Thermodynamic Destabilization And Cycling Performance Of Mg-Based Multicomponent Hydrogen Storage Alloy Systems

Magnesium is a promising hydrogen storage material with high hydrogen capacity?theoretically 7.6 wt.%for MgH₂?and abundant resource.However,the sluggish hydrogen sorption kinetics and high thermodynamic stability of MgH₂ result in an excessively high dehydrogenation temperature?up to 300°C?,which hinders its practical application.Substantial progress has been achieved in improving the hydriding/dehydriding kinetics of Mg-based alloys through nanostructuring,catalyzing,and forming composites.However,destabilizing the thermodynamics is still a great challenge.At present,the main strategies to alter the thermodynamics of Mg/MgH₂ include alloying,nanostructuring,and changing the reaction pathway.Although thermodynamic destabilization has been achieved,it is still far from that required for practical application.Based on the previous work,present research focus on the design of new reversible reaction path for Mg-based alloys with high hydrogen capacity in order to find a potential route for destabilizing the thermodynamics of Mg-based hydrogen storage alloys.The reversible structural transformation and destabilized thermodynamics were realized in the Mg?In?binary solid solution.To further improve its themodynamics and reduce the amount of rare metal In,on the basis of Mg?In?binary solid solution,the Mg?In,Cd?ternary solid solutions and Mg-In-Ni ternary alloys were prepared by sintering and subsequent mechanical milling.The hydrogenated products of Mg?In,Cd?ternary solid solution consisted of MgH₂,MgIn and Mg₃Cd,which could fully transform back to Mg?In,Cd?ternary solid solution after dehydrogenation,which is different from the reaction pathways of pure Mg and Mg-In binary alloys.The maximum hydrogen storage capacities of Mg₉₀In₅Cd₅ and Mg₈₈In₅Cd₇ are 4.3 and 3.2 wt.%,respectively.Compared with pure Mg,the equilibrium pressures for Mg₉₀In₅Cd₅?0.18 MPa?and Mg₈₈In₅Cd₇?0.21 MPa?at 300°C are elevated.Moreover,the Mg₉₀In₅Cd₅ alloy also exhibits enhanced hydrogenation kinetics with decreased hydrogenation activation energy of 61.0 kJ/mol,but the dehydrogenation rate is slow due to the long-range diffusion of In and Cd in MgH₂.Two new ternary intermetallic compounds,Mg₁₄In₃Ni₃ and Mg₂InNi,were discovered for the first time.Both Mg₁₄In₃Ni₃ and Mg₂InNi have orthorhombic crystal structures,as determined by electron diffraction tomography together with X-ray diffraction analysis.In the hydriding process,the Mg₁₄In₃Ni₃ alloy decomposes into a mixture of MgH₂ and Mg₂InNi,which is fully reversible in the dehydrogenation with hydrogen storage capacity of 1.8 wt.%.The reversible formation of these two new Mg-In-Ni ternary phases destabilizes the thermodynamics and improves the kinetics of MgH₂.The dehydriding enthalpy for Mg₁₄In₃Ni₃ was 70.1 kJ/mol H₂,and the dehydrogenation temperature lowered to 230°C.The Mg₁₈In₁Ni₃ alloy has reversible hydrogen storage capacity of 3.8 wt.%,which can release 2.0 wt.%H₂ at 220°C within 120 min.To get rid of expensive In,Mg–Ag–Al and Mg–Ag–Zn ternary alloys were designed and investigated.Mg₈₀Ag₁₅Al₅ alloy exhibits a reaction pathway differs from that in pure Mg,in which the intermediate phase,being a new ternary solid solution MgAg?Al?,reacts with MgH₂during dehydrogenation and contributes to an increase in the dehydriding equilibrium pressure?0.22 MPa at 300°C?and to a reversible hydrogen storage capacity of 1.7 wt.%.Adjusting the composition to Mg₈₅Ag₅Al₁₀ results in a reversible hydrogen storage capacity of approximately3.8 wt.%and an elevated equilibrium pressure?0.26 MPa at 300°C?in the first step of dehydrogenation.These Mg–Ag–Al ternary alloys also show enhanced hydrogen sorption kinetics in comparison to that of Mg,and the apparent activation energies for hydrogenation and dehydrogenation of the Mg₈₅Ag₅Al₁₀ sample are lowered to 74.5 and 124.8 kJ/mol,respectively.Mg₉₀Ag_7.5Zn_2.5 alloy exhibits a maximum hydrogen storage capacity of approximately 4.2 wt.%and two-step dehydrogenation.In the first step of dehydrogenation,a portion of MgH₂ reacts with MgAg and MgZn_1.8Ag_0.2 to transform to Mg₅₄?Ag,Zn?₁₇ solid solution and release hydrogen.After that,the remaining MgH₂ decomposes to form Mg.Because the first step dehydriding pathway is changed,the dehydriding equilibrium pressure of Mg₉₀Ag_7.5Zn_2.5 at 300°C is increased to 0.28 MPa.Besides,the apparent activation energy for dehydrogenation of the Mg₉₀Ag_7.5Zn_2.5 alloy lowered to 118.7 kJ/mol.Mg₇₈Ag_16.5Zn_5.5 alloy exhibits a single-step dehydriding reaction which is the same as that of Mg₉₀Ag_7.5Zn_2.5 in the high pressure region.The maximum hydrogen storage capacity of Mg₇₈Ag_16.5Zn_5.5 is 2.5 wt.%,and the activation energy for dehydrogenation is 116.5 kJ/mol.Combining the results of past and present,two guidelines were proposed for destabilizing dehydriding thermodynamics of Mg-based hydrogen storage alloys by reversibly forming intemetallics:The alloying element could react with Mg to form intermetallic compound,but the reaction enthalpy should be low.The hydride formation enthalpy of the alloying element also should be low.To increase the hydrogen uptake capacity of Mg-3 at.%TiMn₂ composite at room temperature,the hydrogen pressure was decreasd to 0.1 bar,making the material absorb 5.2 wt.%H₂ within 1000 min,higher than those using 1 and 10 bar hydrogen pressures.Because the small grain sizes of MgH₂ and Mg and particle size of TiMn₂ catalysts can be remained,and the distribution of TiMn₂ in the Mg/MgH₂ matrix is uniform unde low hydrogen pressure.A degradation was observed for the temperature oscillation cycling?release H₂ at 300°C and absorb H₂ at 150°C?of the Mg-5 at.%TiMn₂ composite due to the agglomeration and oxidation of catalysts.The hydrogen capacity decreased from 2.5 to 1.2 wt.%after 500 cycles.