Theoretical Study On The Hydrogen Desorption Properties Of MgH₂ By Metal Fluoride

MgH₂ is characteristic of rich resources,low cost,huge hydrogen storage capacity,non-toxicity and lighter quality;hence it has become one of the new-energy resources with widest developing prospect.However,owing to the defects of higher temperature and slower velocity in hydrogen absorption and desorption in the process of dehydrogenating,the practical use of MgH₂ system is considerably restricted.For the sake of perfecting dehydrogenating properties of MgH ₂ system,a lot of researches and tests have been done and certain academic achievements have been achieved.Until now,however,the study on MgH₂ with regard to hydrogen desorption performance is relatively scanty in theory.In order to put this hydrogen storage material into better use,the author is to take MgH₂ as a primary research object in this dissertation by constructing its unit cell models and analyzing its physical properties concerned.Meanwhile,the first-principle computing method is to be adopted in the research on the micro-physical quantities?such as enthalpies of alloy formation,formation of Mg vacancy,dissociation energy of H atom,replacement of solid solution heat,etc.?and the structural stability of alloy.On this basis,with the addition of metal fluorides?NbF₅ and VF₄?and the replacement of Mg by Nb and V atom,the author is to look electronically into the effect of alloying on dehydrogenating properties in MgH ₂system,and the catalytic mechanism of hydrogen desorption is,thereby,to be probed into in theory consequently.Based on the experimental data,MgH₂ unit cell model has been structured.After the calculation of MgH₂ lattice constants,dissociation energy of atom H,alloy combination heat,density of electronic states?DOS?and occupation number of electrons,the following results are to be found that MgH₂ lattice constants and atomic coordinates are in total accordance with other researchers'experimental values.The minus calorific values in the formation of MgH₂ alloy are relatively high,thus dehydrogenating is not easy to be realized.In MgH₂ alloy system,Mg-H bond is,to some extent,covalent in quality.Effected by partial covalent bonds and forceful ionic bonds in it,the MgH₂ alloy system has grown stable,which is inevitably to impede the dehydrogenating dynamics and make dehydrogenating difficult to occur.With Mg atoms in MgH₂ system having been replaced by Nb and V atoms respectively,the author has finished computing bond lengths,H atom dissociation energy,density of electronic states,electron density,heat of solid solution replacement,enthalpies of alloy formation and occupation number of electrons.It is found that subsequent to alloying,Nb-H bond length is getting shorter that of pre-replaced Mg-H,thus becoming more prone to bonding.The energy needed to displace one or two H atoms is less than that for displacing the corresponding atomicity.The new energy gap makes the energy gap around Fermi energy?E _F?in MgH₂ alloy system narrowing down,thus bringing about the movement in the direction of low-energy level.Therefore,the structural stability of the corresponding MgH₂ alloy system is weakened.The inter-actions on each other of partial Mg-H atoms are weakened or even eliminated.On the condition of Mg vacancy,H-H bond length is rather shorter,and the energy consumed to displace two H atoms is reduced.Obviously,replacement of Mg consumes much less energy than the formation of Mg vacancies does,and the minus calorific values in the formation of alloy are much lower than those of the formation of MgH₂ alloy.Hence,from what has been discussed above,it can be concluded that metal fluorides N bF₅ and VF₄ can improve the dehydrogenating properties of MgH₂ system in that the function of Mg-H bond in it is weakened after alloying.