Structural Design And Study Of Electronic Properties And Hydrogen Storage Performance Of Magnesium-Based Ternary Compounds

In order to realize the sustainable use of energy and green development,the development and utilization of new energy sources have attracted great attention,and hydrogen as a clean energy source has great prospects for development,so the storage of hydrogen has become one of the hot spots for research.Among them,MgH2 is an excellent material for hydrogen storage due to its high hydrogen storage density and simple preparation process,however,its high hydrogen discharge temperature,slow hydrogen absorption and release kinetics become the bottleneck problem that restricts its development.Following extensive investigation,it was shown that doping transition metals(TM=Ni,V,Mn)may significantly improve the hydrogen storage characteristics of MgH2,increasing its dehydrogenation efficiency and lowering its dehydrogenation temperature.Compared with the studies on V and Ni doping,the theoretical studies on the doping of MgH2 by the transition metals Sc,Fe and Co elements are scarce.Therefore,this thesis will analyze the effects of doping MgH2 with these three elements on its structure,electronic properties and hydrogen storage property from a theoretical point of view,and the relevant research contents and conclusions are summarized as follows:(1)The crystal structure of the MgTMHx(TM=Sc,Fe and Co;x=8,9 and 10)system are initially predicted using CALYPSO global search software combined with first-principles calculations.Subsequently,the first 50 structures with lower energy were subjected to structural relaxation using a high-precision density functional theory(DFT)method to obtain the most stable structure of the MgTM-H system.The phonon spectrum and enthalpy of formation calculations reveal that for the Mg-Sc-H system,the MgScHg structure with R3 space group is the most stable structure among all the crystal structures of the Mg-Sc-H system,while for the Mg-TM-H(TM=Fe,Co)system,the structures of Fm 3mMg2FeH6 and P4/nmm-Mg2CoH5 are the most stable and agree very well with the experimental results.(2)By calculating the energy band structure and density of states,it is discovered that R3-MgScH8 is an indirect band gap semiconductor material with a band gap value of 2.96 eV.Furthermore,it is found that the doping of Sc element makes its binding with H element stronger than that of Mg atom with H atom in the density of states diagram,which leads to easy dissociation of H.Fw3mMg2FeH6 is a direct bandgap semiconductor with a bandgap value of 1.97 eV,which has a significantly lower bandwidth compared to MgH2,and Fe weakens the strength of the Mg-H bond.P4/nmm-Mg2CoH5 is a narrow bandgap semiconductor with a bandwidth of only 1.33 eV,and the Co element elongates the distance between the Mg atom and the H atom.The above results indicate that the doping of Sc,Fe and Co all weaken the interaction force between Mg atoms and H atoms,resulting in easy separation of H elements.(3)Various possible decomposition pathways for the stable structure R3MgScH8 are investigated,and it is found that MgScH8→ScH6+Mg+H2 is the most favorable decomposition pathway for the R3-MgScH8 structure at pressures below 107.8 GPa,but at the pressures above 107.8 GPa,MgScH8→Mg+Sc+4H2 jumped to be the most thermodynamically stable reaction pathway and released the largest amount of hydrogen.Root Mean Square Deviation calculations show that R3-MgScH8 starts to melt at 400 K.Meanwhile,AIMD simulations show that the structure can easily release hydrogen at 500 K and the amount of hydrogen release can reach 4.04 wt.%,making R3-MgScH8 a potential hydrogen storage material.(4)For the structures of Fm3m-Mg2FeH6 and P4/nmm-Mg2CoH5,the hydrogen storage capacity and hydrogen release temperature are calculated and analyzed using the equations of hydrogen mass density and Gibbs free energy.The results reveal that the hydrogen storage capacity of Mg2FeH6 can reach 5.43 wt.%,which is larger than that of MgH2,and the enthalpy of formation is 83 KJ/mol H2.The decomposition path is Mg2FeH6→2Mg+Fe+3H2,and the hydrogen release temperature of Mg2FeH6 is about 635 K calculated by the Gibbs free energy formula.The hydrogen storage capacity of Mg2CoH5 is 4.45 wt.%,which is slightly higher than that of MgH2.Its enthalpy of formation is 81.05 KJ/mol H2,which eventually decomposes into Mg,Co and H2,and the hydrogen release temperature is 619.65 K.Therefore,the hydrogen evolution temperature of Mg2CoH5 is slightly lower than that of Mg2FeH6.In conclusion,this thesis investigated the effects of doping MgH2 with three transition metal elements(Sc,Fe and Co)on their structure,electronic properties and hydrogen storage performance.Three structurally stable potential hydrogen storage materials were obtained as R3-MgScHg,Fm3m-Mg2FeH6 and P4/nmmMg2CoH5 ternary hydrides,and their energy band structures and density of states were calculated to reveal the specific orbital hybridization,showing that the doping of these three transition metals reduces the structural stability of the traditional hydrogen storage material MgH2 and weakens the binding force between Mg and H atoms.By calculating the enthalpy,it was indicated that the smaller the enthalpy,the lower the decomposition temperature of the compound,and ultimately showed that transition metals had an optimal effect on the hydrogen storage property of MgH2.