Thermodynamics and kinetics of nano-engineered Mg-MgH₂ system for reversible hydrogen storage application

Highlights: • Cyclic performance of nano Fe doped MgH₂ was tested. • Nano Fe doped MgH₂ hydrogenate even below room temperature. • Activation energies of Nano doped MgH₂/Mg reduced remarkably. • Nano Fe does not change the thermodynamics of MgH₂. - Abstract: Thermodynamics and kinetics of hydrogenation-dehydrogenation of nanometric iron (nFe) doped Mg-MgH₂ system have been studied. The nFe-doped Mg could be hydrogenated even at 0 °C up to 45% of the theoretical hydrogen storage capacity within an hour. The dehydrogenation of nFe doped MgH₂ starts below 150 °C. The remarkably improved hydrogenation-dehydrogenation kinetics could be attributed to the nano-engineered surface of MgH₂ by nFe. The enthalpies of hydrogenation-dehydrogenation were found to be 76 kJ/mol, and 77 kJ/mol respectively. The activation energy of hydrogenation was evaluated as 41 ± 2 kJ/mol which is same as the diffusion barrier of hydrogen in Mg matrix. The apparent activation energy of dehydrogenation of nFe-doped MgH₂ was found to be 74 ± 1 kJ/mol which is same as the enthalpy of dehydrogenation. The nFe-doped Mg-MgH₂ system has shown cyclic stability up to 50 cycles without significant changes in the kinetics and hydrogen storage capacity. Three-dimensional diffusion seems to be controlling the dehydrogenation process.