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Influence of Nanoconfinement on Reaction Pathways of Complex Metal Hydrides
Available online at www.sciencedirect.com
Energy Procedia 29 (2012) 731 – 737
World Hydrogen Energy Conference 2012
Influence of Nanoconfinement on Reaction Pathways of Complex Metal Hydrides Zhirong Zhao-Kargera,*, Raiker Wittera,b, Elisa Gil Bardajía, Di Wanga, Daniel Cossementc, Maximilian Fichtnera a
Karlsruhe Institute of Technology(KIT), Institute of Nanotechnology, P.O. Box 3640, D-76021 Karlsruhe, Germany Technomedicum, Tallinn University of Technology Ehitajate tee 5, 19086 Tallinn, Estonia c Institut de recherche sur l’hydrogène, Université du Québec à Trois-Rivières, Trois-Rivières, Québec, Canada G9A 5H7 b
Abstract Nanoconfinement is receiving increasing interest in the field of energy storage and has been applied to different metal hydride systems in order to improve their properties for hydrogen storage. Modified hydrogen sorption kinetics and thermodynamics of metal hydrides have been achieved by nano size effects. In case of metal complex hydrides, in addition to kinetic limitations, competing side reactions and stable intermediates during de/rehydrogenation processes are obstacles for a practical utilization. The impacts of nanoconfinement on dehydrogenation reaction pathways of the eutectic borohydrides LiBH4–Mg(BH4)2 have been studied in the presented work.
© Selection and/or peer-review under responsibility of Canadian Hydrogen and © 2012 2012Published Publishedby byElsevier ElsevierLtd. Ltd. Fuel Cell Association Selection and/or peer-review under responsibility of Canadian Hydrogen and Fuel Cell Association
Keywords: Hydrogen storage; Metal borohydride; Nanoconfinement; Reaction pathways
1. Introduction Hydrogen has been recognized as a clean and environmentally friendly energy carrier to replace traditional fossil fuels. The use of hydrogen for sustainable energy storage technology holds good prospects to address the growing demands for energy supply and simultaneously reduce the CO2 emission. Compared to compressed or liquefied hydrogen, the chemisorbed hydrogen in solids such as metal hydrides can offer increased hydrogen density. Light metal borohydrides have high theoretical gravimetric and volumetric hydrogen densities (e.g. 18.4 wt% and 121 kg/m3 for LiBH4, respectively) and
* Corresponding author. Tel.: +49-721-608-28908; fax: +49-721-608-26368. E-mail address: [email protected]
1876-6102 © 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Canadian Hydrogen and Fuel Cell Association. doi:10.1016/j.egypro.2012.09.085
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Zhirong Zhao-Karger et al. / Energy Procedia 29 (2012) 731 – 737
have attracted much attention as a potential hydrogen storage material.[1] However, unfavorable thermodynamics and/or kinetics hamper the utilization of metal borohydrides in practical hydrogen storage applications. One of the largest obstacles to use of metal borohydrides for reversible hydrogen storage is the formation of some stable closo polyborane intermediates like MB12H12, which may account for the high desorption temperature and the poor reversibility of these materials.[2] In addition, the evolution of diborane during the decomposition of some complex hydrides leads to capacity loss and is also a safety concern.[3] Generally, nanoscale materials may have significantly different properties as compared to the bulk materials owing to increased surface area of the solids, shorter diffusion distances and increased number of atoms at the grain boundary. An alternative approach of nanoconfinement is of increasing interest for fabrication of nano materials by constricting of the active material within a nanoporous scaffold.[4] In addition to the general effects on size reduction, nanoconfinement may also hinder the particle agglomeration and the phase segregation for the composite system through limiting the mobility of the particles within the scaffolds. Nanoconfined MgH2[5] and NaAlH4[6] have shown both improved kinetics and thermodynamics properties. Enhanced dehydrogenation rate and improved reversibility have also been achieved by infiltrating metal borohydrides into the mesoporous carbon scaffolds.[7] The present work is focus on the dehydrogenation reaction mechanism of the LiBH4–Mg(BH4)2 incorporated in a mesoporous carbon support to gain more insight into the influence of the particle restriction or dispersion at nano scale on altering the reaction pathways of the metal borohydrides. 1.1 The eutectic LiBH4–Mg(BH4)2 and its nanocomposite Based on the different electronegativity of the metals, introducing a foreign metal cation into the borohydride system might be an effective method for tuning the thermodynamics properties of the metal borohydrides. The binary system LiBH4–Mg(BH4)2 with an eutectic composition of 1 : 1 has been investigated,[8] which has a theoretical hydrogen capacity of 14.6 mass %. The thermal analysis of the LiBH4–Mg(BH4)2 system revealed the destabilization of the single constituent phases in the eutectic composition. The overall desorption kinetics of the LiBH4–Mg(BH4)2 system has been significantly enhanced compared to the individual components i.e. LiBH4 and Mg(BH4)2. The eutectic LiBH4-Mg(BH4)2 with molar ratio of 1:1 was prepared as previously described.[8] Porous carbon materials with pore width below 4 nm IRH33 was utilized as size controlling template. Because LiBH4-Mg(BH4)2 system has a comparably low eutectic melting point at 182 °C, the LiBH4Mg(BH4)2/IRH33 composite was prepared by a melt impregnation. 0.19 g of LiBH4-Mg(BH4)2 and 0.5 g of IRH33 carbon material were mixed with mortar and pestle, then heated in an autoclave type reactor at 190 °C under 40 bar of H2 for 1 hour. The LiBH4-Mg(BH4)2/IRH33 composite was characterized by means of X-ray diffraction and electron microscopy. The infiltrated LiBH4-Mg(BH4)2/IRH33 composite did not show any Bragg peaks (Fig1), which indicates that the borohydrides have been completely filled into the tiny pores of the carbon host and became amorphous or the crystallite size was too small for coherent scattering of the X-rays.
Zhirong Zhao-Karger et al. / Energy Procedia 29 (2012) 731 – 737
Fig.1. X-ray diffraction patterns
No crystalline borohydride particles were directly observed in the TEM images of the composite and the morphology of the composite resembles that of the activated carbon scaffold. The concentrations of Mg, B, C and O were mapped in an area of 120x120 nm2 by EDX spectra imaging processed by raw quantification as shown Fig. 2. Oxygen originated from oxidation due to the short exposure to ambient conditions during the sample preparation for TEM because the LiBH4-Mg(BH4)2 mixture is highly reactive to air. Nevertheless, the homogeneous distribution of the elements within the map indicates that metal borohydrides were homogeneously infiltrated and dispersed into the carbon scaffold.
Fig. 2. EDX spectra of LiBH4-Mg(BH4)2/IRH33
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1.2 Desorption reactions of LiBH4-Mg(BH4)2/carbon nanocomposite The hydrogen desorption of the LiBH4-Mg(BH4)2/IRH33 composite and the bulk binary LiBH4Mg(BH4)2 was studied by simultaneous thermogravimetric analysis, differential scanning calorimetry and mass spectrometry (TGA-DSC-MS) under the same experimental conditions. The thermogravimetric analysis of LiBH4-Mg(BH4)2 illustrates three distinguished hydrogen desorption steps. The DSC curve of LiBH4-Mg(BH4)2 exhibits four distinct peaks labelled as a, b, c, d in Fig. 3. The transformation steps a at 113 °C and b at 180 °C are assigned to the orthorhombic to hexagonal transformation of LiBH4 and the eutectic melting of the LiBH4-Mg(BH4)2 mixture, respectively. In contrast to the bulk sample, the weight loss of nanoconfined LiBH4-Mg(BH4)2 proceeded by only one step. As shown in Fig 4, there were no notable signal for structural phase transition and melting events in the DSC performance. Two endothermic peaks at 260 and 330°C marked as c’ and d’, respectively, likely correspond to the hydrogen releasing events c and d of the bulk material, but occurred at lower temperatures. These indicate the remarkable changes on the properties of the LiBH4-Mg(BH4)2 by constraining within the carbon scaffold.
Fig. 3. TGA-DSC-MS for LiBH4-Mg(BH4)2
Fig. 4. TGA-DSC-MS for LiBH4-Mg(BH4)2/IRH33 composite
Zhirong Zhao-Karger et al. / Energy Procedia 29 (2012) 731 – 737
Mass spectrometry has been used to simultaneously examine the hydrogen and diborane release upon decomposition of the nanoconfined and bulk sample. For the bulk LiBH4-Mg(BH4)2, four hydrogen releasing steps in the temperature range from 170 to 400 °C are visible in Fig. 3. Compared to the bulk material, the main hydrogen desorption of the nanoconfined LiBH4-Mg(BH4)2 occurred in the temperature range of 220 to 380 °C as shown in Fig 4. By normalizing the amount of active material to the same value for the carbon containing sample, the evolution of B2H6 was compared. As shown in Fig 5, the eutectic LiBH4-Mg(BH4)2 reached their highest B2H6 release rate at the eutectic melting point. In contrast, the infiltrated sample showed no detectable B2H6 trace.
Fig. 5. Mass spectra of diborane release of LiBH4-Mg(BH4)2 and the composite
To further elucidate the reaction paths, several samples at the interstages during their decomposition have been prepared by heating up the bulk and infiltrated materials under the initial H2 pressure of 0.2 bar to 280, 380 °C and holding for 1 hour, subsequently cooled down to ambient temperature, respectively. 11B MAS-NMR displays the changes in the composition of eutectic LiBH4-Mg(BH4)2 during desorption (Fig. 6). Spectrum b corresponds to the products of the first decomposition reaction at 280 °C, where the major peak at –40.8 ppm is assigned to the unreacted LiBH4-Mg(BH4)2 and the appearance of the peak –15.5 ppm indicates the formation of [B12H12]2- anions. At 380 °C, the material is in its third decomposition step according to the H2 release pattern. The concentration of [B12H12]2-anions is increased while the [BH4]groups are vanished and boron is formed in the meantime as shown in spectrum c. In case of the infiltrated sample, spectrum b’ in Fig 7 presents the decomposed products of the composite at 280 °C. The major broad signal centered at about 5 ppm was assigned to amorphous boron. The broad shoulder at about –19.4 ppm is assigned to [B12H12]2- species, which is shifted by about –4 ppm compared to the corresponding one of the bulk material probably due to the influence of the carbon surface. Additionally, the weak peak at –44 ppm implies that a small amount of borohydrides remained at this temperature. When increasing the temperature to 380°C, spectrum c’ shows only a single signal for boron species, which indicates a complete desorption for the composite.
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Fig. 6. 11B MAS-NMR spectra of samples of LiBH4-Mg(BH4)2 at different stages of dehydrogenation. a starting material LiBH4-Mg(BH4)2; b sample desorbed at 280 °C; c sample desorbed at 380°C. *Indicates spinning sidebands.
Fig. 7. 11B MAS-NMR spectra of samples of LiBH4-Mg(BH4)2/IRH33 at different stages of dehydrogenation a’ starting material LiBH4-Mg(BH4)2/IRH33 b’ sample desorbed at 280 °C; c’ sample desorbed at 380°C. *Indicates spinning sidebands
2. Conclusion The eutectic binary LiBH4-Mg(BH4)2 is highly dispersed and amorphized through melt infiltration within the mesoporous carbon support. The decomposition of LiBH4-Mg(BH4)2 confined in the carbon scaffold proceeds via different reaction paths from those of the bulk material. The reactions involving emission of toxic diborane have been found to be suppressed by nanoconfinement. The [B12H12]2- species are strongly destabilized by the influence of the carbon material. The altered reaction pathway and the destabilization of MB12H12 might be the origin of the enhanced desorption rate of nanoconfined LiBH4-Mg(BH4)2 system.
Zhirong Zhao-Karger et al. / Energy Procedia 29 (2012) 731 – 737
Acknowledgements Funding by the EU project NANOHy (“Novel Nanocomposites fro Hydrogen Storage Applications”, contract no. 210092) is gratefully acknowledged. References [1] (a) Schlapbach L, Züttel A. Hydrogen-Storage Materials for Mobile Applications. Nature 2001; 414: 353–358. (b) Eberle U, Felderhoff M, Schüth F. Chemical and Physical Solutions for Hydrogen Storage. Angew. Chem., Int. Ed. 2009; 48: 6608–30. (c) Orimo S, Nakamori Y, Eliseo JR, Züttel A, Jensen CM. Complex Hydrides for Hydrogen Storage. Chem. Rev. 2007, 107: 4111–32. [2] (a) Ohba N, Miwa K, Aoki M, Noritake T, Towata S, Nakamori Y, Orimo S, Züttel A. First-principles study on the stability of intermediate compounds of LiBH4. Phys. Rev. B 2006; 74: 075110. (b) Li H-W, Kikuchi K, Nakamori Y, Ohba N, Miwa K, Towata S, Orimoa S. Dehydriding and rehydriding processes of well-crystallized Mg(BH4)2 accompanying with formation of intermediate compounds. Acta Mater 2008, 56, 1342–47. (c) Hwang S-J, Bowman BC Jr, Reiter JW, Rijssenbeek JR, Soloveichik GL, Zhao J-C, et al. NMR Confirmation for Formation of [B12H12]2- Complexes during Hydrogen Desorption from Metal Borohydrides. J Phy Chem C 2008; 112: 3164–3169. (d) Soloveichika GL, Gao Y, Rijssenbeek J, Andrus M, Kniajanski S, Bowman Jr RC, Hwang S-J, Zhao J-C. Magnesium borohydride as a hydrogen storage material: Properties and dehydrogenation pathway of unsolvated Mg(BH4)2. Int J Hydrogen Energy 2009; 3 4: 916–28. (e) Ozolins V, Majzoub EH, Wolverton C. First-Principles Prediction of Thermodynamically Reversible Hydrogen Storage Reactions in the Li-Mg-Ca-B-H System. J Am Chem Soc 2009; 131: 230–37. [3] (a) 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: 14026–29 (b) Friedrichs O, Borgschulte A, Kato S, Buchter F, Gremaud R, Remhof A, Zuettel A. Low-Temperature Synthesis of LiBH4 by Gas–Solid Reaction. Chem Eur J 2009; 15: 5531–34 (c) Borgschulte A, Callini E, Probst B, Jain A, Kato S, Friedrichs O, Remhof A, Bielmann M, Ramirez-Cuesta AJ, Zuettel A. Impurity Gas Analysis of the Decomposition of Complex Hydrides. J Phys Chem C 2011; 115: 17220-26 [4] (a) Fichtner M. Properties of nanoscale metal hydrides. Nanotechnology 2009; 20: 204009. (b) Fichtner M. Nanoconfinement effects in energy storage materials. Phys Chem Chem Phys 2011; 13: 21186–95; (c) Nielsen TK, Besenbacher F, Jensen TR. Nanoconfined hydrides for energy storage. Nanoscale 2011; 3: 2086–98. (d) Vajo JJ. Influence of nano-confinement on the thermodynamics and dehydrogenation kinetics of metal hydrides. Current Opinion in Solid State and Materials Science 2011; 15: 52–61. [5] Zhao-Karger Z, Hu J, Roth A, Wang D, Kübel C, Lohstroh W, Fichtner M. Altered thermodynamic and kinetic properties of MgH2 infiltrated in microporous scaffold. Chem Commun 2010, 46: 8353–55. [6] (a) Lohstroh W, Roth A, Hahn H, Fichtner M. Thermodynamic effects in nanoscale NaAlH4. Chem Phys Chem 2010; 11:789–92. (b) Gao J, Adelhelm P, Verkuijlen MHW, Rongeat C, Herrich M, van Bentum PJM, et al. Confinement of NaAlH4 in nanoporous carbon: impact on H2 release, reversibility and thermodynamics. J Phys Chem C 2010;114:4675–82. [7] (a) Gross AF, Vajo JJ, Van Atta SL, Olson GL. Enhanced hydrogen storage of LiBH4 in nanoporous carbon scaffolds. J Phys Chem C 2008; 112:5651–7.(b) Fichtner M, Zhao-Karger Z, Hu J, Roth A, Weidler P. Nanotechnology 2009; 20: 204029 [8] Bardají EG, Zhao-Karger Z, Boucharat N, Nale A, van Setten MJ, Lohstroh W, Röhm E, Catti M, Fichtner M. LiBH4Mg(BH4)2: A Physical Mixture of Metal Borohydrides as Hydrogen Storage Material. J Phys Chem C 2011; 115: 6095–101.
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Energy Procedia 29 (2012) 731 – 737
World Hydrogen Energy Conference 2012
Influence of Nanoconfinement on Reaction Pathways of Complex Metal Hydrides Zhirong Zhao-Kargera,*, Raiker Wittera,b, Elisa Gil Bardajía, Di Wanga, Daniel Cossementc, Maximilian Fichtnera a
Karlsruhe Institute of Technology(KIT), Institute of Nanotechnology, P.O. Box 3640, D-76021 Karlsruhe, Germany Technomedicum, Tallinn University of Technology Ehitajate tee 5, 19086 Tallinn, Estonia c Institut de recherche sur l’hydrogène, Université du Québec à Trois-Rivières, Trois-Rivières, Québec, Canada G9A 5H7 b
Abstract Nanoconfinement is receiving increasing interest in the field of energy storage and has been applied to different metal hydride systems in order to improve their properties for hydrogen storage. Modified hydrogen sorption kinetics and thermodynamics of metal hydrides have been achieved by nano size effects. In case of metal complex hydrides, in addition to kinetic limitations, competing side reactions and stable intermediates during de/rehydrogenation processes are obstacles for a practical utilization. The impacts of nanoconfinement on dehydrogenation reaction pathways of the eutectic borohydrides LiBH4–Mg(BH4)2 have been studied in the presented work.
© Selection and/or peer-review under responsibility of Canadian Hydrogen and © 2012 2012Published Publishedby byElsevier ElsevierLtd. Ltd. Fuel Cell Association Selection and/or peer-review under responsibility of Canadian Hydrogen and Fuel Cell Association
Keywords: Hydrogen storage; Metal borohydride; Nanoconfinement; Reaction pathways
1. Introduction Hydrogen has been recognized as a clean and environmentally friendly energy carrier to replace traditional fossil fuels. The use of hydrogen for sustainable energy storage technology holds good prospects to address the growing demands for energy supply and simultaneously reduce the CO2 emission. Compared to compressed or liquefied hydrogen, the chemisorbed hydrogen in solids such as metal hydrides can offer increased hydrogen density. Light metal borohydrides have high theoretical gravimetric and volumetric hydrogen densities (e.g. 18.4 wt% and 121 kg/m3 for LiBH4, respectively) and
* Corresponding author. Tel.: +49-721-608-28908; fax: +49-721-608-26368. E-mail address: [email protected]
1876-6102 © 2012 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Canadian Hydrogen and Fuel Cell Association. doi:10.1016/j.egypro.2012.09.085
732
Zhirong Zhao-Karger et al. / Energy Procedia 29 (2012) 731 – 737
have attracted much attention as a potential hydrogen storage material.[1] However, unfavorable thermodynamics and/or kinetics hamper the utilization of metal borohydrides in practical hydrogen storage applications. One of the largest obstacles to use of metal borohydrides for reversible hydrogen storage is the formation of some stable closo polyborane intermediates like MB12H12, which may account for the high desorption temperature and the poor reversibility of these materials.[2] In addition, the evolution of diborane during the decomposition of some complex hydrides leads to capacity loss and is also a safety concern.[3] Generally, nanoscale materials may have significantly different properties as compared to the bulk materials owing to increased surface area of the solids, shorter diffusion distances and increased number of atoms at the grain boundary. An alternative approach of nanoconfinement is of increasing interest for fabrication of nano materials by constricting of the active material within a nanoporous scaffold.[4] In addition to the general effects on size reduction, nanoconfinement may also hinder the particle agglomeration and the phase segregation for the composite system through limiting the mobility of the particles within the scaffolds. Nanoconfined MgH2[5] and NaAlH4[6] have shown both improved kinetics and thermodynamics properties. Enhanced dehydrogenation rate and improved reversibility have also been achieved by infiltrating metal borohydrides into the mesoporous carbon scaffolds.[7] The present work is focus on the dehydrogenation reaction mechanism of the LiBH4–Mg(BH4)2 incorporated in a mesoporous carbon support to gain more insight into the influence of the particle restriction or dispersion at nano scale on altering the reaction pathways of the metal borohydrides. 1.1 The eutectic LiBH4–Mg(BH4)2 and its nanocomposite Based on the different electronegativity of the metals, introducing a foreign metal cation into the borohydride system might be an effective method for tuning the thermodynamics properties of the metal borohydrides. The binary system LiBH4–Mg(BH4)2 with an eutectic composition of 1 : 1 has been investigated,[8] which has a theoretical hydrogen capacity of 14.6 mass %. The thermal analysis of the LiBH4–Mg(BH4)2 system revealed the destabilization of the single constituent phases in the eutectic composition. The overall desorption kinetics of the LiBH4–Mg(BH4)2 system has been significantly enhanced compared to the individual components i.e. LiBH4 and Mg(BH4)2. The eutectic LiBH4-Mg(BH4)2 with molar ratio of 1:1 was prepared as previously described.[8] Porous carbon materials with pore width below 4 nm IRH33 was utilized as size controlling template. Because LiBH4-Mg(BH4)2 system has a comparably low eutectic melting point at 182 °C, the LiBH4Mg(BH4)2/IRH33 composite was prepared by a melt impregnation. 0.19 g of LiBH4-Mg(BH4)2 and 0.5 g of IRH33 carbon material were mixed with mortar and pestle, then heated in an autoclave type reactor at 190 °C under 40 bar of H2 for 1 hour. The LiBH4-Mg(BH4)2/IRH33 composite was characterized by means of X-ray diffraction and electron microscopy. The infiltrated LiBH4-Mg(BH4)2/IRH33 composite did not show any Bragg peaks (Fig1), which indicates that the borohydrides have been completely filled into the tiny pores of the carbon host and became amorphous or the crystallite size was too small for coherent scattering of the X-rays.
Zhirong Zhao-Karger et al. / Energy Procedia 29 (2012) 731 – 737
Fig.1. X-ray diffraction patterns
No crystalline borohydride particles were directly observed in the TEM images of the composite and the morphology of the composite resembles that of the activated carbon scaffold. The concentrations of Mg, B, C and O were mapped in an area of 120x120 nm2 by EDX spectra imaging processed by raw quantification as shown Fig. 2. Oxygen originated from oxidation due to the short exposure to ambient conditions during the sample preparation for TEM because the LiBH4-Mg(BH4)2 mixture is highly reactive to air. Nevertheless, the homogeneous distribution of the elements within the map indicates that metal borohydrides were homogeneously infiltrated and dispersed into the carbon scaffold.
Fig. 2. EDX spectra of LiBH4-Mg(BH4)2/IRH33
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1.2 Desorption reactions of LiBH4-Mg(BH4)2/carbon nanocomposite The hydrogen desorption of the LiBH4-Mg(BH4)2/IRH33 composite and the bulk binary LiBH4Mg(BH4)2 was studied by simultaneous thermogravimetric analysis, differential scanning calorimetry and mass spectrometry (TGA-DSC-MS) under the same experimental conditions. The thermogravimetric analysis of LiBH4-Mg(BH4)2 illustrates three distinguished hydrogen desorption steps. The DSC curve of LiBH4-Mg(BH4)2 exhibits four distinct peaks labelled as a, b, c, d in Fig. 3. The transformation steps a at 113 °C and b at 180 °C are assigned to the orthorhombic to hexagonal transformation of LiBH4 and the eutectic melting of the LiBH4-Mg(BH4)2 mixture, respectively. In contrast to the bulk sample, the weight loss of nanoconfined LiBH4-Mg(BH4)2 proceeded by only one step. As shown in Fig 4, there were no notable signal for structural phase transition and melting events in the DSC performance. Two endothermic peaks at 260 and 330°C marked as c’ and d’, respectively, likely correspond to the hydrogen releasing events c and d of the bulk material, but occurred at lower temperatures. These indicate the remarkable changes on the properties of the LiBH4-Mg(BH4)2 by constraining within the carbon scaffold.
Fig. 3. TGA-DSC-MS for LiBH4-Mg(BH4)2
Fig. 4. TGA-DSC-MS for LiBH4-Mg(BH4)2/IRH33 composite
Zhirong Zhao-Karger et al. / Energy Procedia 29 (2012) 731 – 737
Mass spectrometry has been used to simultaneously examine the hydrogen and diborane release upon decomposition of the nanoconfined and bulk sample. For the bulk LiBH4-Mg(BH4)2, four hydrogen releasing steps in the temperature range from 170 to 400 °C are visible in Fig. 3. Compared to the bulk material, the main hydrogen desorption of the nanoconfined LiBH4-Mg(BH4)2 occurred in the temperature range of 220 to 380 °C as shown in Fig 4. By normalizing the amount of active material to the same value for the carbon containing sample, the evolution of B2H6 was compared. As shown in Fig 5, the eutectic LiBH4-Mg(BH4)2 reached their highest B2H6 release rate at the eutectic melting point. In contrast, the infiltrated sample showed no detectable B2H6 trace.
Fig. 5. Mass spectra of diborane release of LiBH4-Mg(BH4)2 and the composite
To further elucidate the reaction paths, several samples at the interstages during their decomposition have been prepared by heating up the bulk and infiltrated materials under the initial H2 pressure of 0.2 bar to 280, 380 °C and holding for 1 hour, subsequently cooled down to ambient temperature, respectively. 11B MAS-NMR displays the changes in the composition of eutectic LiBH4-Mg(BH4)2 during desorption (Fig. 6). Spectrum b corresponds to the products of the first decomposition reaction at 280 °C, where the major peak at –40.8 ppm is assigned to the unreacted LiBH4-Mg(BH4)2 and the appearance of the peak –15.5 ppm indicates the formation of [B12H12]2- anions. At 380 °C, the material is in its third decomposition step according to the H2 release pattern. The concentration of [B12H12]2-anions is increased while the [BH4]groups are vanished and boron is formed in the meantime as shown in spectrum c. In case of the infiltrated sample, spectrum b’ in Fig 7 presents the decomposed products of the composite at 280 °C. The major broad signal centered at about 5 ppm was assigned to amorphous boron. The broad shoulder at about –19.4 ppm is assigned to [B12H12]2- species, which is shifted by about –4 ppm compared to the corresponding one of the bulk material probably due to the influence of the carbon surface. Additionally, the weak peak at –44 ppm implies that a small amount of borohydrides remained at this temperature. When increasing the temperature to 380°C, spectrum c’ shows only a single signal for boron species, which indicates a complete desorption for the composite.
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Fig. 6. 11B MAS-NMR spectra of samples of LiBH4-Mg(BH4)2 at different stages of dehydrogenation. a starting material LiBH4-Mg(BH4)2; b sample desorbed at 280 °C; c sample desorbed at 380°C. *Indicates spinning sidebands.
Fig. 7. 11B MAS-NMR spectra of samples of LiBH4-Mg(BH4)2/IRH33 at different stages of dehydrogenation a’ starting material LiBH4-Mg(BH4)2/IRH33 b’ sample desorbed at 280 °C; c’ sample desorbed at 380°C. *Indicates spinning sidebands
2. Conclusion The eutectic binary LiBH4-Mg(BH4)2 is highly dispersed and amorphized through melt infiltration within the mesoporous carbon support. The decomposition of LiBH4-Mg(BH4)2 confined in the carbon scaffold proceeds via different reaction paths from those of the bulk material. The reactions involving emission of toxic diborane have been found to be suppressed by nanoconfinement. The [B12H12]2- species are strongly destabilized by the influence of the carbon material. The altered reaction pathway and the destabilization of MB12H12 might be the origin of the enhanced desorption rate of nanoconfined LiBH4-Mg(BH4)2 system.
Zhirong Zhao-Karger et al. / Energy Procedia 29 (2012) 731 – 737
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