| The excessive development and application of fossil fuels have caused serious environmental pollution.At the same time,it also leads to the danger of energy crunch.Therefore,it is urgent to develop clean,efficient and sustainable new energy sources.Hydrogen energy,an ideal renewable energy source,has the advantages of broad resource,high energy density,and environmental friendliness.The preparation and utilization of hydrogen energy are crucial to solving current environmental and energy problems.Electrocatalytic water splitting and fuel cell are a new type of eco-friendly energy conversion devices.However,hydrogen evolution reaction(HER),oxygen evolution reaction(OER)and oxygen reduction reaction(ORR)involved in them have the sluggish kinetics.The noble metal catalysts have high catalytic activity,however,the scarcity,and high expense of these noble metal have severely limited their large-scale commercial development and application.Therefore,it is of great significance to develop noble metal-free electrocatalysts with high efficiency,low cost and high stability for electrocatalytic water splitting and fuel cells.Metal-organic frameworks(MOFs)are a new kind of three-dimensional network porous materials,which are self-assembled by metal ions/clusters and organic ligands.MOFs-based materials,including pristine MOFs,MOFs complexes and their derivatives,have shown promising applications in various catalytic fields.Compared with the traditional porous materials,MOFs have the unique advantages of high specific surface area,high porosity,three-dimensional ordered metal nodes and adjustable pore size,which provide ideal structural conditions for the design and preparation of efficient electrocatalysts.Through extensive literature research,we constructed a series of efficient electrocatalysts by the adjustion of MOFs growth conditions,morphology and structure based on our full understanding of the structure and composition of MOFs.X-ray diffraction(XRD),scanning electron microscope(SEM),transmission electron microscope(TEM),X-ray photoelectron spectroscopy(XPS),synchrotron radiation and other instruments were used to characterize the physicochemical properties of these as-obtained electrocatalyst.These as-prepared electrocatalysts were used for electrocatalytic overall water splitting(including OER and HER)and ORR.Furthermore,the relationship between physicochemical properties and performance of the catalyst was studied.Combined with theoretical calculation,the catalytic mechanism was deeply explored.Research work in this thesis will provide a new idea and theoretical basis for the controllable preparation of high-efficiency electrocatalysts.The main research contents of this thesis are as follows:(1)The strategy of in-situ growth of MOF nanosheets on the conductive substrate surface was proposed to prepare efficient electrocatalysts.A series of Cu3(PO4)2/Cu-BDC foam electrodes were successfully constructed by using copper foam as substrate via combining sacrificial template method and in-situ growth method.The optimal reaction temperature and time were determined to 150?and 6 h,respectively.The ultrathin Cu-BDC nanosheets were uniformly and vertically grown on the substrate surface.The Cu3(PO4)2/Cu-BDC foam electrode exhibits high OER,HER and overall water splitting electrocatalytic performance due to the well-developed pore structure,high active site exposure ratio and good conductivity,affording the current density of 10 m A/cm2at overpotentials of 241 and 145 m V as well as cell voltage of 1.56 V,respectively.This study provides strategy for the preparation of high performance electrocatalysts based on pristine MOFs.(2)It is proposed to prepare MOF derived high active electrocatalyst by heteroatom doping.Cu-BDC MOF materials were prepared by using cuprous oxide cube as the substrate and copper source and 1,4-dicarboxybenzene as organic ligand via self sacrificial template method.Then,a series of Cu3P@C materials were prepared by using prepared Cu-BDC as precursor through calcination and phosphorization.The effect of reaction temperature on the morphology of Cu-BDC precursor was studied in detail.The cuprous oxide substrate can be completely transformed into a sea urchin-like Cu-BDC when the reaction temperature is 120?.Cu3P@C derived from urchin-like Cu-BDC has well-developed pore structure,high specific surface area and a large number of accessible active sites.The electrocatalytic performance toward OER and HER of Cu3P@C were tested,achieving the current density of 10 m A/cm2 at low overpotentials of 300 and 124 m V for OER and HER,respectively.Meanwhile,Cu3P@C also shows good stability.(3)High-performance electrocatalysts are prepared by combining 2D MOF precursors and heteroatom doping.2D Cu-BDC MOF nanosheets were efficiently prepared by using cuprous oxide cube as substrate and copper source and 1,4-dicarboxybenzene as organic ligand through a bottom-up method.Then,S-doped Cu S@C electrocatalyst with layered structure was prepared by hydrothermal vulcanization and calcination.According to the characterization results,the layered Cu S@C has high specific surface area,percentages of exposed active sites and excellent electrocatalytic performance for HER,delivering a current density of 10m A/cm2 at low overpotential of 125 m V.Moreover,due to the protection of carbon matrix,as-obtained catalysts show high electrocatalytic stability in acidic,alkaline and neutral mediums.(4)Using MOFs as substrate,an enhanced electrochemical performance of Mo S2 were achieved through integrating the synergistic effect of multiple components.A rose-like porphyrin-based MOF(Cu-TCPP)was prepared by solvothermal method,which was used as the substrate material to immobilize Mo ions.Then,3D open and hierarchical porous Cu SNC@Mo S2-Pt electrocatalyst was successfully prepared through sulfuration and Pt-doping.The effects of S,N-doped carbon substrate,Pt-doped Mo S2,overall morphology and their synergistic effects on the electrocatalytic activity of Cu SNC@Mo S2-Pt were studied in detail.Cu SNC@Mo2-Pt showed high alkaline HER activity,which was comparable to that of commercial noble metal Pt/C catalyst,delivering a current density of 10,50 and 100 m A/cm2 at low overpotential of 102.6,165.6 and199.0 m V,respectively.Moreover,the density functional theory(DFT)was used to study the mechanism of synergetic catalysis between active sites and other components.This work provides a new approach for the preparation of MOFs-based composite electrocatalysts with enhanced catalytic activity.(5)In order to improve the atom-utilization efficiency,the preparation of single-atom catalysts(SACs)with high performance was proposed based on MOFs precursor.A porphyrin-based MOF precursor(Co-TPy P)with abundant Co-N coordination structure was constructed by using porphyrin(H2TPy P)and Co2+ions.Meanwhile,urea was introduced into the MOF channel,which not only could increase spatial confinement to prevent metal ion migration and agglomeration,but provide additional N source to increase the N doping ability.Finally,The Co SACs(U-Co SANC)with high Co content was successfully realized.The physicochemical properties of U-Co SANC,such as morphology,crystal structure and coordination environment of Co atom,were systematically studied.U-Co SANC is a rod-like structure composed of nanosheets with well-developed pore structure,high specific surface area and high percentages of exposed active sites.U-Co SANC delivered excellent electrocatalytic activity and stability with half-wave potential and onset potential of 0.960 V and 0.861 V,respectively,which is even superior to commercial Pt/C.Meanwhile,U-cosanc showed high selectivity for four-electron ORR pathway. |
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