Lithium hydrazide as a potential compound for hydrogen storage

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 5 7 5 0 e5 7 5 3

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Lithium hydrazide as a potential compound for hydrogen storage Liang Zeng a, Keiji Shimoda b, Yu Zhang b, Hiroki Miyaoka c, Takayuki Ichikawa a,b,*, Yoshitsugu Kojima a,b a

Department of Quantum Matter, ADSM, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan Institute for Advanced Materials Research, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan c Institute for Sustainable Sciences and Development, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan b

article info

abstract

Article history:

The MetaleNeH system for hydrogen storage has been developed in recent years and is

Received 15 November 2011

considered to be a promising solution. Here we report a potential compound for hydrogen

Received in revised form

storage, LiNHNH2, which is a white solid with 8.0 mass% theoretical hydrogen content and

26 December 2011

can be synthesized from anhydrous hydrazine and n-butyllithium in diethyl ether. The

Accepted 28 December 2011

thermodynamic behaviours and hydrogen storage properties of this compound were firstly

Available online 18 January 2012

investigated and are discussed in this paper. We demonstrate the decomposition pathway of LiNHNH2 and reveal that an alkali metal hydride such as LiH can significantly increase

Keywords:

the hydrogen desorption from LiNHNH2. Moreover, LiNHNH2 can also be used for desta-

Hydrogen storage

bilizing other hydrogen storage systems owing to its instability.

Hydrazine

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

Lithium hydrazide Decomposition pathway

1.

Introduction

With the increasing demand for energy, it is necessary to develop renewable energies that will supersede fossil fuel sources. Hydrogen is considered a promising energy carrier for resolving the energy crisis and reducing environment pollution owing to its highest energy density by weight of any common fuel and its clean combustion with zero CO2 emission. It is an ideal candidate for realizing a low-carbon society. However, the lack of an effective hydrogen storage technique is still a major barrier in the utilization of hydrogen as an energy source [1,2]. Recently, the outstanding hydrogen desorption properties of hydrous hydrazine (N2H4$H2O) were reported by Singh et al. [3e5]. They claimed that hydrous hydrazine can be decomposed to N2 and H2 at room temperature using noble metals as the

catalyst. Nevertheless, solid state materials are generally expected to be more suitable for hydrogen storage owing to their higher density and because of their potential to form mixtures and composites through the partial and entire substitution of other atoms. Therefore, lithium hydrazide (LiNHNH2), a solid mono-substituted hydrazine has attracted our strong interest because of its high hydrogen content of 8.0 mass%. LiNHNH2, which is used as a reducing agent in organic chemistry, can be produced from a reaction between anhydrous hydrazine and nbutyllithium (n-C4H9Li) in diethyl ether [6] or from anhydrous hydrazine and LiH in tetrahydrofuran [7] as follows. N2 H4 þ C4 H9 Li/LiNHNH2 þ C4 H10

(1)

N2 H4 þ LiH/LiNHNH2 þ H2

(2)

* Corresponding author. Institute for Advanced Materials Research, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 7398530, Japan. Tel./fax: þ81 82 424 5744. E-mail address: [email protected] (T. Ichikawa). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.12.144

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TG

NH 3

H2 N2

Weight Loss (%)

90

N2 H4

80

41.5% 70 60 50

13.7%

40

7.2%

50

150

250 T (°C)

350

Experimental details

As starting materials, anhydrous hydrazine (98%, Nacalai Tesque), n-butyllithium (1.6 M in hexane, Nacalai Tesque) and anhydrous diethyl ether (99.7%, SigmaeAldrich) were obtained commercially and used without further purification. LiH (95%, SigmaeAldrich) was ball-milled for 2 h in advance of mixing with LiNHNH2. High purity LiNHNH2 was prepared by the method reported by Glassman et al. [6]. A solution of n-butyllithium in hexane was slowly added to a mixture of anhydrous hydrazine in ether at room temperature. After 30 min the white LiNHNH2 was removed by filtering with a Buchner funnel and then rinsed with ether. Finally, the sample was dried overnight using a rotary pump to remove all the solvents. The entire experimental procedure from preparation to analysis was performed in Ar-filled glove boxes or high-vacuum lines to minimize water adsorption and oxidation. The helium flow rate of thermogravimetry (TG, Rigaku, TG8120) connected to a mass spectrometer (MS, Anelva, MQA200TS) was set to 300 mL/min with a heating rate of 2  C/ min. Fourier transform infrared spectrometry (FTeIR, Spectrum One, PerkineElmer) ran in reflection mode using a diffuse reflectance accessory. The samples used for FTeIR were diluted with KBr to a mass ratio of 1:10 (sample: KBr). The identification of the products was carried out by powder X-ray diffraction (XRD, Rigaku, RINTe2500, Cu Ka) measurements. The samples used for XRD measurements were placed in a sealed capillary or using a polyimide sheet (Kapton, Du PonteToray Co. Ltd.) cover to prevent them from being oxidized during the XRD measurements. 7Li magic-angle spinning nuclear magnetic resonance (MASeNMR, JNMeECA600, JEOL) spectroscopy was performed at a magnetic field of 14.1 T 7Li resonant frequency of 233.2 MHz. Sample powders were packed into 4 mm ZrO2 rotors under Ar atmosphere, which were spun at a speed of 12e15 kHz. The chemical shift was referenced to LiCl aqueous solution as 0.00 ppm. All NMR spectra were obtained at room temperature (R.T.).

3.

Results and discussion

3.1.

Decomposition pathway of LiNHNH2

The decomposition processes of LiNHNH2 was investigated in detail by TGeMS. Fig. 1 reveals that LiNHNH2 decomposes in three main steps under an inert gas flow when the temperature is increased to 400  C. The products in each step (at R.T., 130, 205 and 400  C) were determined by FTeIR and XRD as

100

200 T (°C)

300

400

Fig. 1 e TGeMS profile of as-synthesized LiNHNH2. N2H4 (pink), NH3 (red), N2 (blue) and H2 (green) were desorbed from LiNHNH2 during the heating process.

shown in Figs. 2 and 3, respectively. The diffraction peaks of as-synthesized LiNHNH2 were not sharp owing to the poor crystallinity resulting from the synthesis method. As shown in Fig. 2, various peaks corresponding to NeH stretching between 3100 and 3300 cm1 were detected in the as-synthesized LiNHNH2, while only one peak at 3210 cm1 remained after heating to 130  C. Three peaks were observed at 205  C, which were assigned to LiNH2 (3300 and 3246 cm1) and Li2NH (3178 cm1), respectively. The two peaks at 3256 and 3157 cm1 observed at 400  C corresponded to Li2NH, indicating that the entire sample had been converted to Li2NH [11,12].

as-synthesized

Intensity (a.u.)

2.

100

MS

Intensity (a.u.)

However, a thermodynamic investigation has not yet been carried out on LiNHNH2 and its decomposition properties have not yet been reported. LiNHNH2 has the same elemental constitution and a similar chemical formula to LiNH2 [8e10] but exhibits markedly different thermodynamic behaviours, which needs higher temperature than 300  C for the thermodynamic decomposition. Moreover, the alkali metal hydride has been found to be able to decrease the temperature of hydrogen desorption from LiNHNH2 to as low as 50  C, and to suppress the undesirable gases desorption.

130 °C

205 °C

400 °C

3600

3400

3200

3000

2800

-1

Wave numbers (cm ) Fig. 2 e FTeIR spectra of as-synthesized LiNHNH2 and after heating at 130, 205 and 400  C.

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N2 H4 /N2 þ 2H2

(5) 

Second step: 170e205 C Li2 N2 H2 /xLi2 NH þ ð2  2xÞ LiNH2 þ x=2N2 þ ð3x  2Þ=2H2

(6)



Third step: 205e350 C LiNH2 /Li2 NH þ NH3

(7)

7

Li MASeNMR spectroscopy was also performed to determine the chemical environment of Li in LiNHNH2. Fig. 4 shows the 7Li MASeNMR spectra of as-synthesized LiNHNH2 and its decomposition product at 130  C (Li2N2H2) along with the reference materials LiNH2, Li2NH and LiH. The chemical shifts of the centre of gravity for these materials are 2.1, 2.2, 2.0, 3.5 and 0.0 ppm, respectively. This indicates that the as-synthesized LiNHNH2 and Li2N2H2 have a very similar Li environment, which is close to that of LiNH2.

3.2. Improvement of hydrogen desorption from LiNHNH2 by adding LiH

Fig. 3 e Ex-situ XRD profile of as-synthesized LiNHNH2 and after heating at 130, 205 and 400  C.

The MS result in Fig. 1 shows that N2H4 (m/z ¼ 32), NH3 (m/ z ¼ 17), N2 (m/z ¼ 28) and H2 (m/z ¼ 2) were desorbed from LiNHNH2 during the heating process. The TG result shows that the weight loss (WL) in the first step was 41.5%, and the product at 130  C was confirmed by XRD to contain neither LiNH2 nor Li2NH nor any other known LieNeH phases. This result suggests that LiNHNH2 might decompose to a previously unreported phase, dilithium hydrazide (Li2N2H2), and N2H4 in the first step, for which the theoretical WL is 42.2% for Eq. (3). Hereby the NH3, N2 and H2 desorbed in the first step can be considered as fragment components resulting from the decomposition of N2H4 as shown in Eqs. (4) and (5) [3]. Li2N2H2 then continued to decompose with the desorption of H2 and N2 in the second step (170e205  C), and the XRD pattern at 205  C revealed that the remaining solid product was either LiNH2 or Li2NH. This was in good agreement with the FTeIR result, which clarified the coexistence of LiNH2 and Li2NH at 205  C. The reaction equation of this step can be expressed by Eq. (6), which is a nonstoichiometric reaction. The molar ratio of LiNH2 to Li2NH was varied slightly in the different runs. Herein, the ratio was calculated to be 1:1.3 from the WL of 13.7% in the second step. Finally, the total WL of 62.4% at 400  C indicated that the entire sample had been converted to Li2NH (62.0% in theoretical WL), which is consistent with the FTeIR and XRD results at 400  C shown in Figs. 2 and 3, respectively. Thus, LiNH2 decomposed to Li2NH and NH3 in the third step. From the above results, a possible decomposition pathway of LiNHNH2 is deduced to be as follows. First step: 100e130  C 2LiNHNH2 /Li2 N2 H2 þ N2 H4

(3)

3N2 H4 /N2 þ 4NH3

(4)

To increase the hydrogen desorption amount from LiNHNH2 and suppress the emission of the undesirable gases, it was first necessary to clarify its chemical properties. Hydrogen gas can be easily formed in the presence of Hdþ and Hd [12]. The H atoms in LiNHNH2 are partially positively charged (Hdþ) owing to the greater electronegativity of N than H. Therefore, an alkali metal hydride which contains Hd is an appropriate candidate to mix with LiNHNH2. Herein LiH was selected because of its highest hydrogen capacity (12.7 mass%) in all alkali metal hydrides [13,14]. A 1:5 (molar ratio) mixture of LiNHNH2eLiH was prepared by hand milling for 5 min in an agate mortar. As shown in Fig. 5, the mixture mainly desorbed H2 and N2 during heating. Interestingly, N2H4 desorption was completely suppressed by the addition of LiH and the onset temperature of H2 desorption from the mixture was decreased to as low as 50  C. However, a small amount of NH3 was found in the temperature range from 80 to

Fig. 4 e 7Li MASeNMR spectra of (a) as-synthesized LiNHNH2, (b) LiNHNH2 after heating at 130  C and the reference materials (c) LiNH2, (d) Li2NH and (e) LiH, respectively.

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Storage Materials”. The authors gratefully acknowledge Ms. Xin Jin and Ms. Na Liu for the valuable discussions and suggestions.

references

Intensity (a.u.)

H2

N2

NH 3

N2 H4

100

200

300

400

T (°C) Fig. 5 e MS profile of 1:5 (molar ratio) mixture of LiNHNH2eLiH. H2 (green), NH3 (red), N2 (blue) and N2H4 (pink). 120  C owing to the fact that the reactivity of LiH was not sufficiently high to completely suppress NH3 desorption [15]. The reaction kinetics increases in the order of Li < Na < K for the reaction MH þ NH3 / MNH2 þ H2 [16]. Therefore, NaH and KH may be better candidates for mixing with LiNHNH2 to suppress NH3 desorption. In addition to alkali metal hydrides, other chemical hydrides that contain Hd with a high hydrogen capacity, such as borohydrides and alanates, may also improve the hydrogen desorption properties of LiNHNH2. It should be possible to prepare numerous new complex hydride systems as a result of the discovery of LiNHNH2 as a new fundamental compound for hydrogen storage.

4.

Conclusions

In summary, we have demonstrated that LiNHNH2, which contains 8.0 mass% hydrogen, is a new potential compound for hydrogen storage and its detailed thermal decomposition pathway has been clarified in this paper as well. In addition, an alkali metal hydride was found to be able to decrease the temperature of hydrogen desorption from LiNHNH2 to as low as 50  C, and to suppress undesirable gases desorption. Moreover, the instability of LiNHNH2 can expect to be used for destabilizing other hydrogen storage systems. Although the hydrogen storage properties of LiNHNH2 do not meet the DOE targets [17] so far, it should be possible to prepare numerous promising composites of LiNHNH2 with other hydrides in the near future.

Acknowledgements This work was partially supported by the NEDO project “Advanced Fundamental Research Project on Hydrogen

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