Study On The Preparation And Characterization Of New Polymer Electrolyte-based Composite Proton Exchange Memrbane

Proton exchange membrane (PEM), a core component of Proton exchangemembrane fuel cell (PEMFC), determines the performances of PEMFC to a greatextent. Today, Nafion membrane produced by DuPont is still widely used in PMFC asPEM. Although the Nafion membrane has high proton conductivity, good chemicalstability and other advantages, it also has many disadvantages, such as protonconductivity strongly depends on the water content in membrane, high fuelpenetrability, performances rapidly decline in high temperature, complex productionprocess, high cost etc. These problems are important reasons for limiting the wideapplication of Nafion. Sulfonated aromatic polymer electrolytes are promisingmaterial for PEM due to its good mechanical properties, chemical and thermalstability. Especially, the low methanol permeability and production cost comparedwith Nafion. Therefore sulfonated aromatic polymer electrolytes are widelyconcerned by scientific and energy institutions. As we know, the proton conduction ismainly occurred in the hydrophilic region of PEM, continuous hydrophilic region isadvantageous to the proton conduction. In the sulfonated random copolymermembrane, sulfonic acid groups will aggregate to large clusters and form hydrophilicregions, and the distance between the hydrophilic regions is correspondingly big, sothe proton can not fast conducting in this structure. In order to obtain PEMs withexcellent comprehensive performances and continuous proton transmission channel,many methods have been used for modifying PEM.In this paper, base on sulfonated aromatic polymer electrolyte, we have constructeda serie of PEM with outstanding comprehensive performances, continuous ioniccluster network or long-range proton transmission channel by different approaches.The specific work is divided into four parts:In the first part, we introduced the poly N-isopropylacrylamide (PNIPAm) with alot of amido bonds into SPI via in situ polymerization reaction, and prepared a seriesof semi-interpenetrating polymer networks (semi-IPNs) with different content ofPNIPAm. The hydrogen bond interaction between sulfonic acid groups of SPI andamido bonds of cPNIPAm and the semi-IPNs structure could regulate themicrostructure and properties of composite membranes. When the molar ratio ofsulfonic acid groups and amido bonds is1:1, the SPI-20%-cPNIPAm membrane hassmall size ionic clusters (>6nm) with uniform and continuous distribution which isbeneficial to the proton conduction. At25oC, the proton conductivity ofSPI-20%-cPNIPAm membrane is one time higher than that of the pure SPI membrane, the power density is improved from44mW cm2of SPI to72mW cm2. The resultsshow that the proton transmission channels beneficial to the proton are formed. At thesame time, the semi-IPNs PEMs also have good mechanical properties and thermalstability.In the second part, we synthesized three kinds of GO with different sizes fromvarious sizes of graphite. Different sizes of GO were incorporated into SPI toinvestigate the size effect on the microstructure and performance of SPI/GOcomposite membranes. The study found that in the three kinds of GO, the small sizeGO has the biggest influence on the microstructure and performance of compositePEM at the same content. In the composite PEM with0.5wt%small GO, the ionicclusters are shrinked from25nm of SPI to6nm and dispersed uniformly, resulting inan advantageous microstructure for proton conduction. At25oC, the protonconductivity of this composite PEM is four times higher than that of pure SPI. Themechanical properties, oxidative stability, dimensional stability and methanol--resistance of composite membranes are also improved. This experimental resultsshow that the incorporation of GO can control the microstructure and optimize theperformances of composite PEM.In the third part, we synthesized a kind of organic silane coupling agent(SiSQ)with sulfonic group and N-heterocycle through the reaction between8-hydroxyquinoline-5-sulfonate and3-isocyanatepropyltriethoxysilane. In thepresence of surfactant (template), SiSQ was introduced into SPI matrix using sol-gelmethod, and obtained a series of SPI-MSiSQ composite membranes with differentMSiSQ content. The study found that a better compatibility is formed betweenMSiSQ phase and SPI matrix. The introduction of sulfonic group has diminished thedilution effect of the inorganic component to the concentration of sulfonic acid groupin SPI. The proton conductivities and methanol-resistances of composite membranesare increased with the increasing of MSiSQ content. At25oC, the composite PEMwith40wt%MSiSQ has the best result, high proton conductivity (0.23S cm-1), lowmethanol permeability (1.8×10-8cm2S-1), optimal selectivity (12.8×106Ss cm-3),which is3,6and23times higher than that of pure SPI respectively. The excellentperformances of the composite membranes can be summed up in two points: One isthe interaction between the nitrogen atoms of quinoline in MSiSQ and sulfonic acidgroups in SPI reduced the large size ionic clusters in SPI into smaller one, andforming a uniform and continuous proton transmission channel. Another is theinteraction between sulfonic acid groups of sulfonated organic siloxane and surfactant.When removing the surfactant, the sulfonic acid groups will gather within themesopores, which also provides a long-range proton transmission channel to proton.Cell performance of composite membranes and other properties are all improvedlargely, which is expected to be used in DMFC. In the fourth part, base on the polyester fibers, using the hydrogen-bondinginteraction between GO and sulfonated polyethersulfone (SPES), we layer-by-layerassembled GO and SPES on the surface of polyester fibers, then multilayer structureis formed on the surface of polyester fiber. In each layer, sulfonic acid groups arearranged along the axis of fiber, which provide long-range proton transmissionchannels, promoting rapidly proton conduction. The performances of compositemembranes base on SPES and multilayer assembled polyester fiber were tested. Theexperimental results show that, the proton conductivity of composite membrane isincreased by increasing the assembly layers. At the same time, the mechanicalproperties and methanol-resistances of the composite membranes are obviouslyincreased.