Melting Behavior and Thermolysis of Nabh 4 −mg(bh 4 ) 2 and Nabh 4 −ca(bh 4 ) 2 Composites

The physical properties and the hydrogen release of NaBH4–Mg(BH4)2 and NaBH4−Ca(BH4)2 composites are investigated using in situ synchrotron radiation powder X-ray diffraction, thermal analysis and temperature programmed photographic analysis. The composite, xNaBH4–(1 − x)Mg(BH4)2, x = 0.4 to 0.5, shows melting/frothing between 205 and 220 °C. However, the sample does not become a transparent molten phase. This behavior is similar to other alkali-alkaline earth metal borohydride composites. In the xNaBH4–(1 − x)Ca(BH4)2 system, eutectic melting is not observed. Interestingly, eutectic melting in metal borohydrides systems leads to partial thermolysis and hydrogen release at lower temperatures and the control of sample melting may open new routes for obtaining high-capacity hydrogen storage materials.


Introduction
In order to create a new sustainable energy economy, the storage of renewable energy is essential, e.g., directly as electricity in a battery or indirectly as hydrogen in a solid state metal hydride [1][2][3][4].Metal borohydrides can store considerable amounts of energy as hydrogen in the solid state, but tend to OPEN ACCESS exhibit poor thermodynamic and kinetic properties, which hamper their technological utilization [5,6].In order to improve the properties for reversible solid-state hydrogen storage, continued research within energy storage materials science is required.

In Situ Time-Resolved Synchrotron Radiation Powder X-ray Diffraction
Synchrotron radiation powder X-ray diffraction (SR-PXD) data were collected at beamline I711 at the synchrotron MAX-II in the MAX IV laboratory Lund, Sweden, with a MAR165 CCD detector system, X-ray exposure time of 30 s and selected wavelengths of 0.999991 or 1.00355 Å [34,35].The powdered sample was mounted in a sapphire (Al2O3) single-crystal tube (o.d.1.09 mm, inner diameter (i.d.) 0.79 mm) in an argon-filled glove box p(O2, H2O) < 1 ppm.The temperature was controlled with a thermocouple placed in the sapphire tube 1 mm from the sample.All obtained raw images were calibrated against a standard NIST LaB6 sample and transformed to 2D-powder patterns using the FIT2D program [36].

Thermal Analysis
All samples, including the reactants Mg(BH4)2 and Ca(BH4)2, were studied by simultaneous thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) using a PerkinElmer STA 6000 apparatus.Additionally, the xNaBH4-(1 − x)Mg(BH4)2 samples were studied by mass spectrometry (MS) using a Hiden Analytical HPR-20 QMS sampling system.The samples (approximately 3 mg) were placed in an Al crucible and heated (5 °C/min) in an argon flow of 20 mL/min.The samples exhibit vigorous frothing above 400 °C, preventing further heating of the samples during the thermal analysis experiment.

Temperature Programmed Photographic Analysis
Temperature programmed photographic analysis (TPPA) was performed using a previously described setup [28].Photographs were collected using a digital camera whilst heating the samples from RT to 400 °C (ΔT/Δt = 5 °C/min).The samples (approximately 15 mg) were sealed under argon in a glass vial connected to an argon-filled balloon to maintain an inert atmosphere.A thermocouple was in contact with the sample within the glass vial to monitor the temperature during thermolysis.The glass vial was encased within an aluminum block with open viewing windows for photography, to provide near-uniform heating by rod heaters, interfaced with a temperature controller.

Differential Scanning Calorimetry
Analysis of the melting in the samples is performed using DSC and TPPA from room temperature (RT) to 400 °C.The DSC data of the as-synthesized Mg(BH4)2 (x = 0) reveal a single event with an onset temperature of 197 °C; see Figure 1.NaBH4 is not affected by thermodynamic events below 400 °C [37].The DSC data of the xNaBH4-(1 − x)Mg(BH4)2 composites reveal two endothermic peaks with varying intensity and onset temperatures of 178 and 205 °C; see Figures 1 and S1.The first event at 178 °C is most noticeably observed in the samples of xNaBH4-(1 − x)Mg(BH4)2, x = 0.1 to 0.6; see Figures 1 and S2   The integrated area of the DSC peaks is proportional to the enthalpy change of the thermal events.The heat of reaction (dH) for the different sample compositions is extracted for the thermal events at 178 and 205 °C and shown in Figures S2 and 2, respectively.The integrated area of the peak at 205 °C is largest for the samples 0.4NaBH4-0.6Mg(BH4)2and 0.5NaBH4-0.5Mg(BH4)2.

Thermogravimetric and Mass Spectrometry Analysis
The composites of NaBH4 and Mg(BH4)2 are all destabilized and release hydrogen at lower temperatures as compared to Mg(BH4)2.The largest effect is observed for samples with x = 0.4-0.6;see Figure 4.In the samples with a majority of NaBH4, x = 0.8-0.9, the amount of released hydrogen until 400 °C is minor, since NaBH4 decomposes above 500 °C.The MS data show significant changes in the hydrogen release profile of the composites compared to Mg(BH4)2, in particular at T < 300 °C.The MS data for xNaBH4-(1 − x)Mg(BH4)2, x = 0.4 and 0.6, show two hydrogen release reactions beginning at ~180 and 230 °C, which are not observed for Mg(BH4)2, where only a slight increase in the hydrogen signal is observed at 200 °C.The TGA registers a beginning mass loss at T ~ 240 °C for the composites and T ~ 280 °C for Mg(BH4)2.A total mass loss of 9 wt% is recorded by TGA for Mg(BH4)2 below 380 °C, which suggest partial thermolysis, ρm(Mg(BH4)2) = 14.9 wt% H2.Mass losses of 7 and 5 wt% are observed for 0.4NaBH4-0.6Mg(BH4)2and 0.6NaBH4-0.4Mg(BH4)2between 200 and 400 °C, which also suggests partial thermolysis.The theoretical hydrogen content for these samples is 14.0 and 12.7 wt% H2.

Decomposition Mechanisms Observed by in Situ SR-PXD
The in situ SR-PXD data for the decomposition of 0.665NaBH4-0.335Mg(BH4)2is shown in Figure S6.Normalized diffracted intensities of selected Bragg peaks of the compounds are extracted as a function of temperature; see Figure 5.The diffraction pattern measured at RT has a broad hump in the background in the range 9 < 2θ < 13°, originating from amorphous Mg(BH4)2 [11,38]  At T ~ 400 °C, Bragg peaks from MgH2 are observed, and the diffracted intensity from NaBH4 increases again.MgH2 decomposes at T ~ 465 °C, followed by the formation of Mg metal.At T ~ 525 °C, the Bragg peak at 2θ = 27.5°previously assigned to MgO shows a small shift to lower 2θ values, possibly due to an increased formation of MgB2, which overlaps in peak position with MgO.At T ~ 565 °C, Bragg peaks from NaBH4 vanish, while diffraction peaks from Mg metal, MgO and MgB2 remain until T = 600 °C.

Discussion of the xNaBH4-(1 − x)Mg(BH4)2 Composite
Twelve samples of NaBH4 and Mg(BH4)2 with varying compositions have been studied.An endothermic DSC event with an onset temperature of 178 °C was observed in all of the samples of xNaBH4-(1 − x)Mg(BH4)2.This thermal event likely corresponds to the polymorphic transition of α-to β-Mg(BH4)2.Additionally, the area of the peak is larger for samples with higher Mg(BH4)2 content; see Figures 1 and S2.The polymorphic transition is also observed by in situ SR-PXD at 180 °C; see Figure 5.The transition temperature may be affected by the addition of NaBH4, as the event occurs at a lower temperature compared to the pristine sample; see Figure 1.A similar effect is observed in the LiBH4-LiCl system, where the onset temperature for the orthorhombic to hexagonal transition for LiBH4 is lowered by the addition of LiCl [39].
Consequently, the xNaBH4-(1 − x)Mg(BH4)2, x = 0.4-0.5, system is proposed to be a eutectic melting system with Tm ~ 205 °C.This is the first eutectic system within mixtures of alkali and alkaline metal borohydrides, which may have an excess of the alkaline earth metal borohydride.However, the melting point of Mg(BH4)2 is lower than that of NaBH4 [11,28].The lower melting metal borohydride also makes up the larger part in other eutectic metal borohydride systems [28,30].Like in other eutectic alkali-alkaline earth metal borohydride systems, the molten phase is not a transparent liquid.This may be due to partial thermolysis and gas release of the alkaline earth metal borohydrides during melting/frothing [28].
Weak Bragg peaks assigned to Compound 1 were observed during the in situ SR-PXD experiment after the disappearance of β-Mg(BH4)2.Compound 1 does not show similarities to any of the recently discovered Mg(BH4)2 polymorphs [8,9,12,38].The compound appears to crystallize from the molten phase, possibly due to excess sodium borohydride.A minor increase in the diffracted intensity from NaBH4 occurs during the decomposition of 1, as well as the formation of MgO.Compound 1 may be analogous to compounds in the LiBH4-Ca(BH4)2 system, i.e., Ca3(BH4)3(BO3) and LiCa3(BH4)(BO3)2 [40,41].In situ SR-PXD shows that MgH2 forms and decomposes at T > 465 °C.MgH2 should form after the decomposition of Mg(BH4)2 at T > 300 °C.However, the melting/frothing in the system may make it difficult to observe Bragg peaks from MgH2 before T > 465 °C.
The composites of xNaBH4-(1 − x)Mg(BH4)2 are destabilized, and hydrogen release occurs at lower temperatures, as compared to Mg(BH4)2; and a larger amount of hydrogen is released in between 180 and 300 °C.However, larger amounts of NaBH4 decrease the total amount of released hydrogen in the temperature range of RT to 400 °C.The MS data for hydrogen release from 0.4NaBH4-0.6Mg(BH4)2and 0.6NaBH4-0.4Mg(BH4)2looks very similar to 0.55LiBH4-0.45Mg(BH4)2[28,31].The decomposition mechanism might be similar for the systems, as both melt/froth and contain Mg(BH4)2.However, because of the melting/frothing in the samples, the detailed decomposition mechanism is difficult to establish by in situ SR-PXD.Nanoconfinement has been used to improve the kinetics of other eutectic metal borohydride systems and may also improve the xNaBH4-(1 − x)Mg(BH4)2 system, whereby more hydrogen can be collected at the eutectic melting point [33,[42][43][44].Increased amounts of hydrogen could also be harvested by adding a catalyst [45].
The mass loss observed from the samples containing NaBH4 and Ca(BH4)2 remains below the theoretical content of hydrogen in all samples; see Figure S5.The hydrogen release is lower, since NaBH4 only decomposes above ~500 °C.Therefore, the behavior of the composite xNaBH4-(1 − x)Ca(BH4)2 during thermolysis appears to resemble that of the individual compounds with the exception of the formation of Compound 2. Furthermore, the mixing of NaBH4 and Ca(BH4)2 does not lead to destabilization and hydrogen release at a lower temperatures, as observed in the xNaBH4-(1 − x)Mg(BH4)2 composite.
. The porous structure of γ-Mg(BH4)2 may have collapsed during the ball-milling, as the characteristic Bragg peaks from γ-Mg(BH4)2 are not observed at RT in the in situ SR-PXD experiment.Bragg peaks from NaBH4 are observed at RT, indicating that NaBH4 and Mg(BH4)2 do not react during ball-milling.At T ~ 110 °C, α-Mg(BH4)2 crystallizes, and at T ~ 180 °C the polymorphic phase change from the α-to β-Mg(BH4)2 occurs.