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.