Molecular Design And Structure-Properties Study Of Polymer Electrolyte Membranes

In the majority family of fuel cells, direct methanol fuel cell (DMFC) and high terperature fuel cell (HTFC) are two kinds of fuel cells which were the most thoroughly studied and widely used. On the principle of power generation, the proton exchange membrane (PEM) inside the fuel cells conducted ions, while the closed loop of electrode outside generated the transmission of electrons, the conversion between the chemical energy of fuel and electrical energy was achieved. The efficiency of fuel cells depended on the the proton conduction properities of PEMs, which is the key component of the battery system. How to improve the proton conductivity and reduce fuel permeability, get integrated performance of PEMs were the focus of many researchers’study.DMFC is a kind of low temperature fuel cell, which operated below 80℃, and currently the commercial membrane materials were series of Nafion, which were produced by Dupont. Although the good proton conductivity and chemical stability of Nafion, the high production cost and fuel permeability had made it no longer to meet the development of the DMFC. Currently, the sulfonated poly(arylene ether) copolymer, which has good mechanical properties, high proton conductivity, excellent barrier properties of methanol and low cost, is widely researched as capable DMFC membrane materials. However, the water uptake and dimensional change of the membrane would increase with the ratio of sulfonated degrees, and the properties of methanol permeability and the mechanical properties would decrease. The method to solve this problem is the preparation of cross-linked membranes, and develop the new side chain sulfonated poly(arylene ether) copolymers.HTFC is a kind of fuel cells, the operating temperature of which was above 120℃. Compared with DMFC, HTFC has simple hydrothermal treatment system, high electrochemical reaction rate and strong CO tolerance. As for the R & D of PEMs used in HTFCs, the phosphoric acid doped polybenzimidazole (PA/PBI) composite membranes were the most widely reported. In recent years, with the deeping research of the poly(arylene ether ketone)(PAEK) electrolyte membrane materials, the quaternary ammonium phosphoric acid doped poly(aryl ether ketone) (PA/QPAEK) as a high temperature proton exchange membrane has drawn more and more attention. PAEK has excellent heat resistance, and many research reported that, compared with PBI, the PAEKs containing basic groups has a stronger phosphoric acid doped ability, and thus can achieve higher proton conductivity. Currently, the research on PA/QPAEK were mainly focused on the design of new quaternary ammonium poly(aryl ether ketone) copolymers.In the chapter three, the amino-substituted poly (ether ether ketone) (APEEK) and sulfonated poly(ether ether ketone) (SPEEK, IEC=2.07mequiv/g) have been synthesized via nucleophilic aromatic substitution reaction. The structures of APEEK and SPEEK were characterized by 1H NMR spectra. The composite membranes based on APEEK and SPEEK were confirmed by their FT-IR spectra, indicating the formation of intermolecular ionic cross-linking networks between amino and sulfonic groups. The water uptake, proton and methanol transport properties of composite membranes were also determined for fuel cell applications. The results showed that the composite membranes exhibit high selectivity, appropriate proton conductivities as well as reduced water uptake and methanol permeability when compared with the pristine SPEEK membrane. Furthermore, it should be noted that the intermolecular ionic cross-linking effectively improved the tensile strength, breaking elongation and thermal stabilities of the membranes. In particular, the SPEEK-10 membrane (the weight ratio of APEEK is 10%) showed the tensile strength of 121.2 MPa and breaking elongation of 93.5%, which was 1.5 times and 2.5 times higher than that of pristine SPEEK, respectively. The high selectivity, thermal and mechanical propertied indicate that the composite membranes are promising to be used as proton exchange membranes for direct methanol fuel cells.In the chapter four, in order to improve the proton conductivity, we designed and synthsized a novel side-chain-type sulfonated diphenyl-based poly(arylene ether sulfone)s with hydrogen-bonded network as proton exchange membranes. A novel biphenyl-based bisphenol monomer containing tetra-methoxy groups has been successfully synthesized in high yield by an oxidative-coupling reaction. Sulfonated poly-(arylene ether sulfone)s with two pendant aliphatic sulfonic acid groups and two-OH groups per repeating unit were obtained by sequential polycondensation, demethylation and sulfobutylation. The IEC of the SOPAESs (1.06-1.38 mequiv/g) can be readily controlled by using different-OH content in HOPAES-xx copolymers with 1,4-butanesultone via sulfobutylation reactions. In comparison with Nafion membrane, these membranes exhibited very low water uptake and dimensional change, even at elevated temperatures (80℃). The SOPAES-40 membranes displayed good proton conductivity, which were comparable to Nafion 117 at 100% RH. Moreover, the methanol permeability values of these SOPAES-xx membranes are much lower than Nafion and the relative selectivity are better. The SOPAES-xx membranes showed anisotropic membrane swelling in water with larger swelling in thickness than in plane, which was helpful to improve the dimensional stability. The SAXS profiles revealed that sulfonate groups aggregated into hydrophilic clusters to form a continuous network at 40%(SOPAES-40). The combination of facile synthetic routes for monomer and polymers, good relative proton to methanol transport, relatively low water uptake and swelling ratio makes these membranes attractive as PEM materials for further investigation in fuel cell applications.In the chapter five, a novel side-chain-type naphthalene-based poly(aryl ether ketone) with quaternary ammonium groups (Q-SCT-NPAEK) were prepared as phosphoric acid (PA) doped high temperature proton exchange membranes. The PA/Q-SCT-NPAEK-xx membranes used as HTPEM were prepared successfully from an epoxide ring-opening reaction between HNPAEK and EPTAC. Due to the introduction of naphthalene moieties in the main chain, Q-SCT-NPAEK-xx membranes had high Tg, thermal and mechanical stabilities. What’s more, the PA doped membranes still had good thermal and mechanical properties. Especially, the tensile strengths of PA/Q-SCT-NPAEK-xx membranes were comparable or even higher than PA/PBI membranes, which were prepared by immersing in 85 wt.%PA solution at 80℃ or 24 h. Both the weight uptake and the PA doping level were calculated, and it was found that the IEC values are the main factor to reflect the trend of conductivities. As the IEC increasing, the PA doping level and the proton conductivity increased. The PA/Q-SCT-NPAEK-100% with a highest PA doping level of 16.6 showed the highest proton conductivity of 46 mS cm-1 at 160 ℃. The PA/Q-SCT-NPAEK-80% and PA/Q-SCT-NPAEK-60% membranes still showed much higher proton conductivities than PA/PBI. These results indicated that the structure of rigid moiety in the main chain and functional group in the side chain improved the proton conductivity at high temperature under anhydrous condition, while maintaining a reasonable mechanical and thermal stability at a high PA doping level. The side-chain-type PA doped Q-SCT-NPAEK-xr membranes showed promising potential to be used as high-temperature PEMs.