Hydrogen Desorption Kinetics from Different Structures: The Influence of Short- and Long-Range Orders

This research focuses on the influence of grain size and boundaries on hydrogen diffusion in thin films and powders. The isoconversion kinetic method was applied to investigate the hydrogen desorption properties of Mg–Ni–H thin films and powders. The desorption behavior of Mg–Ni–H films was monitored using in situ optical microscopy and thermal desorption spectroscopy (TDS). In situ investigation of hydrogen release provided valuable insights into heterogeneous nucleation in thin films. The TDS curves of crystalline Mg–Ni–H indicate that desorption occurs in a one-step process, starting at Tonset = 212 °C, with the peak maximum observed at Tdes = 250 °C. The apparent activation energy for the crystalline sample was estimated to be 52.1 ± 0.6 kJ/mol. These findings suggested that the desorption mechanism is strongly influenced by the grain size and the density of defects, such as the grain boundaries. Powders are prepared by mechanical milling of MgH2 with Ni, maintaining the same molar ratio as in the preparation process of thin films. Four samples were prepared with different milling times ranging from 30 min to 2 h. The temperature-programmed desorption coupled with mass spectroscopy (TPD-MS) was used to analyze the prepared powders. Milling-induced defects in the MgH2 crystal structures, combined with the uniform distribution of the catalytic phase, significantly impacted hydrogen desorption kinetic and reduced the desorption temperature by 2 times. In this paper, we compare the amorphous vs the crystalline state, highlighting how the material’s morphology controls the thermodynamics, while the amount and position of defects within the crystal structure influence the desorption kinetics.