Effects Of Doping And Interface Manipulation On Electronic Structure And Electrochemical Performances

With the demand of global sustainable development,energy crisis and environmental pollution become key issues that need to be resolved.For this purpose,the countries all over the world are committed to developing new energy and technologies for the treatment of environmental pollutant.Recently,Electrochemical water splitting and nitrate reduction are widely used in the fields of green energy conversion and nitrate pollution treatment because of the advantages of high energy conversion efficiency,environmental friendliness and simple technique.In the above electrochemical devices,the electrocatalyst determines the reaction efficiency and energy consumption.Unlike the noble metal catalysts with high cost and low reserves,non-precious metal catalysts are more promising in practical application due to their low cost and abundant reserves.Therefore,the development of high-performance non-precious metal materials has become a hot research issue in recent years.In this thesis,we have designed a series of transition metal-based electrocatalytic materials.Through metal doping and constructing heterogeneous interfaces,the electronic structure of such materials is controlled,and their hydrogen evolution reaction（HER）,oxygen evolution reaction（OER）and nitrate reduction reaction（NO3RR）catalytic performances are investigated.Furthermore,the relationship between the electronic structure of active sites and electrocatalytic performance is revealed,and the catalytic mechanism is initially discussed.The main research contents of this thesis are as follows:1.The alkaline hydrogen evolution reaction（HER）is an extremely important reaction in energy conversion and electrolytic industries.Pristine Mo S2 exhibits poor alkaline HER activity due to weak water adsorption and dissociation.Based on the interface engineering,combining active water dissociation components with Mo S2 would be an effective strategy to improve the alkaline HER performance.Herein,Co S2/Mo S2 ultrathin nanosheets have been achieved via a one-step pyrolytic sulfurization of a small-sized Co/Mo-MOF precursor.Rich Co S2-Mo S2 interfaces modulate the interfacial electronic structure,and improve the adsorption and dissociation of H2O.Co S2/Mo S2 heterostructured material exhibits outstanding alkaline HER performance(onset overpotential of 16 m V andη₁₀ of 75 m V),which surpass those of most top-rated Mo S2-based materials and are even proximate to those of commercial Pt/C(onset overpotential of 0 m V andη₁₀ of 60 m V).Under a large current density of 400 m A cm^-2,the complex material exhibited long-term stability.2.With the development of electrochemical green hydrogen production technology,high-performance HER/OER bifunctional electrocatalytic materials are urgently needed.Defect engineering of two-dimensional（2D）heterostructured materials can bring abundant active sites and increase the bifunctional activity.Herein,through pyrolytic sulfurization of a layered Fe-doped Ni/Mo MOF precursor,a series of defect-rich Fe-doped Ni S/Mo S2 ultrathin nanosheets were obtained.Fe atoms induce abundant lattice defects in Ni S as highly active water adsorption sites.Fe doping modulates the electronic structure of Mo and Ni sites.The interface between Ni S and Mo S2 can optimize the adsorption energy of a HER/OER intermediate.As a result,both HER and OER activities are significantly enhanced.0.1Fe-Ni S/Mo S2 reaches lowest overpotential for HER and OER(η₁₀ of 120 m V and 297 m V,respectively).Small Tafel slopes(69.0 m V dec^-1 and 54.7 m V dec^-1)were also achieved.The catalytic performance of this material can be compared with those of commercial Pt/C and Ru O2 catalysts.For overall water splitting,only 1.66 V voltage is required to deliver 10m A cm^-2 with long-term stability.3.Nitrate electroreduction reaction（NO3RR）is an effective method to treat environmental nitrate pollutants.The development of conductive metal-organic frameworks（c MOFs）has opened up new opportunities for the development of high-performance NO3RR electrocatalytic materials.Herein,the 1,3,5-triamino-2,4,6-triphenol（TABTO）containing one benzene ring was coordinated with Cu²⁺to form 2D conductive MOF.And then,Cu Co-TABTO and Cu Ni-TABTO were prepared by metal doping.The effect of metal doping on the electrocatalytic performance of the materials was systematically studied.The results show that both Co and Ni doping can improve the electrocatalytic performance of c MOF,and Co site can further change the reaction pathway,so that the main product can be turned from nitrite to NH3.At the potential of-0.95 V,the NH₃ yield rate of Cu Co-TABTO is 190.04μmol h^-1 cm^-2,and the Faradic efficiency is 67%,which are higher than Cu-TABTO and Cu Ni-TABTO.The cyclic test shows that Cu Co-TABTO has good catalytic stability.4.Herein,hexahydroxytriphenylene benzene（HHTP）with extended conjugated structure was used as organic ligand to prepare a bimetallic c MOF（Cux Coy HHTP）.The NO3RR activity and electronic interaction of metal sites were systematically investigated.As the result,the intrinsic activity of Cu sites is much higher than Co sites.Thus,the Cu sites are the main active center in bimetallic MOF.Meanwhile,Co doping can adjust the electronic structure of Cu sites,and the extended conjugated ligand brings larger pores and high-conductivity.As a consequence,the NO3RR performance and NH3 selectivity of Cux Coy HHTP significantly improve.The optimal Cu1Co1HHTP exhibits an outstanding electrocatalytic activity with a high NH3 yield rate of 299.9μmol h^–1 cm^–2 and a large FE of 96.4%at–0.6 V.The Density functional theory（DFT）calculation reveals that the existing of Co sites can cause the d-band center of Cu site to shift toward Fermi level,which strengthen the adsorption of intermediates on Cu sites,and the energy barrier of potential-determining step（NO*→NOH*）is decreased in nitrate reduction process.Moreover,the Co sites bring a higher selectivity to Cu active sites for reducing*NO2 to*NO,rather than the desorption of NO2^–.5.The construction of heterostructure is an effective strategy to adjust the conductivity and electronic structure of c MOFs.Layered transition metal carbides with excellent electrochemical properties can form stable composite structures with two-dimensional conducting MOF throughπ-πinteraction.Herein,we prepared serial Cu HHTP/Ti3C2 heterostructure materials,and studied the NO3RR electrocatalytic behavior.The influence of Ti3C2 on the electronic structure of Cu site in the heterostructure was revealed.At–0.5 V,the NO3RR product is mainly NO2^–.Cu HHTP/Ti3C2 composite shows NO2^–generation rate and Faradic efficiency of 471.8μmol h^–1 cm^–2 and 86.6%,respectively.DFT calculation results show that the d band center of Cu site in Cu HHTP/Ti3C2 is closer to Fermi level,indicating that Ti3C2combining can enhance the adsorption ability for NO3^–on the Cu sites.Combining Ti3C2 also reduces theΔG of converting*NO2 to NO2^–on the Cu site,and thus greatly improvs the NO2^–selectivity and reaction rate of heterostructured material.