The Role of Ca(BH4)2 Polymorphs

This study compares the structure and decomposition behaviors of the α, β, and γ polymorphs of Ca(BH4)2 for hydrogen storage. Samples with different polymorphic contents are characterized using powder X-ray diffraction and vibrational spectroscopy. Decomposition paths and formation of decomposition...

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Published in:Journal of physical chemistry. C 2012-06, Vol.116 (25), p.13472-13479
Main Authors: Llamas-Jansa, Isabel, Friedrichs, Oliver, Fichtner, Maximilian, Bardaji, Elisa Gil, Züttel, Andreas, Hauback, Bjørn C
Format: Article
Language:English
Subjects:
Condensed matter: structure, mechanical and thermal properties
Crystalline state (including molecular motions in solids)
Crystallographic aspects of phase transformations
pressure effects
Equations of state, phase equilibria, and phase transitions
Exact sciences and technology
Physics
Solubility, segregation, and mixing
phase separation
Structure of solids and liquids
crystallography
Structure of specific crystalline solids
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cited_by cdi_FETCH-LOGICAL-a204t-b0802dd4e53f9a80569e5c839f9956936119bd8aa7dd3e703a8e9d0d8cea7af63
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container_title Journal of physical chemistry. C
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creator Llamas-Jansa, Isabel
Friedrichs, Oliver
Fichtner, Maximilian
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Züttel, Andreas
Hauback, Bjørn C
description This study compares the structure and decomposition behaviors of the α, β, and γ polymorphs of Ca(BH4)2 for hydrogen storage. Samples with different polymorphic contents are characterized using powder X-ray diffraction and vibrational spectroscopy. Decomposition paths and formation of decomposition products are monitored by differential scanning calorimetry and temperature programed desorption as well as in situ synchrotron radiation powder diffraction. Vibrational spectroscopy in the
doi_str_mv 10.1021/jp211289s
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Samples with different polymorphic contents are characterized using powder X-ray diffraction and vibrational spectroscopy. Decomposition paths and formation of decomposition products are monitored by differential scanning calorimetry and temperature programed desorption as well as in situ synchrotron radiation powder diffraction. Vibrational spectroscopy in the &lt;1000 cm–1 range shows different sharp librational bands for α- and γ-, which are not seen in β-Ca(BH4)2. In the 1000–2700 cm–1 range, all three polymorphs show the vibrational features of the C2 local structure corresponding to the internal vibrations of BH4 –. Shifts of these vibrational bands toward larger wavenumbers are observed for γ and β-Ca(BH4)2. The increase in wavenumber coincides with an increase of the decomposition temperatures that can be up to 15 °C between α- and γ-Ca(BH4)2 depending on the polymorphic content. The decomposition temperature of pure β-Ca(BH4)2 is found to be about 6 °C lower than the decomposition of the high-temperature modification obtained via the polymorphic transformation of α-Ca(BH4)2. This confirms that the pure Ca(BH4)2 polymorphs have slightly different kinetic barriers and that the polymorphic content determines the decomposition kinetics of the samples. In addition, simultaneous thermogravimetric and differential scanning calorimetry analyses show increasing mass losses from approximately 7 to 10 mass% depending on the polymorph and the heating rate. The largest hydrogen release occurs for the purest α-Ca(BH4)2 at a heating rate of 10 °C/min. Calculated activation energies lead to 184 (14), 192 (3) and 230 (1) kJ/mol for γ-, α- and β-Ca(BH4)2 samples, respectively. This is in agreement with the observed decomposition behavior. The results illustrate the complexity of the decomposition of Ca(BH4)2 and how the polymorphic content and the formation of intermediates can affect or not affect the decomposition reaction pathways. In particular, the origins of CaB2Hx and the borohydride borate Ca3(BH4)3(BO3) seem to be unrelated to the nature of the polymorph.</description><identifier>ISSN: 1932-7447</identifier><identifier>EISSN: 1932-7455</identifier><identifier>DOI: 10.1021/jp211289s</identifier><language>eng</language><publisher>Columbus, OH: American Chemical Society</publisher><subject>Condensed matter: structure, mechanical and thermal properties ; Crystalline state (including molecular motions in solids) ; Crystallographic aspects of phase transformations; pressure effects ; Equations of state, phase equilibria, and phase transitions ; Exact sciences and technology ; Physics ; Solubility, segregation, and mixing; phase separation ; Structure of solids and liquids; crystallography ; Structure of specific crystalline solids</subject><ispartof>Journal of physical chemistry. 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The increase in wavenumber coincides with an increase of the decomposition temperatures that can be up to 15 °C between α- and γ-Ca(BH4)2 depending on the polymorphic content. The decomposition temperature of pure β-Ca(BH4)2 is found to be about 6 °C lower than the decomposition of the high-temperature modification obtained via the polymorphic transformation of α-Ca(BH4)2. This confirms that the pure Ca(BH4)2 polymorphs have slightly different kinetic barriers and that the polymorphic content determines the decomposition kinetics of the samples. In addition, simultaneous thermogravimetric and differential scanning calorimetry analyses show increasing mass losses from approximately 7 to 10 mass% depending on the polymorph and the heating rate. The largest hydrogen release occurs for the purest α-Ca(BH4)2 at a heating rate of 10 °C/min. Calculated activation energies lead to 184 (14), 192 (3) and 230 (1) kJ/mol for γ-, α- and β-Ca(BH4)2 samples, respectively. This is in agreement with the observed decomposition behavior. The results illustrate the complexity of the decomposition of Ca(BH4)2 and how the polymorphic content and the formation of intermediates can affect or not affect the decomposition reaction pathways. 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C</jtitle></facets><delivery><delcategory>Remote Search Resource</delcategory><fulltext>fulltext</fulltext></delivery><addata><au>Llamas-Jansa, Isabel</au><au>Friedrichs, Oliver</au><au>Fichtner, Maximilian</au><au>Bardaji, Elisa Gil</au><au>Züttel, Andreas</au><au>Hauback, Bjørn C</au><format>journal</format><genre>article</genre><ristype>JOUR</ristype><atitle>The Role of Ca(BH4)2 Polymorphs</atitle><jtitle>Journal of physical chemistry. C</jtitle><addtitle>J. Phys. Chem. C</addtitle><date>2012-06-28</date><risdate>2012</risdate><volume>116</volume><issue>25</issue><spage>13472</spage><epage>13479</epage><pages>13472-13479</pages><issn>1932-7447</issn><eissn>1932-7455</eissn><abstract>This study compares the structure and decomposition behaviors of the α, β, and γ polymorphs of Ca(BH4)2 for hydrogen storage. Samples with different polymorphic contents are characterized using powder X-ray diffraction and vibrational spectroscopy. Decomposition paths and formation of decomposition products are monitored by differential scanning calorimetry and temperature programed desorption as well as in situ synchrotron radiation powder diffraction. Vibrational spectroscopy in the &lt;1000 cm–1 range shows different sharp librational bands for α- and γ-, which are not seen in β-Ca(BH4)2. In the 1000–2700 cm–1 range, all three polymorphs show the vibrational features of the C2 local structure corresponding to the internal vibrations of BH4 –. Shifts of these vibrational bands toward larger wavenumbers are observed for γ and β-Ca(BH4)2. The increase in wavenumber coincides with an increase of the decomposition temperatures that can be up to 15 °C between α- and γ-Ca(BH4)2 depending on the polymorphic content. The decomposition temperature of pure β-Ca(BH4)2 is found to be about 6 °C lower than the decomposition of the high-temperature modification obtained via the polymorphic transformation of α-Ca(BH4)2. This confirms that the pure Ca(BH4)2 polymorphs have slightly different kinetic barriers and that the polymorphic content determines the decomposition kinetics of the samples. In addition, simultaneous thermogravimetric and differential scanning calorimetry analyses show increasing mass losses from approximately 7 to 10 mass% depending on the polymorph and the heating rate. The largest hydrogen release occurs for the purest α-Ca(BH4)2 at a heating rate of 10 °C/min. Calculated activation energies lead to 184 (14), 192 (3) and 230 (1) kJ/mol for γ-, α- and β-Ca(BH4)2 samples, respectively. This is in agreement with the observed decomposition behavior. The results illustrate the complexity of the decomposition of Ca(BH4)2 and how the polymorphic content and the formation of intermediates can affect or not affect the decomposition reaction pathways. In particular, the origins of CaB2Hx and the borohydride borate Ca3(BH4)3(BO3) seem to be unrelated to the nature of the polymorph.</abstract><cop>Columbus, OH</cop><pub>American Chemical Society</pub><doi>10.1021/jp211289s</doi><tpages>8</tpages></addata></record>
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subjects Condensed matter: structure, mechanical and thermal properties
Crystalline state (including molecular motions in solids)
Crystallographic aspects of phase transformations
pressure effects
Equations of state, phase equilibria, and phase transitions
Exact sciences and technology
Physics
Solubility, segregation, and mixing
phase separation
Structure of solids and liquids
crystallography
Structure of specific crystalline solids
title The Role of Ca(BH4)2 Polymorphs
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