Hydrogen–fluorine exchange in NaBH4–NaBF4

Abstract

a Hydrogen-fluorine exchange in the NaBH 4 -NaBF 4 system is investigated using a range of experimental methods combined with DFT calculations and a possible mechanism for the reactions is proposed. Fluorine substitution is observed using in situ synchrotron radiation powder X-ray diffraction (SR-PXD) as a new Rock salt type compound with idealized composition NaBF 2 H 2 in the temperature range T = 200 to 215 1C. Combined use of solid-state 19 F MAS NMR, FT-IR and DFT calculations supports the formation of a BF 2 H 2 complex ion, reproducing the observation of a 19 F chemical shift at 144.2 ppm, which is different from that of NaBF 4 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 B310 1C. This work shows that fluorine-substituted borohydrides tend to decompose to more stable compounds, e.g. NaF and BF 3 or amorphous products such as closo-boranes, e.g. Na 2 B 12 H 12 . The NaBH 4 -NaBF 4 composite decomposes at lower temperatures (300 1C) compared to NaBH 4 (476 1C), as observed by thermogravimetric analysis. NaBH 4 -NaBF 4 (1 : 0.5) preserves 30% of the hydrogen storage capacity after three hydrogen release and uptake cycles compared to 8% for NaBH 4 as measured using 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 Na 2 B 12 H 12 . 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 NaBH 4 -NaF.

Complex and liquid hydrides for energy storage

“…The hydrogen release from KBH 2.8 F 1.2 takes place 433°C lower than it does in pure KBH 4 [63]. However, some of the obtained materials, specifically NaB(H 1-x F x ) 4 , appear to have relatively low thermal stability, and the released hydrogen may be contaminated with significant amounts of boron fluorides and/or borane gases and may lead to the formation of significant amounts of M 2 B 12 H 12 [64][65][66][67].…”

Recent Advances in the Use of Sodium Borohydride as a Solid State Hydrogen Store

“…For example, a recent study on the NaBH4-NaBF4 system showed that hydrogen-fluorine exchange took place in a temperature range of 200-215 °C, leading to a new rock salt-type compound with idealized composition NaBF2H2 [58]. After further heating, the fluorine substituted compound becomes X-ray amorphous and decomposes to NaF at 310 °C.…”

Theoretical study of B12HnF(12−n)2−Sharma1, Sethio2, D’Anna3 et al. 2015International Journal of Hydrogen Energy17070View full textAdd to dashboardCite

“…The synthesis of pure salts with B 12 F 2À 12 ions has been reported recently [25e27]. We have reported previously that isolated BH n F À ð4ÀnÞ (n ¼ 0, 4) species can be formed from mixtures of NaBH 4 and NaBF 4 [13]. Heating these mixtures to 300 C led to the formation of Na 2 B 12 H 12 [12].…”