Article pubs.acs.org/JPCC
First-Principles Studies on Hydrogen Desorption Mechanism of MgnH2n (n = 3, 4) Jianjun Liu,†,‡ James Tyrrell,‡ Lei Cheng,‡ and Qingfeng Ge*,‡ †
State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics (SIC), Chinese Academy of Sciences (CAS), 1295 Dingxi Rd., Shanghai 200050, China ‡ Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901, United States ABSTRACT: 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 minimumenergy 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. mobility.23 Very recently, the transition-metal-doped Mg/ MgH2 nanostructure is starting to attract more attention due to the combined effects of size and doping.25−27 Therefore, understanding hydrogen adsorption/desorption mechanisms of Mg/MgH2 at the nanoscale is expected to help the development of magnesium hydride as a hydrogen storage material. Unfortunately, no potential energy profiles of Mg/MgH2 are available so far. Extensive experimental and theoretical studies have been performed to explore size-dependent thermodynamic properties of Mg/MgH2 nanostructures. Li et al. studied kinetic and thermodynamic properties of nanowires of Mg/MgH2 and found a significant size effect in thermodynamic and kinetic properties. 8 Very recently, Zhao-Karger et al. studied thermodynamic properties of MgH2 nanoparticles with a size of
