Thermodynamics, kinetics and microstructural evolution during hydrogenation of iron-doped magnesium thin films

New results are reported suggesting that with appropriate levels of Fe doping Mg can rapidly and reversibly absorb up to 7 mass fraction (%) hydrogen at moderate temperatures and pressures useful for hydrogen storage applications. Hydrogenation kinetics and thermodynamics of Mg–4Fe at.% (+/− 1 at.%) thin films capped with Pd at temperatures ranging from 363 K to 423 K were studied by a number of different methods: in situ infrared imaging, volumetric pressure-composition isotherm (PCI) measurements, and ex situ X-ray diffraction and transmission electron microscopy. The hydride growth rate was determined by utilizing wedge-shaped films and infrared imaging; assuming formation of a continuous hydride layer, the growth rate was found to range from ≈3.8 nm/s at lower temperature to ≈36.7 nm/s at higher temperature. The apparent activation energy of the thermally activated hydrogenation kinetics was measured to be 56 kJ/mol; this value suggests that at low temperatures hydrogen diffusion along grain boundaries of MgH2 is the mechanism controlling the hydride layer growth.Reproducible PCI measurements of 600 nm-thick uniform films showed a pressure plateau and large hysteresis; from these measurements enthalpy and entropy were estimated as 66.9 kJ/mol and 0.102 kJ/(mol∗K), respectively, which are both slightly less than values for pure magnesium (as either films or bulk). The extremely rapid and cyclable kinetics of Mg-4 at.% Fe films suggest that properly grown Mg–Fe powders of 1–2 μm size can be fully charged with hydrogen within 1 min at temperature near 150 °C (423 K), with possible practical hydrogen storage applications.

► Hydrogenation of Mg-4 at.% Fe films at low temperatures (363 K–423 K) shows very fast kinetics.
► A new method based on IR imaging of a wedge-shaped film was used to evaluate kinetics of the hydride growth.
► Pressure-composition isothermal measurement of a single film was used to evaluate thermodynamic properties.
► Apparent activation energy of hydrogenation kinetics, 56 kJ/mol, suggests hydrogen diffusion along grain boundaries as a controlling mechanism.