Modification Of Cobalt Based Composite Catalyst On The Hydrogen Absorption And Desorption Performance Of Magnesium Hydride

Hydrogen energy,as a clean and pollution-free secondary energy,is an important part of China’s future national energy system,in which the development of hydrogen storage technology directly affects the practical application of hydrogen energy.Among all kinds of hydrogen storage technologies,hydrogen storage with metal hydride has become a promising method for hydrogen storage due to its high hydrogen storage capacity and relative safety.Magnesium hydride（MgH₂）hydrogen storage system has the advantages of high hydrogen capacity（7.6 wt%）,good reversibility and low production cost,but it also has the defects of high temperature and slow kinetics,which limit its large-scale application.In this paper,based on the previous research experience on the improvement of MgH₂ hydrogen storage performance,cobalt-based composite catalysts were designed and prepared,and their improvement effect and mechanism on MgH₂ hydrogen storage performance were systematically investigated.Firstly,CoNi-CoO@C/GO catalyst with Schottky defect structure supported by graphite oxide was designed and prepared.The initial dehydrogenation temperature of MgH₂ was reduced from 345.8℃to 195.5℃.At 290℃,the MgH₂+CoNi-CoO@C/GO composite system can release 7 wt%hydrogen within 30 minutes,and the dehydrogenated composite can start to absorb hydrogen at about 40℃,and the composite can rapidly absorb 5.8 wt%hydrogen within 20 minutes at 150℃.In the cyclic performance test,MgH₂+CoNi-CoO@C/GO system performs well in 20 constant temperature hydrogen absorption and dehydrogenation tests.The mechanism analysis shows that CoO and in situ Mg₂Ni and Mg₂Co co-promote the enhancement of hydrogen absorption and dehydrogenation kinetic properties of MgH₂+CoNi-CoO@C/GO composites.Subsequently,the titanium-cobalt-based catalyst Ti-CoO@C supported by carbon nanocages with multiple catalytic active components was further investigated in this paper,and the initial dehydrogenation temperature of MgH₂ system was successfully reduced to185.6℃.The MgH₂+Ti-CoO@C system can rapidly release 6.3 wt%hydrogen within 5minutes at 275℃.At 175℃,the composite material can rapidly absorb 4.5 wt%hydrogen within 1 minute,showing good thermodynamic and kinetic properties of hydrogen absorption and discharge.In addition,the MgH₂+Ti-CoO@C system showed excellent cyclic stability in20 cycle tests.Microstructure analysis showed that Ti,Ti O₂,MgO,Mg₂Co/Mg₂Co H₅ and other active components of the composite catalyst synergically improved the hydrogen storage performance of MgH₂.Finally,the kinetic properties of pure MgH₂,MgH₂+CoNi-CoO@C/GO and MgH₂+Ti-CoO@C were analyzed by Chou model.At the test temperature,the hydrogen desorption model of the MgH₂ composite doped with two catalysts is the permeation model,while the pure MgH₂ requires a higher temperature to maintain the permeation model.At 225℃,the hydrogen absorption model of pure MgH₂ changes from the diffusion model at high temperature to the permeation model,and the addition of both catalysts can keep the hydrogen absorption model of MgH₂ composite at 125℃as the diffusion model.It can be seen that the addition of catalysts not only reduces the hydrogen absorption temperature of MgH₂,but also changes its hydrogen absorption model from the permeation model to the diffusion model.