Reversibility of the hydrogen desorption from NaBH4 by confinement in nanoporous carbon

Abstract

NaBH₄ is an interesting hydrogen storage material for mobile applications due to its high hydrogen content of 10.8 wt%. However, its practical use is hampered by the high temperatures (above 500 °C) required to release the hydrogen and by the non reversibility of the hydrogen sorption. In this study, we show that upon heating to 600 °C, bulk NaBH₄ decomposed into Na and Na₂B₁₂H₁₂, releasing the expected 8.1wt% of hydrogen. Nanosizing and confinement of NaBH₄ in porous carbon resulted in much faster hydrogen desorption kinetics. The onset of hydrogen release was reduced from 470 °C for the bulk to below 250 °C for the nanocomposites. Furthermore, the dehydrogenated nanocomposites were partially rehydrogenated to form NaBH₄, with the absorption of about 43% of the initial hydrogen capacity at relatively mild conditions (60 bar H₂ and 325 °C). Reversibility in this system was limited due to partial loss of Na during dehydrogenation. The dehydrogenated boron compounds were almost fully rehydrogenated to NaBH₄ (98%) when extra Na was added to the nanocomposites. To the best of our knowledge, this is the first time that reversibility for NaBH₄ has been demonstrated.