NiF2掺杂对MgH2解氢性能影响机制研究

Affecting Mechanism of NiF2 Doping on Improvement of Dehydrogenating Properties of MgH2 Systems

采用第一原理赝势平面波方法,计算分析了NiF2掺杂对MgH2解氢性能的影响,合金形成热计算结果表明:F原子优先占据MgH2中的H原子位,无论F原子占H位还是占居间隙位置都能形成更为稳定的结构。F原子占据MgH2间隙位置时,H原子的解离能明显降低,说明F原子占据间隙位置时,能够改善MgH2体系的解氢性能。Ni原子置换MgH2中的Mg原子后,MgH2体系的形成热与H原子的解离能都降低,说明在MgH2体系中加入Ni原子能够改善MgH2体系的解氢性能。电子态密度,交叠聚居数结果表明:F或Ni原子掺杂后MgH2解氢能力增加,归因于MgH2体系费米能级附近的能隙变窄以及H-H原子的键长变短。合金形成热及交叠聚居数结果表明,Mg与F元素容易成键形成更为稳定的Mg F2化合物,Ni元素与MgH2形成不稳定的Mg2Ni H4化合物,从微观层面证实了NiF2的加入促进化学反应NiF2+3MgH2=Mg F2+Mg2Ni H4+H2的进行,形成更为稳定的Mg F2和不稳定的Mg2Ni H4化合物,从而改善MgH2体系的解氢性能。

A first-principles plane-wave pseudopotential method was used. The mechanism of NiF2 doping on the improvement of dehydrogenating properties of MgH2 systems was studied. Alloy formation heat calculation results showed that F atom preferred to occupy the H atom site of MgH2,whether F atom replaced H atom or occupied interstitial site MgH2 could form a more stable structure. When F atom occupied the interstitial site of MgH2,the dissociation energy of H atom was obviously reduced. It meant that when the F atom occupied the interstitial site,the dehydrogenation properties of the MgH2 system were improved. When Ni atom replaced Mg atom,both the formation heat of the MgH2 system and the dissociation energy of H atom were reduced. It meant that after Ni atom replaced Mg atom,the dehydrogenation properties of the MgH2 system were improved. After analyzing the densities of states( DOS) and overlap populations,it was found that after doped with F or Ni atom the dehydrogenation properties of MgH2 were increased,it was attributed to the narrow of the energy gap near the Fermi energy level and the shortening of H-H bond length. The calculation results of alloy formation heat and overlap populations showed that Mg and F elements tended to form more stable MgF2 compound,and Ni elements and MgH2 formed unstable Mg2 Ni H4 compound. It showed that the addition of NiF2 could accelerate the chemical reaction of NiF2+ 3 MgH2=MgF2+ Mg2 Ni H4+ H2,a more stable MgF2 and unstable Mg2 Ni H4 compound were formed to improve the hydrogenation of the MgH2 system.