Design,Synthsis And Property Of Ion-Conductive Metal-organic Frameworks

With the increasing global energy demands and the serious environme nt a l pollution,the development and utilization of new energies become the only alternat ive to traditional fossil fuels.Proton exchange membrane fuel cel s?PEMFCs?with clean and efficient feature and sodium-ion batteries?SIBs?with safe and low-cost feature have attracted more attentions.As the core of this two kinds of energy conversion or storage the research of electrolyte used in these technology receives much more attention.Recently,metal-organic frameworks?MOFs?have undergone a repid development and become one of the most important research fields in chemistry and materials field.Owing to their easy synthesis,diverse structures,specific properties/functions,abundance in types et al.,these materials have wide applicatio ns ranging from gas sorption and storage,separation,sensing,and catalysis.Recently,MOFs has been realized to have great potential in ion conductivity,due to their crystal ine feature with discernible ion transport pathway.However,the low ion conductivity hinder the practivcal utilization of these MOFs.Considering this fact,the relationship between the structure and conductivity of several MOFs is investigated and thus,two strategies are proposed to tune or improve the proton conductivity of MOFs.Moreover,MOFs firslty applied as sodium-ion solid electrolyte has also been studied,and a method of synthesis MOFs composite as Na⁺solid electrolyte is proposed.The main contents are as follows:First,we choose the stable isostructural UiO-66-X?–X=–SO₃H,–2COOH,–NH₂,–H and–Br?family as a platform to investigate the strategy of tuning proton-conducting properties of MOFs.The proton conductivity of thses MOFs follows the order:UiO-66-SO₃H?UiO-66-2COOH>UiO-66-NH₂>UiO-66>UiO-66-Br.At 25 ^o C and 97%relative humidity?RH?,??UiO-66-SO₃H?and??UiO-66-2COOH?is 3.4×10^–3 and1.0×10^–3 S cm^–1,respectively,obviously higher than those of other three MOFs.Given the different functional groups,the effect of hydrophilicity,acidity,size effects,number and polarity of the functional groups on proton conductivity were investiga ted comprehensively.The interaction between water molecules and MOFs was analyzed by thermogravimetric-mass spectrometry?TG-MS?,and the isosteric heats(-Q_st)of water adsorption and radial distribution functions?RDFs?between the H atoms of functional groups?or Br atoms in UiO-66-Br?and O atoms of adsorbed water molecules in these MOFs were also estimated by molecular simulation.The acidity and hydrophilicity of-SO₃H and-COOH groups have an important influence on the formation of hydrogen bonding networks in MOFs,eventually contributing to high proton conductivity of UiO-66-SO₃H and UiO-66-2COOH.Meanwhile,the results found that the number and/or size effects of functional groups also have a significa nt ly influence the formation of hydrogen bond networks and subsequent conductivity of MOFs.This study not only proves that-SO₃H and-COOH could improve the proton conductivity of MOFs,but also propose a strategy to tune the proton conductivity of MOFs by functional groups.Second,on the basis that–SO₃H group is beneficial for improving proton conductivity,a ligand with two–SO₃H functional groups,4,8-disulfonic acid-2,6-naphthalene dicarboxylic acid,was used to react with Cr?NO₃?₃·9H₂O in N,N'-dimethylformamide?DMF?under solvothermal conditions,and a new MOF,BUT-8?Cr??BUT=Beijing University of Technology?,was obtained.In order to determine the structure,an isostructural Al?III?analogue,BUT-8?Al?,single crystal was also synthesized.The structure analysis shows that BUT-8?M?(M=Al³⁺,Cr³⁺)structure possesses non-protonated sulfonate group functionalized one-dimentional channels with a large amount of NH₂?CH₃?₂⁺as counter-ions.Thus,a new MOF,BUT-8?Cr?_a,with high density–SO₃H group functionalization was obtained by ion-exchange in sulfuric acid solution.High flexibility of this MOFs is revealed by PXRD and gas adsorption,which has a close relationship with RH.For comparison,MIL-101-SO3H with a rigid structure was also synthesized.The alternating current?AC?impedance plots demonstrate the flexible MOF is capable of"self-adapting"its strucuture following the changed humidity,leading to higher proton conductivities.The conductivity of BUT-8?Cr?_a can reach up to 6.23×10^–3 S cm^–1 at 65%RH and 25 ^oC,which is significantly higher than that of most MOFs at low RH.Especailly,the conductivity of BUT-8?Cr?_a ranges up to 1.27×10^–1 S cm^–1 at 100%RH and 80 ^oC,exceeding the proton conductivity of all reported MOFs.The rigid MIL-101-SO₃H,however,only represent a highly-RH dependent but lower proton contuvtivity due to lack of flexibility.Meanwhile,BUT-8?Cr?_a also exhibits high chemical stability,thermal stability and electrochemical stability.In addition,in order to further explore the effect of–SO₃H groups on flexibility and conductivity,BUT-9?Cr?with no–SO₃H group on ligand was synthesized and examined.The results proved that–SO₃H could enhance the sensitivity of flexibility of MOFs,thereby increasing the proton conductivity.Hence,this work provides the great promise of developing flexible MOFs for the extremely high proton conductivities at a wide temperature and humid range,thus facilitating their potential applications in proton exchange membrane fuel cel s.Third,in the light of our previous work related to proton conductive MOFs,we explored a MOF-based sodium ion solid electrolyte.In this work,MIL-101-SO₃H as precursor was used to prepare MIL-101-SO₃Na in Na₂SO₄ solution.Then,ionic liquid?IL?1-ethyl-3-methylimidazolium bis?trifluoromethylsulfonyl?imide?EMIm-TFSI?and sodium bistrifluoromethanesulfonamide?Na-TFSI?were introduced into the pores of MIL-101-SO₃Na by a simple but effective soaking-volatilizing process.A series of IL@MIL-101-SO₃Na composites with different IL contents were obatined.The effects of ionic liquid content on pore volume,stability,morphology and Na⁺conductivit y were investigated.With the increase of IL content,the X-ray diffraction peak of MIL-101-SO₃Na decreased gradually and disappeared eventually,but after being washed,it could be recovered,indicating IL could enter the MOF pores.The Na⁺conductivity test showed that the conductivity of the composites improved with the increase of IL content.Compared to Na-TFSI,EMIm-TFSI has prominent contribution to conductivity.When the amount of EMIm-TFSI and Na-TFSI in 100 mg MIL-101-SO₃Na reaches 100?L and 30 mg,the Na⁺conductivity of the composites reaches up to 6.13×10^–3 S cm^–1 at150 ^oC and N₂ atmosphere.This is the first attempt for Na⁺conductive MOFs,which provides theoretical guidance for the subsequent study of Na⁺conductive MOFs.