Al with hydrogen sorption cycling: Separation of Al and B

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Scripta Materialia 60 (2009) 1089–1092 www.elsevier.com/locate/scriptamat

Microstructural change of 2LiBH4/Al with hydrogen sorption cycling: Separation of Al and B Ji Woo Kim,a,b Oliver Friedrichs,a Jae-Pyoung Ahn,c Do Hyun Kim,b Seul Cham Kim,b Arndt Remhof,a Hee-Suk Chung,b Jehyun Lee,b Jae-Hyeok Shim,d Young Whan Cho,d Andreas Zu¨ttela,* and Kyu Hwan Ohb a

Division of Hydrogen and Energy, Department of Environment, Energy and Mobility, Empa Materials Science and Technology, CH-8600, Switzerland b Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Republic of Korea c Advanced Analysis Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea d Materials Science and Technology Research Division, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea Received 13 December 2008; revised 15 January 2009; accepted 17 January 2009 Available online 29 January 2009

Using transmission electron microscopy, microstructures of 2LiBH4/Al are investigated during the hydrogen cycling (2LiBH4 + Al M 2LiH + AlB2 + 3H2). Mechanically, ball milled 2LiH/AlB2 mixture shows a homogeneous phase distribution. For the hydrogen absorption, the decomposition of AlB2 and the formation of 2LiBH4 occur simultaneously. During the hydrogen cycling, the incomplete formation of AlB2 results in a spatial separation of Al and B. A lack of reactive B from AlB2 results in a limited formation of LiBH4, which leads to the capacity loss of the system. Ó 2009 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. Keywords: Hydrogen storage; Hydrides; Microstructure; Transmission electron microscopy; Focused ion beam

Lithium borohydride (LiBH4) is considered as a promising hydrogen storage material for mobile hydrogen applications due to its great gravimetric hydrogen density (13.9 wt.% H2) according to the hydrogen sorption reaction of LiBH4 M LiH + B + 3/2H2 [1]. However, the practical use of LiBH4 is limited due to the elevated hydrogen desorption temperature (Tdes. = 370°C at 1 bar H2) as a result of its thermodynamic stability (DH = 74 kJ/mol H2) and the kinetic barrier for the re-hydrogenation (Tabs. = 600 °C at 155 bar H2) [2]. To overcome the thermodynamic limitation of LiBH4, significant effort has been made to lower the enthalpy of hydrogen desorption by introducing reactive additives (e.g. MgH2 [3–6], CaH2 [7–9] and Al [10–13]), thus form more stable hydrogen desorption products (e.g. MgB2, CaB6 and AlB2, respectively) upon reaction with LiBH4: 2LiBH4 þ MgH2 $ 2LiH þ MgB2 þ 4H2 ð11:4wt:%H2 Þ

ð1Þ

* Corresponding author. Tel.: +41 44 823 46 92; fax: +41 44 823 40 22; e-mail: [email protected]

6LiBH4 þ CaH2 $ 6LiH þ CaB6 þ 10H2 ð11:6wt:%H2 Þ 2LiBH4 þ Al $ 2LiH þ AlB2 þ 3H2 ð8:3wt:%H2 Þ

ð2Þ ð3Þ

In other words, the additive would lower the hydrogen desorption temperature by destabilizing LiBH4. Indeed, these thermodynamically destabilized systems show lowered hydrogen desorption temperatures together with enhanced hydrogen absorption properties of LiBH4. Recently, we investigated the reversibility of the reaction (3), which starts from the right hand side products (mixture of 2LiH and AlB2) at 450 °C under hydrogen pressure of 13 bar as well as the hydrogen sorption cycling performance without any catalytic additives [14]. With respect to thermodynamics of 2LiBH4/Al system, it was revealed that the enthalpy of reaction (3) (DH = 58.8 kJ/mol H2) is lower than the one for pure LiBH4 (DH = 74 kJ/mol H2) by taking the enthalpy of formation AlB2 (DH = 23.01 kJ/mol AlB2) reported by Mirkovic et al. [15]. However, the expected destabilization effect of Al on the decomposition of LiBH4 was also rather small due to the entropy decrease of AlB2 forma-

1359-6462/$ - see front matter Ó 2009 Published by Elsevier Ltd. on behalf of Acta Materialia Inc. doi:10.1016/j.scriptamat.2009.01.031

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tion (DS = 14 J/mol K) at higher temperatures. With increasing the cycling number, a decomposition of LiBH4, which is not coupled to a formation of AlB2 led into a loss of hydrogen capacity and a degradation of cycling performance. However, the hydrogen absorption process of 2LiBH4/Al is indeed improved and it might be due to the kinetic effects strongly related to microstructures of the system. In order to investigate the effect of microstructures on the hydrogen sorption properties, a number of transmission electron microscopy (TEM) studies have been carried out on complex metal hydrides [16–19]. However, it is extremely difficult to prepare TEM samples due to the high air sensitivity of these materials. In order to overcome the difficulties, we recently proposed a novel method of TEM sample preparation using modified focused ion beam (FIB) which makes it possible to prepare uniformly thin TEM sample which has a wide observable area without air-exposure [20]. In this study, we aim to demonstrate a relation between the microstructural change and the hydrogen sorption properties of 2LiBH4/Al with hydrogen cycling using analytical TEM combined with energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS). The sample preparation of LiH and AlB2 mixture (stoichiometric ratio 2:1) using a high energy ball milling and the hydrogen sorption measurement using a pressure-composition-isotherm (PCI) apparatus are described in detail in Ref. [14]. In this study, three kinds of samples were prepared as presented in Table 1. For the TEM analysis, the cross-sectional TEM samples were obtained from the specific interest region of each sample by a dual-beam FIB (FEI, Nova 200) equipped with a manipulating probe (100.7TM, Omniprobe). A TEM sample preparation process using FIB and airlock loading chamber without air-exposure are described in details in Ref. [20]. Using this technique, TEM samples which have a final thickness of 50 nm and a large observation area (10  5 lm2) were obtained. The prepared TEM sample was loaded into a 200 keV TEM (FEI, Tecnai F20) using a portable glove-bag under Ar atmosphere (99.999%). Scanning transmission electron microscope (STEM) bright field images were collected using a high-angle annular darkfield (HAADF) detector and the chemical composition of the sample was obtained using EDS and EELS. Crystallographic information was obtained by selected area electron diffraction (SAED) patterns. A cross-sectional, scanning electron microscopy (SEM) image of the as-milled 2LiH and AlB2 mixture sample is presented in Figure 1(a), which was observed during the TEM sample preparation using FIB. In order to identify the remarkable phases in the sample, STEM HAADF image (Fig. 1(b)) is measured from the indicated area of Figure 1(a). Indexed SAED pattern (inset of Fig. 1(b)) obtained from the whole area of Figure 1(b) ˚, indicates that AlB2 (PDF No. 39–1483, d100 = 2.6039 A ˚ d101 = 2.0325 A) is the main crystalline phase and LiH ˚ ) phase is also (PDF No. 78–0839, d200 = 2.0304 A clearly observed. From the LiH ring pattern, it can be inferred that LiH phase has a short range ordered crystal structure and the weak diffraction signal is mainly

due to the light weight of LiH phase in contrast to the relatively bright spots of AlB2. Traces of Al (PDF No. ˚ ) and AlB12 (PDF No. 76– 04–0787, d111 = 2.3380 A ˚ ) came from commercial AlB2 1235, d220 = 3.5956 A powder as we measured by XRD in our previous study [14]. In order to investigate the chemical composition, the distinguishable phases of Figure 1(b) are characterized by EDS and EELS. In Figure 1(c), EDS spectra from the each point reveal that point 1 and 2 correspond to AlB2 and Al phase, respectively. On the other hand, the EDS spectrum from point 3 only gives information about Carbon (C) and Oxygen (O) which imply that the region of point 3 is a highly sensitive phase to air. By applying EELS (Fig. 1(d)), point 3 can be identified as a phase containing Li element and it comes to conclusion that the region of point 3 represents LiH phase corresponding with the indexed SAED pattern. From the overview SEM image (Fig. 1(a)) and the identification of the each phase, the representative microstructures of 2LiH and AlB2 mixture after ball milling shows that 300 nm size of AlB2 phase is homogeneously distributed in LiH matrix. To understand the microstructure changes during hydrogen absorption process, the 2LiH and AlB2 mixture sample was quenched from 450 °C to room temperature under 150 bar hydrogen pressure after 7 h of the first hydrogen absorption. STEM HAADF image (Fig. 2(a)) shows representative microstructures of the 2LiH and AlB2 mixture during the hydrogen absorption. From the indexed SAED pattern (Fig. 2(b)), a formation of ˚ ) is LiBH4 phase (PDF No. 27–0287, d111 = 3.3000 A clearly observed together with Al phase according to reaction (3) for the hydrogen absorption. The un-reacted LiH and AlB2 phases are also indicated in the SAED pattern because the absorption process was interrupted before completion. EDS spectra (Fig. 2(c)) from the characteristic regions of STEM HAADF image show the chemical composition of each point. From the chemical composition and the SAED pattern, point 1 is identified as a crystalline Al phase which is originated from decomposed AlB2. Point 2 is regarded as a boron rich-Al phase, which can be produced as an intermediate phase during the decomposition of AlB2 [15] and it has an amorphous structure. The boron rich-Al phase decomposes and it liberates chemically fresh boron source which react with LiH to produce LiBH4. With respect to point 3, EELS spectra (Fig. 2(d)) confirm that the region of point 3 corresponds to the produced LiBH4 phase by reaction (3). According to the phase diagram of Al–B [21], AlB2 intermetallic compound as a single phase starts to decompose at least over 972 °C even though the thermodynamic stability of AlB2 is low. Thus, concerning the hydrogen absorption process of reaction (3), the decomposition of AlB2 and the formation of LiBH4 are indeed coupled. However, this successful hydrogen absorption to produce LiBH4 is contrary to the thermodynamics assessments of 2LiBH4/Al and pure LiBH4. Therefore, it can be explained by the kinetics reasons; (1) a difference of activation energy barrier to produce LiBH4 from LiH and freshly produced boron from AlB2 or chemically stable Boron, and (2) a short range diffusion length between fresh boron and LiH due to the initial homogeneous microstructures of 2LiH/AlB2 mixture.

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Table 1. Sample description. Sample

Experimental condition

Phase composition [14]

2LiH + AlB2 (after ball mill) 2LiH + AlB2 (H2 absorption) 2LiH + AlB2 (after cycling)

Ball milled for 5 h During the 1st hydrogen absorption (Temp. = 450°C, H2 pressure = 150 bar) After the 3rd hydrogen sorption cycling (Temp. = 450°C, H2 pressure = 150 bar)

LiH, AlB2, Al, Al 2O3 LiH, AlB2, LiBH4, Al LiH, AlB2, Al, LiAl

Figure 1. (a) Cross sectional SEM image of the 2LiH/AlB2 mixture after 5 h of ball milling; (b) STEM HAADF image from the indicated area of (a) and SAED pattern (inset); (c) EDS spectra from point 1, 2 and 3 of (b); and (d) EELS spectrum from point 3.

that the formation of AlB2 take place only partially. As can be seen in Figure 3(a) and (b), an aggregation of Al to lager grain (5 lm) is developed after the third hydrogen cycling. EDS spectrum of area 1 of Figure 3(a) and SAED pattern (inset of Fig. 3(c)) from area 1 indicates that the Al is a single crystal phase. In Figure 3(b), the Al signal in the background comes from a FIB sample holder made of Al. The segregation of the Al phase confirms that the formation of AlB2 is incomplete during the hydrogen desorption process of LiBH4. As a result of the incomplete formation of AlB2, it is expected that free boron phase or other boride phase are produced. Figure 4(a) shows an energy filtered TEM (EFTEM) zero loss image of the specific area 2 of Figure 3(a). From the EELS elemental maps of Boron and Al (Fig. 4(b) and (c)), a significant negative correlation between boron and Al map also verify that the reaction to form AlB2 is limited during the hydrogen desorption process of reaction (3). High resolution TEM (HRTEM) image (Fig. 4(d)) from region 1 of Figure 4(a) clearly shows the formation of separated boron phase during the hydrogen sorption cycling. The fast furrier transform (FFT) pattern (inset of Fig. 4(d)) corresponding to the HRTEM lattice fringes is indexed as a rhombohe˚) dral boron phase (PDF No. 85–0409, d020 = 4.4061 A and the crystalline boron phase is chemically stable in contrast to the fresh boron produced from AlB2. In order to re-hydrogenate from LiH and elemental Boron to

Figure 2. (a) STEM HAADF image of the 2LiH/AlB2 mixture during the first hydrogen absorption (450 °C, 150 bar H2) for 7 h; (b) SAED pattern from the whole area of (a); (c) EDS spectra from point 1, 2 and 3 of (a); and (d) EELS spectra from point 3.

In spite of the first successful hydrogen absorption, a significant capacity loss and a degeneration of 2LiBH4/ Al system were observed with increase of the hydrogen cycling number [14]. The third desorption cycle release only 2.4 wt.% of hydrogen and after that a quantity of Al phase appears in the XRD pattern [14] which imply

Figure 3. (a) SEM image of the cross-sectioned 2LiH/AlB2 mixture (after the third desorption cycle) during the preparation of TEM sample using FIB; (b) SEM EDS map of Al of (a); and (c) TEM EDS spectrum and SAED pattern (inset) from area 1 of (a) indicating single crystalline Al phase.

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are generated. With hydrogen cycling, the microstructural segregation of a large size of Al and chemically stable boron crystalline phase leads to an incomplete LiBH4 formation and eventually to a decreased hydrogen capacity of the system. This work has been supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2007-612-D00097) and the 6th Framework Program of the European Commission (NESSHY Contract No.: 518271).

Figure 4. (a) EFTEM zero loss image of the 2LiH/AlB2 mixture after the third desorption cycle; (b) and (c) EELS maps of Boron and Al of (a); (d) high resolution TEM image of area 1 of (a).

LiBH4, instead of the use of fresh boron, an elevated temperature of about 600 °C and a pressure of 155 bar are necessary [2]. Thus, under the experimental conditions (450 °C and 150 bar of hydrogen pressure), the formation of LiBH4 becomes more difficult as the cycle number increase due to the generated chemically stable boron phase and it accelerates the segregation of Al phase. In addition, the liquid phase of LiBH4 during the hydrogen sorption at the given temperature may affect the separation of Al and boron phases, thus, the elements have to diffuse long range to produce AlB2 during the hydrogen desorption. The incomplete LiBH4 formation eventually causes a loss of hydrogen capacity of the 2LiBH4/Al system. The microstructural change of 2LiBH4/Al reversible hydrogen storage system is investigated by analytical TEM and EDS/EELS observation. After ball milling of 2LiH/AlB2 for 5 h, 300 nm size of AlB2 phase is homogeneously distributed in LiH matrix. STEM image and SAED pattern obtained during the first hydrogen absorption show 300 nm size of crystalline Al phase and amorphous boron rich-Al phases, which are developed by the decomposition of AlB2. The successful hydrogen absorption at 450 °C and 150 bar of hydrogen is due to a favorable reaction between LiH and chemically reactive boron from AlB2 as well as a short diffusion length due to the homogeneous phase distribution. For the hydrogen desorption of LiBH4, the destabilization effect on the decomposition of LiBH4 by the formation of AlB2 is rather small due to the thermodynamically unstable AlB2. Therefore, after the third hydrogen sorption cycling, the spatially separated Al and boron phases, which are not participating in the formation of AlB2,

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