Hydrogen Production Behavior And Hydrolysis Mechanism Of Co-Modified Magnesium-Rich Alloys

Limited by green portable hydrogen production technology,hydrogen energy has not been widely used.Large-scale production of hydrogen has become the primary problem in the development of hydrogen energy.To find a fast,efficient and convenient hydrogen production method is the key to solve the problem of hydrogen production.In this thesis,the Mg-rich alloy is taken as the object,and the strategy of ’collaborative modification’ is proposed,that is,the’internal fine’ and ’surface active’ control of the alloy is achieved through multi-element microalloying and surface modification,so as to achieve rapid rate and high capacity of hydrolytic hydrogen production.Multi-element microalloying regulates the solid solubility and the second phase of Mg phase,regulates the electrochemical hydrolysis process,and achieves controlled hydrogen production.By activating the alloy surface with active metal or realizing surface hydrogenation through mechanochemical reaction,the surface characteristics of the Mg-rich alloy can be significantly improved,increasing the activity,accelerating the mass transfer,and laying a foundation for the rapid hydrolytic hydrogen production at room temperature.The effects of rare earth elements addition and surface modification on microstructure evolution and hydrolytic hydrogen production performance of Mg-rich alloys were investigated,the structure-effect relationship between internal/surface microstructure and H₂ production performance was established,and the hydrolytic mechanism and activated H₂ production mechanism of surface activated/mechanochemical reaction microalloying Mg-rich alloys were revealed,which laid a foundation for the design and modification of new Mg-based materials with high H₂ production yield.It provides the possibility for the development of portable hydrolytic H₂ generator.（1）The hydrolytic hydrogen production behavior of Mg10Ni,Mg9Ni10La and Mg9Ni10Ce shows that ’matrix alloying’ is an effective means to improve the hydrolytic hydrogen production performance.The hydrogen production capacities of Mg10Ni,Mg9Ni10La and Mg9Ni10Ce reach 532,511 and 444 mL/g in NaCl solution at 293 K and 2.5 h,and the corresponding hydrogen production yields are 0.63,0.68 and 0.59,and the hydrogen production yields can be increased to 0.88,0.99 and 0.91 at 308 K hydrolysis.When Ni/La/Ce elements are added,active mesophase Mg₂Ni,Mg₁₂Ce or Mg₁₇La₂ can be introduced to increase the phase boundary and form solid solution phases,and improve the microstructure of metallic Mg.The hydrogen production process of metallic Mg is changed from pure chemical hydrolysis to electrochemical hydrolysis coupled with chemical hydrolysis,which accelerated the hydrogen production rate and increased the hydrogen production capacity.The mechanical alloyed Mgrich alloys powder shows better hydrogen production performance due to the fine microstructure,the formation of active mesophase and the introduction of defects.Although the initial hydrolysis rate of Mg-Al Mg-rich alloy is significantly accelerated,the hydrogen production capacity is lower than that of short-time mechanical alloying samples.Long mechanical alloying time,significantly fine sample structure,large specific surface area and high surface activity bring significant advantages in initial hydrogen production rate,but the byproducts are easy to form colloidal film coating by fast nucleation and growth,which affects subsequent mass transfer,and it is difficult to achieve fast and high hydrolytic hydrogen production effect in the short term.（2）Activation of AM50（Mg-Al-Mn）alloy waste on the surface of the intermediate alloy with high active metal content can significantly strengthen the surface electrochemical hydrolysis process and achieve rapid and high-capacity hydrolysis hydrogen production.Mg₂5Ni can effectively regulate the initial hydrolysis kinetics of AM50 alloy.The AM50Mg₂5Ni sample activated by adding 10 wt.%Mg₂5Ni can rapidly produce 500.1 mL/g H₂ at 298 K and 5 min,which is much higher than that of unactivated AM50 alloy（213.9 mL/g H₂）.The AM50-Mg₂5Ni-Mg30Ce sample prepared by the co-activation of AM50 alloy by Mg₂5Ni and Mg30Ce has superior comprehensive hydrolytic properties（initial hydrolysis kinetics and final hydrogen production capacity）.The hydrogen production capacity of AM50-Mg₂5NiMg30Ce in the initial 5 min and 30 min is as high as 427.2 mL/g and 776.5 mL/g H₂,which is much higher than that of unactivated modified AM50 alloy（213.9 and 715.7 mL/g H₂）.The intermediate alloys Mg₂5Ni and Mg30Ce have significant influence on the hydrogen generation thermodynamics of AM50 alloy waste.The apparent activation energy Ea of coactivated AM50-Mg₂5Ni-Mg30Ce is 37.1kJ/mol,which is lower than that of unactivated AM50 alloy（52.0kJ/mol）.Due to the introduction of electrochemical hydrolysis reaction in the activated sample,the nucleation rate and growth rate of AM50-Mg₂5Ni-Mg30Ce hydrogen production byproduct Mg（OH）₂ are effectively coordinated,so that the nucleation,growth and collision process of AM50-Mg₂5Ni-Mg30Ce are orderly and reasonable.The low rate of initial hydrolytic hydrogen production caused by low initial nucleation rate or the low yield caused by difficult mass transfer in the middle and late stages of hydrolytic hydrogen production are avoided.（3）The mechanochemical reaction（MCR）strategy can achieve multi-stage regulation of composition optimization,microstructure regulation and surface modification,and has been proved to be an efficient strategy to activate Mg-based alloy,improve the hydrolytic hydrogen production performance of Mg-based alloy and improve the environmental stability of the system.The hydrolytic hydrogen production performance of Mg₂5Ni,Mg30Ce and Mg30La after environmental exposure is improved,and the final hydrogen production could be increased to 186.1,204.8 and 341.4 mL/g,with the fastest hydrogen production rates being 26.0,24.2 and 41.5 mL/g/min.Compared with Mg₂5Ni and Mg30Ce exposed samples,Mg30La exhibited faster hydrolysis rate and higher hydrogen production capacity after environmental exposure.Mechanochemical reactions of Mg30La exposed samples at 298,308,and 318 K yielded 341,425,and 629 mL/g H₂.The apparent activation energies of hydrolytic hydrogen production of Mg₂5Ni,Mg30Ce and Mg30La were 26.6,30.8 and 23.3 kJ/mol,respectively.The morphology,size,surface defects and hydrogenation degree of magnesium particles in the mechanochemical reaction affect the contact specific surface area,the number and distance of transport channels and the chemical activity of the sample during the hydrogen production process,which will change the initial hydrolysis kinetics and hydrogen production capacity.There is a hydrolysis ’incubation period’ in the single hydrogen filling mechanochemical reaction,but there is no ’incubation period’ in the repeated hydrogen-filling mechanochemical reaction,and the initial hydrolysis response is very fast.The internal microstructure refinement and surface modification of magnesium alloy,moderate activity,suitable particle size,surface defects and reasonable mass transfer channels are the reasons for the rapid hydrolysis kinetics and high hydrogen production yield of Mg-GO composites after 8 h mechanochemical reaction.（4）In order to promote the ’green’ hydrolytic hydrogen production of Mg-based alloys,the hydrolytic hydrogen production behavior of modified Mg alloy was studied in sea salt solution,different monovalent cation solutions,different divalent cation solutions,regulated sea salt solutions and other media.After sea salt solution with a concentration of 3.5 wt.%was selected as the hydrogen-producing seawater solution and the regulation base solution,and halogen anions F^-,Br^-and Cl^-with the same concentration were added,the hydrogen production performance of MG-based alloy deteriorated to different degrees.The inhibition effect of F^-ion is the strongest,followed by Br^-ion,and Cl^-ion is the weakest.The initial kinetic properties of hydrogen production in different hydrolytic media are:NaCl>KCl>NaCl^-KCl;Final hydrolytic hydrogen production capacity:KCl>NaCl>NaCl^-KCl;Temperature fluctuation degree:KCl>NaCl>NaCl^-KCl.The increase of K⁺ion concentration in seawater is more unfavorable to the hydrolysis of as-cast Mg-base alloy for hydrogen production than that of Na⁺ion concentration.After the introduction of CaCl₂,the hydrogen production performance of ascast Mg-rich alloys deteriorates seriously,the degradation degree of MgCl₂ is the least,and the degradation degree of MgCl₂ is in the middle.It can be seen that the hydrogen production performance of Mg-rich alloys will deteriorate to varying degrees with increasing the concentration of monovalent or bivalent cations in seawater.