Microstructure Modification And De-/Hydrogenation Behavior Of Mg-Ni-Based Hydrogen Storage Alloys

With the intensified energy crisis and environmental issues,it is of urgent demand to seek for an efficient and renewable alternative to replace the traditional fossil fuels.Hydrogen is considered one of the most promising energy carriers for the applications of advanced fuel cells and direct combustion,owing to its highest energy density per mass and characteristic of environment friendly.For hydrogen fuel-cell electric vehicles,the on-board hydrogen storage needs light,compact,and affordable system to replace the compressed hydrogen tanks.Mg-based hydride has shown great potential on the commercial applications of solid-state hydrogen storage because of its high gravimetric and volumetric hydrogen storage capacities（7.6 wt.%and 110 g/L）,excellent reversibility,abundant resources,and nontoxicity.Because of the thermodynamically stable Mg-H bond,the poor abilities for the dissociation of H₂ molecules and the recombination of H atoms on Mg/Mg H₂ surface,and the low diffusion coefficient of H atoms in Mg,especially in Mg H₂ matrix,there remains a challenge to prepare a material capable of absorbing and desorbing hydrogen rapidly at ambient temperatures.Herein,the Mg-Ni-based alloys with different composition and structure characteristics are prepared by metallurgical method,and their microstructures and de-/hydrogenation behaviors are systematically studied.The catalytic effect of phases and microstructures on de-/hydrogenation process and the mechanism for the improvement of hydrogen storage properties are revealed in-depth.To understand the effects of primary dendrites and eutectic structures on the better de-/hydrogenation properties of Mg-Ni alloy,hypo-eutectic Mg₉₂Ni₈ alloy and hyper-eutectic Mg₈₅Ni₁₅ alloy are prepared first,their primary phases are Mg and Mg₂Ni,respectively.The Mg₈₅Ni₁₅ alloy shows a preferable activation performance and faster absorption and desorption rates.The hydrogen storage capacity Mg₉₂Ni₈ alloy is higher.The intersections of Mg-Mg₂Ni eutectic boundaries with alloy surface provide active sites for the dissociation of H₂ molecules and excellent paths for the permeation of H atoms.The apparent activation energies for the desorption of Mg₉₂Ni₈ and Mg₈₅Ni₁₅ hydrides are calculated to be 134.67 and 88.34 k J/mol H₂ by Kissinger method,respectively.Synergic desorption occurs in eutectic Mg H₂-Mg₂Ni H₄,and it facilitates the primary Mg H₂ in Mg₉₂Ni₈ hydride to decompose at a lower temperature.The reason why the Mg₈₅Ni₁₅hydride shows faster dehydrogenation kinetics is that its primary phase is Mg₂Ni.To reveal the effects of eutectic structure on the hydrogen storage properties of Mg-Ni-based alloy,Cu is used to substitute Ni in hypo-eutectic Mg-Ni alloy,and the Mg₉₁Ni₆Cu₃ alloy is melted.At the same time,Mg₉₁Ni₉ and Mg₈₈Cu₁₂ alloys are melted for comparation,with nearly the same proportion of primary Mg.With the partial substitution of Cu for Ni,tunable eutectic structure is obtained,and the Mg₉₁Ni₆Cu₃ alloy shows a better activation property and faster absorption rates without any sacrifice of its desorption rates.Its maximum hydrogen storage capacity at 300℃ under 3 MPa H₂ is6.5 wt.%.The dehydrogenation activation energy of the Mg₉₁Ni₆Cu₃ hydride is reduced to 67.34 k J/mol H₂,its onset and peak desorption temperatures also reduce significantly.The diffusion of H atoms is facilitated by the connected Mg₂Ni eutectic and its high-density interfaces.The“hydrogen pump”effect of Mg₂Ni H₄ and the increased dehydrogenation driving force collectively accelerate the hydrogen desorption process.The isothermal hydrogenation curves of the Mg₉₁Ni₆Cu₃ alloy under various conditions are fitted using Johnson-Mehl-Avrami equation,and the dependence of hydrogenation behavior on reaction condition is revealed,which is different from that of pure Mg.Due to the same role of increasedΔP and decreasedΔT on hydride nucleation and their opposite impacts on hydrogen diffusion,reaction temperature and hydrogen pressure affect the absorption process in different mechanism.When the hydrogen pressure reaches a critical value,a three-stage absorption behavior is determined.The diffusion of H atoms in eutectic region is promoted,so more H atoms are absorbed under larger hydrogenation driving force.To reveal the effect of Cu on the hydrogen storage properties of hypo-eutectic Mg-Ni-Cu alloy,the Mg₉₁Ni_9-xCu_x（x=3,4.5,and 6）alloys are prepared.The excessive Cu substitution gradually declines the activation performance and the de-/hydrogenation kinetic property of Mg₉₁Ni_9-xCu_x alloy at moderate temperatures.Cu atoms can replace the Ni atoms in Mg₂Ni lattices,and leading to the increase of lattice constants.The Mg₂Ni/Cu structure can be preserved after hydrogen absorption and desorption,which is in favor of H atoms diffusion.Assisted by the“synergetic effect”for absorption,the nucleation of hydrides in Mg₉₁Ni₆Cu₃ alloy is facilitated,and a higher hydrogen capacity of 5.60 wt.%with the fast kinetics is shown at 175°C under 1 MPa hydrogen pressure.The Mg₉₁Ni₆Cu₃ hydride shows the fastest dehydrogenation kinetics at 225°C,which is attributed to the increased dehydrogenation driving force and the“synergetic effect”.The independent hydrogenation process of Mg₂Ni/Cu phase in Mg₉₁Ni_4.5Cu_4.5 and Mg₉₁Ni₃Cu₆ alloys results in their reduced hydrogen storage capacities.Rare-earth element La can form Mg-La Mg₁₂ eutectic with Mg,and the eutectic point is Mg-2.3 at.%La,which is helpful to optimize the hydrogen storage properties of the hypo-eutectic Mg-Ni alloy with higher Mg content.For this reason,La is used to substitute Ni in Mg₉₈Ni₂ alloy.The role of La on the solidification microstructure and de-/hydrogenation behaviors of the Mg₉₈Ni_2-xLa_x（x=0,0.33,0.67,and 1）alloys are revealed.The Mg₉₈Ni_1.67La_0.33 alloy shows the highest isothermal absorption rate,and the Mg₉₈Ni_1.33La_0.67 hydride shows the highest isothermal desorption rates and lowest desorption temperature of 327°C.The La substitution increases the equilibrium pressure of de-/hydrogenation,thus improving the absorption rates and capacities of alloys.The refined eutectic structure is the key factor that facilitates the desorption of Mg₉₈Ni_1.33La_0.67 and Mg₉₈Ni₁La₁ hydrides.With the further substitution of La,the amount of La₂Mg₁₇ blocks increases,and the hydrogen storage properties of alloys gradually deteriorate.In the hydrogen-induced decomposition of Mg₉₈Ni_2-xLa_x alloys,an Mg-Mg₂Ni-La H_x nanocomposite is in-situ formed from the Mg₉₈Ni_1.67La_0.33 alloy.Owing to the ultrahigh hydrogenation capacity in Stage I,the nanocomposite shows a hydrogen capacity of 7.19 wt.%at 325°C under 3 MPa H₂,and a hydrogen capacity of 5.59 wt.%at 175°C under 1 MPa H₂.Its apparent activation energies for absorption and desorption are respectively determined to be 57.99 and 107.26 k J/mol H₂.The superior kinetic properties are owing to the microstructure of La H_x and Mg₂Ni nanocrystals embedding in eutectic region with tubular nanostructure around.The nano-sized La H_2.49 can catalyze H₂ molecules to dissociate and H atoms to permeate due to its stronger affinity with hydrogen.The interfaces of these nanophases provide preferential nucleation sites,and assist the nanotube structure to alleviate the“blocking effect”.As a result,the diffusion paths for H atoms are retained after the impingement of Mg H₂ colonies,and a superior hydrogenation property is achieved.To clarify the relation between the primary Mg morphology and the hydrogen storage properties of hypo-eutectic Mg-Ni alloy,ultrasonic vibration is used to refine the phases in Mg₉₈Ni₂ alloy.The primary Mg is spheroidized,and the hydrogenation capacity and isothermal de-/hydrogenation rates improve,especially at lower temperatures.Due to the increased eutectic boundaries and the shortened diffusion distance of H atoms in a single grain,the peak desorption temperature of Mg₉₈Ni₂（UST）hydride reduces from392.99 to 345.56°C,and the apparent activation energy for desorption decreases from101.93 to 88.65 k J/mol.The hydrogen pressure hysteresis between absorption and desorption plateaus is reduced,and a better thermodynamic property is determined.Since the primary Mg dendrites are refined,a higher proportion of hydrides is formed in Stage I,and a higher hydrogen capacity is reached.