Evolution of the phase structure and hydrogen storage thermodynamics and kinetics of Mg88Y12 binary alloy

Abstract

Mg88Y12 binary alloy was fabricated by vacuum induction melting technique. The phase compositions and microstructures of the alloy were analyzed by means of X-ray diffraction (XRD) and scanning electron microscopy (SEM). Then the alloy powders were subjected to isothermal hydrogen absorption/desorption characterizations, differential scanning calorimetry (DSC) analysis and pressure-composition isotherms (PCI) measurements. XRD and SEM results showed that the alloy was composed of primary Mg24Y5 intermetallic and fine dendritic eutectic Mg–Mg24Y5 mixture. First hydrogenation process resulted in an irreversible two-step disproportion reaction: Mg24Y5 + H2 → Mg + YH2 → MgH2 + YH2 + YH3, and 6.479 wt.% hydrogen was absorbed at 380 °C. For further absorption/desorption cycles, the alloy had a reversible hydrogen capacity of about 5.6 wt.% due to the fact that the YH2 has a very high thermal stability and can not desorb hydrogen at the present experimental temperature. The hydrogen absorption and desorption rates were gradually improved by increasing hydriding/dehydriding cycle times because the alloy particles significantly cracked and pulverized during the first few cycles. The activated alloy exhibited well isothermal hydrogen absorption kinetics, but the dehydrogenation occurred at relatively high temperature. DSC measurements indicated that the desorption process of the fully hydrogenated alloy began at about 320 °C and peaked at 367 °C when the heating rate was 3 °C/min. The activation energy was determined as 122 and 116 kJ/mol according to the isothermal dehydrogenation kinetics at different temperatures and DSC curves at different heating rates. Moreover, the enthalpy and entropy changes for hydrogen absorption/desorption were calculated to be −79.8 kJ/mol H2, –139 J/K/mol H2 and 81.2 kJ/mol H2, 141 J/K/mol H2, respectively.