An Investigation On The Synthesis, Microstructure And Hydrogen Storage Properties Of Nano/Amorphous Mg-based Composites And Complex Alanate Hydrides

Based on the review of the research and development of Mg-based hydrogen storage materials and NaAlH₄ complex hydrides,the Mg-based amorphous composites and alanate complex hydrides were selected as the objects of the study in this thesis for developing the novel materials with high hydrogen capacities.First,the Mg-Fe-Ni based and Mg-Al-Ni based amorphous composites and the nanocrystal/amorphous Mg₁₇Al₁₂hydrogen storage alloy were prepared by mechanical ball-milling.By means of XRD,SEM/EDS,TEM/EDS, XPS analyses and the electrochemical test methods including the galvanostatic charge-discharge,EIS,linear polarization,anodic polarization and potentialstatic discharge etc.and the gaseous hydrogen storage properties test methods,the relationship among the compositions,microstructures and(electrochemical)hydrogen storage properties of the above materials was systematically investigated.The electrochemical hydrogen storage mechanism of Mg-based amorphous composites was represented.Whereafter,the Ti-doped,Zr-doped and Ti-Zr-doped NaAlH₄ complex hydrides were prepared by hydrogenation of the ball-milled (NaH/Al+Ti),(NaH/Al+Zr)and(NaH/Al+Ti-Zr)composites.Their microstructures, hydrogen storage properties and catalytic mechanism were systematically investigated by means of XRD,SEM/EDS,TG/DSC analyses and the gaseous hydrogen storage properties test methods.Finally,the feasibility and technique for TiF₃-doped NaAlH₄ prepared directly by reactive ball-milling of(NaH/Al+4mol.%TiF₃)composites at ambient temperature and under moderate hydrogen pressure were investigated.The study on the microstructure and electrochemical properties of Mg-Fe-Ni amorphous composites revealed that 2Mg-Fe ball-milled with certain amount(50～200 wt.%)of Ni can charge/discharge reversibly at room temperature.For the Mg-Fe-Ni composites milled under argon,with the Ni amount increasing,the amorphization degree increases accordingly and its particle size increases first and then decreases.The increase of amorphization degree and the decrease of particle size are both advantageous for the improvement of the discharge capacity of the composite.However,the cycling stability increases with increasing the amorphization degree,but decreases with decreasing the particle size.The 2Mg-Fe+150 wt.%Ni composite exhibits the best electrochemical property,its maximum discharge capacity(C_max)is 427.8 mAh/g,and its capacity retention after 20 cycles(S₂₀)is 67.6%.The high rate dischargeability (HRD)at the discharge current density 1000 mA/g(HRD₁₀₀₀)is 52.5%.For the y Mg-Fe+ 100 wt.%Ni composites(y=1.5,2,2.5,3),the amorphization degree and discharge capacity of the composite increase with the increase of Mg content too.The composites modified by THF show the higher degree of amorphorization,smaller particle size and bigger specific surface area,thus have higher C_maxand HRD but lower capacity retention than those milled under argon. The microstructure and electrochemical properties of the ball-milled Mg-Al-Ni amorphous composites were systematically studied.The addition of Ni powder is advantageous for the formation of Mg-Al-Ni amorphous structure and the decrease of particle size,thus improving the electrochemical properties of the composites.The Mg₁₇Al₁₂+200 wt.%Ni composite exhibits the best electrochemical property,its C_maxis 658.2 mAh/g,S₁₀is 50.7%,HRD₁₀₀₀is 77.2%.Moreover,the composites milled with mixed balls have higher amorphorization degree and smaller particle sizes than those milling with the uniform balls, which resulting in the better discharge capacity,capacity retention and high rate dischargeability.Combined with the microstructure and electrochemical properties of the Mg₁₇Al₁₂+200 wt.%Ni composite,it is found that the milled Mg-Al-Ni composite is composed by amorphous Mg-Ni and nanocrystal AlNi alloy.The AlNi nano particles dispersed around the amorphous Mg-Ni base.For the Mg-Al-Ni composite,the discharge capacity is attributed to amorphization degree of Mg-Ni,and the capacity retention is attributed to the nanocrystal AlNi alloy.The investigation on the microstructure and hydrogen storage properties of nanocrystal/amorphous Mg-Al composite system shows that the nanocrystal/amorphous Mg₁₇Al₁₂alloy can be prepared by changing the milling time(t).The nanocrystal n-Mg₁₇Al₁₂ alloy can be formed as t=70 h,and the amorphous a-Mg₁₇Al₁₂alloy can be formed as t=100 h. The ball-milled nanocrystal/amorphous Mg₁₇Al₁₂alloy exhibits the well activate property. The hydriding capacity of the Mg₁₇Al₁₂alloy ball-milled for 70 h and 100 h are 4.27 wt.% and 4.18 wt.%at 350℃,respectively,while the maximum hydrogen storage capacity of the Mg₁₇Al₁₂as-cast alloy is just 2.85 wt.%.The n-Mg₁₇Al₁₂alloy(ball-milled for 70 h)shows the highest dehydriding capacity of 4.19 wt.%.In addition,the a-Mg₁₇Al₁₂+x wt.%TiH₂ (x=5,10)composite system exhibits the better hydriding rate and capacity than those of a-Mg₁₇Al₁₂alloy alone.The hydriding capacity of the composite system increases from 3.51 wt.%(x=5)to 3.73 wt.%(x=10)at 200℃.The a-Mg₁₇Al₁₂alloy is contributing to hydrogen storage,and the TiH₂ addition is contributing to catalytic function in this system.The study on the microstructure and hydrogen storage properties of the Ti-NaAlH₄ composite(Ti-doped NaH/Al)show that the composite prepared under hydrogen has superior hydrding/dehydriding behaviors than those prepared under argon or in THF.The reversible hydrogen storage properties of the NaH/Al+x mol.%Ti(x=0,4,6,10)composites improve with increasing Ti content,its hydriding capacity increases from 3.42 wt.%(x=4)to 4.02 wt.%(x=10)at 120℃.Accordingly,its dehydriding capacity increases from 3.27 wt.%(x=4) to 3.93 wt.%(x=10)at 150℃.The structural analysis reveals that,the activation energy of hydrding/dehydriding reactions can be reduced by Ti-doped ball-milling method.The more Ti content introducing,the better catalytic activity performs in the composite.In the ball-milling process under hydrogen,some of metallic Ti directly reacts with H₂ to form TiH_1.924hydride. After ball-milling process,the residual Ti reacts with Al to form TiAl alloy during the subsequent hydriding/dehydriding process.The nature constituents of the catalysis in hydrogen storage behavior of NaAlH₄ system are TiH_1.924and TiAl active species.The catalytic mechanism for hydrogen storage behavior of Ti-NaAlH₄ is attributed to the presence of active small TiH_1.924and TiAl particles,which are scattered on the surface of much larger NaAlH₄(NaH/Al)globelets,generating more interfaces and defects to accelerate the diffusion of hydrogen,and acting as the catalytic active sites for the complex hydride and playing an important catalytic role in the hydriding/dehydriding process.In order to improve the hydrogen storage properties further,the catalytic and synergistic effects of metallic Ti and Zr as co-dopants on the microstructure and reversible hydrding/dehydriding properties of NaAlH₄ were systematically investigated.The catalysis of metal Ti is better than Zr as a dopant alone,but the proper proportion of Ti and Zr together as co-dopants on hydriding/dehydriding properties is superior to Ti or Zr alone,thus would generate a good synergistic effect of Ti and Zr.The composite doped with proper proportion of Ti and Zr together(8 mol.%Ti+2 mol.%Zr)as co-dopants exhibits the highest hydriding/dehydriding rate and capacity,its hydriding and dehydriding capacities are 4.61 wt.%and 4.34 wt.%,respectively.The hydrided(NaH/Al+Ti-Zr)composite has the lowest activation energy,which can release hydrogen effectively at lower temperature.Moreover,the synchronous lattice and crystal cell expansion could be found in the Ti-Zr co-doped NaAlH₄, and then its enthalpy change decrease accordingly,thus can reduce the stability of hydride and further decrease the hydriding temperature of the system.The catalytic mechanism of Ti-Zr co-doped NaH/Al(NaAlH₄)composite could be ascribed to both the "superficial catalytic process" with the active titanium-hydride and zirconium-hydrides acting as the catalytic active sites on the surface of hydride matrix,and to the "favorable thermodynamic modification" due to the decrease of enthalpy change caused by lattice expansion of the bulk composite hydride.Based on the above results,the NaAlH₄ complex hydride was synthesized by the reactive ball-milling technology using NaH and Al as raw materials and TiF₃ as a dopant,under moderate hydrogen pressure and at room temperature.It is found that the phase structures and hydrogen storage properties of the synthetic product(NaAlH₄)are affected greatly by the ball-milling processing parameter,such as rotational speed,ball-to-powder ratio,milling temperature,hydrogen pressure and milling time etc.The productive rate of NaAlH₄ can be improved by increasing the rotational speed and ball-to-powder ratio,and by decreasing the milling temperature.Further modify the hydrogen pressure and milling time in reactive ball-milling process,the productive rate of NaAlH₄ is increasing more than 75%under 3 MPa hydrogen pressure at room temperature.The NaAlH₄ synthesized by the reactive ball-milling exhibits the excellent reversible hydrding/dehydriding properties,its hydrding and dehydriding capacities are 4.61 wt.%(at 120℃)and 4.34 wt.%(at 150℃),respectively.The synthesis mechanism of NaAlH₄ synthesized in the reactive ball-milling process can be explained by mechanochemistry theory reasonedly.