4.1 Mechanical milling

The mechanical milling process is a powder processing method. This processing technique allows producing homogeneous materials starting from powder mixtures [58]. Moreover, the mechanical milling causes particle and grain size reduction (microstructural refinement). Furthermore, mechanical milling is also applied for the synthesis of hydrides, i.e., mechanochemical synthesis [59]. This process can be carried out at laboratory scale and at industrial scale. Herein, the laboratory scale milling process of hydride compounds and systems are discussed. In general, the milling process for improving the microstructural characteristics of hydrides is performed with amounts of material ranging between 1 and 20 g. The milling vessels are usually made of stainless steel and with a volume between 50 and 250 cm3 . As grinding medium stainless steel balls are used. There are several laboratory milling devices that can be classified by the injected shock power per unit mass of grinding medium (Pg), which is a parameter that determines the reached microstructural refinement of a material (powder) subjected to milling procedure. Indeed, the reached degree of microstructural refinements also depends on the milling time and the numbers of ball (grinding medium), which are parameters that can be changed at the time to perform the milling process. Among the most commonly utilized mill devices for hydride compounds and system preparation at laboratory scale, it is possible to mention: (1) Magneto Uni-ball mill device (low energy, Pg: 0.0003–0.002 W/g), (2) Planetary Fritsch-P6 (middle energy, Pg: 0.01– 0.22 W/g), and Vibratory Spex 8000 M mill device (high energy, Pg < 0.24 W/g) [60, 61]. Owing to the pyrophoric nature of the hydride compounds and their easiness to get hydrolyzed and/or oxidized, the milling process is done under inert atmosphere or hydrogen atmosphere in the case of mechanochemical synthesis. The effects of mechanical milling on the material are: (1) creation of grain boundaries and defects, (2) higher degree of microstructural refinement, and (3) intimate mixture of powders. In this regard, the first two effects improve the activation process of the material for the initial hydrogen absorption and the hydrogenation/ dehydrogenation kinetic processes due to the enhancement of hydrogen diffusion through new pathways and shorter distances. Grain boundaries and defects are created by the mechanical energy provided during the milling process. More grain boundaries provide larger surface area and shorter diffusion distance for hydrogen, while the presence of defects acts as channels for hydrogen diffusion. Both effects ease the contact between hydrogen and fresh hydride forming material, enhancing the kinetic behavior. The third effect contributes to the homogeneous distribution and intermixture of the main components of the material and an additive. Moreover, the milling process can promote interactions between the main components and the additive, leading to the formation of other species with favorable effects on the kinetic behavior. This effect will be addressed in a following section. Figure 9 shows the effects of mechanical milling under hydrogen (0.5 bar) atmosphere for 150 h on a mixture of Mg + 10 wt.%Fe in a Magneto Uni-ball mill device. As seen in the SEM photos (Scanning Electron Microscopy), the particle sizes are notably reduced and the surface has porous characteristic in comparison to the starting material. These results account for the mechanical effects of the milling process as well as the fragilization of Mg (ductile material) because of the in situ formation of MgH2 (brittle material) under H2 atmosphere. The effects of milling on Mg + 10 wt. %Fe markedly improve its kinetic behavior by reducing the diffusion pathways for the hydrogen absorption [62].

causing notable improvements in the hydrogenation/dehydrogenation kinetic behavior. These effects are also noticed in complex hydrides and hydride system [64]. Despite the beneficial effects of milling, there are also some disadvantages. In the case of room temperature hydrides, the effects of milling are not always beneficial. For example, it was also reported that FeTi alloys undergo hydrogen capacity loss after milling process. This is due to the creation of amorphous regions induced by the mechanical deformation and it is not possible to store hydrogen interstitially in the amorphous regions [65]. Furthermore, taking into account that the milling vessel and grinding medium are usually made of stainless steel, Fe contamination is also a concern, particularly at the time to perform the milling process in a highenergy mill device. For instance, as demonstrated by Puszkiel et al., the presence of Fe for the 2LiBH4 + MgH2 hydride system causes detrimental effects in the kinetic behavior since FeB species without any beneficial effects are formed. It was found that an appreciable amount of Fe came mostly from the grinding medium [66]. Despite the described disadvantages, the milling process is quite effective and efficient at the time to prepare and tailor hydride compounds and systems and additionally it is possible to scale-up for practical applications. Reducing the grain and particle sizes, creating more boundary surfaces and defects are the most relevant effects of the milling process. These effects mainly improve the diffusion processes for the hydride formation and decomposition by shortening the diffusion

Tailoring the Kinetic Behavior of Hydride Forming Materials for Hydrogen Storage

DOI: http://dx.doi.org/10.5772/intechopen.82433

SEM photos for (A) Mg starting material and (B) Mg + 10 wt.%Fe after 150 h of milling under 0.5 bar H2 in

Figure 9.

143

a magneto Uni-ball device.

pathways and generating channels for more efficient hydrogen transport.

ered that the addition of transition metal and metal compounds were able to

Since the beginning of the investigations on hydride compounds, it was discov-

4.2 Doping with transition metal and transition metal compounds

In 1997, Zaluska et al. [63] reported a pioneering work about the properties of different as-milled hydride forming materials such as Mg, Mg2Ni, FeTi, and LaNi5. It was found that the materials after milling presented a nanocrystalline nature,

Tailoring the Kinetic Behavior of Hydride Forming Materials for Hydrogen Storage DOI: http://dx.doi.org/10.5772/intechopen.82433

#### Figure 9.

4.1 Mechanical milling

Gold Nanoparticles - Reaching New Heights

the hydrogen absorption [62].

142

250 cm3

The mechanical milling process is a powder processing method. This processing technique allows producing homogeneous materials starting from powder mixtures [58]. Moreover, the mechanical milling causes particle and grain size reduction (microstructural refinement). Furthermore, mechanical milling is also applied for the synthesis of hydrides, i.e., mechanochemical synthesis [59]. This process can be carried out at laboratory scale and at industrial scale. Herein, the laboratory scale milling process of hydride compounds and systems are discussed. In general, the milling process for improving the microstructural characteristics of hydrides is performed with amounts of material ranging between 1 and 20 g. The milling vessels are usually made of stainless steel and with a volume between 50 and

. As grinding medium stainless steel balls are used. There are several labo-

ratory milling devices that can be classified by the injected shock power per unit mass of grinding medium (Pg), which is a parameter that determines the reached microstructural refinement of a material (powder) subjected to milling procedure. Indeed, the reached degree of microstructural refinements also depends on the milling time and the numbers of ball (grinding medium), which are parameters that can be changed at the time to perform the milling process. Among the most commonly utilized mill devices for hydride compounds and system preparation at laboratory scale, it is possible to mention: (1) Magneto Uni-ball mill device (low energy, Pg: 0.0003–0.002 W/g), (2) Planetary Fritsch-P6 (middle energy, Pg: 0.01– 0.22 W/g), and Vibratory Spex 8000 M mill device (high energy, Pg < 0.24 W/g) [60, 61]. Owing to the pyrophoric nature of the hydride compounds and their easiness to get hydrolyzed and/or oxidized, the milling process is done under inert atmosphere or hydrogen atmosphere in the case of mechanochemical synthesis. The effects of mechanical milling on the material are: (1) creation of grain boundaries and defects, (2) higher degree of microstructural refinement, and (3) intimate mixture of powders. In this regard, the first two effects improve the activation process of the material for the initial hydrogen absorption and the hydrogenation/ dehydrogenation kinetic processes due to the enhancement of hydrogen diffusion through new pathways and shorter distances. Grain boundaries and defects are created by the mechanical energy provided during the milling process. More grain boundaries provide larger surface area and shorter diffusion distance for hydrogen, while the presence of defects acts as channels for hydrogen diffusion. Both effects ease the contact between hydrogen and fresh hydride forming material, enhancing the kinetic behavior. The third effect contributes to the homogeneous distribution and intermixture of the main components of the material and an additive. Moreover, the milling process can promote interactions between the main components and the additive, leading to the formation of other species with favorable effects on the kinetic behavior. This effect will be addressed in a following section. Figure 9 shows the effects of mechanical milling under hydrogen (0.5 bar) atmosphere for 150 h on a mixture of Mg + 10 wt.%Fe in a Magneto Uni-ball mill device. As seen in the SEM photos (Scanning Electron Microscopy), the particle sizes are notably reduced and the surface has porous characteristic in comparison to the starting material. These results account for the mechanical effects of the milling process as well as the fragilization of Mg (ductile material) because of the in situ formation of MgH2 (brittle material) under H2 atmosphere. The effects of milling on Mg + 10 wt. %Fe markedly improve its kinetic behavior by reducing the diffusion pathways for

In 1997, Zaluska et al. [63] reported a pioneering work about the properties of different as-milled hydride forming materials such as Mg, Mg2Ni, FeTi, and LaNi5. It was found that the materials after milling presented a nanocrystalline nature,

SEM photos for (A) Mg starting material and (B) Mg + 10 wt.%Fe after 150 h of milling under 0.5 bar H2 in a magneto Uni-ball device.

causing notable improvements in the hydrogenation/dehydrogenation kinetic behavior. These effects are also noticed in complex hydrides and hydride system [64]. Despite the beneficial effects of milling, there are also some disadvantages. In the case of room temperature hydrides, the effects of milling are not always beneficial. For example, it was also reported that FeTi alloys undergo hydrogen capacity loss after milling process. This is due to the creation of amorphous regions induced by the mechanical deformation and it is not possible to store hydrogen interstitially in the amorphous regions [65]. Furthermore, taking into account that the milling vessel and grinding medium are usually made of stainless steel, Fe contamination is also a concern, particularly at the time to perform the milling process in a highenergy mill device. For instance, as demonstrated by Puszkiel et al., the presence of Fe for the 2LiBH4 + MgH2 hydride system causes detrimental effects in the kinetic behavior since FeB species without any beneficial effects are formed. It was found that an appreciable amount of Fe came mostly from the grinding medium [66]. Despite the described disadvantages, the milling process is quite effective and efficient at the time to prepare and tailor hydride compounds and systems and additionally it is possible to scale-up for practical applications. Reducing the grain and particle sizes, creating more boundary surfaces and defects are the most relevant effects of the milling process. These effects mainly improve the diffusion processes for the hydride formation and decomposition by shortening the diffusion pathways and generating channels for more efficient hydrogen transport.

#### 4.2 Doping with transition metal and transition metal compounds

Since the beginning of the investigations on hydride compounds, it was discovered that the addition of transition metal and metal compounds were able to

improve their kinetic characteristics. The addition of amounts from 1 to 10 wt.% of these transition metals and compounds was performed by milling procedures in most of the cases. Therefore, the "doping strategy" also involves the milling process to reach a high degree of intermixture and microstructural refinement. The doping strategy results in reductions of the hydrogenation and/or dehydrogenation activation energies, changes in the reaction paths and consequently different rate-limiting steps. Figure 10 shows the concept of doping via mechanical milling. As an example, the activation energy for the dehydrogenation process for a bulk hydride MHx (M = metal) is reduced after adding a transition metal and transition metal compound by milling process, Ea1 > Ea2.

MgH2 turned to be surface controlled and related to the recombination of the hydrogen molecule [72, 73]. Several transition metals and transition metal compounds exhibited excellent catalytic effect on de-/hydrogenation of MgH2. The main effect of these additives was related to the dehydrogenation mechanism, since they ease the recombination of the hydrogen molecule [74, 75]. One of the most effective additives was Nb2O5, leading to 7 wt.% H2 capacity in about 60 s and fast dehydrogenation in about 130 s. Moreover, the presence of Nb2O5 reduced the dehydrogenation temperature down to 250°C [76]. Nb2O5 had a "pathway effect" mainly on the hydrogenation of MgH2. Upon milling and subsequent hydrogen cycling, the formation of Mg-Nb oxides created diffusion pathways by the formation of metastable niobium hydrides, hence avoiding the large diffusion constraints. The effect and mechanism of Nb2O5 on MgH2 is similar but slower when MgH2 is

Tailoring the Kinetic Behavior of Hydride Forming Materials for Hydrogen Storage

LiBH4 also caught the attention due to the extremely high gravimetric capacity of 18.3 wt.% H2, though just 13.8 wt.% H2 is available because LiH and B are formed upon desorption. However, LiBH4 is quite stable and needs harsh temperature and pressure conditions for de-/re-hydrogenation [79]. Several dopants such as metal oxides and metal halides were tried for improving the kinetic behavior of LiBH4. However, LiBH4 reacts with the dopants because this complex hydride is a strong reduction agent. The interactions between LiBH4 and dopants also lead to gas byproducts such as B2H6. In this regard, complex hydrides such as borohydrides and amides are combined with binary hydrides to form "thermodynamically destabilized systems" and then the kinetic behavior of these hydride systems is tailored by doping [30, 31]. In almost all the cases, the added dopants interact with the hydride system itself by forming other species in situ, and this strategy will be

This strategy is applied to binary hydrides, to complex hydrides and mainly to destabilized hydride systems. In this section, the in situ formation of species with catalytic activity for destabilized hydride system is described. These hydride systems present proper thermodynamic parameters as for example 2LiBH4 + MgH2 and Mg(NH2)2 + 2LiH, as shown in Table 2, because of the exothermic formation of reversible species during the endothermic dehydrogenation. Nonetheless, these hydride systems still present kinetic constraints to reach the operative conditions predicted by the thermodynamic. One of the main strategies to overcome this problem is the addition of dopants. In almost all the cases, these dopants interact

with the hydride system by forming in situ species with catalytic activity.

One of the most representative examples of this approach is the addition of transition metal compounds to the 2LiBH4 + MgH2 hydride system. Under dynamic conditions, this borohydride system uptake hydrogen in one step, but releases hydrogen in two steps as described in reactions (17)–(19), respectively [80].

For the hydrogenation process, temperature and pressures of about 350°C and 50 bar are required. These conditions for the formation of LiBH4 are notably milder than that needed for the re-hydrogenation of LiBH4 from LiH and B [79]. For the dehydrogenation process, temperatures above 350°C and hydrogen overpressures

2LiHð Þ<sup>s</sup> <sup>þ</sup> MgB2 sð Þ <sup>þ</sup> 4H2 gð Þ ! 2LiBH4 lð Þ <sup>þ</sup> MgH2 sð Þ (17) 2LiBH4ð Þ<sup>l</sup> <sup>þ</sup> MgH2 sð Þ ! 2LiBH4 lð Þ <sup>þ</sup> Mgð Þ<sup>s</sup> <sup>þ</sup> H2 gð Þ (18) 2LiBH4 lð Þ <sup>þ</sup> Mgð Þ<sup>s</sup> ! 2LiHð Þ<sup>s</sup> <sup>þ</sup> MgB2 sð Þ <sup>þ</sup> 3H2 gð Þ (19)

physically doped with Nb2O5 [77, 78].

DOI: http://dx.doi.org/10.5772/intechopen.82433

addressed in the next section.

4.3 In situ catalyst formation

145

In the case of room temperature hydrides, the presence of transition metals like Pd and Ni notably improves their activation behavior and kinetic properties. This fact was attributed to the active sites of these transition metals, located on the metal or alloy surface, which facilitated the hydrogen molecule dissociation and penetration across the oxides generated on the metal or alloy surface [67–69]. These hydrides are commonly prepared by arc melting since, as mentioned in Section 4.2, the milling process can cause hydrogen capacity losses.

In 1997, Bogdanović et al. [70] achieved reversible hydrogen uptake and release from NaAlH4 under mild conditions by doping with Ti-based catalyst (TiCl3). However, the catalytic mechanism of the Ti-based catalyst was not clear at those times. In 2015, Züttel et al. [71] reported a work about the catalytic mechanism of Ti-compound in the hydrogen uptake/release of alkali alanates. In this work, based on an atomistic model, it was proposed that Ti works as a bridge to transfer H and M+ (M+ ---Ti---H) from MAlH4 (M = Li, Na, K), reducing their charge separation and thus lowering the activation energies for hydrogenation and dehydrogenation processes.

Among the most interesting hydrides, MgH2 is very attractive because of its low cost and high gravimetric hydrogen capacity (7.6 wt.%). However, its high thermodynamic stability (74 kJ/mol H2) required high operative temperature over 300°C [25]. Pure MgH2 has sluggish kinetic behavior. Upon hydrogenation, the rate-limiting step is three-dimensional diffusion controlled contracting volume. This fact is attributed to the low diffusion coefficient of MgH2 covering fresh Mg; the diffusion coefficient of MgH2 was found to be three orders of magnitude lower that the one for Mg). Upon dehydrogenation, the rate-limiting step for pristine

#### Figure 10.

Concept of doping via mechanical milling. TM = transition metal. MHy = hydride, M = metal, ΔH = enthalpy, Ea = activation energy.

Tailoring the Kinetic Behavior of Hydride Forming Materials for Hydrogen Storage DOI: http://dx.doi.org/10.5772/intechopen.82433

MgH2 turned to be surface controlled and related to the recombination of the hydrogen molecule [72, 73]. Several transition metals and transition metal compounds exhibited excellent catalytic effect on de-/hydrogenation of MgH2. The main effect of these additives was related to the dehydrogenation mechanism, since they ease the recombination of the hydrogen molecule [74, 75]. One of the most effective additives was Nb2O5, leading to 7 wt.% H2 capacity in about 60 s and fast dehydrogenation in about 130 s. Moreover, the presence of Nb2O5 reduced the dehydrogenation temperature down to 250°C [76]. Nb2O5 had a "pathway effect" mainly on the hydrogenation of MgH2. Upon milling and subsequent hydrogen cycling, the formation of Mg-Nb oxides created diffusion pathways by the formation of metastable niobium hydrides, hence avoiding the large diffusion constraints. The effect and mechanism of Nb2O5 on MgH2 is similar but slower when MgH2 is physically doped with Nb2O5 [77, 78].

LiBH4 also caught the attention due to the extremely high gravimetric capacity of 18.3 wt.% H2, though just 13.8 wt.% H2 is available because LiH and B are formed upon desorption. However, LiBH4 is quite stable and needs harsh temperature and pressure conditions for de-/re-hydrogenation [79]. Several dopants such as metal oxides and metal halides were tried for improving the kinetic behavior of LiBH4. However, LiBH4 reacts with the dopants because this complex hydride is a strong reduction agent. The interactions between LiBH4 and dopants also lead to gas byproducts such as B2H6. In this regard, complex hydrides such as borohydrides and amides are combined with binary hydrides to form "thermodynamically destabilized systems" and then the kinetic behavior of these hydride systems is tailored by doping [30, 31]. In almost all the cases, the added dopants interact with the hydride system itself by forming other species in situ, and this strategy will be addressed in the next section.

#### 4.3 In situ catalyst formation

improve their kinetic characteristics. The addition of amounts from 1 to 10 wt.% of these transition metals and compounds was performed by milling procedures in most of the cases. Therefore, the "doping strategy" also involves the milling process to reach a high degree of intermixture and microstructural refinement. The doping strategy results in reductions of the hydrogenation and/or dehydrogenation activation energies, changes in the reaction paths and consequently different rate-limiting steps. Figure 10 shows the concept of doping via mechanical milling. As an example, the activation energy for the dehydrogenation process for a bulk hydride MHx (M = metal) is reduced after adding a transition metal and transition metal com-

In the case of room temperature hydrides, the presence of transition metals like Pd and Ni notably improves their activation behavior and kinetic properties. This fact was attributed to the active sites of these transition metals, located on the metal or alloy surface, which facilitated the hydrogen molecule dissociation and penetration across the oxides generated on the metal or alloy surface [67–69]. These hydrides are commonly prepared by arc melting since, as mentioned in Section 4.2,

In 1997, Bogdanović et al. [70] achieved reversible hydrogen uptake and release


Among the most interesting hydrides, MgH2 is very attractive because of its low cost and high gravimetric hydrogen capacity (7.6 wt.%). However, its high thermodynamic stability (74 kJ/mol H2) required high operative temperature over 300°C [25]. Pure MgH2 has sluggish kinetic behavior. Upon hydrogenation, the rate-limiting step is three-dimensional diffusion controlled contracting volume. This fact is attributed to the low diffusion coefficient of MgH2 covering fresh Mg; the diffusion coefficient of MgH2 was found to be three orders of magnitude lower that the one for Mg). Upon dehydrogenation, the rate-limiting step for pristine

Concept of doping via mechanical milling. TM = transition metal. MHy = hydride, M = metal, ΔH = enthalpy,

and thus lowering the activation energies for hydrogenation and dehydrogenation

from NaAlH4 under mild conditions by doping with Ti-based catalyst (TiCl3). However, the catalytic mechanism of the Ti-based catalyst was not clear at those times. In 2015, Züttel et al. [71] reported a work about the catalytic mechanism of Ti-compound in the hydrogen uptake/release of alkali alanates. In this work, based on an atomistic model, it was proposed that Ti works as a bridge to transfer H and

pound by milling process, Ea1 > Ea2.

Gold Nanoparticles - Reaching New Heights

M+ (M+

processes.

Figure 10.

144

Ea = activation energy.

the milling process can cause hydrogen capacity losses.

This strategy is applied to binary hydrides, to complex hydrides and mainly to destabilized hydride systems. In this section, the in situ formation of species with catalytic activity for destabilized hydride system is described. These hydride systems present proper thermodynamic parameters as for example 2LiBH4 + MgH2 and Mg(NH2)2 + 2LiH, as shown in Table 2, because of the exothermic formation of reversible species during the endothermic dehydrogenation. Nonetheless, these hydride systems still present kinetic constraints to reach the operative conditions predicted by the thermodynamic. One of the main strategies to overcome this problem is the addition of dopants. In almost all the cases, these dopants interact with the hydride system by forming in situ species with catalytic activity.

One of the most representative examples of this approach is the addition of transition metal compounds to the 2LiBH4 + MgH2 hydride system. Under dynamic conditions, this borohydride system uptake hydrogen in one step, but releases hydrogen in two steps as described in reactions (17)–(19), respectively [80].

$$\text{2LiH}\_{(\text{s})} + \text{MgB}\_{2(\text{s})} + 4\text{H}\_{2(\text{g})} \to 2\text{LiBH}\_{4(\text{l})} + \text{MgH}\_{2(\text{s})}\tag{17}$$

$$\text{2LiBH4}\_{\text{(l)}} + \text{MgH}\_{\text{2(s)}} \rightarrow \text{2LiBH4}\_{\text{4(l)}} + \text{Mg}\_{\text{(s)}} + \text{H}\_{\text{2(g)}} \tag{18}$$

$$\text{2LiBH}\_{\mathsf{4(l)}} + \text{Mg}\_{\text{(s)}} \to \text{2LiH}\_{\text{(s)}} + \text{MgB}\_{\text{2(s)}} + \text{3H}\_{\text{2(g)}} \tag{19}$$

For the hydrogenation process, temperature and pressures of about 350°C and 50 bar are required. These conditions for the formation of LiBH4 are notably milder than that needed for the re-hydrogenation of LiBH4 from LiH and B [79]. For the dehydrogenation process, temperatures above 350°C and hydrogen overpressures

higher than 3 bar of hydrogen overpressure are required. These conditions are harsher than those predicted by the thermodynamics (Table 2). The hydrogen overpressure upon dehydrogenation assures the reversibility of the hydride system since MgB2 is only formed under backpressure conditions. As seen in the reactions, LiBH4 is in liquid state because this complex hydride undergoes from solid to liquid state at about 270°C [81]. Furthermore, mainly upon dehydrogenation the kinetic behavior is sluggish, particularly the second step described by reaction (19), taking more than 10 h for the full hydrogen release. Therefore, one of the most effective strategies was the addition of transition metal compounds [82]. Through this strategy, transition metal halides TiCl3, TiF4, NbF5 among others, notably improved the dehydrogenation kinetic behavior of 2LiBH4 + MgH2 hydride system. In this case, the transition metal halide additive interacts with LiBH4 during the preparation stage during milling and subsequent heating to reach the operative temperature for hydrogen interactions. This interaction results in the formation of stable and nanostructure boride species such as TiB2 and NbB2 with similar hexagonal crystal structure as MgB2. The in situ formed nanostructured transition metal boride species act as centers for the nucleation and growth of MgB2, hence, accelerating the second step of the dehydrogenation, reaction (19).

destabilization effect. In situ formed complex amide-borohydrides like Li4(BH4) (NH2)3 and Li2(BH4)(NH2) are responsible for improving the kinetic behavior of

Tailoring the Kinetic Behavior of Hydride Forming Materials for Hydrogen Storage

6 Mg NH ð Þ<sup>2</sup> <sup>2</sup> þ 9LiH þ LiBH4 ⇆ 3Li2Mg2ð Þ NH <sup>3</sup> þ Li4ð Þ BH4 ð Þ NH2 <sup>3</sup> þ 9H2 (22)

Figure 11 depicts the concepts of the in situ catalyst formation applied to "destabilized hydride system." The complex hydride ABHy (A = metal, B = nonmetal) or metal can react with a binary hydride, MHx (M = metal; different from A) to lower the thermodynamic stability by the exothermal formation of MB, thus ΔH1 > ΔH2. The in situ formation of catalytic species lowers the activation energy, i.e., Ea<sup>1</sup> > Ea2, but does not alter the thermodynamic stability of the system.

This strategy consists in confining the dimensions of hydride particles to sizes lower than 25 nm by introducing them into a nanoporous matrix. For a simple hydride formation/decomposition reaction (23), the contribution of the excess surface area given by the nanosize of the metal (M(s)) and metal hydride (MH2(s)) must be taken into account as part of the reaction enthalpy as described in reaction (24), where Vm is the molar volume, r particle radius and E(γ, r, Eads) is the surface energy term which depends on the surface free energies (γ) of the metal hydride and the metal particle, on the molar volumes of the two solid reaction partners, and on an additional energy term Eads, which takes into account that binding of H2 at the surface of both the metal and the hydride reduce the respective surface energy by minimizing the excess of surface energy (γ) arising from not bound surface atoms. Therefore, the classical van't Hoff equation (Section 2.2, Eq. (4)) is corrected by the effects of nanoconfinement by replacing ΔH for ΔH', which takes into account the surface effects owing to the nanometric condition of

Concept of in situ catalyst formation applied to destabilized hydride system. Complex hydride = ABHy (A = metal, B = non-metal or metal); binary hydride, MHx (M = metal; different from A; TM = transition

Further improvement for the 6 Mg(NH2)2 + 9LiH + LiBH4 was achieved by coadding YCl3 and Li3N. The in situ formation of nanostructured YH3 and YBx upon milling and hydrogen interaction leads to faster kinetic behavior with reduced activation energy and capacities of about 4 wt.% at 90°C. On one hand, in situ formed Li4(NH2)3(BH4) enhances the Li<sup>+</sup> transport, providing a source for the fast formation of the reacting species. On the other hand, nanostructured YB<sup>x</sup> species

the hydride system, according to reaction (22) [91]:

contribute to the dissociation of H2 [92].

DOI: http://dx.doi.org/10.5772/intechopen.82433

the particles as shown in Eq. (25) [93].

metal). ΔH = enthalpy, Ea = activation energy.

Figure 11.

147

4.4 Nanoconfinement

Other example with the same 2LiBH4 + MgH2 hydride system is the addition of TiO2, leading to the in situ formation of core-shell LixTiO2 nanoparticles. The mechanism of these core-shell nanoparticles is different from the one described for transition metal boride species. The core and shell of the nanoparticles are composed of Li0.59TiO2 and Li0.59TiO2, respectively. Upon hydrogenation and dehydrogenation, the in situ formed core-shell nanoparticles works as reversible Li<sup>+</sup> pumps, promoting the early decomposition of LiBH4 and providing Li<sup>+</sup> for its rehydrogenation. Furthermore, the core-shell LixTiO2 also improve the dehydrogenation of MgH2, reaction (18). The addition of 1 wt.% of TiO2 to 2LiBH4 + MgH2 hydride system leads to hydrogen capacities of about 10 wt.% H2, markedly shorter uptake (25 min) and release hydrogen times (50 min) and reduced activations energies at 400°C. The rate-limiting step for the hydrogenation process of the 2LiBH4 + MgH2 doped with TiO2 was one-dimensional interface-controlled mechanism (JMA, n = 1), while for the pristine material it was generally a diffusion controlled process [82]. As a novel approach, Puszkiel et al. interpreted the two steps dehydrogenation process by the combination of the JMA model with n = 1 for the fast MgH2 decomposing, reaction (18), and the modified autocatalytic Prout-Tompkins model for the decomposition of LiBH4/formation of MgB2. Therefore, this autocatalytic process accelerated with the further formation of MgB2. The nanostructured core-shell LixTiO2 particles prompt the fast formation of MgB2 seeds by acting as Li<sup>+</sup> sink/source for the early decomposition of LiBH4 and the subsequent formation of LiH, respectively.

Mg(NH2)2 + 2LiH was also investigated as a potential hydrogen storage material due to its hydrogen capacity of about 5 wt.% and operative temperature roughly 220°C [83]. This hydride system uptake/release hydrogen in two steps according to reactions (20) and (21):

$$\text{2 Mg}(\text{NH}\_2)\_{2(\text{s})} + \text{3LiH}\_{(\text{s})} \rightarrow \text{Li}\_2\text{Mg}\_2\text{NH}\_{3(\text{s})} + \text{LiNH}\_{2(\text{s})} + \text{3H}\_{2(\text{g})} \tag{20}$$

$$\rm Li\_2Mg\_2NH\_{3(s)} + LiNH\_{2(s)} + LiH\_{(s)} \to 2Li\_2Mg(NH\_2)\_{2(s)} + H\_{2(g)}\tag{21}$$

Several additives such as halides and hydrides were used to try to improve the kinetic behavior of this system [84–89]. However, it was found that the addition of just 0.1 mol of LiBH4 improves not only the kinetic behavior, but also reduces the reaction enthalpy from 38.9 to 36.5 kJ/mol H2 [31, 90]. Moreover, the 6 Mg (NH2)2 + 9LiH + LiBH4 molar composition is the optimum one for the kineticTailoring the Kinetic Behavior of Hydride Forming Materials for Hydrogen Storage DOI: http://dx.doi.org/10.5772/intechopen.82433

destabilization effect. In situ formed complex amide-borohydrides like Li4(BH4) (NH2)3 and Li2(BH4)(NH2) are responsible for improving the kinetic behavior of the hydride system, according to reaction (22) [91]:

$$\text{6 Mg}(\text{NH}\_2)\_2 + \text{9LiH} + \text{LiBH}\_4 \Leftrightarrow \text{3Li}\_2\text{Mg}\_2(\text{NH})\_3 + \text{Li}\_4(\text{BH}\_4)(\text{NH}\_2)\_3 + \text{9H}\_2 \quad \text{(22)}$$

Further improvement for the 6 Mg(NH2)2 + 9LiH + LiBH4 was achieved by coadding YCl3 and Li3N. The in situ formation of nanostructured YH3 and YBx upon milling and hydrogen interaction leads to faster kinetic behavior with reduced activation energy and capacities of about 4 wt.% at 90°C. On one hand, in situ formed Li4(NH2)3(BH4) enhances the Li<sup>+</sup> transport, providing a source for the fast formation of the reacting species. On the other hand, nanostructured YB<sup>x</sup> species contribute to the dissociation of H2 [92].

Figure 11 depicts the concepts of the in situ catalyst formation applied to "destabilized hydride system." The complex hydride ABHy (A = metal, B = nonmetal) or metal can react with a binary hydride, MHx (M = metal; different from A) to lower the thermodynamic stability by the exothermal formation of MB, thus ΔH1 > ΔH2. The in situ formation of catalytic species lowers the activation energy, i.e., Ea<sup>1</sup> > Ea2, but does not alter the thermodynamic stability of the system.

### 4.4 Nanoconfinement

higher than 3 bar of hydrogen overpressure are required. These conditions are harsher than those predicted by the thermodynamics (Table 2). The hydrogen overpressure upon dehydrogenation assures the reversibility of the hydride system since MgB2 is only formed under backpressure conditions. As seen in the reactions, LiBH4 is in liquid state because this complex hydride undergoes from solid to liquid state at about 270°C [81]. Furthermore, mainly upon dehydrogenation the kinetic behavior is sluggish, particularly the second step described by reaction (19), taking more than 10 h for the full hydrogen release. Therefore, one of the most effective strategies was the addition of transition metal compounds [82]. Through this strategy, transition metal halides TiCl3, TiF4, NbF5 among others, notably improved the dehydrogenation kinetic behavior of 2LiBH4 + MgH2 hydride system. In this case, the transition metal halide additive interacts with LiBH4 during the preparation stage during milling and subsequent heating to reach the operative temperature for hydrogen interactions. This interaction results in the formation of stable and nanostructure boride species such as TiB2 and NbB2 with similar hexagonal crystal structure as MgB2. The in situ formed nanostructured transition metal boride species act as centers for the nucleation and growth of MgB2, hence, accelerating the

Other example with the same 2LiBH4 + MgH2 hydride system is the addition of

hydrogenation. Furthermore, the core-shell LixTiO2 also improve the dehydrogenation of MgH2, reaction (18). The addition of 1 wt.% of TiO2 to 2LiBH4 + MgH2 hydride system leads to hydrogen capacities of about 10 wt.% H2, markedly shorter uptake (25 min) and release hydrogen times (50 min) and reduced activations energies at 400°C. The rate-limiting step for the hydrogenation process of the 2LiBH4 + MgH2 doped with TiO2 was one-dimensional interface-controlled mechanism (JMA, n = 1), while for the pristine material it was generally a diffusion controlled process [82]. As a novel approach, Puszkiel et al. interpreted the two steps dehydrogenation process by the combination of the JMA model with n = 1 for the fast MgH2 decomposing, reaction (18), and the modified autocatalytic Prout-Tompkins model for the decomposition of LiBH4/formation of MgB2. Therefore, this autocatalytic process accelerated with the further formation of MgB2. The nanostructured core-shell LixTiO2 particles prompt the fast formation of MgB2 seeds by acting as Li<sup>+</sup> sink/source for the early decomposition of LiBH4 and the

Mg(NH2)2 + 2LiH was also investigated as a potential hydrogen storage material due to its hydrogen capacity of about 5 wt.% and operative temperature roughly 220°C [83]. This hydride system uptake/release hydrogen in two steps according to

2 Mg NH ð Þ<sup>2</sup> 2 sð Þ <sup>þ</sup> 3LiHð Þ<sup>s</sup> ! Li2Mg2NH3 sð Þ <sup>þ</sup> LiNH2 sð Þ <sup>þ</sup> 3H2 gð Þ (20) Li2Mg2NH3 sð Þ <sup>þ</sup> LiNH2 sð Þ <sup>þ</sup> LiHð Þ<sup>s</sup> ! 2Li2Mg NH ð Þ<sup>2</sup> 2 sð Þ <sup>þ</sup> H2 gð Þ (21)

Several additives such as halides and hydrides were used to try to improve the kinetic behavior of this system [84–89]. However, it was found that the addition of just 0.1 mol of LiBH4 improves not only the kinetic behavior, but also reduces the reaction enthalpy from 38.9 to 36.5 kJ/mol H2 [31, 90]. Moreover, the 6 Mg (NH2)2 + 9LiH + LiBH4 molar composition is the optimum one for the kinetic-

TiO2, leading to the in situ formation of core-shell LixTiO2 nanoparticles. The mechanism of these core-shell nanoparticles is different from the one described for transition metal boride species. The core and shell of the nanoparticles are composed of Li0.59TiO2 and Li0.59TiO2, respectively. Upon hydrogenation and dehydrogenation, the in situ formed core-shell nanoparticles works as reversible Li<sup>+</sup> pumps,

promoting the early decomposition of LiBH4 and providing Li<sup>+</sup> for its re-

second step of the dehydrogenation, reaction (19).

Gold Nanoparticles - Reaching New Heights

subsequent formation of LiH, respectively.

reactions (20) and (21):

146

This strategy consists in confining the dimensions of hydride particles to sizes lower than 25 nm by introducing them into a nanoporous matrix. For a simple hydride formation/decomposition reaction (23), the contribution of the excess surface area given by the nanosize of the metal (M(s)) and metal hydride (MH2(s)) must be taken into account as part of the reaction enthalpy as described in reaction (24), where Vm is the molar volume, r particle radius and E(γ, r, Eads) is the surface energy term which depends on the surface free energies (γ) of the metal hydride and the metal particle, on the molar volumes of the two solid reaction partners, and on an additional energy term Eads, which takes into account that binding of H2 at the surface of both the metal and the hydride reduce the respective surface energy by minimizing the excess of surface energy (γ) arising from not bound surface atoms. Therefore, the classical van't Hoff equation (Section 2.2, Eq. (4)) is corrected by the effects of nanoconfinement by replacing ΔH for ΔH', which takes into account the surface effects owing to the nanometric condition of the particles as shown in Eq. (25) [93].

#### Figure 11.

Concept of in situ catalyst formation applied to destabilized hydride system. Complex hydride = ABHy (A = metal, B = non-metal or metal); binary hydride, MHx (M = metal; different from A; TM = transition metal). ΔH = enthalpy, Ea = activation energy.

$$\mathbf{M}\_{\mathrm{(s)}} + \mathbf{H}\_{\mathrm{2(g)}} \boldsymbol{\Leftrightarrow} \mathbf{M} \mathbf{H}\_{\mathrm{2(s)}} \tag{23}$$

Acknowledgements

Conflict of interest

Author details

149

Julián Atilio Puszkiel

National Council of Scientific and Technological Research (CONICET),

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: julianpuszkiel1979@gmail.com

San Carlos de Bariloche, Río Negro, Argentina

provided the original work is properly cited.

Number: ARG-1187279-GF-P).

There is no conflict of interest.

DOI: http://dx.doi.org/10.5772/intechopen.82433

The author acknowledges CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas) and Alexander von Humboldt Foundation (Fellowship

Tailoring the Kinetic Behavior of Hydride Forming Materials for Hydrogen Storage

$$
\Delta H' = \Delta H + \left[ (\Im \text{V} m \to (\gamma, r, \text{Eads})/r \right] \tag{24}
$$

$$
\ln P\_{eq} = \left[\frac{\Delta H'}{RT}\right] - \left[\frac{\Delta S}{R}\right],
\tag{25}
$$

Nanoconfinement was basically proposed as a promising strategy to make hydride compounds reversible under mild conditions, as for example LiBH4, NaAlH4, and MgH2, and to further destabilized hydrogen systems such as 2LiBH4 + MgH2. However, in most of the cases, the main effect was observed on the kinetic behavior of the hydride compounds and systems. The nanometric range of the particles provide extremely large grain boundaries and notable shorter diffusion paths as well as optimized contacting for the materials, hence these properties account for improvements in terms of kinetic behavior and cycling stability. Carbon frames such as scaffolds, nanotubes, and aerogels are used for confining hydrides due to its light weight and inter condition. Despite the fact that this strategy usually leads to improved kinetic behavior without actually changing the thermodynamics of the hydride compound and system, the main constraint lay on the reduced capacity because of the introduction of the hydride into the nanoporous frame. Furthermore, it is hard to figure out in a possible scale-up for practical application [94–96].
