Progress in improving thermodynamics and kinetics of new hydrogen storage materials

Abstract

Hydrogen storage material has been much developed recently because of its potential for proton exchange membrane (PEM) fuel cell applications. A successful solid-state reversible storage material should meet the requirements of high storage capacity, suitable thermodynamic properties, and fast adsorption and desorption kinetics. Complex hydrides, including boron hydride and alanate, ammonia borane, metal organic frameworks (MOFs), covalent organic frameworks (COFs) and zeolitic imidazolate frameworks (ZIFs), are remarkable hydrogen storage materials because of their advantages of high energy density and safety. This feature article focuses mainly on the thermodynamics and kinetics of these hydrogen storage materials in the past few years.

Graphical abstract

Keywords

ammonia borane / hydrogen storage materials / hydrides / kinetics / metal organic frameworks / thermodynamics

1 Introduction

Hydrogen is an ideal energy carrier which holds tremendous promise as a new renewable and clean energy option because it is light, highly abundant and its oxidation product, water, is environmentally friendly. On the other hand, hydrogen is flammable, particularly explosive, highly diffusive, and its volume density is only one three thousandth of that of gasoline. Therefore, hydrogen storage concerns both hydrogen production and hydrogen application, and assumes a critical role in initiating a hydrogen economy. The ultimate goal related to hydrogen storage is to develop technologies with high energy density, high efficiency and safety [1-3]. Up to now, there are three ways to consider in hydrogen storage: high pressure gas, low temperature liquid and solid-state hydrogen storage materials. Hydrogen storage under high gaseous pressure has been applied with some success but the drawback in this solution is the unresolved safety problem, which makes the solution totally incompatible for large-scale private vehicles. Liquid hydrogen storage at low temperature has a higher volume density, but the energy waste resulting from hydrogen liquefaction can reach up to 50% of the storage. Moreover, liquid hydrogen is readily volatile and still stays significantly unsafe. Hydrogen storage involving chemical reactions or physical adsorption in some solid materials provides devices with high energy density and safety is no longer a problem: it is undeniably the most promising way for hydrogen storage [4]. Great efforts have been made to develop materials that can hold sufficient hydrogen and possess suitable thermodynamic and kinetic properties. The key point is to prepare materials with high capacity that can absorb/desorb hydrogen reversibly. Various hydrogen storage systems, such as metal hydrides, chemical hydrides, complex hydrides, and metal organic frameworks have attracted increasing interest and been explored for onboard hydrogen storage applications [5]. Among the various hydrogen storage systems, thermodynamics and kinetics of complex hydride and metal organic frameworks are discussed in the present paper.

2 Complex hydrides

Compared with traditional metal hydrides (e.g. LiH, LaNi₅H₆), complex metal hydrides have recently got much attention as a promising new kind of hydrogen storage material because of their high hydrogen storage capacity and moderate operation temperature. However, the intensive investigation of complex hydrides did not begin until Bogdanović et al. reported the reversible hydrogen storage of Ti-doped NaAlH₄ in 1997 [6]. Light weight complex hydrides, namely alanates, borohydrides and amides, are mostly attractive candidates for hydrogen storage. They have the similar formula, M_m(XH_x)_n (M=Li, Na, K, Mg, Ca, etc.; X=Al, B, N). Among them, NaAlH₄ has been studied roundly, but its capacity is only 5.6 wt% for practical use. Herein, we will focus on some new materials with higher capacities, like LiAlH₄, LiBH₄ and LiNH₂.

2.1 LiAlH₄

LiAlH₄ releases 7.9 wt% H₂ below 220℃, corresponding to the first two steps [Eq. (1) and Eq. (2)] of decomposition [7, 8]. Apparently, the temperature is still high considering the U. S. Department of Energy (DOE) target of 80℃. In addition, the reversibility of LiAlH₄ is poor. Therefore, many efforts have been done to overcome these disadvantages, such as mechanical milling, doping catalysts, combination with other hydrides and solvent coordination. The corresponding thermodynamics and kinetics investigations after these treatments are essential to the understanding of their influence on LiAlH₄ and, further, on improvement of the hydrogen storage properties of LiAlH₄.

\begin{array}{}\text{(1)}& {\text{3 LiAlH}}_{\text{4}}\stackrel{\text{150-175}℃}{\to }{\text{Li}}_{\text{3}}{\text{AlH}}_{\text{6}}{\text{ + 2 Al + 3 H}}_{\text{2}}\text{, 5}{\text{.3 wt\% H}}_{\text{2}}\end{array}

\begin{array}{}\text{(2)}& {\text{Li}}_{\text{3}}{\text{AlH}}_{\text{6}}\stackrel{\text{180-220}℃}{\to }{\text{3LiH + Al + 3/2 H}}_{\text{2}}\text{, 2}{\text{.6 wt\% H}}_{\text{2}}\end{array}

2.1.1 Mechanical milling

Mechanical milling can decrease the starting temperature of reactions (1) and (2) to some extent, depending on the milling conditions and material supplier. Ares et al. [9] reported that 30 min milling reduces the crystallite size of LiAlH₄ to about 90 nm from the original 300 nm. Hence, the diffusion distance is reduced. The activation energy is lowered and the kinetics is improved for the first step [Eq. (1)] of LiAlH₄ decomposition. But for the second step [Eq. (2)], there exist different results from the influence of milling on the kinetics [9-11]. On the other hand, mechanical milling can hardly change the thermodynamics of LiAlH₄. Too long time milling can lead to the partial decomposition of LiAlH₄. So mechanical milling alone has limited influence on LiAlH₄, and it is usually used as a common method of doping catalysts and combination with other hydrides.

2.1.2 Doping catalysts

Various catalysts have been used on LiAlH₄ and could be classified into five categories: pure metals, such as Ti [12-15], Ni [15, 16], and V [15]; alloys, like TiAl₃[12, 14], Ti₃Al [14]; metal halides, such as TiF₃ [8, 17], TiCl₄ [12, 18], TiCl₃ [19], VCl₃ [20, 21], LaCl₃ [16], ZrCl₄ [22], HfCl₄ [22], {\text{TiCl}}_3\cdot \frac{1}{3}{\text{AlCl}}_3 [13], VBr₃ [21] and NiCl₂ [23]; carbon materials, such as CNFs (carbon nanofibers) and Vulcan XC72R [15, 24]; composites, like VCl₃@CNFs [24]. These catalysts reduce the decomposition temperature of LiAlH₄ more or less and improve the kinetics of dehydrogenation. Among them, metal halides perform best. Doping with 2 mol% {\text{TiCl}}_3\cdot \frac{1}{3}{\text{AlCl}}_3 [13] or 4 mol% HfCl₄ [22] makes LiAlH₄ decompose around 100℃. Moreover, under the catalytic effect of 4 mol% TiF₃ [8], LiAlH₄ starts to decompose at 60℃ and releases 5.0 wt% H₂ at up to 145℃. A thermodynamic change of Li₃AlH₆ was reported by Chen et al. [13] when {\text{TiCl}}_3\cdot \frac{1}{3}{\text{AlCl}}_3 was doped into LiAlH₄, resulting in the partial reversibility.

2.1.3 Combination with other hydrides

Different hydrides combined with LiAlH₄ may form new compounds or just mixtures. Under mechanical milling, LiH and NaH can react with LiAlH₄ to form Li₃AlH₆ [25] and Na₂LiAlH₆ [26], respectively. The occurrence of these reactions indicates that Li₃AlH₆ and Na₂LiAlH₆ are more stable than LiAlH₄, which is also proved by the increase of their dissociation enthalpy [25, 26]. Although their dehydrogenation temperatures increase to some extent, the reversibility is greatly improved as compared with LiAlH₄.

Most hydrides form mixtures rather than react with LiAlH₄ under milling. Then many multiplex systems have been developed, such as LiAlH₄–MgH₂ [27, 28], LiAlH₄–CaH₂ [29], LiAlH₄–LiBH₄ [30] and LiAlH₄–NaBH₄ [31]. The dehydrogenation of LiAlH₄ occurs at lower temperatures and its kinetics has been improved in all the mixtures. But its thermodynamics seems unchangeable. LiAlH₄ provides its Al element to combine with Mg and B to form alloys, and it becomes impossible to regain LiAlH₄ or Li₃AlH₆ under moderate condition of gas-state hydrogen storage.

2.1.4 Solvent coordination

Wang et al. [32] discovered a five-step physiochemical pathway for the rehydrogenation of LiAlH₄ from Li₃AlH₆, LiH and Al, and the reactions are shown in Eq. (3):

\begin{array}{}\text{(3)}& \begin{array}{l}\frac{\text{1}}{\text{3}}\text{L}{\text{i}}_3\text{A}\text{l}{\text{H}}_6+\frac{2}{3}\text{A}\text{l}+{\text{H}}_2+4\text{T}\text{H}\text{F}\underset{\text{B}\text{a}\text{l}\text{l}\text{m}\text{i}\text{l}\text{l}\text{i}\text{n}\text{g}\text{i}\text{n}\text{T}\text{H}\text{F}}{\overset{\text{T}\text{i}\text{C}{\text{l}}_3\mathrm{,}97.5\text{b}\text{a}\text{r}{\text{H}}_2}{\to }}\\ \text{L}\text{i}\text{A}\text{l}{\text{H}}_4\cdot 4\text{T}\text{H}\text{F}\underset{\text{V}\text{a}\text{c}\text{u}\text{u}\text{m}}{\overset{{60}^{\circ }\text{}\text{C}}{\to }}\text{L}\text{i}\text{A}\text{l}{\text{H}}_4+4\text{T}\text{H}\text{F}\end{array}\end{array}

THF is strongly attached to Li⁺ in LiAlH₄ and form the four coordinated lithium solvate, hence the enthalpy changes and makes it thermodynamically favorable to form LiAlH₄·4THF. Graetz et al. [33] used active Al and easily got LiAlH₄ . 4THF after stirring the THF slurry of LiH and Al under 13 bar H₂. The desorption of solvent becomes easier when the solvent is changed to Me₂O because of the thermodynamic unstability of Li–Me₂O bond compared with Li–THF bond [34]. Much work is needed to lower the H₂ pressure and increase the conversion. The solvent coordination method pushes the study of LiAlH₄ as a hydrogen storage material and provides an efficient method for other materials.

2.2 LiBH₄

LiBH₄ releases as high as 13.8 wt% H₂ at a temperature of over 400℃, shown in Eq. (4), and attracts more attention than LiAlH₄. But it is only partially restored even at a rigorous condition of 600℃ and 35 MPa hydrogen [35]. To lower the operation temperature and improve the kinetics and reversibility of LiBH₄, the methods including mechanical milling, doping catalysts and combination with other hydrides have also been applied. Herein, we will not expand these methods.

\begin{array}{}\text{(4)}& \text{L}\text{i}\text{B}{\text{H}}_4\to \text{L}\text{i}\text{H}+\text{B}+\frac{3}{2}{\text{H}}_2\end{array}

The inevitable particle agglomeration and growth lead to the hydrogen storage property degradation. Additionally, it is difficult to obtain freestanding nanopaticles for most hydrides. These obstacles push scientists to seek new methods, like nanoconfinement, which is generally realized by introducing the hydrides into the nanoporous of porous materials through solution impregnation or melt infiltration.

The solution impregnation method is easy to control and used mostly. Fang et al. [36] prepared LiBH₄/AC (activated carbon) sample, which starts to release hydrogen from 220℃. It is 150℃ lower than the onset dehydrogenation temperature of bulk LiBH₄. And the kinetics increases one order of magnitude. The rehydrogenation property is greatly improved and more than 6.6 wt% H₂ can be recovered. These improvements are not limited to kinetics, but are extended to thermodynamic change, which can be implied from the increase of the dissociation pressure. Brun et al. [37] used carbon monoliths with 0.5 nm pores as the host material and got similar results.

Gross et al. [38] applied melt infiltration method to the preparation of nanoconfined LiBH₄ with carbon materials with different pore sizes. The corresponding properties are summarized in Table 1. It is clear that nanoconfinement greatly enhances the properties of hydrogen storage, and smaller pores are inclined to obtain better kinetics and higher reversibility. SBA–15 is also used as an excellent porous material in confining LiBH₄, whose onset dehydrogenation temperature is reduced to 150℃. But LiBH₄/SBA–15 shows poor reversibility due to the formation of Li₂SiO₃ and Li₄SiO₄ [39].

2.3 Li–N–H system

In 1910, Dafert and Miklauz reported that when Li₃N reacted with H₂, Li₃NH₄ was formed [40]. In fact, Li₃NH₄ is the mixture of LiNH₂ and 2LiH. Because lithium amide will decompose to lithium imide and ammonia at increased temperature [41], no investigation on the reverse reaction was done. In 2002, the hydrogenation of Li₃N and the dehydrogenation of the hydrogenated Li₃N were studied, which can be used as a new hydrogen storage material [42]. A large amount of hydrogen (10.4 wt%) were reversibly stored in Li₃N by the following two-step reactions:

\begin{array}{}\text{(5)}& \text{L}{\text{i}}_3\text{N}\text{}+\text{}2{\text{H}}_2\text{}\leftrightarrow \text{}\text{L}{\text{i}}_2\text{N}\text{H}\text{}+\text{}\text{L}\text{i}\text{H}\text{}+\text{}{\text{H}}_2\text{}\leftrightarrow \text{}\text{L}\text{i}\text{N}{\text{H}}_2\text{}+\text{}2\text{L}\text{i}\text{H}\end{array}

However, the standard enthalpy change of the first reaction in Eq. (5) could be calculated to be –148 kJ·mol^-1 H₂ from each standard enthalpy of formation. The large magnitude requires a very high temperature over 430℃ for complete recovery of Li₃N from the hydrogenated state. While the second reaction has a much smaller enthalpy change calculated to be –44.5 kJ·mol^-1 H₂ and still a large amount of hydrogen storage capacity of 6.5 wt% [43]. Therefore, the latter reaction [Eq. (6)] is still considered to be suitable for hydrogen storage and has been investigated extensively [43-50]:

\begin{array}{}\text{(6)}& \text{L}\text{i}\text{N}{\text{H}}_2+\text{L}\text{i}\text{H}\leftrightarrow \text{L}{\text{i}}_2\text{N}\text{H}+{\text{H}}_2\end{array}

2.3.1 Effects of ball milling on (LiNH₂ + LiH) system

Mechanical activation via high-energy ball milling is one way to improve the kinetics of the system. The mechanisms are the particle size refinement through ball milling and the introduction of internal defects.

Varin et al. [51] studied the effects of ball milling time and different molar ratios (1:1, 1:1.2, 1:1.4) of the mixtures of LiNH₂ and LiH on hydrogen storage. During a high-energy ball milling of the 1:1 molar ratio mixture, the grain size decreases with increasing milling time, while the specific surface area (SSA) of powder increases up to 25 h milling and then decreases after milling for 100 h, due to the excessive agglomeration of powder particles (Table 2). The activation energy for hydrogen desorption decreases with increasing SSA of powder, and the lowest activation energy is observed for the molar ratio of LiNH₂:1.2LiH.

The ball milling process causes excessive heat and lithium amide exceeds its melting temperature at the collision site. To combat the effect of the temperature rise at the collision site, Osborn et al. performed the ball milling at 20℃, –40℃, and –196℃ with dry ice chilled ethanol and liquid nitrogen as coolants [52, 53]. The powder at –196℃ exhibited a stable and significant increase in the hydrogen desorption kinetics, and both crystallite size from XRD and equivalent particle size from SSA are larger for the sample milled at –196℃. NMR analysis indicates that what to blame for the increased kinetics of the milled samples at low temperature is the retention of large amounts of lattice and surface defects that results in an increased diffusion rate, while which are annealed at the collision site at higher milling temperatures.

2.3.2 Partial substitution of Li by Mg

In spite of high storage capacity, Li–N–H system possesses unfavorable thermodynamics [54-56]. Several studies reveal that the replacement of Li by Mg (or LiNH₂/MgH₂ mixture) gives rise to a system having favorable combination of thermodynamics, reversible capacity and operating conditions [57-63]. The reversible de/rehydrogenation reactions of this system can be described as:

\begin{array}{}\text{(7)}& \begin{array}{l}\begin{array}{cc}2\text{L}\text{i}\text{N}{\text{H}}_2+\text{M}\text{g}{\text{H}}_2& \to \text{L}{\text{i}}_2\text{M}\text{g}{\text{N}}_2{\text{H}}_2+2{\text{H}}_2\\ & \leftrightarrow \text{M}\text{g}{(\text{N}{\text{H}}_2)}_2+2\text{L}\text{i}\text{H}\end{array}\\ \end{array}\end{array}

Dolci et al. investigated the hydrogen sorption with different ratios of Mg(NH₂)₂/LiH mixtures (1:2, 3:8, 1:4) by in-situ neutron diffraction [64]. Intermediate reaction steps are observed in both absorption and desorption. The thermodynamic properties of the system at 200℃ are not changed by increasing the lithium hydride content of the system. Chen et al. studied the lowest total energy crystal structure of Li₂MgN₂H₂ [65] and the activation energies of the Li–Mg–N–H system with doped TiF₃ by the first principle calculation [66]. The properties of the TiF₃ doped Li–Mg–N–H system had a slight improvement in kinetics of desorption and elucidated a considerable modification in kinetics of rehydrogenation. Wang et al. studied the kinetics of hydrogen desorption of the Mg(NH₂)₂/LiH mixture by measuring desorption rates at various temperatures [67], and suggested the methods of exploring suitable dopants to create more vacancies in the matrix and activating the N–H with an electromagnetic field to improve the desorption kinetics. Then Ma et al. introduced two transition metal nitrides TaN and TiN as catalysts for the Li–Mg–N–H system [68], both nitrides are catalytically active for accelerating the dehydrogenation reaction and the catalytic enhancement well persists without hydrogen capacity loss. V, V₂O₅ and VCl₃ catalysts were admixed with Mg(NH₂)₂/LiH mixture [59], and all these catalysts lead to the reduction in the decomposition temperature and the corresponding enhancement in the desorption kinetics. The optimum catalyst is VCl₃ and the decomposition temperature changes from 80℃ to 50℃. Also, the desorption kinetics is enhanced up to 38%.

2.3.3 Other composition substitution and catalytic modification

To improve the thermodynamic properties of Li–N–H system, many other new class of complex systems besides Li–Mg–N–H are developed [69-76].

Hydrogen desorption properties of different ratios of LiNH₂/LiBH₄/MgH₂ composites (2:1:2, 1:1:1, 2:0.5:1, 2:1:1, 3:1:1.5) were studied by the examination of the effects of reactant stoichiometry [77]. The composites were found to release hydrogen via a complex multistep reaction cascade, which seeded the products of a subsequent reversible hydrogen storage reaction. This so-called autocatalytic reaction sequence was found to generate favorable kinetics, ammonia attenuation, and partial low–temperature reversibility. The optimal ratio (3:1:1.5) releases a total of 9.1 wt% hydrogen. Srinivasan et al. prepared LiNH₂/LiBH₄/MgH₂ composites with a 2:1:1 molar ratio but with varying milling duration [78]. It is found that the samples with MgH₂ crystallite sizes of approximately 10 nm exhibit lower initial hydrogen release at a temperature of 150℃, and the crystallite size of Li–B–N–H has a significant effect on the amount of hydrogen release with an optimum size of 28 nm.

Pan et al. systematically investigated the hydrogen storage properties and mechanisms of a novel Li–Al–N–H system [79]. The Li₃N–AlN mixture milled for 12 h can absorb 5.2 wt% hydrogen with the formation of LiNH₂/LiH/AlN, and the dehydrogenation of the hydrogenated sample proceeds with a two step reaction. The presence of AlN in the LiNH₂/LiH system significantly improves its dehydrogenation kinetics with a 50℃ reduction in the end temperature. Nahm et al. illustrated the dehydrogenation properties of combined LiAlH₄/LiNH₂ (2:1) mixture and LiAlH₄ wet doped with different transition metals (Sc, Ti, and V) in their chloride forms [80]. The LiAlH₄/LiNH₂ mixture showed high hydrogen desorption with 7.9 wt% at a lower temperature between 75℃ and 275℃. Chen et al. added a small amount of HMPA (hexamethylphosphoramide) to LiAlH₄/LiNH₂ mixture. 8.1 wt% H₂ can be released from the system at the ambient temperature, which is 370℃ lower than the corresponding solid state dehydrogenation [81].

Transition metals (Ti, Cr, Fe, or Ni) were used to substitute for one Li sites in LiNH₂ and the interaction was calculated using first principles [82]. The electronic structures, N–H chemical bonds, and thermodynamics of these systems were calculated and analyzed. The Li–Ti–N–H system had a significant improvement in thermodynamics and a reaction enthalpy of –46.6 kJ·mol^-1 is obtained while it is –75.67 kJ·mol^-1 in the Li–N–H system.

3 Ammonia borane

NH₃BH₃ (AB in short) has been a subject of intensive study since 2000 [83] because of its potential to store a significant percent of hydrogen chemically (19.6 wt% H₂) and low molecular weight (30.7 g·mol^-1). However, its stepwise dehydrogenation encounters relatively high kinetic barriers, which render temperatures higher than 100℃ [83, 84]. Other drawbacks of ammonia borane for hydrogen storage include the irreversibility, emission of poisonous side products and severe material foaming in the dehydrogenation. Efficient methods are needed to dehydrogenate ammonia borane to release hydrogen at reasonable rates and moderate temperatures.

Metal catalyzed dehydrogenation, including metal-catalyzed solvolysis and metal-catalyzed thermolysis, is most widely studied among various methods for dehydrogenation of AB. So far, many metals and metal complexes have been found to catalyze ammonia borane solvolysis (Table 3). From these experimental results, we can find that the combination of higher dispersion with smaller particle sizes plays an important role in the active catalyst. While in metal-catalyzed thermolysis, Goldberg et al. reported the “fastest” thermolysis catalyst known for releasing 1 equiv. of hydrogen from ammonia borane by use of an iridium pincer complex, (POCOP)Ir(H)₂ (POCOP = [η³–1, 3–(OP–tert–Bu₂)₂C₆H₃]) [85]. Chen et al. reported that Co and Ni nanoparticles doped ammonia borane samples evolved approximately 1 equiv. of hydrogen at a temperature as low as 59℃ [86] (Fig. 1). In addition, many other metal complexes have also been investigated for AB analogues. That the metal catalysts Rh, Pd and Ru with a 0.5 mol% loading amount can dehydrogenate amine borane at room temperature has been reported by Manners [87].

Several other strategies for the dehydrogenation of AB have also been reported, such as acid-catalyzed dehydrogenation [95], ionic liquid catalyzed dehydrogenation [96] and metal amine activation [97], etc. Gutowska et al. reported the preparation of nanophase AB by incorporating AB into the channels of mesoporous silica SBA-15 [98]. The two notable effects resulting from the nanostructure of AB in SBA-15 scaffold include: 1) the temperature for H₂ release (m/e=2) being remarkably lower than that for neat AB, indicating an enhanced rate of H₂ release; and 2) the yield of the borazine (m/e=80) is significantly lower than that from neat AB. Recent study shows that AB filled in MOFs also enhances the dehydrogenation kinetics and eliminates the production of ammonia [99].

With the high potential driving force between the H^δ+ in –NH₃ and H^δ- in metal hydrides, AB is readily deprotonated by a variety of metal hydrides to form a new family of metal amidoboranes. LiNH₂BH₃ and NaNH₂BH₃ were prepared by ball-milling solid AB with lithium or sodium hydride [100]. Li and Na amidoboranes released hydrogen at considerably lower temperatures than AB and at a dehydrogenation temperature of 90℃, 8 wt% and 6 wt% of hydrogen were released within the first hour without the unwanted by-product borazine. Burrell and co-workers synthesized Ca(NH₂BH₃)₂ by adding AB to CaH₂ in THF [97]. Whereas this compound releases hydrogen slowly at 90℃, only 0.3 equivalents of H₂ being released at this temperature. One more recent example is the synthesis of KNH₂BH₃ reported by Burrell et al. [101]. It is reported that KNH₂BH₃ evolves 6.5 wt% hydrogen at 80℃ in several hours. At present, the main problem for these metal aminoboranes is the lack of facile reversibility.

4 Metal organic frameworks

Metal organic frameworks (MOFs) constructed from metal cation and cluster nodes linked by ligand linkers (as shown in Fig. 2) are an important class of new hybrid solids that show potential applications as storage materials for hydrogen, carbon dioxide and methane. Although physisorption of H₂ in porous hosts can be highly reversible via changes in pressure and/or temperature, the storage involves low binding energies and isosteric heats of adsorption and therefore cryogenic temperatures, typically 77 K, need to be used to achieve reasonable substrate uptake capacities.

The isosteric heat of adsorption (Qst) of a MOFs–H₂ system is a key thermodynamic variable for hydrogen storage and gas separation [103]. The bottleneck of MOFs as hydrogen storage materials for vehicles is the low temperature owing to the weak isosteric heats of adsorption involved (typically 5–8 kJ·mol^-1) [104]. [Mn(DMF)₆]₃[(Mn₄Cl)₃(BTT)₈(H₂O)₁₂]₂ · 42DMF · 11H₂O·20CH₃OH synthesized by Dinca has an adsorption heat of 10.1 kJ·mol^-1, which is the largest one among MOFs applied for hydrogen storage [105]. As shown in Fig. 3(a) and (b) [106], many researches demonstrated that H₂ uptake correlates with the heat of adsorption at low pressure [107].

Furthermore, it has been estimated that an adsorption heat of 15–30 kJ·mol^-1 is required for MOFs materials with high hydrogen storage at room temperature within the pressure range 1.5–20 bar [104, 108, 109]. The isosteric heat of adsorption is a differential quantity usually determined with the Clausius–Clapeyron equation [110]:

\begin{array}{}\text{(8)}& Q_{st}=\frac{R{T}_1{T}_2\ln (P_2/P_1)}{T_2-T_1}\end{array}

The isosteric heat of adsorption at zero surface coverage is related to the energy of hydrogen adsorption and can be considered a measure of the extent of MOFs–H₂ interactions. The Qst values can be calculated using hydrogen-adsorption isotherms measured at different temperatures, otherwise, be simulated or calculated from Grand Canonical Monte Carlo simulations or density functional theory [108].

To enhance the heat of adsorption of H₂, many strategies were investigated, such as narrow pores, open metal centers and doping [111-121]. Using functional ligands to generate MOFs with narrow pores and increase the interaction energy of single H₂ molecule with the overlapping potential from opposite pore walls are considered to be effective [111]. Luo et al. provided structural evidence demonstrating for the first time that an optimal pore size (about 6–7 Å) enhances the interactions between H molecules and pore walls. The pores with optimal size allow single H₂ molecule directly interact with the pore walls with stronger Vander Waals interaction, which is even stronger than the interaction between the H₂ molecule and the open metal site [112]. Yaghi et al. investigated IRMOFs synthesized from different organic linkers with similar net. They demonstrated that the organic linkers play important roles in defining pore sizes, providing organic adsorptive sites and altering the interaction of H₂ with the cluster due to differences in their electronic structures [113, 114].

Recently, MOFs with open metal centers have attracted great attention for their larger heats of adsorption (10 kJ·mol^-1) than classical MOFs (4–5 kJ·mol^-1) [115]. Yaghi and co-workers have already found that both the metal-oxide units and the organic linkers are important features for H₂ binding [113, 114, 116, 117]. Chen and co-workers showed that thermal removal of axial aqua ligands from MOF-505 exposed the unsaturated copper binding sites with a favorable impact on the hydrogen storage capacity [118]. Understanding the relation between the enhanced H₂ binding energy and the open metal sites is important for further research with an aim to improve the hydrogen storage capacity [119].

Beyond, doping with metal ions (Li) has been applied to increasing the binding energy of H₂ to MOFs [120, 121]. The [H₂PPZ]²⁺ cation in the framework of H₂PPZ[In(QPTC)] was found to act as a kinetic trap to give hysteresis in hydrogen uptake. Li⁺ exchange eliminates the hysteresis in hydrogen adsorption completely. Analysis of the adsorption kinetics at various temperatures revealed that the energy barrier for hydrogen diffusion through the small windows is 15 kJ·mol^-1 [122].

Among MOF-based materials, no material can satisfy the criteria delineated by the United States Department of Energy. To meet the criteria, large adsorption heat is necessarily required.

5 Covalent organic frameworks

Apart from MOFs, the covalent organic frameworks (COFs) have been designed and successfully synthesized by Yaghi’s group, which preserve the merits of good thermal stability, large surface area and extremely low mass density (0.17 g·cm^-3 for COF-108). Therefore they are considered as additional prospective candidates for hydrogen storage [123-126]. Previous theoretical investigation predicts that the 3D COF-105 and COF-108 take in hydrogen over 10% at 298 K [127-129], Grand Canonical Monte Carlo simulations of hydrogen showed that the four 3D COFs materials might attain a 30% increase for the uptake compared with analogous simulations performed for metal organic frameworks at 77 K and 298 K [127]. Unfortunately, the hydrogen storage capacity at room temperature is still quite far from the DOE target of 6 wt%. The reason turns out to be the weak interaction between the hydrogen molecular and the materials (about 2~5 kJ·mol^-1) [128, 130, 131].

For practical hydrogen storage at the ambient condition (–30~80℃ and 1~ 100 bars), the hydrogen adsorption energies between 9.6 kJ·mol^-1 to 58 kJ·mol^-1 was suggested [114]. To enhance the hydrogen adsorption energy, some prospective strategies of modifying the COFs materials have been proposed by first principle calculation. One possible way to improve the hydrogen adsorption energy is to simply mix additional metal atoms or ions into the materials [132]. The hydrogen adsorption energies were dramatically increased in Li and Mg ion-decorated COFs, and their saturated hydrogen storage capacities exceed the DOE target. Another way is to use the method of metal functionalization [133]. The calculation predicts that the insertion of lithium alkoxide groups into the COFs leads to the enhancement of the interaction between the H₂ and the materials (7.1 kJ·mol^-1 per H₂ when five H₂ moleculars are being adsorbed. In addition, it is suggested that the metal atoms and B atom be incorporated with the COFs via the replacement of the non-metal atoms [134, 135], which will have improved the hydrogen adsorption energies and increased the hydrogen uptake at room temperature. However, those methods mentioned above do not have advantages in the practical application until now.

Recently, some new COFs materials have been designed with better hydrogen storage properties. Based on the COF-102, the phenylene moieties of COF-102 is substituted by other extended aromatic moieties without changing the net topology. These new COFs materials present superior hydrogen storage properties than the original COF-102. The Grand Canonical Monte Carlo simulation demonstrated that the hydrogen uptake gets as high as 6 wt% at room temperature [136]. Although these new designed COFs materials have not been synthesized in experimental, another modification on the COFs material (composed of phthalocyanine macrocycles joined by phenylene boronic acid linkers) has been implemented [137].

6 Zeolitic imidazolate frameworks

Zeolitic imidazolate frameworks (ZIFs), a new category of metal organic frameworks based on zeolitic topologies, are synthesized from reactions of transition-metal ions and imidazolate/imidazolate-type linkers. The bridging angles of imidazolate (IM) in <M–IM–M units is coincident with the Si–O–Si angle in zeolites and this has led to the use of imidazole and related ligands as tectons to form so-called ZIFs (Fig. 4). These materials possess more tunable structures as compared with traditional zeolites and appear to be promising materials for gas storage through adsorption because of their exceptional chemical and thermal stabilities [138-140].

H₂ adsorption on ZIF-11 was reported to be 1.4 and 1.3 wt% for ZIF-8 at 1 bar and 77 K [139]. Yildirim et al. measured the adsorption of H₂ on ZIF-8 over large temperature and pressure ranges [141]. They also analyzed adsorption sites for D₂ molecules on ZIF-8 using neutron powder diffraction at low temperatures and found that the imidazolate linker is primarily responsible for adsorption. The total maximum gas uptake with these materials are not impressive compared to other more porous metal organic frameworks due to their relative low BET surface areas and pore volumes. However, the ZIF series provides a number of candidates for selective gas capture and adsorption.

7 Summary and outlook

Great efforts have been made to develop and understand solid-state reversible hydrogen storage materials. Although these materials show an encouraging progress, none of them qualify and fulfill all hydrogen storage criteria such as i) high hydrogen content (>6.0 wt%), ii) favorable or tuning thermodynamics (30–55 kJ·mol^-1 H₂), iii) operate below 100℃ for H₂ delivery, iv) onboard refueling option for a hydrogen-based infrastructure, v) cyclic reversibility (about 1000 cycles) at moderate temperatures, and so on. Throughout all the hydrogen storage methods, thermodynamics and kinetics are the most important parameters for investigating the hydrogen uptake capacity. Nanosizing and nanoconfinement are now the new strategies towards meeting the requirement of higher hydrogen storage capacity for chemical storage systems such as metal hydrides and ammonia borane, etc. As for physical storage systems, it is really a challenge to find novel compounds with proper adsorption enthalpies. Theoretical calculation is worthy of deep study in order to understand the mechanism of various hydrogen storage materials, improve their thermodynamics and kinetics, and develop new materials with good performance. In conclusion, there are still challenging issues to be carefully addressed, including high storage capacity, suitable thermodynamics and fast kinetics.

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