3 Results
3.4 Effect of the CaF 2 on the sorption properties of Ca(BH 4 ) 2
3.5.7 Is the hydrogen back pressure influencing the decomposition path of the
In order to obtain a more general understanding of the decomposition paths of the Ca(BH4)2 + MgH2 composite system, several experiments, at different temperatures and pressures, were performed. The decomposition reaction was studied at 400 °C (in vacuum and 1 bar H2
pressure) and at 350 °C (in vacuum, 1 bar and 5 bar H2 pressure).
Thermodynamic calculations (thermodynamic database and first principle calculations) performed by Kim et al.[48] at 350 °C predicts that under 1 bar of hydrogen pressure the (de)hydrogenation of Ca(BH4)2 + MgH2 composite, beside CaH2, would lead to the formation of MgB2 whereas vacuum would promote the formation of CaB6 (and Mg).
Figure 3.52 reports the volumetric analysis of the first hydrogen desorption reaction for the Ca(BH4)2 + MgH2 composite system at 400 °C both in vacuum and at 1 bar H2 pressure.
Figure 3.52. Volumetric measurements showing the desorption curves over the temperature.
Desorption curve of the Ca(BH4)2 + MgH2 composite in vacuum (black) and at 1 bar H2
pressure (blue). The experiments were carried out by heating the samples from room temperature up to 400 °C.
The Figure shows that both samples start desorbing hydrogen at ca. 350 °C. The overall hydrogen capacity (ca. 7 wt. %) is released in 3 hours. The amount of hydrogen delivered in this case is slightly higher compared to that observed in the volumetric curves in Figure 3.38 and 3.45. This is likely related to the absence of side products (e.g. MgO) in the decomposition materials. Figure 3.52 shows that 1 bar hydrogen back pressure does not change the decomposition path of the pure Ca(BH4)2 + MgH2 composite system respect to the vacuum atmosphere. The curves coincide.
XRD analysis on the decomposition products, formed in vacuum and in 1 bar H2 pressure, are reported in Figure 3.53 for the Ca(BH4)2 + MgH2 composite system.
Figure 3.53. XRD spectra of the (de)hydrogenated powders in vacuum and 1 bar H2 pressure at 400 °C. CaH2 (); Mg (); ? (artefact ). The measurements were performed at the Institute for Metallic Materials at the Leibniz Institute for Solid State and Materials Research (Dresden).
Figure 3.53 evidences the Bragg peaks of both the Mg and CaH2 phase. No signals belonging to MgO are visible. This would justify the higher amount of hydrogen delivered during the desorption reaction reported in Figure 3.52 compared to that reported in Figure 3.38.
Furthermore, no reflections belonging to any boron-phase are detectable (e.g. CaB6, CaB12H12
and MgB2). It was already reported that CaB6 cannot be observed in XRD patterns due to its small dimensions. The materials, desorbed at 350 °C in vacuum and at 1 bar H2 pressure, are reported in Figure 3.54.
Figure 3.54. XRD spectra of the (de)hydrogenated powders at 350 °C in vacuum and 1 bar H2
pressure. CaH2 (); Mg (); MgO (). The measurement was performed at the synchrotron Hasylab, DESY (Hamburg), at the beamline D3.
The Bragg peaks shown in Figure 3.54 correspond to the CaH2, Mg and MgO phase. The halo in the scattering vector value range of 1.5-2.5 (Å-1) belongs to the most intense CaB6 peak known to be in nanocrystalline or amorphous-like status. No trace of MgB2 phase is visible.
Due to the fact that, so far, 1 bar hydrogen back pressure did not show any influence on the desorption reaction path of the Ca(BH4)2 + MgH2 composite system, another experiment at 350 °C with 5 bar H2 back pressure was performed. For comparison purposes, the volumetric curve at 5 bar H2 is reported in Figure 3.55 together with that measured in vacuum.
Figure 3.55 shows the slower desorption kinetics for the sample at 5 bar H2 pressure compared to the material measured in vacuum. Although the desorption reaction starts for both samples around 350 °C, with 5 bar H2 pressure, almost 8 hours are necessary to desorb 4.8 wt. % hydrogen. This value represents the 53 % of the theoretical capacity (9.1 wt. % H2) contained within the Ca(BH4)2 + MgH2 composite system. X-ray diffraction on the desorbed materials (shown later) will clarify the reasons for such a lower amount of hydrogen delivered. Instead, the Ca(BH4)2 + MgH2 composite, at 350 °C and in vacuum, desorbs ca. 7 wt. % of hydrogen in 3.5 hours.
Figure 3.55. Volumetric measurements showing the desorption curves over the time.
Temperature (red curve). Desorption curve of the Ca(BH4)2 + MgH2 composite in vacuum (black) and at 5 bar H2 pressure (green). The experiments were carried out by heating the samples from room temperature up to 350 °C.
XRD analysis on the Ca(BH4)2 + MgH2 composite system desorbed at 350 °C and 5 bar H2
pressure is reported in Figure 3.56. For comparison purposes, the XRD pattern of the material desorbed in vacuum is reported as well.
The XRD spectrum of the material desorbed in 5 bar H2 pressure shows the Bragg peaks of the Ca4Mg3H14 and of the MgH2 phase. Presence of the ternary Ca-Mg-H phase is explained by the reaction between CaH2 (formed upon (de)hydrogenation of Ca(BH4)2) and MgH2
phase. MgH2 does not desorb at all in these experimental conditions. Presence of CaB6 cannot be observed.
Figure 3.56. XRD pattern of the (de)hydrogenated powders at 350 °C in vacuum and 5 bar H2
pressure. CaH2 (); Mg (); MgO (); Ca4Mg3H14 (); MgH2 (); ? (artefact). The measurements were performed at the Institute for Metallic Materials at the Leibniz Institute for Solid State and Materials Research (Dresden) (spectrum at 5 bar H2) and at the synchrotron MAX-lab, Lund (Sweden) at the beamline I711 (spectrum in vacuum).
The amount of hydrogen released during the desorption reaction at 5 bar H2 pressure (4.8 wt.
%) corresponds to ca. 2.4 mol H2. Therefore the (de)hydrogenation path can be approximately represented by the following reaction scheme:
(3) Ca(BH4)2 + x MgH2 → y Ca4Mg3H14 + z MgH2 + w CaB6 + 2.4 H2
Reaction 3 does not include the CaB12H12 phase that might have formed upon (de)hydrogenation. 11B{1H} MAS-NMR analysis were not performed on these samples therefore we cannot confirm or exclude its presence among the decomposition products.
However, formation of CaB12H12 upon (de)hydrogenation was already observed in the case of the Ca(BH4)2 + MgH2 composite desorbed in vacuum conditions.