The Study On Catalytic Modification Mechanism Of RE-Mg-based Hydrogen Storage Alloy

The magnesium-based alloys have outstanding advantages,including high ability to store hydrogen,light weight,low price,abundant raw material resources,high reversibility,and stabilized hydride.The Mg-based alloys are believed that the most potential in materials for hydrogen storage.However,the hydrogen storage properties of Mg-based materials are far from satisfactory,attributed to their sluggish kinetics and unfavorable thermodynamics.Therefore,it is very meaningful to adopt suitable methods to improve the hydrogen storage performance of Mg-based hydrogen storage materials.This paper reviews the current status of research on Mg-based hydrogen storage materials and the mechanism of hydrogen storage.A series of studies have been conducted on Mg₉₀Ce₅Y₅ alloy as the base and doping catalyst as the main method,respectively.The mechanism of catalytic modification is explored in depth with the aim of obtaining a more efficient way of improvement.The Mg₉₀Ce₅Y₅+5.0 wt.%MoS₂/MoO₂ composites were produced by the vacuum refining technology.The X-ray diffraction,Transmission Electron Microscopy,Electron Diffraction and Inductively Coupled Plasma Optical Emission Spectrometer were utilized to analyze the phase-composition and microstructure of the specimens.The performance of the hydrogen storage was analyzed by means of a Thermogravimetric and Differential Thermal Analysis curves and a semi-automatic Sievert-type device.The results indicate that the catalytic Mo S₂ as a single phase uniformly distributed in the matrix.However,the doping of Mo O₂ introduces O^2-ions,which combine with Mg,Ce to form Mg O and Ce O₂ phases.The Mo S₂ catalyzed alloy exhibits faster hydrogen absorption and desorption rates,which can absorb 4.5 wt.%H₂ in 120 s（573 K）and release3 wt.%H₂ in 330 s（633 K）.The initial dehydrogenation temperatures of the Mo S₂and MoO₂ catalyzed alloys were 467.7 K and 475.7 K,respectively.The dehydrogenation activation energy of Mo S₂ and Mo O₂ catalyzed alloys were evaluated to be 103.7 k J mol^-1 and 110.2 k J mol^-1 respectively.The addition of Mo O₂ and Mo S₂ modified the decomposition enthalpy and entropy values,but these changes were very limited.Generally,the S^2-is superior to O^2-ions in enhancing the hydrogen storage properties of the alloy.The catalysts Mo,MoO₂ and MoO₃ were incorporated into the Mg₉₀Ce₅Y₅alloy powder by utilizing ball milling equipment.Their microstructure,phase evolution,as well as hydrogen storage thermodynamic and kinetic properties were investigated.The result shows that the internal phases of the Mo catalyzed alloy become Mg H₂,Ce H_2.73,YH₃ and Mo phases after hydrogen absorption,while the hydrogenated alloy consists of Mg H₂,Ce O₂,YH₃,Mg O and Mo phases for the Mo O₂ catalyzed alloy and Mg H₂,Mg O,Ce O₂,Y₂O₃ and Mo phases for the Mo O₃catalyzed alloy.REDOX reaction occurred between Mo O₂,Mo O₃ and the inner phase of the alloy,respectively.In comparison,Mo catalyzed alloy has a higher hydrogen absorption saturation ratio,but the Mo O₂ catalyzed alloy needs a shorter time than Mo and Mo O₃ catalyzed alloys to release H₂,and the dehydrogenation activation energy（E_a）of the samples show an increasing trend in the following order:110.18 kJ mol^-1（Mo O₂）（27）113.72 k J mol^-1（Mo O₃）（27）114.24 k J mol^-1（Mo）.The initial dehydrogenation temperature and peak temperature of the Mo O₂catalyzed alloy are the lowest,which means it has the best catalytic effect on the dehydrogenation kinetics of the alloy.The Mg₉₀Ce₅Y₅+x wt.%ZnF₂（x=3,6,10,15）composites were successfully prepared by mechanical ball-milling.The effects of Zn F₂ contents on the hydrogen absorption and desorption properties of Mg₉₀Ce₅Y₅ alloy were analyzed.The excellent hydriding-dehydriding characteristics of the samples after activation were attributed to the catalytic effect of the Mg F₂ and Mg Zn₂ phases introduced in situ by the addition of Zn F₂.The Mg₉₀Ce₅Y₅+6 wt.%Zn F₂composites exhibit the fastest hydrogen adsorption/desorption kinetics and lower decomposition temperatures.At 633 K,4.12 wt.%H₂ can be absorbed within 180s,and completely dehydrogenated within 740 s.In addition,the activation energy of dehydrogenation is reduced to 89.5 k J mol^-1,the optimal dehydrogenation temperature is reduced to 591 K.It is due to the generation of in situ catalysts that weaken the Mg-H bond energy,refine the grain sizes,expand specific surface area and increase hydrogen diffusion paths.