Controlling the Dehydrogenation Reaction toward Reversibility of the

Article pubs.acs.org/JPCC

Controlling the Dehydrogenation Reaction toward Reversibility of the LiBH4−Ca(BH4)2 Eutectic System Yigang Yan,*,† Arndt Remhof,† Philippe Mauron,† Daniel Rentsch,‡ Zbigniew Łodziana,§ Young-Su Lee,∥ Hyun-Sook Lee,∥ Young Whan Cho,∥ and Andreas Züttel† †

EMPA, Swiss Federal Laboratories for Materials Science and Technology, Hydrogen & Energy, 8600 Dübendorf, Switzerland EMPA, Swiss Federal Laboratories for Materials Science and Technology, Functional Polymers, 8600 Dübendorf, Switzerland § INP, Polish Academy of Sciences, ul. Radzikowskiego 152, 31-342 Kraków, Poland ∥ High Temperature Energy Materials Research Center, Korea Institute of Science and Technology, 136-791 Seoul, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Controlling the dehydrogenation process in a suitable reaction route by choosing the appropriate reaction conditions is crucial for a hydrogen storage system. The dehydrogenation process of the eutectic 0.68LiBH4−0.32Ca(BH4)2 mixture was investigated by dynamic pressure− composition isotherms, X-ray diffraction, and solid-state nuclear magnetic resonance in order to determine the optimal reaction route for the dehydrogenation and to improve the hydrogen absorption reaction. In a temperature range from 330 to 450 °C, the LiBH4−Ca(BH4)2 mixture decomposes in two major steps. First, Ca(BH4)2 decomposes into CaH2, CaB6, CaB12H12, H2, and probably amorphous boron. Second, CaH2 reacts with LiBH4 to CaB6, LiH, and H2. Li2B12H12 has been identified as a byproduct. It was observed that the lower dehydrogenation temperature, the more CaB6 and the less [B12H12]2− containing phases are present in the final dehydrogenation products, resulting in improved absorption performance. The temperature dependence is discussed, providing instructions to improve reversibility for potential applications and new insights into the hydrogen sorption mechanism of metal borohydrides.

1. INTRODUCTION Hydrogen storage in a safe and efficient media is crucial for the realization of a clean hydrogen society in the future.1 Because of the combined high volumetric and gravimetric hydrogen densities, light metal complex hydrides have been widely investigated for solid hydrogen storage.2 Among them, lithium borohydride (LiBH4) and calcium borohydride Ca(BH4)2, exhibiting hydrogen densities of 18.5 and 11.6 wt %, respectively, are two of the currently most discussed lightweight complex hydrides. They form ionic crystals, consisting of positively charged metal cations and [BH4]− anions with covalently bound hydrogen.3 LiBH4 melts at Tm = 280 °C and releases considerable amounts of hydrogen from the liquid state. The decomposition of LiBH4 may follow simultaneously different routes depending on the conditions (pressure, temperature) according to reaction eqs 1 and 2.4 LiBH4 → LiH + B + LiBH4 →

3 H2 2

5 1 13 LiH + Li 2B12H12 + H2 12 12 6

acts as a boron sink and lowers the amount of reabsorbed hydrogen. The stability of LiBH4 with respect to the decomposition into elemental boron and lithium hydride was evaluated to be 74 kJ mol−1 H2,5 and a decomposition temperature of Td = 370 °C was determined at a hydrogen pressure of 1.0 bar. The partial rehydrogenation of LiBH4, from its decomposition products, has been observed under rigorous conditions of 600 °C and 150−350 bar of H2.5,6 The formation enthalpy (ΔHf) of metal borohydrides depends linearly on the Pauling electronegativity (χp) of the metal forming the cation; i.e., the higher χp, the lower ΔHf.3b According to this correlation, the thermodynamic stability tuned by forming mixed cation borohydrides LiBH4−M(BH4)n, where M is a metal with higher χp than that of Li (χp = 1.0), such as Zn (χp = 1.6), Al (χp = 1.5), Zr (χp = 1.4), etc.7 For example, in a system of ZrLim−4(BH4)m, the dehydrogenation temperature varies continuously from 170 to 370 °C with m changing from 4 to 6. Although this method is effective to reduce the dehydrogenation temperature by forming mixed cation borohydrides, it leads to the emission of B2H6.7b,c,8

(1)

(2)

In view of hydrogen storage reaction 1 is more favorable than reaction 2 as it evolves more hydrogen. The stable Li2B12H12 is an unwanted product of the hydrogen desorption reaction that © 2013 American Chemical Society

Received: February 15, 2013 Revised: April 4, 2013 Published: April 4, 2013 8878

dx.doi.org/10.1021/jp401628g | J. Phys. Chem. C 2013, 117, 8878−8886

The Journal of Physical Chemistry C

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[B12H12]2− containing phases under suitable conditions. This study provides new insights into the hydrogen sorption pathway of metal borohydrides and shows the significance of suitable reaction conditions for the reversible hydrogen storage.

Another effective strategy to destabilize LiBH4 was suggested by Vajo et al. which first effectively destabilized LiBH4 by adding MgH2.9 The mutual destabilization of LiBH4 and MgH2 results in the formation of MgB2 during the decomposition, reducing the enthalpy of reaction by 25 kJ mol−1 H2 and preventing the formation of B2H6. Thereafter, many destabilized systems involving borohydrides have been theoretically and experimentally investigated, such as systems of LiBH4 with metals (Mg, Al, etc.) or metal hydrides (CaH2, CeH2, YH3, TiH2, etc.).10 Many of these destabilized systems exhibit reduced reaction enthalpies of 20−50 kJ mol H2−1.11 The destabilized systems do not prevent the formation of [B12H12]2− containing phases. The enthalpy change of LiBH4 decomposing into Li2B12H12 (eq 2) is theoretically predicted to 50 ± 6 kJ mol−1 H2,12 which is comparable to those of metal or metal hydride destabilized systems. Experimentally, the intermediate phase Li2B12H12 has been observed in the decomposition process of LiBH4 as well as in the destabilized systems such as LiBH4−MgH2 and LiBH4−Al.10d,f,13 A similar behavior has also been observed in Ca(BH4)2-based systems. The decomposition of Ca(BH4)2 to CaB6 (eq 3) shows similar enthalpy change to that of reaction from Ca(BH4)2 to CaB12H12 (eq 4).11,12,14 Ca(BH4)2 →

1 2 10 CaB6 + CaH 2 + H2 3 3 3

(35 ± 3 kJ mol−1 H 2) Ca(BH4)2 →

2. METHODS 2.1. Experimental Section. The borohydrides, LiBH4 (95% purity) and Ca(BH4)2 (95% purity), were purchased from KATCHEM and Aldrich, respectively. The sample of 0.68L0.32C was prepared by 90 min ball milling (spex 8000, ball to powder ratio of 20 to 1) of LiBH4 and Ca(BH4)2 in a molar ratio of 0.68 to 0.32. Additionally, CaB6 (99.5%), amorphous boron (95%), and boron acid B(OH)3 (99.5%) were used as reference samples for 11B NMR chemical shift assignments obtained from Aldrich. The dehydrogenation performance of 0.68L0.32C was investigated by temperature-programmed desorption (TPD) and by dynamic pcT measurements. For a TPD experiment, 100 mg of sample was heated up to 500 °C with a ramp of 1.0 °C min−1 in a constant pressure of 1.0 bar of H2. The pressure of the system was controlled by a mass flow controller/meter (connected to the vacuum), which also measured the hydrogen amount released. The dynamic pcT measurements were carried out according to the method in ref 5. 150 mg of sample was filled in a stainless steel autoclave which is air tightly closed and transferred to the pcT apparatus (homemade). The sample was evacuated at room temperature to a vacuum lower than 10−4 mbar before starting the measurement. A starting hydrogen pressure of 30−40 bar was applied to the autoclave, and the sample was subsequently heated up to a required temperature. After the temperature and pressure both became stable, the pcT measurement was started by releasing the hydrogen gas via a mass flow controller/meter at a constant H2 flow such as 0.2 cm3 (STP) min−1 (sccm). The pcT measurement was ended until the pressure was dropped to 0.1 bar. XRD measurements were performed using a Bruker D8 diffractometer equipped with a Goebel mirror selecting Cu Kα radiation (λ = 1.5418 Å) and a linear detector system (Vantec). Samples for XRD measurements were filled into glass capillaries (diameter, 0.7 mm; wall thickness, 0.01 mm), sealed in an inert atmosphere. Solid-state magic angle spinning (MAS) nuclear magnetic resonance (NMR) experiments were performed on a Bruker Avance-400 NMR spectrometer using a 4 mm CP-MAS probe. The 11B NMR spectra were recorded at 128.38 MHz at 12 kHz sample rotation applying the Bruker pulse sequence “hahnecho.av” to suppress the broad 11B resonance of the boron nitride background signal of the probe. Pulse lengths of 1.5 μs (π/12 pulse) and 3.0 μs were applied for the excitation and echo pulses, respectively, and 11B NMR chemical shifts are reported in parts per million (ppm) externally referenced to a 1 M B(OH)3 aqueous solution at 19.6 ppm as external standard. Best results for the 11B quadrupolar powder pattern of the setup sample B(OH)3 were obtained using an echo delay of one rotor cycle (81.1 μs) and applying 50 kHz SPINAL64.17 A left shift corresponding to the echo delay was applied to the raw data before Fourier transformation. We obtained exactly the line shape as described by Klochko et al.18 for the setup sample. For selected samples, 11B CP-MAS NMR experiments were performed using weak radio-frequency powers for spin locking of the 11B nucleus on resonance as described elsewhere.13b,19 In our case the Hartman−Hahn matching condition was typically fulfilled applying nutation frequencies of 28 kHz (1H) and 8

(3)

1 5 13 CaB12H12 + CaH 2 + H2 6 6 6

(36 ± 3 kJ mol−1 H 2)

(4)

The coexistence of CaB6 and CaB12H12 has been detected by solid-state 11B NMR.15 Recently, the combined system of LiBH4 with alkali earth borohydrides (e.g., Ca(BH4)2) was investigated, exhibiting improved dehydrogenation performance compared to the constituent compounds.16 The hydrogen emission starts from the eutectic melt at 200 °C, and the major dehydrogenation processes is completed below 400 °C with hydrogen release of more than 10 wt %. However, only 50% of hydrogen was absorbed in the rehydrogenation reaction.42 The dehydrogenation mechanism and the formation of intermediates are also unclear so far for this combined system. In order to complete the hydrogen absorption reaction for potential applications, the formation of intermediates such as [B12H12]2− containing phases should be suppressed in the decomposition process of metal borohydride. For this reason, it is of significance to clarify the dehydrogenation routes as well as the amount of [B12H12]2− containing phases, which are dependent on reaction conditions. In this paper, we focus on the LiBH4−Ca(BH4)2 system, which shows the eutectic composition close to the molar ratio of 0.68 to 0.3216d and, thereby, the eutectic composition 0.68LiBH4−0.32Ca(BH4)2, labeled as 0.68L0.32C. We investigated the dehydrogenation reactions by dynamic pressure−composition−temperature (pcT) measurements and quantitatively analyzed the formation of intermediates such as [B12H12]2− containing phases by X-ray diffraction (XRD) and 11B magic angle spinning nuclear magnetic resonance (MAS NMR). Phase identification through 11 B MAS NMR was performed by comparison with reference materials and calculated chemical shifts. Finally, we achieved the improved absorption performance by reducing the 8879



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kHz (11B) at a sample rotation of 12 kHz using contact times of 60−100 μs. The deconvolution of 11B spectra was made according to 11B chemical shift of the possible individual components, and line shapes were simulated by applying Gaussian functions. 2.2. Theoretical Calculation of the 11B NMR Shifts. Calculations of 11B chemical shifts were performed within plane wave formulation of density functional theory as implemented in Gauge Including Projector Augmented Waves formulation (GIPAW)20 and Quantum Espresso code.21 The electronic valences states were represented by Troulier−Martins pseudopotentials with valence configurations: 2p1 for B, 2s1 for Li, 1s1 for H 3p64s2 for Ca, and 3s23p3 for P. The Perdew− Bruke−Erzenhof functional was used for exchange correlation function,22 the plane wave cutoff was 100 Ry, and k-point sampling according to Monkhorst−Pack scheme was applied with density no lower than 40 Å−1 in each lattice direction.23 The isotropic shielding is defined as σiso = 1/3 Tr(σii), where σii are diagonal components of magnetic shielding tensor. The experimental isotropic chemical shifts are δiso = −(σiso − σref). Calculations for the reference in solution are still challenging. Therefore, we refer σref to the standard proposed by Hayashi and Hayamizi that is BPO4 at −3.60 ppm.24 The structures were relaxed with respect to internal atomic positions and the volume of the unit cell. A detailed description of the theoretical calculations will be present in a future publication.

Figure 2. pcT isotherms for hydrogen storage of 0.68L0.32C measured at a constant hydrogen flow of 0.2 cm3 (STP) min−1 (sccm).

its high dissociation pressure which is beyond the pressure range of the present measurement. In order to reveal the reactions occurring in isothermal dehydrogenation, the dehydrogenation products after each step were characterized by XRD and 11B MAS NMR, as shown in Figures 3−5. The samples obtained for characterization are listed in Table 1.

3. RESULTS 3.1. Dehydrogenation. As a starting point, we compare the dehydrogenation performance of the eutectic mixture with the constituent compounds. Figure 1 exhibits the respective

Figure 1. TPD curves of LiBH4, Ca(BH4)2, and 0.68L0.32C with a ramp of 1 °C min−1 and external pressure of 1.0 bar of H2.

hydrogen release in an applied hydrogen atmosphere of 1.0 bar at a heat rate of 1.0 °C/min. 0.68L0.32C shows a lower hydrogen release onset temperature of 200 °C, as compared to the constituent compounds. The major dehydrogenation process starts at 310 °C, similar to Ca(BH4)2, and ends below 400 °C with a hydrogen release amount of 9.6 wt %. On the basis of the TPD results, the dynamic pcT measurements were carried out in a temperature range of 310−450 °C. All pcT isotherms in Figure 2 exhibit two major plateaus, as denoted by steps 1 and 2. The isotherms at 330−425 °C show hydrogen release amounts of 9.0−9.5 wt %, which agree well with the TPD measurement shown in Figure 1. The isotherm recorded at 310 °C is an exception. It shows only a hydrogen release of slightly more than 2.0 wt % and no distinct plateau. Obviously, 310 °C is too low to complete the decomposition of 0.68L0.32C. At 450 °C, only a part of step 1 is observed due to

Figure 3. XRD patterns of dehydrogenation products after steps 1 and 2 of the pcT isotherms shown in Figure 2. 8880



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Table 1. Summary of the Names of Individual Samples Obtained after pcT Measurements and Rehydrogenationa temp of pcT measurement (°C)

after step 1

after step 2

rehydrogenated state of samples 2a to 2b

330 350 375 425

1a 1b 1c 1d

2a 2b 2c 2d

3a 3b 3c 3d

a

Samples 1a to 1d and 2a to 2d were obtained by manually stopping pcT measurements after step 1 and step 2 at 330 to 425 °C and cooling down to room temperature rapidly, respectively. Samples 3a to 3d were the rehydrogenated state of samples 2a to 2d at 400 °C and 140 bar. Figure 4. 11B MAS NMR spectra (128.38 MHz) of the as-synthesized 0.68L0.32C (top) and reference compounds.

Figure 3 (upper part) shows the XRD patterns of the dehydrogenation products recorded after the first step of dehydrogenation. The as-synthesized 0.68L0.32C is a physical mixture of LiBH4 and Ca(BH4)2. After step 1, Ca(BH4)2 almost disappears and CaH2 is observed in samples 1a to 1d, yet LiBH4 is still stable and no LiH is formed. CaB6 is also identified by XRD in samples 1b to 1c but not in 1a. Additional Raman spectroscopy measurement (not shown) confirms the formation of CaB6 in sample 1a, which may exist in the amorphous state. These observations indicate that step 1 of the pcT isotherms is attributed to the individual decomposition of Ca(BH4)2. No formation of the intermediate phase CaB2Hx is observed. The XRD patterns of the dehydrogenation products of step 2 are shown in the lower part of Figure 3. Compared to the dehydrogenation products of step 1, the reflections of LiBH4 vanish, those of LiH appear, the diffraction peaks of CaH2 become weaker, and those of CaB6 become stronger after step 2. These observations imply that step 2 of the pcT isotherms is related to the reaction between LiBH4 and CaH2 to form CaB6, LiH, and H2, according to eq 5. The reflections of CaB6 are also observed to become sharper and stronger with increasing the temperature from 330 to 425 °C, implying that the formation of CaB6 is influenced by temperature. 1 1 5 LiBH4 + CaH 2 → LiH + CaB6 + H 2 6 6 3

Table 2. Reference Values for the 11B Chemical Shifts δisoa compounds

referenced to BPO4 δ(11B) (ppm)

LiBH4 α-Ca(BH4)2 β-Ca(BH4)2 γ-Ca(BH4)2 Li2B12H12 CaB12H12 CaB6 α-boron B−O3 B−O4 B(OH)3

−45.2 (4c) −27.3 (16f) −29.6 (4f) −26.9 (8c), −31.0 (8c) −14.0; −14.6 (24d) −9.4; −11.4; −12.6; −13.1; −15.3 (8f) +23.7 (6f) +0.8; +1.1 (18h) +16.0 to +20.9 +3.0 to +5.0 +26.5

exptl δ(11B) (ppm)b −41.3 −29.8 −32.2 −15 −15 +5.0 +1.5 +5.0, +17.7 +1.0 +5.0, +17.7c

(5)

Theoretical values, calculated with BPO4 at −3.60 ppm as reference and the measured values referenced to a 1 M B(OH)3 aqueous solution at 19.6 ppm as external standard. The calculated 11B NMR chemical shifts are based on reported structures for LiBH4,27 α- and βCa(BH4)2,28 Li2B12H12 (Pa-3),29 CaB12H12,30 α-boron,31 and CaB6.31 The range of calculated chemical shifts for three coordinated boron (B−O3) are based on structures CaB2O4, B2O3, α-LiBO2,32 and for tetrahedrally coordinated boron (B−O4) are from γ-LiBO2, CaB4O7, and CaB2O4. The crystallographic positions of the B atoms within the respective compound are given by the Wyckoff notation. bPeak maxima of resonances in 1D 11B MAS NMR spectra. cPeak maxima of ideal second order quadrupolar line shape of B(OH)3.17

Small traces of LiCa3BH4B2O6 and CaO can be identified as impurities. The above XRD results in Figure 3 indicate that the two major steps of the isotherms in Figure 2 correspond to the individual decomposition of Ca(BH4)2 and subsequent reaction between LiBH4 and newly formed CaH2. To clarify the possibly present amorphous phases that are invisible in XRD, 11B MAS NMR measurements were thus carried out. Figure 4 shows the 11 B MAS NMR spectra of as-synthesized 0.68L0.32C and reference compounds Ca(BH4)2, LiBH4, amorphous boron, CaB6, and B(OH)3, and their chemical shifts are summarized in Table 2. LiBH4 shows one resonance at −41.3 ppm. Ca(BH4)2 shows two main resonance attributed to α-Ca(BH4)2 (−29.8 ppm) and β-Ca(BH4)2 (−32.3 ppm).15a 0.68L0.32C contains two main phases of LiBH4 (−41.3 ppm) and α-Ca(BH4)2 (−29.8 ppm). β-Ca(BH4 ) 2 from the starting Ca(BH4) 2 probably transferred into α-Ca(BH4)2 during the ball milling with LiBH4. The γ-Ca(BH4)2 in 0.68L0.32C observed by XRD (Figure 3a) appears to be overlapped by α-Ca(BH4)2 in the 11B NMR spectrum. Amorphous boron and CaB6 show broad

resonances centered at 1.5 ppm (line width of 4100 Hz) and 5.0 ppm (line width of 2950 Hz), respectively. Boric acid shows two peaks at 5.0 and 17.6 ppm, similar to the 11B spectra of B2O3 which is also composed of three coordinated boron atoms in BO3 units.26 The tetrahedrally coordinated boron atoms in BO4 units were reported to show single sharp 11B peak at around 1.0 ppm.26a The measured as well as the calculated 11B NMR chemical shifts are presented in Table 2. For borohydrides and cage boron structures the calculated data are in very good agreement with the measured ones. The B12H12 cage in CaB12H12 is distorted, which results in broad spread of δiso. For boron, the measured spectrum (Figure 4) consists of a broad signal related to its amorphous state. For CaB6, the calculated chemical shift is overestimated due to the ferromagnetically polarized ground state.33 The 11B MAS NMR spectra of dehydrogenation products of steps 1 and 2 in the pcT isotherms (Figure 2) are shown in Figure 5. In the dehydrogenation products of step 1, the residual Ca(BH4)2 (−29.8 ppm) and undecomposed LiBH4

(50 ± 5 kJ mol−1H 2)11,12

a

25

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regarding the broad 11B NMR resonances. Other compounds such as [B3H8]− containing phases with a chemical shift at −30 ppm have not been reported for Ca(BH4)2.34 The broad resonances were accordingly deconvoluted into individual peaks, as shown in Figure 5 (right). The shifting trend of the broad resonances can be explained as follows: less CaB6 and more [B12H12]2− containing phases are formed as the dehydrogenation temperature is increased. It is also interesting to note that CaB6 is the main phase in the dehydrogenation products in samples 2a and 2b. Since the formation of metal borides is considered to be crucial for the rehydrogenation reaction,10 samples 2a and 2b are expected to exhibit superior absorption performance as compared to the dehydrogenated samples 2c and 2d. 3.2. Reversibility. The dehydrogenation products (i.e., samples 2a to 2d) were rehydrogenated at 400 °C in a H2 pressure of 140 bar for 20 h. The rehydrogenated samples of 2a to 2d are accordingly named as 3a to 3d, as displayed in Table 1. The hydrogen release performance of samples 3a to 3d is shown in Figure 6. Sample 3a shows the highest hydrogen

Figure 5. 11B MAS NMR spectra (128.38 MHz) of the dehydrogenation products of steps 1 and 2 in the pcT isotherms in Figure 2. The dark lines, red dotted lines, and gray dotted lines represent the experimental data, the fitting, and the deconvoluted in individual peaks, respectively. Resonances at −29.8 and −41.3 ppm are attributed to Ca(BH4)2 and LiBH4, respectively, those at 5.0 and 17.7 ppm originate from the three coordinated boron atoms of BO3 units in LiCa3BH4B2O6, and the resonance at 1.0 ppm is due to the tetrahedrally coordinated boron atoms of BO4 units.26a CaB6 and possibly formed amorphous boron contribute to the broad background.

(−41.3 ppm) are observed, and the [B12H12]2− containing phase (i.e., CaB12H12) is confirmed at −15 ppm in all dehydrogenation products.30 The resonance of CaB12H12 in sample 1d (obtained at 425 °C) is more pronounced than those in samples 1a to 1c (obtained at 330−375 °C), which suggests that higher temperature facilitate the formation of CaB12H12. CaB6 observed by XRD (Figure 3a) and possibly formed amorphous boron may exist in the broad background between 20 and −20 ppm. Although the quantitative analysis on the different boron containing products is hampered by the broad resonances, the 11B MAS NMR and XRD measurement results (Figure 3) indicate that several reactions occur competitively in the dehydrogenation process of Ca(BH4)2, i.e., the decomposition into CaB 6 , CaB 12 H 12 , and/or amorphous boron. The traces of oxide impurities identified by XRD shown in Figure 3 are also observed in Figure 5. In the dehydrogenation products of step 2 (Figure 5, right), broad resonances between 40 and −30 ppm together with some small shoulders around −30 to −40 ppm are observed. The shoulders are attributed to the residual of LiBH4 and/or Ca(BH4)2. The broad resonances show a high-frequency shift from sample 2a to 2d, as the dehydrogenation temperature is increased from 330 to 425 °C. This shift implies a change in the composition of the boron-containing phases depending on dehydrogenation temperature. These broad resonances are attributed to the mixture of CaB6 observed by XRD (Figure 3) and other boron-containing phases. CaB12H12 and/or amorphous boron, formed in step 1, are thermodynamically stable and will remain in step 2.14c,30 The formation of Li2B12H12 from the individual decomposition of LiBH4 according to eq 2 is also thermodynamically possible. To confirm the existence of the [B12H12]2− species in the broad resonances, a 11B CP-MAS NMR measurement was performed on a selected sample to reduce the signal of the dominating H-free phases such as CaB6. As shown in Figure S1, the shoulder assigned to the [B12H12]2− species in the 11B MAS spectrum is dominating the 11B CPMAS spectrum. Therefore, CaB6, [B12H12]2− containing phases and amorphous boron are the three phases considered

Figure 6. TPD curves of samples 3a to 3d. Heating rate is 1.0 °C min−1, and external hydrogen pressure is 1.0 bar for TPD measurements.

release amount of 6.8 wt % among samples 3a to 3d, and its main desorption temperature is consistent with that of the initial sample of 0.68L0.32C. For samples 3b to 3d, the hydrogen release amounts are reduced to 6.4, 4.8, and 4.5 wt %, respectively. The XRD patterns of samples 3a to 3d are shown in Figure 7. Both LiBH4 and Ca(BH4)2 are regenerated after rehydroge-

Figure 7. XRD patterns of samples 3a to 3d. 8882



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CaH2, CaB6, CaB12H12, and/or amorphous boron in three competing reactions. In the second step, the newly formed CaH2 reacts with molten LiBH4 to generate CaB6, LiH, H2, and the byproduct Li2B12H12. Ca(BH4)2 decomposes prior to the dehydrogenation of LiBH4 due to thermodynamic reasons. The enthalpy changes of Ca(BH4)2 decomposing into CaB6 and CaB12H12 are 35 ± 3 and 36 ± 3 kJ mol−1 H2,11,12 respectively, which are smaller than those of the reaction between LiBH4 and CaH2 (50 ± 5 kJ mol−1 H2) and the reaction of LiBH4 decomposing into Li2B12H12 (50 ± 5 kJ mol−1 H2).11,12,14 After dehydrogenation, LiBH4 and Ca(BH4)2 can be partially regenerated from CaB6, CaH2, and LiH at 400 °C and 140 bar of H2, whereas CaB12H12, Li2B12H12, and amorphous boron act as boron sinks in the rehydrogenation reaction. Although the coexistence of [B12H12]2− and CaB6 is observed in the dehydrogenation products due to thermodynamic reasons, their relative amounts depends on the temperature chosen in the isothermal decomposition. Figure 10 qualitatively

nation in samples 3a and 3b, whereas only LiBH4 is formed in 3c and 3d. The remaining CaH2 and CaB6 in samples 3a to 3d indicates that the rehydrogenation reaction is incomplete. The formation of crystalline Li2B12H12 may originate from the crystallization of amorphous Li2B12H12 formed from the individual decomposition of LiBH4. Oxide impurities such as CaO remain in the rehydrogenation samples. The regeneration of LiBH4 and Ca(BH4)2 in sample 3a is confirmed by 11B MAS NMR measurement, as shown in Figure 8. The resonance attributed to CaB6 almost vanishes, and the

Figure 8. 11B MAS NMR spectrum (128.38 MHz) of sample 3a.

remaining stable [B12H12]2− containing phases is observed. This confirms that CaB6 is the suitable species for the rehydrogenation reaction, and [B12H12]2− containing phases act as the boron sink. The rather broad signal between 20 and −20 ppm is probably attributed to not fully recycled CaB6 and amorphous boron; the same species as already identified in the 11B MAS NMR spectra of the dehydrogenation products shown in Figure 5. Figure 10. Amounts of boron atoms in chemical state of CaB6 and [B12H12]2− in final dehydrogenation products and absorption amounts as a function of temperature. The amounts of boron atoms are calculated based on the peak areas of individual 11B resonances in Figure 5 (right). The boron atoms in the remaining borohydrides are not taken into account in the calculations.

4. DISCUSSION On the basis of pcT, XRD, and 11B NMR measurement results, we propose the dehydrogenation routes of the eutectic mixture 0.68LiBH4−0.32Ca(BH4)2 (0.68L0.32C), shown in Figure 9. In the temperature range from 330 to 450 °C, 0.68L0.32C dehydrogenates in two steps with a total hydrogen release of 9.0−9.5 wt %. In the first step, Ca(BH4)2 decomposes into

shows the distribution of boron atoms in CaB6 and [B12H12]2− in the dehydrogenation products as a function of temperature. After dehydrogenation at 330 °C, about 85 mol % of boron atoms state as CaB6 and 10 mol % of boron atoms as [B12H12]2−. As the dehydrogenation temperature is increased, less CaB6 and more [B12H12]2− are formed, and accordingly, the absorption performance is reduced. This finding indicates the significance of choosing the appropriate dehydrogenation temperature for the reversible hydrogen storage of the eutectic mixture 0.68LiBH4−0.32Ca(BH4)2. It is important to clarify the temperature dependence of the formation of CaB6 and [B12H12]2− containing phases. The attempts to explain it from the viewpoint of thermodynamics (e.g., enthalpy and entropy change) are not successful, which strongly suggests the existence of a kinetic effect. To investigate this kinetic effect, the sample 0.68L0.32C was artificially dehydrogenated at different react rates by applying different H2 flows of 0.2 and 1.0 sccm in the pcT measurements at 350 °C. The initial hydrogen pressures for pcT measurements are set to the same value of 42 bar. The increase of H2 flow from 0.2 to 1.0 sccm increases the average reaction rate by 4 times. The 11B MAS NMR spectra of final dehydrogenation products

Figure 9. Schematic diagram of isothermal dehydrogenation routes of 0.68L0.32C. CaB12H12, Li2B12H12, and amorphous boron in the dashed frames on the right side act as boron sink and are not involved in the rehydrogenation reaction. 8883



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at the two H2 flows are shown in Figure 10. Both spectra consist of broad peaks of dehydrogenation products between 30 and −30 ppm, with shoulders assigned to remaining borohydrides between −30 and −40 ppm. A low-frequency shift from −2.1 to 5.5 ppm of the center of the broad resonance was observed when the H2 flow was increased from 0.2 to 1.0 sccm. This indicates that more CaB6 and less [B12H12]2− containing phases were formed. This result supports the idea that the decomposition pathway of the LiBH4−Ca(BH4)2 combined system is kinetically dependent. According to this idea, selected catalysts, which can improve the reaction kinetics, are expected to further reduce the amount of [B12H12]2− containing phases and thereby improve the reversibility. CaB12H12 crystallizes in monoclinic structure with Ca2+ cations surrounded by five icosahedral [B12H12]2− anions in a unit cell.30 Li2B12H12 shows primitive cubic crystal structure with each Li+ cation surrounded by three [B12H12]2− anions.29 CaB6 exhibits a cubic lattice with Ca2+ cations surrounded by B62− octahedral anions.31 Morphologically, the formation of [B12H12]2− cages involves more atoms to transfer and migrate than that of B62− anions and probably the higher activation energy barrier to be overcome. It may be due to the lower activation energy; the nucleation and growth of CaB6 are relatively easier than those of [B12H12]2− containing phases at lower temperature. As the temperature is increased, more [B12H12]2− containing phases could be formed more easily.

Temperature dependence was observed in the competitive formation of CaB6 with byproducts CaB12H12 and Li2B12H12; i.e., the lower the dehydrogenation temperature, the more CaB6 and the less [B12H12]2− containing phases are formed. We attribute the temperature dependence to the kinetic effect. By performing dehydrogenation at lower temperature (e.g., 330 °C) or higher reaction rate (e.g., by applying a higher H2 flow of 1.0 sccm), the unwanted [B12H12]2− containing phases were reduced and the rehydrogenation performance was improved. This study shows a way to control the dehydrogenation from the viewpoint of kinetics and improve the reversibility of 0.68L0.32C eutectic system and reviews the importance of reaction conditions (e.g., temperature) on dehydrogenation pathway and reversible reaction of metal borohydrides for hydrogen storage.

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ASSOCIATED CONTENT

* Supporting Information S

Comparison of 11B MAS NMR and 11B CP-MAS NMR spectra of a selected sample and the complete author lists of refs 21 and 28. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax +41 58 765 40 22; Tel +41 58 765 40 82; e-mail yigang. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the Korean Research Council and the Polish−Swiss Research Program is gratefully acknowledged. Z.Ł. acknowledges CPU allocation at PL-Grid Infrastructure.

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REFERENCES

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Figure 11. 11B MAS NMR spectra (128.38 MHz) of the dehydrogenation products of 0.68L0.32C at 350 °C of samples prepared by applying H2 flows of 0.2 sccm (above) and 1.0 sccm (below) in the pcT measurements. Dark and red dotted lines represent the experimental and fitting data, respectively, and gray dotted lines represent the individual resonances of remaining LiBH4 (−41.3 ppm) and Ca(BH4)2 (around −30.0 ppm), [B12H12]2− (around −15.0 ppm), CaB6 (5.0 ppm), amorphous boron (around 1.0 ppm), and small amounts of boron oxides between 0 and 20 ppm.

5. CONCLUSIONS We clarified the dehydrogenation pathways and the formation of intermediates for the 0.68LiBH 4 −0.32Ca(BH 4 ) 2 (0.68L0.32C) eutectic system. 0.68L0.32C decomposes in two major steps in a temperature range from 330 to 450 °C. First, Ca(BH4)2 separately decomposes into CaH2, CaB6, CaB12H12, and/or amorphous boron. Second, the newly formed CaH2 reacts with molten LiBH4 to generate CaB6, LiH, H2, and byproduct Li2B12H12. 8884



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Controlling the Dehydrogenation Reaction toward Reversibility of the

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