pubs.acs.org/JPCL
Effect of Hydrogen Back Pressure on Dehydrogenation Behavior of LiBH4-Based Reactive Hydride Composites Jae-Hyeok Shim,*,† Jae-Hag Lim,† Sami-ullah Rather,† Young-Su Lee,† Daniel Reed,‡ Yoonyoung Kim,† David Book,‡ and Young Whan Cho† †
Materials Science and Technology Research Division, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea, and ‡School of Metallurgy and Materials, University of Birmingham, Birmingham B15 2TT, United Kingdom
ABSTRACT Hydrogen back pressure remarkably promotes the formation of metal boride during the dehydrogenation of 4LiBH4 þ YH3, 6LiBH4 þ CeH2 and 6LiBH4 þ CaH2 composites, which seems to be a general phenomenon in LiBH4based reactive hydride composites that enables mutual destabilization between LiBH4 and metal hydride. The formation of metal boride plays a crucial role in the reversible hydrogen storage properties of these composites. The dependence of the dehydrogenation behavior on hydrogen back pressure might be associated with the microstructural evolution of the dehydrogenation products formed by a solid-liquid reaction. SECTION Energy Conversion and Storage
Vajo et al.3 observed the ? effect of hydrogen back pressure on the formation of MgB2 in the 2LiBH4 þ MgH2 composite. The dehydrogenation under vacuum results in the formation of Mg instead of MgB2, whereas a finite hydrogen pressure yields MgB2. Subsequently, Nakagawa et al.8 and Pinkerton et al.9 also confirmed the similar phenomenon in the 2LiBH4 þ MgH2 composite. However, there has been no systematic study of the hydrogen back pressure effect on hydrogen storage properties of LiBH4-based reactive hydride composites. In this paper, we investigate whether this hydrogen back pressure dependence is a general feature, which is valid for other reactive hydride composites. Three LiBH4-based reactive hydride composites, 4LiBH4 þ YH3, 6LiBH4 þ CeH2, and 6LiBH4 þ CaH2, which are expected to react into 4LiH þ YB4 þ 7.5H2 (8.5 wt % theoretical hydrogen capacity), 6LiBH4 þ CeB6 þ 10H2 (7.4 wt %) and 6LiBH4 þ CaB6 þ 10H2 (11.7 wt %), respectively, were prepared, and the influence of hydrogen back pressure on the dehydrogenation path was investigated. Also, the reversibility of the dehydrogenation and rehydrogenation reactions of these composites was investigated. The dehydrogenation profiles of the 4LiBH4 þ YH3 and 6LiBH4 þ CeH2 composites at 350 °C are shown in Figure 1. Although dehydrogenation of the 4LiBH4 þ YH3 composite starting from vacuum presents fast kinetics up to about 1 h, the dehydrogenation rate becomes slow after about 1 h. The amount of hydrogen released after 24 h is less than 2 wt %, which is far less than the theoretical value (8.5 wt %). On the other hand, this composite releases about 7.2 wt % hydrogen
A
mong complex metal hydrides, lithium borohydride (LiBH4) has recently received great attention as a solid-state hydrogen storage material owing to its high gravimetric hydrogen storage density.1 LiBH4 is known to decompose into LiH and B, releasing 13.9 wt % hydrogen. The decomposition temperature of about 370 °C at 1 bar of hydrogen is, however, too high, and the hydrogenation conditions are quite severe (e.g., 600 °C and 155 bar hydrogen).2 Vajo et al.3 first attempted to lower the dehydrogenation (decomposition) temperature of LiBH4 with MgH2. A possible dehydrogenation reaction of the composite is 2LiBH4 þ MgH2 f 2LiH þ MgB2 þ 4H2 with a 11.4 wt % theoretical hydrogen capacity. The formation of MgB2, which is a more stable reaction product than either Mg or B, reduces the overall enthalpy change of the dehydrogenation and eventually decreases the dehydrogenation temperature. This concept is called “destabilization”3 or “reactive hydride composite.”4 It was also shown that 2LiH þ MgB2 with a small amount of TiCl3 as a catalytic additive can be rehydrogenated at more moderate conditions such as 350 °C and 100 bar of hydrogen.3 However, the 2LiBH4 þ MgH2 composite has a serious drawback: dehydrogenation to form 2LiH þ MgB2 does not proceed in a single step. Namely, MgH2 first decomposes to form Mg, and then a subsequent reaction between Mg and LiBH4 produces LiH and MgB2. Therefore, it is unlikely that the dehydrogenation of this composite starts below the dehydrogenation temperature of MgH2 (∼ 288 °C at 1 bar of hydrogen), even though the equilibrium dehydrogenation temperature of this composite to form 2LiH þ MgB2 is estimated to be about 225 °C in 1 bar of hydrogen.3 Currently, a variety of LiBH4-based reactive hydride composites are under investigation, and reversible hydrogen storage at moderate conditions was observed for some reactive hydride systems with appropriate catalytic additives, such as 2LiBH4 þ Al,5 6LiBH4 þ CaH2,6,7 and 6LiBH4 þ CeH2.7
r 2009 American Chemical Society
Received Date: September 21, 2009 Accepted Date: October 28, 2009 Published on Web Date: November 06, 2009
59
DOI: 10.1021/jz900012n |J. Phys. Chem. Lett. 2010, 1, 59–63
pubs.acs.org/JPCL
Figure 2. XRD patterns of the (a) as-milled and dehydrogenated samples of the 4LiBH4 þ YH3 composite under (b) static vacuum and (c) hydrogen back pressure.
Figure 1. Dehydrogenation profiles of the 4LiBH4 þ YH3 composite under (a) static vacuum and (b) hydrogen back pressure, and those of the 6LiBH4 þ CeH2 composite under (c) static vacuum and (d) hydrogen back pressure.
hydrides, which explains why the amount of released hydrogen is so small (
