Mechanisms of partial hydrogen sorption reversibility in a 3NaBH₄/ScF₃ composite

A new hydrogen storage composite containing NaBH₄ and a 3d transition metal fluoride, 3NaBH₄/ScF₃, was synthesized via 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 NaBH₄ to form NaBH F_4- at the early stage, releasing about 0.19 wt% of hydrogen; (ii) formations of Na₃ScF₆, NaBF₄ and ScB₂ through the reaction between NaBH₄ and ScF₃, with 2.52 wt% of hydrogen release and a dehydriding activation energy of 162.67 kJ mol^-1 H₂; (iii) further reaction of residual NaBH₄ and Na₃ScF₆ to form NaF, B and ScB₂, with a dehydriding activation energy of 169.37 kJ mol^-1 H₂. 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^-1 H₂. These results demonstrate that by adding 3d transition metal fluorides into NaBH₄, a partial reversibility in NaBH₄ can be achieved.