Study On The Hydrogen Storage Properties Of Doped Light-weight Metal Hydride MgH₂ With High-capacity

Magnesium hydride MgH₂ is one of the most practical candidates of solid-state hydrogen materials,due to the high capacities and low cost.However,the stable thermodynamics and sluggish kinetics of MgH₂,hardly satisfied practical demands.Based on the previous works on magnesium-based hydrogen materials by our research group,the material was chosen as the research subject.Mechanical milling was applied to prepare the Nb₂O₅-doped MgH₂,Graphite-doped MgH₂and Activated Carbon-doped MgH₂system.The hydrogen storage property and the role of different dopants were studied comprehensively.In this paper,six groups of MgH₂ samples were prepared,commercial MgH₂,milled MgH₂,and four groups of doped MgH₂ prepared by mechanical milling.Characterization of the microstructure,morphology and surface chemical state were tested via X-ray diffraction(XRD),scanning Electron microscope(SEM),and X-ray Photoelectron spectroscopy(XPS).To investigate the desorption kinetics performance of the MgH₂samples,TG-DSC,isothermal desorption and PCT were applied.Furthermore,to study the desorption kinetic mechanisms of the two carbon materials doped sample,the rapid selection approach of kinetic models initiated by Sharp and Jones was also fitted.For different contents of Nb₂O₅ doped MgH₂ samples,the desorption property improves as the content increase.The desorption performance of MgH₂-Nb₂O₅ system was affected by the mutual impact of particle size and the dopant.For MgH₂₊5wt%Nb₂O₅,the onset desorption temperature decreased by 77.9?compare to the commercial MgH₂,the maximum desorption content is 6.6wt%,the apparent activation decreased to-58.07k J/mol,and it can release 4.63wt%H₂ in 20 minutes at 380?under vacuum.The PCI curves indicate that the reversible capacity of MgH₂+5wt%Nb₂O₅is 6.09wt%;The desorption platform pressure of three group is similar,while the absorption platform pressure for the milled MgH₂,MgH₂+2wt%Nb₂O₅ and MgH₂+5wt%Nb₂O₅ is different,which of each group is 1.04,1.02 and 1.3MPa.The result of XPS shows that Nb₂O₅ was reduced by MgH₂ to Nb and Nb O₂due to the reaction between Nb₂O₅ and MgH₂ in the desorption process.The multiple-valance Nb element can facilitate the charge transport between Mg²⁺and H^-,which weaken the Mg-H bond and improve the desorption property.The two kinds of carbon materials improve the desorption property of MgH₂,but the specific desorption kinetic mechanism is different for graphite and activated carbon doped sample,respectively.Graphite is on the MgH₂ matrix uniformly,while activated carbon is in an amorphous state.The latter one with greater refinement effect on MgH₂.The desorption temperature,hydrogen capacity and desorption kinetics of the two kind of carbon doped MgH₂ are affected by mutual influence of the dopants and particle size.The activated carbon doped MgH₂ has better desorption kinetic performance,it can release3.6wt%H₂ in 20 minutes at 380?.The fitting results of various kinetic models indicate that the dehydrogenation kinetic mechanism of the milled and the graphite doped MgH₂is first order reaction,which implies the desorption rate of MgH₂ is determined by its concentration.As for the activated carbon doped MgH₂,the mechanism is three-dimensional diffusion,the desorption kinetics is controlled by the diffusion of H atoms within the bulk MgH₂.Above all,the catalytic effect of Activated carbon is more superior,the maximum dehydrogenation content is 5.8wt%,the onset temperature of dehydrogenation is 386.6?,and the apparent desorption active energy is-137.29k J/mol.The result of PCI curves demonstrates that the reversible hydrogen capacity of MgH₂+5wt%Graphite is highest,4.86wt%H₂.The aborption platform pressure of milled MgH₂,MgH₂+5wt%Graphite and MgH₂+5wt%Activated Carbon are 1.04,1.32,and 1.34MPa,respectively.Though the desorption platform pressure is nearly same.