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Superior Hydrogen Exchange Effect in the MgH2-LiBH4 System Liang Zeng,† Hiroki Miyaoka,‡ Takayuki Ichikawa,*,†,‡ and Yoshitsugu Kojima†,‡ Department of Quantum Matter, ADSM, and Institute for AdVanced Materials Research, Hiroshima UniVersity, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8530, Japan ReceiVed: May 10, 2010; ReVised Manuscript ReceiVed: June 13, 2010
The MgH2-LiBH4 system is one of the promising hydrogen storage materials. In this system, it was found that there was a mutual interaction between the two hydrides, but its detailed mechanism has not been clarified yet. In this work, we found an “H-D” exchange between MgD2 and LiBH4 during heating. IR absorption spectroscopy revealed that the peak of “B-D” vibration appeared at 275 °C below the melting point, indicating that this exchange proceeded even in solid phases. The hydrogen desorption properties of the composite of catalyst-doped MgH2 and LiBH4 under inert gas were investigated by mass spectrometry. The results showed that the hydrogen desorption temperature of the first step was over 300 °C, in spite of catalyst-doped MgH2 being able to release hydrogen at 200 °C. The above results suggest the hydrogen desorption from catalystdoped MgH2 is somehow suppressed by the hydrogen exchange effect between MgH2 and LiBH4. MgH2 + 2LiBH4 T 2LiH + Mg + 2B +
Introduction Hydrogen energy is one of the candidates for secondary energy to be efficiently converted from alternative energies, such as solar power, hydroelectric, wind power and so on.1 The research on hydrogen storage materials is thought to be the most important part to realize the hydrogen energy society at present. However, there are still a lot of technical barriers to be overcome as soon as possible. In the materials-based hydrogen storage systems with an on-board reversibility, the barriers are listed as follows: lack of understanding of hydrogenation and dehydrogenation, reproducibility of performance, regeneration processes, byproduct material removal, and so forth.2 LiBH4 is one of the promising materials for hydrogen storage due to its high gravimetric and volumetric hydrogen densities of 18.5 mass % and 121 kg H2/m3, respectively.3 It is reported that LiBH4 can release hydrogen up to ∼13.5 mass % according to the following reaction equation:4,5
3 LiBH4 T LiH + B + H2 2
(1)
However, the hydrogen desorption from LiBH4 starts at round 400 °C and only releases half the hydrogen below 600 °C.4 Furthermore, the hydrogenation to LiBH4 from LiH and B is significantly difficult unless under extreme conditions of 35 MPa H2 and 600 °C.6 Therefore, many improvements have been achieved to enhance the dehydrogenation and rehydrogenation properties of LiBH4, such as reducing the particle size,7 doping oxides,8,9 and reacting with metal hydrides10,11 or hydride mixtures.12 It has been well established that LiBH4 can be effectively destabilized by MgH210,13,14 and completely decomposed below 500 °C.3 It was found that there was a mutual interaction between the two hydrides, which increased the overall rates of hydrogen diffusion due to the reduction of diffusion distances.13 The reaction is expressed as follows: * To whom correspondence should be addressed. Phone: +81-82-4245744. Fax: +81-82-424-5744. E-mail:
[email protected]. † Department of Quantum Matter, ADSM, Hiroshima University. ‡ Institute for Advanced Materials Research, Hiroshima University.
4H2(under dynamic vacuum10) (2) At present, the detailed mechanism of hydrogen generation of the above reaction is still unknown, and the source of the released hydrogen that comes from the MgH2-LiBH4 system is not clear yet. Therefore, in this work, we use catalyst-doped MgD2/MgH2 instead of neat MgH2 in order to clarify this mechanism by isotopic method. Experimental Details As starting materials, LiBH4 (95%, Sigma-Aldrich), Mg (99.9%, Rare Metallic), MgH2 (98%, Alfa Aesar), Nb2O5 (99.5%, Sigma-Aldrich), KBr (99.9%, Sigma-Aldrich), H2 (>99.99999%), and D2 (99.95%) were obtained commercially and used without further purification. MgD2 was self-made by ball-milling method, and the purity of MgD2 was evaluated to be 90% (see the Supporting Information). The Nb2O5 (1 mol %) powder as a catalyst was doped into MgH2/MgD2 by ballmilling the mixture of MgH2/MgD2 and Nb2O5 under 1 MPa H2/D2 for 20 h at room temperature, where this technique revealed the best kinetic properties on MgH2.15,16 The composite of MgD2-2LiBH4 was made by ball-milling under 0.1 MPa Ar for 2 h. Furthermore, the Nb2O5-doped MgH2/ MgD2 and LiBH4 (molar ratio 1:2) were mixed by ball-milled apparatus under 0.1 MPa Ar for 2 h or in agate mortar by hand for 5 min in an Ar filled glovebox, where the ball-milled and hand-milled samples were named as “composite” and “mixture”, respectively. In calculating the 1:2 molar ratio of MgH2/MgD2 with LiBH4, we assumed that the purities of MgH2 and MgD2 were, respectively, 100 and 90% and the solid phase reaction between MgH2/MgD2 and oxide catalyst did not occur. All the ball-milling processes were performed with 20 ZrO2 balls (8 mm in diameter) by using a planetary ball-mill apparatus (P7, Fritsch) at 370 rpm. The thermal analysis was performed by a thermogravimetry with differential thermal analysis equipment (TG-DTA, Rigaku, TG8120) connected to a mass spectrometer (MS, Anelva, M-QA200TS) under an Ar gas flow. A flow rate of 300 mL/
10.1021/jp1042443 2010 American Chemical Society Published on Web 07/12/2010
MgH2-LiBH4 Hydrogen Exchange Effect
Figure 1. TG-DTA-MS results of the MgD2-2LiBH4.composite, ballmilled for 2 h. The measurement was performed under Ar flow with a heating rate of 5 °C min-1.
min was used with Ar as the carrier gas at a heating rate of 5 °C/min. The characteristic B-D and B-H stretching modes in LiBD4 and LiBH4 were examined via a Fourier transform infrared spectrometer (FT-IR, Spectrum One, Perkin-Elmer) that ran in reflection mode by using a diffuse reflectance accessory with a heated chamber (PIKE, DiffusIR). The samples for FTIR were diluted by KBr with a mass ratio of 1:10 (sample/ KBr). The identification of the products was carried out by X-ray powder diffraction (XRD) measurements (Rigaku, RINT-2500, Cu KR). All the processes from preparation to analyses were carried out in Ar filled gloveboxes with a recycling purification system to minimize water adsorption and oxidation. Regarding the samples for XRD measurement, a polyimide sheet (Kapton, Du Pont-Toray Co. Ltd.) was used to cover them to avoid oxidation during the XRD measurement. Results and Discussion Thermal Analyses and Infrared Spectroscopy of MgD2-2LiBH4 Composite. TG-DTA-MS results of the MgD2-2LiBH4 composite are shown in Figure 1. Four endothermic peaks were found on the DTA profile. The first two peaks at 115 and 285 °C were ascribed to the phase transition and melting of LiBH4, respectively.3 The hydrogen desorption corresponding to H2 was started from 270 °C from MgD2, and the peak was found at 330 °C; the peak at 461 °C was corresponding to the hydrogen desorption of LiBH4, where the products after each reaction were confirmed by XRD measurements as shown in Figure 2. A shift to higher 2θ values can be observed in the XRD pattern of Mg after heating to 500 °C, suggesting that MgxLiy was formed around 500 °C, which agrees well with the findinds of Yu et al.11 The continued weight loss at 500 °C in Figure 1 can be understood by the reaction of xMg + yLiH/LiD f MgxLiy + y/2 H2/D2. According to the above results, D2 should be observed at 330 °C by mass spectra. However, D2 gas was hardly detected in MS measurement while H2 and HD were found as the main desorption gases at this temperature. This result indicated that HD exchange should occur between MgD2 and LiBH4 below 330 °C. Figure 3 shows the results of in situ FT-IR spectroscopy for the MgD2-2LiBH4 composite. At room temperature, only the peak corresponding to the B-H stretching vibration (∼2280 cm-1) was detected, and it remained until 130 °C. As the temperature is increasing, the peak of the B-D stretching vibration (∼1688 cm-1) was found at 275 °C, which was below
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Figure 2. XRD patterns of the MgD2-2LiBH4 composite heating up to (a) 300 °C, (b) 350 °C, and (C) 500 °C.
Figure 3. In-situ FT-IR spectra of the MgD2-2LiBH4 composite, performed under Ar flow with a heating rate of 5 °C min-1.
the melting temperature of LiBH4 (285 °C). This result indicated H atoms of [BH4]- anions were exchanged for D atoms in MgD2 to form [BH4-xDx]- (x ) 1, 2, 3, or 4) during the increase in temperature; thus, MgD2 became MgHD and MgH2 at the same time. In other words, a protium-deuterium (H-D) exchange reaction occurred between MgD2 and LiBH4 in solid phases without any gas desorption during the heating of the composite. This phenomenon explained why H2 and HD were mainly detected with a hydrogen desorption from MgD2 in the composite at 330 °C by MS. Nb2O5-Doped MgH2/MgD2 with LiBH4. In order to understand the influence of the exchange effect in the MgD2-LiBH4 system, we used Nb2O5-doped MgH2/MgD2 to synthesize the composite with LiBH4. The Nb2O5-doped MgH2/MgD2 can release H2/D2 at around 200 °C, while the neat MgH2/MgD2 decomposes over 350 °C.15,16 In this case, we can understand whether the exchange effect will affect the hydrogen desorption of Nb2O5-doped MgH2/MgD2 in the composite or not. To achieve this goal, we designed a series of samples as shown in Figure 4. First, we prepared the Nb2O5-doped MgH2/MgD2 samples by ball-milling MgH2/MgD2 with 1 mol % Nb2O5 under 1 MPa H2/D2 for 20 h. Then, the products were mixed with LiBH4 by hand-mill and ball-mill methods. The ball-milled samples were named “composite” while the hand-milled samples were named “mixture” hereafter.
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Figure 4. Experiment scheme of (a) the MgH2-2LiBH4 system and (b) the MgD2-2LiBH4 system.
Figure 6. In-situ FT-IR spectra of Nb2O5-doped MgD2 + 2LiBH4, (a) composite (ball-milled) and (b) mixture (hand-milled).
Figure 5. MS results of (a) Nb2O5-doped MgD2 + 2LiBH4 systems and (b) Nb2O5-doped MgH2 + 2LiBH4 systems, performed under Ar flow with a heating rate of 5 °C min-1.
Figure 5(a) shows the mass spectra of the composite and mixture together with the mass spectra of Nb2O5-doped MgD2. As mentioned before, due to the H-D exchange effect between MgD2 and LiBH4, D2 (m/z ) 4) desorption in the case of the composite was changed to H2 (m/z ) 2), whereas the main desorption gas of MgD2 itself was D2. Therefore, the mass numbers shown in Figure 5(a) for Nb2O5-doped MgD2 and the composite/mixture are 4 and 2, respectively. Nb2O5-doped MgD2 desorbed D2 at around 200 °C. On the other hand, the hydrogen desorption temperature increased to over 300 °C after mixing with LiBH4 by both ball-milling and hand-milling. This result indicated the H-D exchange effect between MgD2 and LiBH4 suppressed the hydrogen desorption of Nb2O5-doped MgD2. Strictly speaking, the D atoms could not form D2 to release from the mixture and the composite but only exchanged with the H atoms in LiBH4. Regarding this phenomenon, it may be caused by a thermodynamic reason. Generally, H atoms in hydrides form an adsorbed state on the material surface before H2 desorption. We assume the entropy changes of hydrogen desorption from MgD2-2LiBH4 and MgD2 are ∆S1 and ∆S2,
respectively. In the case of MgD2-LiBH4, the configuration number of hydrogen atoms was increased due to the H-D exchange effect, leading to ∆S1 < ∆S2. According to ∆G ) ∆H - T∆S, we can easily deduce that T1 > T2, in which the hydrogen desorption from MgD2-2LiBH4 and MgD2 are occurring around T1 and T2, respectively. That is to say, the hydrogen desorption temperature was increased by the H-D exchange effect. The reaction between Nb2O5-doped MgH2 and LiBH4 showed almost the same result as shown in Figure 5(b). The hydrogen desorption temperature of the Nb2O5-doped MgH2 + 2LiBH4 mixture was also over 300 °C even though Nb2O5-doped MgH2 can release hydrogen at round 200 °C. This result suggested there was also an exchange effect between MgH2 and LiBH4; it was H-H exchange in this case. Then, the similar thermodynamic properties were obtained in the MgD2-LiBH4 and MgH2-LiBH4 composites. In principle, the H-H exchange should be faster and more effective than the H-D exchange due to the higher mobility of H atoms. However, we could not observe the difference between H-D and H-H exchanges directly due to the limitation of the conventional laboratory apparatus. Therefore, the comparison of these two exchange effects is not easy to achieve at present. Figure 6 shows IR spectra of the composite and the mixture with Nb2O5-doped MgD2 obtained by in situ measurements. The peak corresponding to the B-D stretching vibration (∼1688 cm-1) was found at 100 °C in the composite. This temperature was 175 °C lower than that of the composite without using catalyst as shown in Figure 3. In the case of the mixture, Figure 6(b) showed the appearance of the B-D peak started from 200 °C, which was 100 °C higher than the composite. The above results revealed that the diffusive motion of hydrogen was increased by both the catalytic and milling effects.17
MgH2-LiBH4 Hydrogen Exchange Effect Conclusions In this work, we found an H-D exchange occurred in the MgD2-2LiBH4 system during the heating even though they were solid phases below 300 °C. The exchange effect between the two hydrides would suppress the hydrogen desorption of catalyst-doped MgD2 thermodynamically. The same phenomenon was also confirmed in the case of the MgH2-LiBH4 system. Furthermore, we found that Nb2O5 and ball-milling can lower the starting temperature and accelerate the exchange reaction. Acknowledgment. This work was partially supported by NEDO under “Advanced Fundamental Research Project on Hydrogen Storage Materials”. Supporting Information Available: TG-DTA-MS results and XRD pattern of the self-made MgD2 sample. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Schlapbach, L.; Zu¨ttel, A. Hydrogen-storage materials for mobile applications. Nature 2001, 414, 353–358. (2) Freedom Car Hydrogen Storage Roadmap; U.S. Department of Energy, 2005. (3) Nakagawa, T.; Ichikawa, T.; Hanada, N.; Kojima, Y.; Fujii, H. Thermal analysis on the Li-Mg-B-H systems. J. Alloys Compd. 2007, 446447, 306–309. (4) Zu¨ttel, A.; Wenger, P.; Rentsch, S.; Sudan, P.; Mauron, P.; Emmenegger, C. LiBH4 a new hydrogen storage material. J. Power Sources 2003, 118, 1–7. (5) Zu¨ttel, A.; Rentsch, S.; Fischer, P.; Wenger, P.; Sudan, P.; Mauron, P.; Emmenegger, C. Hydrogen storage properties of LiBH4. J. Alloys Compd. 2003, 356, 515–520.
J. Phys. Chem. C, Vol. 114, No. 30, 2010 13135 (6) 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. (7) Yu, X. B.; Wu, Z.; Chen, Q. R.; Li, Z. L.; Weng, B. C.; Huang, T. S. Improved hydrogen storage properties of LiBH4 destabilized by carbon. Appl. Phys. Lett. 2007, 90, 034106. (8) Yu, X. B.; Grant, D. M.; Walker, G. S. Low-temperature dehydrogenation of LiBH4 through destabilization with TiO2. J. Phys. Chem. C 2008, 112, 11059–11062. (9) Yu, X. B.; Grant, D. M.; Walker, G. S. Dehydrogenation of LiBH4 destabilized with various oxides. J. Phys. Chem. C 2009, 113, 17945–17949. (10) Vajo, J. J.; Skeith, S. L.; Mertens, F. Reversible storage of hydrogen in destabilized LiBH4. J. Phys. Chem. B 2005, 109, 3719–3722. (11) Yu, X. B.; Grant, D. M.; Walker, G. S. A new dehydrogenation mechanism for reversible multicomponent borohydride systems-The role of Li-Mg alloys. Chem. Commun. 2006, 3906–3908. (12) Niemann, M. U.; Srinivasan, S. S.; Kumar, A.; Stefanakos, E. K.; Goswami, D. Y.; McGrath, K. Processing analysis of the ternary LiNH2MgH2-LiBH4 system for hydrogen storage. Int. J. Hydrogen Energy 2009, 34, 8086–8093. (13) Vajo, J. J.; Salguero, T. T.; Gross, A. F.; Skeith, S. L.; Olson, G. L. Thermodynamic destabilization and reaction kinetics in light metal hydride systems. J. Alloys Compd. 2007, 446-447, 409–414. (14) Bo¨senberg, U.; Doppiu, S.; Mosegaard, L.; Barkhordarian, G.; Eigen, N.; Borgschulte, A.; Jensen, T. R.; Cerenius, Y.; Gutfleisch, O.; Klassen, T.; Dornheim, M.; Bormann, R. Hydrogen sorption properties of MgH2-LiBH4 composites. Acta Mater. 2007, 55, 3951. (15) Hanada, N.; Ichikawa, I.; Fujii, H. Catalytic effect of Ni nanoparticle and Nb oxide on H-desorption properties in MgH2 prepared by ball milling. J. Alloys Compd. 2005, 404-406, 716–719. (16) Hanada, N.; Ichikawa, T.; Hino, S.; Fujii, H. Remarkable improvement of hydrogen sorption kinetics in magnesium catalyzed with Nb2O5. J. Alloys Compd. 2006, 420, 46–49. (17) Corey, R. L.; Ivancic, T. M.; Shane, D. T.; Carl, E. A.; Bowman, R. C., Jr.; Bellosta von Colbe, J. M.; Dornheim, M.; Bormann, R.; Huot, J.; Zidan, R.; Stowe, A. C.; Conradi, M. S. Hydrogen motion in magnesium hydride by NMR. J. Phys. Chem. C 2008, 112, 19784–19790.
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