Aug 15, 2006 - following reaction; MCln Ñ nLiBH4/nNaBH4 ! M(BH4)n Ñ nLiCl/nNaCl. Then the thermal desorption properties of M(BH4)n were investigated by ...
Materials Transactions, Vol. 47, No. 8 (2006) pp. 1898 to 1901 #2006 The Japan Institute of Metals
Syntheses and Hydrogen Desorption Properties of Metal-Borohydrides M(BH4 )n (M ¼ Mg, Sc, Zr, Ti, and Zn; n ¼ 2{4) as Advanced Hydrogen Storage Materials Yuko Nakamori1 , Haiwen Li1 , Kazutoshi Miwa2 , Shin-ichi Towata2 and Shin-ichi Orimo1 1 2
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Toyota Central R&D Labs., Nagakute, Aichi 480-1192, Japan
Metal-borohydrides M(BH4 )n (M ¼ Mg, Sc, Zr, Ti, and Zn; n ¼ 2{4) were synthesized by mechanical milling process according to the following reaction; MCln þ nLiBH4 /nNaBH4 ! M(BH4 )n þ nLiCl/nNaCl. Then the thermal desorption properties of M(BH4 )n were investigated by gas-chromatography and mass-spectroscopy combined with thermogravimetry. The results indicate that the hydrogen desorption temperature Td of M(BH4 )n correlates with the Pauling electronegativity P of M; that is, Td decreases with increasing value of P . The components of desorbed gas for M ¼ Mg, Sc, Zr and Ti (P 5 1:5) are hydrogen only, while that for M ¼ Zn (P ¼ 1:6) contains borane besides hydrogen. The Pauling electronegativity P of M is an indicator to estimate Td of M(BH4 )n as candidates for advanced hydrogen storage materials with high gravimetric hydrogen densities and low desorption temperatures. [doi:10.2320/matertrans.47.1898] (Received April 17, 2006; Accepted June 5, 2006; Published August 15, 2006) Keywords: borohydride, mechanical milling, electronegativity, thermal stability, hydrogen storage
1.
Table 1 Gravimetric hydrogen densities of M(BH4 )n (M ¼ Mg, Sc, Zr, Ti, and Zn).
Introduction
Complex hydrides have attracted considerable attention as hydrogen storage materials, because of their high gravimetric hydrogen densities. Since Bogdanovic´ and Schwickardi have reported that the catalyzed sodium alanate (NaAlH4 ) showed reversible hydrogen desorption and absorption reactions at moderate conditions,1) many researchers have studied alkali complex hydrides mainly from the viewpoint of kinetics.2–10) Among the alkali complex hydrides, lithium borohydride (LiBH4 ) is one of the candidates for hydrogen storage materials because of its extremely high gravimetric hydrogen density (18 mass%). LiBH4 desorbs approximately 14 mass% of hydrogen through the decomposition reaction as follows;11) LiBH4 ! LiH þ B þ 3/2H2
ð1Þ
However, the hydrogen desorption temperature is higher than the required one (below 423 K) for solid state hydrogen storage materials. Zu¨ttel et al. have reported that LiBH4 mixed with SiO2 desorbs hydrogen below 673 K that is lower than pure LiBH4 does.12) In order to control the stability of borohydride for lowing the hydrogen desorption temperature Td , the systematic understanding of the material properties is of great important. We have recently performed first-principles calculations on the stabilities of M(BH4 )n (M ¼ Li, Na, K, Cu, Mg, Zn, Sc, Zr, and Hf).13,14) The results revealed that M(BH4 )n are stabilized by ionic bond between Mnþ and [BH4 ] . In other word, the charge transfer from Mnþ to [BH4 ] is a key feature for the stability of M(BH4 )n . The ability of the charge transfer can be estimated by value of the Pauling electronegativity P ; charge transfer from the cation Mnþ to the anion [BH4 ] becomes smaller with increasing value of P for M, which makes ionic bond weaker, and M(BH4 )n less stable. In fact, our first-principles calculations predicted that the heat of formation H of M(BH4 )n becomes smaller negative value with increasing of value of P . In this case, the hydrogen desorption temperature Td is expected to be lower.
Mg(BH4 )2 Sc(BH4 )3 Zr(BH4 )4 Ti(BH4 )3 Zn(BH4 )2
M Gravimetric hydro. density (mass%)
14.9
13.5
10.7
13.1
8.5
Furthermore, M with larger P becomes di-, tri- or tetravalent cation of Mnþ , and the Mnþ combines with n[BH4 ] to form M(BH4 )n , so the gravimetric hydrogen densities of M(BH4 )n are larger than the required value for hydrogen storage materials (more than 5.5 mass%), as shown in Table 1. However, the hydrogen storage properties of M(BH4 )n with larger P and n = 2 have not been systematically investigated yet. In this study, M(BH4 )n with various electronegativities of 1.2–1.6 (M ¼ Mg, Sc, Zr, Ti, and Zn, n ¼ 2{4) are synthesized by mechanical milling of the mixture of MCln and nLiBH4 /nNaBH4 . And then, their hydrogen desorption properties are investigated by thermal desorption spectroscopy. The primary aim of this study is to clarify the correlation between Td of M(BH4 )n and P of M, which has been predicted by our first-principles calculation.13,14) 2.
Experimental
Starting materials, anhydrous MCln (M ¼ Mg, Sc, Zr, Ti, and Zn) with 99.9%–99.999% purities, LiBH4 and NaBH4 with 95%–99.9% purities, were purchased from Aldrich Co., LTD. Then the mixture of MCln and nLiBH4 /nNaBH4 was mechanically milled under a 0.1 MPa argon gas atmosphere for 5 hours, in order to synthesize M(BH4 )n (M ¼ Mg, Sc, Zr, Ti, and Zn). Mechanochemical syntheses have been reported for preparation of Zn(BH4 )2 15) and Zr(BH4 )4 .16) The expected reaction during milling is expressed as follows; MCln þ nLiBH4 ! M(BH4 )n þ nLiCl
ð2Þ
MCln þ nNaBH4 ! M(BH4 )n þ nNaCl
ð20 Þ
The samples thus prepared were examined by powder X-ray
Syntheses and Hydrogen Desorption Properties of Metal-Borohydrides M(BH4 )n (M ¼ Mg, Sc, Zr, Ti, and Zn; n ¼ 2{4)
LiBH4
LiCl
(a) M Cln + nLiBH4
1899
Intensity (a.u.)
M = Mg
MCln + nLiBH4 M = Mg
Intensity (a.u.)
Sc Zr Ti
Sc Zr
Zn NaCl
(b) M Cln + nNaBH4
Ti
Intensity (a.u.)
M = Mg Sc
Zn
1000
2000
3000
-1
4000
Zr
Raman Shift /cm
Ti
Fig. 2 Raman spectra of the samples measured after mechanical milling of MCln þ nLiBH4 (M ¼ Mg, Sc, Zr, Ti, and Zn).
Zn
10
20
30
40 2θ
50
60
70
Fig. 1 Powder x-ray diffraction profiles of the samples measured after mechanical milling of (a) MCln þ nLiBH4 , and (b) MCln þ nNaBH4 (M ¼ Mg, Sc, Zr, Ti, and Zn).
diffraction measurement (PANalytical X’PERT with the CuK radiation), Raman spectroscopy (Nicolet, Almega-HD, 532 nm-laser with back scattering geometry) and thermal desroption spectroscopy detected by gas chromatography (GL Science GC323, Ar flow rate of 40 ml/min and heating rate of 5 K/min) and by mass spectroscopy (Anelva MQA200TS, combined with thermogravimetry (Rigaku, TG8120, He flow rate of 150 ml/min and heating rate of 5 K/min). The samples were always handled in a glove box filled with purified Ar/He (dew point below 183 K) without exposing to air. The experimental details were described in elsewhere.17) 3.
Results and Discussion
3.1 Syntheses of M(BH4 )n Figure 1 shows the powder x-ray diffraction profiles of the samples after mechanical milling of (a) MCln þ nLiBH4 , and (b) MCln þ nNaBH4 . There are broad diffraction peaks around 20 degree in all the profiles of Fig. 1(a), originating from the diffraction of tape covering the samples to avoid (hydro-)oxidation by exposing to air. In Fig. 1(a), all the diffraction peaks measured after mechanical milling of MCln þ nLiBH4 are identified as LiCl, although the diffraction peaks of LiCl after the milling of MgCl2 þ 2LiBH4 shift to lower angle, probably due to the partial substitution of Mg for Li in LiCl. There are no diffraction peaks of the starting
materials of MCln þ nLiBH4 , showing the progression of reaction of eq. (2) to be completed. On the other hand, in Fig. 1(b), the diffraction peaks of NaCl were scarcely observed after mechanical milling of MCln þ nNaBH4 for M ¼ Sc and Mg. For M ¼ Zn, Ti, and Zr, the peak position shifts to lower angle than that from NaCl. Therefore, the reaction of eq. (20 ) is not thought to proceed completely under the milling condition in this experiment. These results indicated that the progression of eq. (2) (starting materials of MCln þ nLiBH4 ) is easier than that of eq. (20 ) (starting materials of MCln þ nNaBH4 ). One of the reasons for easier progression of the reaction of eq. (2) is the similarity of ionic radius between Liþ and Mnþ ; the ionic radius of Liþ (0.076 nm) is closer to those of Mg2þ (0.072 nm), Sc3þ (0.075 nm), Zr4þ (0.072 nm), Ti3þ (0.067 nm), and Zn2þ (0.074 nm) than that of Naþ (0.102 nm).18) There are no diffraction peaks of M(BH4 )n , which is due to the absence of any long range ordering in the structure. Therefore, the atomistic vibrations are investigated by Raman spectroscopy in order to confirm the existence of M(BH4 )n in the samples prepared by mechanical milling. The Raman spectra of the samples measured after mechanical milling of MCln þ nLiBH4 are shown in Fig. 2. The spectrum of LiBH4 is also shown as a reference. The Raman peaks for LiBH4 are observed at around 2300 and 1300 cm1 , each corresponding to the stretching (1 ) and bending (2 , and also 2 0 ) modes of B-H in the BH4 anion.19) The Raman peaks for M ¼ Mg, Sc, Zr, Ti, and Zn can be found around similar wave number region, which agree with the experimental and theoretical vibrating modes of M(BH4 )n in Refs. 20–22, but not the same as those of LiBH4 . So, we confirmed the presence of M(BH4 )n and LiCl in the sample as the products of eq. (2) by Raman spectroscopy and powder x-ray diffraction measurement, respectively.
Y. Nakamori, H. Li, K. Miwa, S. Towata and S. Orimo
Mg
1.6
Ti
400
0.9
1.3
Zn
Thermal Desorption (a.u.)
Na
Sc
1.5
300
Ar 0.1 MPa 5 K/min
1.2
Li
1.0
K
Zr
0.8
1.4
500 600 700 Temperature, T/K
Fig. 3 Thermal desorption profiles of the samples of MCln þ nLiBH4 (M ¼ Mg, Sc, Zr, Ti, and Zn) after mechanical milling detected by gas chromatography. The results of MBH4 (M ¼ Li, Na, and K) are also shown as references. Pauling electronegativity is shown below each M.
3.2 Thermal desorption properties of M(BH4 )n Figure 3 shows the thermal desorption profiles of the mixture of MCln þ nLiBH4 after mechanical milling detected by gas chromatography. Although the samples contain not only M(BH4 )n but also LiCl, the thermal desorption profiles are originating only from M(BH4 )n , because LiCl decomposes only at the temperature higher than 878 K. The results of MBH4 for M ¼ Li, Na, and K are also shown as references. MBH4 for M ¼ Li, Na, and K desorb hydrogen at the temperature higher than 700 K, in which alkali hydrides remained up to 873 K such as eq. (1). There are multi desorption peaks of M(BH4 )n for M ¼ Mg, Sc, Zr, and Ti, which indicated that the hydrogen desorption reactions from borohydride to hydride, and from hydride to metal and/or boride as was reported in Mg(BH4 )2 .20) There is a single desorption peak in Zn(BH4 )2 , because it desorbs directly to elemental Zn due to instabilities of the zinc hydride and boride. It should be emphasized that there is a good correlation between desorption temperature Td and Pauling electronegativity P ; Td decrease with increasing values of P except for M ¼ Ti.13,14) This indicated that our theoretical prediction, H of M(BH4 )n becomes smaller negative value with increasing of value of P , is experimentally supported for the first time. The exception of M ¼ Ti might be originated from the stabilities of decomposed products, i.e., Ti hydride and/or boride. The products after hydrogen desorption reaction could not be clarified in this study, because only LiCl is also confirmed in the powder x-ray diffraction profiles after hydrogen desorption reaction. Further investigations are needed to determine the precise reaction path and the H for the hydrogen desorption reaction. In order to investigate the components of desorbed gas, experiments using mass-spectroscopy and thermogravimetry have been carried out. The results are shown in Fig. 4(a) and (b), respectively. From the results of mass spectroscopy, the components of desorbed gas are confirmed to be only hydrogen from M(BH4 )n for M ¼ Mg, Sc, Zr, and Ti
(a)
-1
M Cln + nLiBH4 He 0.1 MPa 5 K/min
Zn
Mg
Sc
Ti Zr mass. 2 mass. 28 0 Ti, 5.5mass%
0
800
Weight Loss (mass%)
Thermal Desorp. (a.u.)
M Cln + nLiBH4
(b) Mg
Zr
5.0mass%
5.8mass%
-5
-2 -3 -10 Sc
-4
5.6mass%
Zn 4.5mass%
-5 300
400 500 600 700 Temperature, T/K
Weight Loss (mass%)
1900
-15
Fig. 4 Thermal desorption profiles of the samples of MCln þ nLiBH4 (M ¼ Mg, Sc, Zr, Ti, and Zn) after mechanical milling detected by (a) mass-spectroscopy and (b) thermogravimetry.23) The calculated value of weight loss from the mixture of M(BH4 )n þ nLiCl is shown below each M. The values are smaller than the expected ones from M(BH4 )n without nLiCl in Table 1.
(P 5 1:5). However, components of desorbed gas contain also borane for M ¼ Zn (P ¼ 1:6).24) The larger weight loss than the theoretical value for M ¼ Zn is also due to the desorption of borane. For M ¼ Ti (P ¼ 1:5) and Zr (P ¼ 1:4), the measured weight losses are smaller than those of the theoretical values, corresponding to 5% and 30%, respectively. This seems to be originated from the dehydriding reaction during mechanical milling, because their hydrogen desorption temperatures are lower than 400 K. On the other hand, the weight losses of M(BH4 )n for M ¼ Sc (P ¼ 1:3) and Mg (P ¼ 1:2) are 80% and 60% of the theoretical values, because their decomposition temperatures are relatively high. Therefore, the suitable P of M(BH4 )n was expected to be approximately 1.3–1.5 for hydrogen storage materials without desorbing borane and with low desorption temperature. The electronegativity P of M is an useful indicator to estimate the hydrogen desorption temperature of M(BH4 )n as candidates for advanced hydrogen storage materials with high gravimetric hydrogen densities and low desorption temperatures.
Syntheses and Hydrogen Desorption Properties of Metal-Borohydrides M(BH4 )n (M ¼ Mg, Sc, Zr, Ti, and Zn; n ¼ 2{4) Table 2 Summary of hydrogen desorption properties of M(BH4 )n þ nLiCl (M ¼ Mg, Sc, Zr, Ti, and Zn). M
Mg
Sc
Zr
Ti
Zn
1.2
1.3
1.4
1.5
1.6
Td (peak temp. in Fig. 3) Desorbed gases
705 K H2
548 K H2
468 K H2
395 K H2
433 K H2 , B2 H6
Weight loss; exp./cal.
60%
80%
30%
5%
Electronegativity of M
4.
Summary
In order to clarify the correlation between hydrogen desorption temperature Td of M(BH4 )n and the Pauling electronegativity P of M, M(BH4 )n (M ¼ Mg, Sc, Zr, Ti, and Zn; n ¼ 2{4) are synthesized by mechanical milling and the thermal desorption properties are investigated for the first time. The mixture of MCln and nLiBH4 changes into M(BH4 )n with nLiCl by mechanical milling, which was confirmed by Raman spectroscopy and by powder x-ray diffraction measurement, respectively. On the other hand, for the mixture of MCln and nNaBH4 , the formation into M(BH4 )n and nNaCl did not proceed completely under the milling condition used in this experiment. The thermal desorption properties of the mixture of MCln and nLiBH4 after mechanical milling, that is M(BH4 )n and nLiCl, are summarized in Table 2. A good correlation between Td and P was observed experimentally, which is in good agreement with our theoretical prediction.13,14) The components of desorbed gas from M(BH4 )n for M ¼ Mg, Sc, Zr, and Ti (P 5 1:5) are hydrogen only, while those for M ¼ Zn (P ¼ 1:6) contain borane besides hydrogen. Appropriate P in M(BH4 )n for hydrogen storage is expected to be approximately 1.3–1.5 because of the lower decomposition temperature without releasing borane. Therefore, electronegativity P of M is found to be an useful indicator to estimate the decomposition temperature Td , which shows good correlation with the heat of formation H of M(BH4 )n predicted theoretically. Further investigations to determine the precise reaction path and H for hydrogen desorption reaction are now in progress. Acknowledgements The authors would like to thank T. Noritake, M. Aoki, and S. Hyodo for valuable discussion, and N. Warifune for her technical support. This study was partially supported by the New Energy and Industrial Technology Development Organization (NEDO), under the ‘‘Development for Safe
1901
Utilization and Infrastructure of hydrogen’’ Project, by the Ministry of Education, Science, Sports and Culture, ‘‘Grantin-Aid for Encouragement of Young Scientists (B), #17760555’’ and for Scientific Research (A), #18206073’’.
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