Study On Preparation And Dehydrogenation Performance Of Aluminum Hydrides Composites By Doping

Alanate is considered as one of the promising solid-state hydrogen storage materials because of its high capacity and low cost of raw material.Among them,LiAlH₄ and NaAlH₄,as two representative metal alanates,have attracted extensive attention.However,the high dehydrogenation temperatures,slow kinetics and relatively poor reversibility restrict its practical application.It was found that doping catalysts can improve the dehydrogenation properties of LiAlH₄ and NaAlH₄ by analyzing numerous studies.To systematically study catalytic effect on the dehydrogenation properties of LiAlH₄ and NaAlH₄,we prepared three kinds of transition metal catalysts and studied its catalysis on dehydrogenation properties of LiAlH₄ and NaAlH₄.Main results are summarized as follows:Firstly,based on the first-principles calculation of the double metal oxide CoTiO₃,it was found that CoTiO₃,as a dopant,has an apparent effect on improving the dehydrogenation performance of LiAlH₄ hydrogen storage materials.The porous nano rod-like CoTiO₃ was prepared by co-precipitation method,and then doped it into LiAlH₄ by ball-mill.The results showed that the dehydrogenation temperature of LiAlH₄-5 wt%CoTiO₃ is 62°C,92°C lower than pure LiAlH₄.And 7.45 wt%hydrogen released below 300°C.Approximately3.40 wt%H₂ was released from LiAlH₄-5 wt%CoTiO₃ within 30 min and 3.80 wt% within1 hour at 150°C.The E_a values were reduced to 58.09 and 40.49kJ/mol during the first two stage reactions,50.0%and 69.6%lower than than pure LiAlH₄ respectively.The theoretical calculation results show that the doping of CoTiO₃ reduces the dissociation energy barrier of the Al-H bond.The interface charge transfer and the dehybridization of Al-H clusters promote the weakening and fracture of the Al-H bond.Secondly,composite nanoparticles（NiO@Fe₂O₃）were synthesized successfully through hydrothermal method,then doped it into LiAlH₄ by ball-milling.We found that the dehydrogenation temperature of LiAlH₄-5 wt%NiO@Fe₂O₃ sample was decreased to 71°C and released 7.31 wt%hydrogen.The 5 wt% NiO@Fe₂O₃ added LiAlH₄ sample released approximately 4.36 wt%H₂ at 150°C within 1 hour.The calculation results revealed that the activity energy of the first two dehydrogenation reactions were 76.49 and 104.28kJ/mol,which were 34.2% and 21.6% lower than pure LiAlH₄ respectively.The mechanism of dehydrogenation shows that NiO@Fe₂O₃ has a larger specific surface area,exposing more active sites on the surface of the material,providing a channel for hydrogen diffusion and facilitating the dissociation of LiAlH₄.Finally,two-dimensional MXene material Ti₂C was prepared by etching method and then doped it into NaAlH₄ by ball-mill.It showed that the NaAlH₄-2 wt% Ti₂C started to release hydrogen at 47°C and 5.07 wt% hydrogen desorbed.In addition,3.75 wt% of hydrogen could be desorbed from NaAlH₄-2 wt% Ti₂C at 180°C,within 1 hour.The activity energies for the first two dehydrogenation steps were decreased to 56.53 and 96.16kJ/mol,respectively.It was obvious that Ti₂C significantly enhanced the hydrogen-storage properties of NaAlH₄.The analysis of the dehydrogenation mechanism shows that the chemical reaction between Ti₂C and NaAlH₄ during ball milling lead to the fracture of the Ti-C bond and the transformation of Ti⁴⁺into Ti³⁺and Ti⁰,which promotes a good catalytic effect on the hydrogen release of NaAlH₄.