Features of magnesium hydride affecting its thermolytic hydrogen desorption

Magnesium hydride is a solid-phase hydrogen carrier and a candidate for energy storage applications. However, slow kinetics and high desorption temperatures hinder its practical use. Despite numerous attempts to improve Mg/MgH₂ kinetic and thermodynamic performance, a sound model of the hydrogen desorption process is still lacking. The current study investigated the thermolysis kinetics of MgH₂ produced by various methods and characterized critical rate-determining steps. The thermolysis kinetics of H₂ release from MgH₂ produced by reactive ball milling under hydrogen of MgH₂ (pristine [MGH-RBM]) of nanometric size particles was found to be substantially different from the commercial MgH₂ produced using hydridic atomization (pristine [MGH-HAT]) of micrometric particle size. Hydrogen desorption of [MGH-RBM] was faster and required lower temperatures, while hydrogen desorption of [MGH-HAT] included an extended incubation step, which does not exist for [MGH-RBM]. The delayed hydrogen release is explained by the trapping of hydrogen within the grain cores of [MGH-HAT] following the initial decomposition process of MgH₂. Further delayed hydrogen desorption is also caused by exposure to air before thermolysis.