Synthesis And Property Study Of Polyoxometalate-Based Hybrid Materials As Well As The Initial Exploration In Lithium-Sulfur Batteries

Polyoxometalates(POMs for short)and Polyoxometalate(POM)-based hybrid materials have become one of the hot topics in current coordination chemistry and materials due to their rich structural features and diverse spatial structures,as well as their excellent properties in the fields of photocatalysis,electrochemistry,energy,and optoelectronics.POMs are strong Lewis acids with tunable redox activity,and can form POM-based hybrid materials by coordinating with transition metals owing to their rich surface oxygen atoms.POM-based hybrid materials are usually formed by bridging polyoxoanions and metal-organic fragments by virtue of covalent bonds or supramolecular forces that are further expanded into chain-like,layered,and three-dimensional framework structures.At present,POM-based hybrid materials based on classic Keggin-type polyoxoanions have accounted for more than 50%of such compounds.This type of materials combine the advantages of POMs and transition metals/lanthanides,thereby regulating the energy level structures and functional properties of hybrid materials at the molecular level,all of which render them excellent performance in visible light catalysis,electrocatalysis,and photoelectrochemistry.In this paper,a series of redox-active metal clusters material were prepared by using a hydrothermal synthesis method based on different transition/rare earth metal clusters,N—containing ligands and hetropolyoxotungstates.These novel compounds are obtained by adjusting different pH values,stoichiometric ratios,and temperature.These conditions play important roles in the synthesis process.In addition,according to the structural characteristics of these compounds,their applications were explored in the fields of electrochemistry,photocatalysis,and fluorescence.1.One novel polyoxometalate—based hybrid 1 with multinuclear zinc clusters was synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffractions,PXRD,IR,TG and UV—vis spectra.Compound 1 contains a large cage consisting of two mutually symmetrical triangles and six quadrangles,which encapsulates the POM anions.The results showed that compound 1 was a mesoporous material with a pore size of 3.1 nm.To solve the problem that classic POMs can only absorb ultraviolet light,the transition metal fragments are introducted into the POMs so as to effectively adjust the energy level structures of the resulting compounds.As such hybrid materials can generate absorbance in the visible light region,the photo—reduction activities of CO2 under visible light are investigated for these compounds.Based on compound 1,two novel POM-based coordination polymers 2 and 3 were synthesized by using the photocatalytic activity of Co instead of Zn.Compound 2 exhibited a 3D structure with 4—connected[SiW12O40]and 3—connected[Co2(Htrz)3(H2O)3]secondary building units(SBUs).The 1D chain of compound 3 was constructed from[SiW12O40]and[Co3(Htrz)6(H2O)3Cl]SBUs.Furthermore,the photo—reduction of CO2 under visible light by the two cobalt—based POMs was investigated using[Ru(bpy)3]Cl2·6H2O as a photosensitizer.The CO yields of compounds 2 and 3 were 15705 and 18501 μmol·g-1 for the CO2 photocatalytic reduction under three hour irradiation at 293 K,respectively.The difference in the photocatalytic performance of 2 and 3 was explained by comparing the energy of the valence band,band gaps and conduction band.The results showed that the photocatalysts incorporated with multinuclear Co clusters could effectively improve photocatalytic activities.[Zn27(trz)36(Htrz)15(H2O)2(SiW12O40)4]·Cl2·xH2O(1)[Co2.67(SiW12O40)(H2O)4(Htrz)4]·Cl1.33(2)[Co3(SiW12O40)(H2O)(Htrz)6Cl]Cl·6H2O(3)Ligand=1,2,4-triazole(Htrz,C2N3H3)2.Based on the aboved research work,we introduced lanthanide metal ions to regulate the luminescent properties of POM-based hybrid materials.A discrete Tb-Keggin compound 4 was obtaind in high yield under the hydrothermal conditions.Structural analysis of 4 reveals that all the N atoms of ina ligands are uncoordinated and can serve as potential coordination sites to bridge transition metals.So,the copper cation was introduced into the reaction system and 5—8 and this realized the structural extension from 0D to 3D pillared structures in which the 2D sheets were composed of[Ln12(Hina)4(ina)4(H2O)2]or[Ln22(Hina)4(ina)4(H2O)4]and[SiW12O40]acted as pillars.In addition,all these compounds are characterized by XRD,IR,TG,UV-Vis and solid-state photoluminescence spectra.[Tb2(ina)2(Hina)6(H2O)2](SiW12O40)(4)[Tb2Cu(SiW12O40)(Hina)4(ina)4(H2O)2]·(Hina)2·(I2)0.5(5)[Eu2Cu(SiW12O40)(Hina)4(ina)4(H2O)2]·(Hina)2·(I2)0.5(6)[Tb2Cu(SiW12O40)(Hina)4(ina)4(H2O)3]-(Hina)2·H2O(7)[Eu2Cu(SiWI2O40)(Hina)4(ina)4(H2O)3]-(Hina)2·H2O(8)Ligand=isonicotinic acid(Htrz,C6NH5O2)3.Under hydrothermal conditions,by use of Pressler—type K12.5Na1.5[NaP5W30O110](P5W30),lanthanum nitrate and 2,5-pyridinedicarboxylic acid,we successfully synthesized five discrete Ln—containing POMs(9—13).In addition,all these compounds are characterized by XRD,IR,TG and solid-state photoluminescence spectra.KNa{[Tb4(H2O)32](NaP5W30O110)}(9)KNa{[Eu4(H2O)32](NaP5W30O110)}(10)KNa{[La4(H2O)24](NaP5W30O110)}·6H2O(11)KNa{[Gd4(H2O)30](NaP5W30O110)}(12)KNa{[Sm4(H2O)24](NaP5W30O110)}·3H2O(13)4.In the last part of this thesis,we carried out the initial exploration of POMs/POM-based hybrid materials in lithium-sulfur batteries.We designed and fabricated a bifunctional composite separator including a polypropylene(PP)matrix layer and Keggin polyoxometalate[PW12O40]3-/Super P composite retarding layer by utilizing the Coulombic repulsion between polyanion and polysulfides.Such a binary composite separator shows the effects in enhancing the Coulombic efficiency and cycling stability.Compared with the polypropylene(PP)matrix separator,the capacity is improved by 41%after 120 cycles when using the PW12/Super P separator.It is the first time that the polyoxometalate(POM)matrix is used as a bifunctional separator for lithium—sulfur(Li—S)batteries,demonstrating the promise of POM—based separators in reducing the shuttling effect of Li—S battery.