Hydrogen Storage Capacity Loss in a LiBH 4 –Al Composite

Article pubs.acs.org/JPCC

Hydrogen Storage Capacity Loss in a LiBH4−Al Composite Bjarne R. S. Hansen,† Dorthe B. Ravnsbæk,†,‡ Daniel Reed,§ David Book,§ Carsten Gundlach,∥ Jørgen Skibsted,⊥ and Torben R. Jensen*,† †

Center for Materials Crystallography, Interdisciplinary Nanoscience Center (iNANO), and Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark ‡ Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States § School of Metallurgy and Materials, University of Birmingham, Birmingham, B15 2TT, United Kingdom ∥ MAX-II Laboratory, Lund University, Ole Römers väg 1, 223 63, S-22100 Lund, Sweden ⊥ Instrument Centre for Solid-State NMR Spectroscopy, Department of Chemistry, and Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark S Supporting Information *

ABSTRACT: A detailed investigation of the decomposition reactions and decay in the hydrogen storage capacity during repeated hydrogen release and uptake cycles for the reactive composite LiBH4−Al (2:3) is presented. Furthermore, the influence of a titanium boride, TiB2, additive is investigated. The study combines information from multiple techniques: in situ synchrotron radiation powder X-ray diffraction, Sieverts measurements, simultaneous thermogravimetric analysis, differential scanning calorimetry and mass spectroscopy, solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR), and Raman spectroscopy. The decomposition of LiBH4−Al results in the formation of LiAl, AlB2, and Li2B12H12 via several reactions and intermediate compounds. The TiB2 additive appears to have a limited effect on the decomposition pathway of the samples, but seems to facilitate formation of intermediate species at lower temperatures compared to the sample without additive. Solid solutions of LixAl1−xB2 or Al1−xB2 are observed during decomposition and from Rietveld refinement the composition of the solid solution is estimated to be Li0.22Al0.78B2. The intercalation of Li in the AlB2 structure is further investigated by 11B and 27Al MAS NMR spectra of the LiH-AlB2 and AlB2 samples (presented in Supporting Information). Hydrogen release and uptake for LiBH4−Al reveals a significant loss in the hydrogen storage capacity, that is, after four cycles a capacity of about 45% remains, and after 10 cycles, the capacity is degraded to approximately 15% of the theoretically available hydrogen content. This capacity loss may be due to the formation of Li2B12H12, as observed by 11B MAS NMR and Raman spectroscopy. Formation of Li2B12H12 has previously been observed during the decomposition of LiBH4, but it has not been reported earlier in the LiBH4−Al (2:3) system. significant interest.9−12 However, its relatively poor thermodynamic and kinetic properties have hampered its utilization in technological applications. At ambient temperature, LiBH4 exists as an orthorhombic polymorph denoted o-LiBH4 and transforms to a hexagonal polymorph, h-LiBH4 at 110 °C. hLiBH4 melts at 275 °C and decomposes at temperatures above 375 °C, partly due to the large enthalpy change (ΔH = −74 kJ/ mol H2) associated with the following reaction,10−12

1. INTRODUCTION Renewable energy sources like wind, solar, and hydropower are environmentally friendly and inexhaustible alternatives to fossil fuels. However, the utilization of these sources is hampered by their fluctuation in time and nonuniform geographical distribution. A solution to this problem would be a safe, efficient, cheap, and clean method to store the harvested energy. The supply of clean and sustainable energy is among the most important scientific and technical challenges facing humanity in the 21st century. Hydrogen is suggested as a future energy carrier of renewable energy, however, safe, compact, and efficient storage is challenging.1−3 Metal borohydrides, alanates, and amides fulfill some of the requirements for successful hydrogen storage, and these materials have therefore been subject to much scientific attention.4−8 Lithium borohydride, LiBH4, has a high gravimetric hydrogen capacity of 18.5 wt % H2 and has therefore received © 2013 American Chemical Society

LiBH4(l) → LiH(s) + B(s) + 3/2H 2(g)

(1)

Even at these conditions only three-quarters of the hydrogen content is available as indicated by eq 1, that is, ρm = 13.9 wt % H2. Furthermore, elevated temperatures above 600 °C and Received: December 18, 2012 Revised: March 4, 2013 Published: March 12, 2013 7423

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pressures above 155 bar H2 are required for reversal of eq 1.12 This may be due to formation of stable intermediate species such as amorphous boron or lithium dodecahydro-closododecaborate, Li2B12H12.13−17 One approach, which leads to more energetically favorable decomposition pathways, is the utilization of reactive hydride composites (RHC).18−34 This has been demonstrated for several compounds, for example, LiBH4−MgH2 composites, where full dehydrogenation and rehydrogenation is achieved by formation of MgB2 and LiMg alloys.18−22,33 It has been suggested that MgB2 promotes the reformation of the BH4 complex, which is otherwise difficult to obtain from hydrogenation of the decomposition products. Similarly, the conditions for decomposition and the reversible formation of LiBH4 can be significantly improved by the addition of aluminum, which results in formation of more favorable decomposition products, that is, AlB2 and LiAl. The LiBH4− Al composite releases hydrogen in two steps as shown in eqs 2 and 3 below. Furthermore, addition of a Ti-additive is suggested to promote the formation of LiAl in LiBH4−Al (2:1).31 2LiBH4(l) + Al(s) → AlB2 (s) + 2LiH(s) + 3H 2(g)

(2)

2LiH(s) + 2Al(s) → 2LiAl(s) + H 2(g)

(3)

2. EXPERIMENTAL DETAILS 2.1. Sample Preparation. Lithiumborohydride, LiBH4 (Aldrich, ≥95%), aluminum, Al (Strem Chemicals, 99.7%), and titanium diboride, TiB2 (Aldrich), were purchased commercially and used without further purification. LiBH4 and Al powders were mixed in a relative molar ratio of 2:3 (S1 and S2). Additionally, 4.5 mol % TiB2 with respect to LiBH4 was added to sample S2 (see Table 1). All handling was Table 1. Composition of the LiBH4−Al Samples and Their Theoretical Gravimetric H2 Contents, ρm (wt% H2) composition (mol %) samples

LiBH4

Al

TiB2

ρm (wt% H2)

S1 S2

40.0 39.3

60.0 58.9

1.8

6.4 6.1

conducted under an argon atmosphere in a glovebox with a circulation purifier (p(O2, H2O) < 1 ppm). All samples were mechanically milled using a Fritsch Pulverisette No. 4, 80 mL of tungsten carbide (WC) vials, and 10 mm WC balls with a ballto-powder mass ratio of approximately 35:1. All samples were milled for 120 min at a milling rate of 400 rpm under an argon atmosphere. Furthermore, samples of AlB2 and LiH−AlB2 were prepared and heated to 500 °C for 5 h under inert conditions, as described in Supporting Information. 2.2. Synchrotron Radiation Powder X-ray Diffraction (SR-PXD). In situ time-resolved SR-PXD data was collected at beamline I711 at the synchrotron MAX-II, Lund, Sweden, in the research laboratory MAX-Lab with a MAR165 CCD detector system.39 The selected wavelengths were 1.10205 Å (S1) and 0.98922 Å (S2). The used sample cell was specially developed for studies of gas/solid reactions and allows variable pressures and temperatures to be applied.40 The powdered samples were mounted in a sapphire single-crystal tube (Al2O3, outer diameter 1.09 mm, inner diameter 0.79 mm) in an argonfilled glovebox (p(O2 , H2 O) < 1 ppm). The sample temperature was controlled with a thermocouple placed in the sapphire tube approximately 1 mm from the sample. A gas supply system was attached to the sample cell, which allowed a change in gas and pressure via a vacuum pump during the X-ray experiment. The system was flushed with argon and evacuated three times before the valve to the sample was opened and the X-ray experiment began. During desorption measurements the samples were heated to 500 °C (ΔT/Δt = 10 °C/min) at dynamic vacuum with the samples connected to a vacuum pump. The temperature was kept at 500 °C for 15 min before the samples were cooled to room temperature (RT). During absorption measurements the samples were heated to 400 °C (ΔT/Δt = 15 °C/min) under a pressure of p(H2) = 100 bar and then kept at a constant temperature for 1 h before cooling to RT. The FIT2D program was used to remove diffraction spots originating from the single-crystal sapphire tubes and subsequently to transform raw data to powder patterns.41 Rietveld refinement was performed on selected PXD data using the program Fullprof.42 The background scattering was described by linear interpolation between selected points, while pseudo-Voigt profile functions were used to fit the diffraction peaks. In general, the unit cell parameters, zero shift, profile parameters, and the overall temperature factors, Bov, were refined. In some cases, preferred orientation corrections were included in the refinements.

The enthalpy change for eq 2 has been calculated to ΔH = −59.3 kJ/mol H2,12 which is significantly lower than the enthalpy change of the decomposition of pristine LiBH4 according to eq 1. In addition, eq 3 indicates that the full H2 capacity of LiBH4 is available in the LiBH4−Al (2:3) composite. A further advantage is that the rehydrogenation of the LiBH4− Al composite can be achieved using more moderate conditions than for pristine LiBH4, that is, T = 400 °C and p(H2) = 100 bar.23,27,28 However, it should be noted, that the addition of 1.5 mol equiv of Al relative to LiBH4 lowers the gravimetric H2 capacity to ρm = 6.4 wt % H2. A recent study reveals a more complex decomposition of the LiBH4−Al composite than previously anticipated, which occurs via an unknown intermediate compound.31 Furthermore, repeated hydrogen release and uptake has shown that the LiBH4−Al composite suffers from a loss in hydrogen storage capacity. Previous studies suggest loss of diborane gas from the system or formation of Li2B12H12 or amorphous boron.12,35−37 Hence, the mechanism of hydrogen release and uptake reactions including analysis of the capacity loss needs to be further investigated. Additionally, titanium additives such as TiCl3 have been shown to improve the Alanate-system significantly. However, TiCl3 reacts with LiBH4 and forms LiCl prompting the use of more inert additives such as TiB2,38 which was already shown to improve hydrogen storage properties for the composite LiBH4−Al (2:1).31 The present investigation focuses on the detailed mechanism for hydrogen release and uptake and the decay in storage capacity during multiple cycles of hydrogen release and uptake for the LiBH4−Al (2:3) composite. This investigation utilizes complementary methods, that is, in situ synchrotron radiation powder X-ray diffraction (SR-PXD), simultaneous thermal analysis and mass spectroscopy (TGA/DSC-MS), Raman and solid-state 11B magic-angle spinning (MAS) NMR spectroscopy, and Sieverts method measured over 10 cycles of hydrogen release and uptake. 7424

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The solid solution, LixAl1−xB2, was modeled by introducing Li on the Al position in the structure of AlB2 and the sum of Li and Al occupancies were constrained to one and refined. Analysis of the sample compositions during hydrogen release and uptake is challenging because several compounds are isostructural and have similar unit cell parameters, that is, Al and LiH crystallize in space group Fm-3m and AlB2 and TiB2 in P6/mmm (see Table 2).

Table 3. Overview of the Notation Used for the Different Samples in This Study and the Performed Measurementsa name S1 S2

Table 2. Crystallographic Data for Compounds Observed in the LiBH4−Al System compound

space group

a (Å)

b (Å)

c (Å)

ref

o-LiBH4 h-LiBH4 LiH Al TiB2 AlB2 LiAl WC

Pnma P63mc Fm-3m Fm-3m P6/mmm P6/mmm Fd-3m P6-m2

7.179 4.276 4.083 4.049 3.028 3.006 6.376 2.907

4.437

6.803 6.948

10 10

DS1

X

DS2

X

AS1

X

AS2

X

description ball-milled LiBH4−Al (2:3) ball-milled LiBH4−Al−TiB2 (2:3, 4.5 wt % TiB2) S1 after 10 desorption and 9 absorption experiments S2 after 10 desorption and 9 absorption experiments S1 after 10 desorption and 10 absorption experiments S2 after 10 desorption and 10 absorption experiments

Additionally, all samples are investigated by PXD. Samples in a dehydrogenated (desorbed) state or a hydrogenated (absorbed) state are denoted DS or AS, respectively.

56 57

relaxation delay, and 1H decoupling (γB2/2π ≈ 50 kHz) during acquisition. The experiments were performed at ambient temperature using airtight end-capped zirconia rotors packed with the samples in an argon-filled glovebox. Isotropic chemical shifts are relative to neat F3B·O(CH2CH3)2, employing a 0.1 M H3BO3 aqueous solution (δiso = 19.6 ppm) as a secondary, external standard sample. 2.7. Raman Spectroscopy. Raman spectra were obtained for samples DS1 and DS2 using a Renishaw inVia Raman Microscope with a 488 nm excitation laser (2 mW power). A microscope objective was used to focus the laser beam onto the sample with a spot-diameter of about 50 μm. Samples were measured under 1.0 bar flowing Ar, using an Instec HCS621V sample cell stage.

58 59

2.837

Raman NMR

X X

a

55

3.228 3.247

SRPXD

60

2.3. In-House Powder X-ray Diffraction. In-house PXD measurement was performed in Debye−Scherrer transmission geometry using a STOE Stadi P diffractometer equipped with a curved Ge(111) monochromator (Cu Kα1 radiation) and a curved position sensitive detector. Data was collected at RT between 4 and 127° 2θ with a counting time of 180 s per step. The samples were mounted in 0.5 mm special glass No. 0140 or 0.7 mm quartz capillaries sealed with glue under argon atmosphere. 2.4. Thermal Analysis and Mass Spectroscopy (TGA/ DSC-MS). Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) was measured using a PerkinElmer STA 6000 connected to a mass spectrometer (MS; Hiden Analytical HPR-20 QMS sampling system). Samples of S1 and S2 were transferred to Al2O3 crucibles under argon atmosphere. The samples were heated from 40 to 550 °C (ΔT/Δt = 5 °C/ min) with an argon purge rate of 65 mL/min. The outlet gaseous species H2 and B2H6 were monitored using mass spectroscopy. 2.5. Sieverts Measurement. The samples were transferred to a stainless steel high-temperature autoclave, sealed under argon and attached to the Sieverts apparatus (Hy-Energy PCTPro-2000).43 During desorption measurements the sample was heated to 300 °C (ΔT/Δt = 10 °C/min) and then slowly from 300 to 500 °C (ΔT/Δt = 1 °C/min) at p(H2) = 10−2 bar. This temperature was maintained for 2.5 h followed by natural cooling to RT. During hydrogen absorption measurements the sample was heated to 400 °C (ΔT/Δt = 5 °C/min) at p(H2) = 100 bar, kept at a constant temperature, 400 °C, for 2 h and then allowed to cool naturally to RT. After the first de- and absorption measurements the samples were characterized with PXD without exposure to air. After 10 desorptions and nine absorptions the samples were denoted DS1 and DS2, and after 10 desorptions and 10 absorptions, they were denoted AS1 and AS2 (see Table 3) 2.6. 11B MAS NMR. 11B MAS NMR spectra were obtained for samples AS1 and AS2 on a Varian Direct-Drive VNMRS600 spectrometer (14.1 T) using a home-built CP/MAS NMR probe for 4 mm outer diameter rotors. The experiments employed a spinning speed of νR = 13.0 kHz, a 0.5 μs excitation pulse for a 11B rf field strength of γB1/2π ≈ 60 kHz, a 4 s

3. RESULTS AND DISCUSSION 3.1. In Situ Synchrotron Radiation Powder X-ray Diffraction Measurements. The in situ SR-PXD desorption measurements for S1 are presented in Figure 1 and the Rietveld refinement of the pattern at RT in Figure S1. At T ≈ 100 °C, oLiBH4 undergoes a transformation to the hexagonal high-

Figure 1. In situ SR-PXD desorption measurement for LiBH4−Al (S1, 2:3) in the temperature range from RT to 500 °C, at p(H2) = 10−2 bar (ΔT/Δt = 10 °C/min, λ = 1.10205 Å). The temperature was maintained at 500 °C for 15 min. Symbols: (open square) o-LiBH4, (gray square) h-LiBH4, (star) WC, (black square) Al and LiH, (black circle) 1, (gray circle) 2, (open triangle) LixAl1−xB2, (open circle) LiAl, (gray diamond) 3. 7425

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temperature polymorph h-LiBH4, which disappears at T ≈ 250 °C due to melting. At approximately 360 °C an unidentified compound (denoted 1) is formed, possibly from a reaction between molten LiBH4 and Al, which form LiH and 1. The formation of 1 is accompanied by a sharp decrease in the diffracted intensity of peaks from Al and LiH (at T ≈ 360 °C). This is further highlighted in Figure 2 where integrated

Al2B3, and other Li/Al borides. LiAl is formed at 400 °C from Al and LiH, which indicates that the sample undergoes both decomposition eqs 2 and 3. The diffraction from LixAl1−xB2 overlaps with the diffraction from Al at 2θ ≈ 31° in the temperature range T = 460−500 °C. When 2 disappears at T = 475 °C, diffraction from Al and LiH increase in intensity, hence, 2 may contain Al and LiH. An unidentified compound (denoted 3) is observed at T = 500 °C with no significant changes in the diffracted intensity for the remainder of the experiments including cooling and the hydrogen absorption experiment (described below). Hence, 3 may be a stable impurity. Rietveld refinement of PXD data obtained after the desorption at RT (shown in Figure 3) provides a solid solution with composition Li0.22Al0.78B2 (provided in Figure S2). The (001)-reflection is nearly extinct, which may be due to Lidoping in AlB2 in accord with previous investigations.31,45 It is noted that the composition is very similar to that obtained in a previous study, that is, Li0.18Al0.82B2.45

Figure 2. (a) Integrated intensities of selected Bragg peaks from the in situ SR-PXD desorption measurement of LiBH4−Al (S1, 2:3) shown in Figure 1. (b) Enlargement of the section marked with a dashed box in (a). Symbols: (black square) diffracted intensity from Al and LiH at 2θ ≈ 27°, (gray triangle) diffracted intensity from Al, LixAl1−xB2 and LiH, (open square) o-LiBH4, (gray square) h-LiBH4, (black circle) 1 at 2θ ≈ 35°, (gray circle) 2 at 2θ ≈ 34°, (open circle) LiAl, (gray diamond) 3 at 2θ ≈ 9°.

diffracted intensities from the observed compounds are presented. Compound 1 has previously been observed as an intermediate in the decomposition of LiBH4−Al and LiBH4− NaAlH4.31,32 Compound 1 may be a metal boride with composition and structure different from AlB2 (P6/mmm) or an intermetallic compound. At approximately 380 °C, 1 possibly undergoes a change in composition. This is observed as a rapid shift of several Bragg peaks Δ2θ ≈ 1° toward lower 2θ value. However, most of the peaks from 1 decrease in intensity and vanish. After this structural change the compound is denoted 2. Compound 2 possibly transforms to a solid solution of LixAl1−xB2. Compounds 1 and 2 appear different from all known compounds, for example, Al2Li3, Al4Li9, Al3Li,44

Figure 3. In situ SR-PXD hydrogen absorption measurement of LiBH4−Al (S1, 2:3) in the temperature range from (a) RT to 400 °C (ΔT/Δt = 15 °C/min) and (b) cooling from 400 °C to RT (ΔT/Δt = 15 °C/min) at p(H2) = 100 bar. The temperature was maintained at 400 °C for 60 min prior to cooling (λ = 1.10205 Å). Symbols: (open square) o-LiBH4, (gray square) h-LiBH4, (star) WC, (black square) Al and LiH, (open triangle) LixAl1−xB2, (open circle) LiAl, (gray diamond) 3. 7426

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The hydrogen absorption measurement of S1 during heating to 400 °C is presented in Figure 3a. Peaks from LiAl disappear at approximately T = 200 °C simultaneously with the appearance of diffracted intensity from Al and LiH. The sample is kept at 400 °C for 1 h, and during the subsequent cooling, shown in Figure 3b, Bragg peaks from h-LiBH4 crystallize from the melt at T = 240 °C. The phase transformation of h- to o-LiBH4 is observed at T ≈ 100 °C. Hereby it is shown that LiBH4−Al (S1, 2:3) can be rehydrogenated under relatively mild physical conditions. Rietveld refinement was performed for the first diffractogram obtained before the desorption measurement (see Figure S1) and on the last diffractogram measured after the absorption experiment (see Figure S3), providing the sample compositions presented in Table 4. The sample composition before the Table 4. Fractional Composition of LiBH4−Al (S1, 2:3) before Desorption and after Absorption Extracted by Rietveld Refinement of SR-PXD Data Measured at RT (see Figures 1 and 3, Diffraction from 3 is Neglected) before desorption

Figure 5. Integrated intensities for selected Bragg peaks from the in situ SR-PXD desorption measurement of LiBH4−Al−TiB2 (S2, 2:3, 4.5 mol %). Symbols: (black square) contribution from Al and LiH (at 2θ ≈ 24°), (gray inverted triangle) contribution from Al, LixAl1−xB2, LiH, and TiB2 (at 2θ ≈ 28°), (open square) o-LiBH4, (gray square) hLiBH4, (black circle) 1 at 2θ ≈ 31°, (gray circle) 2 at 2θ ≈ 30°, (open circle) LiAl.

after absorption

compound

wt%

mol %

wt%

mol %

LiBH4 Al WC

31.49 68.00 0.51

36.43 63.51 0.07

20.86 78.95 0.19

24.65 75.32 0.02

decomposition in several steps, as diffraction from Al and LiH decreases simultaneously with the formation of 1 and LiAl. The Bragg peak at 2θ ≈ 31° from 1 shifts abruptly to 2θ ≈ 30° at T ≈ 380 °C due to formation of 2. Formation of LixAl1−xB2 expected when 2 disappears is not clearly visible in Figure 4. Increasing diffracted intensity from Al and LiH is observed when 2 disappears, which is similar to the observations for sample S1. However, at T = 500 °C, the diffraction from Al at 2θ ≈ 24° is decreasing, while diffraction from both Al and LixAl1−xB2 at 2θ ≈ 28° is constant. This indicates insertion of excess aluminum in LixAl1−xB2, which becomes more Al-rich due to extraction of lithium and formation of LiAl. The TiB 2 additive does not seem to impact the decomposition of LiBH4−Al (S2, 2:3, 4.5 mol %) to the same extent as previously observed for LiBH4−Al (2:1, 2.0 mol % TiB2).31 However, a notable difference is the formation temperature of the intermediates. Compound 1 appears at T ≈ 290 °C in S2, which is approximately 80 °C, lower than observed in S1 (T ≈ 370 °C). Likewise, compound 2 is formed at approximately 400 and 350 °C in S1 and S2, respectively. Accurate determination of quantitative sample compositions and formation temperatures, for example, for LixAl1−xB2, is hampered by multiple overlapping Bragg diffraction peaks (see Table 2). Apparently, LixAl1−xB2 forms at 470 and 440 °C in S1 and S2, respectively, which may be an additive effect from TiB2 causing formation of intermediates at lower temperatures in S2. In both samples LiAl is formed at ∼400 °C. Samples of LiH−AlB2 (1:2) have been investigated by PXD and MAS NMR before and after heat treatment at 500 °C for 5 h (see Supporting Information). PXD and NMR reveal formation of LiAl in LiH−AlB2 during heating, partly due to a reaction between LiH and an Al impurity in AlB2, in accord with a recent study of the Al−LiBH4 system.35 The 11B MAS NMR spectra of AlB2, AlB2 after heating, and the ground LiH− AlB2 sample are identical and contain a centerband resonance at −4.7 ppm from AlB2, with indications of the two singularities for a second-order quadrupolar line shape (Figure S5a). However, for the heated LiH−AlB2 sample, the 11B centerband resonance is shifted to higher frequency and exhibits a slightly

desorption corresponds well with the anticipated as-milled LiBH4−Al (S1, 2:3) composite, whereas the sample composition after hydrogen absorption shows that approximately 67% of the LiBH4 is reformed. The in situ SR-PXD data for LiBH4−Al−TiB2 (S2, 2:3, 4.5 mol %) is shown in Figure 4. At T ≈ 100 °C, o-LiBH4 transforms to h-LiBH4 and melts at T ≈ 250 °C. Compound 1 is observed at T ≈ 290 °C, which transforms to 2 at T = 380 °C, and at T = 390 °C, the intermetallic compound LiAl is formed. The integrated intensities presented in Figure 5 reveal

Figure 4. In situ SR-PXD desorption measurement of sample LiBH4− Al−TiB2 (S2, 2:3, 4.5 mol %) in the temperature range from RT to 500 °C, at p(H2) = 10−2 bar (ΔT/Δt = 10 °C/min, λ = 0.98922 Å). The sample was kept at a constant temperature of 500 °C for 15 min. Symbols: (open square) o-LiBH4, (gray square) h-LiBH4, (star) WC, (black square) Al and LiH, (black triangle) TiB2, (black circle) 1, (gray circle) 2, (gray inverted triangle) LixAl1−xB2 and TiB2, (open circle) LiAl. 7427

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lower line width (Figure S5b), indicating that the local environments of the B atoms have been modified/perturbed. Thus, the present results indicate that LiH and AlB2 react upon heating, forming either Al1−xB2 or a LixAl1−xB2 solid solution. Thus, the PXD data presented above suggest that a Li/Al exchange may occur in the layered structure (Li/Al)B2 and that 1 and 2 may be new Li−Al−B compounds, which change composition and form LixAl1−xB2 as a function of sample composition and temperature. 3.2. TGA/DSC-MS Measurements. The simultaneous thermal analysis and mass spectroscopy measurements of S1 and S2 are presented in Figure 6. For both samples the Figure 7. Sieverts desorption measurements of LiBH4−Al (S1, 2:3). The dashed line is the temperature profile and the 10 desorption measurements are numbered (1)−(10). The sample was heated to 300 °C (ΔT/Δt = 10 °C/min) and, subsequently, to 500 °C (ΔT/Δt = 1 °C/min) and then kept at 500 °C for 150 min.

Figure 8. Sieverts desorption measurements of LiBH4−Al−TiB2 (S2, 2:3, 4.5 mol %). The dashed line is the temperature profile and the ten desorption measurements are numbered (1)−(10). The sample was heated to 300 °C (ΔT/Δt = 10 °C/min) and, subsequently, to 500 °C (ΔT/Δt = 1 °C/min) and then kept at 500 °C for 150 min.

Figure 6. TGA/DSC-MS profiles of LiBH4−Al (S1, 2:3) and LiBH4− Al−TiB2 (S2, 2:3, 4.5 mol %) in the temperature range RT to 550 °C (5 °C/min).

polymorphic transformation of o- to h-LiBH4 and the melting is observed as endothermic events at T = 106 ± 1 and 278 ± 1 °C, respectively. At approximately 350 °C, another endothermic event is observed followed by a gravimetric loss of approximately 4.5 and 4.0 wt % for S1 and S2, respectively. This is accompanied by a release of hydrogen shown in the MS data. This thermal event is likely due to the formation of 1, followed by the formation of 2 and AlB2. At T ≈ 410 °C, LiAl is formed in another endothermic event associated with a minor gravimetric mass loss and release of hydrogen. The total gravimetric loss for S1 and S2 is 6.07 and 5.42 wt %, corresponding to 94.8 and 88.9% of the theoretical hydrogen content, respectively (see Table 1). Hydrogen is released in two steps, which indicates that eqs 2 and 3 occur. Diborane, B2H6, is not detected by MS during the heating of the samples. The results from TGA/DSC-MS are in good agreement with the in situ SR-PXD measurements of S1 and S2. 3.3. Sieverts Measurements. The desorption measurements from the 10 cycles of Sieverts measurements of S1 and S2 are shown in Figures 7 and 8, where the hydrogen release (in wt% H2) for each desorption cycle is presented. Sample S1 releases 4.7 wt % (75% of the total hydrogen capacity) in the first dehydrogenation step, while 1.0 wt % H2 is released in the second step, that is, 5.7 wt % in total corresponding to 89% of the hydrogen content of the sample. The two-step release indicates that both decomposition eqs 2 and 3 occur, which is in good agreement with the mass spectroscopy measurements of the samples. However, the data reveal that numerous hydrogen release and uptake cycles result in a dramatic loss in

hydrogen storage capacity and only 1.8 wt % H2 is released in the tenth desorption measurement. The seventh desorption measurement deviates slightly from the other measurements due to an artifact. During the 10 cycles, the second hydrogen release has a shift toward lower onset temperature possibly due to decreasing amounts of hydrogen released during the first step, which may shift the gas−solid equilibrium in the closed system and enable the second gas release at lower temperatures. A gradual decrease in hydrogen storage capacity is also observed during the ten desorption measurements for S2, which is shown in Figure 8. The first desorption of S2 releases 4.6 wt % and a total hydrogen releases of 5.5 wt % corresponding to 90% of the theoretical hydrogen content. The tenth desorption releases only 1.6 wt %, revealing a significant decay in storage capacity and low cyclic stability of the system. The hydrogen release for S2 occurs at slightly lower temperatures as compared to S1 possibly due to an additive effect from TiB2. The total normalized gas release from sample S1 and S2 during each Sieverts desorption measurements is presented in Figure 9. The decay in hydrogen storage capacity is nearly identical for the two samples, which indicates that the additive TiB2 in the LiBH4−Al (2:3) composite only has a minor kinetic effect, that is, TiB2 appears to facilitate the formation of the intermediate compounds 1 and 2, which are observed at slightly lower temperatures for S2 as compared to S1 during hydrogen release. 7428

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Figure 9. Comparison of the gas release and decay in hydrogen storage capacity of LiBH4−Al (S1, 2.3) and LiBH4−Al−TiB2 (S2, 2:3, 4.5 mol %) during the PCT desorption measurements. The data is normalized using the absolute release in the first cycle of 5.7 and 5.5 wt % for S1 and S2, respectively.

Figure 11. SR-PXD data for the decomposed samples DS1 and DS2 after 10 Sieverts hydrogen desorption experiments (λ = 1.00355 Å, DS1 and λ = 0.98922 Å, DS2). Symbols: (black square) Al and LiH, (gray triangle) AlB2, (black triangle) TiB2, (star) WC, (open circle) LiAl, and (open diamond) 4.

from TiB2 in AS2. Crystalline LiBH4 is not observed in any of the two samples.

Diffraction data measured before and after the first desorption and after the first absorption of hydrogen performed in the Sieverts apparatus is shown in Figure 10. In both samples AlB2 is observed in the decomposed samples. In contrast, the intermetallic LiAl is not observed possibly due to the moderate hydrogen pressure generated during the first desorption (p ≈ 0.15 bar). This may also explain why LiAl was not observed by ex situ PXD of desorbed samples in other previous studies.23,27 During the absorption measurement LiBH4 is formed in both samples. Diffraction data 2 and 3 for both samples were measured using quartz capillaries as sample holders, which gives a broad hump in the background in the 2θ range 15 to 25°. The decomposed samples after 10 hydrogen desorptions, DS1 and DS2, were characterized using SR-PXD (Figure 11), in which both samples reveal diffraction from an unknown compound denoted 4. This compound may play a role in the loss of hydrogen storage capacity. Diffraction from Al and LiH are observed for sample DS1, whereas additional diffraction from LiAl and AlB2 is detected for sample DS2. The PXD data measured for samples after 10 hydrogen absorptions, AS1 and AS2, are shown in Figure 12, and both samples reveal diffraction from Al and 4 along with additional diffraction

Figure 12. PXD data measured for AS1 and AS2 after the tenth hydrogen absorption in the Sieverts apparatus (λ = 1.54056 Å). Symbols: ▲ TiB2, ■ Al, and ◇ 4.

Figure 10. PXD data of (A) LiBH4−Al (S1, 2:3) and (B) LiBH4−Al−TiB2 (S2, 2:3, 4.5 mol %) showing diffractograms from (1) as milled, (2) after the first desorption, and (3) after the first hydrogen absorption (λ = 1.54056 Å). Symbols: ○ o-LiBH4; ▲ TiB2; ■ Al; and △ AlB2. 7429

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chemical shift consistent with Li2B12H12 (δ = −15 ppm) is observed.13,36,37 The center of gravity for Li2B12H12 in AS1 is δcg = −8.7 ppm and the intensities correspond to the molar sample composition LiBH4/Li2B12H12 = 63:37, considering the central and satellite transition intensities for LiBH4 and the central transition intensity only for Li2B12H12. For AS2 the center of gravity for Li2B12H12 is δcg = −9.1 ppm and the molar sample composition LiBH4/Li2B12H12 = 52:48. Thus, more Li2B12H12 is formed in the sample containing the TiB2 additive, which further supports that TiB2 has no beneficial effect for the hydrogen storage stability of the LiBH4−Al (2:3) composite. TiB2 is not observed in the spectrum for AS2 at δ ≈ −4.9 ppm, as expected, since this resonance may be blurred by overlap with the broad resonance from Li2B12H12. In both spectra there is a low-intensity resonance at δ = 13 ppm, which may originate from 4 or a borate impurity containing BO3 species. Thus, compound 4 may be assigned to impurities (e.g., oxides) not relevant for the mechanism for release and uptake of hydrogen. 3.5. Raman Spectroscopy. Raman spectra of DS1 and DS2 (Figure 14) show peaks at 2515 and 756 cm−1, which are consistent with Raman peaks reported for Li2B12H12.48−50 The vibrational mode observed at 954 cm−1 corresponds to the B− B vibrations in AlB2.51 In addition, an unknown peak is observed at ∼500 cm−1, which may originate from 4 or a borate impurity, as it is in the B−O region. Thus, this peak may originate from the same compound as observed by 11B MAS NMR (the δ = 13 ppm resonance), considering the fact that this 11B NMR resonance falls in the spectral region for BO3 sites. TiB2 is Raman-active, but no peaks (i.e., 598 and 409 cm−1)52 are observed. The observation of AlB2 is in contrast to the 11B MAS NMR spectra measured for different samples in the hydrogenated state. However, both the dehydrogenated (DS1 and DS2) and hydrogenated (AS1 and AS2) samples show the presence of Li2B12H12 by Raman spectroscopy and 11 B MAS NMR, respectively. Lithium dodecahydro-closo-dodecaborate (Li2B12H12) is highly stable and may be responsible for the loss in hydrogen storage capacity in the LiBH4−Al system, as it acts as a “borontrap” under the physical conditions used in this study.

3.4. 11B MAS NMR Measurements. The 11B MAS NMR spectra for AS1 and AS2 after the 10 hydrogen absorption experiments are presented in Figure 13. LiBH4 shows a

Figure 13. 11B MAS NMR spectra (14.1 T, νR = 13.0 kHz) for the samples (a) AS1 and (b) AS2, illustrating resonances from the central and satellite transitions for LiBH4 (δ = −41.3 ppm) and the central transition for Li2B12H12 (δ ≈ −9 ppm). A low-intensity resonance at 13 ppm (asterisk) is also observed, which may arise from 4 or a borate (BO3) impurity.

centerband resonance at −41.3 ppm for both samples in accordance with earlier results.13,46,47 This shows that LiBH4 is formed after 10 de- and absorptions, although it was not observed by PXD. This may indicate a crystalline-to-amorphous transformation of LiBH4 upon repeated hydrogen desorption/ absorption. Furthermore, a centerband resonance with a

Figure 14. Raman spectra of DS1 and DS2 measured at two different positions on the samples. Li2B12H12 and AlB2 are observed, together with an unknown signal, which may arise from 4 or a borate (BO3) impurity. 7430

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The Journal of Physical Chemistry C Furthermore, Raman spectra measured at different locations within the same sample are different, which could indicate inhomogeneity and phase separation during cycling of hydrogen release and uptake, as proposed earlier.29 The presence of amorphous boron has previously been proposed to be responsible for the hydrogen capacity loss, however, it is not detected by either Raman or 11B MAS NMR spectroscopy in this study. Formation of Li2B12H12 has earlier been observed during the decomposition of pristine LiBH4,53,54 but is here reported in the LiBH4−Al (2:3) system for the first time. The selected physical conditions appear to suppress release of diborane, but may facilitate formation of the more stable closoborane, Li2B12H12, which clearly hampers hydrogen uptake. Further research within efficient catalytic additives may solve this problem or the use of sufficiently high hydrogen back pressure during decomposition of the composite. The present work reveals the importance of full control of possible side reactions, which may occur during release and uptake of hydrogen and may lead to decreasing storage capacity.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Rietveld refinement plots as well as synthesis and characterization using PXD and solid-state 11B MAS NMR of a sample of LiH−AlB2. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

The work was supported by the Danish National Research Foundation (DNRF93), Center for Materials Crystallography, the Danish Strategic Research Council (The research project HyFillFast), the Carlsberg Foundation and by the Danish Research Council for Nature and Universe (Danscatt). The access to beam time at the MAX-II synchrotron, Lund, Sweden, in the research laboratory MAX-lab is gratefully acknowledged. The use of the facilities at the Instrument Centre for Solid-State NMR Spectroscopy, Department of Chemistry, Aarhus University, sponsored by the Danish Natural Science Research Councils and the Carlsberg Foundation, is acknowledged. We are also grateful for funding from the European Community’s Seventh Framework Program, The Fuel Cells and Hydrogen Joint Undertaking (FCH JU), Project BOR4STORE (303428).

4. CONCLUSIONS The hydrogen storage capacity loss for a LiBH4−Al (2:3) composite has been investigated during 10 hydrogen release and uptake cycles. The hydrogen storage capacity decreases from 6.4 to ∼1 wt %, possibly due to the formation of relatively stable closo-borane, Li2B12H12, as observed by 11B MAS NMR and Raman spectroscopy. The samples of LiBH4−Al (2:3) with and without TiB2 additive show a similar decrease in hydrogen storage capacity after each cycle, that is, addition of TiB2 offers no improvement to the system. The capacity loss is largest in the first four cycles after which only about 45% hydrogen storage capacity remains. The decomposition mechanisms for the LiBH4−Al composite have been studied using in situ SRPXD and TGA/DSC-MS, which clearly shows that the decomposition occurs via several chemical reactions, in which only hydrogen is released. The decomposition occurs via the intermediate compounds denoted 1 and 2 below temperatures of 500 °C, which may be new Li−Al−B compounds. Furthermore, a minor kinetic effect from the additive TiB2 appears to facilitate the formation of the intermediate compounds 1 and 2 at slightly lower temperatures. Additionally, Rietveld refinement reveals solid solutions of Li0.22Al0.78B2 or Al0.78B2. This work shows that detailed knowledge of the reaction mechanisms for hydrogen release and uptake is essential and may be tailored by the physical conditions, such as temperature, pressure, and sample compositions and possibly also influenced by additives.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 7431

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