Preparation and Properties of High-performance Crosslinked Proton Exchange Membranes Based on Sulfonated Poly(Arylene Ether Ketone)s

Author:Liu Zhen Chao

Supervisor:liu bai jun


Degree Year:2019





The fuel cell is considered as a promising future energy conversion technology because of their inherent advantages of electrochemical conversion over thermal combustion processes.Among the various types of fuel cells,the proton exchange membrane fuel cell(PEMFC)is gaining much interest because of its capability of producing high-power densities working at low temperature.A proton exchange membrane(PEM)is a key component in the development of PEMFCs,and Nafion-type membranes are the most commonly used materials as PEMs in these devices.Nafion membranes have good chemical and physical properties suitable for the harsh operating conditions of fuel cells;however,their high cost and fuel crossover limit the extent of their further application and commercialization.Much effort is currently focused on the development of alternative and economically viable sulfonated polymers,such as sulfonated poly(arylene ether ketone)s(SPAEK),sulfonated polysulfone,sulfonated polyimides,and sulfonated polybenzimidazoles,because of their respective advantages.Among the numerous potential alternatives,SPAEKs have drawn considerable attention due to their excellent properties and cost competitiveness.Obtaining a high degree of sulfonation(DS)is generally necessary for sulfonated aromatic polymers to attain the required conductivity comparable to that of Nafion.However,excessive swelling ratio(SR)caused by high DS causes poor dimensional stability and mechanical properties under high temperature and humid circumstance.Thus,it is quite necessary to resolve this contradiction.In the first part,a kind of amino-functionalized SPAEK with high degree of substitution(DS)was designed and synthesized(Am-SPAEK-1.0).Due to the high swelling ratio(74%,100℃)of the membrane based on Am-SPAEK,we chose to crosslink the membranes to construct a three-dimensional network structure.A kind of aldehyde-functionalized polysulfone(PSF-CHO)was used as the crosslinker to avoid the residues of small molecular compounds and increase the crosslinking efficiency.We used lithiation chemistry under low temperature to complete the chemical modification for the conversion of PSF into soluble PSF-CHO.The conductivity of the crosslinked sample SPAEK/CHO-15 was 0.0925 S/cm at 80℃and even reached0.1581 S/cm at 100℃.The corresponding swelling ratios were 21.8%and 48.7%,which declined 39%and 35%comparing with the pristine Am-SPAEK-1.0.Because of the high DS of Am-SPAEK-1.0,the crosslinked membranes of last chapter could not reach an ideal level.In this part,another amino-functionalized SPAEK with low DS was designed and synthesized(Am-SPAEK-0.8).The membrane based on Am-SPAEK-0.8 with low swelling ratio(17.0%at 100℃)had a low conductivity of 0.0805 S/cm at 100℃.A new kind of SPAEK modified with organic siloxane was synthesized by the reaction of Am-SPAEK-0.8 andγ-(2,3-epoxypropoxy)propytrimethoxysilane(GPTMS).Three series of hybrid membranes were prepared by blending the inorganic particles(TiO2,SiO2 and PWA)and silane-grafted SPAEK.The side chain of organic siloxane served as a“bridge”between organic phase and inorganic phase.Compared with the pristine membrane,all the hybrid membranes showed an improved conductivity.The highest conductivities at100℃of every series could reach 0.1472 S/cm,0.1480 S/cm and 0.1463 S/cm,respectively.As the inorganic particle amount added reached a certain value,proton conductivity showed an obvious decline.This phenomenon may be due to the aggregation of particles,loss of water-soluble particles and dilution of IEC value.In consideration of the negative effects caused by the inorganic particles,we attempted to introduce more–SO3H griups into the system to increase the IEC value directly.In this part,3-mercaptopropyl trimethoxysilane(MPTMS)was used as supplier of extra–SO3H groups.The SC-SPAEK/SH-x membranes containing different contents of MPTMS were immersed in 3%H2O2 at 60℃after film-forming process.The highest conductivities of the SC-SPAEK/SO3H-6 sample were 0.0910S/cm at 80℃and 0.1539 S/cm at 100℃.Because of the crosslinked structure after hydrolysis of the organic siloxane,all of the membranes possessed low swelling properties.The highest swelling ratio of this series of membranes was even less than26.4%,which is as expected.In the last part,a kind of amino-functionalized SPAEK having fluorinated main chains(Am-6F-SPAEK)was designed and synthesized,and its IEC value was similar with that of Am-SPAEK-0.8.Besides,a water-soluble SPAEK(Am-SPAEK-H)with high IEC was synthesized to work as the second component to increase the proton conductivity.In order to avoid the loss of water-soluble oligomer and improve dimensional stability,two kinds of three-dimensional networks were constructed by using epoxy resin and silane coupling agent respectively.The introduction of the Am-SPAEK-H was helpful in improving the samples’conductivities.The highest conductivities at 100℃of the two series of crosslinked membranes,EP-SPAEK-15and SC-SPAEK-15,were 0.1845 S/cm and 0.1876 S/cm.Compared with the EP-SPAEK-x series membranes,the SC-SPAEK-x series membranes showed better performance in respects of water uptakes and conductivities,which could be explained as the result of extra hydrophilic–OH groups.A methanol-O2 fuel cell based on the sample SC-SPAEK-10 showed a peak power density of 31.67 mW/cm2at 80℃,which was much higher than that of Am-6F-SPAEK.In summary,by adjusting the balance between proton conductivity and dimensional stability,we prepared crosslinked PEMs based on SPAEK with tunable properties.