Bifunctionalized Metal-organic Framework As Fillers To Modify Proton Exchange Membranes

The utilization of clean and sustainable energy is the trend of global energy technology in the future,and the development and application of proton exchange membrane fuel cells（PEMFCs）are highly expected.Before the large-scale commercial application of PEMFCs can be realized,many key material components and infrastructure problems need to be overcome urgently.Among them,one of the most critical challenges is to develop low-cost,high-performance proton exchange membrane materials.Sulfonated polyether ether ketone（SPEEK）with high degree of sulfonation（DS）has proton conductivity comparable to Nafion（perfluorosulfonic acid ionomer membrane）,but its dimensional stability is poor and even soluble in water.SPEEK with low DS has good dimensional stability,but it has many end-closed channels and a low degree of microphase separation,which leads to a proton conductivity much lower than that of commercial Nafion.Therefore,the key to realizing SPEEK applications is to improve proton conductivity without sacrificing other properties of SPEEK.In order to connect the end-blocked proton transport channels and increase the degree of microphase separation in the low-DS SPEEK membrane,an amino-alkylsulfonic acid bifunctional metal-organic frameworks（MOFs）UNC3S（short for Ui O-66-NH-CH₂CH₂CH₂SO₃H）was prepared by using a post-synthesis step（Chapter 3）,and then as a filler for modified SPEEK.The characterization results of the composite membrane SPEEK/UNC3S-X showed that the UNC3S filler constructed a long-range conducting proton transport channel in the SPEEK membrane,increased the separation degree of the hydrophilic phase and shortened the distance of proton conduction.In addition,the acid-base pair interaction between the amino group of the filler and the sulfonic acid group of SPEEK enhanced the dimensional stability of the composite membrane and also provided more hopping sites for proton conduction.The proton conductivity of the composite membrane SPEEK/UNC3S-3 was increased from 140.41m S·cm^-1 of SPEEK to 186.388 m S·cm^-1,which was also higher than that of Nafion 117under the same conditions.On the basis of Chapter 3,in order to enhance the proton conductivity of SPEEK with high DS and coordinate its dimensional stability,another longer pendant alkylsulfonic acid chain-amino bifunctionalized MOFs filler（UNC4S,short for Ui O-66-NH-CH₂CH₂CH₂CH₂SO₃H）was prepared in Chapter 4 by grafting 1,4-butanesultone,and then incorporated into the SPEEK matrix to prepare a series of new composite membrane SPEEK/UNC4S-X.At 60°C and 100%RH,the proton conductivity of the composite membrane SPEEK/UNC4S-2 increased from 106.71 m S·cm^-1 of SPEEK to125.27 m S·cm^-1,and its dimensional stability was improved by 2.07%.In addition,SPEEK/UNC4S-2 exhibited comparable proton conductivities to SPEEK/UNC3S-2 at70°C and 100%RH,which were 229.10 m S·cm^-1 and 232.01 m S·cm^-1,respectively.Both of them were higher than that of recast SPEEK(183.41 m S·cm^-1)and Nafion 117(135.82 m S·cm^-1).Based on the effectiveness of the incorporation of amino-alkylsulfonic acid bifunctional MOFs fillers into the SPEEK matrix to improve membrane proton conductivity in the first two chapters of this paper,in order to reduce the energy and solvent consumption during the preparation and cleaning of MOFs nanofillers.Two organic ligands with equal ratios of amino-functionalized and sulfonic-functionalized organic ligands were added to react with metal salts,and then an amino-sulfonic acid bifunctional MOFs filler（NUS,short for NH₂-Ui O-66-SO₃H）was directly hydrothermally synthesized.Compared with the same loading of dual MOF filler UN（Ui O-66-NH₂）+US（Ui O-66-SO₃H）,NUS has better dispersion in SPEEK.The proton conductivity of the composite membrane SPEEK/NUS membrane increased from 151.27m S·cm^-1 of pure SPEEK to 177.76 m S·cm^-1,which was higher than that of SPEEK/UN+US and Nafion 117.