On the hydrogen desorption entropy change of modified MgH₂

Reducing hydrogen desorption temperature (T_d) is a current challenge for application of Mg-based hydrogen storage alloys. The equilibrium T_d is directly determined by the thermodynamics, characterized by the enthalpy (ΔH) and the entropy (ΔS), of hydrogen desorption reaction of MgH₂. While the change of ΔH has been proved due to the size effect and alloying effect, the ΔS is customary considered as entropy of H₂, being a constant of 130.5 Jmol^-1·K⁻¹. Very recently, however, the ΔS is experimentally reported to change in some cases as well. In this work, for the first time, we proposed a model to illustrate theoretically that the vary of ΔS, which significantly influences the T_d, of MgH₂ with the changing particle/grain size, covering catalyst on surface and forming solid solution of MgH₂ that facilitates the hydrogen molecule dissociation. The model is supported by the experimental results.