Materials Transactions, Vol. 50, No. 7 (2009) pp. 1855 to 1858 #2009 The Japan Institute of Metals
Optimum Hydrogen Desorption Properties in LiH-LiOH Composites
Masatsugu Kawakami1;*, Takahiro Kuriiwa1, Atsunori Kamegawa1, Hitoshi Takamura1, Masuo Okada1 and Tomohiro Kaburagi2
1Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan 2Nissan Motor Co., Ltd., Yokosuka 237-8523, Japan
The effect of variation of LiOH content on hydrogen desorption properties in LiH-(30–60) mol%LiOH composites was investigated. The addition of LiOH destabilized LiH in desorbing hydrogen below 300 C for all composites while pure LiH desorbs hydrogen above 650 C. The hydrogen desorption temperature of these composites decreased with decreasing the content of LiOH. The onset temperature of hydrogen desorption lowered to 262 C for the sample of LiH-30 mol%LiOH. In the TDS measurement, the generation of water was observed around 420 C for the samples of LiH-(40–60) mol%LiOH due to decomposition of unreacted LiOH. The intensity of the water peak from these composites in TDS decreases with decreasing the content of LiOH. The water generation was unobserved from LiH-30 mol%LiOH composite. These results indicate that LiH-(30 and 40) mol%LiOH is a suitable composition for hydrogen desorption in this study. [doi:10.2320/matertrans.M2009055]
(Received February 13, 2009; Accepted April 21, 2009; Published June 10, 2009) Keywords: hydrogen storage materials, solid-solid reactions, LiH-LiOH composites, lowering desorption temperature
1. Introduction 2LiH þ NaOH ! Li2O þ NaH þ H2 Lightweight materials such as used for hydrogen storage H ð298 KÞ¼ 48:1 kJ/mol-H2 ð4Þ media or on-board type hydrogen generator are required 2LiBH4 þ MgH2 , 2LiH þ MgB2 þ 4H2 for fuel cells in vehicles or portable electronic devices. H ð298 KÞ¼ 25 kJ/mol-H2 ð5Þ Then, some light metal hydrides such as LiH and MgH2 are paid much attention with its high hydrogen storage These reactions consist of hydrogen desorption exothermi- capacity. The hydrogen storage capacity for LiH and MgH2 cally which could lead to reduction of capacity of the heater are theoretically calculated to be 12.7 mass% and for hydrogen generation system. Some of these reactions 7.7 mass%, respectively. However, LiH melts at 689 C show reversibility of hydrogenation and dehydrogenation. without hydrogen desorption.1) Its desorption temperature is From a point of view for reduction of heat for hydrogen higher than its melting point. So, it might be inconvenient desorption, the hydrolysis reactions of hydrides are paid for practical use as hydrogen source. On the other hand, much attentions as hydrogen source. Hydrolysis reactions MgH2 is well known to generate hydrogen about 300 C could be sorted out as two groups by means of water supply. under ambient pressure.2,3) Working temperature of both One is direct addition of water for hydrolysis reaction, and materials is high and which caused major difficulty for the other is indirect supply of water by means of such as practical on-board use of these materials. The other major decomposition and water generation, i.e. dehydration, of problems for these materials for practical utilization are metal hydroxides. its large reaction of heat for hydrogen desorption which Hydrolysis reactions of hydrides, aluminum hydrides and requires hydrogen storage system for large capacity of borohydrides of Na, Li, Ca and Mg for hydrogen desorption heater. were reported by V. C. Y. Kong et al.7) Then, the destabilization of these ionic hydrides for MH þ xH O ! M(OH) þ xH ð6Þ decrement of working temperature as well as reduction of x 2 x 2 heat for hydrogen generation has been attracting strong Where M represents a metal of valence x. Hydrolysis reaction interests as for use for on-board type fuel cells and so on. produces hydrogen gas and alkaline hydroxide as water J. J. Vajo et al. reported the destabilization of LiH or solutions. The hydrolysis reaction rates of some hydrides are 4) 5) MgH2 by additives such as pure Si, some hydroxides and high. In addition, it is possible to steady constant reaction rate 6) LiBH4 as follows. during a wide range of yield, and overall yields of hydrolysis reaction over 96% and 90% were reported for CaH2 and LiH, 4LiH þ Si , Li4Si þ 2H2 7) respectively. H ð298 KÞ¼ 120 kJ/mol-H2 ð1Þ 8,9) Some catalysts for hydrolysis reaction of NaBH4 and 10) 2MgH2 þ Si , Mg2Si þ 2H2 NH3BH3 were also investigated in order to improve H ð298 KÞ¼ 38:9 kJ/mol-H2 ð2Þ reaction rate, although, the disadvantage of this reaction is producing high pH solution as byproduct. LiH þ LiOH ! Li2O þ H2 The hydride-hydroxide reaction could take place without H ð298 KÞ¼ 23:3 kJ/mol-H2 ð3Þ addition of water, and produces hydrogen gas and solid oxide.
MH þ MOH ! MO þ H2 ð7Þ *Graduate Student, Tohoku University 1856 M. Kawakami et al.
The byproducts of hydride-hydroxide reactions are easier to 6 LiH-50 mol%LiOH 5 ºC/min be handled than those of the hydrolysis reactions. Ar flow
As for hydrogen generation, solid-state reactions of ionic -1 4 hydrides with alkaline hydroxides was reported by J. J. Vajo 5) / Wg 412 ºC et al. For example, the reaction of lithium hydride with Q 5.6 kJ/mol lithium hydroxide is described as follows. 2
LiH þ LiOH ! Li2O þ H2 0 H ð298 KÞ¼ 23:3 kJ/mol-H2 ð3Þ Heat Flow, 248 ºC 19.8 kJ/mol The composite of LiH-LiOH decomposed into Li2O and H2 -2 with exothermic reaction. The amount of H2 generation 0 100 200 300 400 500 through this reaction is measured to be 5.73 mass% from R.T. Temperature, T / ºC up to 250 C. J. J. Vajo et al. suggested that eq. (3) consist of the two-step solid-gas reactions: Fig. 1 DSC curve of LiH-50 mol%LiOH composite. The sample was heated up to 420 C with a rate of 5 C/min under flowing argon.
2LiOH ! Li2O þ H2O(g) ð Þ¼ ð Þ LiH-50 mol%LiOH LiH H 298 K 129.4 kJ/mol-H2O 8 LiOH Li2O 2LiH þ H2O(g) ! Li2O þ 2H2 H ð298 KÞ¼ 87:9 kJ/mol-H2 ð9Þ after DSC measurement Equation (8) seems to dominate the hydrogen desorption LiOH Li2O temperature of the composite.
On the other hand, J. M. Kiat et al. investigated the prepared composite 11) dehydration of LiOH into Li2O. Equation (8) occurs LiH LiOH above 350 C under the partial pressure of water vapour: Intensity (arbitrary units) PH2O ¼ 0{150 mmHg. With this result, eq. (3) below 350 C 10º 20º 30º 40º 50º 60º 70º 80º 90º should be considered to be a solid-solid reaction. The 2θ (Cu-Kα) difference between working temperature of LiH-LiOH composite reported by Vajo and dehydration temperature of Fig. 2 XRD patterns of LiH-50 mol%LiOH composite of as-prepared and after DSC measurement up to 420 C. LiOH reported by Kiat is large. So there could exist some probability of dependency of working temperature of LiH- LiOH composite on LiOH contents. Also, other properties 3. Results and Discussions might be changed with changing of LiOH ratio in LiH-LiOH composite. Figure 1 shows DSC curve of LiH-50 mol%LiOH. An The purpose of this study is to investigate the effect of exothermic reaction occurred in the temperature range of variation LiOH content for hydrogen desorption properties 250–410 C. The enthalpy change of this reaction is calcu- of LiH-(30–60) mol%LiOH composites. lated as 19:8 kJ/mol-H2. This value is in good agreement with that of eq. (3). Around 400 to 450 C, a couple of 2. Experimental Procedures endothermic and exothermic reactions were observed on heating and cooling processes, respectively. Heat flow peaks The starting materials, LiH (95% purity), LiOH (98% like these imply occurrences of a phase transformation and in purity) were purchased from ALDRICH. Before preparation this sample, which is derived from that of LiOH.2) It is of samples, XRD patterns of these starting materials were notable that onset temperature of exothermic reaction is quite taken, and no phase was observed other than that of starting lowered than melting point (689 C) of LiH. material, LiH or LiOH respectively. The hydride-hydroxide Figure 2 shows XRD patterns of LiH-50 mol%LiOH, of composites, LiH-x mol%LiOH (x ¼ 30{60) were prepared as-prepared and after DSC measurement up to 420 C. Li2O using hand milling in an argon filled glove box with the and LiOH were observed in the sample after DSC measure- milling time of 30 min. ment. The exothermic reaction in Fig. 1 around 250–400 C The decomposition and reaction temperatures and heats of could be attributed to the occurence of reaction (3). reactions of the starting materials and LiH-LiOH composites The effect of variation of LiOH content on hydrogen were measured by using differential scanning calorimetry in desorption properties of LiH-(0–100) mol%LiOH composites a glove box filled with argon. The samples were heated up to was investigated. Figure 3 shows TDS curves of LiH- 420–500 C with a rate of 5 C/min under flowing argon. The (0–100) mol%LiOH composites, in which (a) mass#2 and hydrogen desorption temperatures of LiH-LiOH composites (b) #18 correspond to emission of hydrogen gas and water, were measured by thermal desorption mass spectroscopy respectively. For pure LiH, small amount of hydrogen combined with thermogravimetry and differential thermal desorption was obserbed to be around 100–150 C. It may analysis heating up to 550 C with a rate of 5 C/min under be able to attribute the hydrogen desorption derived from flowing helium. During sample preparation for TDS, samples LiH to contact with air. For the sample of LiH-(30–60) were exposed to air for several tens of minutes. The phase mol%LiOH, the hydrogen desorption started around 200– identification was carried out by X-ray diffraction. 300 C. J. J. Vajo reported that hydrogen desorption starts at Optimum Hydrogen Desorption Properties in LiH-LiOH Composites 1857
(a) LiH-x mol%LiOH mass#2: H2 (b) LiH-x mol%LiOH mass#18: (x = 0-100) (x = 0-100) H2O
x = 100
292 ºC x = 60 x = 100 422 ºC
278 ºC x = 60 394 ºC x = 50
x = 50 378 ºC 285 ºC x = 45 x = 45 374 ºC 263 ºC Ion Current (arbitrary units) x = 40 Ion Current (arbitrary units) x = 40 340 ºC 262 ºC x = 30 x = 30
x = 0 x = 0 105 ºC 0 100 200 300 400 500 600 0 100 200 300 400 500 600 Temperature, T / ºC Temperature, T / ºC
Fig. 3 TDS curves of LiH-(0–100) mol%LiOH composites. The samples were heated up to 550 C with a rate of 5 C/min under flowing helium.
50–60 C for mechanically milled LiH-50 mol%LiOH with- 10 out TiCl added as a catalyst. The differance of hydrogen LiH-x mol%LiOH 3 (x = 30-60) desorption temperature was caused by the sample prepara- 5 tion. In general, the mechanical milling refines the particle 0 x = 30 size of the sample, so that it is possible to raise reaction rate. (mass%) x = 40 W LiOH decomposed and generated of water at 422 C. This -5 x = 45 result conformed with that reported by J. M. Kiat.11) For the x = 50 -10 sample of LiH-60 mol%LiOH, intensity of water generation x = 60 was stronger than any other composite. From the view point x = 50 -15 x = 45 x = 60 of hydrogen generation materials or hydrolysis, detection of x = 40 Weight Loss, x = 30 water during reaction means some amount of water was -20 0 100 200 300 400 500 600 unutilized for the hydrogen generation, or in other word, it Temperature, T / ºC could be regarded as excessive addition of water beyond optimum content (in this case, excess of hydroxide) and Fig. 4 TG curves of LiH-(30–60) mol%LiOH composites. The samples which could lead to deteriorating of gravimetrical efficiency were heated up to 550 C with a rate of 5 C/min under flowing helium. as hydrogen generating materials. Meanwhile, amount of water genaration from LiH-LiOH Table 1 Weight losses after TG measurement and calcurated hydrogen composites decreased with decreasing of LiOH contents contents of LiH-(30–60) mol%LiOH composites. significantly from LiH-(30–50) mol%LiOH. Composition of Weight loss Hydrogen content Figure 4 shows TG curves of LiH-(30–60) mol%LiOH. LiH-x mol%LiOH (x ¼ 30{60) (mass%) (mass%) Measured weight loss after TG measurement and calculated x ¼ 60 16.91 5.74 hydrogen content of LiH-(30–60) mol%LiOH are summariz- 50 9.21 6.32 ed in Table 1. The weight loss of sample after TG measure- 45 5.67 6.65 ment increased with increasing of the LiOH content. In the 40 4.24 7.02 composites of LiH-(50 and 60) mol%LiOH, measured weight 30 3.16 7.91 losses were larger than the theoretical hydrogen contents of these compositions. This result could be due to detection of water from LiH-LiOH composites caused by excess of LiOH was observed after DSC measurement. Existance of Li2Oin content since molecular weight of water is nine times as LiH-LiOH composites after DSC measurement was attrib- heavy as that of hydrogen. uted to an exothermic reaction in the temperature range Figure 5 shows XRD patterns of LiH-(30–60) mol%LiOH of 250–410 C of a solid-solid reaction between LiH and composites after DSC measurements. From all samples, Li2O LiOH. 1858 M. Kawakami et al.
300 LiH-x mol%LiOH LiH LiOH LiH-x mol%LiOH / ºC
(x = 30-60) Li2O T (x = 30-60) 290 after DSC measurement x = 60 LiOH Li O 2 280
x = 50 270 LiOH Li2O
260
x = 45 Hydrogen desorption temperature, Li2O 250 30 40 50 60 Content of LiOH (mol%) x = 40 Intensity (arbitrary units) Li O 2 Fig. 6 Hydrogen desorption temperatures of LiH-LiOH composites as a x = 30 function of content of LiOH. LiH Li2O
10ºººººººº20 30 40 50 60 70 80 90º (30–60) mol%LiOH, the addition of LiOH destabilized 2θ (Cu-Kα) LiH to generate hydrogen below 300 C. Especially, LiH- 30 mol%LiOH composite shows the lowest hydrogen de- Fig. 5 XRD patterns of LiH-(30–60) mol%LiOH composites after DSC sorption temperature at 262 C in this study. The hydrogen measurement up to 420 C. desorption temperature of these composites decreases with decreasing the LiOH content. In the composites of LiH-(50 and 60) mol%LiOH or LiH- From results of TDS measurements, intensity of water of 30 mol%LiOH, unreacted LiOH or LiH was also observed. LiH-60 mol%LiOH sample was stronger than any other These results indicate that these composite were unoptimum sample. Intensity of water decreased with decreasing the ratio composition for LiH-LiOH composite. Excess amount of LiH of LiOH. No peak of generation of water was observed from (or LiOH) could increase amount of unreacted LiH (or LiOH) LiH-30 mol%LiOH sample. after hydrogen generation and deteriorate hydrogen gener- From XRD patterens of samples after DSC measurement, ation gravimetrical efficiency. samples of LiH-(40 and 45) mol%LiOH were confirmed as In the composites of LiH-x mol%LiOH (x ¼ 40 and 45), consisting of Li2O. Meanwhile, unreacted starting material of only Li2O was observed. It can be said that LiH and LiOH LiH or LiOH was observed besides Li2O in some samples reacted well at these composition ratios. But, detection of after DSC measurement, the former was confirmed in the LiH is relatively difficult, so there exists some possibility that sample of LiH-30 mol%LiOH, the latter was confirmed in the these samples (x ¼ 40 and 45) still contained LiH after DSC samples of LiH-(50 and 60) mol%LiOH, respectively. measurement. Judging from the results of XRD, TDS and TG, LiH-(30 In this study, judging from the results of XRD, TDS and and 40) mol%LiOH is a suitable composition with regard to TG, suitable content of LiH-x mol%LiOH composites for hydrogen desorption properties. This result could be attrib- hydrogen desorption is around x ¼ 30{40. As mentioned uted to enhancement of the contact area between LiH and above, hydrogen desorption temperature of LiH-LiOH LiOH. composites are lowered significantly than that of LiH without LiOH. In the next, effects of LiOH contents on reduction of REFERENCES onset temperature of hydrogen desorption are studied. Figure 6 shows hydrogen desorption temperatures of LiH- 1) C. E. Messer, E. B. Damon, P. C. Maybury, J. Mellor and R. A. Seales: J. Phys. Chem. 62 (1958) 220–222. LiOH composites as a function of the molar content of LiOH. 2) F. H. Ellinger, C. E. Holley, Jr., B. B. McInteer, D. Pavone, R. M. Hydrogen desorption temperature of LiH-LiOH composite Potter, E. Staritzky and W. H. Zachariase: J. Am. Chem. Soc. 77 (2000) decreased with decreasing of the molar content of LiOH in 157–166. LiH-LiOH composites. 3) J. F. Stampfer, C. E. Holley and J. F. Suttle: J. Am. Chem. Soc. 82 In general, solid-solid reaction proceeds from the surface (1960) 3504–3508. 4) J. J. Vajo, F. Mertens, C. C. Ahn, R. C. Bowman, Jr. and B. Fultz: J. of each particles. The mechanical milling and addition of a Phys. Chem. B 108 (2004) 13977–13983. catalyst were offen used in order to enhance these surface 5) J. J. Vajo, S. L. Skeith, F. Mertens and S. W. Jorgensen: J. Alloy. states. In the case of the LiH-LiOH composite, dependence of Compd. 390 (2005) 55–61. the hydrogen desorption temperature on the content of LiOH 6) J. J. Vajo, S. L. Skeith and F. Mertens: J. Phys. Chem. B 109 (2005) was observed in this study. The contact area between LiH and 3719–3722. 7) V. C. Y. Kong, F. R. Foulkes, D. W. Kirk and J. T. Hinatsu: Int. J. LiOH might been increases with increasing the contents of Hydrogen Energ. 24 (1999) 665–675. LiH. 8) S. C. Amendola, S. L. Sharp-Goldman, M. S. Janjua, N. C. Spencer, M. T. Kelly, P. J. Petillo and M. Binder: Int. J. Hydrogen Energ. 25 4. Summary (2000) 969–975. 9) S. Suda, Y.-M. Sun, B.-H. Liu, Y. Zhou, S. Morimitsu, K. Arai, M. Uchida, Y. Candra and Z.-P. Li: Appl. Phys. A 72 (2001) 209–212. The effect of variation of LiOH content on hydrogen 10) M. Chandra and Q. Xi: J. Power Sources 156 (2006) 190–194. desorption properties of LiH-(30–60) mol%LiOH composites 11) J. M. Kiat, G. Boemare, B. Rieu and D. Aymes: Solid State Commun. was investigated. For the studied compositions of LiH- 108 (1998) 241–245.