The Study On The Design Of Iron-based Metal-organic Frameworks And Their Performances In Electrocatalysis

Since the term"Metal Organic Framework（MOF）Material"was put forward in the1990s,it has been in the forefront of multidisciplinary research,from the initial structural design synthesis to the development of functional properties and practical applications.MOFs have the characteristics of large surface area,adjustable pore structure and uniformly distributed unsaturated active sites,which show attractive application prospects in gas separation and storage,catalysis,proton conduction and electrochemical energy storage.At present,research on MOF as raw materials（MOF source materials,including pure MOF materials,MOF based composites,MOF derivatives,MOF gels,etc.）has attracted increasing attention,such as MOF matrix composites,MOF derivative materials and other advantages in electrocatalysis,energy storage and other fields,which has expanded the application field of MOF materials.In the field of electrocatalysis,iron-based metal elements have good activity,so it is urgent to develop the function of iron-based MOF materials.Therefore,on the basis of the original work of our research group,taking the electrocatalytic function as the guide,and keeping the original structure of MOF unchanged,we designed and synthesized polycarboxylic iron-based MOF crystalline materials and composites,and explored the application of iron-based MOF source materials in the fields of electrocatalytic oxygen evolution（OER）,alcoholic oxidation（AOR）.The main research contents are as follows:The first part is the investigation and summary of the related literature in this research field.Firstly,the characteristics,classification and classical iron-based MOF materials are introduced.Secondly,their work in the field of oxygen evolution from electrolytic water and published classical literature are discussed.Secondly,the related work in the field of methanol oxidation is listed.In addition,the iron-based MOF materials are electrocatalyzed.The advantages and disadvantages in this field are elaborated to lay the foundation and summarize the experience for further design and synthesis.Finally,the basis and research direction of this paper are clarified.Part II:Seven iron-based MOF materials were constructed under hydrothermal conditions with iron-based metal ions Fe²⁺,Co²⁺,Ni²⁺,triangular ligand H₃TATB of pyromellitic acid derivatives as organic components,and different types of short-chain small molecules as auxiliary components.The synthesis and crystal structure of these materials were discussed with emphasis,and the regulation of short-chain small molecules on the final structure of organic components was analyzed and summarized.At the same time,the related basic characterization was made.The crystalline materials of 7MOF systems were as follows:{Co₆（μ₃-O）₂（μ₂-O）（TATB）_8/3（HCO₃）（TRZ）（H₂O）}_n 1{(Co₄（μ₃-OH）₄（TATB）_4/3（dpe）₂)}n 2{(Ni₄（μ₃-OH）₄（TATB）_4/3（dpe）₂)}n 3{[Ni ^IINi ₂^III（μ₃-O）（μ₂-OH）_0.5（TATB）（Ina）_2.5（H₂O）]}_n 4{[Co ₂^IICo ^III（μ₃-O）（TATB）（Nic）₂（H₂O）₃]}_n 5{[Fe ₂^IIFe ^III（μ₂-OH）（TATB）₂]}_n 6{[Co（TATB）（bib）]·H₂O}_n 7The third part:Choosing crystalline material 1 as the main material to explore the OER properties of itself and the composites based on it;crystalline material 1 has certain OER properties,its initial overpotential is 376 m V,Tafel slope is 77 mV dec^-1,and the overpotential 10 corresponds to 10 m A cm^-2is 454 m V;because of the low conductivity of MOF material,the introduction of conductive material can improve its conductivity,and then improve its conductivity.In order to improve its OER performance,three types of conductive materials are selected:conductive material-Commercial graphene（Gr）,semiconductor material with OER activity-commercial cobalt tetroxide and semiconductor material without OER activity-Commercial tin oxide with different mass ratio.Among them,commercial graphene（Gr）has the best effect when the mass ratio is4%,and the initial overpotential decreases.48 m V,η₁₀decreased by 64 m V;commercial cobalt tetroxide（Co₃O₄）with OER activity had the best enhancement effect when the mass ratio was 20%,the initial overpotential decreased by 77 mV andη₁₀decreased by 69 m V;commercial tin dioxide without OER activity had the best enhancement effect when the mass ratio was 15%,the initial overpotential decreased by 62 mV andη₁₀decreased by 66m V;and through impedance,based on the comprehensive analysis of the value of TOF,double-layer capacitance and I-T curve,the construction of conductive agent-MOF composites improves the conductivity of MOF source materials,widens the area of catalytic active region,speeds up the conversion frequency of reactants per unit time and activity,and improves the stability of materials.Some of the results were published in Nature Index Journal Inorganic Chemistry（Inorg.Chem.2019,58,5837-5843）.The other work of this chapter chooses the classical ZIF-67 as the main material to explore its OER performance and the OER performance based on the construction of multi-metal matrix composites.ZIF-67 has certain OER performance,its initial overpotential is 382 m V,Tafel slope is 122 mV dec^-1,and the overpotential＿10 is 507 mV corresponding to 10 m A cm^-2.Due to the single metal center of ZIF-67 material and the low OER performance,the introduction of heterogeneous metals can improve the OER performance.The catalytic activity of Fe₂Co metal salts is mainly selected as the heterogeneous metal source to control the different proportion of Fe₂Co metal salts and cobalt nitrate as the raw material.The influence of different proportion of cobalt nitrate on the properties of materials is investigated.Firstly,the effects of different proportion of cobalt nitrate on the properties of materials are studied.It is found that the OER performance of materials is improved in varying degrees,and the effect is relatively best when the mass ratio is 25%.The potential decreases by 52 mV andη10 decreases by 126 m V.Secondly,the influence of different morphologies on the properties of materials is studied.When the mass ratio is 15%,the morphology of ZIF-67 material changes from flat to petal,the initial overpotential decreases by 52 m V,η10 decreases by 85 mV and TOF increases by 0.0321 S^-1.It shows that the construction of multi-metal matrix composites improves the MOF source material on the one hand.On the other hand,the OER performance of MOF material has some relationship with its morphology.The petal morphology is better than the flat morphology.This work has certain reference value for studying the relationship between morphology and oxygen evolution performance.Part IV:Based on the analysis of methanol oxidation reaction,the electrocatalytic methanol oxidation performance of Tetranuclear cube nickel-based crystalline material 3was explored.The results showed that tetranuclear nickel-based crystalline material 3 had high methanol oxidation mass activity of 203 m A mg^-1_catalystand low initial oxidation potential（0.83 V）.The activity of KB-crystalline material 3 for methanol oxidation increased by 2 times,reaching 527 m A mg^-1_catalyst,and the initial oxidation potential decreased to 0.803 V.This study provides valuable information for further study of the electrocatalytic methanol oxidation performance of Ni-MOF source materials,and also provides a meaningful reference for further study of the choice of catalyst materials for methanol oxidation.