Melting Behavior and Thermolysis of NaBH4−Mg(BH4)2 and NaBH4−Ca(BH4)2 Composites

Morten Ley,Elsa Roedern,Torben Jensen,Peter Thygesen

doi:10.3390/en8042701

Abstract

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.

Highlights

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
Synchrotron radiation powder X-ray diffraction (SR-PXD), temperature programmed photographic analysis and thermal analysis combined with mass spectrometry
The composite 0.4NaBH4–0.6Mg(BH4)2 shows eutectic melting with Tm ~ 205 °C

Summary

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 Energies 2015, 8 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.The structural flexibility observed for metal borohydrides is highlighted by magnesium borohydride with seven structurally different polymorphs: α-, β-, β'-, γ-, ε-, δ- and ζ-Mg(BH4)2 [7,8,9,10,11,12]. 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 Energies 2015, 8 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. Calcium borohydride exists in several structural polymorphs, α/α'-, β- and γ-Ca(BH4)2 [15,16] Mg(BH4) is among the more promising materials for hydrogen storage applications with a high gravimetric hydrogen content of 14.9 wt% H2 and possible reformation from the decomposition products [13,14].

Results

Discussion

Conclusion