- Email: [email protected]
Ternary nitrides for hydrogen storage: Li–B–N, Li–Al–N and Li–Ga–N systems
Journal of Alloys and Compounds 466 (2008) 287–292
Ternary nitrides for hydrogen storage: Li–B–N, Li–Al–N and Li–Ga–N systems Henrietta W. Langmi, G. Sean McGrady ∗ Department of Chemistry, University of New Brunswick, 30 Dineen Drive, Fredericton, New Brunswick E3B 6E2, Canada Received 2 August 2007; accepted 11 November 2007 Available online 21 November 2007
Abstract This paper reports an investigation of hydrogen storage performance of ternary nitrides based on lithium and the Group 13 elements boron, aluminum and gallium. These were prepared by ball milling Li3 N together with the appropriate Group 13 nitride—BN, AlN or GaN. Powder X-ray diffraction of the products revealed that the ternary nitrides obtained are not the known Li3 BN2 , Li3 AlN2 and Li3 GaN2 phases. At 260 ◦ C and 30 bar hydrogen pressure, the Li–Al–N ternary system initially absorbed 3.7 wt.% hydrogen, although this is not fully reversible. We observed, for the first time, hydrogen uptake by a pristine ternary nitride of Li and Al synthesized from the binary nitrides of the metals. While the Li–Ga–N ternary system also stored a significant amount of hydrogen, the storage capacity for the Li–B–N system was near zero. The hydrogenation reaction is believed to be similar to that of Li3 N, and the enthalpies of hydrogen absorption for Li–Al–N and Li–Ga–N provide evidence that AlN and GaN, as well as the ball milling process, play a significant role in altering the thermodynamics of Li3 N. © 2007 Elsevier B.V. All rights reserved. Keywords: Lithium nitride; Mechanochemical synthesis; Hydrogen storage
1. Introduction The development of a practical hydrogen storage technology remains one of the major obstacles to the implementation of a hydrogen economy. Many different solid-state materials such as metal hydrides and their complexes; borohydrides; metalorganic frameworks; zeolites; and carbonaceous materials have been considered as potential candidates for hydrogen storage [1]. In the past few years there have been increasing research interests in lithium nitride, Li3 N, as a hydrogen storage material [2]. Li3 N absorbs hydrogen at high temperatures in a twostep process, transforming into LiH and LiNH2 in an overall reaction that corresponds to a take-up of 11.5 wt.% hydrogen [3]. However, owing to incomplete hydrogenation, less than 6 wt.% hydrogen is reversibly stored below 280 ◦ C [4]. Many attempts have been made to enhance the hydrogen storage properties of this promising system, such as the addition of various catalysts [5–7], and the partial substitution of Li by Mg [8–10].
∗
Corresponding author. E-mail address: [email protected] (G.S. McGrady).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.11.043
Lithium nitride exists in three polymorphs: ␣-Li3 N (the low pressure phase), and - and ␥-Li3 N (the high pressure phases) [11]. The structure of ␣-Li3 N consists of graphite-like layers of Li hexagons with a nitride ion at the centre of each hexagon. The Li6 N hexagons are capped by two Li ions above and below the plane forming hexagonal bipyramids [12,13]. The Group 13 binary nitride, BN also adopts a layered structure, analogous to that of graphite [14]. Due to the structural similarity between BN and graphite there have been several investigations on hydrogen in nanostructured h-BN and BN nanotubes [15–18]. Lithium nitride has an extensive chemistry; reacting, for instance, with many elements or binary nitrides to form ternary nitrides [19]. Synthesis procedures for ternary nitrides often involve solid-state reactions that require high temperatures [20]. Ternary nitrides containing Li and Group 13 elements (i.e. Li3 BN2 , Li3 AlN2 and Li3 GaN2 ) have been the subject of a variety of research efforts for the past several decades [21–24]. Previous research regarding applications of these systems and other ternary nitrides containing Li have focused on the Li-ion conductivity of these materials for solid electrolyte applications such as solid-state Li batteries [20,25]. However, in spite of their long-established existence and potential materials applications, these ternary nitrides remain
288
H.W. Langmi, G.S. McGrady / Journal of Alloys and Compounds 466 (2008) 287–292
under-explored in the field of hydrogen storage. To the best of our knowledge, there exist only a few reports in the literature on the interaction of hydrogen with ternary nitrides [26–28]. Nakamori et al. [26] investigated mixtures of Li3 N and Mg3 N2 prepared by heating at 560–820 ◦ C in 1 bar nitrogen. At 250 ◦ C and 350 bar H2 the LiMgN-type structures formed were difficult to hydrogenate, although the 5:1 mixture of Li3 N and Mg3 N2 that was annealed at 560 ◦ C underwent hydrogenation. In another study, Yamane et al. [27] synthesized Li3 XN2 (where X = B, Al or Ga) by solid-state reaction of Li3 N and XN at 700–800 ◦ C. At 300 ◦ C and 100 bar H2 , Li3 GaN2 absorbed 3.4 wt.% hydrogen while Li3 BN2 and Li3 AlN2 showed no hydrogen uptake. More recently, Xiong et al. [28] reported reversible 5 wt.% hydrogen absorption at 50–500 ◦ C and 80 bar H2 by Li3 AlN2 that was generated from the reaction between LiNH2 and LiAlH4 . Here we report, for the first time, ternary nitrides based on Li and the Group 13 elements B, Al and Ga prepared by mechanical milling of Li3 N and the corresponding Group 13 nitrides—BN, AlN and GaN. The ternary products obtained are not the Li3 XN2 systems described above, and we have tentatively designated them as Li–X–N (where X = B, Al or Ga). We also report an investigation of the hydrogen absorption–desorption behaviour of these Li–X–N ternary nitrides. Recent studies on the quaternary hydrides Li–B–N–H and Li–Al–N–H have shown that these systems release large amounts of hydrogen upon heating [29–31], although subsequent re-absorption of hydrogen does not occur. Our approach contrasts with these recent studies, in which LiBH4 –LiNH2 , LiAlH4 –LiNH2 or Li3 AlH6 –LiNH2 were used as starting materials to generate the quaternary hydride phases [28–31]. Thus, instead of using hydrogenous starting materials we react non-hydrogenated binary nitrides and obtain ternary systems, which we subsequently subject to hydrogenation–dehydrogenation procedures.
Q20P DSC instrument from TA Instruments. Operations were carried out in a constant volume mode. Typically, a sample of mass 9–10 mg was loaded in an aluminum pan and placed in a pressure cell. The cell was pressurized at 30 bar with H2 gas and the sample was heated at 10 ◦ C/min. Hydrogen absorption/desorption of each sample was monitored by kinetic curves and pressure–composition–temperature (P–C–T) plots obtained using a commercial PCTPro-2000 Sieverts-type instrument provided by HyEnergy LLC. Prior to hydrogen absorption 0.5 g sample was loaded into a sample holder in a nitrogen-filled glovebox. The sample holder was then attached to the PCT instrument without exposing the sample to air. The sample was pretreated in situ by heating in vacuum at 100 ◦ C for 1 h. Static P–C isotherms were recorded at 195 ◦ C for all samples, and additionally at 260 ◦ C for Li–Al–N, and at 225, 255 and 300 ◦ C for Li–B–N. The total measurement time for an absorption–desorption cycle was set at 40 h and the interval between data points on the isotherm was 1 h. High purity hydrogen (99.999%) was used in all experiments.
3. Results and discussion X-ray diffraction patterns for all samples are shown in Figs. 1–3. When Li3 N was mechanically milled with BN, a new phase which could not be indexed to the known Li3 BN2 phases was generated after 18 h of milling (Fig. 1). If the mixture
2. Experimental The starting materials were Li3 N (STREM, 99.5%), h-BN (STREM, 99+%), AlN (Aldrich, 98+%) and GaN (Aldrich, 99.99+%). All materials and samples were handled in a nitrogen-filled glovebox to minimize contact with air and moisture. Ternary nitrides were prepared by ball milling stoichiometric amounts of Li3 N and either BN, AlN or GaN. In a typical experiment about 3 g of both nitrides, corresponding to 1:1 molar ratio, was loaded in a 250 mL tungsten carbide milling vessel containing five tungsten carbide balls with a diameter of 20 mm. The ball-to-powder mass ratio was 104:1. The powder mixtures were mechanically milled at room temperature in a N2 atmosphere at a rotational speed of 200 rpm for 18 and 48 h using a Retsch PM 100 planetary ball mill. After every 5 min of milling there was a 10 s pause and rotation was automatically reversed. Phase crystallinity of all samples was checked through powder X-ray diffraction (XRD) measured using a Rigaku MiniFlex diffractometer equipped with a Cu K␣ radiation source. Identification of products was carried out by referring to the Joint Committee for Powder Diffraction Studies (JCPDS) values in the database of the diffractometer. Samples for XRD analysis were mounted on a PVC plastic disc and covered with parafilm to protect the samples from contact with air or moisture during the experiment. This arrangement resulted in diffraction peaks at ca. 21.6◦ and 24◦ arising from the parafilm. These diffraction peaks did not interfere with those of the sample. Solid-state 1 H MAS NMR experiments on hydrogenated samples were carried out using a Bruker Avance NMR spectrometer with a 16.45 T magnet (700.25 MHz 1 H Larmor frequency). The chemical shift scale was referenced externally to H2 O at 4.65 ppm. Differential Scanning Calorimetry (DSC) measurements were performed using a
Fig. 1. XRD patterns for Li–B–N milled for 6, 12, 18, 24 and 48 h. The XRD patterns for the starting materials Li3 N and BN are also shown.
Fig. 2. XRD patterns for Li–Al–N milled for 18 and 48 h. The XRD patterns for the starting materials Li3 N and AlN are also shown.
H.W. Langmi, G.S. McGrady / Journal of Alloys and Compounds 466 (2008) 287–292
Fig. 3. XRD patterns for Li–Ga–N milled for 18 and 48 h. The XRD patterns for the starting materials Li3 N and GaN are also shown.
was initially milled for 6 h, all BN diffraction peaks disappeared while a few Li3 N diffraction peaks remained. As the milling time increased a new set of peaks began to emerge. These new features were clearly visible after milling for 18 h. Further milling up to 48 h did not produce any new peaks in the XRD pattern, implying that the formation of the new Li–B–N phase was complete. The interaction between Li3 N and BN might have been facilitated because of the structural resemblance between the two compounds; both ␣-Li3 N and h-BN consist of graphite-like hexagonal layers [12–14]. During milling the hexagonal layers were broken to create new bonds. Unlike for Li–B–N, the XRD pattern for Li–Al–N was dominated by AlN reflections (Fig. 2). This may be due to the greater hardness of AlN when compared with h-BN. After milling for 18 h only AlN peaks were present in the XRD pattern. However, when the sample was subjected to extended milling for 48 h, new diffraction peaks that could not be indexed to either AlN or Li3 N were observed at ca. 32.1◦ , 48.5◦ and 53.7◦ , in addition to AlN peaks. The XRD pattern could not be assigned to the known Li3 AlN2 phase. In the case of Li–Ga–N, one new diffraction peak was present at ca. 54.2◦ alongside the GaN peaks in the XRD pattern after milling for both 18 and 48 h (Fig. 3). Once again, the XRD pattern did not match that of the known Li3 GaN2 phase. Hydrogen sorption studies were carried out on Li–B–N (milled for 18 h), Li–Al–N (milled for 48 h), and Li–Ga–N (milled for 18 h). Fig. 4 shows hydrogen absorption–desorption isotherms at 195 and 260 ◦ C for Li–Al–N. In the first cycle of measurements hydrogen absorption reached 2.4 wt.% for Li–Al–N at 195 ◦ C and ca. 30 bar (Fig. 4). Approximately 0.4 wt.% of the absorbed hydrogen was desorbed at 195 ◦ C. After 40 h elapsed the absorption–desorption cycle was terminated and a second cycle of measurements was carried out on the same sample under similar conditions. An additional 0.7 wt.% hydrogen was absorbed, part of which was desorbed at 195 ◦ C. At 260 ◦ C and ca. 30 bar a fresh Li–Al–N sample absorbed 3.7 wt.% hydrogen and desorbed 1.2 wt.% of the absorbed hydrogen at same temperature, indicating that some of the hydrogen was still strongly bound to the material. In comparison, at 195 ◦ C and ca. 30 bar, Li–Ga–N absorbed 1.5 wt.% hydrogen in the first
289
Fig. 4. Hydrogen absorption–desorption isotherms at 195 and 260 ◦ C for Li–Al–N.
cycle and 0.6 wt.% in the second cycle (Fig. 5). The absorbed hydrogen was only partially desorbed in the first cycle. Nevertheless, almost all hydrogen absorbed in the second cycle was desorbed before the 40 h absorption–desorption cycle measurement time elapsed. For both Li–Al–N and Li–Ga–N the P–C isotherms showed a large hysteresis. The hydrogen storage capacities obtained for Li–Al–N in this study are remarkable; for the first time, we observe hydrogen uptake by a pristine ternary nitride of Li and Al that was prepared from the binary nitrides of the elements. A previous study reported no hydrogen uptake by the known ternary nitride Li3 AlN2 synthesized from Li3 N and AlN [27]. However, Xiong et al. [28] reported that Li3 AlN2 reversibly stored 5 wt.% hydrogen. The Li3 AlN2 sample was generated after dehydrogenation of 2LiNH2 –LiAlH4 that had been previously ball milled. Hydrogen uptake by Li–B–N was close to zero at temperatures up to 300 ◦ C and a hydrogen pressure of ca. 30 bar (Fig. 6). Interestingly, the results obtained for Li–B–N corroborate those for hydrogenation of the known Li3 BN2 ternary nitride [27,29]. Pinkerton et al. [29] synthesized a new quaternary hydride by reacting LiNH2 and LiBH4 in a 2:1 molar ratio. The quaternary hydride released more than 10 wt.% hydrogen above 250 ◦ C and formed a mixture of Li3 BN2 polymorphs, which could not reabsorb any significant amount of hydrogen. It has also been shown that Li3 BN2 pre-
Fig. 5. Hydrogen absorption–desorption isotherms at 195 ◦ C for Li–Ga–N.
290
H.W. Langmi, G.S. McGrady / Journal of Alloys and Compounds 466 (2008) 287–292
binds to these ternary nitrides in a similar fashion, and one akin to that of Li3 N [3]. Consider Li–Al–N, which is in fact synthesized from equimolar amounts of Li3 N and AlN. For complete hydrogenation of this system the, following reaction is proposed: Li3 N–AlN + 2H2 → LiNH2 + 2LiH + AlN
(1)
Similarly, for the Li–Ga–N system the hydrogenation reaction may be written as Li3 N–GaN + 2H2 → LiNH2 + 2LiH + GaN
(2)
pared by high temperature solid-state synthesis does not absorb hydrogen at temperatures up to 300 ◦ C and 100 bar hydrogen [27]. Powder XRD patterns for all samples were measured after hydrogenation at 195 ◦ C, and it was noted that no new diffraction peaks emerged in the patterns. This implies that the hydrogenated phase within the Li–X–N system was amorphous, or that it existed as a nanophase. Xiong et al. [28] observed no significant change in the XRD pattern after hydrogenation of Li3 AlN2 at 200 ◦ C. Solid-state 1 H NMR spectra were also obtained for these samples (Fig. 7). The results revealed that the 1 H signal of hydrogenated Li–B–N was very weak, in agreement with the observed low hydrogen uptake capacity for the material. On the other hand, the signals for hydrogenated Li–Al–N and Li–Ga–N were quite strong, and the good resolution of the spectra is an indication of hydrogen mobility within the systems. The spectra also revealed that hydrogen existed in two distinct chemical environments in each case. The line intensities centered at −2.377 and −1.897 ppm for Li–Al–N–H and Li–Ga–N–H, respectively, are most likely associated with amide hydrogens of LiNH2 . The other line intensities centered at 2.763 and 2.748 ppm for Li–Al–N–H and Li–Ga–N–H, respectively, are then due to the presence of LiH. This implies that hydrogen
The theoretical hydrogen storage capacity obtained from reaction (1) is 5.3 wt.%. Hydrogen storage capacities observed for Li–Al–N were 2.4 and 3.7 wt.% at 195 and 260 ◦ C, respectively. Clearly, hydrogen uptake did not reach completion in these materials. It is likely that the newly generated Li–Al–N phase absorbed hydrogen and transformed into LiNH2 , LiH and AlN, while residual AlN did not absorb any hydrogen. It cannot also be ruled out that some Li3 N was present in an amorphous or nanocrystalline state in the Li–Al–N sample and, upon hydrogenation, reacted to generate LiNH2 and LiH. The powder XRD patterns provided compelling evidence for the presence of unreacted material. Though the ternary nitrides apparently store hydrogen in a similar manner to Li3 N and at comparable conditions, hydrogen storage capacities for the Li–X–N systems were considerably lower than that of Li3 N [3]. As shown by reaction (1) the Al component and some of the N atoms in the Li–Al–N system seemed reluctant to undergo hydrogenation and remained as AlN, which is thermodynamically very stable. AlN contributes substantially to the total mass of the system, yet makes zero contribution to its hydrogen uptake, leading to relatively lower wt.% hydrogen absorption by the ternary nitrides. If hydrogenation occurred in a similar manner to that observed for Li3 N, then the partial desorption of hydrogen by these ternary systems may be accounted for by the intermediate reaction that generates Li2 NH. Thus, dehydrogenation of LiNH2 at 195 and 260 ◦ C will generate Li2 NH, and this intermediate will not undergo further dehydrogenation at these temperatures, accounting for the relatively lower hydrogen uptake in the second cycle of measurements.
Fig. 7. 1 H MAS NMR spectra of hydrogenated Li–B–N, Li–Al–N and Li–Ga–N.
Fig. 8. Hydrogen absorption kinetic curves for Li–Al–N and Li–Ga–N over a series of temperatures from 160 to 295 ◦ C.
Fig. 6. Hydrogen absorption–desorption isotherms at 195 ◦ C for Li–B–N.
H.W. Langmi, G.S. McGrady / Journal of Alloys and Compounds 466 (2008) 287–292
291
to −39.6 kJ/mol H2 ). These enthalpy values can be compared with those for the hydrogenation reactions of the known Li3 AlN2 and Li3 N (Eqs. (3) and (4)) [28]. Li3 AlN2 + 2H2 → LiNH2 + 2LiH + AlN, H = −50.1 kJ/molH2
(3)
Li3 N + 2H2 → LiNH2 + 2LiH, ΔH = −80.5 kJ/molH2
Fig. 9. DSC plots of hydrogen absorption by Li–B–N, Li–Al–N and Li–Ga.
Fig. 8 presents kinetic plots for hydrogen absorption by Li–Al–N and Li–Ga–N over a series of temperatures from 160 to 295 ◦ C. The initial hydrogen pressure at 160 ◦ C was 30 bar. The sample was held for 2 h at each of the following temperatures: 160, 195, 230, 265 and 295 ◦ C, and its hydrogen storage capacity was monitored as a function of time. Clearly, hydrogen uptake in Li–Al–N generally exceeds that in Li–Ga–N. For both materials the rate of hydrogen absorption was greatest at 195 ◦ C. At higher temperatures the rate was relatively lower. This behaviour can be accounted for by the generation of the hydrogenated phase. Initially at 160 ◦ C the reaction rate was slow, as the temperature was not sufficiently high to enhance the reaction kinetically. As the temperature was increased to 195 ◦ C in the presence of many unreacted Li–X–N species, the rate of hydrogenation increased. However, as time progressed the number of Li–X–N species that were available for reaction diminished as the material became more saturated with hydrogen, and the hydrogenation rate decreased. The shapes of the kinetic curves at 160 ◦ C are noticeably different. An examination of the hydrogen absorption isotherms for both Li–Al–N and Li–Ga–N at 160 ◦ C clearly reveals the distinct nature of the absorption curves. Further study is needed to clarify this behaviour. Enthalpies of absorption were determined by DSC. Fig. 9 shows the DSC plots for hydrogen absorption by the ternary nitrides. It can be seen that the curve is different for each material. In the case of Li–B–N, DSC measurement did not show any heat evolution at temperatures up to 400 ◦ C, implying that Li–B–N is stable towards hydrogenation at these temperatures. This observation correlates very well with the near zero hydrogen uptake displayed by this material (Fig. 6). On the other hand, the heat flow for both Li–Al–N and Li–Ga–N revealed the occurrence of an exothermic event. An exothermic process attributed to the absorption of hydrogen started at about 75 ◦ C and peaked at 224 ◦ C for Li–Al–N. The exothermic hydrogen absorption reached a maximum at about 321 ◦ C for Li–Ga–N. Clearly, Li–Al–N absorbs most of its hydrogen at a relatively lower temperature than Li–Ga–N. The enthalpies of absorption for Li–Al–N and Li–Ga–N calculated from the DSC measurements are −77.1 kJ/mol for Li3 N–AlN (equivalent to −38.5 kJ/mol H2 ), and −79.2 kJ/mol for Li3 N–GaN (equivalent
(4)
On the basis of these values, it can be said that hydrogenation of the Li–Al–N and Li–Ga–N systems in this work is less thermodynamically favourable than hydrogenation of Li3 AlN2 or Li3 N. However, under comparable conditions dehydrogenation of hydrogenated Li–Al–N and Li–Ga–N ternary nitrides would be more favoured thermodynamically. This demonstrates the role played by the ball milling process as well as by the Group 13 nitrides (AlN and GaN) in modifying the thermodynamics of the Li3 N system. 4. Conclusions For the first time, ternary nitrides have been synthesized from Li3 N and the Group 13 nitrides (BN, AlN and GaN) by mechanical milling. The ternary nitrides obtained are not the known Li3 BN2 , Li3 AlN2 and Li3 GaN2 systems. The Li–B–N and Li–Ga–N phases were generated after milling for 18 h, while the Li–Al–N phase was generated after 48 h of milling. We have also observed, for the first time, hydrogen uptake by a pristine ternary nitride of Li and Al synthesized from the binary nitrides of the elements. Hydrogen storage capacity was greatest for Li–Al–N, reaching 3.7 wt.% (at 260 ◦ C and 30 bar) and near zero for Li–B–N. At 195 ◦ C and 30 bar, Li–Ga–N absorbed 1.5 wt.% hydrogen while Li–Al–N absorbed 2.4 wt.% hydrogen. Hydrogen uptake by these ternary nitrides was partially reversible, and the hydrogenation reaction appeared to be similar to that of Li3 N but the enthalpy of absorption was less exothermic for the ternary nitrides. Ternary nitrides are interesting materials that are under-explored as hydrogen storage materials, and warrant more attention. Acknowledgements The authors would like to thank Dr. Ulrike Werner-Zwanziger for carrying out solid-state NMR experiments. This research was supported by NSERC, CFI and HSM Systems Inc. References [1] M. Felderhoff, C. Weidenthaler, R. von Helmolt, U. Eberle, Phys. Chem. Chem. Phys. 9 (2007) 2643. [2] H.W. Langmi, G.S. McGrady, Coord. Chem. Rev. 251 (2007) 925. [3] P. Chen, Z. Xiong, J. Luo, J. Lin, K.L. Tan, Nature 420 (2002) 302. [4] Y.H. Hu, N.Y. Yu, E. Ruckenstein, Ind. Eng. Chem. Res. 44 (2005) 4304. [5] S. Isobe, T. Ichikawa, N. Hanada, H.Y. Leng, M. Fichtner, O. Fuhr, H. Fujii, J. Alloys Compd. 404–406 (2005) 439.
292
H.W. Langmi, G.S. McGrady / Journal of Alloys and Compounds 466 (2008) 287–292
[6] T. Ichikawa, N. Hanada, S. Isobe, H.Y. Leng, H. Fujii, J. Alloys Compd. 404–406 (2005) 435. [7] T. Ichikawa, S. Isobe, N. Hanada, H. Fujii, J. Alloys Compd. 365 (2004) 271. [8] S. Orimo, Y. Nakamori, G. Kitahara, K. Miwa, N. Ohba, T. Noritake, S. Towata, Appl. Phys. A 79 (2004) 1765. [9] Y. Nakamori, S. Orimo, Mater. Sci. Eng. B 108 (2004) 48. [10] T. Ichikawa, K. Tokoyoda, H. Leng, H. Fujii, J. Alloys Compd. 400 (2005) 245. [11] D.H. Gregory, Coord. Chem. Rev. 215 (2001) 301. [12] E. Zintl, G. Brauer, Z. Elektrochem. Angew. Phys. Chem. 41 (1935) 102. [13] A. Rabenau, H. Schulz, J. Less Common Met. 50 (1976) 155. [14] R.S. Pease, Acta Crystallogr. 5 (1952) 356. [15] P. Wang, S. Orimo, T. Matsushima, H. Fujii, G. Mayer, Appl. Phys. Lett. 80 (2002) 318. [16] P. Wang, S. Orimo, H. Fujii, Appl. Phys. A 78 (2004) 1235. [17] C. Tang, Y. Bando, X. Ding, S. Qi, D. Golberg, J. Am. Chem. Soc. 124 (2002) 14550.
[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
T. Oku, M. Kuno, Diamond Relat. Mater. 12 (2003) 840. R. Juza, K. Langer, K. von Benda, Angew. Chem. Int. Edit. 7 (1968) 360. H. Yamane, S. Kikkawa, M. Koizumi, J. Power Sources 20 (1987) 311. V.J. Goubeau, W. Anselment, Z. Anorg. Allg. Chem. 310 (1961) 248. R.C. DeVries, J.F. Fleisher, Mater. Res. Bull. 4 (1969) 433. R. Juza, F. Hund, Z. Anorg. Allg. Chem. (1948) 13. H. Yamane, S. Kikkawa, M. Koizumi, J. Solid State Chem. 71 (1987) 1. H. Yamane, S. Kikkawa, M. Koizumi, Solid State Ionics 15 (1985) 51. Y. Nakamori, G. Kitahara, K. Miwa, S. Towata, S. Orimo, Appl. Phys. A 80 (2005) 1. H. Yamane, T. Kano, A. Kamegawa, M. Shibata, T. Yamada, M. Okada, M. Shimada, J. Alloys Compd. 402 (2005) L1. Z. Xiong, G. Wu, J. Hu, Y. Liu, P. Chen, W. Luo, J. Wang, Adv. Funct. Mater. 17 (2007) 1137. F.E. Pinkerton, G.P. Meisner, M.S. Meyer, M.P. Balogh, M.D. Kundrat, J. Phys. Chem. B 109 (2005) 6. J. Lu, Z.Z. Fang, J. Phys. Chem. B 109 (2005) 20830. Y. Kojima, M. Matsumoto, Y. Kawai, T. Haga, N. Ohba, K. Miwa, S. Towata, Y. Nakamori, S. Orimo, J. Phys. Chem. B 110 (2006) 9632.
Ternary nitrides for hydrogen storage: Li–B–N, Li–Al–N and Li–Ga–N systems Henrietta W. Langmi, G. Sean McGrady ∗ Department of Chemistry, University of New Brunswick, 30 Dineen Drive, Fredericton, New Brunswick E3B 6E2, Canada Received 2 August 2007; accepted 11 November 2007 Available online 21 November 2007
Abstract This paper reports an investigation of hydrogen storage performance of ternary nitrides based on lithium and the Group 13 elements boron, aluminum and gallium. These were prepared by ball milling Li3 N together with the appropriate Group 13 nitride—BN, AlN or GaN. Powder X-ray diffraction of the products revealed that the ternary nitrides obtained are not the known Li3 BN2 , Li3 AlN2 and Li3 GaN2 phases. At 260 ◦ C and 30 bar hydrogen pressure, the Li–Al–N ternary system initially absorbed 3.7 wt.% hydrogen, although this is not fully reversible. We observed, for the first time, hydrogen uptake by a pristine ternary nitride of Li and Al synthesized from the binary nitrides of the metals. While the Li–Ga–N ternary system also stored a significant amount of hydrogen, the storage capacity for the Li–B–N system was near zero. The hydrogenation reaction is believed to be similar to that of Li3 N, and the enthalpies of hydrogen absorption for Li–Al–N and Li–Ga–N provide evidence that AlN and GaN, as well as the ball milling process, play a significant role in altering the thermodynamics of Li3 N. © 2007 Elsevier B.V. All rights reserved. Keywords: Lithium nitride; Mechanochemical synthesis; Hydrogen storage
1. Introduction The development of a practical hydrogen storage technology remains one of the major obstacles to the implementation of a hydrogen economy. Many different solid-state materials such as metal hydrides and their complexes; borohydrides; metalorganic frameworks; zeolites; and carbonaceous materials have been considered as potential candidates for hydrogen storage [1]. In the past few years there have been increasing research interests in lithium nitride, Li3 N, as a hydrogen storage material [2]. Li3 N absorbs hydrogen at high temperatures in a twostep process, transforming into LiH and LiNH2 in an overall reaction that corresponds to a take-up of 11.5 wt.% hydrogen [3]. However, owing to incomplete hydrogenation, less than 6 wt.% hydrogen is reversibly stored below 280 ◦ C [4]. Many attempts have been made to enhance the hydrogen storage properties of this promising system, such as the addition of various catalysts [5–7], and the partial substitution of Li by Mg [8–10].
∗
Corresponding author. E-mail address: [email protected] (G.S. McGrady).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.11.043
Lithium nitride exists in three polymorphs: ␣-Li3 N (the low pressure phase), and - and ␥-Li3 N (the high pressure phases) [11]. The structure of ␣-Li3 N consists of graphite-like layers of Li hexagons with a nitride ion at the centre of each hexagon. The Li6 N hexagons are capped by two Li ions above and below the plane forming hexagonal bipyramids [12,13]. The Group 13 binary nitride, BN also adopts a layered structure, analogous to that of graphite [14]. Due to the structural similarity between BN and graphite there have been several investigations on hydrogen in nanostructured h-BN and BN nanotubes [15–18]. Lithium nitride has an extensive chemistry; reacting, for instance, with many elements or binary nitrides to form ternary nitrides [19]. Synthesis procedures for ternary nitrides often involve solid-state reactions that require high temperatures [20]. Ternary nitrides containing Li and Group 13 elements (i.e. Li3 BN2 , Li3 AlN2 and Li3 GaN2 ) have been the subject of a variety of research efforts for the past several decades [21–24]. Previous research regarding applications of these systems and other ternary nitrides containing Li have focused on the Li-ion conductivity of these materials for solid electrolyte applications such as solid-state Li batteries [20,25]. However, in spite of their long-established existence and potential materials applications, these ternary nitrides remain
288
H.W. Langmi, G.S. McGrady / Journal of Alloys and Compounds 466 (2008) 287–292
under-explored in the field of hydrogen storage. To the best of our knowledge, there exist only a few reports in the literature on the interaction of hydrogen with ternary nitrides [26–28]. Nakamori et al. [26] investigated mixtures of Li3 N and Mg3 N2 prepared by heating at 560–820 ◦ C in 1 bar nitrogen. At 250 ◦ C and 350 bar H2 the LiMgN-type structures formed were difficult to hydrogenate, although the 5:1 mixture of Li3 N and Mg3 N2 that was annealed at 560 ◦ C underwent hydrogenation. In another study, Yamane et al. [27] synthesized Li3 XN2 (where X = B, Al or Ga) by solid-state reaction of Li3 N and XN at 700–800 ◦ C. At 300 ◦ C and 100 bar H2 , Li3 GaN2 absorbed 3.4 wt.% hydrogen while Li3 BN2 and Li3 AlN2 showed no hydrogen uptake. More recently, Xiong et al. [28] reported reversible 5 wt.% hydrogen absorption at 50–500 ◦ C and 80 bar H2 by Li3 AlN2 that was generated from the reaction between LiNH2 and LiAlH4 . Here we report, for the first time, ternary nitrides based on Li and the Group 13 elements B, Al and Ga prepared by mechanical milling of Li3 N and the corresponding Group 13 nitrides—BN, AlN and GaN. The ternary products obtained are not the Li3 XN2 systems described above, and we have tentatively designated them as Li–X–N (where X = B, Al or Ga). We also report an investigation of the hydrogen absorption–desorption behaviour of these Li–X–N ternary nitrides. Recent studies on the quaternary hydrides Li–B–N–H and Li–Al–N–H have shown that these systems release large amounts of hydrogen upon heating [29–31], although subsequent re-absorption of hydrogen does not occur. Our approach contrasts with these recent studies, in which LiBH4 –LiNH2 , LiAlH4 –LiNH2 or Li3 AlH6 –LiNH2 were used as starting materials to generate the quaternary hydride phases [28–31]. Thus, instead of using hydrogenous starting materials we react non-hydrogenated binary nitrides and obtain ternary systems, which we subsequently subject to hydrogenation–dehydrogenation procedures.
Q20P DSC instrument from TA Instruments. Operations were carried out in a constant volume mode. Typically, a sample of mass 9–10 mg was loaded in an aluminum pan and placed in a pressure cell. The cell was pressurized at 30 bar with H2 gas and the sample was heated at 10 ◦ C/min. Hydrogen absorption/desorption of each sample was monitored by kinetic curves and pressure–composition–temperature (P–C–T) plots obtained using a commercial PCTPro-2000 Sieverts-type instrument provided by HyEnergy LLC. Prior to hydrogen absorption 0.5 g sample was loaded into a sample holder in a nitrogen-filled glovebox. The sample holder was then attached to the PCT instrument without exposing the sample to air. The sample was pretreated in situ by heating in vacuum at 100 ◦ C for 1 h. Static P–C isotherms were recorded at 195 ◦ C for all samples, and additionally at 260 ◦ C for Li–Al–N, and at 225, 255 and 300 ◦ C for Li–B–N. The total measurement time for an absorption–desorption cycle was set at 40 h and the interval between data points on the isotherm was 1 h. High purity hydrogen (99.999%) was used in all experiments.
3. Results and discussion X-ray diffraction patterns for all samples are shown in Figs. 1–3. When Li3 N was mechanically milled with BN, a new phase which could not be indexed to the known Li3 BN2 phases was generated after 18 h of milling (Fig. 1). If the mixture
2. Experimental The starting materials were Li3 N (STREM, 99.5%), h-BN (STREM, 99+%), AlN (Aldrich, 98+%) and GaN (Aldrich, 99.99+%). All materials and samples were handled in a nitrogen-filled glovebox to minimize contact with air and moisture. Ternary nitrides were prepared by ball milling stoichiometric amounts of Li3 N and either BN, AlN or GaN. In a typical experiment about 3 g of both nitrides, corresponding to 1:1 molar ratio, was loaded in a 250 mL tungsten carbide milling vessel containing five tungsten carbide balls with a diameter of 20 mm. The ball-to-powder mass ratio was 104:1. The powder mixtures were mechanically milled at room temperature in a N2 atmosphere at a rotational speed of 200 rpm for 18 and 48 h using a Retsch PM 100 planetary ball mill. After every 5 min of milling there was a 10 s pause and rotation was automatically reversed. Phase crystallinity of all samples was checked through powder X-ray diffraction (XRD) measured using a Rigaku MiniFlex diffractometer equipped with a Cu K␣ radiation source. Identification of products was carried out by referring to the Joint Committee for Powder Diffraction Studies (JCPDS) values in the database of the diffractometer. Samples for XRD analysis were mounted on a PVC plastic disc and covered with parafilm to protect the samples from contact with air or moisture during the experiment. This arrangement resulted in diffraction peaks at ca. 21.6◦ and 24◦ arising from the parafilm. These diffraction peaks did not interfere with those of the sample. Solid-state 1 H MAS NMR experiments on hydrogenated samples were carried out using a Bruker Avance NMR spectrometer with a 16.45 T magnet (700.25 MHz 1 H Larmor frequency). The chemical shift scale was referenced externally to H2 O at 4.65 ppm. Differential Scanning Calorimetry (DSC) measurements were performed using a
Fig. 1. XRD patterns for Li–B–N milled for 6, 12, 18, 24 and 48 h. The XRD patterns for the starting materials Li3 N and BN are also shown.
Fig. 2. XRD patterns for Li–Al–N milled for 18 and 48 h. The XRD patterns for the starting materials Li3 N and AlN are also shown.
H.W. Langmi, G.S. McGrady / Journal of Alloys and Compounds 466 (2008) 287–292
Fig. 3. XRD patterns for Li–Ga–N milled for 18 and 48 h. The XRD patterns for the starting materials Li3 N and GaN are also shown.
was initially milled for 6 h, all BN diffraction peaks disappeared while a few Li3 N diffraction peaks remained. As the milling time increased a new set of peaks began to emerge. These new features were clearly visible after milling for 18 h. Further milling up to 48 h did not produce any new peaks in the XRD pattern, implying that the formation of the new Li–B–N phase was complete. The interaction between Li3 N and BN might have been facilitated because of the structural resemblance between the two compounds; both ␣-Li3 N and h-BN consist of graphite-like hexagonal layers [12–14]. During milling the hexagonal layers were broken to create new bonds. Unlike for Li–B–N, the XRD pattern for Li–Al–N was dominated by AlN reflections (Fig. 2). This may be due to the greater hardness of AlN when compared with h-BN. After milling for 18 h only AlN peaks were present in the XRD pattern. However, when the sample was subjected to extended milling for 48 h, new diffraction peaks that could not be indexed to either AlN or Li3 N were observed at ca. 32.1◦ , 48.5◦ and 53.7◦ , in addition to AlN peaks. The XRD pattern could not be assigned to the known Li3 AlN2 phase. In the case of Li–Ga–N, one new diffraction peak was present at ca. 54.2◦ alongside the GaN peaks in the XRD pattern after milling for both 18 and 48 h (Fig. 3). Once again, the XRD pattern did not match that of the known Li3 GaN2 phase. Hydrogen sorption studies were carried out on Li–B–N (milled for 18 h), Li–Al–N (milled for 48 h), and Li–Ga–N (milled for 18 h). Fig. 4 shows hydrogen absorption–desorption isotherms at 195 and 260 ◦ C for Li–Al–N. In the first cycle of measurements hydrogen absorption reached 2.4 wt.% for Li–Al–N at 195 ◦ C and ca. 30 bar (Fig. 4). Approximately 0.4 wt.% of the absorbed hydrogen was desorbed at 195 ◦ C. After 40 h elapsed the absorption–desorption cycle was terminated and a second cycle of measurements was carried out on the same sample under similar conditions. An additional 0.7 wt.% hydrogen was absorbed, part of which was desorbed at 195 ◦ C. At 260 ◦ C and ca. 30 bar a fresh Li–Al–N sample absorbed 3.7 wt.% hydrogen and desorbed 1.2 wt.% of the absorbed hydrogen at same temperature, indicating that some of the hydrogen was still strongly bound to the material. In comparison, at 195 ◦ C and ca. 30 bar, Li–Ga–N absorbed 1.5 wt.% hydrogen in the first
289
Fig. 4. Hydrogen absorption–desorption isotherms at 195 and 260 ◦ C for Li–Al–N.
cycle and 0.6 wt.% in the second cycle (Fig. 5). The absorbed hydrogen was only partially desorbed in the first cycle. Nevertheless, almost all hydrogen absorbed in the second cycle was desorbed before the 40 h absorption–desorption cycle measurement time elapsed. For both Li–Al–N and Li–Ga–N the P–C isotherms showed a large hysteresis. The hydrogen storage capacities obtained for Li–Al–N in this study are remarkable; for the first time, we observe hydrogen uptake by a pristine ternary nitride of Li and Al that was prepared from the binary nitrides of the elements. A previous study reported no hydrogen uptake by the known ternary nitride Li3 AlN2 synthesized from Li3 N and AlN [27]. However, Xiong et al. [28] reported that Li3 AlN2 reversibly stored 5 wt.% hydrogen. The Li3 AlN2 sample was generated after dehydrogenation of 2LiNH2 –LiAlH4 that had been previously ball milled. Hydrogen uptake by Li–B–N was close to zero at temperatures up to 300 ◦ C and a hydrogen pressure of ca. 30 bar (Fig. 6). Interestingly, the results obtained for Li–B–N corroborate those for hydrogenation of the known Li3 BN2 ternary nitride [27,29]. Pinkerton et al. [29] synthesized a new quaternary hydride by reacting LiNH2 and LiBH4 in a 2:1 molar ratio. The quaternary hydride released more than 10 wt.% hydrogen above 250 ◦ C and formed a mixture of Li3 BN2 polymorphs, which could not reabsorb any significant amount of hydrogen. It has also been shown that Li3 BN2 pre-
Fig. 5. Hydrogen absorption–desorption isotherms at 195 ◦ C for Li–Ga–N.
290
H.W. Langmi, G.S. McGrady / Journal of Alloys and Compounds 466 (2008) 287–292
binds to these ternary nitrides in a similar fashion, and one akin to that of Li3 N [3]. Consider Li–Al–N, which is in fact synthesized from equimolar amounts of Li3 N and AlN. For complete hydrogenation of this system the, following reaction is proposed: Li3 N–AlN + 2H2 → LiNH2 + 2LiH + AlN
(1)
Similarly, for the Li–Ga–N system the hydrogenation reaction may be written as Li3 N–GaN + 2H2 → LiNH2 + 2LiH + GaN
(2)
pared by high temperature solid-state synthesis does not absorb hydrogen at temperatures up to 300 ◦ C and 100 bar hydrogen [27]. Powder XRD patterns for all samples were measured after hydrogenation at 195 ◦ C, and it was noted that no new diffraction peaks emerged in the patterns. This implies that the hydrogenated phase within the Li–X–N system was amorphous, or that it existed as a nanophase. Xiong et al. [28] observed no significant change in the XRD pattern after hydrogenation of Li3 AlN2 at 200 ◦ C. Solid-state 1 H NMR spectra were also obtained for these samples (Fig. 7). The results revealed that the 1 H signal of hydrogenated Li–B–N was very weak, in agreement with the observed low hydrogen uptake capacity for the material. On the other hand, the signals for hydrogenated Li–Al–N and Li–Ga–N were quite strong, and the good resolution of the spectra is an indication of hydrogen mobility within the systems. The spectra also revealed that hydrogen existed in two distinct chemical environments in each case. The line intensities centered at −2.377 and −1.897 ppm for Li–Al–N–H and Li–Ga–N–H, respectively, are most likely associated with amide hydrogens of LiNH2 . The other line intensities centered at 2.763 and 2.748 ppm for Li–Al–N–H and Li–Ga–N–H, respectively, are then due to the presence of LiH. This implies that hydrogen
The theoretical hydrogen storage capacity obtained from reaction (1) is 5.3 wt.%. Hydrogen storage capacities observed for Li–Al–N were 2.4 and 3.7 wt.% at 195 and 260 ◦ C, respectively. Clearly, hydrogen uptake did not reach completion in these materials. It is likely that the newly generated Li–Al–N phase absorbed hydrogen and transformed into LiNH2 , LiH and AlN, while residual AlN did not absorb any hydrogen. It cannot also be ruled out that some Li3 N was present in an amorphous or nanocrystalline state in the Li–Al–N sample and, upon hydrogenation, reacted to generate LiNH2 and LiH. The powder XRD patterns provided compelling evidence for the presence of unreacted material. Though the ternary nitrides apparently store hydrogen in a similar manner to Li3 N and at comparable conditions, hydrogen storage capacities for the Li–X–N systems were considerably lower than that of Li3 N [3]. As shown by reaction (1) the Al component and some of the N atoms in the Li–Al–N system seemed reluctant to undergo hydrogenation and remained as AlN, which is thermodynamically very stable. AlN contributes substantially to the total mass of the system, yet makes zero contribution to its hydrogen uptake, leading to relatively lower wt.% hydrogen absorption by the ternary nitrides. If hydrogenation occurred in a similar manner to that observed for Li3 N, then the partial desorption of hydrogen by these ternary systems may be accounted for by the intermediate reaction that generates Li2 NH. Thus, dehydrogenation of LiNH2 at 195 and 260 ◦ C will generate Li2 NH, and this intermediate will not undergo further dehydrogenation at these temperatures, accounting for the relatively lower hydrogen uptake in the second cycle of measurements.
Fig. 7. 1 H MAS NMR spectra of hydrogenated Li–B–N, Li–Al–N and Li–Ga–N.
Fig. 8. Hydrogen absorption kinetic curves for Li–Al–N and Li–Ga–N over a series of temperatures from 160 to 295 ◦ C.
Fig. 6. Hydrogen absorption–desorption isotherms at 195 ◦ C for Li–B–N.
H.W. Langmi, G.S. McGrady / Journal of Alloys and Compounds 466 (2008) 287–292
291
to −39.6 kJ/mol H2 ). These enthalpy values can be compared with those for the hydrogenation reactions of the known Li3 AlN2 and Li3 N (Eqs. (3) and (4)) [28]. Li3 AlN2 + 2H2 → LiNH2 + 2LiH + AlN, H = −50.1 kJ/molH2
(3)
Li3 N + 2H2 → LiNH2 + 2LiH, ΔH = −80.5 kJ/molH2
Fig. 9. DSC plots of hydrogen absorption by Li–B–N, Li–Al–N and Li–Ga.
Fig. 8 presents kinetic plots for hydrogen absorption by Li–Al–N and Li–Ga–N over a series of temperatures from 160 to 295 ◦ C. The initial hydrogen pressure at 160 ◦ C was 30 bar. The sample was held for 2 h at each of the following temperatures: 160, 195, 230, 265 and 295 ◦ C, and its hydrogen storage capacity was monitored as a function of time. Clearly, hydrogen uptake in Li–Al–N generally exceeds that in Li–Ga–N. For both materials the rate of hydrogen absorption was greatest at 195 ◦ C. At higher temperatures the rate was relatively lower. This behaviour can be accounted for by the generation of the hydrogenated phase. Initially at 160 ◦ C the reaction rate was slow, as the temperature was not sufficiently high to enhance the reaction kinetically. As the temperature was increased to 195 ◦ C in the presence of many unreacted Li–X–N species, the rate of hydrogenation increased. However, as time progressed the number of Li–X–N species that were available for reaction diminished as the material became more saturated with hydrogen, and the hydrogenation rate decreased. The shapes of the kinetic curves at 160 ◦ C are noticeably different. An examination of the hydrogen absorption isotherms for both Li–Al–N and Li–Ga–N at 160 ◦ C clearly reveals the distinct nature of the absorption curves. Further study is needed to clarify this behaviour. Enthalpies of absorption were determined by DSC. Fig. 9 shows the DSC plots for hydrogen absorption by the ternary nitrides. It can be seen that the curve is different for each material. In the case of Li–B–N, DSC measurement did not show any heat evolution at temperatures up to 400 ◦ C, implying that Li–B–N is stable towards hydrogenation at these temperatures. This observation correlates very well with the near zero hydrogen uptake displayed by this material (Fig. 6). On the other hand, the heat flow for both Li–Al–N and Li–Ga–N revealed the occurrence of an exothermic event. An exothermic process attributed to the absorption of hydrogen started at about 75 ◦ C and peaked at 224 ◦ C for Li–Al–N. The exothermic hydrogen absorption reached a maximum at about 321 ◦ C for Li–Ga–N. Clearly, Li–Al–N absorbs most of its hydrogen at a relatively lower temperature than Li–Ga–N. The enthalpies of absorption for Li–Al–N and Li–Ga–N calculated from the DSC measurements are −77.1 kJ/mol for Li3 N–AlN (equivalent to −38.5 kJ/mol H2 ), and −79.2 kJ/mol for Li3 N–GaN (equivalent
(4)
On the basis of these values, it can be said that hydrogenation of the Li–Al–N and Li–Ga–N systems in this work is less thermodynamically favourable than hydrogenation of Li3 AlN2 or Li3 N. However, under comparable conditions dehydrogenation of hydrogenated Li–Al–N and Li–Ga–N ternary nitrides would be more favoured thermodynamically. This demonstrates the role played by the ball milling process as well as by the Group 13 nitrides (AlN and GaN) in modifying the thermodynamics of the Li3 N system. 4. Conclusions For the first time, ternary nitrides have been synthesized from Li3 N and the Group 13 nitrides (BN, AlN and GaN) by mechanical milling. The ternary nitrides obtained are not the known Li3 BN2 , Li3 AlN2 and Li3 GaN2 systems. The Li–B–N and Li–Ga–N phases were generated after milling for 18 h, while the Li–Al–N phase was generated after 48 h of milling. We have also observed, for the first time, hydrogen uptake by a pristine ternary nitride of Li and Al synthesized from the binary nitrides of the elements. Hydrogen storage capacity was greatest for Li–Al–N, reaching 3.7 wt.% (at 260 ◦ C and 30 bar) and near zero for Li–B–N. At 195 ◦ C and 30 bar, Li–Ga–N absorbed 1.5 wt.% hydrogen while Li–Al–N absorbed 2.4 wt.% hydrogen. Hydrogen uptake by these ternary nitrides was partially reversible, and the hydrogenation reaction appeared to be similar to that of Li3 N but the enthalpy of absorption was less exothermic for the ternary nitrides. Ternary nitrides are interesting materials that are under-explored as hydrogen storage materials, and warrant more attention. Acknowledgements The authors would like to thank Dr. Ulrike Werner-Zwanziger for carrying out solid-state NMR experiments. This research was supported by NSERC, CFI and HSM Systems Inc. References [1] M. Felderhoff, C. Weidenthaler, R. von Helmolt, U. Eberle, Phys. Chem. Chem. Phys. 9 (2007) 2643. [2] H.W. Langmi, G.S. McGrady, Coord. Chem. Rev. 251 (2007) 925. [3] P. Chen, Z. Xiong, J. Luo, J. Lin, K.L. Tan, Nature 420 (2002) 302. [4] Y.H. Hu, N.Y. Yu, E. Ruckenstein, Ind. Eng. Chem. Res. 44 (2005) 4304. [5] S. Isobe, T. Ichikawa, N. Hanada, H.Y. Leng, M. Fichtner, O. Fuhr, H. Fujii, J. Alloys Compd. 404–406 (2005) 439.
292
H.W. Langmi, G.S. McGrady / Journal of Alloys and Compounds 466 (2008) 287–292
[6] T. Ichikawa, N. Hanada, S. Isobe, H.Y. Leng, H. Fujii, J. Alloys Compd. 404–406 (2005) 435. [7] T. Ichikawa, S. Isobe, N. Hanada, H. Fujii, J. Alloys Compd. 365 (2004) 271. [8] S. Orimo, Y. Nakamori, G. Kitahara, K. Miwa, N. Ohba, T. Noritake, S. Towata, Appl. Phys. A 79 (2004) 1765. [9] Y. Nakamori, S. Orimo, Mater. Sci. Eng. B 108 (2004) 48. [10] T. Ichikawa, K. Tokoyoda, H. Leng, H. Fujii, J. Alloys Compd. 400 (2005) 245. [11] D.H. Gregory, Coord. Chem. Rev. 215 (2001) 301. [12] E. Zintl, G. Brauer, Z. Elektrochem. Angew. Phys. Chem. 41 (1935) 102. [13] A. Rabenau, H. Schulz, J. Less Common Met. 50 (1976) 155. [14] R.S. Pease, Acta Crystallogr. 5 (1952) 356. [15] P. Wang, S. Orimo, T. Matsushima, H. Fujii, G. Mayer, Appl. Phys. Lett. 80 (2002) 318. [16] P. Wang, S. Orimo, H. Fujii, Appl. Phys. A 78 (2004) 1235. [17] C. Tang, Y. Bando, X. Ding, S. Qi, D. Golberg, J. Am. Chem. Soc. 124 (2002) 14550.
[18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
T. Oku, M. Kuno, Diamond Relat. Mater. 12 (2003) 840. R. Juza, K. Langer, K. von Benda, Angew. Chem. Int. Edit. 7 (1968) 360. H. Yamane, S. Kikkawa, M. Koizumi, J. Power Sources 20 (1987) 311. V.J. Goubeau, W. Anselment, Z. Anorg. Allg. Chem. 310 (1961) 248. R.C. DeVries, J.F. Fleisher, Mater. Res. Bull. 4 (1969) 433. R. Juza, F. Hund, Z. Anorg. Allg. Chem. (1948) 13. H. Yamane, S. Kikkawa, M. Koizumi, J. Solid State Chem. 71 (1987) 1. H. Yamane, S. Kikkawa, M. Koizumi, Solid State Ionics 15 (1985) 51. Y. Nakamori, G. Kitahara, K. Miwa, S. Towata, S. Orimo, Appl. Phys. A 80 (2005) 1. H. Yamane, T. Kano, A. Kamegawa, M. Shibata, T. Yamada, M. Okada, M. Shimada, J. Alloys Compd. 402 (2005) L1. Z. Xiong, G. Wu, J. Hu, Y. Liu, P. Chen, W. Luo, J. Wang, Adv. Funct. Mater. 17 (2007) 1137. F.E. Pinkerton, G.P. Meisner, M.S. Meyer, M.P. Balogh, M.D. Kundrat, J. Phys. Chem. B 109 (2005) 6. J. Lu, Z.Z. Fang, J. Phys. Chem. B 109 (2005) 20830. Y. Kojima, M. Matsumoto, Y. Kawai, T. Haga, N. Ohba, K. Miwa, S. Towata, Y. Nakamori, S. Orimo, J. Phys. Chem. B 110 (2006) 9632.