Structure Design And Performance Research Of Electrocatalysts Derived From Metal Organic Frameworks

Energy crisis and environmental pollution have gradually become serious problems that restrict the development of modern society.Due to its high calorific value,environmental friendliness,and renewable characteristics,hydrogen has become one of the most promising clean energy sources to replace traditional fossil fuels.However,the development of clean energy conversion devices,such as electrolyzers and fuel cells for hydrogen production and consumption is facing severe challenges.The hydrogen evolution reaction(HER),oxygen evolution reaction(OER)and oxygen reduction reaction(ORR)on the electrodes are kinetically sluggish,which limits the further application of these devices.Therefore,it is undoubtedly of great significance to explore high-efficiency electrocatalysts to increase the reaction rate.In recent years,nanomaterials derived from metal-organic frameworks(MOFs)have been widely used in the field of electrocatalysis because of their high specific surface area,abundant pores and controllable composition structure.Based on the full understanding of MOFs and their derivatives,a series of high-efficiency electrocatalysts with different structures were designed and prepared using MOFs as precursors for ORR,HER and OER.Moreover,a variety of characterization and testing methods are used to conduct in-depth researches on the structure-activity relationship,active sites,and changes in the catalytic process,providing theoretical guidance and material support for the subsequent design and synthesis of high-efficiency catalysts.The main research contents are listed as follows.(1)Study on the active site purification and performance of cobalt single-atom electrocatalyst derived from MOFs.The research reveals the coupled evolution of N-doped carbon-encapsulated cobalt nanoparticle sites(Co@CN)and single-atom cobalt sites(SA-Co)in the catalyst(Py-ZIF)obtained by direct pyrolysis of the MOFs precursor.Based on this,a method of sulfur-assisted pyrolysis of MOFs precursors was further proposed to remove cobalt particles for purifying single-atom cobalt sites,and successfully prepared single-atom cobalt electrocatalyst(Co-SAC).It is found that compared with Py-ZIF catalyst containing two different sites,Co-SAC catalyst with completely removal of the Co@C-N sites shows no obvious change of activity for either ORR or HER,while significant enhancement of ORR selectivity,indicating that the SA-Co sites play dominate role in both the ORR and HER processes of MOF-derived catalysts,and show higher ORR catalytic selectivity.(2)Study on the hydrogen production performance of chromium oxide and ruthenium/carbon nanocomposites derived from MOFs in neutral media.Ru-loaded porous carbon material composited with Cr₂O₃(2-Ru@C/Cr₂O₃)was prepared by a one-step pyrolysis method using MOFs loaded with Ru Cl₃ as the precursor.The results indicate that the introduction of Cr₂O₃ significantly reduces the higher energy barrier for the adsorption and activation of water molecules of the ruthenium-carbon catalyst in the neutral media,and accelerates the dissociation of water molecules and the generation of H₂.Also,the porous carbon substrate obtained by the pyrolysis of the MOFs precursor plays a key role in the improvement of HER performance owing to the improved electrochemical surface area and conductivity.2-Ru@C/Cr₂O₃ exhibits excellent HER catalytic activity in neutral media,which is better than commercial Pt/C catalysts in the high current density range greater than 20 m A cm^-2.(3)Study on the oxygen production performance of in situ generated nickel-doped cobalt oxyhydroxide nanosheets with a 3D network structure in neutral and alkaline media.A convenient solvothermal method was developed to synthesize polymetallic phosphate nanorods aligned on nickel foam(Na_xCo_yNi_zPO₄/NF)using MOFs as precursor.The high Na content in Na_xCo_yNi_zPO₄/NF enables it rapidly and completely reconstitute during the OER process in alkaline media to form highly active metal hydroxide nanosheets with a large quantity of accessible active sites(Ni-Co OOH/NF).The nickel foam is not only acted as a conductive support during the synthesis process,but also can enter into the catalyst as a small amount of Ni source,thus effectively modulate the electronic structure to optimize catalytic performance.The research results show that the in situ generated Ni-Co OOH/NF catalyst exhibits better OER catalytic activity than commercial Ru O₂in alkaline and neutral media.