Microstructure,Hydrogen Absorption/Desorption Behavior And Catalytic Modification Of Mg-Ni Based Alloys

Mg-based alloys have become a promising system for hydrogen storage due to high density of hydrogen storage and light weight.However,the activation issue,the high temperature for desorption and the sluggish hydrogen absorption kinetics as the main bottlenecks prevent its practical applications.For modification of Mg-based hydrogen storage alloys,the means currently used are single,most of them centered on alloying,nanocrystallization/amorphization,surface catalysis tend to improve a certain aspect hydrogen storage performance of the alloys.The strategy based on overall controlling is seldom involved in the exploitation of Mg-based alloys and the investigation on the relationship between microstructures and hydrogen storage properties of Mg-based alloys and the catalytic modified activation mechanism for Mg-Ni alloy system as well as the hydrogen absorption/desorption process are still not enough in-depth.Investigation of these problems can not only understand the relationship between the microstructure controlling and hydrogen storage properties,and the function of catalysts during the activation process and the subsequent hydrogen absorption/desorption process can be clarified.It can also provide ideas and lay foundations for the development of high capacity Mg-based hydrogen storage alloys with excellent comprehensive performance.Due to the complexity of the hydrogen absorption/desorption reaction,this article is based on the overall controlling,namely the internal microstructure controlling and subsequently surface catalytic modification to obtain alloys with excellent comprehensive hydrogenation properties.Considering the urgent demand for the large capacity hydrogen storage material in the hydrogen powered vehicles,the magnesium rich alloy Mg10 Ni with high theory capacity is designed and the potentialities for traditional Mg₂ Ni alloy are excavated.The overall controlling is achieved by melt-spinning for internal microstructure optimization and surface catalysis to improve the surface properties.For modified samples prepared based on the overall controlling,the relationship between microstructures and hydrogen storage properties is discussed,the activation mechanism for catalytic modified samples is revealed,the thermodynamics for catalytic absorption/desorption are investigated and the destabilization effect for catalysts is discussed.In this paper,the main research contents and the innovation results are as follows:SEM,TEM,EDS and XRD methods are employed to investigate the microstructure and phase composition for Mg₂ Ni alloys before and after microstructure optimization and surface catalysis.The as-cast Mg₂ Ni alloy is mainly consisted by peritectic Mg₂ Ni phase and little residual MgNi2 as well as Mg phases.The bulky microstructure for as-cast Mg₂ Ni alloy is unfavorable to rapidly hydrogen absorption/desorption completely.High-energy ball milling technique can effectively refine the microstructure for as-cast Mg₂ Ni alloy and different microstructure of Mg₂ Ni can be obtained after various ball milling time and ball to powder ratio.Amorphous Mg₂ Ni is obtained when high-energy ball milled for 38 h with 20:1 ball-to-powder ratio.The refined microstructure and active surface achieve when amorphous Mg₂ Ni alloy high-energy ball milled with MWCNTs.The hydrogenation property of amorphous Mg₂ Ni is superior and the absorption capacity is 3.5wt% within 30 min.The absorption rate for amorphous Mg₂ Ni after surface modified is fastest and the final hydrogenation capacity is 3.8wt%?The microalloying Mg10 Ni alloy prepared by metallurgical method is consisted of first precipitated phase Mg dendrite and layer flake Mg-Mg₂ Ni.The microstructure is bulky and the specific surface area is small,which is lack of channels for H-atoms diffusion.After internal microstructure controlling by melt-spinning,the Mg matrix phase and second phase Mg₂ Ni in Mg-rich Mg10 Ni alloy were nanocrystallized.The microstructure is refined effectively and the the amounts of grain/phase boundaries increase.A large number of micro-twins are introduced during the nanocrystallization of Mg₂ Ni and the autocatalytic effect of the Mg-Ni alloy system is effectively improved.Short time high-energy ball milling surface catalytic modification can realize the uniform distribution of catalysts on the surface of the particles of the Mg-Ni alloys and the highly active particles are introduced for the hydrogen absorption/desorption process and the alloy surface properties are improved.Compared with Mg10 Ni alloy,MWCNTs and TiF3 can improve the activation kinetics of Mg-rich Mg10 Ni alloy to different degrees.Mg10Ni-MWCNTs-TiF3 sample after compound adding of MWCNTs and TiF3 shows extremely excellent activation performance and there is no incubation period.It can quickly absorb hydrogen and achieve ideal hydrogen storage capacity during the first cycle.XPS means reveals that samples before and after surface catalytic modification for Mg-based,the passivation layer composed by MgO and Mg?OH?2 on particles surface reduces the surface activity and hinder the adsorption & dissociaton of H₂ molecules on the surface of the alloy particles,thereby impede the hydrogen absorption process.The surface passivation layer is the main reason for the difficult activation of alloy particles.The improvement for activation by catalysts of Mg10 Ni alloy is not only the accelerated broken and peeling process of the formed surface passivation layer with the aid of lattice strain caused by the improved absorption/desorption processes,thus exposed the fresh surface of the alloy,but also hindering the further formation of surface passivation layer.The main function for TiF3 is providing the interface with low energy barriar for H₂ adsorption & dissociation and MgH₂ heterogeneous nucleation during the hydrogenation,accelerating the process of hydrogen absorption.MWCNTs mainly accelerate the adsorption of H₂ and rapid diffusion of atomic H.During hydrogenation of Mg10Ni-MWCNTs-TiF3,the mutual effect of ancillary diffusion for H-atoms by MWCNTs with adsorption & dissociation of H₂ and MgH₂ heterogeneous nucleation by Ti F3 plays a role of synergetic catalytic effect and exhibits an excellent activation properties.The hydrogen absorption kinetics fitting index m is obatained for different hydrogenation cycle,especially for first hydrogen absorption of samples before and after modification.It shows that the rate-limiting steps for the first absorption process are different and the corresponding mechanisms are different.At 250 oC,2.5MPa,the hydrogen uptake capacities are 5.39wt%,6.52wt%,5.06wt% of Mg10 Ni,Mg10Ni-MWCNTs and Mg10Ni-TiF3,respectively.The hydrogen absorption capacities for Mg10Ni-MWCNTs-TiF3 within the initial 1 and 5min are 5.93wt% and 5.99wt%,respectively.After activation,the absorption capacities for Mg10 Ni,Mg10Ni-MWCNTs,Mg10Ni-TiF3 and Mg10Ni-MWCNTs-TiF3 at 300 oC and 2.5 MPa reach to 6.01wt%,6.64wt%,5.89wt% and 5.92wt%,respectively.More than 5.0wt% of hydrogen can be uptaken within 300 s.The capacity for activated Mg10Ni-TiF3 is lower than that of activated Mg10 Ni alloy without any catalys.TiF3 strongly promotes the dissociation of H₂ molecules and the formation of active H-atoms,which make many sites on the surface of Mg-Ni alloy particles to reach to the critical hydrogen concentration for hydride nucleation.Free surface of TiF3 and the interface between matrix and TiF3 particles can provide interfaces for MgH₂ nucleation and the heterogeneous nucleation rate of hydride is promoted.Eventually a large number of hydride cores form and limited growth up takes place before impingement at the initial hydrogen absorption.The core-shell structure is formed and the subsequent absorption process is hindered due to sluggish diffusion of H-atoms in MgH₂ layer and the low hydrogen absorption capacity is obtained.Separately adding MWCNTs,due to the high surface activity and special tubular structure,H₂ molecular dissociation and H-atoms diffusion can be prompted effectively,and the dissociated H-atoms can be transfered to other parts of the surface or the subsurface of Mg-Ni alloy.The initial nucleation rate for MgH₂ is reduced and the diffusion driving force is increased.Hydride particles can fully grow up at low nucleation and higher hydrogen absorption capacity can be reached.Compound adding of MWCNTs and TiF3,numerous H-atoms dissociated by TiF3 can be spread by MWCNTs to other parts of the surface or subsurface of Mg10 Ni paticles and the heterogeneous nucleation rate is reduced and the completely saturated hydrogen absorption is finally achieved.Absorbed at 300 oC under 2.5MPa hydrogen pressure,the Mg phase and unsaturated Mg₂NiH0.3 phase are observed in the hydrogen absorbing products of Mg10 Ni and Mg10Ni-TiF3,while there are only saturated phases MgH₂ and Mg₂NiH₄ in the hydrogenated Mg10Ni-MWCNTs and Mg10Ni-MWCNTs-TiF3 samples.With the temperatures rising,the pressure differences between hydrogen absorption/desorption thermodynamic P-C-T curves decrease and the hysteresis effect gradually reduce.The hydrogenation /dehydrogenation platforms become more flat and the hysteresis between modified samples are different.The hysteresis for Mg10Ni-MWCNTs is smaller,while the hysteresis for Mg10Ni-TiF3 is bigger and the hysteresis for Mg10Ni-MWCNTs-TiF3 is minimal.The dehydrogenation P-C-T curves for experimental samples at different temperatures are fitted by Van't Hoff equation,and the formation enthalpy/entropy for hydrides are calculated.The effects of catalysts are evaluated from the thermodynamics.Comparing to Mg10 Ni sample,the formation enthalpies for hydride are reduced from-77.36kJ/mol H₂ to-73.75kJ/mol H₂ and-75.51kJ/mol H₂,respectively by adding MWCNTs and TiF3.When adding MWCNTs and TiF3 simultaneously,the formhation enthalpy is reduced to-73.37kJ/mol H₂,suggesting that the addition of catlysts and alloying element Ni can decrease the hydride stability of Mg-Ni alloy system and improve the performance of hydrogen desorption.