Thermal Dehydrogenation Characteristics of Li-Sr-Al-N-H Hydrogen Storage System

Abstract

Thermolysis behavior of the Li-Sr-Al-N-H hydrogen storage system prepared by ball milling of Sr₂AlH₇ + LiNH₂ mixture was investigated in this paper. The results show that thermal decomposition of the Li-Sr-Al-N-H system proceeds mainly in two steps with only hydrogen desorption. The thermal stability of this system is lowered as compared to the individual starting material, resulting in the hydrogen desorption initiating from about 125 °C. In addition, about 0.91 and 1.53 wt.% of hydrogen can be isothermally desorbed within 180 min at 180 and 330 °C, respectively. The decreased thermal stability of the Li-Sr-Al-N-H system might be attributed to the chemical reactions between the starting materials during the heating process with the formation of LiSrH₃ and N-containing amorphous phases.

Keywords:
Hydrogen storage material; Metal-N-H system; Dehydrogenation property; Reaction mechanism

1. Introduction

Hydrogen has been regarded as a promising alternative for the conventional fossil energy sources due to its high calorific value, low environmental impact and abundant reserves. For the wide applications of hydrogen energy, an effective and safe storage technology must be developed. Compared to the gas- and/or liquid-state hydrogen storage, the storage of hydrogen in solid state based on the physical or chemical interaction between hydrogen and hydrogen storage materials usually has a higher hydrogen density. Among the developed hydrogen storage materials, the metal aluminum hydrides such as LiAlH₄, NaAlH₄ and Mg(AlH₄)₂ are some of the most promising candidates for the on-board hydrogen storage owing to their high hydrogen capacity^1-6. For example, Liu et al. found that the Ti-doped LiAlH₄ can release 7 wt.% of hydrogen commencing at temperature as low as 80 °C, and that the dehydrogenated product can be re-hydrogenated almost completely under 100 bar of hydrogen and room temperature by employing liquid Me₂O as a solvent³. In 2002, an alkaline-earth metal aluminum hydride with a special crystal structure, Sr₂AlH₇, was obtained by the hydrogenation of Zintl compound SrAl₂⁷. Further studies indicate that it can also be synthesized by the reaction of SrH₂, Al and hydrogen^8,9.

In addition, growing attention has also been paid to the metal-N-H systems as hydrogen storage materials in the last decade^10-28. Chen et al. first reported that the binary LiNH₂-LiH (1:2) system exhibits reversible de-/hydrogenation reactions in a two-step process, which can be expressed as¹⁰:

(1) ${\mathrm{LiNH}}_2+2\mathrm{LiH}\leftrightarrow {\mathrm{Li}}_2\mathrm{NH}+\mathrm{LiH}+H_2\leftrightarrow {\mathrm{Li}}_{3}N+2H_2$

This Li-N-H system has a hydrogen storage capacity as high as 10 wt.%. Unfortunately, its dehydrogenation temperature is too high for practical applications. To destabilize the Li-N-H system, Li was partially substituted by Mg or Ca, and the ternary Li-Mg-N-H^11-17 and Li-Ca-N-H^18-20 hydrogen storage systems were developed. Moreover, the combined alkali metal aluminum hydride-metal amide systems such as LiAlH₄-LiNH₂^21-24, Li₃AlH₆-LiNH₂^25,26, LiAlH₄-Mg(NH₂)₂²⁷ and Na₂LiAlH₆-Mg(NH₂)₂²⁸ were intensively studied. However, the metal-N-H systems consisting of alkaline-earth metal aluminum hydride and metal amide were rarely reported. In the present work, the multinary Li-Sr-Al-N-H system was constructed by combining Sr₂AlH₇ with LiNH₂ in a molar ratio of 1:1, and its thermal decomposition properties as well as the chemical reactions involved in the heating process were investigated.

2. Experimental

2.1 Sample preparation

The starting material LiNH₂ (95%, J&K Chemical) was purchased and used as-received. The hydride Sr₂AlH₇ was prepared by the same method reported in our previous paper²⁹. Rietveld analysis (see Table 1) shows that the Sr₂AlH₇ hydride is composed of 85 wt.% Sr₂AlH₇, 13 wt.% SrH₂, 1 wt.% Al and 1 wt.% SrO. The Li-Sr-Al-N-H system was prepared by ball milling the mixture of Sr₂AlH₇ and LiNH₂ powders in a molar ratio of 1:1. The milling operation was performed under 0.5 MPa of hydrogen atmosphere at a rotation speed of 400 rpm for 2 h by using a QM-1SP2 planetary mill. Stainless steel vials (250 mL in volume) and balls (10 mm in diameter) were used. The ball to powder weight ratio was 20:1. To prevent the sample from air-exposure, all the sample handling was carried out in an Ar-filled glove box equipped with a purification system, in which the typical O₂/H₂O levels were below 1 ppm.

Table 1
Structural parameters and phase abundance of the Sr₂AlH₇ hydride refined by Rietveld analysis.

Phase	Space Group	Lattice Parameters		Abundance (wt.%)
		a (Å)	c (Å)
Sr2AlH7	I2/a	12.5812 (6)	7.9992 (4)	85
SrH2	Pnma	6.3881 (5)	7.3277 (8)	13
Al	Fm-3m	4.0518 (7)		1
SrO	Fm-3m	5.1594 (5)		1

2.2 Determination of thermal decomposition properties

Thermolysis process of the as-milled Sr₂AlH₇ + LiNH₂ mixture was monitored using a simultaneous thermal analyzer (NETZSCH STA 449) equipped with a quadrupole mass spectrometer (QMS 403C) in the temperature range of 25-500 °C. Differential thermal analysis (DTA), thermogravimetry (TG) and mass spectrometry (MS) measurements were carried out under argon flow (30 ml/min) at a heating rate of 2 °C/min. In addition, isothermal dehydrogenation for the Li-Sr-Al-N-H system was measured based on the volumetric method by using a carefully calibrated Sieverts-type apparatus. Two separate experiments with two separate samples were carried out. One sample was exposed to 180 °C and the other was exposed to 330 °C. Prior to hydrogen desorption, the testing system of the apparatus was evacuated.

2.3 Phase composition characterization

To analyze the phase compositions of the Sr₂AlH₇ + LiNH₂ mixture as-milled and after heat treatment at different temperatures, X-ray diffraction (XRD) measurements were performed using a Rigaku D/Max 2500VL/PC diffractometer with Cu Kα radiation at 50 kV and 200 mA. The XRD samples were loaded and sealed in an air-tight holder that can keep the sample under argon atmosphere during the measurement. In addition, the XRD pattern of the heat-treated product was analyzed with the Rietveld refinement program RIETAN-2000³⁰.

3. Results and Discussion

3.1 Thermal decomposition properties

Fig. 1 gives the DTA/TG curves for the as-milled Sr₂AlH₇ + LiNH₂ mixture during the heating process at a rate of 2 °C/min. It is observed that there are two prominent DTA endothermic peaks centered at about 190 and 355 °C, respectively ( see Fig. 1a), indicating that there exists two obvious endothermic events in the temperature range of 25-500 °C. The TG curve (see Fig. 1b) shows that the thermal decomposition process of the Li-Sr-Al-N-H system was accompanied by obvious mass loss, and that the total mass loss is about 1.97 wt.% up to 500 °C. Furthermore, Fig. 2 presents the simultaneous MS analysis of the released gas during the heating process. It is clear that there are two peaks originating from H₂ desorption, and that NH₃ release was not detected from the Li-Sr-Al-N-H system. These two H₂-MS peaks correspond to the two endothermic peaks observed in Fig. 1a. On the basis of the above results, it can be concluded that the thermal decomposition of the Sr₂AlH₇ + LiNH₂ mixture ball milled for 2 h is a two-step process with hydrogen desorption only. The first step occurs in the temperature range of 125-270 °C, and releases about 1.03 wt.% of hydrogen; and the second one starts following the first step and ends at about 400 °C, with about 0.94 wt.% of hydrogen released.

Figure 1
DTA/TG curves of the as-milled Sr₂AlH₇ + LiNH₂ mixture.

Figure 2
H₂- and NH₃-MS curves of the as-milled Sr₂AlH₇ + LiNH₂ mixture

According to the DTA/TG-MS results, two different temperatures of 180 and 330 °C were selected to further investigate the isothermal dehydrogenation properties of the Li-Sr-Al-N-H system. As shown in Fig. 3, about 0.91 wt.% of hydrogen can be desorbed within 180 min at 180 °C. When the Li-Sr-Al-N-H system was exposed to 330 °C, the hydrogen amount desorbed within 180 min increases to about 1.53 wt.%. In contrast, the individual Sr₂AlH₇ ball milled for 2 h (in the absence of LiNH₂) did not release any noticeable amount of hydrogen at the temperature below 180 °C. In addition, the decomposition reaction of LiNH₂ has been found to happen only at the temperature higher than 220 °C^11,21,31. The results indicate that Sr₂AlH₇ and LiNH₂ can be chemically destabilized by combination to each other. Similar to the Li-Mg-N-H^11-16 and Li-Al-N-H^21-26 systems, the chemical reactions between Sr₂AlH₇ and LiNH₂ rather than the catalytical role may be the reason for the decreased dehydrogenation temperature in the present case. The dehydrogenation reactions involved in the Li-Sr-Al-N-H system will be discussed below.

Figure 3
Time dependences of dehydrogenation for the as-milled Sr₂AlH₇ + LiNH₂ mixture at 180 and 330 °C, respectively

3.2 Phase compositions

Fig. 4 displays the XRD patterns of the Sr₂AlH₇ + LiNH₂ mixture after ball milling, as well as after heat treatment at different temperatures. It can be seen from Fig. 4a that the as-milled Sr₂AlH₇ + LiNH₂ mixture is composed of the starting materials, implying that no obvious reactions occurred during ball milling for 2 h. When the as-milled Sr₂AlH₇ + LiNH₂ mixture was heated to 180 °C, as indicated in Fig. 4b, the characteristic diffraction peaks from Sr₂AlH₇ and LiNH₂ are still dominant in the XRD pattern. However, a new diffraction peak (2θ = 32.8°) assigned to LiSrH₃ appears in Fig. 4b, indicating the occurrence of chemical reactions in the Li-Sr-Al-N-H system. Increasing the heat treatment temperature to 330 °C, as shown in Fig. 4c, the diffraction peaks from LiSrH₃ are enhanced drastically, while those from Sr₂AlH₇ are weakened. In addition, SrH₂ can be observed, together with a small amount of Al and SrO in the product dehydrogenated at 330 °C. However, there is no diffraction peak that can be unambiguously ascribed to any N-containing phases.

Figure 4
XRD patterns of the Sr₂AlH₇ + LiNH₂ mixture as-milled (a) and after heat treatment at 180 (b), 330 (c) and 500 °C (d), respectively.

To further confirm the presence of these crystalline phases, the XRD pattern in Fig. 4c was refined by Rietveld method. It can be seen from Fig. 5 that the diffraction pattern calculated from the structure models of the above five phases (LiSrH₃, SrH₂, Sr₂AlH₇, Al and SrO) is in good agreement with that measured. Because no NH₃ was released during the heating process, it is reasonable to consider that the N-containing phases exist in the predominantly amorphous form. Similar phenomena of the formation of amorphous phases were also reported in other combined systems of metal aluminum hydride and metal amide^21,27,32. As shown in Fig. 4d, the product dehydrogenated at 500 °C is composed of LiSrH₃, SrH₂, SrAl₄ and SrO. Again, the N-containing crystalline phases cannot be observed from the XRD pattern.

Figure 5
Rietveld refinement of the XRD pattern for the as-milled Sr₂AlH₇ + LiNH₂ mixture dehydrogenated at 330 °C. The vertical bars (from above) indicate the positions of Bragg diffraction for LiSrH₃, Al, SrH₂, Sr₂AlH₇ and SrO, respectively. (The reliability factors for refinement are R_wp = 5.81%, R_p = 4.59% and S = 2.42)

3.3 Dehydrogenation reactions

As indicated in Section 3.1, thermal decomposition of the Sr₂AlH₇-LiNH₂ (1:1) system is a two-step dehydrogenation process without NH₃ emission. Based on the above phase composition analysis, the dehydrogenation reactions during the heating process can be deduced. Since SrH₂ can react with LiNH₂ to form LiSrH₃ and SrNH at temperatures up to 180 °C according to reaction (2) ³³ and that 13 wt.% SrH₂ is present in the starting material Sr₂AlH₇, the first dehydrogenation step for the Li-Sr-Al-N-H system was thought to be originated from the reaction between SrH₂ and LiNH₂.

(2) $2{\mathrm{SrH}}_2+{\mathrm{LiNH}}_2\to {\mathrm{LiSrH}}_3+\mathrm{SrNH}+H_2$

Based on reaction (2), the hydrogen amount desorbed from Li-Sr-Al-N-H system can be calculated to be about 0.13 wt.%. This value is much lower than that experimentally obtained (~ 0.91 wt.%) as shown in Fig. 3, indicating that a certain amount of Sr₂AlH₇ must also react with LiNH₂ to release hydrogen at 180 °C. Thus, the dehydrogenation reaction between SrH₂ and LiNH₂, as well as that between part Sr₂AlH₇ and LiNH₂ both contribute to the first dehydrogenation step as observed in Figs. 1 and 2.

With the increase of decomposition temperature, LiNH₂ reacted with remaining Sr₂AlH₇ completely, resulting in the formation of LiSrH₃, N-containing amorphous phases, SrH₂ and Al as shown in Fig. 4c. Meanwhile, SrH₂ reacted with Al to directly form SrAl₄ and the residual Sr₂AlH₇ decomposed into SrH₂ and SrAl₄ based on the following reaction⁹:

(3) $4{\mathrm{Sr}}_2{\mathrm{AlH}}_7\to 7{\mathrm{SrH}}_2+{\mathrm{SrAl}}_4+7H_2$

These reactions contribute to the second dehydrogenation peak shown in Figs. 1 and 2. According to the results and discussion above, the overall reaction taken place during the heating process up to 500 °C for the Sr₂AlH₇-LiNH₂ (1:1) system can be described qualitatively as:

(4) $\begin{array}{c}{\mathrm{Sr}}_2{\mathrm{AlH}}_7+{\mathrm{LiNH}}_2\to {\mathrm{LiSrH}}_3+{\mathrm{SrH}}_2+{\mathrm{SrAl}}_4+\hfill \\ \mathrm{N-containing}\mathrm{amorphus}\mathrm{phases}+H_2\hfill \end{array}$

4. Conclusions

In this paper, the thermal decomposition properties and chemical reactions involved in the heating process for the combined Sr₂AlH₇-LiNH₂ (1:1) hydrogen storage system were studied. It was found that thermal decomposition of the Li-Sr-Al-N-H system is a two-step dehydrogenation process without detectable NH₃ emission. This system begins to release hydrogen at about 125 °C, and can isothermally desorb about 0.91 and 1.53 wt.% of hydrogen within 180 min at 180 and 330 °C, respectively. The chemical reactions between the starting materials during the heating process to form LiSrH₃ and N-containing amorphous phases may be the reason for the decreased dehydrogenation temperature. Further investigations on the effect of composition change on the decomposition behaviors might be of significance for the improvement of the hydrogen storage properties of the Sr₂AlH₇-LiNH₂ system.

5. Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (No. 51371008) and the Key Project of Outstanding Young Talents in Universities of Anhui Province (No. gxyqZD2016067).

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Publication Dates

• Publication in this collection
  2018

History

• Received
  04 Aug 2017
• Reviewed
  20 Oct 2017
• Accepted
  19 Nov 2017