Density functional theory on reaction mechanism between p-doped LiNH<sub>2</sub> clusters and LiH and a new hydrogen storage and desorption mechanism

Abstract

Hydrogen energy is considered a clean energy with great development prospects. In the field of hydrogen energy applications, the solid-state chemical hydrogen storage method using hydrogen storage materials as media has received widespread attention due to its safety and high hydrogen storage density. In the research on metal-N-H system hydrogen storage materials, current studies focus on improving the kinetic conditions for hydrogen storage. In this study, the B3LYP hybrid functional method of density functional theory is used to investigate the reaction mechanism between P-doped LiNH<sub>2</sub> clusters and LiH at a cluster level, and explore the effects of doping, in addition a new hydrogen storage mechanism called “secondary hydrogen transfer” is proposed. The full-geometry optimization of (LiNH<sub>2</sub>)<sub><i>n</i></sub> (<i>n</i> = 1–4) clusters and their P-doped clusters at the 6-31G(d,p) basis set level are carried out, and their corresponding most stable configurations are obtained. The distribution and stability of the frontier orbitals of the relevant reactants are calculated. Using the same method and basis set, the theoretical calculation and analysis of the reaction mechanism between P-doped (LiNH<sub>2</sub>)<sub><i>n</i></sub> (<i>n</i> = 1–4) clusters and LiH are conducted, including the configuration optimization of the stationary points in each reaction path, and the correctness of the connection between the stationary points is determined through frequency and intrinsic reaction coordinate calculations. The results show that P doping has a small effect on the lowest unoccupied molecular orbital, while the highest occupied molecular orbital has a significant transition towards the doping atom, and the electron-deficient region is concentrated at the P atom. P doping reduces the stability of the lithium amide clusters and enhances their ability to participate in chemical reactions and reaction activity, and the reaction dehydrogenation energy barrier decreases. The reaction dehydrogenation energy barrier between P-doped LiNH<sub>2</sub> clusters and LiH is significantly lower than that between LiNH<sub>2</sub> and LiH, which is consistent with the analysis of reactant stability. Additionally, it is found that the reaction between P-doped LiNH<sub>2</sub> clusters and LiH tends to dehydrogenate through the —PH<sub>2</sub> functional group and store hydrogen at the —NH<sub>2</sub> functional group. Therefore, a new idea of “secondary hydrogen transfer” is proposed, in which effective transfer of hydrogen between —NH<sub>2</sub> and —PH<sub>2</sub> functional groups takes place during the cyclic hydrogen storage process, thus the reversibility of hydrogen storage is further improved and the hydrogen desorption energy barrier of the material is reduced.