ARTICLE pubs.acs.org/JPCC
Optimizing the Destabilization of LiBH4 for Hydrogen Storage and the Effect of Different Al Sources M. Meggouh,† D. M. Grant,‡ and G. S. Walker*,† †
Energy and Sustainability Division and ‡Materials Mechanics Structures Division, Faculty of Engineering, University of Nottingham, Nottingham, NG7 2RD, U.K. ABSTRACT: Due to its high hydrogen storage capacity of 18.5 wt %, LiBH4 has gained much attention as a potential onboard hydrogen storage medium for automotive applications. Unfortunately, LiBH4 only decomposes fully above 600 °C, and hydrogenation does not occur below 600 °C and requires hydrogen pressures of at least 350 bar. However, these conditions can be significantly improved by thermodynamic destabilization. In this study, LiBH4 was augmented with two different Al-sources (metallic Al and from the decomposition of LiAlH4) and the effectiveness of the different Al sources on the dehydrogenation/hydrogenation behavior was investigated along with the efficacy of using a Ti-based catalyst. Ball milling conditions were also investigated to study the effect of premilling the individual components. It has been found that longer ball-milling times (3 h) for 2LiBH4:LiAlH4 in the presence of TiCl3 resulted in higher H2 release than reported in the literature. In addition, a lower dehydrogenation temperature and improved reversibility (under 85 bar H2 and at 350 °C) were achieved for the LiAlH4-containing samples than in the case of metallic Al. The TiCl3 was found to catalyze the decomposition of LiAlH4 during ball milling, resulting in highly dispersed Al through the LiBH4. This proved to be a more effective route to deliver the Al destabilization agent, leading to higher capacities and improved reversibility of the system. Premilling the individual components together prior to the addition of the Ti-catalyst was found to be detrimental to the system, resulting in higher dehydrogenation temperatures than achieved by comilling all the reagents. The enthalpy of dehydrogenation was found to be 38.2 kJ mol1 (H2) and the temperature for a 1 bar equilibrium pressure was calculated to be in the range 240300 °C.
’ INTRODUCTION One of the main challenges for a hydrogen based economy is the development of a suitable on-board hydrogen storage material. Recently, complex hydrides have gained much attention due to their high hydrogen storage capacities, e.g., 18.5 wt % for LiBH4, which exceeds the 2015 gravimetric hydrogen storage target (5.5 wt %) set by the U.S. Department of Energy.1 However, on its own, LiBH4 only decomposes fully above 600 °C24 and hydrogenation does not occur below 600 °C and under a hydrogen pressure of less than 350 bar.3 The dehydrogenation of LiBH4, as presented in reaction 1, does not release all the hydrogen because LiH is formed, which is a more thermally stable metal hydride (LiH has a T (1 bar) of 910 °C, the temperature needed in order to generate a 1 bar H2 equilibrium pressure).5 2LiBH4 f 2LiH þ 2B þ 3H2
ð1Þ
One strategy to decrease the decomposition temperature of LiBH4 is by thermodynamic destabilization. Here, the addition of a second phase, e.g., a binary hydride,4 oxide,6 or a metal,2 reduces the enthalpy of dehydrogenation and results in a lower decomposition temperature. Reversible systems have been found for binary hydride4,7,8 and metal additions.9 One such destabilization agent is Al, predicted by Cho et al.2 to destabilize LiBH4 r 2011 American Chemical Society
forming AlB2 upon decomposition (eq 2) and yield up to 8.6 wt % of hydrogen and a calculated T(1 bar) of 188 °C (cf. 459 °C for LiBH410). 2LiBH4 þ Al / 2LiH þ AlB2 þ 3H2
ð2Þ
However, there is some uncertainty in the literature as to the correct enthalpy of formation for AlB2 with Friedrichs et al. having estimated the T(1 bar) for the destabilized reaction to be only 334 °C.11 Kang et al.12 have shown that ball-milled LiBH4 with Al powder in a 2:1 mol ratio formed AlB2 and LiH upon dehydrogenation at 450 °C and released 7.2 wt % of hydrogen in the first dehydrogenation step after 3 h. Rehydrogenation to a capacity of 5.1 wt % was only achieved at 400 °C under 100 bar of H2. X-ray diffraction (XRD) analysis showed the disappearance of the LiH and AlB2 peaks and the reappearance of LiBH4. Unfortunately, it was found that further dehydriding and rehydriding resulted in a capacity loss down to 3 wt %. Friedrichs et al.11 also showed the reversible reaction pathway by the synthesis of LiBH4 from LiH and AlB2 at 500 °C under 150 bar H2. Again, severe capacity loss was found upon cycling, Received: August 19, 2011 Revised: October 3, 2011 Published: October 04, 2011 22054
dx.doi.org/10.1021/jp208009v | J. Phys. Chem. C 2011, 115, 22054–22061
The Journal of Physical Chemistry C
ARTICLE
from 8 wt % down to 2.4 wt % after just three cycles. In both cases, after dehydrogenation AlB2 was formed, but much unreacted Al still remained, indicating poor kinetics for the desired destabilized reaction. This incomplete formation of AlB2 is often cited to be the cause of the partial formation of LiBH4 and the resulting loss in hydrogen capacity with cycling.11 An in-depth review on these systems is outside the scope of this article; however, the interested reader is directed to reviews on boronbased hydrogen storage materials13,14 and multicomponent hydrogen storage systems.15 The incomplete conversion of Al to its boride may be due to the inherent passivating oxide layer that as-received Al particles will have. LiAlH4 can be used as a source of Al in order to produce highly dispersed, oxide-free Al through the in situ decomposition of LiAlH4.3 The thermal decomposition of LiAlH4 consists of three steps (reactions 35): 3LiAlH4 f Li3 AlH6 þ 2Al þ 3H2
ð3Þ
Li3 AlH6 þ 2Al f 3LiH þ Al þ 3=2H2
ð4Þ
3LiH þ 3Al f 3LiAl þ 3=2H2
ð5Þ
Figure 1. XRD patterns for the as-milled samples of catalyzed and uncatalyzed 2LiBH4:Al and 2LiBH4:LiAlH4.
ratio of 80:1. The catalyzed samples were prepared by the addition of 3 mol % TiCl3. Thermal analysis of 1.54.0 mg samples was undertaken by thermogravimetric analysis (TGA; Netzsch 209 F1) and a differential scanning calorimeter (DSC; Netzsch 204 HP). Both techniques were run using a heating ramp of 10 °C min1 under 1 bar of either argon or hydrogen with a flow rate of 100 mL min1. The samples were loaded into alumina crucibles and hermitically sealed within an aluminum pan and lid. The sample lid was pierced immediately before loading the sample into the instrument to minimize air contamination of the sample. The reversibility of the decomposed samples for both catalyzed and uncatalyzed 2LiBH4:Al and 2LiBH4:LiAlH4 systems were conducted on the DSC, heating to 350 at 10 °C min1 under a pressure of 85 bar of H2, holding at this temperature and pressure for 2 h. After this, the sample was cooled to 50 °C, and a DSC experiment was run under an argon carrier gas and monitored to identify the thermal events for any reformed LiBH4. Powder XRD was recorded on a Bruker D8 Advance, with a 2θ range of 1080°, a step size of 0.02°, and dwell time of 4 s with a CuKα source (λ = 1.5418 Å). For the measurements, a Si single crystal wafer was used and covered with an amorphous polymer film to protect against oxidation during analysis of the sample. Diffuse reflectance infrared spectroscopy (DRIFTS) measurements were performed on a Bruker Tensor 27, using a Pike environment cell (HC-900) with a DiffusIR accessory. Samples were loaded into the environment cell in the glovebox, sealed and transferred to the spectrometer. Measurements were taken at room temperature.
The first two steps (3 and 4) occur between 150 and 210 °C and release 5.3 and 2.6 wt % of hydrogen, respectively. The last step (5), proceeds above 400 °C and releases 2.6 wt % of hydrogen.16 The addition of LiAlH4 to LiBH4 (1:2.2 mol ratio) combined with 10 mol % TiF3 as a catalyst, has been studied3 with a hydrogen release of 7.5 wt % in the first dehydrogenation cycle starting at 350 °C under 0.9 bar of hydrogen. Dehydrogenation/ hydrogenation cycles showed a capacity loss reaching 2.8 wt % in the fourth dehydrogenation cycle. Hydrogenation was achieved at 290 °C under 100 bar of hydrogen. The addition of TiCl3 instead of TiF3 to a 2LiBH4:LiAlH4 mixture and mechanically milling for 30 min also resulted in a low 2.02.4 wt % H2 capacity upon rehydrogenation.17,18 For both these cases, the addition of LiAlH4 to LiBH4 in the presence of a catalyst resulted in partial formation of AlB2, and hence the material was only partially reversible. These results indicate that more detailed studies are needed to understand the 2LiBH4:LiAlH4 reaction pathway, to identify the barrier to the destabilization reaction and reversibility. Also, there has been no direct comparison between sources of Al for the destabilization reaction, i.e., from metal powder and from an alanate source. The work reported here is an investigation of the effect of the Al-source (Al powder and LiAlH4) on the dehydrogenation reaction and cyclability of the multicomponent system. Ball milling conditions such as the order of milling the components has also been investigated, along with the use of TiCl3 as a catalyst precursor. These variables have been correlated with the dehydrogenation/hydrogenation behavior of the materials.
’ RESULTS
’ EXPERIMENTAL SECTION LiBH4 (95%, Sigma Aldrich), LiAlH4 (95%, Alfa Aesar), Al (93%, Sigma Aldrich), and TiCl3 (99.995%, Sigma Aldrich) were used as received without any further purification. All handling and storage of samples were performed in an inert atmosphere (Ar) glovebox with
