Destabilization effect of transition metal fluorides on sodium borohydride

Abstract

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.