Abstract: The magnesium based metal hydrogen storage composite system Mg(NH₂)₂-2LiH has a theoretical hydrogen storage capacity of 5.6 wt.% and is a promising hydrogen storage material for vehicles. However, its application is limited due to serious thermodynamic and kinetic barriers. Introducing efficient catalysts is an effective method to improve the hydrogen storage performance of Mg(NH₂)₂-2LiH. This article investigates for the first time the use of nano rare earth oxide CeO₂ (∼44.5 nm) as an efficient modifier, achieving comprehensive regulation of the hydrogen storage performance of Mg(NH₂)₂-2LiH composite system through oxygen vacancy driven catalysis. The modification mechanism of nano CeO₂ is also systematically studied using density functional theory (DFT) calculations and experimental results. Research has shown that the comprehensive hydrogen storage performance of the Mg(NH₂)₂-2LiH-5 wt.% CeO₂ composite system is optimal, with high hydrogen absorption and desorption kinetics and reversible performance. The initial hydrogen absorption and desorption temperatures of the composite system were significantly reduced from 110/130 °C to 65/80 °C, and the release of by-product ammonia was significantly inhibited. Under the conditions of 170 °C/50 min and 180 °C/100 min, 4.37 wt.% of hydrogen can be rapidly absorbed and released. After 10 cycles of hydrogen release, the hydrogen cycle retention rate increased from 85 % to nearly 100 %. Further mechanistic studies have shown that the nano CeO_2-x generated in situ during hydrogen evolution can effectively weaken the Mg-N and N-H bonds of Mg(NH₂)₂, exhibiting good catalytic effects. Meanwhile, oxygen vacancies provide a fast pathway for the diffusion of hydrogen atoms in the composite system. In addition, nano CeO_2-x can effectively inhibit the polycrystalline transformation of the hydrogen evolving product Li₂MgN₂H₂ in the system at high temperatures, reducing the difficulty of re-hydrogenation of the system. This study provides an innovative perspective for the efficient modification of magnesium based metal hydrogen storage composite materials using rare earth based catalysts, and also provides a reference for regulating the comprehensive hydrogen storage performance of hydrogen storage materials using rare earth catalysts with oxygen vacancies.

Key words: Hydrogen storage, Rare earth oxide, Magnesium based material, CeO₂, Hydrogen storage mechanism