Enhanced hydrogen sorption of LiBH4–LiAlH4 by quenching dehydrogenation, ball milling, and doping with MWCNTs

Enhanced hydrogen sorption of LiBH4–LiAlH4 by quenching dehydrogenation, ball milling, and doping with MWCNTs

Journal of Physics and Chemistry of Solids 136 (2020) 109202 Contents lists available at ScienceDirect Journal of Physics and Chemistry of Solids jo...

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Journal of Physics and Chemistry of Solids 136 (2020) 109202

Contents lists available at ScienceDirect

Journal of Physics and Chemistry of Solids journal homepage: http://www.elsevier.com/locate/jpcs

Enhanced hydrogen sorption of LiBH4–LiAlH4 by quenching dehydrogenation, ball milling, and doping with MWCNTs Sukanya Meethom a, Dechmongkhon Kaewsuwan c, Narong Chanlek c, Oliver Utke d, Rapee Utke a, b, * a

School of Chemistry, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima, 30000, Thailand Research Network NANOTEC-SUT on Advanced Nanomaterials and Characterization, School of Chemistry, Suranaree University of Technology, Nakhon Ratchasima, 30000, Thailand c Synchrotron Light Research Institute (Public Organization), Nakhon Ratchasima, 30000, Thailand d Mechanical System Division, Synchrotron Light Research Institute (Public Organization), Nakhon Ratchasima, 30000, Thailand b

A R T I C L E I N F O

A B S T R A C T

Keywords: Hydride composites Aluminum diboride Lithium aluminum alloy Reversibility 27 Al MAS NMR

Poor reversibility of LiBH4 and LiAlH4 (or Li3AlH6) is found in LiBH4–LiAlH4 composite due to the agglomeration of Al upon dehydrogenation, resulting in the ineffective formation of AlB2 and LiAl. A new strategy of quenching the first-step dehydrogenation of LiBH4–LiAlH4 composite (T ¼ 220 � C) and particle size reduction via ball milling, leading to good dispersion of all species especially Al, is proposed for the first time. Additionally, multiwalled carbon nanotubes (MWCNTs) are doped into the milled sample to enhance hydrogen diffusion and thermal conductivity, favoring hydrogen sorption of hydrides. The LiBH4–LiAlH4 composite shows the individual decomposition of LiAlH4 and LiBH4 to LiH, Al, amorphous B, and Li2B12H12; and decomposition of the milled sample quenched during dehydrogenation proceeds through reactions of Al with LiBH4 and LiH to form AlB2 and LiAl, respectively. This leads to three times faster kinetics, reduction of onset temperature by 120 � C, and reversibility of LiBH4, LiAlH4, and Li3AlH6. For the MWCNTs-doped sample, although LiAlH4 and Li3AlH6 cannot be reproduced, kinetics is enhanced due to positive effects of MWCNTs.

1. Introduction Lithium borohydride (LiBH4) has high gravimetric and volumetric hydrogen densities of 18.5 wt % H2 and 121 kg H2/m3, respectively; however, it suffers from high thermal stability (fully dehydrogenation at T > 600 � C) and severe temperature and pressure conditions for hydro­ genation (T ¼ 600 � C and p (H2) ¼ 350 bar) [1–6]. Among several stra­ tegies to solve these problems, composites of LiBH4 with metallic Al [7] and Al-containing compounds (e.g., AlH3, NaAlH4, Na3AlH6, LiAlH4, and Li3AlH6 [8–15]) have been intensively investigated. The LiBH4-1.5Al desorbed hydrogen via several reaction steps through un­ known intermediates and produced LiAl and Li1-xAlxB2, encouraging reversibility of LiBH4 upon rehydrogenation (T ¼ 400 � C and p (H2) ¼ 100 bar). Nevertheless, another composite with lower Al content (LiBH4-0.5Al) showed full dehydrogenation to form LiAl and Li1-xAlxB2 only after doping with 2 mol % TiB2. In addition, amorphous B detected after dehydrogenation due to deficient Al content led to incomplete rehydrogenation [7]. Theoretical calculation of the LiBH4–Al system

showed Al replacement at Li and B atoms and occupation in interstitial sites of LiBH4 [16]. This resulted in weaker B–H bonding interaction and formation of Al–B bonds, benefiting hydrogen desorption kinetics. Furthermore, hydrogen sorption kinetics of LiBH4 were enhanced by up to 10% together with released hydrogen content of 11.2 wt % H2 (T � 500 � C), after compositing with AlH3 (2LiBH4–AlH3) [8]. Kinetic improvement was described by the formation of AlB2 and Li–Al–B via reactions between LiBH4 and oxide-free Al obtained from the decom­ position of AlH3. Other composites of LiBH4–LiAlH4 and LiBH4–Li3AlH6 with and without transition metal-based catalysts (e.g., TiF3, TiCl3, and Tiisopropoxide) [9–11,17,18] showed comparable reaction pathways of (i) dehydrogenation of LiAlH4 and Li3AlH6, (ii) reaction between LiBH4 and Al, (iii) individual dehydrogenation of LiBH4, and (iv) reaction between LiH and Al. Upon dehydrogenation, the formation of several phases, such as Al, LiH, amorphous B, and Li2B12H12 has been observed together with active species of AlB2, Li–Al–B, and LiAl, benefiting hydrogen sorption kinetics and reversibility. When LiBH4–LiAlH4 was

* Corresponding author. School of Chemistry, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima, 30000, Thailand. E-mail address: [email protected] (R. Utke). https://doi.org/10.1016/j.jpcs.2019.109202 Received 30 April 2019; Received in revised form 12 September 2019; Accepted 13 September 2019 Available online 13 September 2019 0022-3697/© 2019 Elsevier Ltd. All rights reserved.

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Fig. 1. PXD (A), FTIR (B), and B 1s XPS (C) spectra of as-prepared LB-LA (a), LB-LA (220) (b), and LB-LA (220)-CNT (c).

doped with 5 mol% TiF4, it showed reductions of dehydrogenation enthalpy and onset temperature by 14 kJ/mol H2 and 150 � C, respec­ tively, with respect to pristine LiBH4 [9]. The LiBH4–Li3AlH6 composite released a total capacity of 8.5 wt % H2 at T < 450 � C as well as showing decreases in onset dehydrogenation temperatures of Li3AlH6 and LiBH4 by 20 and 50 � C, respectively [10]. However, formation of an AlB2 layer around Al particles during dehydrogenation resulted in poor reversibility of LiBH4 due to reduction of the Al-free surface for reacting with LiBH4 and/or LiH to produce AlB2, Li–Al–B, and LiAl [17]. Although LiAlH4 showed good dispersion in LiBH4 matrix after ball milling, this advantage was lost when it melted upon heating and led to agglomeration of metallic Al [17]. Moreover, the considerable amount of metallic Al formed after dehydrogenation of LiBH4–LiAlH4 (or LiBH4–Li3AlH6) with and without catalysts resulted in irreversibility of LiAlH4 and/or Li3AlH6 despite high temperature and pressure applied for rehydrogenation: T ¼ 400–600 � C and p (H2) ¼ 40–80 bar [9–11]. In the present work, we propose a new strat­ egy to enhance surface area and particle distribution of all phases, especially Al, by ball milling the dehydrogenated sample of LiBH4–LiAlH4 quenched at the first-step reaction (decomposition of LiAlH4 at 220 � C). In addition, multi-walled carbon nanotubes (MWCNTs), benefiting hydrogen diffusion and thermal conductivity as well as preventing particle agglomeration and/or sintering upon cycling [19–23] are doped into the milled sample.

2. Experimental details The LiBH4 (�90% hydrogen-storage grade, Sigma-Aldrich) and LiAlH4 (>99.95% hydrogen-storage grade, Sigma-Aldrich) were milled with a molar ratio of 1:1 in a stainless-steel vial (Evico Magnetic, Ger­ many) using a SPEX SamplePrep 8000D DUAL Mixer/Mill to obtain LiBH4–LiAlH4 composite, denoted as LB-LA. Milling time and ball-topowder weight ratio (BPR) were 5 h and 10:1, respectively. The LB-LA was dehydrogenated at 220 � C for 15 min and milled for 5 h with BPR of 10:1, and denoted as LB-LA (220). The MWCNTs (Nano Generation Co. Ltd., Thailand) were treated at 100 � C under vacuum for 1 h to remove oxygen and moisture. Treated MWCNTs (5 wt %) were milled with LB-LA (220) for 30 min with BPR of 10:1 to obtain LB-LA (220) doped with 5 wt % MWCNTs, denoted as LB-LA (220)-CNT. All samples were handled under a nitrogen atmosphere in a glove box (99.9% N2). Powder X-ray diffraction (PXD) experiments of as-prepared and de/ rehydrogenated samples were carried out using a Bruker D2 PHASER with Cu Kα radiation (λ ¼ 1.5406 Å). The sample was packed in an airtight sample holder, covered by a poly (methyl methacrylate) dome. All experiments were carried out at room temperature with scanning step and 2θ range of 0.02� /s and 10–80� , respectively. Fourier transform infrared spectroscopy (FTIR) spectra were recorded using a Bruker Tensor 27-Hyperion 2000. The powder sample was ground with anhy­ drous KBr in a mortar and pressed into pellets for the experiment. The FTIR spectra were collected at room temperature in the wavenumber 2

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Fig. 2. Micrographs of LB-LA (A), LB-LA (220) (B), and LB-LA (220)-CNT (C).

range of 4000–400 cm 1 with 64 scans for both sample and background. The X-ray photoelectron spectroscopy (XPS) was carried out at the SUTNANOTEC-SLRI joint research facility, Synchrotron Light Research Institute (Public Organization), Thailand, using a PHI5000 Versa Probe II (ULVAC-PHI Inc., Japan) with Al Kα (1.486 keV) radiation as an excitation source. The powder samples were deposited on the sample holder using carbon glue tape under an N2 atmosphere in the glove box and placed in a high vacuum chamber (1 � 10 8 mbar) for 2 h prior to the experiment. The high-resolution scan of each element was collected using pass energy and a step size of 46.95 and 0.05 eV, respectively. The binding energy was calibrated with respect to the C 1s peak (284.8eV). The data were processed and analyzed using MultiPak software version 9.6.0 (ULVAC-PHI, Japan). Peak fitting was performed after Shirley background subtraction and a symmetrical Gaussian–Lorentzian func­ tion was used to approximate the line shapes of the fitting components. Solid-state 27Al, magic-angle spinning nuclear magnetic resonance (MAS NMR) experiments were carried out using a Bruker ASCEND™ 500 spectrometer. The powder sample was tightly packed in a zirconia endcapped tube. All experiments were performed at 302 K using a BL4 VTN probe for 4 mm outer diameter rotors. The 27Al chemical shifts were detected in parts per million (ppm) relative to neat aluminum oxide (Al2O3). Spinning speed (ʋR), excitation pulse length, and the number of scans were 8 kHz, 9.8 μs, and 1000, respectively. Morphology of samples was characterized using an optical micro­ scope (BX51, Olympus America Inc., USA). As-prepared LB-LA, LB-LA (220), and LB-LA (220)-CNT were sandwiched between microscope glass slides wrapped with Kapton tape to prevent oxidation from air and moisture. The experiments were carried out under LBD, ND25, and ND6 transmitted light and all micrographs were collected using an Olympus XC50 camera with a color charge-coupled device. Dehydrogenation kinetics and reversibility were investigated using a laboratory-scale setup of a Sievert-type apparatus [24]. The powder sample of 50–100 mg was packed in a high-pressure stainless-steel sample holder (SS316, Swagelok) and transferred to the Sievert-type apparatus. During the experiments, K-type thermocouples

( 250–1300 � C, SL heater) and pressure transducers (0–500 and 0–3000 psig C206, Cole Parmer) were used to measure temperatures and pres­ sures, respectively. Thermocouples and pressure transducers were con­ nected to an AI210I module converter data logger (Wisco) measuring and transferring (every 1 s) the pressure and temperature changes to the computer. Dehydrogenation was carried out at 400 � C under 7 mbar H2 and rehydrogenation was at 400 � C under 80 bar H2 for 16 h. Once the pressure of the system was constant, the amount of hydrogen released was calculated by the pressure change (Δp) and the following equations: (Δp)V ¼ nRT

(1)

H2 desorbed (wt. %) ¼ [(n � 2.0158)/sample weight] � 100

(2)

Where p, V, and T are H2 pressure (atm), volume of system (L), and temperature (K), respectively; and n and R are moles of hydrogen (mol) and the universal gas constant (0.0821 L atm K 1 mol 1), respectively. 3. Results and discussion Phase compositions in as-prepared LB-LA, LB-LA (220), and LB-LA (220)-CNT are investigated by PXD, FTIR, and B 1s XPS techniques. The diffraction pattern of LB-LA shows the signals of LiAlH4, LiBH4, Li3AlH6, and Al/LiH (Fig. 1A(a)), while those of LB-LA (220) and LB-LA (220)-CNT reveal comparable phases of LiBH4 and Al/LiH (Fig. 1 A (b and c)). Due to small and broad diffraction peaks of LiBH4 observed in PXD patterns, especially those of LB-LA (220) and LB-LA (220)-CNT (Fig. 1A(b and c)), FTIR and B 1s XPS experiments are further carried out to track B-containing phases. The FTIR spectra of all as-prepared sam­ ples show vibrational peaks of B–H stretching and bending of LiBH4 (2387–2227 and 1122 cm 1, respectively [25]), O–H bending of air and/or moisture contamination during experiments (1636 cm 1 [24]), and B–O asymmetric stretching due to oxidation of LiBH4 (1600–1300 cm 1 [26,27]) (Fig. 1B). Moreover, B 1s XPS spectra of all as-prepared samples show characteristic peaks of B–H bond of LiBH4 (187.8–188.3 eV) and B–O bond of B2O3 due to oxidation of LiBH4 3

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and/or amorphous state due to ball milling. The formation of Li3AlH6 and Al/LiH detected in LB-LA suggests partial dehydrogenation of LiAlH4 (equation (3)) during ball milling. In the case of as-prepared LB-LA (220) and LB-LA (220)-CNT, the signal of Al/LiH hints at com­ plete dehydrogenation of LiAlH4 to LiH and Al at 220 � C (equation (3) and (4)). Furthermore, the morphology of all as-prepared samples is characterized by optical microscopy. The LB-LA (220) and LB-LA (220)-CNT (Fig. 2B and C) show smaller particles and less agglomera­ tion compared with LB-LA (Fig. 2A). This confirms that particle size reduction can be obtained after ball milling of dehydrogenated LB-LA quenched at 220 � C, leading to high reactive surface area and good dispersion of all phases. 3LiAlH4(l) → Li3AlH6(s) þ 2Al(s) þ 3H2(g)

(3)

Li3AlH6(s) → 3LiH(s) þ Al(s) þ 3/2H2(g)

(4)

Furthermore, kinetics during the 1st dehydrogenation (T ¼ 400 � C and p (H2) ¼ 7 mbar H2) of all as-prepared samples is investigated. The LB-LA releases hydrogen in two steps with total storage capacity of 6.2 wt % H2 (Fig. 3) approaching the results of previous studies (6.10 wt % H2) [30]. The first-step reaction (T ¼ 100–250 � C) rapidly liberating ~4.0 wt % H2 within 2 h agrees with decompositions of LiAlH4 and Li3AlH6 (equations (3) and (4)). For the 2nd step corresponding to decomposition of LiBH4, onset temperature at 364 � C is observed with storage capacity of 2.2 wt % H2 within 6 h. For LB-LA (220) and LB-LA

Fig. 3. The 1st dehydrogenation kinetics of as-prepared LB-LA, LB-LA (220), and LB-LA (220)-CNT.

(192 eV) (Fig. 1C) [20,28,29]. The signal of B–O bond implies oxidation with air and/or moisture of B-containing phases, i.e. LiBH4 and/or amorphous B obtained from decomposition of LiBH4 during ball milling. Thus, the weak LiBH4 diffraction peaks in LB-LA (220) and LB-LA (220)-CNT (Fig. 1A(b and c)) can be explained by nanocrystallite size

Fig. 4. PXD (A) and FTIR (B) spectra of dehydrogenated LB-LA (a), LB-LA (220) (b), and LB-LA (220)-CNT (c) as well as solid-state dehydrogenated LB-LA (220) (a) and LB-LA (220)-CNT (b). 4

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Al MAS NMR (C) spectra of

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kinetics and hydrogen content released from LB-LA (220) and LB-LA (220)-CNT compared to LB-LA (Fig. 3). In addition, Al-containing pha­ ses in dehydrogenated LB-LA (220) and LB-LA (220)-CNT are investi­ gated by solid-state 27Al MAS NMR technique. Both samples reveal characteristic peaks of metallic Al (1638 ppm) and doublet of β-LiAlO2 (13 and 80 ppm; Fig. 4C), approaching values in previous reports of 1640 ppm and doublet at 11.9 and 77.7 ppm for Al and β-LiAlO2, respectively [18,32]. Moreover, the resonance peaks of AlB2 and β-LiAl (or γ-[Li3Al3]) at 865 and 393 ppm, respectively [18,32], are observed in LB-LA (220) (Fig. 4C(a)). The formation of AlB2 is due to dehydroge­ nation of LiBH4 via reacting with Al (equation (7)), while that of LiAl is from reaction between LiH and Al (equation (8)) [32]. LiBH4(l) → LiH(s) þ B(s) þ 3/2H2(g)

(5)

LiBH4(l) → 1/12Li2B12H12(s) þ 5/6LiH(s) þ 13/12H2(g)

(6)

2LiBH4(l) þAl(s) → 2LiH(s) þ AlB2(s) þ 3H2(g)

(7)

2LiH(s) þ 2Al(s) → 2LiAl(s) þ H2(g)

(8)

Considering phase composition in dehydrogenated products, severalstep decompositions of LB-LA (220) and LB-LA (220)-CNT are summa­ rized as follows. Both samples start with complete dehydrogenation of LiAlH4 to produce LiH and Al (equation (3) and (4)) and melting of LiBH4. Afterward, molten LiBH4 proceeds by different mechanisms of (i) individual decomposition to LiH with either amorphous B (equation (5)) or Li2B12H12 (equation (6)) and (ii) reaction with Al to form LiH and AlB2 (equation (7)) (only LB-LA (220)). The LiH further reacts with Al to produce LiAl, which is possible at T > 400 � C under low hydrogen pressure (equation (8)) [8,17,32]. The formations AlB2 and LiAl in dehydrogenated LB-LA (220), benefiting reversibility of LiBH4 and LiAlH4 [8,33], can be achieved due to enhanced surface interaction between Al with molten LiBH4 and LiH obtained from particle size reduction via ball milling. In the case of LB-LA (220)-CNT, the disap­ pearance of AlB2 might be due to the fact that dispersed MWCNTs in hydride matrices prevent contact between molten LiBH4 and metallic Al. Furthermore, dehydrogenation kinetics and reversibility of LB-LA, LB-LA (220), and LB-LA (220)-CNT (T ¼ 400 � C and p (H2) ¼ 7 mbar) are characterized by titration measurements. Hydrogen content pro­ duced from LB-LA during the 2nd cycle is 2.3 wt % H2 (Fig. 5). For modified samples, LB-LA (220) releases comparable hydrogen content in the range of 2.5–2.8 wt% H2 within 3 h for three de/rehydrogenation cycles, while LB-LA (220)-CNT liberates 2.0–3.0 wt% H2 within 3–5 h (Fig. 5). During the 1st cycle, LB-LA (220) and LB-LA (220)-CNT reveal comparable onset dehydrogenation temperatures of 246 � C, corre­ sponding to decomposition of LiBH4. During the 2nd and 3rd cycles, LBLA (220) shows two-step decomposition at onset temperatures of 160 and 318 � C with storage capacities of ~0.5 and 2.0 wt % H2, respectively (Fig. 5A), corresponding to dehydrogenation of LiAlH4 (and/or Li3AlH6) and LiBH4. For LB-LA and LB-LA (220)-CNT, only decomposition of LiBH4 (2.0–2.3 wt % H2) is detected at a comparable onset temperature of 318 � C (Fig. 5B). Thus, LiAlH4 (and/or Li3AlH6) and LiBH4 can be reproduced after rehydrogenation of LB-LA (220), whereas LB-LA and LB-LA (220)-CNT show recovery of only LiBH4. To further confirm the reversibility of hydride composite, chemical compositions in rehydrogenated LB-LA (220) and LB-LA (220)-CNT are characterized by PXD, FTIR, and 27Al MAS NMR techniques. Compara­ ble diffraction peaks of Al/LiH and Li2O observed in rehydrogenated LBLA (220) and LB-LA (220)-CNT indicate incomplete reversibility of LiAlH4 and oxidation of Li-containing phases, respectively (Fig. 6A). In the case of FTIR results, rehydrogenated LB-LA (220) and LB-LA (220)CNT reveal strong B–H vibrations of LiBH4 (2388–2226 and 1128 cm 1 for stretching and bending, respectively) with respect to other phases (Fig. 6B), suggesting reversibility of LiBH4. However, the clear vibra­ tional peak of Li2B12H12 at 2482 cm 1 hints at irreversibility of this thermally stable phase, corresponding to deficient hydrogen content

Fig. 5. Dehydrogenation kinetics and reversibility of LB-LA (220) (A) and LBLA (220)-CNT (B) with respect to LB-LA.

(220)-CNT, single-step dehydrogenation is detected (Fig. 3). The 1st step reaction is not observed due to complete dehydrogenation of LiAlH4 during sample preparation, in accordance with PXD results (Fig. 1A(b and c)). During the 2nd step, decomposition of LiBH4 releases 2.7–3.0 wt % H2 within 2 h (~three times faster kinetics) at significant lower onset temperature of 244 � C with respect to LB-LA (ΔT ¼ 120 � C). Although LB-LA (220) and LB-LA (220)-CNT liberate less hydrogen compared with LB-LA due to complete decomposition of LiAlH4 during sample prepa­ ration, faster dehydrogenation kinetics and significant reduction of onset dehydrogenation temperature of thermally stable LiBH4 can be obtained. By ball milling of LB-LA (220), particle size reduction and good distribution of all phases (Fig. 2B) result in de/rehydrogenation kinetic improvement [31]. Moreover, good particle dispersion, espe­ cially Al in LiBH4 matrix favors the formation of AlB2 and other active species during dehydrogenation of LiBH4 as also shown in Figs. 4 and 5. These active phases lead not only to sorption kinetic improvement of LiBH4 (Fig. 3) but also reversibility of LiAlH4 and/or Li3AlH6. Reaction mechanisms during dehydrogenation of all samples are further characterized by PXD, FTIR, and solid-state 27Al MAS NMR techniques. All dehydrogenated samples show comparable diffraction patterns of Al/LiH together with LiAlO2 and Li2O (LB-LA (220) and LBLA (220)-CNT) due to oxidation of Li-containing phases and LiAl, respectively (Fig. 4A(b and c)). The FTIR spectra of all samples reveal vibrational peaks of B–H stretching and bending of LiBH4 (2388–2226 and 1126 cm 1, respectively), O–H bending of contamination (1634 cm 1), B–O asymmetric stretching due to oxidation of LiBH4 and/ or amorphous B (1600–1300 cm 1), and [B12H12]2 of Li2B12H12 (2486 cm 1) (Fig. 4B). The formation of Al/LiH, amorphous B, and Li2B12H12 observed in dehydrogenated powder of LB-LA (Fig. 4A(a) and B(a)) suggests individual decompositions of LiAlH4 (equation (3) and (4)) and LiBH4 (equation (5) and (6)). Moreover, the relative peak area of LiBH4 vibrations (both stretching and bending at 2388–2226 and 1126 cm 1, respectively) with respect to other phases in dehydro­ genated LB-LA (220) and LB-LA (220)-CNT is significantly lower than that of LB-LA. This implies effective decomposition of thermally stable LiBH4 in LB-LA (220) and LB-LA (220)-CNT, corresponding to superior 5

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Fig. 6. PXD (A), FTIR (B), and solid-state

27

Al MAS NMR (C) spectra of rehydrogenated LB-LA (220) (a) and LB-LA (220)-CNT (b).

released from the decomposition of LiBH4 during the 2nd–3rd cycles (onset at 318 � C) (Fig. 5). For 27Al MAS NMR spectra (Fig. 6C), rehy­ drogenated LB-LA (220) and LB-LA (220)-CNT show comparable char­ acteristic peaks of metallic Al at 1640 ppm as well as α- and β-LiAlO2 at 16.2 and 83.4 ppm, respectively [32]. Moreover, the 27Al MAS NMR spectrum of rehydrogenated LB-LA (220) exhibits the signals of LiAlH4 and Li3AlH6 at 108.3 and 31.2 ppm, respectively (Fig. 6C(a)), while that of rehydrogenated LB-LA (220)-CNT reveals the chemical shift of six-coordinated oxidized AlIII derivatives (Alo) at 65 ppm from the re­ action of highly reactive Al and oxygen impurities (Fig. 6C(b)) [34]. The reversibility of LiAlH4 and Li3AlH6 after rehydrogenation of LB-LA (220) is consistent with the dehydrogenation at low onset temperature (160 � C) detected in the 2nd cycle (Fig. 5A). Considering dehydroge­ nation kinetics and reversibility of LB-LA (220) and LB-LA (220)-CNT (Fig. 5), reversibility of LiBH4 in the 2nd and 3rd cycles is comparable (hydrogen content released of ~2.0–2.3 wt % H2 for both samples). Although the formation of AlB2, favoring reversibility of LiBH4 via reverse reaction of equation (8), cannot be achieved after dehydroge­ nation of LB-LA (220)-CNT, catalytic effects as well as enhanced hydrogen diffusion and thermal conductivity of carbon materials (MWCNTs in this study) [19,21,23] benefit the reproducibility of LiBH4. For LB-LA (220), the simple approach of reducing particle size of Al obtained after decomposition of LiAlH4 via ball milling encourages the

formation of AlB2 and LiAl. The latter leads to effective reversibility of LiBH4, LiAlH4, and Li3AlH6, and enhances hydrogen content desorbed upon cycling. Nevertheless, irreversible phases of metallic Al and Li2B12H12 lead to lower hydrogen contents released and reproduced by both LB-LA (220) and LB-LA (220)-CNT. 4. Conclusions Reversibility of LiBH4 and LiAlH4 (or Li3AlH6) in LB-LA composite was improved by ball milling of dehydrogenated LB-LA quenched at 220 � C (LB-LA (220)). This resulted in particle size reduction and good dispersion of all species, especially metallic Al; and effective formation of AlB2 and LiAl upon dehydrogenation. In addition, MWCNTs enhancing hydrogen diffusion and thermal conductivity for de/rehy­ drogenation were doped into LB-LA (220) to obtain LB-LA (220)-CNT. The LB-LA showed two-step decomposition of LiAlH4 and LiBH4 together with storage capacities of ~4.0 and 2.2 wt % H2, respectively, while LB-LA (220) and LB-LA (220)-CNT revealed only decomposition of LiBH4 with 2.7–3.0 wt % H2. With respect to LB-LA, reduction of onset temperature (ΔT ¼ 120 � C) and rapid kinetics (~three times) during dehydrogenation of the thermally stable phase of LiBH4 were observed for LB-LA (220) and LB-LA (220)-CNT. Dehydrogenation of LB-LA pro­ ceeded through individual reactions of LiAlH4 and LiBH4 to produce Al, 6

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Journal of Physics and Chemistry of Solids 136 (2020) 109202

LiH, amorphous B, and Li2B12H12 without any active species. For LB-LA (220) and LB-LA (220)-CNT, in addition to comparable phases with LBLA, the formation of AlB2 (only LB-LA (220)) and LiAl suggested effec­ tive reaction of Al with molten LiBH4 and LiH, respectively, due to high surface area and good distribution of Al. Owing to the formation of AlB2 and LiAl, reversibility of LiBH4, LiAlH4, and Li3AlH6 was found in LB-LA (220), but LB-LA (220)-CNT showed only reversibility of LiBH4. This might be explained by the fact that contact between Al and other phases was obstructed by dispersion of MWCNTs in hydride matrices, leading to deficient formation of active species for reversibility.

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