Abstract
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 NaBHxF4−x 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.
Highlights
The composite shows no reaction during milling and its dehydriding process can be divided into three steps upon heating: (i) partial substitution of HÀ by FÀ in NaBH4 to form NaBHxF4Àx 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 molÀ1 H2; (iii) further reaction of residual NaBH4 and Na3ScF6 to form NaF, B and ScB2, with a dehydriding activation energy of 169.37 kJ molÀ1 H2
These results demonstrate that by adding 3d transition metal fluorides into NaBH4, a partial reversibility in NaBH4 can be achieved
The 3NaBH4/ScF3 composite was prepared through mechanical milling
Summary
Hydrogen is one of the most promising alternative and attractive clean energy sources that can substitute fossil fuels, with sufficient energy density and environment-friendliness.[1,2] almost a century since the concept of “hydrogen economy” was introduced by Jules Velne,[3,4] it is still challenging to nd reliable, exible and cost-efficient hydrogen media for on-board, stationary and portable applications.[5,6] In the past few decades, great attention has been paid to both hydrogen production technologies and a variety of hydrogen storage methods,[7,8,9,10] including the use of different compounds,[11,12] especially complex hydrides, of which borohydrides are typical representative.[13,14,15] These solid-state hydrogen storage materials offer some advantages over high pressure gaseous storage and low temperature liquid storage, such as high capacity, high safety, and low cost. J. Urgnani et al investigated the thermal decomposition behaviors of NaBH4, and proposed that NaBH4 would decompose in two steps according to the following reactions:[22]. Paper facilitates hydrogen release and improves the reversibility to some extent.[29] the chemical reaction that regenerates borohydrides from metal–borides occurs much easier over the regeneration from boron since less energy is required for breaking the chemical bond between B–M (M means metal) relative to the B–B's.30. Many research works regarding hydrogen storage composite systems have been carried out, and some of them have explored how rare earth element (RE) addition can effect the thermodynamics and kinetics properties of metal borohydrides based systems, i.e., NaBH4–YF3,31 NaBH4–ScCl3,32 LiBH4–YCl3,33 etc. Hydrogen sorption reversibility was achieved in 3NaBH4/LnF3 systems with good thermodynamic and kinetic properties.[34]. We conducted a detailed study of hydrogen sorption behaviors of the 3NaBH4/ ScF3 system, and proposed mechanisms of hydrogen sorption in this composite, depending on experimental analyses
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