Design,preparation And Oxygen Reduction Of MOF-Derived Fe/N/C Electrocatalysts

Fuel cell and metal-air battery are considered as the ideal power source in the future because of their environmental friendliness and high energy density.Oxygen reduction reaction（ORR）,an important half-reaction,still depends on precious metal catalyst（Pt/C）.However,the low reserve and high price of platinum hinder the large-scale application of these energy-conversation technologies.In order to solve this problem,great efforts have been made to search for non-noble metal catalysts with high activity and low cost.Among various candidate materials,iron/nitrogen co-doped carbon-based materials（Fe/N/C）are considered as one of the most promising non-platinum catalysts.However,the current Fe/N/C ORR catalysts still suffer from insufficient activity and poor stability.Because metal-organic frameworks（MOFs）have great advantages compared to other precursors including ultrahigh surface area,intrinsically porosity,diverse composition/structures and easy preparation,MOF-derived Fe/N/C electrocatalysts for ORR are emerging as one of the current research hot topics.In this dissertation,four Fe/N/C ORR electrocatalysts have been prepared by rational design of MOF precursors,and the effects of the microstructure and active sites of the materials on ORR electrocatalytic activity have been also discussed.The dissertation is mainly divided into the following four parts:（1）There are many ways to dope iron into metal-organic framework materials.In general,researchers only use iron source to create Fe N_x active sites,but less attention is paid to the impact on carbon texture of iron doping.The core-shell Fe（OH）₃@ZIF-8 is used as the precursor of Fe/N/C electrocatalysts.During the carbonization process,Fe（OH）₃ encapsulated in ZIF-8 gradually evolves into iron oxide as the temperature increases.The in-situ generated iron oxide can not only create Fe N_x active sites but also adjust the structure of carbon materials.The best-performing C-Fe（OH）₃@ZIF-1000 catalyst has a hollow polyhedron structure.In alkaline solution,the ORR activity of C-Fe（OH）₃@ZIF-1000 exceeds that of Pt/C.In acidic solution,the half-wave potential of C-Fe（OH）₃@ZIF-1000 reaches 0.80 V.H₂-O₂ proton exchange membrane fuel cell（PEMFC）with C-Fe（OH）₃@ZIF-1000 as the cathode catalyst delivers a maximum power density of 411 m W cm^-2.（2）Ionic metal-organic frameworks possess exchangeable ions which can be used to introduce exterior iron-containing ions through ion exchange.The cationic metal-organic framework（Cd-TTPBA-4）was selected for anion exchange with potassium ferricyanide to obtain the precursor{K₂[Fe（CN）₆]}^-@Cd-TTPBA-4,and the Fe/N/C ORR electrocatalyst with iron carbide embedded into the carbon matrix was obtained through carbonization.Both the anion exchange process and the Fe doping amount can be accurately controlled by tracking the changes in the UV-vis absorption intensity of K₃[Fe（CN）₆]solution.In alkaline electrolyte,the best ORR performance of Fe/N/C catalyst is better than that of commercial Pt/C.（3）In order to solve the problems with the traditional template method such as complicated removal steps and unfriendly environment,an nonporous eight-fold interpenetrated metal-organic framework（Fe-Zn-TTPA）containing dense Zn（II）-carboxylate coordination units was used to prepare Fe/N/C ORR electrocatalysts.During pyrolysis,Zn（II）-carboxylate units are gradually decomposed into nanoscale Zn O particles.Owing to its thermal-removal nature,Zinc oxide can adjust the structure of carbon materials through carbothermal reduction.The best catalyst Fe/N/C-1000-0.05 contains well-dispersed iron carbide（Fe₃C）nanoparticles and Fe N_xactive sites.In alkaline electrolyte,the ORR activity of the catalyst is higher than Pt/C.In addition,the peak power density of Zn-air battery with Fe/N/C-1000-0.05 as the cathode catalyst is 130 m W cm^-2,and the discharge continues at the current density of 5 m A cm^-2 for 70hours with a slight attenuation of 3.1%.（4）MOF-5 is an excellent self-sacrificing template,with high thermal stability,specific surface area and easy preparation.MOF-5 is widely used to prepare porous carbon materials,but the Fe/N/C catalyst derived from MOF-5 has not been explored.First,Fe/Zn-MOF-5 is obtained by solid-liquid cation exchange between MOF-5 and Fe（BF₄）₂ solution,the cation exchange method can not only easily control the doping amount of Fe atom but also well ensure the uniform distribution of Fe atom throughout the MOF-5 framework,which is beneficial for inhibition of Fe atom agglomeration during the pyrolysis.Second,N-containing imidazole molecules were encapsulated into activated MOF-5,making full use of 3-D interconnected channels of MOF-5.Under optimal conditions,the direct pyrolysis of imidazole@Fe/Zn-MOF-5 produces porous Fe/N/C catalysts embedded with metal carbide（Fe₃C）nanoparticles.The ORR catalytic performance of Fe/N/C catalyst(E_1/2=0.86 V vs RHE)is superior to Pt/C,the catalyst has good stability in alkaline electrolyte.In zinc-air batteries,the maximum power density is 85 m W cm^-2.The strategies reported in this work have broad versatility and can be generalized to other transition-metal-doped carbon electrocatalysts,providing a new entry for MOF-derived carbon-based electrocatalysts.