Phase evolution,hydrogen storage thermodynamics,and kinetics of ternary Mg_(98)Ho_(1.5)Fe_(0.5) alloy

Rare earth elements and transition metals have been found to improve the hydrogen storage characteristics of magnesium-based alloys.This study investigated the Mg-Ho-Fe(MHF) ternary alloy prepared using the vacuum induction melting technique.X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM),pressure-composition-temperature(PCT),and differential scanning calorimetry(DSC) were used to analyze the alloy's phase transitions,microstructure,thermodynamics,and kinetic properties.The results reveal that the Mg_(98)Ho_(1.5)Fe_(0.5) alloy forms a solid solution with Ho and Fe in the magnesium matrix.Upon hydrogen absorption,the activated alloy transforms into a mixture of Mg/MgH_(2) phases and nanoscale HoH_(2) phases.Notably,only the MgH_(2) phase decomposes during hydrogen desorption,while the HoH_(2) phase remains unchanged,exhibiting a positive catalytic effect.The alloy demonstrates excellent hydrogen absorption kinetics,achieving a capacity of 5.56 wt% H_(2) within 10 min at 360℃,owing to the combined catalytic effects of Ho and Fe.The activation energy for hydrogen desorption is found to be 135.87 kJ/mol,which is lower than that of the activation energies of pure MgH_(2) and MgFe alloys,indicating an enhancement in desorption kinetics.Moreover,the enthalpy and entropy changes for hydrogen absorption and desorption are determined to be-70.51 kJ/mol H_(2),-125.62 J/(K·mol) H_(2),72.83 kJ/mol H_(2),and 128.95 J/(K·mol) H_(2),respectively.Furthermore,it is worth noting that the thermodynamic properties of the alloy are improved due to the catalytic effect of Ho and Fe.