X-ray and Synchrotron FTIR Studies of Partially Decomposed Magnesium Borohydride

Abstract

Magnesium borohydride (Mg(BH4)2) is an attractive compound for solid-state hydrogen storage due to its lucratively high hydrogen densities and theoretically low operational temperature. Hydrogen release from Mg(BH4)2 occurs through several steps. The reaction intermediates formed at these steps have been extensively studied for a decade. In this work, we apply spectroscopic methods that have rarely been used in such studies to provide alternative insights into the nature of the reaction intermediates. The commercially obtained sample was decomposed in argon flow during thermogravimetric analysis combined with differential scanning calorimetry (TGA-DSC) to differentiate between the H2-desorption reaction steps. The reaction products were analyzed by powder X-ray diffraction (PXRD), near edge soft X-ray absorption spectroscopy at boron K-edge (NEXAFS), and synchrotron infrared (IR) spectroscopy in mid- and far-IR ranges (SR-FTIR). Up to 12 wt% of H2 desorption was observed in the gravimetric measurements. PXRD showed no crystalline decomposition products when heated at 260–280 °C, the formation of MgH2 above 300 °C, and Mg above 320 °C. The qualitative analysis of the NEXAFS data showed the presence of boron in lower oxidation states than in (BH4)−. The NEXAFS data also indicated the presence of amorphous boron at and above 340 °C. This study provides additional insights into the decomposition reaction of Mg(BH4)2.