Study on the mechanism of co-catalyzing Mg–Al–Y hydrogen storage alloys with Co@C and multiple fluorides

Abstract

In this study, the different kinds of fluorides (NiF2, CrF2, and ZrF4) are combined with the carbonaceous catalyst Co@C to explore their co-catalytic effect on the Mg–Al–Y-based hydrogen storage material. The obtained composites are named as Mg91Al5Y4-5 wt.%TmFx (TmFx = NiF2, CrF2 and ZrF4)-5 wt.%Co@C. The isothermal and non-isothermal hydrogen storage properties of the composites were measured by Sievert apparatus and differential scanning calorimeter, respectively. X-ray diffraction (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM) were used for characterizing the phase component and observing microtopography. To make sense of the action mechanism of fluorides, the MgH2 supercell was established to study the occupation of F and Ni atoms and the first-principle pseudopotential plane wave method was utilized to calculate the heat of formation (ΔHsys) of these systems and the dissociation energy (ΔE) of single H atom. According to the results, the Mg91Al5Y4-5 wt.%NiF2-5 wt.%Co@C specimen has the minimal enthalpy change of hydrogen evolution (75.7 kJ/mol H2) and activation energy of dehydrogenation (81.4 ± 1.09 kJ/mol), showing the optimal hydrogen storage performances. The calculation results are consistent with the experiments that the introduction of Ni leads to the decline in ΔHsys and ΔE, suggesting the destabilization of MgH2 and the enhancement of dehydrogenation. The F atom occupying H or the gap position will lead to the increase of ΔHsys and induce the system to be more stable.