Preparation And Properties Of Highly Efficient Polymer Electrolyte Membranes With Long Side Chain

Polymer electrolyte membrane fuel cells,which include proton exchange membrane fuel cells(PEMFC)and alkaline exchange membrane fuel cells(AEMFC),are a kind of clean and efficient new-energy sources power generation device.They are widely used in transportation,aerospace,submarine,power station and portable power.As one of the key components of fuel cells,proton exchange membrane(PEM)and alkaline exchange membrane(AEM)prevent the penetration of fuel and oxidant while selectively transporting protons or hydroxide ions.Considering the ionic conductivity and chemical stability of the current PEM and AEM need to be improved,polymer electrolyte membranes with long side chain functional groups were synthesized through molecular design in this paper.The purpose of this paper is to develop polymer electrolyte membranes with good comprehensive properties.The main contents and results are described as follows:Initially,a monomer(MO-DHPZ)containing methoxy and phthalazinone was synthesized.Then,poly(phthalazinone ether ketone)containing methoxy groups(PPEK-OMe)with a viscosity of 1.14 dL/g was prepared by the nucleophilic condensation reaction of MO-DHPZ and 4,4'-difluorobenzophenone(DFK).Finally,a series of long flexible alkylsulfonated side chains poly(phthalazinone ether ketone)(SFGPPEK-x,x is the number of-CH2-in the side chain)with different alkyl chain lengths were prepared by a demethylation procedure of PPEK-OMe and following sulfobutylation grafting reaction.The proton conductivities of SFG-PPEK-x membranes significantly improved.Especially,the SFG-PPEK-4 membrane(IEC=1.70 mmol/g)has a proton conductivity of 0.184 S/cm at 80? and 100%relative humidity,which is higher than the commercial Nafion(?)115 membrane(IEC=0.91 mmol/g,0.163 S/cm).This is because the SFG-PPEK-4 membrane has a higher IEC and well-defined microphase separation structure.The effect of side chain length on the performance of the membranes were systematically studied.As the length of the side chain increases,the IECs of the membrane increase,the water absorption and swelling degree decrease,the toughness of the membranes decrease,and the oxidation stability is enhanced.The effect of side chain length on the proton conductivities of the membranes is related to relative humidity.The proton conductivities at higher relative humidity(70%and 100%)decrease with increasing side chain length while those at lower relative humidity(30%and 50%)increase with increasing side chain length.The poly(phthalazinone ether ketone)copolymers containing methoxy groups(MPPEK-x,x is the molar ratio of MO-DHPZ to DFK)were prepared by the copolymerization of MO-DHPZ,4,4'-(hexafluoroisopropylidene)bisphenol,and DFK.Then,a series of poly(phthalazinone ether ketone)copolymers bearing flexible pendant quaternary ammonium groups(QAPPEK-x)were prepared by a demethylation procedure of MPPEK-x and following Williamson reaction.The QAPPEK-x membranes were prepared by solution casting.The structure of the as-prepared copolymers were confirmed by FT-IR and 1H NMR spectroscopy.The QAPPEK-x membranes with IECs in range of 1.22?1.62 mmol/g showed hydroxide conductivity of 0.026?0.048 S/cm and 0.044?0.092 S/cm at 30? and 80?,respectively.The swelling ratio of the QAPPEK-x membranes are 6.0-21.8%at 30? and 9.6-37.1%at 60?,respectively.The morphology of the membranes characterized by atomic force microscope(AFM)and transmission electron microscope(TEM),showed well-defined microphase separation structure.As the IEC increases,the ionic conductivity of QAPPEK-x membrane increases first and then decreases,reaching the maximum value when the IEC is 1.53 mmol/g.This may be because the water uptake increases with the increasing IEC.The large water uptake give rise to dilution effects that limited the charge carrier concentration,and thus the conductivity.The resulting QAPPEK-x membranes demonstrated excellent thermal stability,mechanical properties and alkaline stability.High molecular weight and ether-bond free poly(arylene piperidine)s(PAPips)were synthesized by the hydroxyalkylation polymerization of N-methyl piperidone with m-terphenyl using super-acid as catalyst.AEMs based on PAPips tethered with heterocyclic ammonium and quaternary ammonium via long alkyl chain were successfully prepared by functionalization with side groups.Changing the type of cationic groups(quaternary ammonium ion,piperidinium,morpholinium)have a great influence on the properties of the AEMs.These properties include micromorphology,ionic conductivity,mechanical properties and chemical stability.The membrane attached with quaternary ammonium group(Q-PAPip membrane)exhibited a highest hydroxide conductivity of 0.127 S/cm at 80? and 100%relative humidity.The membranes attached with piperidinium(P-PAPip membrane,IEC=1.64 mmol/g)exhibited a hydroxide conductivity of 0.082 S/cm at 80?.The P-PAPip membrane exhibited significant chemical stability due to its stable polymer backbone(without ether linkages)and stable heterocyclic ammonium group.The P-PAPip membrane maintained 80.1%of its conductivity after immersion in 1 M NaOH at 80? for 1500 h.The P-PAPip membrane maintained 62.8%of its weight after immersion in Fenton's reagent at 80? for 880 min.The swelling ratio of P-PAPip membrane is similar to that of the commercial Nafion(?)117 membrane.The resulting HEMs also showed excellent mechanical properties,thermal stability,and well-defined phase separation.The P-PAPip membrane also presented a peak power density of 76 mW/cm2 at 80? in a H2/O2 AEMFC.In order to further improve the chemical stability,while suppressing the swelling of the AEM,the Q-PAPip/PTFE pore-filled composite membrane was prepared by filling Q-PAPip in the pores of polytetrafluoroethylene(PTFE)microporous membrane.It can be observed in SEM images that the pores in the PTFE microporous membrane were filled by Q-PAPip.The properties of the composite membrane were studied.The swelling ratio of the composite membrane at 80? was about 50%of that of the Q-PAPip membrane.The dimensional stability of the composite membrane was significantly improved.The ionic conductivity of the composite membrane at 80? is higher than 0.1 S/cm,slightly lower than that of the Q-PAPip membrane.It's worth noting that the chemical stability of the Q-PAPip/PTFE composite membrane was significantly improved.The composite membrane maintained 96.5%of its ionic conductivity after immersion in 1 M NaOH solution for 300 h,which was about 1.5 times that of the Q-PAPip membrane.The Q-PAPip/PTFE composite membrane was still strong and tough after soaking in Fenton's reagent at 80? for 24 h while the Q-PAPip membrane began to break after soaking in Fenton's reagent at 80? for 5.3 h.The Q-PAPip/PTFE composite membrane maintained 81.3%of weight after soaking in Fenton's reagent at 80? for 24 h.The Q-PAPip/PTFE composite membrane which has good comprehensive performance is a potential candidate as AEM for AEMFC.