| Fuel cell?FC?is a novel energy conversion technology,which convert hydrogen,methanol,methane,and other kinds of renewable resources to electric energy efficiently,but without any pollution.Recently,FC has been developed rapidly for green energy vehicles with the strong advocacy and huge subsidies of government.Since the storage and transportation of methanol is much safer than hydrogen,direct methanol fuel cell?DMFC?is considered as one of the most practicably portable FCs.The internal composition and materials for DMFC are similar to those of hydrogen-oxygen fuel cell?PEMFC?.However,the current proton exchange membrane?PEM?used for PEMFC is not suitable for DMFC,because the perfluorinated sulfonic acid membrane?Nafion?suffers from severe methanol crossover,which will results in poor performance of DMFC.Hence,developing alternative PEMs with better methanol resistance for DMFC has drawn much attention in the past decades.Particularly,sulfonated poly?arylene ether?s?SPAEs?polyelectrolyte seems to be the most promising candidate or modifier for Nafion,because it possesses high proton conductivity,low methanol crossover,satisfactory chemical stability,and low cost.Common main-chain-type SPAEs have rigid molecular structure,and the sulfonic acid groups are attached to the backbone of them.According to previous research,these two structural features hinder the motion of polymer chain so much that main-chain-type SPAEs can't form distinct phase separation,which means much narrower ionic transport channels in SPAEs material.The narrow channels restrict the methanol diffusion and proton transport in the polyelectrolyte,conducing enhancement of methanol resistance as well as declining proton conductivity.Thus SPAEs need high degree of sulfonation?DS?for high proton conductivity to meet the requirement used as PEM.However,high DS will damage the dimensional stability of SPAEs-based PEM,leading to defective stability and short service life of DMFC.Therefore,developing a SPAEs-based polyelectrolyte membrane with both high proton conductivity and good dimensional stability is the prior task.At present,methods such as cross-linking,grafting alkyl sulfonic acid,and incooperating with inorganic nano-filler,et al have been carried out to improve dimensional stability of SPAEs-based PEMs.Among them,hybrid with inorganic nano-filler not only brings down swelling ratio,but also combines the advantages of inorganic and polymeric materials,which will conduce overall enhancement to the PEMs.Besides,SPAEs polyelectrolyte can also serve as modifier to Nafion matrix,for its good methanol resistance.The blending Nafion/SPAEs membranes show lower methanol permeability but higher proton conductivity,promoting their performance in DMFC.In chapter two,a sulfonated ionic liquid impregnated metal-organic-framework?IL@NH2-MIL-101?was prepared and introduced into sulfonated poly?arylene ether ketone?with pendent carboxyl groups?SPAEK?as the nano-filler for achieving hybrid PEMs?IL@MOF?.The IL@NH2-MIL-101 was anchored in SPAEK by the formation of amido linkage between pendent carboxyl groups attached to polymer chain and amino groups on the organic framework under hyperthermal-anhydrous condition.The massive amido linkage generates cross-linking structure in IL@MOF,which results in the enhancement in mechanical properties,chemical stability and methanol resistance.Besides,hybrid membranes exhibit superior proton conductivity up to0.184 S·cm-1 due to the sulfonated ionic liquid in the nano-cages of inorganic filler forming a high-speed proton passway.Moreover,the syphonage and electrostatic force between ionic liquid and NH2-MIL-101 contributes to a low leakage,consuming stable proton conductivity of IL@MOF membranes under fully hydrated condition.Furthermore,as a result of cross-linking structure formed by inorganic nano-filler,IL@MOF-1 shows a methanol permeability of 7.53×10-7 cm2 s-1,almost 26.5%lower than pristine SPAEK membrane.As a consequence,IL@MOF-1 performs the best single cell performance with a peak power density of 37.5 mW cm-2,which is 2.3-fold to that of SPAEK.According to the result of chapter two,the chemical bond between inorganic filler and polymeric matrix improves the compatibility of the two materials.Besides,the cross-linking structure induced by the massive chemical bond enhances the dimensional stability and methanol resistance of hybrid PEMs.Furthermore,the conductive medium in the nano-filler facilitates proton conductivity,as well as promotes single cell performance.However,the leakage of conductive medium usually causes a decline on durability of hybrid membranes.Hence,a novel amino-sulfo-bifunctionalized metal-organic framework?MNS?was synthesized via a hydrothermal technology and post-modification as the nano-filler.MNS itself serves as a proton conductive medium for the organic framework is covered with sulfonic acid groups,increasing proton conductivity of hybrid membranes but avoiding the loss when long-term use.In addition,the amino groups attached to the surface of MNS have electrostatic force with the sulfonic acid groups belonging to the side-chain-type sulfonated poly?arylene ether ketone??SNF-PAEK?polymeric matrix,so as to form ionic cross-linking structure in the hybrid PEMs?MNS@SNF-PAEK?.The ionic cross-linking structure is helpful to improve the mechanical properties,methanol resistance,and dimensional stability of MNS@SNF-PAEK.The introduction of MNS also leads to the rearrangement of polymer segment,where the hydrophilic groups of SNF-PAEK are induced by the electrostatic force and surrounding the MNS,generating larger ionic clusters for proton conducting.Among all samples,MNS@SNF-PAEK-3 exhibits the highest proton conductivity of 0.192S·cm-1 and selectivity of 2.88×105 S s cm-3 at 80°C,because the acceleration of proton conducting will be deducted if too much MNS forming over cross-linking in the hybrid PEMs.Since the good combination properties,MNS@SNF-PAEK-3performs a peak power density of 79.5 mW cm-2,which is 33%higher than pristine SNF-PAEK.On the basis of chapter three,the introduction of amino-sulfo-bifunctionalized metal-organic framework nano-fillers can facilitate proton conducting,hinder methanol transport,and reduce swelling ratio.However,the synthesis procedure of MNS is complex,and the yield is very low.Hence,an easily prepared nano-filler,possessing the similar feature with MNS,is necessary for amplifying the application prospect of hybrid PEMs.In chapter four,an amino functionalized metal-organic framework was prepared via hydrothermal synthesis,and flexible alkyl sulfonic acid is grafted to the framework only with one-step post-modification,and the received amino-sulfo-bifunctionalized inorganic nano-filler is named as MNCS.The same polyelectrolyte SNF-PAEK and preparation method to chapter three was used to obtain MNCS@SNF-PAEK hybrid membranes.According to results of characterization of MNCS@SNF-PAEK,we found the PEM MNCS@SNF-PAEK-1.5 could attain the equal properties to MNS@SNF-PAEK-3while the additive amount of MNCS is just half of MNS.It is account for the denser amino groups on the surface of MNCS form more ion pairs with SNF-PAEK,which will induce enough ionic cross-linking to enhance dimensional stability and methanol resistance.Since the sulfonic acid groups are on the end of the branched alkyl of the MNCS framework,it can form more continuous and wider ionic transport channel due to the larger free volume.Thus the proton transports more easily in the channel formed by MNCS,leading to the promoting proton conductivity of the hybrid PEM.Among all samples,MNCS@SNF-PAEK-1.5 exhibits the highest proton conductivity of 0.188 S·cm-1 and best selectivity of 3.04×105 S s cm-3.The peak power density of MNCS@SNF-PAEK-1.5 is 90.8 mW cm-2,almost 14%higher than that of MNS@SNF-PAEK-3 in chapter three.In brief,the incorporation of functionalized metal-organic-framework nano-filler to prepare hybrid SPAEs based membrane is proven to be suitable for application in direct methanol fuel cells.Though SPAEs polyelectrolyte used as modifier to improve the performance Nafion has been reported,the influence caused by SPAEs with various features has not been well studied.So in the chapter five,three kinds of sulfonated poly?arylene ether ketones?with different structures are introduced into Nafion as blenders to enhance the properties,especially methanol resistance of Nafion.Characterizations such as transmission electron microscope?TEM?,proton conductivity,methanol crossover,single cell performance,et al are carried out to evaluate these blending membranes?SPAEK@Nafion?,so as to identify what kind of structure of SPAEs can further enhance the properties of the composite Nafion PEMs.By investigating the microstructure of composite membranes,p-BPAF@Nafion is found to have the most homogeneous distribution and sea-island structure phase separation among all samples.The pendent sulfobutyl side-chain and fluorinated main chain of p-BPAF make it most similar structure with Nafion compared with m-BPAF and p-BPA,and such Nafion-liked structure provides p-BPAF a good compatibility with Nafion,which can facilitate its enhancement for Nafion.Consequently,when introducing the same amount of blenders,p-BPAF@Nafion-7.5 exhibits the highest proton conductivity of 0.256 S cm-1,and lowest methanol permeability of 1.87×10-6 cm2 s-1compared with other membranes.Besides,the mechanical properties of SPAEK@Nafion membranes are improved due to reinforce of SPAEs blenders,and the entanglement network formed by SPAEs and Nafion brings down the swelling ratio of composite membranes.Owing to high proton conductivity and good methanol resistance,p-BPAF@Nafion-7.5 possesses a better DMFC performance with a peak power density of 111.53 mW cm-2,when fed with 2 M methanol at 80°C,which is almost 3.1-fold to recast Nafion.According to the conclusion of chapter five,SPAEs blender with sulfobutyl side-chains and fluorinated main-chains can further facilitate the enhancement for Nafion matrix,due to its similar structure with Nafion,which provides a good compatibility with Nafion.In addition,it has been found a thinner PEM can lead to a lower Ohm resistance which contributes to higher cell efficiency and lower cost,but usually accompanying higher methanol crossover.Therefore,we proposed to further improve the performance of S@N via reducing the membrane thickness?55?m?in chapter six.Meanwhile,a cross-linking structure was induced into SPAEs blender via reacting with a fluorinated epoxy resin monomer?fEO?to form a semi-interpenetrating polymer network?semi-IPN?with Nafion for mitigating high methanol crossover of those thinner blending membranes?S@N/fEO?.Then a series of Nafion based composite membranes?S@N/fEO?with different fEO contents were prepared,characterizations including proton conductivity,methanol permeability,single cell performance,et al were carried out to seek the best dosage of fEO via casting method and treatment at high temperature to complete the cross-link reaction.On account of the semi-IPN in blending membranes,both mechanical properties and dimensional stability of S@N/fEO are enhanced.Besides,the proton conductivity decreases from 0.202 S cm-11 to 0.178 S cm-1 and the methanol permeability declines from 1.82×10-66 cm2 s-11 to 1.13×10-66 cm2 s-1,while the selectivity rises from 1.11×105 S s cm-3 to 1.57×105 S s cm-3 with more addition of fEO.So it proves that the semi-IPN restricts the mass transfer of methanol rather than proton.Although S@N/fEO-6 has a slightly lower proton conductivity(0.198 S cm-1)than S@N membrane(0.202 S cm-1),it was found to possess the best single cell performance with a peak power density of 180.9 mW cm-2,which is 30%higher than Nafion 212and 62%higher than p-BPAF@Nafion-7.5 in above chapter.In summary,the thinner composite Nafion PEM with semi-IPN structure by blending cross-linked SPAEs shows good performance in DMFC and has the strong application value. |
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