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MATERIALS AND INTERFACES Highly Effective Li2O/Li3N with Ultrafast Kinetics for H2 Storage Yun Hang Hu* and Eli Ruckenstein† Department of Chemical Engineering, State University of New York at Buffalo, Amherst, New York 14260
The refueling time and safe hydrogen storage are critical factors in the development of a hydrogen economy for transportation. The finding of a fast and high-capacity H2 storage material represents a worldwide challenge. The short-term target of the U.S. Department of Energy for a hydrogen storage material is a 4.5 wt % reversible hydrogen capacity. In this paper, we report that reversible 5 wt % H2 can be reached in only 3 min even at the relatively low temperature of 180 °C by a Li2O/Li3N solid material. Furthermore, this material has excellent stability because successive cycles of absorption-desorption did not decrease the reversible hydrogen capacity and absorption rate. 1. Introduction
2. The Basic Idea
Hydrogen is viewed as a promising clean fuel of the future. The refueling time, the safe hydrogen storage, and the handling facilities are critical factors in the development of a hydrogen technology for transportation. An effective hydrogen storage technology is required to make this source of energy economically viable. The technologies to store hydrogen can be classified into three classes, which involve liquefying hydrogen, compressing hydrogen, and hydrogen absorption/adsorption in a solid material. The former two are unacceptable for transportation because the liquid hydrogen is too expensive and the compressed hydrogen requires a too large space.1 Therefore, the attention was focused on solid storage materials, which include metal hydrides,2,3 complex hydrides,4-7 nanotubes and fibers,8-16 microporous metal-organic materials,17 and lithium nitride.18-21 A hydrogen storage technology that can carry enough hydrogen onboard a vehicle to enable a 300-mile vehicle range is critical for the hydrogen fuel initiative, which President George W. Bush announced in his 2003 State of the Union Address. Furthermore, the short-term target of the U.S. Department of Energy for onboard hydrogen storage material is a 4.5 wt % reversible hydrogen capacity with fast kinetics. At the present time, no existing hydrogen storage material meets these targets to make the hydrogen-powered automobiles competitive with the traditional vehicles. Therefore, new ideas are required to find an effective storage material with a high capacity and fast kinetics as well as high stability. Here, we report that a Li2O/ Li3N ultrafast H2 storage material has a high reversible hydrogen capacity and high stability even at relatively low temperatures.
A critical factor, which must be taken into account in the design of an ultrafast hydrogen storage material, is its stability, because being an exothermic reaction, the hydrogen absorption with fast kinetics can lead to the generation of hot spots. Our experiments have shown that Li3N has an ignition temperature of about 180 °C. However, as soon as the reaction was ignited, the material acquired a temperature above 400 °C, and this jump in temperature resulted in its deactivation for the reabsorption of hydrogen because of sintering. To avoid this deactivation, our idea was to partially oxidize the surface layer of the Li3N material, followed by a hydrogenation-dehydrogenation pretreatment. Compared with the pure Li3N, the partially oxidized Li3N will have a lower hydrogenation rate because Li2O formed in the surface layer (which cannot be hydrogenated) will cover most of the active sites of Li3N, thus reducing the rate of hydrogenation. The slowing down of the hydrogenation will significantly reduce the temperature jump and hence will prevent the sintering of the material. After a long hydrogenation of Li2O/Li3N, the material will be subjected to dehydrogenation. This hydrogenation-dehydrogenation pretreatment will generate rearrangements, which will stimulate the migration of Li2O from the surface layer to the bulk of Li3N. As a result, Li2O will no longer cover the Li3N surface and the fast hydrogen absorption of Li3N will be recovered. The dispersion of Li2O in the bulk will protect Li3N from sintering. Consequently, after pretreatment, the material will recover the fast kinetics and the hydrogen absorption will be stabilized. On the basis of this material design idea, we carried out the experiments described below.
* To whom correspondence should be addressed. Tel.: (716) 645-2911, ext. 2266. Fax: (716) 645-3822. E-mail: yhu@ buffalo.edu. † Tel.: (716) 645-2911, ext. 2214. Fax: (716) 645-3822. E-mail:
[email protected].
3. Experimental Section Li3N was first partially oxidized by its exposure for 30 min to air to absorb H2O, followed by heating in a vacuum to 230 °C for the decomposition of Li-H2O to Li2O and H2. Then, the material was pretreated with hydrogen at 230 °C for at least 48 h and dehydrogenated
10.1021/ie049947q CCC: $27.50 © 2004 American Chemical Society Published on Web 04/10/2004
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Figure 1. Volumetric measurement unit.
at 280 °C for 24 h to ensure the dispersion of Li2O. The obtained material, which contained about 17 wt % Li2O (estimated from X-ray diffraction, XRD), will be further denoted as Li-N-O. It is worth noting that the exposure time to wet air is critical. Both too long and too short exposure times are not satisfactory. Powder XRD was carried out with a Siemens D500 XRD instrument, equipped with a Cu KR source, at 40 kV and 30 mA. During the measurement, the sample was covered with a plastic film to prevent its oxidation by air. The crystal sizes of the various phases in the samples were estimated from the widths at half peaks of XRD. To accurately examine the hydrogen absorption by Li-N-O, we have employed a volumetric method (Figure 1), which can be described as follows: A solid storage material (0.25 g) was loaded in a reactor located inside an electrical tubular furnace. The reservoir was filled with H2. The pressure of H2 in the reservoir was measured using a digital pressure gauge with two cutoff valves closed at both ends of the reservoir. The cutoff valve between the reservoir and the reactor containing the sample was opened to allow H2 to enter into the reactor, which was heated to a selected reactor temperature (which does not account for the hot spots formed during the hydrogenation). To examine the effect of the hydrogen absorption-desorption cycles, the hydrogenated sample was exposed to a vacuum to desorb the hydrogen at 280 °C for 3-12 h, followed by reabsorption. An online mass spectrometer (HP quadrupole mass selective detector) equipped with a fast-response inlet capillary system was used to confirm that, except hydrogen, no other compounds were present during the hydrogenation and dehydrogenation. The hydrogen capacity was calculated on the basis of Li-N-O.
Figure 2. Hydrogen absorption by Li-N-O. Conditions: initial pressure ) 7 atm and final pressure ) 4 atm, Li3N ) 0.25 g.
Figure 3. Stability of Li-N-O for H2 absorption at 198 °C. Conditions: initial pressure ) 7 atm and final pressure ) 4 atm, Li3N ) 0.25 g.
Figure 4. Hydrogen absorption versus reaction time at 198 °C and 7 atm (initial pressure): (a) the first absorption by fresh Li3N without any treatment; (b) the second absorption after desorption of the 24 h hydrogenated Li3N; (c) hydrogen absorption by Li3N treated by hydrogenation-dehydrogenation at 230 °C without previous partial oxidation; (d) hydrogen absorption by the Li3N partially oxidized by its exposure to air for 30 min, followed by heating to 200 °C at the rate of 5 °C/min in a vacuum without hydrogenation-dehydrogenation pretreatment.
4. Results and Discussion As shown in Figure 2, the Li-N-O material could reach 5 and 5.2 wt % hydrogen capacities in only 3 min at 180 and 198 °C, respectively. Furthermore, we found that during six absorption-desorption cycles the absorption curves coincided with each other at 198 °C (Figure 3). This indicates that the Li-N-O material possesses not only an ultrafast kinetics but also a high stability for hydrogen storage. In contrast, at the same pressure, we found that magnesium, which is the bestknown metal for hydrogen storage, can hardly absorb any H2 at temperatures below 300 °C. Furthermore, as shown in Figure 4, although the freshly pure Li3N could initially absorb 6.5 wt % hydrogen in 20 min, the hydrogen capacity dropped to 4 wt % during the second absorption after the first cycle. This indicates that the pure Li3N has a low stability. Figure 4 also shows that, for either the pure Li3N pretreated by hydrogenation-
dehydrogenation or the partially oxidized Li3N without pretreatment, the hydrogen absorption was as low as 1 wt % at 198 °C. This indicates that only the combination of the partial oxidation with the hydrogenation-dehydrogenation pretreatment could make Li3N active and stable for hydrogen absorption. Furthermore, the much faster hydrogen absorption by the partially oxidized Li3N subjected to pretreatment rather than free of the latter process implies that Li2O diffused from the surface into the bulk of the sample during the pretreatment. The estimation of crystal sizes from XRD patterns indicated that the average crystal size of the fresh Li3N was about 42 nm, while the average crystal sizes of various phases in the Li-N-O sample before and after hydrogenation were below 38 nm. The average size of Li2O in Li-N-O was about 20 nm.
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Figure 5. XRD pattern of Li-N-O: (a) after hydrogen absorption; (b) after hydrogen desorption.
Dafert and Miklauz indicated that the reaction between Li3N and H2 generated Li3NH4,18 which is a mixture of 2LiH and LiNH2.19
Li3N + 2H2 ) Li3NH4 Furthermore, it was found that the dehydrogenation of the hydrogenated Li3N generated Li2NH and LiH at temperatures below 350 °C.18,20 Therefore, after the hydrogenation-dehydrogenation pretreatment, LiO-N became a mixture of Li2O and Li3NH2 (which, in turn, is a mixture of Li2NH and LiH), with very good contacts between them. Therefore, starting from LiN-O, it is likely that most of the absorption and desorption has taken place via the reversible reaction Li3NH2 + H2 ) Li3NH4. Indeed, as shown by XRD patterns (Figure 5), Li-N-O consists of Li3NH4 (LiH and LiNH2 mixture) and Li2O after hydrogen absorption and Li3NH2 (LiH and Li2NH mixture) and Li2O after desorption. By assuming that the reversible reaction (Li3NH2 + H2 ) Li3NH4) is responsible for the hydrogen capacity of Li-N-O and taking into account that the latter contains 17 wt % Li2O, one obtains that the H2 capacity is 4.6 wt % if Li2O does not adsorb hydrogen. The about 0.5 wt % higher hydrogen capacity obtained experimentally indicates that, besides the main reaction Li3NH2 + H2 ) Li3NH4, other reactions, probably catalyzed by Li2O, have also taken place. Therefore, Li2O may play not only the role of a stabilizer but also the role of a catalyst. It should be noted that Li in Li3N-based hydrogen storage materials is easily oxidized by H2O or O2 at the temperatures required for hydrogen absorption or desorption. Consequently, even trace amounts of H2O and O2 impurities in H2 can reduce the hydrogenation storage capacity during the long cycling operations required in practice. 5. Conclusions In conclusion, the partially oxidized Li3N associated with a relatively high-temperature hydrogenationdehydrogenation pretreatment is highly effective for hydrogen storage with fast kinetics and high reversible hydrogen capacity as well as excellent stability. These
important characteristics could make this material attractive for its use in fuel cell vehicles. Acknowledgment Bo Hu, who was a summer intern, participated in this work. Literature Cited (1) Trudeau, M. L. Advanced materials for energy storage. MRS Bull. 1999, 24, 23. (2) Genossar, J.; Rudman, P. S. Catalytic role of Mg2Cu in the hydriding and dehydriding of Mg. Z. Phys. Chem., Neue Folge 1979, 116, 215. (3) Buchner, H. Energiespeichrung in Metallhydriden; SpringerVerlag: Wien, Austria, 1982. (4) Wiswall, R. In Hydrogen in Metals II; Alefeld, G., Vo¨lkl, J., Eds.; Springer-Verlag: Wien, Austria, 1978; p 201. (5) Bogdanovic, B.; Schwickardi, M. Ti-doped alkali metal aluminium hydrides as potential novel reversible hydrogen storage materials. J. Alloys Compd. 1997, 253, 1. (6) Bogdanovic, B.; Sandrock, G. Catalyzed complex metal hydrides. MRS Bull. 2002, 27, 712. (7) Gross, K. J.; Thomas, G. J.; Jensen, C. M. Catalyzed alanates for hydrogen storage. J. Alloys Compd. 2002, 330, 683. (8) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethuune, D. S.; Heben, M. J. Storage of hydrogen in single-walled carbon nanotubes. Nature 1997, 386, 377. (9) Ye, Y.; Ahn, C. C.; Witham, C.; Fultz, B.; Liu, J.; Rinzler, A. G.; Colbert, D.; Smith, K. A.; Smalley, R. E. Hydrogen adsorption and cohesive energy of single-walled carbon nanotubes. Appl. Phys. Lett. 1999, 74, 2307. (10) Liu, C.; Fan, Y. Y.; Cheng, H. M.; Dresselhaus, M. S. Hydrogen storage in single-walled carbon nanotubes at room temperature. Science 1999, 286, 1127. (11) Yang, R. T. Hydrogen storage by alkali-doped carbon nanotubes-revisited. Carbon 2000, 38, 623. (12) Hirscher, M.; Becher, M.; Haluska, M.; Dethlaff-Weglikowska, U.; Quintel, A.; Duesberg, G. S. Hydrogen storage in sonicated carbon materials. Appl. Phys. A 2001, 72, 129. (13) Dresselhaus, M. S.; Williams, K. A.; Eklund, P. C. Hydrogen adsorption in carbon materials. MRS Bull. 1999, 24, 45. (14) Ma, R.; Bando, Y.; Zhu, H.; Sato, T.; Xu, C.; Wu, D. Hydrogen uptake in boron nitride nanotubes at room temperature. J. Am. Chem. Soc. 2002, 124, 7672. (15) Chen, J.; Kuriyama, N.; Yuan, H.; Takeshita, H. T.; Sakai, T. Electrochemical hydrogen storage in MoS2 nanotubes. J. Am. Chem. Soc. 2001, 123, 11813. (16) Chen, J.; Li, S. L.; Tao, Z. L.; Shen, Y. T.; Cui, C. X. Titanium disulfide nanotubes as hydrogen-storage materials. J. Am. Chem. Soc. 2003, 125, 5284.
Ind. Eng. Chem. Res., Vol. 43, No. 10, 2004 2467 (17) Rosi, N. L.; Eckert, J.; Eddaoudi, M.; Vodak, D. T.; Kim, J.; O’Keefe, M.; Yaghi, O. M. Hydrogen storage in microporous metal-organic frameworks. Science 2003, 300, 1127. (18) Dafert, F. W.; Miklauz, R. New compounds of nitrogen and hydrogen with lithium. Monatsh. Chem. 1910, 31, 981. (19) Ruff, O.; Goeres, H. Li imide and some compounds of N, H and Li. Ber. Dtsch. Chem. Ges. 1910, 44, 502. (20) Hu, Y. H.; Ruckenstein, E. H2 storage in Li3N. Temperature-programmed hydrogenation and dehydrogenation. Ind. Eng. Chem. Res. 2003, 42, 5135.
(21) Hu, Y. H.; Ruckenstein, E. Ultrafast reaction between LiH and NH3 during H2 storage in Li3N. J. Phys. Chem. A 2003, 107, 9737.
Received for review January 16, 2004 Revised manuscript received March 1, 2004 Accepted March 8, 2004 IE049947Q
