A Systematic Approach to the Synthesis, Thermal Stability and Hydrogen Storage Properties of Rare-Earth Borohydrides

Chapter 13

A Systematic Approach to the Synthesis, Thermal Stability and Hydrogen Storage Properties of Rare-Earth Borohydrides Fabiana C. Gennari1, 2, 3, Julio J. Andrade-Gamboa2, 4 1

National Scientific and Technical Research Council (CONICET), Buenos Aires, Argentina; Department of Physicochemistry of Materials, National Atomic Energy Commission (CNEA) Bariloche Atomic Centre (CAB), S.C. de Bariloche, Rı´o Negro, Argentina; 3National University of Cuyo, S.C. de Bariloche, Rı´o Negro, Argentina; 4National University of Comahue, S.C. de Bariloche, Rı´o Negro, Argentina 2

Chapter Outline 13.1 Introduction 13.2 Synthesis 13.2.1 Exchange Reactions in Solution 13.2.2 Metathesis Reactions in Solid-State: Mechanochemical Synthesis 13.3 Decomposition of Rare-Earth Borohydrides and Hydrogen Storage Reversibility

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13.4 Destabilization of LiBH4 by Rare-Earth Hydrides and Rare-Earth Borohydrides 13.4.1 Destabilization of LiBH4 by Rare-Earth Hydride 13.4.2 Destabilization of LiBH4 by Rare-Earth Borohydrides 13.5 Conclusions Acknowledgments References

445 446

449 455 456 456

13.1 INTRODUCTION Metal borohydrides (M(BH4)n) (M ¼ metal; n ¼ the valence of M), also named tetrahydroborates, comprise one kind of complex hydrides in which complex anion [BH4] and metal cation Mnþ form an ionic solid structure. The borohydride anion [BH4] has applications spanning neutron capture to Emerging Materials for Energy Conversion and Storage. https://doi.org/10.1016/B978-0-12-813794-9.00013-2 Copyright © 2018 Elsevier Inc. All rights reserved.

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chemical reduction; lithium and sodium borohydrides are the most common reducing agents used [1]. Because of their high gravimetric hydrogen content, metal borohydrides are promising materials for use in hydrogen storage [2,3]. For this application, hydrogen release has to occur at a low temperature through a decomposition reaction in which the thermodynamic stability of the borohydride is a key factor. The stability of M(BH4)n is related to the stability of the anion BH4  within the crystalline structure. The polarization effects in the ionic interaction MnþBH4  tend to enhance the MnþH affinity, with consequent lowering in the integrity of BH4  . The properties of the metal M are critical to regulating the stability of M(BH4)n. In fact, irrespective of the crystal structure or valence n of metal, there is an inverse dependence of the dehydrogenation temperature (Td), and a direct variation of the formation enthalpy (DHf) with the electronegativity of M [4]. The latter is given by the following linear relationship (where Xp is the electronegativity of M in Pauling units): (13.1) DHf ¼ 253.6 XP  398.0 (kJ mol1) from which it can be predicted that DHf ¼ 0 for XP ¼ 1.6. Therefore, the prediction is made that stable borohydrides will be formed by metals with an electronegativity lower than 1.6. Eq. (13.1) shows that DHf typically increases (and Td decreases) as the valence of metal M increases. This is a manifestation of the polarizing power of Mnþ, which contributes to destabilize the anion BH4  . In particular, the electronegativity of rare-earth (RE) metals has a mean value (XP ¼ 1.2), which is in the range of electronegativities corresponding to stable borohydrides. Hence, rare-earth (RE) metal borohydrides (RE(BH4)3) should exhibit moderate stability. Consequently, they are considered to be promising materials for the technology of hydrogen storage. In this chapter, the main features of RE metal borohydrides (RE(BH4)3) are analyzed. A comparative overview of the synthesis procedure, the crystal structures obtained, the thermal stability, and the dehydrogenation behavior of several RE borohydrides is presented. Hydrogen storage properties of different RE borohydrides as well as composites formed by LiBH4-RE borohydrides are analyzed, with an emphasis in their reversibility behavior.

13.2 SYNTHESIS Different synthetic approaches have been developed to produce RE borohydrides. Among them, the most extended method of synthesis is mechanochemistry [5,6]. Chemical reactions such as double substitution and addition can be performed in different media (solidesolid, gasesolid, and solvent) to produce RE metal borohydrides. In general, reactions taking place in organic solvents produce halide- and solvent-free monometallic borohydrides [7e15]. Mechanochemical reactions favor the formation of monometallic borohydrides,

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bimetallic borohydrides, or mixed-metal borohydride halides [13e30]. In the case of gasesolid reactions, the solvent-free synthesis of monometallic borohydrides is possible [18,31,32]. Previous synthesis procedures involved the use of reagents sensitive to moisture and air. For this reason, all reactants are handled in a glove box or use standard Schlenk and vacuum techniques.

13.2.1 Exchange Reactions in Solution RE metal borohydrides can be synthesized by metathesis reaction from the corresponding metal halide and LiBH4 in an organic solvent. The different solubilities of these precursors in organic solvents allow salt-free products to be obtained. Weak solvent coordination is a key factor for solvent removal at moderate conditions. A series of complexes of RE(BH4)3$mTHF (where m is a nonintegral number) was prepared by treatment of RECl3 with an excess of diborane in tetrahydrofuran (THF) [7]. Several RE(BH4)3 (in which RE ¼ Sm, Eu, Gd, Tb, Dy, Ho, Er, Eu, Tm, Yb, and Lu) were synthesized in this way, but because of the insoluble nature of these compounds, they were not studied further and purification was not possible. In subsequent work, the synthesis of Gd(BH4)3 using O-donor solvents such as THF gave different adducts in which the solvent was difficult to remove without decomposition of the product [8]. In general, solvent-based methods lead to the production of the most stable compound, which is usually the monometallic borohydride. The synthesis of bimetallic borohydrides, which is possible via a mechanochemical reaction, is often unsuccessful. New solvent-based synthesis using dimethyl sulfide (S(CH3)2) to produce RE(BH4)3 (with RE ¼ Y, La, Ce, or Gd) has been developed [9,10]. The use of S(CH3)2 provides new solvates as intermediates, RE(BH4)3-S(CH3)2 (in which RE ¼ Y, La, Ce, or Gd), which finally transform to a-Y(BH4)3, Gd(BH4)3 or Ce(BH4)3 at about 140 C. This reaction, shown for RE ¼ Ce, can be expressed as: Toluene; filt.

CeCl3 ðsÞ þ 3LiBH4 ðsÞ þ mSðCH3 Þ2ðlÞ ƒƒƒƒƒ! CeðBH4 Þ3$m SðCH3 Þ2ðsolÞ þ3LiClðsÞY 140 C; vac.

CeðBH4 Þ3$mSðCH3 Þ2ðsolÞ ƒƒƒƒƒ! CeðBH4 Þ3ðsÞ þ mSðCH3 Þ2ðgÞ[ (13.2) Several advantages appeared when S(CH3)2 was used as solvent: the reaction was fast, the solvent removal was possible under moderate conditions, and the desired product could easily be separated from the by-products by filtration. On the other hand, the reactions between RECl3 (where RE ¼ La and Ce) and LiBH4 in diethyl ether (Et2O) first produced an ether solvate, LiRE(BH4)3Cl$nEt2O, which upon removal of the coordinated solvent under

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vacuum yielded LiRE(BH4)3Cl (where RE ¼ La and Ce) according to reaction (shown for RE ¼ Ce): filt.

CeCl3 ðsÞ þ 3LiBH4 ðsÞ þ mEt2 OðlÞ ƒ! LiCeðBH4 Þ3Cl$mEt2 OðsolÞ þ 2LiClðsÞY 110 C; vac.

LiCeðBH4 Þ3Cl$mEt2 OðsolÞ ƒƒƒƒƒ! LiCeðBH4 Þ3ClðsÞ þ mEt2 OðgÞ[ (13.3) 

During heating at 110 C, the release of Et2O from LiRE(BH4)3Cl$nEt2O(M ¼ La, Ce) was very quick, and solid material could easily be separated from the Schlenk equipment. Similarly, an exchange reaction between LiBH4 and YCl3 was performed in solution, using the solubility of both reactants in Et2O. The resulting Y(BH4)3 was dissolved in Et2O whereas the by-product LiCl was separated by precipitation [11]. The final product was contaminated with 15% of LiCl. On the other hand, 13 different synthetic approaches to synthesizing pure solventfree Y(BH4)3 were tested [12]. In these methods, the exchange reaction using YCl3, Y(OCH3)3, or YH3 as an yttrium source in different solvents (THF (C4H9)4NBH4, R3HBH3, etc.), or in the absence of solvents was unsuccessful [12]. The main problem was the elimination of the solvent. Its presence was undesirable for practical use owing to the elimination of gas impurities and the reduction in the hydrogen storage density. In other work [13], Eu(BH4)2 and Sm(BH4)2 were produced free of side products and their structures were investigated (both compounds crystallized in orthorhombic space group Pbcn, a-PbO2 type). The synthesis involved the individual activation of the reactants, the mixture of LiBH4 and RECl3 (where RE ¼ Eu and Sm) in a 3:1 ratio to which Et2O was added, agitated overnight, and removed under vacuum. After these steps, S(CH3)2 was added. The resultant mixture was filtered by standard solvent-based extraction techniques and RE(BH4)2 compounds were obtained. It appeared that the synthesis of Eu(BH4)2 was promoted by the reduction of Eu3þ to Eu2þ by LiBH4 dissolved in Et2O before extraction of the product RE(BH4)2 by S(CH3)2, resulting in a crystalline solvate. An alternative procedure for Eu(BH4)2 synthesis by milling mixtures of EuH2:2(C2H5)3NBH3 was reported [14]. The mixture was cooled to room temperature and washed with cyclohexane. The residue was dried under dynamic vacuum up to 100 C for a few hours. After this procedure, the compound crystallized in the a-PbO2 type structure and was transformed at 395 C to a t-ZrO2-type structure (tetragonal space group P42/nmc), and at 425 C to a fluorite-type structure (cubic space group Fm3m). The first direct synthesis of Pr(BH4)3 was performed using a 3PrCl3:LiBH4 mixture by the solvation method [15]. The use of Et2O facilitated the production of a-Pr(BH4)3, LiPr(BH4)3Cl, and minor amounts of b-Pr(BH4)3. However, when recrystallization from S(CH3)2 followed by heat treatment was used, almost pure a-Pr(BH4)3 phase was produced. Along the same lines,

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Er(BH4)3 was produced by milling of a 3ErCl3:LiBH4 mixture and later dissolution in S(CH3)2 [15]. LiCl was filtered from the solution and Er(BH4)3 was precipitated under vacuum. To remove residual solvents, the product was heated at 145 C in an oil bath or at 150 C under dynamic vacuum. The final phases obtained were a- and b-Er(BH4)3.

13.2.2 Metathesis Reactions in Solid-State: Mechanochemical Synthesis Mechanochemical synthesis is a processing technique of solids in which mechanical and chemical phenomena are coupled on a molecular scale. It is possible to produce a desired product using only a mechanical action (high pressure and mechanical stress between reactants and balls) at room temperature or at temperatures lower than traditional solid-state synthesis [5,6]. Mechanochemical synthesis can be performed under different conditions, e.g., using a reactive atmosphere (reactive ball milling [BM]), under cryogenic conditions (cryomilling), or in a solvent [5,6]. In addition, other experimental parameters can be controlled that influence the characteristics of the final material: milling time, powder to ball weight ratio, milling temperature, milling frequency, milling atmosphere and pressure of the selected gas, etc. Depending on the synthesis parameters, different products can be obtained, such as metastable phases, high-pressure phases, and amorphous and disordered phases, leading to the development of ultrafine-grained and nanostructured compounds with homogeneous composition [6]. Hence, mechanochemical processing is a versatile technique that can be used to prepare a wide range of materials. Possible disadvantages of the mechanochemical method are the structurally amorphous product that is obtained and the formation of undesired products as a result of competing reactions. Mechanochemical processing is the most extended synthesis procedure of RE metal borohydrides. The metathesis reaction is promoted by BM of an alkali metal borohydride (mainly LiBH4) and a metal halide (mainly chlorides). This procedure leads to the synthesis of different products, depending on the ionic radius of RE3þ: (1) mixed-cation mixed-anion borohydride chlorides based on RE elements, LiRE(BH4)3Cl, for the biggest cations in which RE3þ ¼ La, Gd, Ce, Pr, Nd, and Sm (by mechanochemical synthesis or a combination of mechanochemistry and heat treatment); (2) a monometallic RE borohydride such as RE(BH4)3 for a slightly smaller RE3þ ¼ Y, Sm, Gd, Tb, Dy, Ho, Er, and Tm. In addition, when Sm, Eu and Yb adopt the oxidation state RE2þ, the stoichiometry of the rare earth borohydrides results RE(BH4)2; and (3) mixed-cation borohydrides such as Li[RE(BH4)4] for the smallest RE3þ ¼ Yb, Lu, and Sc, which act as the strongest Lewis acids and form complex [RE(BH4)4]. Table 13.1 summarizes these different RE borohydrides and the chemical reactions involved during mechanochemical synthesis using LiBH4 as a reactant.

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TABLE 13.1 Synthesis of Rare-Earth Borohydrides by Mechanochemical Processing Using 3LiBH4:RECl3 Mixtures as a Starting Reactant Reaction Type/ Compound Type

Stoichiometry

Cases

3LiBH4 þ RECl3/LiRE(BH4)3Cl þ 2LiCl

RE3þ ¼ La, Gd, Ce, Pr, Nd, Sm

3LiBH4 þ RECl3/ RE(BH4)3 þ 3LiCl

RE3þ ¼ Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb

3LiBH4 þ RECl3/Li[RE(BH4)4] þ 2LiCl

RE3þ ¼ Yb, Lu, Sc

Metathesis 1 Mixed-cation mixed-anion borohydride chloride Metathesis 2 Monometallic RE borohydride Metathesis 3 Mixed-cation borohydride RE, rare earth.

By BM some RE chlorides and LiBH4 it was possible to produce the RE monometallic borohydride RE(BH4)3 (in which RE3þ ¼ Y, Sm, Gd, Tb, Dy, Er, Yb, and Ho), according to the reaction (Table 13.1) [12,16e22]: (13.4) 3LiBH4 þ RECl3 / RE(BH4)3 þ 3LiCl where the RE3þ and BH4  ions are arranged in a distorted ReO3-type structure at room temperature (a-RE(BH4)3, cubic space group Pm3m). Higher temperatures favor transformation to a face-centered cubic polymorph (b-RE(BH4)3, cubic space group Fm3c). The undesired metal halide formed with the borohydride can be separated using solvents. For example, LiCl was separated from Y(BH4)3 by using Et2O [11]. The b-RE(BH4)3 phase can be obtained at room temperature (retained as metastable) by quenching [18,19] or it can be formed directly as a metastable phase during BM through reaction (13.4) [20,23,24]. During the mechanochemical synthesis of Ce(BH4)3 [25] and Gd(BH4)3 [26], a new phase was reported to form in the as-milled LiBH4:RECl3 mixture (for RE ¼ Ce) or during further thermal treatment on the a-phase (for RE ¼ Gd). This new structure was partially solved (RE3þ and BH4  positions) based on cubic space group I43m [26]. Later, this new phase was identified as LiRE(BH4)3Cl and solved in the same space group for RE ¼ Ce [27,28] and ˚ (RE ¼ La, Ce, Pr, La [29]. For metal ions with a radius larger than 0.983 A

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Nd, and Sm), mechanochemical synthesis leads to the formation of LiRE(BH4)3Cl (Table 13.1): 3LiBH4 þ RECl3 / LiRE(BH4)3Cl þ 2LiCl

(13.5)

This last reaction could be considered an addition reaction that promotes the formation of a solid solution, facilitated by mechanochemical treatment [20,25e30]. In addition, LiRE(BH4)3Cl can also be formed after thermal treatment of a-RE(BH4)3 through: RE(BH4)3 þ 3LiCl(s) / LiRE(BH4)3Cl(s) þ 2LiCl(s)

(13.6)

Reactions (13.5) and (13.6) represent the behavior of Ce [25,27,28] and Gd [26,29], respectively (more details are provided in Section 13.4). A third possible product of synthesis using mechanochemical synthesis from LiBH4-RECl3 is Li(RE(BH4)4), (tetragonal space group P42c), first observed for Sc [31] and reported for Yb and Lu [20], obtained by: 4LiBH4(s) þ RECl3(s) þ / Li[RE(BH4)4] (s) þ 3LiCl(s)

(13.7)

The BM of mixtures containing RECl3 (in which RE ¼ Sm, Eu, and Yb) and LiBH4 display a behavior different from the other RE samples [20]. This is because of the ability of these elements to form compounds with different oxidation states, leading to the synthesis of RE(BH4)2 after heating of the RE(BH4)3:LiCl mixture below 200 C. In all cases, this reduction from RE3þ to RE2þ was accompanied by diborane release. For example, in the case of Sm, the final products after mechanochemical synthesis were a- and b-Sm(BH4)3 and LiSm(BH4)3Cl, which become Sm(BH4)2 after heating above 200 C. These transformations can be generically represented by reactions (13.6) and (13.7), followed by the reduction of RE: 2RE(BH4)3(s) / 2RE(BH4)2(s) þ B2H6(g) þ H2(g)

(13.8)

On the contrary, when Sm(BH4)2 (a-PbO2 type) was synthesized from the same starting LiBH4 and SmCl3 reactants by metathesis reaction in ethereal solution combined with solvent extraction, nonintermediate phases were observed [13,20]. Table 13.2 shows observed phases from the mechanochemical reaction of mixtures 3LiBH4-RECl3. As pointed out in [33,34], there is a dependence of crystal structures and RE3þ radii, which is schematized in Fig. 13.1. An attractive synthesis procedure involving a gasesolid reaction is reactive BM, which avoids the use of a solvent both during the reaction and/or for later elimination of by-products. Solvent-free synthesis of a-Y(BH4)3 was demonstrated by the direct reaction of diborane gas and YH3 during reactive BM, with yields above 75% [21,35]: YH3(s) þ 3/2B2H6(g) / Y(BH4)3(s)

(13.9)

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TABLE 13.2 Crystal Structures for Products of Milled Samples in System 3LiBH4RECl3

La3þ Ce

3þ

3þ

Pr

a-RE(BH4)3 Pa3

1.032

[20]

[20,29]

1.01

[25,27]

[25,27]

0.99

[15]

[15,20]

3þ

0.983

3þ

0.958

Nd Sm Eu

Radius of REnþ (A˚) [32]

3þ 3þ

b-RE(BH4)3 Pm3m

[20]

[20]

[20]

0.947 0.938

[16]

0.923

[20]

3þ

0.912

[16]

3þ

0.901

[22]

[22]

0.90

[16,17,23]

[17,23]

0.89

[17,24]

[17,24]

[33]

[33]

Tb

Dy

Ho

3þ

Y

3þ

Er

3þ

Tm

Li[RE(BH4)4] P42c

[20]

3þ

Gd

LiRE(BH4)3Cl I43m

[29]

0.88

3þ

0.868

3þ

0.861

[20]

3þ

0.745

[31]

Yb Lu Sc

FIGURE 13.1 Phase occurrence according to RE3þ radii.

[20]

LiRE(BH4)3Cl

α-RE(BH4)3 β-RE(BH4)3 Li[RE(BH4)4]

Decreasing ionic radii

Moreover, BM of Y(BH4)3 with diborane induced the formation of Y(B3H8)3 and Y2(B12H12)3, which suggests the presence of a superior borane species during the decomposition of Y(BH4)3 [36]. This type of solidegas reaction may facilitate the synthesis of a wide range of solvent-free RE borohydrides in the future.

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13.3 DECOMPOSITION OF RARE-EARTH BOROHYDRIDES AND HYDROGEN STORAGE REVERSIBILITY Several RE borohydrides were identified and their structures elucidated. Some are attractive in different application fields such as solid-state electrolytes, hydrogen storage, luminescence, and magnetic properties [23,28e30,37,38]. RE borohydrides receive increasing interest as hydrogen storage materials because of their high hydrogen content and expected lower decomposition temperature compared with alkali metal borohydrides [16,39]. In fact, RE borohydrides can release most hydrogen between 200 and 300 C, which is lower than temperatures for alkali metal borohydrides (e.g., LiBH4 > 380 C [40]). For this reason, it is interesting to study the decomposition reaction pathway and evaluate to possibility of rehydrogenating this type of compound. The decomposition of RE borohydrides occurs under heating, producing mainly metal hydride and releasing hydrogen, and in some cases diborane. Depending on the identity of RE, possible dehydrogenation reactions are: RE(BH4)3(s) / REHn(s) þ 3B(s) þ (6  1/2n) H2(g)

(13.10)

RE(BH4)3(s) / (1 e 3/m) REHn(s) þ 3/m REBm(s) þ (6  n/2 þ 3n/2m) H2(g)

(13.11)

As presented earlier, the easiest and simplest synthesis procedure for the production of RE borohydrides is a metathesis reaction (Table 13.1) promoted by the mechanochemical processing of an alkali metal borohydride (mainly LiBH4) and metal halides (usually RE chlorides). During mechanochemical synthesis, the halide side product obtained simultaneously with RE borohydride is often difficult to remove. This salt may hinder the reversible hydrogenation of RE borohydride because of the formation of ternary chlorides, or favor reversibility by an unknown mechanism. In fact, it was shown that the presence of LiCl influences the decomposition temperature and/or the decomposition pathway [9,15,21]. In contrast, the production of almost pure RE borohydride by solvent-based synthesis allows an accurate determination of its physical properties [9,15]. Table 13.3 shows some examples of RE borohydrides obtained by different synthesis procedures, their onset decomposition temperature, and the amount of hydrogen released/taken up. An exemplary case evidencing the effect of LiCl is Y(BH4)3. Theoretical calculations indicate that Y(BH4)3 is thermodynamically unstable at room temperature with a low decomposition reaction enthalpy of DH ¼ 22.536.2 kJ mol1 H2 [41]. The thermal decomposition pathway of Y(BH4)3 produced by milling of the YCl3:LiBH4 mixture differs from that obtained by a gasesolid reaction involving B2H6 and YH3. In the first case, the milling product contains a mixture of a-Y(BH4)3 with a minor amount of b-Y(BH4)3, and in the range 160 to 180 C, an a / b transformation takes place [18]. Upon heating, b-Y(BH4)3 decomposes at 190 C to YH3, which transforms

Rare-Earth Borohydride/Hydrogen Storage (wt%) Y(BH4)3/8.6 or 7.5

c

Rehydrogenation Conditions P(MPa)/ T( C)/Time (h)

References

Synthesis Procedure

Tonset ( C)

BM of 3LiBH4:YCl3 in Et2O solution (15% of LiCl remains)

190

7.8

1.1e1.3

3.5/250e300

[11]

BM of 3LiBH4:YCl3

190 w210e220 (TG)

4.64a a ¼ 3.73a,b b ¼ 3.55a,b

e e

e e

[18] [19]

Cryo-BM of 4LiBD4:YCl3

200

e

e

e

[17]

YH3 þ B2H6 (gas solid reaction, yield of 77%)

190

6.8b

e

e

[35]

4.6a 6.2

e

e

[21]

BM of 3LiBH4:YCl3 YH3 þ B2H6 (gasesolid reaction)

LiCe(BH4)3Cl/6.8 or 6.2d

Hydrogen Recharged (wt%)

Hydrogen Released (wt%)

Transformation of Y(BH4)3.S(CH3)2 to Y(BH4)3 at 140 C

200 (TG) Tpeak ¼ 285 (DSC)

6.7b (TG)

Formation YH3

155/300/24

[9]

BM of 3LiBH4:CeCl3

220 w220 220

3.5a 3.05a 6.2b

w1.0a w0.7a 0.8

6.5/350 10/350/24 8.0/340

[25,27,30]

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TABLE 13.3 Hydrogen Storage Properties of Some Rare-Earth Borohydrides

BM of 3LiBH4:GdCl3

220 Tpeak ¼ 258 (DSC) 220 Tpeak ¼ 261 (DSC)

3.1 2.58 (TG); 3.6 (Sieverts)a

e e

e e

[26] [29]

Gd(BH4)3/6.0e

BM of 3LiBH4:GdCl3 and solvent extraction

200 Td ¼ 225e250

5.75 (Sieverts)

e

e

[29]

LiLa(BH4)3Cl/5.3e

BM of 3LiBH4:LaCl3

225

3.87a (Sieverts)

e

Er(BH4)3/5.7

Eu(BH4)2/4.4e e

Eu(BH4)2/4.4

e

[29]

w0.7

6.0/400

[24]

b

BM of 3LiBH4:ErCl3

230

BM of 3LiBH4:ErCl3 and wet chemistry

Tpeak ¼ 281 (DSC)

4.7 (TG)

Not successful

10/400

[15]

BM of 3LiBH4:EuCl3

334

4.4a

1.25

10/400/8

[13]

350

b

3.5 (cycle 2) >3.0 (cycle 3)

10/400/8

[13]

3LiBH4:EuCl3 and solvent extraction

3.2

e

a

6.0 (TG)

BM, ball milling; DSC, differential scanning calorimetry; TG, thermogravimetric. a The value refers to the starting mixture considering side products. b The value considers minor amounts of diborane emission simultaneous with hydrogen in the decomposition process. c Hydrogen storage capacity was calculated according to reactions (13.12) and (13.13), respectively. d Hydrogen storage capacity was calculated according to reactions (13.15) and (13.16), respectively. e Hydrogen storage capacity by formula unit.

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LiGd(BH4)3Cl/4.9e

439

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to YH2 at 270 C. In addition, an unidentified compound was observed between 215 and 280 C. Similar results were found by other authors regarding the final products and intermediate phases, without clear evidence of YB4 formation [11,17]. Instead, YB4 was detected in the final product of decomposition obtained from YCl3eLiBH4 mixtures with an excess of LiBH4 [21]. The authors considered that the YB4 product could be formed by reaction between LiBH4 and YH3 instead of the direct decomposition of Y(BH4)3. Then, during decomposition, 4.64 wt% of hydrogen would be released and the solid products would be YB4 and amorphous boron-containing compounds. Possible reactions that would occur are: Y(BH4)3(s) / 0.25YH2(s) þ 0.75YB4(s) þ 5.75H2(g) (8.6 wt% of H2)

(13.12)

Y(BH4)3(s) / YH2(s) þ 3B(s) þ 5H2(g) (7.5 wt% of H2)

(13.13)

A typical dehydrogenation curve obtained for Y(BH4)3 using volumetric measurements is presented in Fig. 13.2A [23]. In the presence of LiCl, an endothermic event before decomposition Y(BH4)3 was observed and related with the melting of Y(BH4)3 [21] (Fig. 13.2B). As a new result, Gennari [23]

H desorbed (wt%)

(A)

0 LiBH4 -1 -2 -3 -4

(a) Y(BH4)3 (b) Er(BH4)3

Heat Flow (watt/g)

(B) LiBH4

endo 100

200

300

400

Temperature (oC) FIGURE 13.2 A) Nonisothermal hydrogen desorption and (B) thermal decomposition of as-synthesized RE(BH4)3 from as-milled 3LiBH4-RECl3 mixtures (in which RE ¼ Y and Er). For comparison, as-milled LiBH4 curves are included. A heating ramp of 5 C min1 was used. Endothermic, down.

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detected the presence of [B12H12]2 in the final dehydrogenated product. Reversibility was limited and only 1 wt% was rehydrogenated (20% of the initial hydrogen capacity) under 65 bar at 400 C after 6 h. A similar hydrogen content was recharged (1.1e1.3 wt%) using 35 bar at 300 C for 24 h [11]. In contrast, when Y(BH4)3 was synthesized free of side products such as LiCl, a-Y(BH4)3 remained up to 250 C without transforming into b-polymorph [35]. The absence of a thermal event caused by Y(BH4)3 melting demonstrates that it is not an inherent characteristic of the Y borohydride. Similar behavior was observed for a-Y(BH4)3 produced by desolving dimethyl sulfide [9]. In that work, no crystalline phases were detected during heating of a-Y(BH4)3 in the range 190 to 250 C. Decomposition of a-Y(BH4)3 started at about 285 C, with a mass loss of 6.7 wt%, and continued up to 300 C, releasing mainly hydrogen and small amounts of diborane. Moreover, the addition of LiBH4 or LiCl to Y(BH4)3 induced melting in both cases at 190 and 220 C. These observations demonstrated that LiCl influences the phase transitions as well as the dehydrogenation path [21]. Upon heating of Y(BH4)3 under hydrogen back-pressure (at the range of 1e10 bar), the formation of Y(B3H8)3 as an intermediate was identified, along with minor amounts of Y2(B12H12)3 [36]: Y(BH4)3(s) / 1/3Y(B3H8)3(s) þ 2/3YH3(s) þ H2(g)

(13.14)

It is expected that triborane could act as a favorable intermediate for the rehydrogenation of [BH4] under moderate conditions [42]. However, Y(BH4)3 did not reform even under 1550 bar at 300 C; instead YH3 was obtained [9]. Gennari and Esquivel [25] reported for the first time the synthesis of cubic Ce(BH4)3 by milling of the 3LiBH4:CeCl3 mixture. Thermal decomposition started at 220 C and displayed a multistep process, with 3.5 wt% of hydrogen release (considering LiCl as part of the total mass). The formation of an intermediate unknown phase and CeH2 was confirmed, and the decomposition pathway also suggested the formation of CeB6 based on thermodynamic considerations. Ce borohydride was partially rehydrogenated under 6 MPa at 350 C, with 28% of the total hydrogen capacity. Later, Zhang et al. [30] studied the thermal behavior of ball-milled 3LiBH4:RECl3 mixtures (in which RE ¼ La and Ce). The production of both borohydrides was observed using RE chlorides. When RE fluorides were used as reactants, no formation of RE borohydrides was detected. After the 3LiBH4:RECl3 mixture was dehydrogenated up to 600 C and kept at this temperature, CeB6 was detected. These early works assumed that RE(BH4)3 (in which RE ¼ La and Ce) was the material synthesized instead of LiRE(BH4)3Cl. Frommen et al. [27] demonstrated that BM of 3LiBD4:CeCl3 produces LiCe(BD4)3Cl instead of Ce(BD4)3; this was the first mixed-cation mixed anion RE borohydride reported. During decomposition, a similar behavior was found: formation of CeD2 and an intermediate phase was detected, with no evidence of crystalline

442 PART j V Emerging Materials for Hydrogen Storage

CeB6. Taking these findings into account, the thermal decomposition was suggested to proceed via the following reactions: LiCe(BD4)3Cl(s) / 0.5CeD2(s) þ 0.5CeB6(s) þ 5.5D2(g) þ LiCl(s) (4.6 wt% of D2) (13.15) LiCe(BD4)3Cl(s) / CeD2(s) þ 3B(s) þ 5D2(g) þ LiCl(s) (4.2 wt% of D2)

(13.16)

Deuterium storage reversibility of LiCe(BD4)3Cl was also explored [28]. After deuterium absorption at 340 C under 80 bar of D2 for 24 h, 0.8 wt% of deuterium was absorbed. This amount was 13% of the initial gas release obtained after deuterium desorption and demonstrates that LiCe(BD4)3Cl is only partially reversible. Andrade Gamboa et al. [26] studied the dehydrogenation path of Gd(BH4)3 produced by BM of 3LiBH4:GdCl3. They found that thermal evolution was a multistep process with three endothermic events. The first one did not involve gas release; it was originally associated with an irreversible structural transition to a new Gd(BH4)3 polymorph [26]. However, according to later work by Ley et al. [29], the event must be related to the formation of LiGd(BH4)3Cl through reaction (13.5). For the second endothermic event, dehydrogenation started at 230 C, releasing a total of 3.1 wt% up to 400 C. The final solid products detected were GdH2 and GdB4. The global dehydrogenation reaction can be represented by: Gd(BH4)3(s) þ LiCl(s) / LiGd(BH4)3Cl(s) / 0.5GdH2(s) þ 0.5GdB4(s) þ 5.5H2(g) þ LiCl(s) (4.5 wt% of H2) (13.17) Afterward, Ley et al. [29] pointed out that LiLa(BH4)3Cl instead of La(BH4)3 was the product obtained after BM of 3LiBH4:LaCl3. Upon heating, a mass loss of 2.87 wt% was observed between 200 and 400 C (theoretically 3.89 wt% including LiCl). Decomposition of LiLa(BH4)3Cl generated only hydrogen gas without indicating diborane formation. In situ diffraction studies showed that LiLa(BH4)3Cl peaks disappeared at 185 C owing to melting. Evidence of an unknown compound appeared at 165 C; this compound may have been a ternary salt. In addition, both LaH2 and LaB6 were observed at the end of decomposition, similar to LiCe(BH4)3Cl: LiLa(BH4)3Cl(s) / 0.5LaH2(s) þ 0.5LaB6(s) þ 5.5H2(g) þ LiCl(s) (4.9 wt% of H2) (13.18) In the same investigation, the authors showed that the Gd(BH4)3:3LiCl mixture produced by milling was transformed by heating at 200 C into LiGd(BH4)3Cle2LiCl. The Gd borohydride released 2.58 wt% of hydrogen between 200 and 450 C (theoretically 3.67%). At 240 C, diffraction peaks from LiGd(BH4)3Cl vanish as a result of decomposition and an unknown phase was formed and disappeared at 290 C. At the end of heating, GdB4 and

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an unknown compound similar to that observed for La was detected, with no evidence of GdH2 formation. No rehydrogenation tests were performed on LiGd(BH4)3Cl. The first report on the synthesis and decomposition of a-Er(BH4)3 was provided by Gennari [24]. BM of the 3LiBH4:ErCl3 mixture led to the formation of a-Er(BH4)3e3LiCl; the synthesis was unsuccessful when NaBH4 was used instead of LiBH4. During the decomposition of a-Er(BH4)3 3.2 wt% of hydrogen was released (taking into account the LiCl mass), as determined from nonisothermal measurements in volumetric equipment (Fig. 13.2A). Differential scanning calorimetry curves for the Er(BH4)3:LiCl mixture (Fig. 13.2B) combined with X-ray powder diffraction studies showed the multistep nature of the Er(BH4)3 decomposition [24]. The first endothermic peak at 220 C was reversible and was related to the structural transition of Er(BH4)3 [24]. However, the shape of this first peak suggested that two thermal events could be superimposed and the simultaneous Er(BH4)3 melting was not discarded. Heating up to 250 C led to the formation of an intermediate compound containing hydrogen. Finally, after the last endothermic peak, Er(BH4)3 decomposition was complete and ErB4 was detected at 500 C. Rehydrogenation by applying p(H2) at 60 bar and 400 C was partial (20%). A possible decomposition pathway is: Er(BH4)3(s) / 0.25ErH2(s) þ 0.75ErB4(s) þ 5.75H2(g) (5.4 wt% of H2)

(13.19)

By combining mechanochemistry and wet chemistry, halide-free Er(BH4)3 and Pr(BH4)3 were synthesized for the first time [15]. In the case of RE ¼ Er, two phases were present in the samples, which were identified as a-Er(BH4)3 and b-Er(BH4)3, respectively. Decomposition of the sample released 4.7 wt% of gas (mainly hydrogen, with minor amounts of diborane and S[CH3]2) between room temperature and 400 C. In the absence of LiCl, the decomposition pathway indicated that the reaction products were completely amorphous. Rehydrogenation of the decomposition products at 400 C under 100 bar was unsuccessful. The temperature decomposition determined by thermal analysis was about 281 C, which was 10 to 20 C higher than in earlier reports [24]. On the other hand, Olsen et al. [20] reported that the decomposition temperature of Er(BH4)3 was 240 C in the presence of 6LiBH4. The difference from the decomposition temperature reported for halide-free Er(BH4)3 probably resulted from to the co-melt/decomposition of Er(BH4)3 and LiBH4. Similar destabilization behavior was found when pure RE(BH4)3 (in which RE ¼ Eu and Sm) were compared with compounds containing residual LiCl [13]. Apparently, the influence of the presence of LiH rather than LiCl or Li-ion in the material synthesized by Gennari [24] had a favorable influence on the hydrogen storage reversibility of the system and on the formation of crystalline final phases.

444 PART j V Emerging Materials for Hydrogen Storage

In the case of Pr(BH4) synthesized by wet chemistry, two different Pr(BH4)3 samples were studied [15]. The first, using Et2O, contained a-Pr(BH4)3 and LiPr(BH4)3Cl, evidencing that LiCl was partly removed. The second, using S(CH3)2, consisted of pure a- and b-Pr(BH4)3. The decomposition pathways of both samples displayed different behavior, which was assumed to be an effect of the residual LiCl. In the presence of LiCl, decomposition was a complex process with at least six major endothermic events. Hydrogen release began at 175 C and continued until 375 C through multiple events. Minor amounts of Et2O (75 to 150 C) and B2H6 (175 to 200 C) release were detected. In opposition, without LiCl, the Pr(BH4)3 decomposition path looked simple, reinforcing the idea that LiCl influenced the decomposition path and was not an inert compound. Heating of Pr(BH4)3 from room temperature to 400 C produced a mass loss of 8.3 wt%, which could be mainly assigned to hydrogen desorption and minor amounts of B2H6 and S(CH3)2. The release of these gases was probably responsible for the difference between the theoretical and measured hydrogen capacity. The proposed reaction is: Pr(BH4)3(s) / 0.5PrH2(s) þ 0.5PrB6(s) þ 5.5H2(g) (5.7 wt% of H2)

(13.20)

During heating to 204 C, the only crystalline phase detected was a-Pr(BH4)3. At 204 C a phase transition was observed. The X-ray powder diffraction intensities of the b-Pr(BH4)3 increased until 217 C and vanished at 275 C. No additional crystalline products were detected. Mechanochemical reaction of HoCl3 and xLiBH4(3.15 < x < 12) induced the formation LiCl and Ho(BH4)3 in a- and b-Ho(BH4)3 polymorphic structures as products [22]. These novel compounds were isostructural to the related RE borohydrides, adopting a-Y(BH4)3 and b-Y(BH4)3 structures. Milling of HoCl3-3LiBH4 led to a-Ho(BH4)3 polymorph formation, whereas the excess of LiBH4 in the starting mixture favored the appearance of a b-Ho(BH4)3 polymorph. Mixed-cation borohydrides were not detected after milling of HoCl3 and xLiBH4, whereas the addition of KBH4 resulted in the preparation of K [Ho(BH4)4], which was isostructural with an Na[Sc(BH4)4]-type structure [43]. The thermal decomposition of a-Ho(BH4)3 started above 170 C, with the fastest rate within 250 to 260 C. Samples a-Ho(BH4)3 heated to 240 C, i.e., before the main event of mass loss, showed no diffraction signal from either polymorph of Ho borohydride. However, several weak peaks from an unidentified phase were observed simultaneously with LiCl. Experimental evidence indicated the presence of a [B12H12]2 anion. The final products of decomposition were amorphous; only LiCl was detected, without evidence of BeH bonds. In other work, Humphries et al. [13] prepared pure Eu(BH4)2 and Sm(BH4)2 by a metathesis reaction of trivalent RE metal chlorides and LiBH4

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in ethereal solution, combined with solvent extraction. The decomposition pathways of both Eu and Sm borohydrides were studied using different experimental techniques and compared against the same compound materials prepared by mechanochemical synthesis. It was demonstrated that the presence of LiCl as a side-product in powder material destabilized the RE(BH4)2 complex and promoted the reduction of at least 20 C of the onset temperature of decomposition compared with the pure material [33]. The thermal decomposition of Sm(BH4)2 had no crystalline intermediates, which was different from the situation observed for the ball-milled 6LiBH4:SmCl3 mixture. In this last case, the starting material consisted of a/b-Sm(BH4)3, SmCl3, and some LiSm(BH4)3Cl; during heating, a/b-Sm(BH4)3 transformed to LiSm(BH4)3Cl, which became Sm(BH4)2 above 200 C [33]. The destabilization effect of impurities was observed for Sm(BH4)2: it decomposed at 335 C when it was obtained in ethereal solution, whereas the presence of LiCl caused decomposition at about 300 C. In the case of Eu(BH4)2, after decomposition an amorphous phase appeared followed by an unknown compound, before the detection of EuB6. An Eu(BH4)2 compound was observed at 105 C from EuCl3:LiH4 mixtures in ethereal solution (compositions of 1:3 and 1:6) and was stable to 185 C. Hydrogen desorption occurred above 350 C for the pure compound and above 334 C for the mixed material. Apparently, between 185 C and 350 C, an intermediate phase was formed before decomposition. As an interesting result, both pure and ball-milled Eu(BH4)2 materials were rehydrogenated at 400 C and 100 bar for 8 h. Moreover, 3.5 wt % of hydrogen was reversibly absorbed in pure Eu(BH4)2 during three cycles, compared with 1.25 wt% for the mechanically milled material. This study highlighted the importance of synthesizing high-purity RE borohydrides, which could provide improved hydrogen storage properties compared with those containing LiCl as a by-product.

13.4 DESTABILIZATION OF LiBH4 BY RARE-EARTH HYDRIDES AND RARE-EARTH BOROHYDRIDES The development of energy-efficient, safe, and reliable hydrogen storage media is a goal that needs to be reached to allow the use of hydrogen as an energy carrier. Solid-state complex hydrides such as borohydrides constitute a leading class of material for hydrogen storage. LiBH4 exhibits one of the highest gravimetric and volumetric hydrogen capacities (18.4 wt% and 122 kg m3, respectively). However, LiBH4 decomposition starts at 380 C, releasing 13.5 wt% of hydrogen according to the reaction [40]: LiBH4(s) / LiH(s) þ B(s) þ 2H2(g) (13.8 wt% of H2)

(13.21)

Rehydrogenation of LiBH4 can be achieved only under extreme conditions (>10 MPa of hydrogen pressure and >400 C) owing to its high thermodynamic stability (DH values between 52 and 76 kJ mol1 H2); this limits its

446 PART j V Emerging Materials for Hydrogen Storage

application as a hydrogen storage material [2,40]. Considering that the dehydrogenation temperature at atmospheric pressure is determined by the thermodynamic stability of the hydride compound, complex hydrides and multicomponent systems emerge as attractive alternatives to traditional metal hydrides. In particular, the combination of LiBH4 with another hydride such as MgH2 allows the thermodynamic stability of the starting hydride to be modified, maintaining a high gravimetric capacity by a strategy known as “reactive hydride composites” (RHCs) [44,45]: 2LiBH4 þ MgH2 $ 2LiH þ MgB2 þ 4H2

(13.22)

The formation of MgB2 during desorption reduces the enthalpy of the reaction (DH ¼ 40 kJ mol1 H2) by stabilizing the dehydrated state, i.e., destabilizing both LiBH4 and MgH2. Vajo et al. [45] showed that 2LiBH4:MgH2 mixtures can be reversibly hydrogenated and rehydrogenated, and that hydrogen storage capacities greater than 10 wt% were achieved using 2e3 mol% TiCl3 catalyst. Based on these encouraging results, the 2LiBH4:MgH2 system was thoroughly investigated with respect to the influence of experimental conditions on the reaction pathway and the effect of additives on the reaction kinetics [46e52]. Among the new RHCs proposed, the combination of LiBH4 with RE hydrides has been studied from a theoretical as well as experimental point of view [53e62]. Theoretical investigations establish whether the RHC constitutes a new destabilized system by calculating DH and the theoretical storage capacity [53,54]. However, these studies are not sufficient to confirm whether the composite would store hydrogen in a reversible form or what the experimental conditions are involved in the hydrogenation/dehydrogenation processes. Experimental studies on LiBH4 destabilized by the addition of RE hydride [55e61] or RE borohydride [20,23,63e66] are useful to determine the role of the kinetic barriers. Experimentally, destabilized LiBH4 with different RE hydrides and RE borohydrides have been explored; the results are summarized in the following section.

13.4.1 Destabilization of LiBH4 by Rare-Earth Hydride Different RHCs were produced by combining LiBH4 with RE hydrides using BM. Most of these RE hydrides were produced through the hydrogenation of pure metals or alloys. Significant destabilization effects were obtained; progress is summarized in Table 13.4. The 6LiBH4:CeH2 composite catalyzed by 0.2TiCl3 was introduced for the first time in 2008 by Jin et al. [55]. Both its theoretical hydrogen capacity of 7.4 wt% and the predicted reaction enthalpy of 44.1 kJ mol1 constitute promising properties for hydrogen storage applications. The experimental evidence showed that the 6LiBH4:CeH2 composite followed the thermodynamically expected dehydrogenation path, forming LiH

Hydrogen Capacity (wt%)

Reaction

Theoretical

Dehydride (cycle 1)

Dehydride (cycle 2)

Temperature ( C)/Pressure (MPa) Dehydride (cycle 1) 

Rehydride (cycle 1) 

DH (kJ mol1 H2)

References

6LiBH4 þ CeH2 / 6LiH þ CeB6 þ 10H2

7.4

6.0

6.0

380 to 400 C/ static vacuum

350 C/10 MPa for 20 h

44.1 58  3b

[55,56]

2LiBH4 þ ScH2 / 2LiH þ ScB2 þ 4H2

8.9

<5.0

e

350 to 450 C/ static vacuum and 0.7 MPa

e

34.1a

[53,57]

4LiBH4 þ YH3 / 4LiH þ YB4 þ 7.5H2

8.5

7.0

5.2

350 C/ 0.5 MPa

350 C/9.0 MPa for 24 h

48a 51b

[58,59]

6LiBH4 þ LaH3 þ 8.5MgH2 / LaB6 þ 6LiH þ 8.5 Mg þ 19 H2

7.7

w6.8

w6.8

260 to 460 C/ vacuum

400 C/10 MPa for 24 h

a

Calculated value. Experimental value.

b

a

[61]

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TABLE 13.4 Hydrogen Storage Properties of LiBH4 Destabilized by Combination With Rare-Earth Hydrides

447

448 PART j V Emerging Materials for Hydrogen Storage

and CeB6 as products. During dehydrogenation under vacuum and later rehydrogenation at 350 C under 100 bar for 20 h, the RHC released/took up about 6.0 wt% of hydrogen reversibly. Later, Mauron et al. [56] determined the stability of the 6LiBH4:CeH2 composite by dynamic pressure, composition, and temperature measurements. The equilibrium pressures were estimated using different constant hydrogen flows and by extrapolating ln (Pdes/P0) linearly to equilibrium at zero flow. Using this extrapolating model and by applying the van ’t Hoff equation, the following thermodynamic parameters were obtained: DH ¼ 58  3 kJ mol1 H2 and DS ¼ 113  4 J K1 mol1 H2, which implied a decomposition temperature of 240  32 C at 1 bar. Although these values were higher than the theoretical ones, the 6LiBH4:CeH2 composite constitutes a destabilized system compared with pure LiBH4 (decomposition temperature of 380 C [40]). Experimental limitations such as the slow kinetics of the material and equilibrium pressures obtained by extrapolating to zero flow could have affected the thermodynamic parameters obtained experimentally. In the case of the LiBH4:ScH2 composite (8.9 wt%), thermodynamic calculations predicted a destabilized system with a reaction enthalpy of about DH ¼ 34 kJ mol1 H2 [57]. However, experimental results showed that less than 5 wt% of hydrogen was released up to 450 C, without evidence of ScH2 participation and/or ScB2 formation during dehydrogenation. The addition of 2 mol% TiCl3 catalyst improved the desorption kinetics but did not promote the destabilization reaction. The formation of Li2B12H12 as an intermediate phase during decomposition was confirmed, which restricted the hydrogen storage reversibility. The authors indicated that the milled LiBH4:ScH2 mixture segregated back into distinct phases above the melting point of LiBH4, and even when the LiBH4:Sc mixture was used, formation of ScH2 was observed rather than ScB2. These results suggest that ScH2 is not effective in destabilizing LiBH4. Another destabilized system reported is the 4LiBH4:YH3 composite. About 7.0 wt% of hydrogen (theoretically 8.4 wt%) was released at 350 C under 0.5 MPa of hydrogen back-pressure through the formation of YB4 and LiH. It was shown that the increase in hydrogen back-pressure enhances the hydrogen desorption process, avoiding the incubation period observed in a vacuum [58]. Rehydrogenation of the RHC was 70% at 350 C and 9.0 MPa for 24 h, without using a catalytic additive. In addition, argon back-pressure showed a similar effect on dehydrogenation because of the suppression of the formation of diborane [59]. According to previous work [60], diborane reacts with LiBH4 to form Li2B12H12. Apparently, the formation of this closoborane retards the progress of rehydrogenation by restricting contact between YH3 and liquid LiBH4. In later work, Shim et al. [58] studied the 4LiBH4:YH3 composite both theoretically and experimentally. The reaction enthalpy obtained from the van ’t Hoff plot was 52 kJ mol1, which was in agreement with the calculated

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value of 48 kJ mol1. The decomposition temperature at 1 bar of hydrogen estimated from the measured data was 232 C, which was higher than the calculated value (180 C) but significantly lower than that of LiBH4 [58]. Using the transformation of La2Mg17 into MgH2 and LaH3 as a strategy during hydrogenation, the LiBH4eLa2Mg17 system was explored as an RHC [61]. The dehydrogenation pathway consisted of two reactions: first, MgH2 decomposed in the temperature range of 230 of 300 C; during subsequent heating (300 to 470 C), LiBH4 reacted with LaH3 to produce LaB6 and LiH. The authors [61] showed that the as-milled RHC was able to desorb and absorb about 6.8 wt% under 400 C and 10 MPa, with a high reaction rate. Mechanistic analyses revealed that the improved behavior resulted from a combination of destabilization and a catalysis effect produced by MgH2 and LaH3 formed during reactive BM.

13.4.2 Destabilization of LiBH4 by Rare-Earth Borohydrides Several RE borohydrides were synthesized by mechanochemical reaction between RE chlorides and LiBH4, which is a solvent-free procedure. In these cases, the final product consisted of a mixture of the RE borohydride and LiCl. Sometimes the milling induced partial halide substitution in the RE borohydride synthesized. As mentioned in the previous section, the LiCl by-product influences the dehydrogenation path of RE borohydride and usually constitutes a dead mass of about 40e60 wt% for hydrogen storage purposes. Table 13.5 summarizes LiBH4-based systems destabilized by RE borohydrides studied experimentally, in which partial reversibility was obtained. The first investigation into destabilization of LiBH4 by adding RE borohydrides was reported by Gennari et al. [63]. The mixture of LiBH4 and RE borohydrides was prepared by milling the 6LiBH4-RECl3 powders (RE ¼ Ce, Gd), according to the following metathesis reaction: 6LiBH4(s) þ CeCl3(s) / LiCe(BH4)3Cl(s) þ 3LiBH4(s) þ 2LiCl(s)

(13.23)

6LiBH4(s) þ GdCl3(s) / Gd(BH4)3(s) þ 3LiBH4(s) þ 3LiCl(s) (13.24) The RHCs containing RE borohydride displayed superior hydrogen storage properties compared with LiBH4, as shown in Fig. 13.3. Hydrogen release started at 200 C owing to RE borohydride decomposition, generating RE hydrides and RE borides. For example, for RE ¼ Ce, the first step can be expressed as: LiCe(BH4)3Cl(s) / 1/2CeH2þx(s) þ 1/2CeB6(s) þ LiCl(s) þ (11  x/2)/2H2(g) (13.25) in which LiCe(BH4)3Cl was produced by milling the 3LiBH4:CeCl3 mixture (Fig. 13.3). As the temperature increased, the in situ REH2þx that was formed

TABLE 13.5 Hydrogen Storage Properties of LiBH4 Destabilized by Combination With Rare-Earth Borohydrides Hydrogen Capacity (wt%) Reactive Hydride Composites from Mechanochemical Reaction Between LiBH4 and RECl3

Theoreticala

Dehydride (cycle 1)

Dehydride (cycle 2)

Temperature ( C)/Pressure (MPa) Dehydride (cycle 1) 

Rehydride (cycle 1) 

References

3LiBH4 þ LiCe(BH4)3Cl þ 2LiCl / CeB6 þ 3LiH þ 3LiCl þ 10.5H2

5.6

5.3 5.5e

2.3 2.6e

400 C/0.02 MPa 400 C/0.5 MPa

400 C/6.0 MPa for 2 h

[63]

6LiBH4 þ CeH2 þ 3LiCl / CeB6 þ 6LiH þ 3LiCl þ 10H2b

5.6

4.6d

4.6d

400 C/0.02 MPa

400 C/6.0 MPa for 4 h

[64]

6LiBH4 þ LaH2 þ 3LiCl / LaB6 þ 6LiH þ 3LiCl þ 10 H2b

5.6

5.1d

3.6d

400 C/0.02 MPa

400 C/6.0 MPa for 4 h

[64]

3LiBH4 þ LiLa(BH4)3Cl þ 2LiCl / LaB6 þ 3LiH þ 3LiCl þ 10.5 H2

5.6

5.3d (TG)

1.9d

430 C/static vacuum

415 C/10 MPa

[20]

LiBH4 þ Gd(BH4)3 þ 3LiCl / GdB4 þ LiH þ 3LiCl þ 7.5H2c

4.3

5.0d 4.9e

2.0d 1.7e

400 C/0.02 MPa 400 C/0.5 MPa

400 C/6.0 MPa for 2 h

[63]

LiBH4 þ Y(BH4)3 þ 3LiCl / YB4 þ LiH þ 7.5H2 LiBH4 þ Y(BH4)3 þ 3LiCl / YH2 þ B þ 7H2 þ 3LiCl

5.3 5.0

4.6

1.0

400 C/0.5 MPa

400 C/6.5 MPa for 6 h

[23]

4LiBH4 þ YH2 þ 3LiCl / YB4 þ 4LiH þ 7H2b

4.6

4.5d 4.4e

4.1d 4.1e

400 C/0.02 MPa 400 C/0.5 MPa

400 C/6.5 MPa for 6 h

[23]

4LiBH4 þ NdH2 þ 3LiCl / NdB4 þ 4LiH þ 7H2 þ 3LiCl

3.9

3.9

3.9d

370 C/vacuum

400 C/10 MPa for 24 h

[65]

LiBH4 þ Er(BH4)3 þ 3LiCl / ErB4 þ 4LiH þ 7H2c

3.9

e

1.4d

430 C/static vacuum

415 C/10 MPa for 20 h

[20]

4LiBH4 þ ErH3 / ErB4 þ 4LiH þ 7.5H2b

5.9

4.2

3.7

400 C/0.3e0.5 MPa

340 C/10 MPa for 12 h

[66]

TG, thermogravimetric. a The value refers to the starting mixture considering side products. b Addition of 3LiH to the starting mixture. c The starting mixture was 6LiBH4-3RECl3 and resulted in an excess of 2LiBH4. d Dehydrogenation under vacuum or low hydrogen pressure. e Dehydrogenation under high hydrogen back-pressure.

d

d

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0

H desorbed (wt%)

-1 -2 -3 -4 -5

as-milled LiBH4 3LiBH4:CeCl3 mixture 6LiBH4:CeCl3 mixture 6LiBH4: (ex-situ milled CeCl3: LiH) mixture

-6

100

200

300

400

o

Temperature ( C) FIGURE 13.3 Dehydrogenation curves of as-milled: LiBH4, 3LiBH4:CeCl3 mixture, 6LiBH4:CeCl3 mixture, and 6LiBH4 plus (ex situ milled CeCl3:LiH) mixture. A heating ramp of 5 C min1 was used.

promoted LiBH4 decomposition, with additional hydrogen release up to 400 C (80% of the theoretical value): 3LiBH4(s) þ 1/2CeH2þx(s) þ 1/2CeB6(s) þ 3LiCl(s) / CeB6(s) þ 3LiH(s) þ 3LiCl(s) þ (5 þ x/4)/2H2(g)

(13.26)

The overall destabilization reaction can be expressed as: 3LiBH4(s) þ LiCe(BH4)3Cl(s) þ 2LiCl / CeB6(s) þ 3LiH(s) þ 3LiCl(s) þ 10.5H2(g) (13.27) Thus, both REH2þx (in which RE ¼ Ce and Gd) hydrides were effective in reducing the dehydrogenation temperature of LiBH4, which alone released 11% of its total hydrogen content from 300 to 400 C [63]. The dehydrogenation products of RHCs consisted of the mixture of metal borides (CeB6 and GdB4) and LiH, which could be rehydrogenated under moderate conditions, i.e., 400 C and 6.0 MPa for 2 h. To improve reversibility, the RHC obtained after mechanochemical processing the 6LiBH4:CeCl3 mixture was milled again with an extra amount of 3LiH: RELi(BH4)3Cl(s) þ 3LiBH4(s) þ 2LiCl(s) þ 3LiH(s) / 6LiBH4(s) þ REH3(s) þ 3LiCl(s) (RE ¼ Ce, Gd)

(13.28)

The 6LiBH4:CeH2þx composite showed promising hydrogen storage properties via the formation of CeH2þx during milling, reaching 80% of reversibility without using catalysts under mild conditions (400 C, 6.0 MPa for 2 h).

452 PART j V Emerging Materials for Hydrogen Storage

To analyze the role of the nanostructure on destabilizing a 6LiBH4:CeH2þx composite, CeH2þx was produced by in situ and ex situ mechanochemical activated reactions [64]. Clearly, when CeH2þx was formed in situ, both the sorption rate and hydrogen storage reversibility were superior as a result of the nanostructured nature of the composite. Fig. 13.3 shows the dehydrogenation curve of the 6LiBH4:CeH2þx composite in which CeH2þx was formed ex situ compared with LiBH4. The onset temperature of dehydrogenation was lower than that of LiBH4, and was evidence of a destabilization effect of RE hydride. Moreover, the destabilization effect of LaH2þx produced ex situ on LiBH4 decomposition was reported for the first time [64]. The starting temperature of hydrogen release was 260 C for both 6LiBH4:LaH2þx and 6LiBH4:CeH2þx composites. However, 6LiBH4:LaH2þx displayed improved hydrogen desorption performance and inferior hydrogen storage reversibility. In general, the thermodynamic destabilization effect and kinetic improvement of these RHCs deteriorated with cycling, showing that a kinetic barrier operated related to the RE hydride. Two different RHCs, LiBH4:Y(BH4)3 and 4LiBH4:YH2þx, were prepared by mechanochemical processing of 4LiBH4:YCl3 and the as-milled 4LiBH4:YCl3 plus 3LiH, respectively [20]. The representative consecutive reactions are: 4LiBH4(s) þ YCl3(s) / Y(BH4)3(s) þ 3LiCl(s) þ LiBH4(s, unreacted)

(13.29)

Y(BH4)3(s) þ 3LiCl(s) þ LiBH4(s) þ 3LiH(s) / 4LiBH4(s) þ YH2þx(s) þ 3LiCl(s) (13.30) The milling stimulated the in situ reconstruction of 3LiBH4 and the formation of YH2þx from Y(BH4)3 and LiH. Heating the LiBH4eY(BH4)3 mixture obtained from reaction (13.29) induced the decomposition of Y(BH4)3 at 250 C with the consequent in situ formation of YH2þx. Subsequently, YH2þx destabilized LiBH4, leading to the formation of minor amounts of YB4. Only 20% of rehydrogenation was obtained from YB4e4LiHe3LiCl at 400 C and 6.5 MPa. In the case of the 4LiBH4eYH2þx system produced by the consecutive reactions (13.29) and (13.30), the hydrogen storage properties depended strongly on the experimental conditions used. The increase in the hydrogen back-pressure during dehydrogenation favored the formation of YB4 and suppressed that of diborane, whereas a reduction in the hydrogen backpressure to less than 0.1 MPa induced the formation of Li2(B12H12). The nanostructured 4LiBH4:YH2þx composite displayed a good hydrogen storage reversibility of 80%. It was also found that NdH2 thermodynamically destabilized LiBH4 by forming NdB4 in the dehydrogenated state. The estimated decomposition enthalpy change was 64 kJ mol1 H2. Experimentally, w6.0 wt% of hydrogen (roughly the theoretical value) was quickly liberated at 370 C within 1.5 h by

A Systematic Approach to the Synthesis, Thermal Stability Chapter j 13

453

the nanosized NdH2þx particles [65]. This RHC possessed a superior cyclic dehydrogenation capacity, recharging 6.0 and 5.2 wt% of hydrogen in 3 and 2 h for the second and third cycles, respectively. Despite this good reversible behavior, the kinetics deteriorated during later cycling as a result of NdH2þx coarsening. The authors commented on the relevance of reducing and stabilizing the particle size of NdH2þx to reach a better destabilized RHC system for hydrogen storage applications. In other work, the thermal decomposition behavior of the as-milled 6LiBH4eRECl3 (in which RE ¼ La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Er, Yb, and Lu) mixtures was studied [20]. The dehydrogenation process of the LiBH4eRE borohydride (in which RE ¼ La, Ce, Pr, and Nd) is complex. It starts below 200 C and proceeds by forming unknown intermediate phases. As the temperature increases, LiBH4 decomposes and releases all hydrogen below 350 C, i.e., at a lower temperature than pure LiBH4. The observed mass loss for the composite with RE ¼ Ce was 5.35 wt%, which confirmed that the RE compound destabilizes the excess LiBH4. Gas analysis showed the absence of diborane or other borane species in the gas phase [20]. Moreover, the thermal decomposition of samples with Gd, Tb, Er, and Lu resembled the general behavior of the La, Ce, and Pr systems: RE-borohydride decomposed around 250 C, releasing hydrogen followed by hydrogen release from the remaining LiBH4. On the contrary, RHCs containing Sm, Eu, and Yb behaved differently, with one major mass loss at temperatures lower than 200 C. For Yb it was shown that Yb(BH4)3 formed by BM reduced to Yb(BH4)2 upon heating around 100 C, with the release of diborane [33]. Similar behavior was assumed to occur for Sm and Eu: the major mass loss observed at T less than 200 C was associated with a reduction from the trivalent to the divalent state of the RE metal and diborane gas emission. In the same investigation, a preliminary evaluation of hydrogen storage reversibility of 6LiBH4eRECl3 (for which RE ¼ La and Er) composite was performed. Partial rehydrogenation (18% and 25%) was obtained for RE ¼ La and Er at 300 and 415 C, respectively, using 10 MPa of hydrogen, after complete dehydrogenation under vacuum. New experiments to evaluate the effect of adding LiH on dehydrogenation and the reversibility properties of LiBH4:RECl3 composites (in which RE ¼ La and Er) were carried out by Frommen et al. [34]. Hydrogen desorption started at 300 C as a result of the destabilization of LiBH4 by REH2þx formed from the parent RE borohydrides. For both RHCs, simplified thermal decomposition was observed with LiH compared with the composites without LiH [20]. Rehydrogenation was performed at 340 C under 10 MPa hydrogen pressure for 12 h. The RHC with La showed limited reversibility (20%). In the case of the RHC of Er, desorption against 0.5 MPa of hydrogen back-pressure improved reversibility (80%) compared with vacuum (66%) [34].

454 PART j V Emerging Materials for Hydrogen Storage

In other work [66], the effect of Er(BH4) free of LiCl upon LiBH4 decomposition was evaluated for the first time. Two new RHCs were studied: Er(BH4)3:6LiH and 3LiBH4:Er(BH4)3:3LiH. In the first composite, milling of the reactants induced the following transformation: Er(BH4)3(s) þ 3LiH(s) / ErH3(s) þ 3LiBH4(s)

(13.31)



After heating at 400 C, a mass loss of 3.4 wt% of H2 was measured using thermogravimetric analysis, according to the reaction: 4LiBH4(s) þ ErH3(s) / ErB4(s) þ 4LiH(s) þ 7.5H2(g)

(13.32)

Reabsorption at 340 C under 10 MPa of H2 for 12 h resulted in 3.0 wt% of hydrogen storage via the reformation of LiBH4 and ErH3. For the second composite, BM of 3LiBH4:Er(BH4)3:3LiH favored an incipient ErH2 formation via reaction between Er(BH4)3 and LiH. During 5 h decomposition at 400 C and 3e5 bar, about 4.2, 3.7, and 3.5 wt% of H2 was released during the first, second, and third cycle, respectively. Both composites reacted during milling and heating in a two-step reaction. In a first step, Er hydrides and LiBH4 were formed; in the second step, thermal decomposition occurred and ErB4 appeared. The composites were rehydrogenated and reached about 88% of the initial hydrogen storage capacity and seemed stable after the third cycle. In fact, the hydrogen storage capacity was almost double compared with the first investigated systems, containing LiCl as a side product. Finally, as summarized in Tables 13.4 and 13.5, both RE hydrides and RE borohydrides are effective additives to destabilize LiBH4. RHCs formed by combining LiBH4 with RE hydrides by BM had a relatively simple dehydrogenation process. Hydrogen was released at temperatures near the LiBH4 melting point but below its normal dehydrogenation temperature. Superior reversible hydrogen storage ability was demonstrated for some RHCs under mild conditions (in which RE ¼ Ce, Y, and LaeMg), which is encouraging. The RHCs containing RE borohydrides were produced by milling an xLiBH4:RECl3 mixture. Decomposition of these composites displayed a complex multistep process that involved several intermediate phases until the formation of the final products, i.e., the metal borides and RE hydrides. Hydrogen release started at about the decomposition temperature of the RE borohydrides, with the consequent in situ formation of RE hydrides. In this way, the RE hydride formed destabilized the excess LiBH4 and led to hydrogen release at lower temperatures than the ball-milled LiBH4. This behavior was demonstrated for Ce, Y, La, Gd, Er, and Ho among other RE metals. When 3LiH was added to the initial xLiBH4:RECl3 composite, milling promoted the formation of the in situ RE hydride. The resulting RHC showed a simplified thermal decomposition compared with the composite (containing RE borohydride) without LiH. Although dehydrogenation started at higher temperatures, the kinetics of hydrogen desorption seemed to be enhanced compared with the xLiBH4:RECl3 composite.

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As a general behavior, reversibility of RHC with RE hydride or RE borohydride is sensitive to different variables, such as the experimental conditions selected for rehydrogenation (temperature, time, and hydrogen pressure), the hydrogen back-pressure used during dehydrogenation, the presence of LiBH4 and/or LiH in the final product, the microstructure of the material, the presence of a catalyst, etc.

13.5 CONCLUSIONS RE borohydrides are complex hydrides formed by the [BH4] anion and the RE metal cation. Considering the intermediate electronegativity of RE metals with respect to other metals in the periodic table and the nature of their interaction with the complex anion, these compounds possess intermediate thermodynamic stability in comparison with traditional alkali metal or alkaline earth metal borohydrides. Because of their moderate decomposition temperatures and their high densities of stored hydrogen, RE borohydrides are regarded as potential candidates for hydrogen storage applications. Several RE borohydrides were synthesized using exchange reactions in solution, solidestate or gasesolid media. The exchange reaction in solution allows the production of RE borohydrides free of by-products, which is relevant for determining the decomposition temperature of pure compounds. Similarly, reactive BM emerges as an attractive procedure for obtaining pure RE borohydride, but the use of toxic diborane as a reactant constrains its extended application. Moreover, the metathesis reaction promoted by mechanochemical processing allows the synthesis of diverse borohydrides such as monometallic RE borohydrides (in which RE3þ ¼ Y, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, and Yb), mixed-cation mixed-anion borohydride chlorides (in which RE3þ ¼ La, Gd, Ce, Pr, Nd, and Sm) and mixed-cation borohydrides (in which RE3þ ¼ Yb, Lu, and Sc), with a diverse chemistry. These materials have a wide range of unexpected properties, with high potential for novel applications in the future. The progress reached in these synthesis procedures allows the study of the decomposition behavior of RE borohydrides as well as their hydrogen storage reversibility. Thermal studies showed that the decomposition reaction of RE borohydrides is endothermic and the onset temperature is about 200 C. In general, it consists of a multistep pathway with the formation of intermediate compounds. The gas released under heating is mainly hydrogen with minor amounts of diborane. When the synthesis procedure involves solvents, it is also possible to detect the minor emission of other gases. Hydrogen storage reversibility for RE borohydrides was unsuccessful; instead LiBH4 and/or REHx reformed. Thus, the direct applicability of RE borohydrides is limited from a technological point of view. In addition, it was demonstrated that the presence of by-products such as LiCl or unreacted LiBH4 influences the physical properties and the dehydrogenation pathway of RE borohydrides.

456 PART j V Emerging Materials for Hydrogen Storage

Destabilization of LiBH4 by their combination with RE hydrides or RE borohydrides proved to be effective for some composites. This approach improves the thermodynamics and kinetics of hydrogen sorption by modification of the reaction pathway, resulting in more reversible hydrogen storage. Reactive hydride composites containing RE hydrides or RE borohydrides enhance hydrogen uptake and release rates and allow rehydrogenation under mild experimental conditions with respect to the pure LiBH4. Hydrogen storage reversibility is promoted by dehydrogenation at lower temperatures or under hydrogen back-pressure, which restricts the formation of higher boranes. Despite the good properties displayed by these materials, none can simultaneously fulfill all requirements for mobile hydrogen storage applications. Therefore, continued efforts are required to develop novel RE borohydrides with improved hydrogen storage properties, by tailoring their composition, structure, and properties. RE borohydrides have shown high potential for new applications owing to their ionic conductivity, photoluminescence, and magnetic properties.

ACKNOWLEDGMENTS The authors thank CONICET (Consejo Nacional de Investigaciones Cientı´ficas y Te´cnicas), ANPCyT (Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica) and CNEA (Comisio´n Nacional de Energı´a Ato´mica). F. C. G gratefully acknowledges the financial support from L’Ore´al-UNESCO National Award for Women in Science, in collaboration with CONICET (2016).

REFERENCES [1] W. Buchner, H. Niederprum, Sodium borohydride and amine-boranes, commercially important reducing agents, Pure Appl. Chem. 49 (June 1977) 733e743. [2] H.-W. Li, et al., Recent progress in metal borohydrides for hydrogen storage, Energies 4 (January 2011) 185e214. [3] M. Paskevicius, et al., Metal borohydrides and derivatives e synthesis, structure and properties, Chem. Soc. Rev. 46 (February 2017) 1565e1634. [4] Y. Nakamori, S-i. Orimo, Borohydrides as hydrogen storage materials, in: G. Walker (Ed.), Solid-state Hydrogen Storage, Materials and Chemistry, CRC Press, New York, 2008. [5] J. Huot, et al., Mechanochemical synthesis of hydrogen storage materials, Prog. Mater. Sci. 58 (January 2013) 30e75. [6] C. Suryanarayana, Mechanical alloying and milling, Prog. Mater. Sci. 46 (January 2001) 1e184. [7] E. Zange, Entwicklungeines Mikroverfahrens zur Darstellung von Boranaten der schweren Lanthaniden, Chem. Ber. 93 (March 1960) 652e657. [8] A. Badalov, et al., Thermal stability and thermodynamic characteristics of gadolinium tetrahydroborate tetrahydrofuranate, Russ. J. Coord. Chem. 18 (1992) 559e565 (in Russian). [9] M.B. Ley, et al., Novel solvates M(BH4)3S(CH3)2 and properties of halide-free M(BH4)3 (M ¼ Y or Gd), Dalton Trans. 43 (July 2014) 13333e13342. [10] M.B. Ley, et al., From m(BH4)3 (M ¼ La, Ce) borohydride frameworks to controllable synthesis of porous hydrides and ion conductors, Inorg. Chem. 55 (October 2016) 9748e9756.

A Systematic Approach to the Synthesis, Thermal Stability Chapter j 13 [11] [12] [13] [14] [15]

[16] [17] [18] [19]

[20]

[21] [22] [23] [24] [25] [26] [27]

[28] [29] [30] [31] [32]

457

Y. Yan, et al., Dehydriding and rehydriding properties of yttrium borohydride Y(BH4)3 prepared by liquid-phase, Int. J. Hydrogen Energy 34 (July 2009) 5732e5736. T. Jaro n, W. Grochala, Y(BH4)3 an old-new ternary hydrogen store aka learning from a multitude of failures, Dalton Trans. 39 (January 2010) 160e166. T.D. Humphries, et al., Crystal structure and in situ decomposition of Eu(BH4)2 and Sm(BH4)2, J. Mater. Chem. 3 (March 2015) 691e698. M. Sharma, et al., Halide free m(BH4)2 (M ¼ Sr, Ba, and Eu) synthesis, structure, and decomposition, Inorg. Chem. 55 (June 2016) 7090e7097. M. Heere, et al., The influence of LiH on the rehydrogenation behavior of halide free rareearth (RE) borohydrides (RE ¼ Pr, Er), Phys. Chem. Chem. Phys. 18 (August 2016) 24387e24395. T. Sato, et al., Experimental and computational studies on solvent free rare-earth metal borohydrides R(BH4)3 (R ¼ Y, Dy, and Gd), Phys. Rev. B 77 (March 2008) 104114 [1-8]. C. Frommen, et al., Crystal structure, polymorphism, and thermal properties of yttrium borohydride Y(BH4)3, J. Alloys Compd. 496 (April 2010) 710e716. D.B. Ravnsbæk, et al., Thermal polymorphism and decomposition of Y(BH4)3, Inorg. Chem. 49 (April 2010) 3801e3809. T. Jaro n, et al., Phase transition induced improvement in H2 desorption kinetics: the case of the high-temperature form of Y(BH4)3, Phys. Chem. Chem. Phys. 13 (April 2011) 8847e8851. J.E. Olsen, et al., Structure and thermal properties of composites with RE-borohydrides (RE ¼ La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Er, Yb or Lu) and LiBH4, RSC Adv. 4 (June 2014) 1570e1582. K. Park, et al., Thermal properties of Y(BH4)3 synthesized via two different methods, Int. J. Hydrogen Energy 38 (July 2013) 9263e9270. W. Wegner, et al., Polymorphism and hydrogen discharge from holmium borohydride, Ho(BH4)3, and KHo(BH4)4, Int. J. Hydrogen Energy 39 (December 2014) 20024e20030. F.C. Gennari, Improved hydrogen storage reversibility of LiBH4 destabilized by Y(BH4)3 and YH3, Int. J. Hydrogen Energy 37 (December 2012) 18895e18903. F.C. Gennari, Mechanochemical synthesis of erbium borohydride: polymorphism, thermal decomposition and hydrogen storage, J. Alloys Compd. 581 (December 2013) 192e195. F.C. Gennari, M.R. Esquivel, Synthesis and dehydriding process of crystalline Ce(BH4)3, J. Alloys Compd. 485 (October 2009) L47eL51. J. Andrade-Gamboa, et al., A novel polymorph of gadolinium tetrahydroborate produced by mechanical milling, Int. J. Hydrogen Energy 35 (October 2010) 10324e10328. C. Frommen, et al., Synthesis, crystal structure, and thermal properties of the first mixedmetal and anion-substituted rare-earth borohydride LiCe(BH4)3Cl, J. Phys. Chem. C 115 (October 2011) 23591e23602. M.B. Ley, et al., LiCe(BH4)3Cl, a new lithium-ion conductor and hydrogen storage material with isolated tetra nuclear anionic clusters, Chem. Mater. 24 (April 2012) 1654e1663. M.B. Ley, et al., New Li ion conductors and solid state hydrogen storage materials: LiM(BH4)3Cl, M ¼ La, Gd, J. Phys. Chem. C 116 (September 2012) 21267e21276. B.J. Zhang, et al., Destabilization of LiBH4 by (Ce,La)(Cl,F)3 for hydrogen storage, J. Alloys Compd. 509 (January 2011) 751e757. H. Hagemann, et al., LiSc(BH4)4: a novel salt of Liþ and discrete Sc(BH4) 4 complex anions, J. Phys. Chem. 112 (July 2008) 7551e7555. R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Crystallogr. A 32 (September 1976) 751e767.

458 PART j V Emerging Materials for Hydrogen Storage [33] J.E. Olsen, et al., Crystal structures and properties of solvent-free LiYb(BH4)4-xClx, Yb(BH4)3 and Yb(BH4)2-xClx, RSC Adv. 3 (May 2013) 10764e10774. [34] M. Frommen, et al., Hydrogen storage properties of rare-earth (RE) borohydrides (RE ¼ La, Er) in composite mixtures with LiBH4 and LiH, J. Alloys Compd. 645 (October 2015) S155eS159. [35] A. Remhof, et al., Solvent-free synthesis and decomposition of Y(BH4)3, Scr. Mater. 66 (March 2012) 280e283. [36] Y. Yan, et al., Is Y2(B12H12)3 the main intermediate in the decomposition process of Y(BH4)3? Chem. Commun. 49 (April 2013) 5234e5236. [37] P. Schouwink, et al., Structure and properties of complex hydride perovskite materials, Nat. Commun. 5 (December 2014) 5706. [38] P. Schouwink, et al., Structural and magnetocaloric properties of novel gadolinium borohydrides, J. Alloys Compd. 664 (April 2016) 378e384. [39] Y. Nakamori, et al., Correlation between thermodynamical stabilities of metal borohydrides and cation electronegativites: first-principles calculations and experiments, Phys. Rev. B 74 (July 2006) 045126. [40] A. Zu¨ttel, et al., Hydrogen storage properties of LiBH4, J. Alloys Compd. 356e357 (August 2003) 515e520. [41] Y.-S. Lee, et al., Polymorphism and thermodynamics of Y(BH4)3 from first principles, J. Phys. Chem. C 114 (July 2010) 12833e12837. [42] M. Chong, et al., Reversible dehydrogenation of magnesium borohydride to magnesium triborane in the solid state under moderate conditions, Chem. Commun. 47 (November 2011) 1330e1332.  ´, et al., NaSc(BH4)4: a novel scandium-based borohydride, J. Phys. Chem. C 114 [43] R. Cerny (December 2010) 1357e1364. [44] U. Bo¨senberg, et al., Hydrogen sorption properties of MgH2-LiBH4 composites, Acta Mater. 55 (June 2007) 3951e3958. [45] J.J. Vajo, et al., Reversible storage of hydrogen in destabilized LiBH4, J. Phys. Chem. B 109 (February 2005) 3719e3722. [46] U. Bo¨senberg, et al., Pressure and temperature influence on the desorption pathway of the LiBH4 - MgH2, J. Phys. Chem. C 114 (August 2010) 15212e15217. [47] F. Cova, et al., New insights into the thermodynamic behavior of 2LiBH4eMgH2 composite for hydrogen storage, J. Phys. Chem. C 119 (June 2015) 15816e15822. [48] K.-B. Kim, et al., Dehydrogenation reaction pathway of the LiBH4eMgH2 composite under various pressure, J. Phys. Chem. C 119 (April 2015) 9714e9720. [49] U. Bo¨senberg, et al., Role of additives in LiBH4eMgH2 reactive hydride composites for sorption kinetics, Acta Mater. 58 (May 2010) 3381e3389. [50] J.A. Puszkiel, et al., Effect of Fe additive on the hydrogenation-dehydrogenation properties of 2LiHþMgB2/2LiBH4þMgH2 system, J. Power Sources 284 (February 2015) 606e616. [51] F. Cova, et al., CNT addition to the LiBH4eMgH2 composite: the effect of milling sequence on the hydrogen cycling properties, RSC Adv. 5 (October 2015) 90014e90021. [52] J.A. Puszkiel, et al., A novel catalytic route for hydrogenation-dehydrogenation of 2LiHþMgB2 via in-situ formed core-shell LixTiO2 nanoparticles,, J. Mater. Chem. A 5 (May 2017) 12922e12933. [53] S.V. Alapati, et al., Large-scale screening of metal hydride mixtures for high-capacity hydrogen storage from first-principle calculations, J. Phys. Chem. C 112 (March 2008) 5258e5262.

A Systematic Approach to the Synthesis, Thermal Stability Chapter j 13 [54]

[55] [56] [57]

[58] [59] [60] [61] [62] [63] [64]

[65] [66]

459

D.J. Siegel, et al., Thermodynamics guidelines for the prediction of hydrogen storage reactions and their application to destabilized hydride mixtures, Phys. Rev. B 76 (October 2007) 134102. S.-A. Jin, et al., Reversible hydrogen storage in LiBH4-MH2 (M ¼Ce, Ca) composites, J. Phys. Chem. C 112 (June 2008) 9520e9524. P. Mauron, et al., Stability of the LiBH4/CeH2 composite system determined by dynamic PCT measurements, J. Phys. Chem. C 114 (September 2010) 16801e16805. J. Purewal, et al., Hydrogen sorption behavior of the ScH2-LiBH4 system: experimental assessment of chemical destabilization effects, J. Phys. Chem. C 112 (May 2008) 8481e8485. J.-H. Shim, et al., Effect of hydrogen back pressure on dehydrogenation behavior of LiBH4based reactive hydride composites, J. Phys. Chem. Lett. 1 (January 2010) 59e63. K.-B. Kim, et al., Pressure-enhanced dehydrogenation reaction of the LiBH4-YH3 composite, Chem. Commun. 47 (August 2011) 479831e479833. O. Friedrichs, et al., Role of Li2B12H12 for the formation and decomposition of LiBH4, Chem. Mater. 22 (May 2010) 3265e3268. Y. Zhou, et al., Improved hydrogen storage properties of LiBH4 destabilized by in situ formation of MgH2 and LaH3, J. Phys. Chem. C 116 (January 2012) 1588e1595. I. Saldan, A prospect for LiBH4 as on-board hydrogen storage, Cent. Eur. J. Chem. 9 (October 2011) 761e775. F.C. Gennari, et al., Reversible hydrogen storage from 6LiBH4-MCl3 (M¼ Ce, Gd) composites by in-situ formation of MH2, Int. J. Hydrogen Energy 36 (January 2011) 563e570. F.C. Gennari, Destabilization of LiBH4 by MH2 (M¼ Ce, La) for hydrogen storage: nanostructural effects on the hydrogen sorption kinetics, Int. J. Hydrogen Energy 36 (November 2011) 15231e15238. W. Cai, et al., Nanosize-controlled reversibility for a destabilizing reaction in the LiBH4NdH2þx system, J. Phys. Chem. C 117 (April 2013) 9566e9572. M. Heere, et al., Hydrogen sorption in erbium borohydride composite mixtures with LiBH4 and/or LiH, Inorganics 5 (April 2017) 31e45.