Recent research results :

Hydrogen-fluorine exchange in the NaBH₄–NaBF₄ system is investigated with a range of experimental methods combined with DFT calculations and a possible mechanism for the reactions is proposed. Fluorine substitution is observed by in-situ synchrotron radiation powder X-ray diffraction (SR-PXD) as a new Rock salt type compound with idealized composition NaBF₂H₂ in the temperature range T = 200 to 215 °C. Combined use of solid-state ¹⁹F MAS NMR, FT-IR and DFT calculations supports the formation of a BF₂H₂⁻ complex ion, reproducing the observation of a ¹⁹F chemical shift at 144.2 ppm, which is different from that of NaBF₄ at 159.2 ppm, along with the new absorption bands observed in the IR spectra. After further heating, the fluorine substituted compound becomes X-ray amorphous and decomposes to NaF at ~310 ºC. This work shows that fluorine-substituted borohydrides tend to decompose to more stable compounds, e.g. NaF, BF₃ or amorphous products such as closo-boranes, e.g. Na₂B₁₂H₁₂. The NaBH₄-NaBF₄ composite decomposes at lower temperatures (300 °C) compared to NaBH₄ (476 °C), as observed by thermogravimetric analysis. NaBH₄-NaBF₄ (1:0.5) preserves 30 % of the hydrogen storage capacity after three hydrogen release and uptake cycles compared to 8 % for NaBH₄ measured by the Sievert’s method under identical conditions, but more than 50 % using prolonged hydrogen absorption time. The reversible hydrogen storage capacity tends to decrease possibly due to the formation of NaF and Na₂B₁₂H₁₂. On the other hand, the additive sodium fluoride appears to facilitate hydrogen uptake, prevent foaming, phase segregation and loss of material from the sample container for samples of NaBH₄-NaF.