Towards the endothermic dehydrogenation of nanoconfined magnesium borohydride ammoniate

Abstract

The utilization of metal borohydride ammoniates as practical hydrogen storage materials is hindered by their unfavorable exothermic dehydrogenation thermodynamics. Here, we report a first successful attempt to tailor the dehydrogenation thermodynamics of magnesium borohydride hexaammoniate (Mg(BH₄)₂·6NH₃) through nanoconfinement into microporous activated carbon (AC). The onset temperature for hydrogen release from the nanoconfined Mg(BH₄)₂·6NH₃ is dramatically decreased to approximately 40 °C, and more encouragingly, hydrogen desorption is endothermic in nature. The relationship between pore size and dehydrogenation behavior is established, and the critical pore size for the endothermic dehydrogenation of the nanoconfined Mg(BH₄)₂·6NH₃ is found to be less than 4 nm. The nanoconfinement effect of carbon scaffolds is believed to be the primary reason for the change in the dehydrogenation pathway caused by incorporating Mg(BH₄)₂·6NH₃ into microporous activated carbon. This finding opens up the possibility to achieve reversible hydrogen storage in metal borohydride ammoniates.