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

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 degrees 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 degrees 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 degrees C for the bulk to below 250 degrees 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 H-2 and 325 degrees 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.