Preparation And Properties Of Electrocatalysts Derived From MOFs

With the rapid increase in global energy demand in this century,global energy consumption has been growing at an exponential rate and is expected to increase by 56%in 2040.The energy crisis has become a more serious social problem under the influence of the current international situation,and China is in a disadvantageous position in this“energy war”,how to achieve energy transformation is a major livelihood issue that needs to be solved in China.The“carbon peak,carbon neutral”dual carbon goal is an important strategic direction for the country.China’s demand for traditional fossil energy is great,and it needs a variety of auxiliary energy to fill the gap.The development of new energy technology can not only break the technical barriers,but also achieve a bend in the technology suppression in Europe and the United States for many years.So the development of sustainable energy has gradually become a top priority.People focus their development on electrochemical energy storage and conversion device technologies with sustainable energy,among which rechargeable metal-air batteries and hydrogen fuel production have received widespread attention.The core reactions are oxygen reduction reaction（ORR）,oxygen precipitation reaction（OER）and hydrogen precipitation reaction（HER）.However,the performance of these core reactions is strictly dependent on the electrode materials,which has drawn much attention to the development of new electrode materials.Due to the advantages of metal organic framework compounds（MOFs）derived materials such as huge porosity,large contact area,and high electron conductivity,it makes them promising for energy storage and conversion.Therefore,in this thesis,MOFs-derived materials were investigated,and materials with good catalytic performance of ORR/OER/HER were prepared by optimizing the precursor components and synthesis conditions,and their applications to zinc-air batteries and electrolytic water were further explored.The key research results are shown as follows:（1）MOFs-derived S-axially doped Fe N₄ catalyst materials for ORR performance in alkaline electrolytes:Different MOF-derived single-atom Fe materials（Fe-N₄S₁,Fe-D,Fe-SO_x,Fe-N_x）were prepared by introducing different heteroatoms（S,N）to Al/Fe-MOF during pyrolysis as precursors by a simple wet chemical method to exploit the effect of heteroatoms on the charge around the Fe sites and thus achieve excellent ORR performance.The alternating linkage of metal ions with organic ligands in MOF can avoid metal agglomeration and the metal ions are trapped on the carbon carrier during the pyrolysis of MOF,which ensures high dispersion of the catalyst and also improves the stability of the catalyst.S and N heteroatoms are introduced during the thermal transformation process,leading to a large size three-dimensional carbon structure.This structure ensures that the catalyst can expose more active sites and an abundant three-phase interface,facilitating the efficient transport of reaction-related substances.The introduction of S atoms also increases the loading of Fe providing more active sites and solving the low loading problem of most single-atom catalysts.In addition,the results of X-ray absorption spectroscopy（XAS）and theoretical（DFT）calculations revealed that the introduction of axial S heteroatoms could reshape the electronic configuration of the Fe-N₄S₁ active center,and the axial introduction of S atoms in the Fe active center moved its d-band center away from the Fermi energy level compared to Fe N₄,weakening the intensity of adsorption to intermediates and leading to high activity.The catalyst was shown to display high ORR half-wave potential（0.88 V）and limiting current density(-5.71 m A cm^-2)in alkaline electrolyte with good stability and methanol tolerance,and it also showed good performance when assembled as a cathode material for zinc-air batteries.（2）Hydrogen precipitation performance of Co-doped Cu₃P catalyst materials derived from MOFs in full p H pure water and seawater electrolytes:Cu（OH）₂nanoarrays were grown on copper foam substrates by a simple wet chemical process using copper foam as the substrate material and copper source,followed by the addition of Co source and ligand to grow Co-Cu-MOF on Cu（OH）₂ nanoarrays in an oriented manner by solvothermal method.Then phosphorylation was performed to obtain 1D/3D structured Co-Cu₃P/CF catalysts.The construction of this 1D/3D structure not only increases the surface area of the catalyst and exposes more active sites,but also promotes the rapid transport of reaction-related substances.Cu（OH）₂NWs as sacrificial templates can provide Cu sources and specific metal sites on which the MOFs can grow directionally,exposing more active sites and effectively preventing the aggregation of MOFs.The doping of Co induces more defects that introduce additional active sites and affect the electronic properties,which contributes to the performance improvement.The catalyst was shown to have good hydrogen precipitation performance in both pure water and seawater electrolytes at full p H.Co-Cu₃P/CF required 230 and 371 m V overpotential currents to reach 1000 m A cm^-2in alkaline and acidic electrolytes,respectively,and 230 m V overpotential to reach 100m A cm^-2 in neutral dielectrics.Co-Cu₃P/CF also shows good performance and stability in seawater of different p H values（from the Yellow Sea of China）,especially in alkaline seawater.The assembly of this catalyst into an electrolytic water system also has excellent performance.