Simultaneous improvement of kinetics and thermodynamics based on SrF2 and SrF2@Gr additives on hydrogen sorption in MgH2

Abstract

Herein we describe and discuss the effect of significant advantages of the alkaline earth fluoride additive SrF₂ on the improvement of the kinetics, thermodynamics, and cyclability of the frontier hydrogen storage material MgH₂. Strontium fluoride, SrF₂, which has been used as an additive for the first time in the present study, has an elemental electronegativity difference of 3.04 (the highest amongst the fluorides) as compared to 2.38 for NbF₅, one of the best-known catalysts so far for MgH₂. Therefore, SrF₂ is highly ionic and will readily react with MgH₂, which is not only ionic but also has a polar covalent character. SrF₂ reacts with MgH₂ to yield magnesium fluoride (MgF₂) and strontium hydride (SrH₂) which act as catalysts. The present investigations have revealed that both (MgF₂ + SrH₂) and their graphene templated version, (MgF₂ + SrH₂)@Gr, work as better catalysts for MgH₂ than NbF₅. Thus the desorption of H₂ from ball-milled MgH₂ catalyzed by (MgF₂ + SrH₂) corresponds to 5.30 wt% in 15 min, and for (MgF₂ + SrH₂)@Gr the desorption is 6.01 wt% in 15 min at 290 °C. Also, the onset desorption temperatures for MgH₂ with (MgF₂ + SrH₂) and (MgF₂ + SrH₂)@Gr catalysts are 261 °C and 231 °C, respectively, which are 95 °C and 125 °C lower than that for ball-milled MgH₂. The H₂ absorption for (MgF₂ + SrH₂) catalyzed MgH₂ is found to be 6.00 wt% in 5 min at 290 °C. For (MgF₂ + SrH₂)@Gr catalyzed MgH₂ the hydrogen absorption is 6.16 wt% in 2 min at 290 °C. The change in desorption enthalpy for MgH₂–(MgF₂ + SrH₂)@Gr is 67.60 kJ mol⁻¹ as compared to 74.84 kJ mol⁻¹ for MgH₂–(MgF₂ + SrH₂). The storage capacity for MgH₂–(MgF₂ + SrH₂)@Gr remains ∼6.00 wt% even after 15 cycles, which corresponds to excellent cyclability. A feasible catalytic mechanism arising from (MgF₂ + SrH₂) and (MgF₂ + SrH₂)@Gr catalysts on hydrogen sorption in MgH₂ has been proposed based on X-ray diffraction, Raman spectroscopy, Fourier transmission infrared spectroscopy, and transmission/scanning electron microscopic studies. The present study is the first of its type where absorption/desorption kinetics, thermodynamics, and cyclability for MgH₂ have all been improved by the use of the single additive SrF₂ and the derived catalyst MgF₂ + SrH₂.