Preparation And Properties Of Functionalized Metal-organic Frameworks/Sulfonated Poly(Arylene Ether Ketone Sulfone) Composite Proton Exchange Membranes

Sulfonated aromatic polymers are considered as the most potential materials of proton exchange membranes(PEMs)that could replace Nafion^?membranes due to their excellent thermodynamic stability and good proton conductivity.Howere,with the increasing of the degree of sulfonation,the proton conductivity would increase,but the mechanical property and chemical stability of sulfonated aromatic polymers decreased.Therefore,it is neccessry to modify the sulfonated aromatic polymers to achieve the balance between high proton conductivity and good mechanical property.Organic-inorganic composite modification is a common method.In recent years,metal-organic frameworks(MOFs),as a new kind of organic-inorganic composite materials,have drawn many attention.MOFs are constructed by metal ions or metal-oxygen clusters coordinated with organic ligands.The adjustable structures of frameworks,pore diameter,surface properties,particle sizes,controllable guest molecules,and abundant active sites of MOFs have attracted much more attentions in the field of proton conduction.Functional groups such as-COOH and-OH of MOFs materials can build hydrogen bond networks and construct new proton transport channels.The porous structures of MOFs can also accommodate guest molecules as proton carriers to promote proton conductivities.However,it is difficult for MOFs to be used as proton exchange membranes directly.Therefore,the organic-inorganic proton exchange composite membranes fabricated by polymer matrixes and MOFs have attracted researcher's attention.The organic-inorganic composite proton exchange membranes which prepared by using polymer matrixes with MOFs have aroused researcher's concern.In this paper,the sulfonated poly(arylene ether ketone sulfone)containing carboxyl groups(C-SPAEKS)was selected as the polymer matrixes,and different functionalized MOFs were synthesized and doped into C-SPAEKS.The influence of the introduced MOFs materials on properties of composite membranes was studied.The C-SPAEKS was prepared by nucleophilic substitution polycondensation.MOF-801 with structure defects was synthesized through the solvothermal method by adding excess formic acids as the regulator.These structure defects confer MOF-801 high surface area(2476.34 m² g^-1).Organic-inorganic composite membranes were synthesized by combining the MOF-801 with structure defects and C-SPAEKS.Hydrogen-bond networks which formed between the MOF-801 and C-SPAEKS promoted the dispersion of MOF-801 in composite membranes and enhanced the proton conductivity of composite membranes.These composite membranes showed outstanding chemical and thermal stabilities.Meanwhile,at 90 ^oC and 100%relative humidity(RH),the highest proton conductivity of composite membranes reached to 0.100 S cm^-1,which was similar to that of Nafion 117.To further improve the proton conductivity of membranes,two kinds of imidazole functionalized MOFs were synthesized based on MOF-801 blank-tamplate.And composite membranes were fabricated with functionalized MOFs as fillers and C-SPAEKS as matrixes.The structures and morphologies of the prepared MOFs and composite membranes were characterized by XRD,FT-IR and SEM.Both Im@MOF-801 and Im-MOF-801 exhibited excellent stabilities in water and acid/base solutions.The proton conductivities of composite membranes were 0.050-0.128 S cm^-1 at 90 ^oC and 100%RH.Phosphotungstic acid can also act as proton carriers.The phosphotungstic acid-encapsulated MIL-100(Fe)(HPW@MIL-100(Fe))was synthesized by the in-situ direct hydrothermal method.Due to the large mesoporous cages and small microporous windows of MIL-100(Fe),HPW could be well accommodated in the cavities of MIL-100(Fe).Furthermore,novel composite proton exchange membranes were fabricated by incorporating HPW@MIL-100(Fe)into C-SPAEKS.The structures and properties of membranes were characterized.The HPW@MIL-100(Fe),with a large amount of phosphotungstic acid in cavities,could enhance the proton conductivities of composite membranes.The composite membrane with 4%content of HPW@MIL-100(Fe)achieved a high proton conductivity of 0.072 S cm^-1 at 80 ^oC and 100%RH.Meanwhile,the introduced HPW@MIL-100(Fe)fillers improved the dimensional stability of composite membranes.The swelling ratios of composite membranes at 80 ^oC were slightly lower than that of C-SPAEKS.In addition,the ionic liquid([EMIM][N(Tf)₂])was chosen to futher modify the HPW@MIL-100(Fe).And the phosphotungstic acid-ionic-liquid(HPW-ILs)modified MIL-100(Fe)(HPW-ILs@MIL-100(Fe))was obtained.The final product was introduced into polymer matrixes to prepare composite membranes.This series of membranes exhibited increasing proton conductivities from 0.043 S cm^-1 to 0.138 S cm^-1 at100 ^oC and 100%RH.Furthermore,these composite membranes have enhanced mechanical properties and dimensional stabilities,the Young's modulus were1884.73-2330.73 MPa,and tensile strength were 39.82-48.72 MPa.Meanwhile,HPW-ILs@MOF-4 has excellent performance of fuel cells,with a maximum power density of 269.71 m W/cm² at 80 ^oC and 100%RH.