Sulfonated Poly(Aryl Ether)s Containing Bis-phthalazinone Moieties For Proton Exchang Membranes

Confronted with the increasingly serious energy situation and environmental crisis,the world spotlights the development of new energy and clean energy.As a kind of clean energy device,proton exchange membrane fuel cells（PEMFCs）are developing rapidly.As the core component of cells,the research of proton exchange membranes has attracted much attention.Perfluorosulfonic acid（PFSA）membranes are favored with PEMFCs for their outstanding proton conduction capacity and oxidation resistance stability;however,their high cost limits the widespread application of PFSA membranes in PEMFCs,so inexpensive sulfonated poly（aryl ether）s（SPAEs）proton exchange membranes are developing rapidly.Because of the inherent characteristics of the molecules of membrane materials,the proton conductivity of SPAEs proton exchange membranes is slightly inadequate,and the free radicals generated from the electrode reaction will attack the ether bonds and other connecting units in the membranes.The performance of the membrane will be eroded under the attacks of radicals,which restricts their practical application and promotion in fuel cells.Therefore,it is significant to adjust the molecular structure of membrane materials and prepare SPAEs proton exchange membranes with high proton conductivity and stability.In this investigation,poly（aryl ether ketone）s containing bis-phthalazinone moieties were synthesized and modified by post-sulfonation to prepare sulfonated poly（aryl ether ketone）s containing bis-phthalazinone moieties with low ether bond content based on molecular design.The introduction of pendant-benzenesulfonic groups depresses the hydrated degree of the main chain of the sulfonated poly（aryl ether）,and decreases the attack of free radicals on the main chains.And the content of ether bonds in molecular chains is decreased by increasing the content of phthalazinone units.Combining the above two approaches improve the oxidation stability of the membrane materials to prepare the proton exchange membranes with excellent proton conduction capacity and oxidation stability.The poly（bis-phthalazinone ether ketone）s containing pendant phenyl moieties were prepared with 4-（3′-phenyl-4′-hydroxyphenyl）-2,3-phthalazinone（DHPZ-P）,4,4′-di（4′′-1′′（2H）-phthalazinone）-1,1′-biphenyl（BPBN）,and 4,4’-difluorodiphenyl ketone（DFK）,which performed favorable thermo-resistance and mechanical properties.The poly（bis-phthalazinone ether ketone）s containing sulfonated pendant phenyl moieties（SPBPEK-Ps）with low ether bond content were prepared by modification of PBPEK-Ps with concentrated sulfuric acid,the ion exchange capacity（IEC）of which was in the range of 0.80-1.69 mmol g^-1.The SPBPEK-Ps proton exchange membranes were prepared by the solution-casting method.The SPBPEK-Ps membranes exhibited proton conductivity of ranged from 3.9 to 125.0 m S cm^-1,favorable dimensional stability,and mechanical properties.SPBPEK-Ps membranes performed enhanced oxidation stability with the rupture time longer than 2.5 h in Fenton’s reagent at 80°C,which was longer than that of sulfonated poly（ether ether ketone）s membrane（SPEEK membrane,0.75h）and sulfonated poly（phthalazinone ether ketone）s membrane（SPPEK membrane,1.5h）.The oxidation stability of SPBPEK-Ps membranes was enhanced along with the improvement of BPBN units in the chains,while IEC decreased resulting in a reduction of proton conduction capacity.The molecular chains were broken at the links of C-S bonds,ether bonds,Ph-CO-Ph,C-N-C bonds,etc.,under the attack of radicals,causing decay in the comprehensive performance of SPBPEK-Ps membranes.The SPBPEK-Ps membrane electrodes were prepared and assembled into H₂-O₂ fuel cells with a maximum power density(P_max)in the range from175 to 1210 m W cm^-2 at 80°C,and the SPBPEK-Ps membranes exhibited favorable cell performance.The enhancement of flexibility of the molecular chain would enhance the proton conduction ability of the membrane by introducing 1,4-dibenzoylbenzene units in the chains.Poly（bis-phthalazinone ether ketone ketone）s containing pendant phenyl moieties（PBPEKK-Ps）were designed and prepared from DHPZ-P,BPBN,and 1,4-bis（4’-fluorobenzoyl）benzene.The thermal resistance and mechanical strength of PBPEKK-Ps were slightly lower than those of PBPEK-Ps due to the enhanced flexibility of molecular chains.The poly（bis-phthalazinone ether ketone ketone）s containing sulfonated pendant phenyl moieties（SPBPEKK-Ps）with low ether bond content were achieved by post-sulfonated modification of PBPEKK-Ps,performing IEC in the range from 0.90 to 1.46 mmol g^-1.SPBPEKK-Ps proton exchange membranes are prepared by a solution casting method.The enhancement in flexibility led to an increase in the stacking density of the molecular chains resulting in a decrease in the water absorption and swelling behavior of the SPBPEKK-Ps membrane.The SPBPEKK-Ps membranes performed proton conductivity ranging from 4.4-137.0 m S cm^-1,the proton conduction performance of which was higher than SPBPEK-Ps membranes.The rupture time of SPBPEKK-Ps membranes was longer than 3 h in Fenton’s test at 80°C.And the performance decay of SPBPEKK-Ps membranes was more serious than that of SPBPEK-Ps membranes with the similar IEC under the same condition resulting from BFBB units improving the attack sites of benzoyl groups in chains.The SPBPEKK-Ps membrane electrodes were prepared and assembled into H₂-O₂ fuel cells with a maximum power density of between 800 and 1790 m W cm^-2 at 80°C,and the fuel-cell performance of SPBPEKK-Ps membranes was better than SPBPEK-Ps and Nafion membranes.A combination of pendant benzenesulfonic groups and biphthalazin-dione units could endow the sulfonated polymer with enhanced IEC and reduced ether bond content,which would improve the proton conduction ability and oxidation resistance stability of the membranes at the same time.So copoly（aryl ether ketone ketone）s containing diphenyl-biphthalazin-dione units（PDPEKKs）with excellent thermal resistance and mechanical performance were created from BFBB,diphenyl-biphthalazin-dione（DBD）,and 4-（4′-hydroxyphenyl）-2,3-phthalazinone（DHPZ）.Sulfonated poly（aryl ether ketone ketone）s containing diphenyl-biphthalazin-dione moieties（SPDPEKKs）with IEC of between 0.91-2.19 mmol g^-1 were prepared by post-sulfonated modification of PDPEKKs with fuming sulfuric acid,the pendant benzenesulfonic groups of which were not adjacent to the ether bond.SPDPEKKs proton exchange membranes were prepared by a solution-casting method with the performance of water absorption of between 17.7 and 71.3%,swelling ratio of between 6.3 and 26.1%,which were better than Nafion membrane（20.6-29.5%,and 32.8-37.9%）,and tensile strength of above 31 MPa,exhibiting good dimensional stability and mechanical properties.Improving the density of pendant benzenesulfonic acid groups enlarged the difference in thermodynamic states between the main chain and side groups,which induced a developed proton conduction network and enhanced the proton conduction capacity of the membrane.So SPDPEKKs membrane performed a proton conduction performance with the proton conductivity of between 9.1 and256.0 m S cm^-1.The rupture time of SPDPEKKs membranes was longer than 4 h in Fenton’s reagent at 80°C.The mass retention,molecular weight retention,IEC retention,and proton conduction retention of SPDPEKKs membranes were better than those of SPBPEK-Ps and SPBPEKK-Ps membranes under similar conditions,and exhibited a wonderful oxidation stability.The SPDPEKKs membrane electrodes were prepared and assembled into H₂-O₂ fuel cells with P_max scattering from 1040 to 2300 m W cm^-2 at 80°C.The SPDPEKKs membranes exhibited the cell performance better than Nafion membrane,presenting excellent fuel-cell performance.