Study On Hydrogen Storage Performance And Mechanism Of Magnesium With In-situ Doping Nickel And Ceria

Mg has received considerable attention for its high theoretical hydrogen storage density(7.6 wt.%),low cost,abundant resource storage.However,its practical application in the filed of hydrogen energy is primarily hindered by sluggish hydrogen sorption kinetics and high operation temperature.To overcome these problems,kinds of additives are introduced to modify the hydrogen sorption kinetics of Mg-based materials.Based on the systematically investigation of the main development in Mg/MgH2 based hydrogen storage materials at home and abroad,doping several additives,such as Ni-based compounds,nano CeO2 with different morphologies,Ni and CeO2 bicomponent,are designed to enhance the hydrogen sorption kinetics of Mg/MgH2 system by in-situ alloying and oxygen vacancy effect.Moreover,based on the experimental results and theoretical calculation data,plausible modified mechanisms are proposed.Firstly,nano NiO and NiCl2 were introduced into Mg by mechanical milling,and the effect of NiO and NiCl2 on hydrogen sorption kinetics of Mg was investigated.Results show that compared with pure Mg,the hydrogen conversion of Mg-NiO and Mg-NiCl2composites is increased from 53.8%to 89.0%and 77.7%,and the corresponding apparent activation energies are decreased from 165.3 kJ/mol H2 to 114.7 and 149.7 kJ/mol H2.It suggests that the hydrogen sorption kinetics of Mg-NiO and Mg-NiCl2composites can be improved.During the hydrogen absorption and desorption cycles in Mg-NiO and Mg-NiCl2 composites,Mg2Ni can be in-situ formed and distributed uniformly in Mg matrix.From the results we have obtained,it can be inferred that the in-situ formed Mg2Ni on the surface of Mg particles can promote the dissociation of H2 molecule during the hydrogen absorption process and weaken the Mg-H bond during the hydrogen desorption process by transferring electrons.Moreover,the uniform dispersion of Mg2Ni in Mg/MgH2 matrix can provide high density interfaces between Mg2Ni and Mg,and numerous boundaries of MgH2 nanocrystal,which can be the nucleation sites of Mg and H diffusion channels.Secondly,nano CeO2 powders with morphologies of rod(rCeO2),cubic(cCeO2),and octahedron(oCeO2)were synthesized by hydrothermal method,and then were introduced into MgH2 by mechanical milling.From the analyses of Raman,H2-TPR,and XRD,it can be found that rCeO2 and cCeO2 have higher concentration of oxygen vacancy defects,exhibit higher activity to H2,and can be reduced to dark blue Ce6O11.During the mechanical milling process,three kinds of CeO2 are transformed into Ce6O11 with higher concentration of oxygen vacancies and remain stable in the system.From the DSC and isothermal hydrogen desorption kinetics,it can be found that for hydrogen desorption,the peak temperatures of composites are reduced,the rate and H2 amount are increased by the in-situ formed Ce6O11,among which MgH2-cCeO2 has the fastest dehydrogenation rate.According to the Kissinger equation,the calculated apparent activation energies for double peaks of MgH2-cCeO2 are 62.52%and 42.45%lower than that of pure MgH2(173.49 kJ/mol H2).On the basis of our findings,it can be inferred that more oxygen vacancies and active sites are exposed on the surface of Ce6011 particles,which can enhance the interaction between adsorbents and Ce6011,and beneficial to improve the hydrogen sorption performance of the composites.To further enhance the hydrogen storage performance,Ni-CeO2 were introduced into Mg by mechanical milling.Results show that compared with pure Mg(434.7?,138.3 kJ/mol H2),the peak temperature and apparent activation energy for dehydrogenation of Mg-20(Ni-CeO2)are reduced by 115.8 ? and 63.89 kJ/mol H2.Mg-20(Ni-CeO2)composite has higher hydrogen desorption rate and amount,lower peak temperature and apparent activation energy for dehydrogenation when comparing that of Mg with Ni or CeO2.This suggests that Mg-20(Ni-CeO2)system has excellent hydrogen sorption kinetics.Microstructural analyses suggest that Mg2Ni and Ce6O11 can be in-situ formed on the surface of Mg particles during the hydrogen activation process,which exhibit a synergistic catalysis for hydrogen sorption.During the hydrogenation process,the H2 molecules are trapped by the oxygen vacancies on Ce6011 surface,and then dissociated by Mg2Ni on Mg/Mg2Ni interface,then H atoms can gain electrons from Mg to form Mg-H bond with the assistance of electron donor Ce6O11 by Ce3+/Ce4+transfer.During the dehydrogenation process,electron transfer can be enhanced by Ni,the Mg-H bond can be weakened by Mg2Ni,which is decomposed from Mg2NiH4 on MgH2 matrix and play a role in transferring H atoms for H2 formation with the assistance of electron acceptor Ce6O11 by Ce3+/Ce4+ transfer.In addition,the in-situ formed Mg2Ni and Ce6O11 can restrain the growth of Mg crystal and stabilize the composites,and result in a good stability of Mg-Ni-CeO2 composites.Finaly,based on the first principle calculation of density functional theory,the structural stability and oxygen vacancy effect of CeO2(111),(110),and(100)surfaces were investigated,and the adsorption behavior of H2 molecule on clean and CeO2(111)with oxygen vacancy was simulated.The calculation results of surface energy and vacancy formation energy show that Ceo2(111)is the most stable surface,followed by CeO2(110),and the CeO2(100)surface has the highest activity.The adsorption energies of H2 molecule on clean and CeO2(111)with oxygen vacancy surfaces are-0.10 eV and-0.16 eV,respectively.It suggests that the local electron environment formed by oxygen vacancy is favorable for H2 adsorption.Besides,the adsorption and dissociation of H2 molecue on Ni doped Mg(001)surface were studied.Results show that after Ni doping the adsorption energy of H2 molecule is increased from-0.09 eV to-0.14 eV,the corresponding dissociation energy barrier is sharply decreased from 1.51 eV to 0.41 eV.This indicates that Ni doping can significantly facilitate the adsorption and dissociation of H2 molecule.Combing the analyses of density of states and charge density,it can be inferred that Ni provides electrons to H2 molecule in the form of orbital hybridization to promote the crack of H-H bond.In the investigation of hydrogen desorption from Ni doped MgH2(001),it can be found that after Ni doping the reaction energy barrier for the recombination of H atoms to generate H2 is significantly reduced from 1.83 eV to 0.41 eV.This indicates that Ni doping can promote the recombination of H atoms.Combing the analyses of density of states and charge density,it can be inferred that Ni can capture electrons from H atoms nearby by orbital hybridization to accerlate the formation of H-H bond.From the above analyses,it can be preliminary verified that surface oxygen vacancy and alloying effect can significantly improve the hydrogen sorption performance of Mg/MgH2 system.