First-Principles Studies on Hydrogen Desorption Mechanism of MgnH2n (n = 3, 4)

In an attempt to decrease the hydrogen desorption temperature of MgH2, great efforts have been made to improve kinetic and thermodynamic properties by reducing the particle sizes. However, these improvements were impeded because the hydrogen desorption mechanism even for the small MgH2 clusters is not clear. Herein, density functional theory studies have been performed to investigate hydrogen desorption mechanisms in MgnH2n (n = 3, 4) systems. In both Mg3Hx and Mg4Hx systems, hydrogen desorption barriers are higher than hydrogen transfer ones and the first hydrogen desorption is the rate-determining step. Electronic structure analysis showed that many important intermediates along the minimum-energy pathways are combinations of metallic Mg and ionic MgH2, suggesting that phase separation into metallic Mg and ionic MgH2 may happen in a large nanoparticle. The overall hydrogen desorption process from the MgH2 cluster comprises hydrogen transfer, phase separation, and hydrogen separation.