Mechanisms of partial hydrogen sorption reversibility in a 3NaBH/ScF composite.

A new hydrogen storage composite containing NaBH and a 3d transition metal fluoride, 3NaBH/ScF, was synthesized ball milling. The composite shows no reaction during milling and its dehydriding process can be divided into three steps upon heating: (i) partial substitution of H by F in NaBH to form NaBH F at the early stage, releasing about 0.19 wt% of hydrogen; (ii) formations of NaScF, NaBF and ScB through the reaction between NaBH and ScF, with 2.52 wt% of hydrogen release and a dehydriding activation energy of 162.67 kJ mol H; (iii) further reaction of residual NaBH and NaScF to form NaF, B and ScB, with a dehydriding activation energy of 169.37 kJ mol H. The total hydrogen release of the composite reaches 5.54 wt% at 530 °C. The complete dehydrided composite cannot be rehydrogenated while the products after the second dehydriding step can be hydrogenated with an absorption activation energy of 44.58 kJ mol H. These results demonstrate that by adding 3d transition metal fluorides into NaBH, a partial reversibility in NaBH can be achieved.