Thermodynamic calculation of LiH ↔ Li3AlH6 ↔ LiAlH4 reactions

Journal of Alloys and Compounds 420 (2006) 286–290

Thermodynamic calculation of LiH ↔ Li3AlH6 ↔ LiAlH4 reactions Je-Wook Jang a , Jae-Hyeok Shim b , Young Whan Cho b , Byeong-Joo Lee a,∗ a

Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea b Nano-Materials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea Received 26 August 2005; received in revised form 21 October 2005; accepted 22 October 2005 Available online 5 December 2005

Abstract Thermodynamic properties of hydrides, LiH, Li3 AlH6 and LiAlH4 which can be a promising candidate for a hydrogen storage material system, were critically assessed by analyzing available piecewise literature information on thermodynamic properties using the CALPHAD method. The optimized thermodynamic descriptions were used to compute the phase equilibria between Li3 AlH6 and LiAlH4 as a function of temperature and hydrogen partial pressure. It was predicted that more than 103 bar of hydrogen partial pressure is necessary to induce the hydrogen absorption reaction of Li3 AlH6 → LiAlH4 above room temperature. It could be concluded that a reversible reaction between Li3 AlH6 and LiAlH4 is thermodynamically impossible in a practically accessible temperature and hydrogen pressure range for a hydrogen storage material without any attempt to change the thermodynamic properties of the hydrides. © 2005 Elsevier B.V. All rights reserved. Keywords: Hydrogen storage materials; Thermodynamic calculations; Chemical reactions; Li3 AlH6 ; LiAlH4

1. Introduction A high capacity of hydrogen storage and a reversible hydrogen absorption/desorption reaction are key material properties of hydrogen-powered devices. Light-metal hydride is one of the promising solid-state hydride hydrogen storage media. Especially, the (Na,Li)–Al-hydrides have attracted a significant scientific interest because of the high capacity of hydrogen storage 5.5–7.4 wt.% [1] and the possibility of reversible reactions by catalyst [2]. In the case of the Li–Al-hydrides system, a theoretical hydrogen storage capacity of 5.5 wt.% is obtained from the Li3 AlH6 → LiH reaction. The hydrogen storage capacity can be increased up to 7.9 wt.% if the LiAlH4 → Li3 AlH6 reaction can be further utilized. However, a reversible reaction between LiAlH4 and Li3 AlH6 has not been experimentally reported yet. A large number of experimental and theoretical investigations have been performed on thermodynamic properties (heat of reactions, reaction temperature vs. hydrogen pressure) and reversibility of the reactions, LiAlH4 ↔ Li3 AlH6 ↔ LiH [3–13]. However, thermodynamic data from various sources are mostly

∗

Corresponding author. Fax: +82 54 2792399. E-mail address: [email protected] (B.-J. Lee).

0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.10.040

piecewise and can provide only limited information on the reaction thermodynamics. In addition, it is not clearly known yet whether the no observation of reversible reaction between LiAlH4 and Li3 AlH6 comes from a thermodynamic reason or from a kinetic reason. If it comes from a kinetic reason, further efforts to find an adequate catalyst in order to promote the reaction should be made. However, if it turns out that the no observation of reversible reaction originates from intrinsic thermodynamic properties of the relevant materials, then efforts should be concentrated on the stabilization of the Li3 AlH6 ↔ LiH reversible reaction or on finding other materials systems to increase the hydrogen storage capacity. The purpose of the present study is to clarify the reason for the experimental difficulty in observing reversible reactions between LiAlH4 and Li3 AlH6 , by an empirical thermodynamic calculation technique. The thermodynamic properties of all relevant hydride phases were critically assessed by analyzing available piecewise literature information using the CALPHAD method [14]. By this, self-consistent Gibbs energy expressions for hydride phases, LiAlH4 , Li3 AlH6 and LiH, could be obtained and phase equilibria in regions where experimental information is not available (for example, the LiAlH4 /Li3 AlH6 equilibrium) could be provided by thermodynamic calculations. The thermodynamic model, procedure

J.-W. Jang et al. / Journal of Alloys and Compounds 420 (2006) 286–290

and results of thermodynamic parameter optimization and the calculated van’t Hoff diagram among LiAlH4 , Li3 AlH6 and LiH will be presented. The finally assessed thermodynamic parameters used for thermodynamic calculations will also be presented.

287

of temperature: 0

Gi (T ) − HiSER = a + bT + cT ln T + dT 2 + eT 3 + fT −1 + · · ·

(1)

Here, HiSER is the enthalpy of pure element i at the “Stable Element References (SER)”, 298.15 K and 1 bar.

2. Thermodynamic model In order to be able to predict hydride reactions under thermodynamic conditions where experimental information is not available and to predict phase equilibria in higher order systems, it is essential to construct a self-consistent thermodynamic parameter set of the individual hydride systems. This can be performed effectively by using the CALPHAD method where thermodynamic properties (Gibbs free energy) of individual phases are modeled and the model parameters are optimized by fitting all available experimental (or theoretical) thermodynamic information. The hydrides of interest for the present purpose were LiAlH4 , Li3 AlH6 and LiH. All these hydrides were modeled as line compounds, and the functional forms of Gibbs energy of these phases were similar to those for pure elements. The following is a typical expression of Gibbs energy of a pure element i as a function

3. Evaluation of model parameters The evaluation of various parameters was made by means of a computer program for the optimization of thermodynamic parameters, PARROT, developed by Jansson [15]. The optimization was performed with the selected set of experimental data which will be described later in the present section. Each piece of information was given a certain weight reflecting the experimental uncertainty. The weight could be changed until a satisfactory description of most of the selected experimental information was achieved. All calculations were carried out by using a computer program, Thermo-Calc, developed by Sundman et al. [16]. The thermodynamic parameter for pure elements, Li, Al and H2 could be obtained from the SGTE solution [17] and pure substance database [18]. These descriptions for pure elements

Table 1 List of experimental literature data for the reaction thermodynamics among LiH, Li3 AlH6 , LiAlH4 and pure elements, used in the present thermodynamic assessment Reaction

Property

Temp. (K)

Calculated Set 1

Calculated Set 2

Experiment

Reference

Li + Al + 2H2 (g) → LiAlH4


HR (kJ/mol)

298.15

−114.8

−114.8


SR (J/mol K)
GR (kJ/mol)

298.15 298.15

−221.3 −48.87

−221.3 −48.9


HR (kJ/mol)

298.15

−26.5

−26.5


HR (kJ/mol)

298.15

−8.5

−8.9

3LiH + Al + 23 H2 (g) → Li3 AlH6


GR (kJ/mol)
HR (kJ/mol)

437 298.15 298.15

−5.1 24.4 −53.9

−6.8 17.4 −52.9

Li + 21 H2 (g) → LiH


HR (kJ/mol)

298.15

−88.3

−88.3


SR (J/mol K)
GR (kJ/mol)

750 800 298.15 298.15

−94.3 −93.7 −64.3 −69.1

−94.3 −93.7 −64.3 −69.1


HR (kJ/mol)

460 500 600 700 298.15

−56.3 −53.0 −44.6 −36.6 −319.1

−56.3 −53.0 −44.6 −36.6 −318.0

−118.8 −117.0 −106.6 −113.4a −107.2 −218.9 −48.4 −35.5 −53.6 −28.5 −22.4 −9.8a −5.3 −4.7 27.7 −61.7 −47.2a −90.7 −90.7 −91.2 −96.2 −87.6 −68.8 −68.5 −70.2 −56.8 −53.4 −45.1 −36.7 −310.9a −298.5 −311.0

[4] [7] [8] [5] [6] [4] [7] [8] [4] [4] [3] [5] [6] [6] [8] [9] [5] [4] [7] [10] [10] [11] [4] [7] [4] [12] [12] [12] [12] [5] [8] [6]

LiH + Al + 23 H2 (g) → LiAlH4 1 3 Li3 AlH6

+ 23 Al + H2 (g) → LiAlH4

3Li + Al + 3H2 (g) → Li3 AlH6

Two sets of calculated values using two different parameter sets (see text) are given for comparison. a First-Principles calculation.

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were accepted in the present study without any modification. Thermodynamic descriptions for the LiH and LiAlH4 were also available from the SGTE pure substance database [18]. These descriptions could be good starting values during the assessment procedure. Because the thermodynamic description of the Li3 AlH6 was not available, this had to be newly optimized in the present work. The optimization of Gibbs energies of the hydrides, LiAlH4 , Li3 AlH6 and LiH was performed by fitting to all available experimental (or theoretical) thermodynamic information on the hydride phases and finding an optimized thermodynamic parameter set that reproduces most of the piecewise thermodynamic information in good agreements. The published thermodynamic data are mostly on the enthalpy, entropy and Gibbs energy of formation of the hydride phases [4–8,10–12] and those of reactions among the hydrides [3–6,8–9], as summarized in Table 1. The temperature dependence of equilibrium hydrogen pressure between Li3 AlH6 and LiH [9,13] could be valuable information for the present optimization. First, the reliability of the SGTE description for the LiH binary hydride was examined by comparing the calculated enthalpy, entropy and Gibbs energy of formation of LiH at 298.15 K with corresponding experimental values by Smith and Bass [4]. The agreement was satisfactory as shown in Table 1. Further, the calculated temperature dependences of the enthalpy and Gibbs energy of formation were also in good agreement with corresponding literature values by several authors [4,7,10–12]. These are demonstrated in Fig. 1. Based on the good agreements between the present calculations using the SGTE parameters and corresponding literature information on the thermodynamic properties of LiH, the SGTE description of the LiH hydride was accepted in the present study without any modification. The Gibbs energy parameters for Li3 AlH6 and LiAlH4 phases were optimized simultaneously by fitting to all the other litera-

ture information. During the optimization, it was found that the enthalpy, entropy and Gibbs energy of formation of the LiAlH4 hydride could be reproduced well in reasonable agreements with literature data (see Table 1) using the SGTE parameters. Therefore, the SGTE description of the LiAlH4 hydride was also accepted in the present study without any modification. Only the thermodynamic description of the Li3 AlH6 phase was newly determined so that the enthalpies of reactions between hydrides [5,6,8,9] and the LiH/Li3 AlH6 two phase equilibrium data [9,13] are correctly reproduced. However, it was also found that the LiH/Li3 AlH6 two phase equilibrium data from the two independent literatures, one by Chen et al. [9] and the other by Brinks et al. [13], were too much different from each other. Usually, during thermodynamic assessments, different weights are given to individual data sets depending on the reliability of the experimental data. However, in the present case, it was difficult to evaluate the reliability of the individual experimental results. Further, the present authors could not explain why so different results were obtained by the two research groups. It seemed meaningless to simply put both experimental data sets into the assessment procedure and obtain an average value. Therefore, it was finally determined to perform two separate assessments, one fitting to only Chen et al.’s data [9] and the other fitting to only Brinks et al.’s data [13] for the LiH/Li3 AlH6 two phase equilibrium. Two separate Gibbs energy expressions were obtained for the Li3 AlH6 phase and were used to calculate two different temperature-hydrogen pressure diagrams for equilibria among LiH, Li3 AlH6 and LiAlH4 . Even though the two parameter sets for the Li3 AlH6 yielded different results for the equilibrium between Li3 AlH6 and LiAlH4 , the same conclusion could be drawn concerning the possibility of hydrogen absorption reaction from Li3 AlH6 to LiAlH4 as will be discussed in the next section. All the literature thermodynamic data used in the present optimization except the LiH/Li3 AlH6 two phase equilibrium data are listed in Table 1. Here, two sets of calculated values using the two different thermodynamic parameter sets are given to each item for comparison. It is shown that most of the experimental data are reasonably reproduced by the present thermodynamic descriptions. The finally optimized thermodynamic parameter sets are given in Table 2. The parameters of which values are determined in the present study are designated with an asterisk. It should be noticed here again that two different parameter sets for the Gibbs energy of Li3 AlH6 are given. “Set 1” is obtained by fitting to Chen et al.’s data and “Set 2” is obtained by fitting to Brinks et al.’s data for the LiH/Li3 AlH6 two phase equilibrium. 4. Results and discussion

Fig. 1. Calculated enthalpy and Gibbs energy of formation of LiH, in comparison with experimental data [4,7,10–12].

Fig. 2 is the stability diagram of LiH, Li3 AlH6 and LiAlH4 , calculated using the present thermodynamic parameter sets in a form of the van’t Hoff diagram. Here, two sets of diagrams are presented, one by solid curves and the other by dotted curves. The solid curves are those calculated using the parameter set 1 which was obtained by fitting to Chen et al.’s data for the LiH/Li3 AlH6 two phase equilibria, while the dot-

J.-W. Jang et al. / Journal of Alloys and Compounds 420 (2006) 286–290

289

Table 2 The thermodynamic descriptions for hydrides and pure elements in the Li–Al–H systems 0

GLiH ∗ = 0 GSGTE LiH

0

GLiAlH4 ∗ =0 GSGTE LiAlH4

Set 1 [fitting to Chen et al.] 0 0 fcc 0 gas GLi3 AlH6 ∗ = −288265 − 486.219T + 126.4216T ln T + 30 Gbcc Li + GAl + 3 GH2 Set 2 [fitting to Brinks et al.] 0 0 fcc 0 gas GLi3 AlH6 ∗ = −296438 − 211.214T + 95.4862T ln T + 30 Gbcc Li + GAl + 3 GH2 0

2 −9 3 −1 GSGTE LiH = −100373.459 + 135.665824T − 21.00559T ln T − 0.021857085T − 1.651466 × 10 T + 226236.7T

= −112352.348 + 360.842441T − 55T ln T + 2.96472 × 10 0

−18 2

T + 2.06655667 × 10

−21 3

T − 1.6389435 × 10

T

2 −4 3 −1 GSGTE LiAlH4 = −330533.598 + 4817.71342T − 811.3445T ln T + 1.2320415T − 3.9569 × 10 T + 8948260T

= −132001.663 + 206.902854T − 38.11737T ln T − 0.08073365T 2 + 1.4195685 × 10−5 T 3 − 461924.05T −1 0

−8 −1

Gbcc Li

= −10583.817 + 217.636843T − 38.940488T ln T + 0.035466931T − 1.9869816 × 10 2

−5 3

T + 159994T

0

Gfcc Al

= −7976.15 + 137.071542T − 24.3671976T ln T − 0.001884662T − 8.77664 × 10 2

−7 3

T + 74092T

= −11276.24 + 223.02695T − 38.5844296T ln T + 0.018531982T 2 − 5.764227 × 10−6 T 3 + 74092T −1 = −11277.683 + 188.661987T − 31.748192T ln T − 1.234264 × 1028 T −9 0

gas GH2

(965 < T < 2000)

(298.15 < T < 500)

(500 < T < 800) −1

= −565846.534 + 10668.0378T − 1722.32564T ln T + 2.28509159T 2 − 5.77968153 × 10−4 T 3 + 34264368T −1 = −9062.949 + 179.277549T − 31.2283718T ln T + 0.002633221T 2 − 4.38058 × 10−7 T 3 − 102387T −1

(298.15 < T < 965)

(200 < T < 453.69) (453.69 < T < 500)

(500 < T < 3000) −1

(298.15 < T < 700)

(700 < T < 933.6)

(933.6 < T < 2900)

= −9522.9741 + 78.5273879T − 31.35707T ln T + 0.0027589925T − 7.46390667 × 10−7 T 3 + 56582.3T −1 2

= +180.108664 − 15.6128256T − 17.84857T ln T − 0.00584168T 2 + 3014618667 × 10−7 T 3 − 1280036T −1

(298.15 < T < 1000)

(1000 < T < 2100)

= −18840.1663 + 92.3120255T − 32.05082T ln T − 0.0010728235T 2 + 1.14281783 × 10−8 T 3 + 3561002.5T −1

(2100 < T < 6000)

All values are given in SI units and for one mole. The parameters of which values are determined in the present study are designated with an asterisk “*”.

ted curves are calculated using the parameter Set 2 obtained by fitting to Brinks et al.’s data. The two lines of each calculation represent temperature–hydrogen pressure relation for the equilibrium between two hydrides, LiH/Li3 AlH6 and Li3 AlH6 /LiAlH4 , respectively. Experimentally reported two phase equilibrium data [9,13] between LiH and Li3 AlH6 are also compared with the present calculations. Certainly, the two

Fig. 2. Calculated stability diagram of LiH, Li3 AlH6 and LiAlH4 , in comparison with experimental data [9,13] for the LiH/Li3 AlH6 equilibrium. The solid curves are those calculated using the parameter Set 1 obtained by fitting to Chen et al.’s data [9], while the dotted curves are using the parameter Set 2 obtained by fitting to Brinks et al.’s data [13].

calculations based on the two different experimental data sets give different temperature–hydrogen pressure relations for the equilibrium between Li3 AlH6 and LiAlH4 as well as for that between LiH and Li3 AlH6 . As already mentioned, currently it is difficult to say which calculation is more reliable between the two, based on parameter Set 1 and 2. However, concerning the Li3 AlH6 /LiAlH4 phase equilibria, practically the same conclusion can be made from the two different calculations. The purpose of the present study is to check whether the hydrogen absorption/desorption reaction between Li3 AlH6 and LiAlH4 would be thermodynamically possible in a practically accessible temperature and hydrogen pressure range, as is the one between LiH and Li3 AlH6 . Before, it has been only known that more than 90 bar of hydrogen pressure is necessary to keep the LiAlH4 un-decomposed at 53 ◦ C [13]. Using this information, one could not say whether the hydrogen absorption reaction from Li3 AlH6 to LiAlH4 could be realized at a higher hydrogen pressure, 150 bar for example. Fig. 2 shows that the hydrogen absorption reaction Li3 AlH6 → LiAlH4 can be possible only when the hydrogen partial pressure is well above 103 bar. About 103 bar of hydrogen partial pressure is a practically inaccessible value for a hydrogen storage material. The present thermodynamic calculations clearly show that a reversible reaction between Li3 AlH6 and LiAlH4 cannot be realized in a practical condition, not because of kinetic reason but because of thermodynamic reason. An attempt should be made to change the thermodynamic properties of the hydrides. As an attempt to change the thermodynamic properties of the hydrides, alloying of a fourth element (Na, for example) or using nano-sized particles [19] can be considered. A thermodynamic calculation study to confirm the possibility of these approaches is now being carried out.

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5. Conclusion Thermodynamic descriptions of the hydrides, LiH, Li3 AlH6 and LiAlH4 , were critically assessed based on piecewise experimental thermodynamic information, and were used to compute the phase equilibrium between Li3 AlH6 and LiAlH4 . The present thermodynamic calculations predict that more than 103 bar of hydrogen partial pressure is necessary to induce the hydrogen absorption reaction of Li3 AlH6 → LiAlH4 above room temperature. This means that a reversible reaction between Li3 AlH6 and LiAlH4 is thermodynamically impossible in a practically accessible temperature and hydrogen pressure range for a hydrogen storage material, without any attempt to change the thermodynamic properties of the hydrides. Acknowledgement This work has been financially supported by the Hydrogen Energy R&D Center, one of the 21st Century Frontier R&D Programs, funded by the Ministry of Science and Technology of Korea. References [1] B. Bogdanovi´c, R.A. Brand, A. Marjarnovi´c, M. Schwickardi, J. T¨olle, J. Alloys Compd. 302 (2000) 36.

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