 150 | 60–155 | [134] |
| AlH3 | TiO2 and Pr6O11 | Ball milling | 8.3 | – | – | 43 | [135] |
| NaAlH4 | TiCl3 | Composition | / | – | 148–220 | 148–220 | [136] |
| Na3AlH6 | Ti/TiCl3 | Doping | 1.7 | 25 | 170 | 170 | [137] |
3.5 Other alloys
3.5.1 High entropy alloy (HEA)
HEA consists of five or more equal or approximately equal amounts of metals, and it is found that the specific strength, fracture resistance, tensile strength, corrosion resistance, and oxidation resistance properties of some HEAs are better than those of the conventional alloys. These advantages are helpful to improve the performance of metal hydrides.
The preparation methods of the HEAs are various, based on those of the normal alloy. Dissolution by arc light heating in vacuum, dissolution by resistance heating in vacuum, and dissolution by high-frequency induction heating in vacuum are realized in the laboratory. The basic methodology is that metal components melt at high temperature over their melting point together, then stirred and combined in the vacuum vessel and finally cooled to the ambient temperature. In recent years, this method in combination with mechanical milling and annealing has enjoyed great popularity in synthesizing samples. Sleiman and Huot [30] prepared TiVZrNbHf by arc melting after mixing all raw elements in equal proportion. Montero et al. [139] applied ball milling in an Ar atmosphere to deal with the low melting temperature and high vapor pressure of the Mg component. Moreover, another approach was that raw metals were directly ball milled at a hydrogen pressure of 70 bar for 1 h and the hydride composite was prepared, while it has the risk of high pressure of hydrogen in the preparation process. Target HEA could be restored after desorption. The lattice structure and the component composition of the HEA were proved to maintain by characterization analysis with X-ray diffraction, SEM, electron backscatter diffraction (EBSD), etc.
Edalati et al. [140] prepared the TiZrCrMnFeNi alloy to test the microstructural features and the sorption capacity. In Fig.16, the results exhibited the formation of coarse grains of the C14 Laves phase and fine grains of the Ti- and Ni-rich cubic phase in high-entropy alloy TiZrMnCrFeNi. The reversible hydrogen storage capacity was w(hydrogen) = 1.7% at ambient temperature without activation treatment. The (Ti0.8Zr0.2)1.1Mn1.2Cr0.55Ni0.2V0.05 alloy was developed by Tu et al. [141] for hybrid hydrogen storage of fuel cell bikes. It had excellent overall hydrogen storage performances, including the hydrogenation pressure of 27.34 bar at 333 K and the dehydrogenation pressure of 1.42 bar at 273 K. The hydrogen storage capacity was w(hydrogen) = 1.82% at 298 K, and other thermodynamic properties could meet the operating temperature and pressure requirements of a hybrid hydrogen storage tank. Kunce et al. [142] used laser engineered net shaping to synthesize two kinds of TiZrNbMoV HEA with different microstructures by a laser power of 300 W and 1 kW. A higher laser power led to a fuller interfusion of all components inside the HEA, as the powder quality and other parameters of the synthesis equipment were modified, resulting in the expected preliminary BCC phase of the alloy. A low laser power generated a two-phase mixture composed of a BCC phase and an orthorhombic NbTi4-type phase, and Zr-rich precipitates occurred. However, the stable BCC phase brought out a lower capacity of w(hydrogen) = 0.59%, while a two-phase mixture with more defects could adsorb a maximum amount of hydrogen of w(hydrogen) = 2.3% at ambient temperature. Liu et al. [143] doped Ni, Fe, and Cu into HEA (Ti0.85Zr0.15)1.05Mn1.2Cr0.6V0.1 with target proportion and investigated the hydrogen storage capacity in the ambient temperature zone from 0 to 45°C in detail. All samples maintained C14 Laves phase and minished crystallite particle size after the sorption and desorption. The obvious hysteresis phenomenon occurred at each test point in the temperature zone due to the lattice inflation of sorption, and higher temperature resulted in a higher pressure plateau and lower capacity. The best performance belonged to (Ti0.85Zr0.15)1.05Mn1.2Cr0.6V0.1Cu0.1 which could sorb and desorb w(hydrogen) = 1.81% of hydrogen at 273 K and w(hydrogen) = 1.58% of hydrogen at 318 K, respectively. Hu et al. [144] and Shen et al. [145] conducted density functional theory calculations to study the hydrogen storage properties of HEA TiZrVMoNb modified from TiZrHfMoNb. The BCC phase transformed to the FCC phase when the mass fraction of hydrogen reached w(hydrogen) = 1.5% during the sorption process, and hydrogen occupied octahedral and tetrahedral interstitial sites before and after phase transformation, respectively. This transformation led to an extra capacity up to w(hydrogen) = 2.65%. With the small atomic weight of V, different site occupation, large lattice distortion and charge redistribution, the substitution of Hf for V in HEA improved the hydrogen storage capacity and promoted the desorption of its hydrides.
HEA is an extension study of alloys, resulting in a complex microstructure. A kind of HEA may have a few lattice structures with hydrogen storage sites meanwhile, and the solid phase change occurs during the cycle with some reversible or irreversible fluctuation of the hydrogen storage capacity. Many efforts are made with the adjacent elements of those known representative metals and those elements with the same main group which is shown in Tab.9. Existing hydrogen storage HEAs are competitive as high energy additives to propellants or explosives.
Tab.9 HEA and properties |
| Based material | Additive material | Method | Capacity/(wt. %) | Pressure/bar | Tsorption/°C | Tdesorption/°C | Ref. |
|---|---|---|---|---|---|---|---|
| TiVZrHfNb | – | Composition | 1.2 | 2–20 | 300 | 100–400 | [30] |
| TiVZrNb | Mg | Doping | 2.7 | 25 | 25 | 25 | [139] |
| TiZrCrMnFeNi | – | Alloying | 1.7 | 100 | 30 | 30 | [140] |
| TiZrNbMoV | – | Laser engineerednet shaping | 2.3 | 85 | 50 | 29 | [142] |
| (Ti0.85Zr0.15)1.05Mn1.2Cr0.6V0.1 | Ni, Fe and Cu | Doping | 1.81/1.58 | 40 | 0/45 | – | [143] |
| TiZrVMoNb | – | First principles calculations | 2.65 | 50 | – | 302 | [144,145] |
3.5.2 Metal film
Film and membrane metal hydride can also be applied to hydrogen separation and alarm devices, and there exists a mass transfer process in all hydrogen storage materials, leading the application of thin films for hydrogen storage to becoming a very important method to achieve large surface area with a fast sorption/desorption rate for hydrogen, slower pulverization, significantly lower critical pressure, and critical temperature and better heat transfer coefficient. A volume expansion always happens during each chemisorption and physisorption process, resulting in internal stress preventing the mass transfer. In addition, protective surface coating is needed from the reaction with other gas impurities for sensitive materials such as Ti-based materials.
Except for the pure material film, composite films satisfy the demand for wider application with a better performance and capacity, and nonreactive material can modify the performance of the based hydrogen storage material. de Almeida Neto et al. [31] produced polyetherimide-LaNi5 composite films to improve the performance and stability of LaNi5 in the air. As the modification in many papers, ball milling and vacuum annealing were applied to prepare the LaNi5. Both the polyetherimide and LaNi5 powder were solubilized, stirred, and sonicated in the organic solvent. The films were dried in an oven at 100°C under vacuum condition for 48 h and finally peeled off (Fig.17). It is hard to define the relationship between particle size and rotation parameters (time, speed, number of balls, and material of balls), which would change with different devices and materials. The original material had pulverization which gradually cut down the sorption capacity. Similarly, high-intensity milling could decrease the capacity of hydrogen storage due to the defects where the regular hydrogen storage sites were destroyed. However, a better reaction kinetics could be obtained which benefited the performance under the condition of composite films. The sorption capacity of composite films was proportional to the mass ratio of the ball-milling LaNi5. It changes to a capacity of w(hydrogen) = 0.6% from the original capacity of w(hydrogen) = 1%. Compared with pure LaNi5, composite films had a slower kinetics and stronger stability with the property of the polymer, and the operation temperature maintained ambient which is the same as that of the basic metal hydride.
Composite films with multiple reactive materials can take the advantage of each material. Higuchi et al. [146] prepared nano-composite three-layered Pd/Mg/Pd thin films for hydrogen storage. The results showed that the desorption temperature decreased with the increase of the Mg thickness in Pd/Mg/Pd films, for which the minimum temperature was lower than 100°C when the Mg layer was 800 nm and the hydrogen sorption capacity was w(hydrogen) = 5%. Reddy and Kumar [147] similarly synthesized Pd/Mg/Pd/Mg/Pd films and got a reversible capacity of about w(hydrogen) = 3.5% in at least 15 cycles. The minimum sorption and desorption temperature was 70°C with the catalytic properties of Pd, which would increase with operation cycles due to the drawback for stability. Han et al. [148] prepared Mg85Ni14Ce1 amorphous alloy films with different thicknesses by direct current magnetron sputtering. The nanosized amorphous alloys reversibly absorbed and desorbed hydrogen with almost unchanged desorption kinetics at a low temperature of 120 °C. The structure of the amorphous alloy could be fully recovered after hydrogen sorption and dehydrogenation cycles.
The thin structure and limited volume doom result in that film is not a large-scale hydrogen storage material. On the other hand, this unique structure and property make it have the potential for the gas filtration or serve as the wall surface of the hydrogen storage vessel. Especially, in the application of the small-scale energy supply system, the sorption storage film can provide a reliable structure and both the adequate gravimetric and volumetric hydrogen storage capacity. The hydrogen storage capacity varies with the specific materials in Tab.10.
4 Conclusions and perspectives
The solid chemisorption hydrogen storage technology, owing to its potentially high gravimetric/volumetric storage capacity and low energy consumption, presents exciting and promising prospects for near-ambient-temperature technologies for large-scale hydrogen storage. Nowadays, the feasibility of solid chemisorption hydrogen storage materials at the near ambient temperature has been demonstrated at the laboratory-scale level and exhibits good performance. Although some market products have been launched in the past few years, they can be integrated with a completed energy storage system and the solid sorption part works as the hydrogen storage unit without a strict requirement of performance or capacity. The promotion of the energy storage capacity, power, and continuous operation period can be attributed to the addition of the amount of the hydrogen which can be reflected in the huge size and mass of the total system. The efforts and challenges mentioned herein indicate that efficient hydrogen storage by chemisorption materials at present cannot reach the optimal hydrogen storage performance, which means that near ambient temperature is not the best operation temperature of the material. The main efforts and challenges in developing the advanced solid chemisorption hydrogen storage materials and systems in recent years are aimed at:
1) Chemisorption rules including the thermodynamic and mechanical processes are microscopically explained for chemisorption hydrogen storage. Based on the metal classification, the mechanism, performance, and defects of hydrogen storage materials at present are classified and expounded.
2) Additions of various catalysts, metal compounds, and carbon materials can also improve the hydrogen storage performance for near-ambient-temperature-available chemisorption materials. Theoretical model calculations and experimental synthetic methods with processes such as high-temperature melting, annealing, high-energy ball milling, and modification by different reagents are utilized.
3) The adjustment of the operation temperature zone of the chemisorption materials always causes an inevitable decline in sorption capacity, cyclic stability, and reaction kinetics. In most cases, the principle behind the enhancement with a certain ensemble, fraction, processing or structure is hard to be understood and interpreted.
At present, the directions are pointed out toward future investigation of high-value composites and cost-effective hydrogen storage systems. Some hydrogen storage materials are promising such as the Mg hydrides by the ultrasound-driven liquid-solid phase metathesis strategy, CaNi5 with Cr/Zr substitute, TiFe with organics or hydrogen reactive metals, and other advanced materials like incompletely etched multilayer Ti2CTx with an unprecedented hydrogen uptake of w(hydrogen) = 8.8% at room temperature and 60 bar H2 [149]. For the entire hydrogen energy industry, the control of the temperature and pressure can realize the promotion of long-term and large-scale practical applications of hydrogen energy for different regions and working conditions. Although the comprehensive performance of hydrogen storage materials has been improved significantly over the past several decades, materials with a higher hydrogen storage density, longer cycle life, better kinetics, and lower cost are still urgently needed, especially in the ambient temperature range. When designing the modified material, some innovative preparation methods such as the unique ultrasound-driven liquid-solid phase metathesis strategy [63] can effectively improve the property and performance of the hydrogen storage materials, thus some interdisciplinary research methods are promising to overcome the bottleneck of the current materials. When testing the performance, advanced characterization methods should be applied to the sample before, after, and even during the hydrogenation/dehydrogenation so that the completed microscopic morphologic process and property evolution can be revealed especially in the solid-gas interface. Advanced characterization should not be confined to the conventional methods such as X-ray diffraction (XRD), SEM, P-C-T, TPD, but some new methods such as wide angle X-ray diffraction (WAXD) and small angle X-ray scattering (SAXS). From the perspective of hydrogen storage system design, the coupling problems of new materials in actual large-scale storage scenarios should be optimized. The design of the external system has a great feedback effect on the working condition of hydrogen storage materials, and the thermal management system and mass transfer system in the reaction process directly affect the overall energy consumption and efficiency. Under the background of carbon neutrality, near ambient temperature chemisorption hydrogen storage materials and the corresponding energy system can be extensively used in transportation, energy utilization, and other fields, leading to the sustainable and healthy development of industry and social life.