Hydrogen De-/absorption improvement of NaBH ₄ catalyzed by titanium-based additives

NaBH ₄ 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, TiH ₂, and TiF ₃, on the dehydrogenation and rehydrogenation of NaBH ₄ (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 NaBH ₄, and, among them, TiF ₃ possessed the highest catalytic activity. The whole dehydrogenation process for the NaBH ₄-0.05TiF ₃ sample can be regarded as a two-step process: (i) a preferential reaction (3NaBH ₄ + TiF ₃ → 3NaF + TiB ₂ + B + 6H ₂) occurring at around 300 °C, and (ii) the formation of Ti- and F-containing species catalyze the dehydrogenation of the remaining NaBH ₄. It was also indicated that the F anion can substitute for anionic H in NaH to form NaF _1-xH _x in the case of NaH-B-0.05TiF ₃ during the hydrogenation process. Therefore, the observed promotion effect of TiF ₃ on the reversible dehydrogenation of NaBH ₄ should be understood as arising from the combined effects of active Ti- and F-containing species. Also, FTIR spectroscopy confirmed the presence of amorphous Na ₂B ₁₂H ₁₂, in both the dehydrogenated and the rehydrogenated states, which may play a role in the partial dehydrogenation and reversibility observed in NaBH ₄ with and without catalyst doping.