Analyses of hydrogen sorption kinetics and thermodynamics of magnesium–misch metal alloys

The multi-component Mg–x wt.% Mm alloys are synthesized using the mechanical ball-milling technique and their hydrogen storage capacities, absorption/desorption kinetics, and thermodynamic parameters are quantified. The analysis of kinetic properties presented here is based on the method of exponential peeling. It is seen that the presence of misch metal (Mm) in the alloy samples dramatically decreases their rate of decrepitation and increases their cyclic stability. However, the increased concentration of misch metal in the samples has an adverse effect on their hydrogen storage capacity and their reaction rate. The hydrogen storage properties also vary with reaction temperature. The best hydrogen absorption kinetics are observed at temperatures around 300 °C and the desorption kinetics are quite fast at temperatures of 400 °C and above. The hydrogen desorption activation energy of Mg–x wt.% Mm hydride is much lower than that of MgH₂. The pressure–composition–isotherm (P–C–T) plots of the samples at 300–420 °C indicate that the alloys possess good cyclic stability but very poor reversibility, making the study of their thermodynamic properties difficult. The P–C–T plots also indicate that the increase of the concentration of the misch metal's rare earths leads to an increase of the hydrogen equilibrium pressure and decrease of hydrogen storage capacity.