Recently we demonstrated the ability to cycle between Mg(BH4)2 and Mg(B3H8)2 under relatively mild conditions of temperature and hydrogen pressure. This was one of the first examples of reversible H2 storage at temperatures below 200 °C in complex metal hydrides. Subsequently a computational study was published that questioned the formation of Mg(B3H8)2 intermediate at these temperatures based on DFT calculations. The reaction enthalpy for the decomposition of Mg(BH4)2 to Mg(B3H8)2 was predicted to be higher, by an order of magnitude, than for MgB2 and MgB12H12. However, the distinctive 11B and 1H NMR spectrum of B3H8 anion in our experimental study was unambiguous. To address the apparent discrepancy between the experimental observations of Mg(B3H8)2 and the theoretical predictions we have revisited the thermodynamics calculations – extending the earlier studies to include vibrational and thermal corrections to calculate the free energy for Mg(B3H8)2 using supercell approach. Furthermore, we have measured the enthalpy for regeneration of Mg(BH4)2 from hydrogenation of Mg(B3H8)2 in the presence of excess magnesium hydride and followed the conversion in situ by solid state 11B NMR. Pacific Northwest National Laboratory, a multi-program national laboratory operated by Battelle for the U.S. Department of Energy.

Theory can’t be that wrong: Resolving the discrepancies to describe the reaction pathways leading to magnesium triborane

2019 Spring National Meeting

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