Improvement Of Hydrogen Storage Properties Of Light-weight LiAlH₄ By Transition Metal Catalysts

Lithium aluminum hydride(LiAlH4)is a promising solid-state hydrogen storage material for on-board hydrogen fuel cell systems due to its high hydrogen storage capacity,mild operating temperature,and low cost.However,the sluggish dehydrogenation kinetics and poor reversibility of LiAlH4 limit its practical application.Thus,based on an overall review of the research and the development of LiAlH4,transition metal nano-catalysts are introduced into the LiAlH4 hydrogen storage systems through high-energy ball milling.The structural characteristics of the catalyst and the dehydrogenation performance of the LiAlH4 composites are systematically investigated and the corresponding mechanisms are also revealed.The main results are as follows:(1)The Co3O4@CoNi-LDO catalyst is prepared using hydrothermal synthesis combined with calcination,and its effect on the dehydrogenation performance of LiAlH4 is investigate.The dehydrogenation experiments show that the doping of double metal oxides can effectively improve the dehydrogenation performance of LiAlH4.LiAlH4 doped with 7 wt%Co3O4@CoNi-LDO shows two-step hydrogen dehydrogenation temperatures of 85.1℃ and 177.0℃,respectively,releasing about 6.87 wt%hydrogen at 300℃.The activation energy of the two-step dehydrogenation phase is 20.0%and 46.3%lower than that of as-received LiAlH4,respectively.The dehydrogenated sample can absorb about 0.64 wt%hydrogen within 5 hours,exceeding the adsorption performance of as-received LiAlH4.Al13Co4 and Al0.9Ni4.22 are identified as two catalytic active substances that improve the dehydrogenation performance of LiAlH4.(2)The g-C3N4@NiFe-LDH catalyst is prepared via a solvothermal method combined with high-temperature pyrolysis and introduced into the LiAlH4 system by ball milling.The effect of the catalyst on the hydrogen desorption performance of LiAlH4 and the related mechanisms are studied.LiAlH4 doped with 7 wt%g-C3N4@NiFe-LDH shows two-step dehydrogenation temperatures of 78.1℃ and 179.9℃,respectively,releasing about 7.19 wt%hydrogen.Moreover,the dehydrogenation activation energy is reduced by 43.0%and 54.8%compared with as-received LiAlH4,respectively.In addition,the smaller particle size and higher specific surface area of the dopant are conducive to fully reacting with LiAlH4 during ball milling and improving the catalytic activity.In situ formed Al5Fe2 and FeNi3 active components during the ball milling process are identified as catalytic active centers for the hydrogen desorption reaction of LiAlH4,which improved the hydrogen absorption and desorption kinetics.(3)The Ni/C@Ti3C2 nanocomposite is prepared using etching and hydrothermal methods,and its improvement and influence mechanism on the hydrogen storage performance of LiAlH4 are studied.LiAlH4 doped with 7 wt%Ni/C@Ti3C2 showed twostep initial dehydrogenation temperatures of 56.1℃ and 127.1℃,respectively,releasing about 4.14 wt%hydrogen within 40 min at 120℃.Its two-step dehydrogenation activation energy is 34.5%and 53.2%lower than that of as-received LiAlH4.The sample after dehydrogenation at 40 bar H2 and 300℃ could absorb about 0.58 wt%hydrogen.Combining DFT calculations,one reveals that the in-situ formed Al2Ti during ball milling weakened the Al-H bond and promoted the decomposition of LiAlH4.It is further proved that Al2Ti enhanced the adsorption and dissociation of hydrogen,and promoted the activation of hydrogen molecules during hydrogenation.