Preparation Of MIL-derived Transition Metal Oxides And Their Electrocatalytic Properties

The energy crisis and environmental degradation have seriously threatened the survival and development of human beings,and countries all over the world are committed to devel-oping new energy sources to alleviate the energy problem.Hydrogen energy is a clean and sustainable energy source with the advantages of high energy density,the high calorific value of combustion and zero carbon emission.Compared with solar energy,wind energy and tidal energy,which are limited by geographical and seasonal restrictions,hydrogen energy un-doubtedly has the great advantage of stable storage and transportation.Hydrogen production from electrolytic water is one of the most popular hydrogen production methods,in which the Oxygen evolution reaction（OER）involves multiple electron transfer and proton coupling processes,resulting in slow kinetics and a much higher actual decomposition voltage than the theoretical voltage.Therefore,electrolysis of water requires the use of appropriate catalysts to reduce the reaction overpotential and improve the energy conversion efficiency.Currently,noble metal catalysts possess excellent catalytic performance,but they are limited by high cost and low reserves and cannot be used on a large scale.Transition metal oxides are prom-ising catalytic materials with abundant reserves and high activity,while metal-organic frame-work（MOF）materials have unique advantages such as high specific surface area,porous structure,and tunable chemical composition.Therefore,the derivation of transition metal ox-ides by pyrolysis of MOF materials is one of the effective technical routes to obtain efficient and inexpensive electrocatalysts.In this thesis,metal-organic framework material MIL（Fe）is used to construct highly active and stable oxygen precipitation electrocatalysts by enhancing the electrical conductiv-ity,exposing more active sites,improving the electron transport capacity and establishing stable catalytic structures.The details are as follows:Using Fe³⁺as the central atom and 2-aminoterephthalic acid as the ligand,pure phase MIL（Fe）and its derived Fe₂O₃ particles were synthesized in one step using the solution com-bustion method,focusing on the effects of the ratio of the central atom to the ligand and the solution combustion calcination temperature on the structure and catalytic performance of the derived oxides.When the ratio of metal atoms to organic ligands was 5:8,the particles were smaller in size and exposed more active area.MIL（Fe）underwent a crystalline phase transi-tion from the MOF structure to Fe₂O₃ when the calcination temperature reached 300°C.The MIL（Fe）5-8/NF catalyst at 400°C exhibited higher OER catalytic activity in 1 M KOH al-kaline electrolyte with an overpotential of 319 m V at a current density of 10 m A cm^-2 and a Tafel slope of 95 mV dec^-1,possessing higher catalytic kinetics,and maintained the electro-lytic water stability for 30 h.Derived spinel oxide catalysts(MFe₂O₄)（M=Cu,Co,Ni,Zn）were obtained by pyrolysis with MIL（Fe）as the precursor and the introduction of metal cations.It was found that the Zn-doped spinel-type oxide had the weakest crystallinity under the same conditions,while the OER electrocatalytic performance showed the best performance with 279 m V in 1 M KOH alkaline solutionη₁₀ and 76.43 m V dec^-1 of Tafel slope.It was further found that the crystal-linity of Zn Fe₂O₄ increased sequentially with the increase of calcination temperature,and the catalyst containing Zn-doped MIL（Fe）spindle was gradually pyrolyzed to form Zn Fe₂O₄spherical nanoparticles and its OER electrocatalytic performance was reduced.The data sug-gest that the Zn-doped MIL（Fe）-containing spindle formed at 400°C provides the main active site,and when the spindle is pyrolyzed,the electrochemically active area decreases and the catalytic performance decreases.The studies in both directions together suggest that samples tending to amorphous forms can expose more active sites and enhance the electrochemical performance.The P-doped Ni O wrapped with P-doped Fe₂O₃ in a core-shell structure（P-Ni O@Fe₂O₃/NF）catalytic material was also prepared by the solution combustion method through the introduction of ammonia hypophosphite.XRD showed that the introduction of P element could further reduce the crystallinity of the material,and TEM results revealed its unique core-shell structure.The derivative P2.0-Ni O@Fe₂O₃/NF electrocatalytic performance was optimal at an ammonia hypophosphite addition of 2 mmol,with an overpotential of 208m V at a current density of 10 m A cm^-2 in 1 M KOH solution and a Tafel slope of 69.64 m V dec^-1.In addition,the electrode material has excellent catalytic stability at an OER of 20 m A cm^-2 The electrode material has excellent catalytic stability and can operate stably for more than 100 h at an OER of 20 mA cm^-2 without current density degradation.