Fe3O4@graphene as a superior catalyst for hydrogen de/absorption from/in MgH2/Mg

Abstract

The present investigation describes the hydrogen sorption (de/absorption) behavior of MgH₂ catalyzed by graphene sheet templated Fe₃O₄ nanoparticles (Fe₃O₄@GS). Hydrogen sorption studies reveal that MgH₂ catalyzed by Fe₃O₄@GS (MgH₂:Fe₃O₄@GS) offers improved hydrogen storage behavior as compared to stand-alone MgH₂ catalyzed by graphene sheets (GS) (MgH₂:GS) or Fe₃O₄ nanoparticles (MgH₂:Fe₃O₄). The MgH₂:Fe₃O₄@GS has an onset desorption temperature of ∼262 °C (∼142 °C lower than pristine MgH₂), while MgH₂:GS and MgH₂:Fe₃O₄ have onset desorption temperatures of ∼275 °C and ∼298 °C respectively. In contrast to this, MgH₂:GS absorbs 4.40 wt% and MgH₂:Fe₃O₄ absorbs 5.50 wt% in 2.50 minutes at 290 °C under 15 atm hydrogen pressure. On the other hand, MgH₂:Fe₃O₄@GS absorbs 6.20 wt% hydrogen in 2.50 minutes (which is considerably higher than recently studied catalyzed MgH₂ systems) under identical temperature and pressure conditions. The MgH₂ catalyzed with Fe₃O₄@GS shows negligible degradation of the storage capacity even after 25 cycles. Additionally, the desorption activation energy for MgH₂:Fe₃O₄@GS has been found to be 90.53 kJ mol⁻¹ (which is considerably lower as compared to metal/metal oxide catalyzed MgH₂ and fluorographene catalyzed MgH₂). The formation enthalpy for MgH₂:Fe₃O₄@GS is 60.62 kJ per mole of H₂ (13.44 kJ mol⁻¹ lower than bulk MgH₂). The catalytic effect of Fe₃O₄@GS has been described and discussed with the help of structural (X-ray diffraction (XRD)), micro structural (electron microscopy) and Raman spectroscopic studies.