The Role of Ca(BH4)(2) Polymorphs

This study compares the structure and decomposition behaviors of the alpha, beta, and gamma 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 <1000 cm(-1) range shows different sharp librational bands for alpha- and gamma-, which are not seen in beta-Ca(BH4)(2). In the 1000-2700 cm(-1) range, all three polymorphs show the vibrational features of the C-2 local structure corresponding to the internal vibrations of BH4-. Shifts of these vibrational bands toward larger wavenumbers are observed for gamma and beta-Ca(BH4)(2). The increase in wavenumber coincides with an increase of the decomposition temperatures that can be up to 15 degrees C between alpha- and gamma-Ca(BH4)(2) depending on the polymorphic content. The decomposition temperature of pure beta-Ca(BH4)(2) is found to be about 6 degrees C lower than the decomposition of the high-temperature modification obtained via the polymorphic transformation of alpha-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 alpha-Ca(BH4)(2) at a heating rate of 10 degrees C/min. Calculated activation energies lead to 184 (14), 192 (3) and 230 (1) kJ/mol for gamma-, alpha- and beta-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 CaB(2)Hx and the borohydride borate Ca-3(BH4)(3)(BO3) seem to be unrelated to the nature of the polymorph.