On the hydrogen desorption entropy change of modified MgH2

Reducing hydrogen desorption temperature (Td) is a current challenge for application of Mg-based hydrogen storage alloys. The equilibrium Td 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 Td, 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.