Hydrogen De-/absorption improvement of NaBH4 catalyzed by titanium-based additives

NaBH4 is considered as a promising candidate material for solid-state hydrogen storage due to its high hydrogen content of 10.6 wt %. However, its practical use is hampered by its high thermodynamic stability and slow H-exchange kinetics. In the present work, the effects of Ti-based additives, including Ti, TiH2, and TiF3, on the dehydrogenation and rehydrogenation of NaBH4 (NaH+B) were investigated. It was revealed that all of the titanium-based additives were effective in improving the hydrogen desorption and absorption reactions of NaBH4, and, among them, TiF3 possessed the highest catalytic activity. The whole dehydrogenation process for the NaBH4–0.05TiF3 sample can be regarded as a two-step process: (i) a preferential reaction (3NaBH4 + TiF3 → 3NaF + TiB2 + B + 6H2) occurring at around 300 °C, and (ii) the formation of Ti- and F-containing species catalyze the dehydrogenation of the remaining NaBH4. It was also indicated that the F anion can substitute for anionic H in NaH to form NaF1–xHx in the case of NaH–B–0.05TiF3 during the hydrogenation process. Therefore, the observed promotion effect of TiF3 on the reversible dehydrogenation of NaBH4 should be understood as arising from the combined effects of active Ti- and F-containing species. Also, FTIR spectroscopy confirmed the presence of amorphous Na2B12H12, in both the dehydrogenated and the rehydrogenated states, which may play a role in the partial dehydrogenation and reversibility observed in NaBH4 with and without catalyst doping.