Preparation And Application Of Iron-Based Metal-Organic Framework Composites For Electrocatalytic Water Splitting

Social progress drives economic development,economic development drives energy consumption,and energy consumption leads to serious ecological problems.To solve these problems,researchers are investing more and more energy in efficient and clean energy development.Among them,the production of hydrogen and oxygen by electrolysis of water is one of the focuses of research.As a green,renewable and clean energy,hydrogen energy is expected to become a new energy carrier to replace fossil fuels.As the most efficient catalysts for hydrogen and oxygen production,Pt and Ru O₂/Ir O₂ are less abundant in the earth and are severely limited in practical applications.In addition,the water resource commonly used in electrolysis water is pure water,which will limit large-scale production and utilization to a certain extent.As the water resource with the largest reserves on the earth,seawater can become the mainstream electrolysis resource.On the one hand,it can not only desalinate seawater,on the other hand.Abundant hydrogen resources can be obtained.Based on the above problems,the development of low-cost,high-efficiency catalysts for water electrolysis has become the main contradiction.Metal-organic frameworks（MOFs）are new types of catalysts with a certain spatial structure formed by metal sources or metal clusters and organic ligands through coordination bonding,which is characterized by their ordered and diverse structure and adjustable pore size,making them suitable for many applications.The unique spatial structure and excellent water stability of iron-based organic frameworks have been recognized by researchers,but their poor electrical conductivity and poor intrinsic activity of the active sites in electrolytic water applications have seriously hampered their use in practice.Based on the above analysis,this thesis uses iron-based metal-organic frameworks as the main body and uses different approaches to investigate their electronic structure,crystalline surface structure,synergistic effects and other factors to apply them to the study of electrolytic water under alkaline electrolyte conditions.The study includes the following main aspects:（1）Bimetallic electrocatalysts with different Zn/Fe molar ratios were prepared by hydrothermal methods using anhydrous zinc chloride（Zn Cl₂）,ferric chloride hexahydrate（Fe Cl₃·6H₂O）as the metal source,and 2-aminoterephthalic acid（NH₂-BDC）as the organic ligand.The physical characterization revealed the morphology,electronic structure and elemental distribution of the materials.Electrochemical tests showed that the best electrocatalytic hydrolysis performance was achieved for Zn:Fe=0.75 in alkaline and alkaline seawater environments,with OER overpotentials of 295 m V and 293 m V at a current density of 100 m A cm^-2,respectively,much better than the Fe-based MOF itself.The results indicate that the modulation of the electronic structure of the Fe-based MOF by Zn accelerates the efficiency of electron transfer during the catalytic reaction while reducing the adsorption energy to the intermediate,thus promoting the efficiency of water electrolysis.（2）In this part,replace the Zn ions with two divalent main（sub）group metal ions(Mg²⁺and Cd²⁺),and use the hydrothermal synthesis method to grow M（Mg,Zn,Cd）Fe bimetallic MOF electrode materials on nickel foam,to explore the regulation of divalent metals with different ionic radii on the performance of catalysts for water electrolysis.The results show that the adjustment of the electronic structure of the iron-based MOF by Cd is more conducive to the OER,HER,and overall water-splitting reactions,in which the voltage of the water splitting is 1.628 V@10 m A cm^-2.In addition,we analyzed the energy change and the mechanism of action during the electrolysis of water through theoretical calculations.The Gibbs free energies of the reaction between OER and HER are 1.59 e V and 1.17 e V,which are much lower than those of other MOF electrode materials.No matter in the OER reaction,HER reaction,or water-splitting reaction,the Cd Fe bimetallic MOF electrode material can maintain stability for more than 12 h.（3）In this part,based on the iron-based metal-organic framework prepared in content（1）,the iron-based hydrotalcite material was compounded with the secondary hydrothermal method.The larger specific surface area of iron-based hydrotalcite materials provides more catalytically active sites,which is beneficial to the adsorption of reaction intermediates.On the other hand,the recombination of the two shortens the transport distance of electrons during the reaction,thus reducing the required energy of the reaction.Among them,the Fe-BDC composited with Co Fe-LDH exhibits the best water electrolysis performance.Under the two-electrode alkaline test conditions,its overpotential is 389 m V at a current density of 10 m A cm^-2.Stability remains around 95%.Therefore,the composite structure of MOF and LDH provides effective help for the development of bifunctional water electrolysis catalysts in the future.This thesis aims to develop efficient,inexpensive and stable catalysts for the electrolysis of water based on an iron-based organic framework and successfully prepares efficient catalysts for the electrolysis of water through three different perspectives.Finally,it is hoped that the design ideas and research lines of this thesis will be a breakthrough for future electrolytic water applications.