In situ characterization of the decomposition behavior of Mg(BH4)2 by X-ray Raman scattering spectroscopy

Abstract

We present an in situ study of the thermal decomposition of Mg(BH₄)₂ in a hydrogen atmosphere of up to 4 bar and up to 500 °C using X-ray Raman scattering spectroscopy at the boron K-edge and the magnesium L_2,3-edges. The combination of the fingerprinting analysis of both edges yields detailed quantitative information on the reaction products during decomposition, an issue of crucial importance in determining whether Mg(BH₄)₂ can be used as a next-generation hydrogen storage material. This work reveals the formation of reaction intermediate(s) at 300 °C, accompanied by a significant hydrogen release without the occurrence of stable boron compounds such as amorphous boron or MgB₁₂H₁₂. At temperatures between 300 °C and 400 °C, further hydrogen release proceeds via the formation of higher boranes and crystalline MgH₂. Above 400 °C, decomposition into the constituting elements takes place. Therefore, at moderate temperatures, Mg(BH₄)₂ is shown to be a promising high-density hydrogen storage material with great potential for reversible energy storage applications.