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The effect of exposure of the H-storage system (LiNH2 + MgH2) to water-saturated air
Journal of Alloys and Compounds 440 (2007) L13–L17
Letter
The effect of exposure of the H-storage system (LiNH2 + MgH2) to water-saturated air S. Luo a , Ted B. Flanagan a,∗ , W. Luo b a
Chemistry Department, University of Vermont, Burlington, VT 05401, USA b Sandia National Laboratory, MS 9403, Livermore, CA 94550, USA
Received 16 June 2006; received in revised form 31 August 2006; accepted 2 September 2006 Available online 18 October 2006
Abstract The H-storage system, (LiNH2 + MgH2 ), has been exposed to water-saturated air and its subsequent rates of H2 desorption and absorption have been determined. Not only are there no adverse effects of the exposures, but also there is a small positive effect on the rates and final wt% H. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydrogen absorbing materials; Kinetics
1. Introduction The Li–Mg–N–H system has been shown to have promise as a hydrogen-storage system because of its high wt% H, i.e., 9%, and its decomposition pressure of 2.8 MPa at 200 ◦ C [1–7]. A ball-milled mixture of (2LiNH2 + MgH2 ) is a specific example of a Li–Mg–N–H system which converts to (Mg(NH2 )2 + 2LiH) at, e.g., 220 ◦ C and a H2 pressure of 10 MPa [6]. This conversion however generally takes place during the initial desorption/absorption cycle. The subsequent reversible storage reaction is that shown in reaction (1), which does not involve the starting reactants. Mg(NH2 )2 + 2LiH = Li2 Mg(NH)2 + 2H2 .
(1)
In this paper, the storage material will be referred to as the (LiNH2 + MgH2 ) storage system with the understanding that the reversible storage process is given by reaction (1). Generally, this system can reversibly store about 5.4 wt% H and has an estimated plateau pressure of 1 MPa at 140 ◦ C [4]. It shows very encouraging cycle stability with a 0.005 wt% loss/cycle over 100 cycles [5]. The components are inexpensive and can be made in several different ways. In this work, the effect of exposure to water-saturated air (H2 O-sat.air) on the subsequent H-storage properties of this system will be reported. ∗
Corresponding author. Tel.: +1 802 656 0199; fax: +1 802 656 8705. E-mail address: [email protected] (T.B. Flanagan).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.09.026
This contaminant has been chosen because it is the most likely gaseous contaminant during routine handling. If it were to be only slightly or not affected at all by H2 O-sat.air, its ease of handling would greatly increase its usefulness as an H storage system. 2. Experimental The initial materials, LiNH2 and MgH2 , were purchased from Aldrich and used without any pre-treatment. The LiNH2 :MgH2 ratio was 2:1.1, i.e, a 10% excess of MgH2 was added so that the amide did not undergo decomposition to NH3 (g) during cycling [7]. A 30 g mixture of LiNH2 and MgH2 was mechanically milled in a glove box under an argon atmosphere using a Fritsch mill with 40 WC milling balls each weighing 8 g. The milling conditions were 5 repetitions at 300 rpm (30 min milling interspersed with periods of rest time of 90 min) then 12 repetitions at 350 rpm (30 min milling interspersed with periods of rest time of 90 min) followed by 50 repetitions at 350 rpm (30 min milling interspersed with a periods of rest time totaling 150 min). The purpose of alternating the milling with rest times is to keep the sample from being overheated during the milling. The apparatus for the H2 desorption/absorption measurements were a standard Sieverts apparatus with a Setra pressure gauge. Generally, the kinetic runs were determined at 220 ◦ C. This temperature was chosen because it had been previously found that the kinetics are reasonably fast but not so fast as to obscure comparisons of the kinetics before and after exposure to H2 O-sat.air. For desorption, the furnace must be raised around the reaction vessel and the temperature increased from 25 to 220 ◦ C and therefore there is a time delay before the sample reaches 220 ◦ C. There was no time delay for absorption because H2 was admitted at 220 ◦ C following a previous H2 desorption. The ball-milled (2LiNH2 + MgH2 ) mixture yielded two samples, which were loaded under an argon atmosphere in a high quality glove box. Sample 1 was employed directly for the studies of the effects of H2 O-sat.air. Sample 2
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S. Luo et al. / Journal of Alloys and Compounds 440 (2007) L13–L17
desorbed/absorbed H2 three times after which it was “soaked” in H2 at ∼11.8 MPa at 220 ◦ C for 144 h prior to the determination of further desorption/absorption kinetics. The “soaking” was to ensure complete conversion to Mg(NH2 )2 and LiH [5]. After this “soaking”, Sample 2 was subjected to a desorption/absorption cycle, evacuated, and then the contamination studies were undertaken. The samples were exposed to H2 O-sat.air by slowly bubbling air through liquid water into the initially evacuated reaction vessel at room temperature. After a 10 min exposure to the H2 O-sat.air, H2 was introduced into the reaction vessel at room temperature and then evacuated three times in order to flush out any gaseous impurities. The temperature was increased to 220 ◦ C and the kinetics determined. Sample 1 was exposed to wet air (indicated as H2 O-sat.air) for 10 min at room temperature prior to the H2 desorption. A desorption run was carried out after being evacuated. These are the only data obtained for Sample 1. Most data were taken using Sample 2 and as an aid in the interpretation of the results a flow chart for Sample 2 is shown below giving the sequence of kinetic runs.
Note:
desorption at 220 ◦ C was then determined and the result is shown in Fig. 1 where it is compared with similar results for Sample 2 prior to its exposure to H2 O-sat.air. It can be seen (Fig. 1) that there is no adverse effect of the exposure to H2 O-sat.air and, in fact, there is a small acceleration of the rate. This is a very encouraging result and it leads to the more detailed study as described below using Sample 2. 3.2. Effect of exposure to H2 O-sat.air on H2 absorption by Sample 2 Sample 2 was subjected to four absorption/desorption cycles (220 ◦ C) before any exposures to H2 O-sat.air; the kinetics were
3. Results and discussion
found to be quite reproducible. After the fourth desorption (see flow chart), the sample was exposed to H2 O-sat.air (25 ◦ C) and the subsequent absorption (fifth) (220 ◦ C) was found to be quite fast, i.e., the exposure had no adverse effects. After the fifth desorption, the sample was re-exposed to H2 O-sat.air and the sixth absorption kinetics were determined. The same procedure was repeated for the seventh absorption. All of these absorption runs had faster kinetics than before admitting the H2 O-sat.air. A comparison of absorption profiles before and after an exposure to air is given in Fig. 2.
3.1. Effect of exposure of (2LiNH2 + MgH2 ) to H2 O-sat.air on H2 desorption
3.3. Effect of exposure to H2 O-sat.air on H2 desorption by Sample 2
Sample 1 was evacuated at room temperature and then exposed to H2 O-sat.air for 10 min. Following this exposure treatment, it was flushed with H2 and evacuated. The initial H2
After Sample 2 had absorbed H (sixth and seventh absorptions), it was cooled and evacuated at room temperature (25 ◦ C). The sample was then exposed to H2 O-sat.air at 25 ◦ C and the
• For desorption, the sample was always heated from room temperature to 220 ◦ C which required about 14 min. • All absorption measurements were carried out at 220 ◦ C. At the completion of absorption, the sample was cooled to room temperature under hydrogen pressure. The sample was evacuated before the subsequent desorption. • All H2 O-sat.air was introduced into the system at room temperature (RT). • The box labeled “H2 O + air” indicates treatment with water-saturated air as described in Section 2.
S. Luo et al. / Journal of Alloys and Compounds 440 (2007) L13–L17
Fig. 1. Comparison of initial desorption kinetics of 2LiNH2 + MgH2 (220 ◦ C) during the first hour for Sample 2 (dashed blue line) and Sample 1 (heavy red solid line). The light black solid line at the top is the sample temperature over the experimental period. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 2. Comparison of absorption kinetics (220 ◦ C) for Sample 2 over the first 2 h before (fourth absorption, dashed blue line) and after introduction of H2 Osat.air (fifth, sixth and seventh absorptions, solid red lines). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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Fig. 3. Comparison of desorption kinetics (220 ◦ C) for the first hour before (fourth desorption, dashed blue line) and after (sixth and seventh desorptions, heavy red solid lines) introduction of H2 O-sat.air. The thin solid line at the top is the sample temperature over the experimental period. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
subsequent desorption results at 220 ◦ C (Fig. 3) were analogous to those shown in Fig. 2, i.e., there were no adverse effects due to the exposure. It can also be seen from Fig. 3 that the kinetics are faster after exposure to H2 O-sat.air than before. The results are repeatable (see the overlap of solid lines for the sixth and seventh runs). Furthermore, the enhanced rates for either absorption or desorption found following exposure to H2 O-sat.air persists without further exposures as shown in Figs. 4–5. In addition, the final wt% H is slightly enhanced (4.8 wt%) after exposure to the H2 O-sat.air as shown in Figs. 6 and 7. It can be seen that there appears to be a small shift of the subsequent desorption profiles towards the one before the H2 O-sat.air exposure, however, the subsequent ones are still faster than before the exposure and they remain significantly faster especially at longer reaction times as shown in Fig. 6 for absorption and Fig. 7 for desorption. For many subsequent cycles after exposure to H2 O-sat.air, the rates over long times remain faster than before exposure. It should be noted that 16 absorptions and 15 desorptions were carried out with Sample 2 with no indication of any degradation and, in fact, the wt% H increased slightly, 4.6–4.8 wt%. This suggests that the 0.005 wt% loss noted above [5] may not be applicable to all sample preparations. A sample from another ball-milled batch was exposed to dry air at room temperature and at 220 ◦ C and it was found that subsequent H2 absorption/desorption was not affected although there was no clear indication of an increase in rate as found for the H2 O-sat.air exposure. Hu and Ruckenstein [8] investigated the Li2 O/Li3 N system for H storage purposes. They prepared the system by exposing
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Fig. 4. Comparison of absorption kinetics (220 ◦ C) before (fourth absorption, dashed blue line) and after (seventh absorption, heavy red solid line) introduction of H2 O-sat.air. The light solid green lines are for subsequent (eighth, ninth and tenth) absorptions without further introduction of H2 O-sat.air. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 5. Comparison of desorption kinetics (220 ◦ C) during the first hour before (fourth desorption, blue dashed line) and after (sixth desorption, heavy solid red line) introduction of H2 O-sat.air. The thin solid green lines are for the subsequent (eighth and ninth) desorptions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 6. Comparison of absorption kinetics (220 ◦ C) over a longer time than Figs. 1–4. The dashed blue line is before and the heavy solid red line is after introduction of H2 O-sat.air. The thin solid green lines are for subsequent absorptions. The abrupt changes in H capacity correspond to the introduction of greater pressures for absorption. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 7. Comparison of desorption kinetics (220 ◦ C) over a longer time duration than in Figs. 1–5. The blue dashed line is the desorption (fourth) before exposure to H2 O-sat.air. The solid red line is the desorption (seventh) after exposure to H2 O-sat.air and the thin solid green lines are subsequent desorptions (eighth and ninth). The abrupt changes in weight loss correspond to the reduction of pressures during desorption. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
S. Luo et al. / Journal of Alloys and Compounds 440 (2007) L13–L17
Li3 N to air containing some H2 O, which formed some Li2 O (17 wt%) under the conditions which they employed. The Li2 O was dispersed in the system stabilizing it and they also believed it acts as a catalyst. Although the present system differs from that employed by Hu et al. [8], Li2 O presumably also forms here from the H2 O and O2 and it may act as a catalyst similar to the Li2 O/Li3 N system. In contrast to the results of Hu and Ruckenstein, however, there was no indication of any loss of H storage capacity after exposure to the H2 O-sat.air. For the present system, Li2 O may only form on the granule surfaces, which may be why a greater amount of Li2 O does not form. 4. Conclusions The important conclusions are that the (2LiNH2 + MgH2 ) system can be exposed to H2 O-sat.air (25 ◦ C) in all three of its forms, i.e., the starting mixture (2LiNH2 + MgH2 ), fully dehydrided or fully hydrided material (equation 1), with no degradation of its H-storage properties. The effect of H2 O-sat.air exposure is nearly the same whether the sample is exposed in its hydrided or dehydrided forms. The “final” wt% H obtained was either unchanged or slightly enhanced after exposure of the sample to H2 O-sat.air at 25 ◦ C. The results for these samples were quite reproducible. There is also a decrease of the times for reaching the same H wt% after exposure to the H2 O-sat.air for both absorption and desorption. In addition, an accelerating effect is present not just for the immediately succeeding run but for subsequent ones, i.e., those not immediately following the exposure to the H2 O-sat.air.
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Although the (2LiNH2 + MgH2 ) samples studied here absorb/ desorb only ≈4.8 wt% H and not e.g., >6%, its resistance to atmospheric contamination increases its viability of as an H storage system because of the greater safety and convenience in loading and handling. It also offers the possibility of the use of impure H2 . These characteristics balance, to some extent, the smaller than desired wt%. Acknowledgements Funding provided by the Department of Energy, Office of Energy Efficiency and Renewable Energy under the Hydrogen Storage Grand Challenge, Center of Excellence within DOE’s National Hydrogen Storage Project. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. References [1] [2] [3] [4] [5]
P. Chen, Z. Xiong, J. Luo, J. Lin, K. Tan, Nature 420 (2002) 302. P. Chen, Z. Xiong, J. Luo, K. Tan, J. Phys. Chem., B 107 (2003) 10967. Z. Xiong, G. Wu, J. Hu, P. Chen, Adv. Mater. 16 (2004) 1522. W. Luo, J. Alloys Compd. 381 (2004) 284. J. Wang, DOE Program Annual Review, http://www.hydrogen.energy.gov/ annual review05 storage.html#metal, 2005. [6] W. Luo, S. Sickafoose, J. Alloys Compd. 407 (2006) 274–281. [7] T. Ichikawa, S. Isobe, N. Hanada, H. Fujii, J. Alloys Compd. 365 (2004) 271–276. [8] Y. Hu, E. Ruckenstein, Ind. Eng. Chem. Res. 43 (2004) 2464.
Letter
The effect of exposure of the H-storage system (LiNH2 + MgH2) to water-saturated air S. Luo a , Ted B. Flanagan a,∗ , W. Luo b a
Chemistry Department, University of Vermont, Burlington, VT 05401, USA b Sandia National Laboratory, MS 9403, Livermore, CA 94550, USA
Received 16 June 2006; received in revised form 31 August 2006; accepted 2 September 2006 Available online 18 October 2006
Abstract The H-storage system, (LiNH2 + MgH2 ), has been exposed to water-saturated air and its subsequent rates of H2 desorption and absorption have been determined. Not only are there no adverse effects of the exposures, but also there is a small positive effect on the rates and final wt% H. © 2006 Elsevier B.V. All rights reserved. Keywords: Hydrogen absorbing materials; Kinetics
1. Introduction The Li–Mg–N–H system has been shown to have promise as a hydrogen-storage system because of its high wt% H, i.e., 9%, and its decomposition pressure of 2.8 MPa at 200 ◦ C [1–7]. A ball-milled mixture of (2LiNH2 + MgH2 ) is a specific example of a Li–Mg–N–H system which converts to (Mg(NH2 )2 + 2LiH) at, e.g., 220 ◦ C and a H2 pressure of 10 MPa [6]. This conversion however generally takes place during the initial desorption/absorption cycle. The subsequent reversible storage reaction is that shown in reaction (1), which does not involve the starting reactants. Mg(NH2 )2 + 2LiH = Li2 Mg(NH)2 + 2H2 .
(1)
In this paper, the storage material will be referred to as the (LiNH2 + MgH2 ) storage system with the understanding that the reversible storage process is given by reaction (1). Generally, this system can reversibly store about 5.4 wt% H and has an estimated plateau pressure of 1 MPa at 140 ◦ C [4]. It shows very encouraging cycle stability with a 0.005 wt% loss/cycle over 100 cycles [5]. The components are inexpensive and can be made in several different ways. In this work, the effect of exposure to water-saturated air (H2 O-sat.air) on the subsequent H-storage properties of this system will be reported. ∗
Corresponding author. Tel.: +1 802 656 0199; fax: +1 802 656 8705. E-mail address: [email protected] (T.B. Flanagan).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.09.026
This contaminant has been chosen because it is the most likely gaseous contaminant during routine handling. If it were to be only slightly or not affected at all by H2 O-sat.air, its ease of handling would greatly increase its usefulness as an H storage system. 2. Experimental The initial materials, LiNH2 and MgH2 , were purchased from Aldrich and used without any pre-treatment. The LiNH2 :MgH2 ratio was 2:1.1, i.e, a 10% excess of MgH2 was added so that the amide did not undergo decomposition to NH3 (g) during cycling [7]. A 30 g mixture of LiNH2 and MgH2 was mechanically milled in a glove box under an argon atmosphere using a Fritsch mill with 40 WC milling balls each weighing 8 g. The milling conditions were 5 repetitions at 300 rpm (30 min milling interspersed with periods of rest time of 90 min) then 12 repetitions at 350 rpm (30 min milling interspersed with periods of rest time of 90 min) followed by 50 repetitions at 350 rpm (30 min milling interspersed with a periods of rest time totaling 150 min). The purpose of alternating the milling with rest times is to keep the sample from being overheated during the milling. The apparatus for the H2 desorption/absorption measurements were a standard Sieverts apparatus with a Setra pressure gauge. Generally, the kinetic runs were determined at 220 ◦ C. This temperature was chosen because it had been previously found that the kinetics are reasonably fast but not so fast as to obscure comparisons of the kinetics before and after exposure to H2 O-sat.air. For desorption, the furnace must be raised around the reaction vessel and the temperature increased from 25 to 220 ◦ C and therefore there is a time delay before the sample reaches 220 ◦ C. There was no time delay for absorption because H2 was admitted at 220 ◦ C following a previous H2 desorption. The ball-milled (2LiNH2 + MgH2 ) mixture yielded two samples, which were loaded under an argon atmosphere in a high quality glove box. Sample 1 was employed directly for the studies of the effects of H2 O-sat.air. Sample 2
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desorbed/absorbed H2 three times after which it was “soaked” in H2 at ∼11.8 MPa at 220 ◦ C for 144 h prior to the determination of further desorption/absorption kinetics. The “soaking” was to ensure complete conversion to Mg(NH2 )2 and LiH [5]. After this “soaking”, Sample 2 was subjected to a desorption/absorption cycle, evacuated, and then the contamination studies were undertaken. The samples were exposed to H2 O-sat.air by slowly bubbling air through liquid water into the initially evacuated reaction vessel at room temperature. After a 10 min exposure to the H2 O-sat.air, H2 was introduced into the reaction vessel at room temperature and then evacuated three times in order to flush out any gaseous impurities. The temperature was increased to 220 ◦ C and the kinetics determined. Sample 1 was exposed to wet air (indicated as H2 O-sat.air) for 10 min at room temperature prior to the H2 desorption. A desorption run was carried out after being evacuated. These are the only data obtained for Sample 1. Most data were taken using Sample 2 and as an aid in the interpretation of the results a flow chart for Sample 2 is shown below giving the sequence of kinetic runs.
Note:
desorption at 220 ◦ C was then determined and the result is shown in Fig. 1 where it is compared with similar results for Sample 2 prior to its exposure to H2 O-sat.air. It can be seen (Fig. 1) that there is no adverse effect of the exposure to H2 O-sat.air and, in fact, there is a small acceleration of the rate. This is a very encouraging result and it leads to the more detailed study as described below using Sample 2. 3.2. Effect of exposure to H2 O-sat.air on H2 absorption by Sample 2 Sample 2 was subjected to four absorption/desorption cycles (220 ◦ C) before any exposures to H2 O-sat.air; the kinetics were
3. Results and discussion
found to be quite reproducible. After the fourth desorption (see flow chart), the sample was exposed to H2 O-sat.air (25 ◦ C) and the subsequent absorption (fifth) (220 ◦ C) was found to be quite fast, i.e., the exposure had no adverse effects. After the fifth desorption, the sample was re-exposed to H2 O-sat.air and the sixth absorption kinetics were determined. The same procedure was repeated for the seventh absorption. All of these absorption runs had faster kinetics than before admitting the H2 O-sat.air. A comparison of absorption profiles before and after an exposure to air is given in Fig. 2.
3.1. Effect of exposure of (2LiNH2 + MgH2 ) to H2 O-sat.air on H2 desorption
3.3. Effect of exposure to H2 O-sat.air on H2 desorption by Sample 2
Sample 1 was evacuated at room temperature and then exposed to H2 O-sat.air for 10 min. Following this exposure treatment, it was flushed with H2 and evacuated. The initial H2
After Sample 2 had absorbed H (sixth and seventh absorptions), it was cooled and evacuated at room temperature (25 ◦ C). The sample was then exposed to H2 O-sat.air at 25 ◦ C and the
• For desorption, the sample was always heated from room temperature to 220 ◦ C which required about 14 min. • All absorption measurements were carried out at 220 ◦ C. At the completion of absorption, the sample was cooled to room temperature under hydrogen pressure. The sample was evacuated before the subsequent desorption. • All H2 O-sat.air was introduced into the system at room temperature (RT). • The box labeled “H2 O + air” indicates treatment with water-saturated air as described in Section 2.
S. Luo et al. / Journal of Alloys and Compounds 440 (2007) L13–L17
Fig. 1. Comparison of initial desorption kinetics of 2LiNH2 + MgH2 (220 ◦ C) during the first hour for Sample 2 (dashed blue line) and Sample 1 (heavy red solid line). The light black solid line at the top is the sample temperature over the experimental period. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 2. Comparison of absorption kinetics (220 ◦ C) for Sample 2 over the first 2 h before (fourth absorption, dashed blue line) and after introduction of H2 Osat.air (fifth, sixth and seventh absorptions, solid red lines). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
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Fig. 3. Comparison of desorption kinetics (220 ◦ C) for the first hour before (fourth desorption, dashed blue line) and after (sixth and seventh desorptions, heavy red solid lines) introduction of H2 O-sat.air. The thin solid line at the top is the sample temperature over the experimental period. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
subsequent desorption results at 220 ◦ C (Fig. 3) were analogous to those shown in Fig. 2, i.e., there were no adverse effects due to the exposure. It can also be seen from Fig. 3 that the kinetics are faster after exposure to H2 O-sat.air than before. The results are repeatable (see the overlap of solid lines for the sixth and seventh runs). Furthermore, the enhanced rates for either absorption or desorption found following exposure to H2 O-sat.air persists without further exposures as shown in Figs. 4–5. In addition, the final wt% H is slightly enhanced (4.8 wt%) after exposure to the H2 O-sat.air as shown in Figs. 6 and 7. It can be seen that there appears to be a small shift of the subsequent desorption profiles towards the one before the H2 O-sat.air exposure, however, the subsequent ones are still faster than before the exposure and they remain significantly faster especially at longer reaction times as shown in Fig. 6 for absorption and Fig. 7 for desorption. For many subsequent cycles after exposure to H2 O-sat.air, the rates over long times remain faster than before exposure. It should be noted that 16 absorptions and 15 desorptions were carried out with Sample 2 with no indication of any degradation and, in fact, the wt% H increased slightly, 4.6–4.8 wt%. This suggests that the 0.005 wt% loss noted above [5] may not be applicable to all sample preparations. A sample from another ball-milled batch was exposed to dry air at room temperature and at 220 ◦ C and it was found that subsequent H2 absorption/desorption was not affected although there was no clear indication of an increase in rate as found for the H2 O-sat.air exposure. Hu and Ruckenstein [8] investigated the Li2 O/Li3 N system for H storage purposes. They prepared the system by exposing
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S. Luo et al. / Journal of Alloys and Compounds 440 (2007) L13–L17
Fig. 4. Comparison of absorption kinetics (220 ◦ C) before (fourth absorption, dashed blue line) and after (seventh absorption, heavy red solid line) introduction of H2 O-sat.air. The light solid green lines are for subsequent (eighth, ninth and tenth) absorptions without further introduction of H2 O-sat.air. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 5. Comparison of desorption kinetics (220 ◦ C) during the first hour before (fourth desorption, blue dashed line) and after (sixth desorption, heavy solid red line) introduction of H2 O-sat.air. The thin solid green lines are for the subsequent (eighth and ninth) desorptions. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 6. Comparison of absorption kinetics (220 ◦ C) over a longer time than Figs. 1–4. The dashed blue line is before and the heavy solid red line is after introduction of H2 O-sat.air. The thin solid green lines are for subsequent absorptions. The abrupt changes in H capacity correspond to the introduction of greater pressures for absorption. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
Fig. 7. Comparison of desorption kinetics (220 ◦ C) over a longer time duration than in Figs. 1–5. The blue dashed line is the desorption (fourth) before exposure to H2 O-sat.air. The solid red line is the desorption (seventh) after exposure to H2 O-sat.air and the thin solid green lines are subsequent desorptions (eighth and ninth). The abrupt changes in weight loss correspond to the reduction of pressures during desorption. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
S. Luo et al. / Journal of Alloys and Compounds 440 (2007) L13–L17
Li3 N to air containing some H2 O, which formed some Li2 O (17 wt%) under the conditions which they employed. The Li2 O was dispersed in the system stabilizing it and they also believed it acts as a catalyst. Although the present system differs from that employed by Hu et al. [8], Li2 O presumably also forms here from the H2 O and O2 and it may act as a catalyst similar to the Li2 O/Li3 N system. In contrast to the results of Hu and Ruckenstein, however, there was no indication of any loss of H storage capacity after exposure to the H2 O-sat.air. For the present system, Li2 O may only form on the granule surfaces, which may be why a greater amount of Li2 O does not form. 4. Conclusions The important conclusions are that the (2LiNH2 + MgH2 ) system can be exposed to H2 O-sat.air (25 ◦ C) in all three of its forms, i.e., the starting mixture (2LiNH2 + MgH2 ), fully dehydrided or fully hydrided material (equation 1), with no degradation of its H-storage properties. The effect of H2 O-sat.air exposure is nearly the same whether the sample is exposed in its hydrided or dehydrided forms. The “final” wt% H obtained was either unchanged or slightly enhanced after exposure of the sample to H2 O-sat.air at 25 ◦ C. The results for these samples were quite reproducible. There is also a decrease of the times for reaching the same H wt% after exposure to the H2 O-sat.air for both absorption and desorption. In addition, an accelerating effect is present not just for the immediately succeeding run but for subsequent ones, i.e., those not immediately following the exposure to the H2 O-sat.air.
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Although the (2LiNH2 + MgH2 ) samples studied here absorb/ desorb only ≈4.8 wt% H and not e.g., >6%, its resistance to atmospheric contamination increases its viability of as an H storage system because of the greater safety and convenience in loading and handling. It also offers the possibility of the use of impure H2 . These characteristics balance, to some extent, the smaller than desired wt%. Acknowledgements Funding provided by the Department of Energy, Office of Energy Efficiency and Renewable Energy under the Hydrogen Storage Grand Challenge, Center of Excellence within DOE’s National Hydrogen Storage Project. Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. References [1] [2] [3] [4] [5]
P. Chen, Z. Xiong, J. Luo, J. Lin, K. Tan, Nature 420 (2002) 302. P. Chen, Z. Xiong, J. Luo, K. Tan, J. Phys. Chem., B 107 (2003) 10967. Z. Xiong, G. Wu, J. Hu, P. Chen, Adv. Mater. 16 (2004) 1522. W. Luo, J. Alloys Compd. 381 (2004) 284. J. Wang, DOE Program Annual Review, http://www.hydrogen.energy.gov/ annual review05 storage.html#metal, 2005. [6] W. Luo, S. Sickafoose, J. Alloys Compd. 407 (2006) 274–281. [7] T. Ichikawa, S. Isobe, N. Hanada, H. Fujii, J. Alloys Compd. 365 (2004) 271–276. [8] Y. Hu, E. Ruckenstein, Ind. Eng. Chem. Res. 43 (2004) 2464.