Destabilization effect of transition metal fluorides on sodium borohydride
The effect of transition metal fluorides on the decomposition of NaBH4 has been investigated for NaBH4 ball milled with TiF3, MnF3 or FeF3. The compounds were examined by thermal programmed desorption with residual gas analysis, thermo gravimetric analysis and volumetric measurements using a Sieverts-type apparatus. The phase formation process during thermal decomposition was studied by in situ synchrotron radiation powder X-ray diffraction on the as-milled powders. NaBF4 was among the products in all mechano-chemical reactions. 11B-NMR spectra analysis gave NaBF4 : NaBH4 ratios of 1 : 150 for Na–Ti, 1 : 40 for Na–Mn, and 1 : 10 for Na–Fe. Pure NaBH4 possessed a hydrogen release onset temperature of 430 °C. The hydrogen release in the NaBH4–MnF3 system began as low as 130 °C. FeF3 decreased the onset temperature to 161 °C and TiF3 to 200 °C. TiF3 reacted completely with NaBH4 below 320 °C. All the examined systems have negligible emissions of diborane species. H-sorption studies performed at selected temperatures above 300 °C exhibited relatively fast desorption kinetics. Partial hydrogen re-absorption was observed for the Na–Mn and Na–Fe samples.