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High Reversible Hydrogen Capacity of LiNH2/Li3N Mixtures Yun Hang Hu* and Eli Ruckenstein Department of Chemical Engineering, State University of New York at Buffalo, Amherst, New York 14260
Although Li3N can theoretically absorb as much as about 10 wt % hydrogen, its reversible hydrogen capacity is only about 5.5 wt % because only a fraction of the hydrogen absorbed can be desorbed at relatively low temperatures. In the present paper, it is shown that the addition of LiNH2 to Li3N constitutes an effective method to increase the reversible hydrogen capacity. It was found that the reversible hydrogen capacity of the LiNH2/Li3N mixture depends on its composition. A maximum reversible hydrogen capacity of 6.8 wt % could be achieved when the pre-added LiNH2 content in the LiNH2/Li3N mixture was between 28 and 50 mol %. Furthermore, the LiNH2/Li3N mixtures have fast hydrogenation kinetics and excellent cyclibility. 1. Introduction A low-cost hydrogen storage technology that provides a high storage capacity and fast kinetics is a critical factor in the development of a hydrogen economy for transportation. Solid-state storage is now considered the safest and most effective way of routinely handling hydrogen,1,2 and the attention is focused on metal hydrides,3 complex hydrides,4-7 nanotubes and -fibers,8-16 microporous metal-organic materials,17 and lithium nitride.18-21 A hydrogen storage technology which can economically carry enough hydrogen on-board a vehicle to enable a 300-mile vehicle range is critical to make hydrogen-powered automobiles competitive with traditional vehicles. Furthermore, the DOE mid-term target for on-board hydrogen storage material is 6 wt % reversible hydrogen capacity with fast kinetics at temperatures below 100 °C. At the present time, no existing hydrogen storage material meets this target. As early as 1910, Dafert and Miklauz reported that Li3N can absorb 10.4 wt % hydrogen to form Li3NH422 (Li3N + 2H2 ) Li3NH4) and Li3NH4 can decompose to release hydrogen. Furthermore, Ruff and Goeres reported that Li3NH4 is a mixture of LiNH2 and 2LiH.23 Therefore, Li3N can be a useful storage material. However, it did not attract attention for about a century probably because of the suspicion that it can generate NH3, which, indeed, is a thermodynamically favorable process at temperatures below 400 °C.18a However, recent experiments showed that no NH3 could be detected during the hydrogenation of Li3N and the dehydrogenation of hydrogenated Li3N.18,21 Furthermore, recent experiments demonstrated that an ultrafast reaction between NH3 and LiH enables LiH to capture the entire NH3 generated during hydrogenation and dehydrogenation.18a,19b Therefore, recently, Li3N started to attract attention as a material for hydrogen storage.18-21 However, a critical issue is that its reversible hydrogen capacity is only about 5.5 wt %. This occurs because LiNH2 and 2LiH, which are the products of Li3N hydrogenation, dehydrogenate in two steps: LiH + LiNH2 ) Li2NH + H2 and LiH + Li2NH ) Li3N + H2. The first step, which provides about 5.5 wt % hydrogen capacity, takes place easily even at temper* To whom correspondence should be addressed. Tel.: 716645-2911, ext. 2266. Fax: 716-645-3822. E-mail: yhu@ buffalo.edu.
atures below 200 °C, whereas the second step requires high temperatures (>400 °C). Fujii et al. used a 1:1 mixture of LiH and LiNH2 to evaluate, by temperatureprogrammed dehydrogenation (TPD), the dehydrogenation that occurs during the first step.19a They found that the mixture can release 5.5 wt % hydrogen during the temperature-programmed dehydrogenation process.19a In the present paper, the basic strategy for increasing the reversible hydrogen storage of Li3N is to change the composition of the hydrogenated Li 3N by adding LiNH2 to Li3N before hydrogenation. It was found that this method increases the reversible hydrogen capacity of Li3N up to 6.8 wt %. 2. Experiments 2.1. Material Design and Preparation. The molar ratio of LiNH2/LiH of the hydrogenated Li3N free of added LiNH2 is 0.5. Consequently only half of the LiH can release hydrogen during the first step LiH + LiNH2 ) Li2NH + H2 at reasonable temperatures. In contrast, in the LiNH2-preadded to Li3N, the LiNH2/LiH ratio can be adjusted. As the content of LiNH2 increases, the consumption of LiH generated by the hydrogenation of Li3N increases during the dehydrogenation process. Furthermore, when the LiNH2 added to Li3N reaches 50 mol %, the entire LiH generated by the hydrogenation of Li3N can release hydrogen via the first step (LiH + LiNH2 ) Li2NH + H2). Starting from this material design idea, LiNH2/Li3N mixtures with various LiNH2/ Li3N molar ratios were prepared, by mixing powders of LiNH2 and Li3N (about 80 mesh) with an agate mortar and pestle, by hand, in air for 5 min. The grinding of the sample in air (at room temperature) generated a small amount of LiOH on the surface layer of the sample. This was followed by its decomposition to Li2O on the surface layer of Li3N by heating in a vacuum at 230 °C. Furthermore, the sample was subjected to an in-situ pretreatment consisting of hydrogenation (at 230 °C for 24 h), followed by dehydrogenation (at 280 °C for 12 h) before the reversible hydrogen storage measurements. This partial oxidation and hydrogenationdehydrogenation pretreatment is important for the following reason. The hydrogenation of Li3N (Li3N + H2 ) Li2NH + LiH, ∆H ) -116 kJ/mol) is a highly exothermic reaction with fast kinetics, which generates hot spots and the sintering of Li3N. However, when a part of the Li3N surface is transformed into LiOH in
10.1021/ie0492799 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/04/2005
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Figure 1. Blank experiment for hydrogen storage when 0.25 g of quartz wool at 230 °C and 7 atm (initial hydrogen pressure) was employed.
air at room temperature and is further decomposed into Li2O in a vacuum at a higher temperature, Li2O covers most of the surface active sites of Li3N, resulting in the lowering of the hydrogenation rate. During the hydrogenation-dehydrogenation pretreatment, Li2O diffuses into the bulk of the sample and no longer covers the surface. In addition, Li3N is transformed into Li2NH after the hydrogenation-dehydrogenation pretreatment. In other words, the hydrogen absorption of the hydrogenation-dehydrogenation pretreated sample is provided by the reaction Li2NH + H2 ) LiNH2 + LiH, which has a much lower heat of reaction (-45 kJ/mol) than the hydrogenation of Li3N (-116 kJ/mol). In addition, the dispersed Li2O may play the role of a stabilizer. The combination between the lower reaction heat and the dispersion of Li2O prevented the pretreated material from sintering during the hydrogen absorption that followed. For comparison purposes, Li3N free of added LiNH2, the mechanical mixture of LiH/LiNH2 (1: 1), and the decomposed LiNH2, were also employed as hydrogen storage materials, and subjected to the grinding, decomposition, and hydrogenation-dehydrogenation pretreatment. 2.2. Hydrogen Absorption. It is important to accurately measure the H2 capacity during H2 adsorption. One of the conventional methods is the thermogravimetry, which determines the H2 capacity via the weight change. The major disadvantage of thermogravimetry is that the H2O impurity can lead to a significant error. This happens because the weight of a H2O molecule is equal to the weight of 9 H2 molecules. Even though the concentration of H2O impurity in H2 is usually as low as several ppm, the sample is usually kept in a H2 flow for a certain time to determine the weight change. As a result, the sample can adsorb a significant amount of H2O, particularly in small samples and during long-time measurements. For example, 0.5 wt % H2O adsorbed can be thought of as 4.5 wt % H2 capacity. In contrast, the volumetric method determines the pressure change of H2 during absorption in a closed chamber. As a result, the adsorption of H2O leads to less than 0.01 wt % error in the hydrogen capacity in the volumetric method. However, in the volumetric method, one must ensure that the unit is free of leakage. Our leakage test experiments showed that the pressure change in our volumetric equipment was 0.1 psi during 10 h, which means 0.02 wt % hydrogen capacity for 0.25 g of storage material. Furthermore, in our blank experiments, 0.25 g of quartz wool, which replaced the hydrogen storage material in the volumetric test unit, produced 0.06 wt % hydrogen capacity at 230 °C and 7 atm (see Figure 1). Because the quartz wool cannot absorb hydrogen,
Figure 2. Effect of LiNH2/Li3N composition on reversible H2 capacity at 230 °C and 7 atm initial hydrogen pressure (the samples were previously subjected to a hydrogenation-dehydrogenation pretreatment before the determination of the reversible hydrogen capacity).
it’s 0.06 wt % hydrogen capacity is the equipment error, which is small enough for our hydrogen storage experiments. The hydrogen absorption by a sample was determined with the volumetric method described in a previous paper.18b A solid sample (0.25 g) was loaded into a reactor located inside an electrical tubular furnace. A solid storage material (0.25 g) was loaded into 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 the 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). The change of H2 pressure during absorption was determined using the digital pressure gauge, which could detect changes in pressure as small as 0.007 atm. The same initial H2 pressure of 7 atm was used in all absorption experiments, and only the final equilibrium pressures were different when the samples had different adsorption capacities. To examine the effect of hydrogen absorption-desorption cycles, the hydrogenated sample was exposed to vacuum to desorb the hydrogen at 230 °C for 12 h (unless otherwise mentioned), followed by reabsorption. An on-line mass spectrometer was used to confirm that, except hydrogen, no other compounds were present during hydrogenation and dehydrogenation. It should be noted that because the temperature was measured outside the reactor, the reaction temperature did not account for the hot spots generated during absorption. The reversible hydrogen capacity was determined as the amount of hydrogen absorbed after the sample was subjected to the hydrogenation-dehydrogenation pretreatment. The hydrogen capacity is defined as the percentage of hydrogen absorbed based on the total weight of the solid sample before any treatment. 2.3. X-ray Powder Diffraction (XRD). The X-ray powder diffractions of LiNH2-added Li3N samples, before and after hydrogenation, were determined with a Siemens D500 X-ray diffraction 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. 3. Results and Discussion Reversible hydrogen capacity of LiNH2/Li3N was determined by the volumetric method at 230 °C. As shown in Figure 2, the reversible hydrogen capacity of
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Figure 5. Effect of cycles on reversible H2 capacity in 28 mol % LiNH2/Li3N at 230 °C and 7 atm initial hydrogen pressure.
Figure 3. XRD-patterns of Li3N and LiNH2/Li3N: (a) LiNH2/Li3N (28 mol % LiNH2); (b) after hydrogenation of (a); (c) after dehydrogenation of (b); (d) after dehydrogenation of hydrogenated Li3N. (Note, two types of Li3N phases were detected.)
Figure 4. Comparison between various samples for hydrogenation at 230 °C and 7 atm (initial hydrogen pressure): (a) 28 mol % LiNH2/Li3N previously subjected to a hydrogenation-dehydrogenation cycle; (b) Li3N previously subjected to a hydrogenationdehydrogenation cycle; (c) LiH/LiNH2 (1:1) mixture; (d) dehydrogenated LiNH2 in a vacuum at 280 °C for 12 h.
LiNH2/Li3N was strongly dependent on its composition. When the amount of added LiNH2 was above 50 mol %, the amount of reversible hydrogen increased with increasing Li3N content. When the amount of added LiNH2 was less than 50 mol %, but larger than 28 mol %, the reversible hydrogen capacity remained almost constant at 6.8 wt %. Even for 14 mol % LiNH2, the reversible hydrogen capacity could still reach 6.6 wt %. The reversible hydrogen capacity of Li3N free of added
LiNH2 was 5.7 wt %. According to theoretical calculations based on the assumption that the reversible hydrogen was generated just via the first step (LiH + LiNH2 ) Li2NH + H2), the highest reversible hydrogen capacity should be 6.85 wt %, which can be reached only when Li3N is mixed with LiNH2 at a mole ratio of 1:1, because at this composition, the total number of moles of LiNH2 added plus generated during the Li3N hydrogenation becomes equal to the number of moles of LiH generated through Li3N hydrogenation. However, the experimental results differed from this theoretical prediction. When Li3N was mixed with 28 mol %LiNH2, the theoretical reversible hydrogen capacity was 6.3 wt %, whereas its real capacity was 6.8 wt %. Furthermore, when Li3N was mixed with 14 mol % LiNH2, its theoretical reversible hydrogen capacity was 6 wt %, whereas the real one was 6.6 wt %. A reversible hydrogen capacity higher than predicted implies that besides the hydrogen produced through the first-dehydrogenation step (LiH + LiNH2 ) Li2NH + H2), additional reversible hydrogen was generated through the second-hydrogenation step (LiH + Li2NH ) Li3N + H2). To verify this possibility, XRD was employed to examine the hydrogenated LiNH2/Li3N mixture before and after dehydrogenation. The XRD patterns revealed that the hydrogenated Li3N, which was mixed with 28 mol % LiNH2, consisted of LiH and LiNH2 (Figure 3b). After dehydrogenation, both LiH and LiNH2 disappeared from the XRD patterns (Figure 3c). This is an interesting result. If the hydrogen desorption occurs just via the first step (LiH + LiNH2 ) Li2NH + H2), the LiNH2 generated through the hydrogenation of Li3N should consume only 50% of the LiH produced, because the number of moles of LiH generated via the hydrogenation of Li3N (Li3N + 2H2 ) LiNH2 + 2LiH) is twice that of LiNH2. Furthermore, the total amount of LiNH2 added and generated through the Li3N hydrogenation of a 28 mol % LiNH2/Li3N mixture consumes only 78% of the total LiH generated, if the hydrogen desorption occurs only through the first step (LiH + LiNH2 ) Li2NH + H2). In other words, 22% of LiH generated via the hydrogenation of the 28 mol % LiNH2/Li3N mixture should remain after the hydrogen desorption, if the hydrogen desorption occurs only through the first step. A possible explanation for the LiH disappearance in the XRD pattern of the hydrogenated 28 mol % LiNH2/Li3N mixture after hydrogen desorption is that, after the first dehydrogenation step (LiH + LiNH2 ) Li2NH + H2), the remaining LiH becomes highly dispersed in the Li2NH phase, the reason for which the XRD could not
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Literature Cited
Figure 6. Rehydrogenation of a 28 mol % LiNH2-added Li3N at 230 °C and 7 atm initial hydrogen pressure (after 0.5, 1, 3, and 12 h dehydrogenation of hydrogenated LiNH2/Li3N, at 230 °C, respectively).
detect its presence. The highly dispersed LiH reacted with Li2NH at their interface to generate hydrogen and Li3N. Because the interface between LiH and Li2NH is small, only small amounts of hydrogen and Li3N could be produced through the second-dehydrogenation step. Consequently, the amount of Li3N regenerated during the second-dehydrogenation step is not only small but also highly dispersed in the Li2NH phase, so that XRD could not detect its presence. As shown in Figure 4, a 28 mol % LiNH2/Li3N mixture absorbed 6.0 wt % hydrogen in only 7 min at 230 °C. After 40 min, the hydrogen capacity became 6.5 wt % and finally 6.8 wt %. In contrast, under the same reaction conditions, LiNH2 free of Li3N, which was previously subjected to dehydrogenation in a vacuum at 280 °C for 12 h, could just achieve 1 wt % reversible hydrogen capacity in 7 min and a final capacity of only 2.3 wt %. Furthermore, although Li3N free of added LiNH2 has a faster absorption rate and a higher hydrogen capacity than LiNH2 free of Li3N (about 2 wt % capacity) and the mechanical mixture of LiH/LiNH2 (1:1) (about 3.8 wt % capacity), its hydrogen capacity is still lower than that of the LiNH2-added Li3N. This indicates that the LiNH2-added Li3N has both high reversible hydrogen capacity and fast absorption kinetics. Usually, the low cyclibilty is a critical issue for most hydrogen storage materials. However, one can see from Figure 5 that, during 4 absorption-desorption cycles, the absorption curves coincided with each other. This observation shows that LiNH2-added Li3N has a high stability for hydrogen storage. Furthermore, we also evaluated the dehydrogenation by determining the rehydrogenation. Figure 6 shows that 62% of the total reversible hydrogen could desorb in only 30 min and near 80% after 60 min from the hydrogenated LiNH2added Li3N. This indicates that this material has also a reasonable dehydrogenation kinetics. 4. Conclusion In conclusion, the mixture of Li3N with LiNH2 provides a highly effective hydrogen storage material, being able to store as much as 6.8 wt % reversible hydrogen with ultrafast hydrogenation kinetics and excellent cyclibility, but the temperatures used are higher than the DOE target.
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Received for review August 10, 2004 Revised manuscript received December 15, 2004 Accepted December 17, 2004 IE0492799
