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 300C for all composites while pure LiH desorbs hydrogen above 650C. The hydrogen desorption temperature of these composites decreased with decreasing the content of LiOH. The onset temperature of hydrogen desorption lowered to 262C for the sample of LiH-30 mol%LiOH. In the TDS measurement, the generation of water was observed around 420C 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 ÁH ð298 KÞ¼48:1 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. ÁH ð298 KÞ¼25 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 689C 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. ÁH ð298 KÞ¼À120 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 ÁH ð298 KÞ¼À38:9 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 ÁH ð298 KÞ¼À23:3 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 ÁH ð298 KÞ¼À23:3 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 250C. 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 420C with a rate of 5C/min under flowing argon. 2LiOH ! Li2O þ H2O(g) ð Þ¼ ð Þ LiH-50 mol%LiOH LiH ÁH 298 K 129.4 kJ/mol-H2O 8 LiOH Li2O 2LiH þ H2O(g) ! Li2O þ 2H2 ÁH ð298 KÞ¼87:9 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 420C. 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 250–410C. The enthalpy change of this reaction is calcu- of LiH-(30–60) mol%LiOH composites. lated as À19:8 kJ/mol-H2. This value is in good agreement with that of eq. (3). Around 400 to 450C, 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 (689C) 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 250–400C 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- 420–500C with a rate of 5C/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 100–150C. It may analysis heating up to 550C with a rate of 5C/min under be able to attribute the hydrogen desorption derived from flowing helium.