Role of Early-Stage Atmosphere in the Dehydrogenation Reaction of

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Role of Early-Stage Atmosphere in the Dehydrogenation Reaction of the LiBH4−YH3 Composite Kee-Bum Kim,†,‡ Jae-Hyeok Shim,*,† Kyu Hwan Oh,‡ and Young Whan Cho† †

High Temperature Energy Materials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea Department of Materials Science and Engineering, Seoul National University, Seoul 151-742, Republic of Korea

‡

ABSTRACT: This article investigates the dehydrogenation behavior of the LiBH4−YH3 composite under various early-stage Ar back-pressure conditions. It is clearly observed that a minute change in early-stage atmosphere greatly affects the overall dehydrogenation reaction of the composite. Free boron and Li2B12H12 start to form in turn as the partial dehydrogenation products around 400 °C under static vacuum or low Ar back pressure. The formation of Li2B12H12 greatly increases the activation energy for the dehydrogenation reaction between LiBH4 and YH3 into LiH and YB4, hence significantly retarding the reaction. The formation of B just slightly increases the incubation period of the reaction. The formation of Li2B12H12 is effectively suppressed by initially applying Ar back pressure above 0.1 MPa.

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INTRODUCTION LiBH4 has received great attention as one of the most promising candidates for a solid-state hydrogen storage material due to its high gravimetric (18.5 wt % H2) and volumetric (121 kg H2/m3) hydrogen storage capacities.1 However, its impractical temperature and pressure conditions for hydrogen sorption reactions, low reaction reversibility, and poor reaction kinetics have been regarded as major drawbacks to practical applications.2,3 Extensive efforts have been attempted so far to overcome these drawbacks, and the concept of reactive hydride composites (or destabilization) proposed by Barkhordarian et al.4 and Vajo et al.5 is one of the efforts. In the concept, the enthalpy for hydrogen sorption reactions is effectively reduced and the reaction reversibility is significantly improved by the formation of metal boride instead of free boron after dehydrogenation.4−6 The LiBH4−YH3 composite is an example of the reactive hydride composites. The composite is expected to theoretically release 8.5 wt % H2 according to the following reaction: 4LiBH4 + YH3 → 4LiH + YB4 + 7.5H 2

regarded as an undesired intermediate phase during the dehydrogenation of LiBH4 due to its high stability,14,15 before reacting with metal hydride.10−12 Also, argon back pressure has been shown to play a similar role to hydrogen back pressure in the dehydrogenation reaction of the LiBH4−YH3 composite.8 The positive effect of hydrogen and argon back pressure in the LiBH4−YH3 composite seems to be related to the prevented formation of Li2B12H12 during the dehydrogenation. Li2B12H12 is believed to retard the dehydrogenation reaction between LiBH4 and YH3 (reaction 1) by forming on the surfaces of YH3 particles in liquid LiBH4 and then hindering the direct contact between LiBH4 and YH3. In our previous studies, we found that the formation of Li2B12H12 is effectively prevented during the dehydrogenation of the LiBH4−YH3 composite under hydrogen and argon back pressure higher than 0.2 MPa.7,8 This phenomenon implies that gas back pressure suppresses the release of diborane (B2H6) gas during the dehydrogenation of LiBH4,8,16 eventually forming Li2B12H12 by reacting with undecomposed LiBH4.17 In this study, we focus on the role of early-stage atmosphere in the dehydrogenation of the LiBH4−YH3 composite. The dehydrogenation behavior of the composite is investigated under various early-stage Ar back-pressure conditions. Special attention is paid to the formation of Li2B12H12 during the dehydrogenation of the composite.

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Our previous studies have shown that the composite releases ∼7 wt % H2 at 350 °C through the expected reaction with a reaction enthalpy of 51 kJ/mol·H2, and the reverse reaction can take place under relatively mild rehydrogenation conditions (e.g., 350 °C and 9 MPa of H2).7−9 The role of hydrogen back pressure that enhances the dehydrogenation reactions of the LiBH4−MgH2,10−13 LiBH4− CaH2,7 LiBH4−CeH2,7 and LiBH4−YH37,8 composites has been reported. It has been attributed that hydrogen back pressure enhances the dehydrogenation reaction between LiBH4 and metal hydride by preventing the direct decomposition of LiBH4 into LiH and B or Li2B12H12, which has been © 2013 American Chemical Society

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EXPERIMENTAL PROCEDURE Lithium borohydrides (LiBH4, Acros, 95% purity) were used as a raw material without any purification. YH3 was hydrogenated Received: January 2, 2013 Revised: March 3, 2013 Published: April 3, 2013 8028

dx.doi.org/10.1021/jp4000208 | J. Phys. Chem. C 2013, 117, 8028−8031

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from yttrium powders (Sigma-Aldrich, 99.9% purity) at 350 °C under 9 MPa of H2 (99.9999% purity) for 2 h. The LiBH4− YH3 mixture (3 g) with a 4:1 molar ratio was ball-milled using a Retsch PM200 with 650 rpm for 12 h. Thirteen 12.7 mm diameter and twenty-four 7.9 mm diameter Cr-steel balls were employed together with a 140 mL hardened steel bowl, sealed in an argon atmosphere with a lid having a Viton O-ring. The ball-to-powder weight ratio was approximately 50:1. We dehydrogenated 0.3 g of the composite at 400 °C using a Sievert-type volumetric apparatus with a 110 mL reactor. In some cases, pressure modifications were carried out from static vacuum to 0.3 MPa of Ar (99.9999%) in various reaction stages. The heating rate was 30 °C/min and the pressure in the reactor was monitored during the dehydrogenation. XRD measurement of the dehydrogenated samples was performed using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation. Raman spectroscopy measurement of the dehydrogenated samples was carried out using a Horiba Jobin-Yvon LabRam ARAMIS with a 514 nm Ar laser. To prevent the samples from air exposure during the XRD and Raman spectroscopy measurements, we used borosilicate capillary tubes and specially designed alumina holders with a cover glass, respectively. High-pressure DSC measurement of 2 mg of the composite was performed up to 580 °C with heating rates 2, 10, 20, and 40 °C/min under 0.02, 0.3, and 0.5 MPa of Ar using a Netzsch DSC 204 HP Phoenix. The whole sample handlings were carried out inside an argon-filled glovebox (mBraun, UniLab), in which oxygen and water vapor levels were kept below 1 ppm.

measured amount of dehydrogenated hydrogen is 7.7 wt %. This result is consistent with our previous report showing that the composite releases about 7 wt % H2 through reaction 1 at 350 °C after an incubation period under higher than 0.2 MPa of Ar, although the incubation period at 400 °C is much shorter than that at 350 °C (>4 h).8 When the exposure time to static vacuum becomes a very little bit longer to 14 min, the composite starts to drastically dehydrogenate ∼27 min after the Ar back pressure is applied, despite the increase in incubation period (Figure 1b). The drastic increase in the pressure is not observed within 70 min, indicating that the dehydrogenation proceeds little when the exposure time increases to 15 min (Figure 1c). Instead of static vacuum, one sample initially undergoes 0.1 MPa of Ar for 15 min, followed by the application of 0.3 MPa of Ar, as shown in Figure 1d. In this case, the drastic increase in pressure starts ∼10 min after 0.3 MPa of Ar is applied, which is in contrast with the result for the exposure to static vacuum for 15 min shown in Figure 1c. The pressure profiles of the composite under the various conditions of Ar back pressure imply that the delicate difference in the exposure time to static vacuum around 400 °C can greatly affect the subsequent dehydrogenation behavior. X-ray diffraction (XRD) patterns of the reaction products after the dehydrogenation described in Figure 1 are compared in Figure 2. The patterns of the samples initially exposed to

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RESULTS AND DISCUSSION Figure 1 shows pressure and temperature profiles of the LiBH4−YH3 composite during the dehydrogenation at 400 °C

Figure 2. XRD patterns of the samples dehydrogenated according to the conditions described in Figure 1.

static vacuum for 12 and 14 min exhibit that LiH and YB4 are main reaction products (Figure 2a,b), confirming that the dehydrogenation reactions follow reaction 1, although YH2 that is partially dehydrogenated still remains. The pattern of the sample initially exposed to static vacuum for 15 min is quite different from those of the samples exposed to static vacuum for the shorter times, showing that starting materials LiBH4 and YH3 remain as major phases together with YH2 (Figure 2c). The pattern of the sample initially exposed to 0.1 MPa of Ar for 15 min is similar to that of the samples exposed to static vacuum for 12 and 14 min. As shown in Figure 3, Raman spectroscopy of the dehydrogenated samples to confirm the presence of amorphous dehydrogenation products that cannot be identified by XRD is performed. In the sample exposed to static vacuum for 12 min, only peaks of YB4 are observed (Figure 3a). However, a small peak of B (free boron), which is believed to be amorphous, is found together with the peaks of YB4, when the exposure time

Figure 1. Pressure and temperature profiles during dehydrogenation of the LiBH4−YH3 composite. 0.3 MPa of Ar is applied (a) 12, (b) 14, and (c) 15 min after static vacuum is initially maintained. In the case of panel d, 0.3 MPa of Ar is applied 15 min after 0.1 MPa of Ar is initially applied.

under three different pressure conditions, where 0.3 MPa of Ar is applied as back pressure after static vacuum is initially maintained for 12, 14, and 15 min. Practically, the composite is exposed to static vacuum at 400 °C for 0, 2, and 3 min because it takes ∼12 min for temperature to reach 400 °C. When the composite is initially exposed to static vacuum for 12 min, the pressure drastically increases ∼15 min after Ar back pressure is applied at 400 °C (Figure 1a). The increase in pressure indicates the dehydrogenation of the composite and the 8029

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Figure 3. Raman spectra of the samples dehydrogenated according to the conditions described in Figure 1. The inset shows the magnified Raman spectrum in the range from 2000 to 2800 cm−1 of (c).

to static vacuum becomes a little bit longer to 14 min (Figure 3b). When the exposure time is 15 min, the peaks of Li2B12H12 newly appear together with the peaks of B and the starting materials (Figure 3c). These results show that B first forms and subsequently Li2B12H12 forms during the early-stage dehydrogenation when the composite is at 400 °C under static vacuum before the application of Ar back pressure (0.3 MPa). This implies that the dehydrogenation starting sequence of LiBH4 around 400 °C under static vacuum is the direct decomposition of LiBH4 into LiH and B and the release of B2H6, which eventually turns into Li2B12H12 through a reaction with LiBH4.17 Although the formation of B at 400 °C under static vacuum seems to slightly retard reaction 1, comparing the dehydrogenation curves in Figure 1a,b, it does not seem to significantly suppress reaction 1 (Figures 2b and 3b) in contrast with Li2B12H12, which is believed to significantly suppress reaction 1 (Figures 2c and 3c). However, the formation of B and Li2B12H12 at 400 °C seems to be effectively suppressed by the initial application of Ar back pressure above 0.1 MPa instead of static vacuum, as shown in Figure 3d. The suppression of Li2B12H12 seems to be related to the role of Ar back pressure in suppressing diborane gas during the dehydrogenation of LiBH4, which eventually forms Li2B12H12 by reacting with undecomposed LiBH4, as Kostka et al.16 showed that inert gas flow can reduce the amount of diborane released from LiBH4 for a kinetic reaction compared with vacuum conditions. To understand the effect of Ar back pressure on the activation energy for the dehydrogenation reaction of the composite, we applied the Kissinger method18 under various Ar back pressures according to the following equation: ⎡ ⎤ ⎛ RA ⎞ E β ln⎢ 2 ⎥ = − a + ln⎜ ⎟ ⎢⎣ Tp ⎥⎦ RTp ⎝ Ea ⎠

Figure 4. DSC curves of the LiBH4−YH3 composite dehydrogenated under (a) 0.02, (b) 0.3, and (c) 0.5 MPa of Ar with various heating rates.

transformation and melting of LiBH4, respectively,1−3 which are not very dependent on the heating rate. The peaks above 380 °C indicate the dehydrogenation reaction of the composite (reaction 1), which tends to increase with increasing heating rate. It is also noted that the onset temperature of the dehydrogenation reaction under 0.3 and 0.5 MPa of Ar is quite lower than that under 0.02 MPa of Ar. To construct the Kissinger plots, the temperature corresponding to the minimum value of the heat flow for the endothermic, dehydrogenation reaction above 380 °C is chosen as Tp. The Kissinger plots for the dehydrogenation reaction under various

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where β is heating rate, Tp is peak temperature, Ea is activation energy, R is the gas constant, and A is a pre-exponential factor. T p is measured by high-pressure differential scanning calorimetry (DSC) with different heating rates (2, 10, 20, and 40 °C/min) under 0.02, 0.3, and 0.5 MPa of Ar (Figures 4). In the DSC curves, three major peaks corresponding to endothermic reactions are observed irrespective of Ar back pressure. The peaks around 120 and 280 °C indicate the phase 8030

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Ar back pressures, which represent ln(β/Tp2) against the inverse of Tp, are given in Figure 5. According to eq 2, the

ACKNOWLEDGMENTS This study has been supported by Korea Institute of Science and Technology (Grant Nos. 2E24043 and 2E24022). REFERENCES

(1) Züttel, A.; Wenger, P.; Rentsch, S.; Sudan, P.; Mauron, P.; Emmenegger, C. LiBH4 a New Hydrogen Storage Material. J. Power Sources 2003, 118, 1−7. (2) Orimo, S.; Nakamori, Y.; Kitahara, G.; Miwa, K.; Ohba, N.; Towata, S.; Züttel, A. Dehydriding and Rehydriding Reactions of LiBH4. J. Alloys Compd. 2005, 404−406, 427−430. (3) Mauron, P.; Buchter, F.; Friedrichs, O.; Remhof, A.; Bielmann, M.; Zwicky, C. N.; Zuttel, A. Stability and Reversibility of LiBH4. J. Phys. Chem. B 2008, 112, 906−910. (4) Barkhordarian, G.; Klassen, T.; Dornheim, M.; Bormann, R. Unexpected Kinetic Effect of MgB2 in Reactive Hydride Composites Containing Complex Borohydrides. J. Alloys Compd. 2007, 440, L18− L21. (5) Vajo, J. J.; Skeith, S. L.; Mertens, F. Reversible Storage of Hydrogen in Destabilized LiBH4. J. Phys. Chem. B 2005, 109, 3719− 3722. (6) Cho, Y. W.; Shim, J. -H.; Lee, B. J. Thermal Destabilization of Binary and Complex Metal Hydrides by Chemical Reaction: A Thermodynamic Analysis. CALPHAD 2006, 30, 65−69. (7) Shim, J. -H.; Lim, J. -H.; Rather, S. -U.; Lee, Y. -S.; Reed, D.; Kim, Y.; Book, D.; Cho, Y. W. Effect of Hydrogen Back Pressure on Dehydrogenation Behavior of LiBH 4-Based Reactive Hydride Composites. J. Phys. Chem. Lett. 2010, 1, 59−63. (8) Kim, K. -B.; Shim, J. -H.; Cho, Y. W.; Oh, K. H. PressureEnhanced Dehydrogenation Reaction of the LiBH4−YH3 Composite. Chem. Commun. 2011, 47, 9831−9833. (9) Shim, J. -H.; Lee, Y. -S.; Suh, J. -Y.; Cho, W.; Han, S. S.; Cho, Y. W. Thermodynamics of the Dehydrogenation of the LiBH4−YH3 Composite: Experimental and Theoretical Studies. J. Alloys Compd. 2012, 510, L9−L12. (10) Nakagawa, T.; Ichikawa, T.; Hanada, N.; Kojima, Y.; Fujii, H. Thermal Analysis on the Li−Mg−B−H Systems. J. Alloys Compd. 2007, 446−447, 306−309. (11) Pinkerton, F. E.; Meyer, M. S.; Meisner, G. P.; Balogh, M. P.; Vajo, J. J. Phase Boundaries and Reversibility of LiBH4/MgH2 Hydrogen Storage Material. J. Phys. Chem. C 2007, 111, 12881−12885. (12) Bösenberg, U.; Ravnsbaek, D. B.; Hagemann, H.; D’Anna, V.; Minella, C. B.; Pistidda, C.; Beek, W. V.; Jensen, T. R.; Bormann, R.; Dornheim, M. Pressure and Temperature Influence on the Desorption Pathway of the LiBH4-MgH2 Composite System. J. Phys. Chem. C 2010, 114, 15212−15217. (13) Yan, Y.; Li, H. -W.; Maekawa, H.; Miwa, K.; Towata, S.; Orimo, S. Formation of Intermediate Compound Li2B12H12 during the Dehydrogenation Process of the LiBH4−MgH2 System. J. Phys. Chem. C 2011, 115, 19419−19423. (14) Ohba, N.; Miwa, K.; Aoki, M.; Noritake, T.; Towata, S.; Züttel, A. First-Principles Study on the Stability of Intermediate Compounds of LiBH4. Phys. Rev. B 2006, 74, 075110−1−7. (15) Hwang, S. -J.; Bowman, R. C., Jr.; Reiter, J. W.; Rijssenbeek, J.; Soloveichik, G. L.; Zhao, J. -C.; Kabbour, H.; Ahn, C. C. NMR Confirmation for Formation of [B12H12]2‑ Complexes during Hydrogen Desorption from Metal Borohydrides. J. Phys. Chem. C 2008, 112, 3164−3169. (16) Kostka, J.; Lohstroh, W.; Fichtner, M.; Hahn, H. Diborane Release from LiBH4/Silica-Gel Mixtures and the Effect of Additives. J. Phys. Chem. C 2007, 111, 14026−14029. (17) Friedrichs, O.; Remhof, A.; Hwang, S. -J.; Züttel, A. Role of Li2B12H12 for the Formation and Decomposition of LiBH4. Chem. Mater. 2010, 22, 3265−3268. (18) Kissinger, H. E. Reaction Kinetics in Differential Thermal Analysis. Anal. Chem. 1957, 29, 1702−1706.

Figure 5. Kissinger plots of the LiBH4−YH3 composite dehydrogenated under 0.02, 0.3, and 0.5 MPa of Ar.

activation energy of the dehydrogenation reaction is determined from the slope of the plots. The activation energies for the dehydrogenation reaction under 0.3 and 0.5 MPa of Ar are similar, being estimated at 110 and 116 kJ/mol, respectively. The activation energy significantly increases to 168 kJ/mol, when the composite is dehydrogenated under 0.02 MPa of Ar. This increase in activation energy seems to be associated with the formation of Li2B12H12 under very low Ar back pressure and supports the conjecture that the formation of Li2B12H12 significantly retards the dehydrogenation reaction between LiBH4 and YH3 by hindering the contact between YH3 and LiBH4.

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CONCLUSIONS The dehydrogenation of the LiBH4−YH3 composite under various early-stage Ar back-pressure conditions is carried out. A minute change in early-stage atmosphere is found to control the overall dehydrogenation reaction of the composite. When the composite is exposed to static vacuum (or low Ar back pressure below 0.02 MPa) around 400 °C, the partial dehydrogenation products B and Li2B12H12 start to form in turn in a short time (∼3 min). The formation of Li2B12H12 significantly retards the subsequent dehydrogenation reaction between LiBH4 and YH3 into LiH and YB4, whereas the formation of B slightly increases the incubation period of the subsequent dehydrogenation reaction. The initial application of Ar back pressure above 0.1 MPa is effective in suppressing the formation of Li2B12H12 around 400 °C. The activation energy for the dehydrogenation reaction of the composite is estimated at 110 kJ/mol under 0.3 MPa of Ar. However, the activation energy significantly increases to 168 kJ/mol under 0.02 MPa of Ar due to the formation of Li2B12H12. These results imply that it is very important to carefully control the early-stage atmosphere to obtain a proper reaction pathway during the dehydrogenation of the composite.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +82-2-958-6760. Notes

The authors declare no competing financial interest. 8031

dx.doi.org/10.1021/jp4000208 | J. Phys. Chem. C 2013, 117, 8028−8031