Mechanism of Thermodynamic Destabilization and Fast Desorption Kinetics in a Mechanically Alloyed MgH2–In Composite

Improving desorption kinetics and achieving thermodynamic destabilization of MgH2 is of great importance for its practical applications. In the present study, a MgH2–In composite was synthesized through high-energy ball milling of MgH2 and In, and microstructures; hydrogen ab/desorption properties of the composite were carefully investigated with respect to the identically processed pure MgH2. The thermodynamic destabilization of MgH2 was achieved through reversible phase transformation of Mg(In) solid solution due to the partial solubility of In in Mg lattice (11 atom %), which resulted in lower enthalpy values and reduced desorption temperatures. X-ray diffraction peak analysis revealed the presence of internal stress/strain fields caused by the lattice mismatch between Mg and indium, which could play an important role in improving the desorption properties of the composite. Hydrogen evolution of the MgH2–In composite was directly observed via an in situ high-resolution transmission electron microscope (TEM), which shows the “hydrogen pump” effect of a Mg3In phase which is stable only under a hydrogen environment. The formation mechanism of the Mg3In phase and its role as “hydrogen pump” in the improvements of desorption kinetics are thoroughly discussed based on the TEM observations and density functional theory (DFT) calculations.