Destabilization effect of transition metal fluorides on sodium borohydride

The effect of transition metal fluorides on the decomposition of NaBH 4 has been investigated for NaBH 4 ball milled with TiF 3 , MnF 3 or FeF 3 . 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. NaBF 4 was among the products in all mechano-chemical reactions. 11 B-NMR spectra analysis gave NaBF 4 : NaBH 4 ratios of 1 : 150 for Na-Ti, 1 : 40 for Na-Mn, and 1 : 10 for Na-Fe. Pure NaBH 4 possessed a hydrogen release onset temperature of 430 °C. The hydrogen release in the NaBH 4 -MnF 3 system began as low as 130 °C. FeF 3 decreased the onset temperature to 161 °C and TiF 3 to 200 °C. TiF 3 reacted completely with NaBH 4 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. Ball-milling sodium borohydride with transition metal fluorides significantly lowers the onset temperature of hydrogen release.