Theoretical Study On Hydrogen Storage Properties Of Ti Doped MgH₂

The increasing demand for the energy resources leads to the rapid depletion of fossil fuels in particular especially oil and gas,means that we are facing tremendous challenges concerning our energy systems.Our current energy infrastructure dominated by fossil fuel use also leads to greenhouse gas emissions that pose serious threats to our living environment.Thus,it is of great importance to develop clean and nonpolluting renewable energy as a substitute for fossil fuels.Hydrogen is considered as the most promising alternative energy carrier for future the because of its wide variety of sources,high combustion heat value（it almost triples the heat of combustion of gasoline）and clean combustion（no CO₂emissions,just H₂O）.However,the storage of hydrogen is a key technology realizing the commercialization of hydrogen energy.Comparing with high-pressure gas storage and low-temperature liquefaction storage,solid state storage has outstanding advantages,such as high hydrogen storage density,low packing cost and safety guarantee.Among them,Mg H₂is one of the most promising hydrogen storage materials.Although Mg H₂has high hydrogen storage capacity of 7.6 wt%,the high desorption temperature（over 300℃）and poor kinetics hamper severely its commercial application.Recent studies show that the dehydrogenation kinetics of Mg H₂system is improved by adding catalysts and producing the nanosized composites.In this paper,we studied how the transition metal Ti affects the hydrogenation/dehydrogenation kinetics of Mg H₂.By using unbiased evolutionary algorithm combined with density functional theory PAW-PBE and perturbation theory MP2 models,we investigates the geometric and electronic structures of pure Mg_nH_2nand Mg_n-1Ti H_2n（+2）（n=4,5,6）clusters and the effect of Ti-doping on the hydrogen storage properties.Analyses on the bond lengths,charge distributions and the energetics show that Ti-doping weakens the Mg-H bonds slightly but forms strong Ti-H bonds.The Ti-doped clusters are much more stable than the pure Mg_nH_2nclusters.The dehydrogenation energies increase from 0.63-0.86 e V of the pure Mg_nH_2nto 0.98-1.13 e V of Mg_n-1Ti H_2n+2.The energy barriers to desorb H₂molecules from the Ti-doped clusters are also higher.Thus,Ti-doping favours the initial dehydrogenation of Mg H₂at low titanium content,but it is unfavorable for the complete dehydrogenation due to the strong Ti-H bonds.For the nanosize clusters,Ti-doping elevates the dehydrogenation temperature.