Mechanisms of partial hydrogen sorption reversibility in a 3NaBH(4)/ScF(3) composite

A new hydrogen storage composite containing NaBH(4) and a 3d transition metal fluoride, 3NaBH(4)/ScF(3), 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 NaBH(4) to form NaBH(x)F(4−x) at the early stage, releasing about 0.19 wt% of hydrogen; (ii) formations of Na(3)ScF(6), NaBF(4) and ScB(2) through the reaction between NaBH(4) and ScF(3), with 2.52 wt% of hydrogen release and a dehydriding activation energy of 162.67 kJ mol(−1) H(2); (iii) further reaction of residual NaBH(4) and Na(3)ScF(6) to form NaF, B and ScB(2), with a dehydriding activation energy of 169.37 kJ mol(−1) H(2). 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) H(2). These results demonstrate that by adding 3d transition metal fluorides into NaBH(4), a partial reversibility in NaBH(4) can be achieved.