Triphenyl Phosphine Oxide Sulfonic Acid Proton Exchange Membrane Of Preparation And Characterization

Proton exchange membrane (PEM) is one of the key components of proton exchange membrane fuel cells (PEMFC). At present, Nafion is the state-of-the-art PEM. However, it shows some drawbacks such as low conductivity at high temperatures or low humidity, high methanol permeability, and expensive cost, which impede its broad applications. Therefore, great efforts have been made to develop non-fluorinated aromatic polymer PEM with excellent overall properties.The objective of this dissertation aims at the shortcomings of aromatic polymer proton exchange membranes, i.e., the poor oxidative stability and high swelling at high ion exchange capacity (IEC). A series of triphenyl phosphine oxide-containing PEM were designed and prepared according to our previous studies related to the oxidative stability and swelling of PEM. At the same time, the relationship between the structure and the properties was investigated in detail. The comparison studies of oxidative stability between sulfonated poly(arylene ether phosphine oxide)s and sulfonated poly(arylene thioether phosphine oxide)s demonstrated that the latter showed much better resistance to oxidation, and the reason was expounded; The triphenyl phosphine oxide-containing PEM with low swelling was prepared by incorporation of phthalazinone units; Finally, the triphenyl phosphine oxide based PEM, with low swelling as well as high oxidative stability, was synthesized by introduction of functional groups. The detailed content is narrated as follows.Bis(4-fluorophenyl)phenyl phosphine oxide (BFPPO) was synthesized by Grignard reaction, and sulfonated bis(4-fluorophenyl)phenyl phosphine oxide (sBFPPO) was prepared by sulfonation of BFPPO.A series of sulfonated poly(arylene thioether/ether phosphine oxide)s was prepared by direct polycondensation of sBFPPO and BFPPO with the diphenol-type or dimercapto-type monomers such as hydroquinone (HQ), bisphenol-A (Bis A), 4,4'-biphenol (BP), 4,4'-(Hexafluoroisopropylidene)diphenol (6F), 1,5-dihydroxynaphthalene (NA), 4,4'-dihydroxybiphenyl (DB), and 4,4'-thiobisbenzenethiol (TB), respectively. The structural effect of the diphenol-type or dimercapto-type monomers on the properties of the products was investigated. Sulfonated poly(arylene ether phosphine oxide)s derived from Bis A and NA show the worst oxidative stability among the products because the former possess theα-H with low bond strength and the latter have naphthalene rings with low energy of conjugation. In contrast, sulfonated poly(arylene ether phosphine oxide)s derived from HQ, BP, Bis F, and DB indicate better oxidation stability than the products from Bis A and NA. The structure of sulfonated poly(arylene ether phosphine oxide)s from DB is similar to that of sulfonated poly(arylene thioether phosphine oxide)s (sPATPO) from TB. The latter could be regarded as the"product"of the former provided that the ether unit of the former was replaced by the thioether unit. The studies illustrated that the latter exhibited much better oxidative stability than the former. This is due to the fact that the thioether unit of the former was oxidized into the sulfone or sulfoxide unit in Fenton reagent, which inactivated the adjacent benzene rings and thus retarded the degradation reaction (addition reaction). Similar to sulfonated poly(arylene thioether sulfone/ketone)s, sulfonated poly(arylene thioether phosphine oxide)s denote better oxidative stability than sulfonated poly(arylene ether phosphine oxide)s.A series of sulfonated poly(arylene thioether phosphine oxide)s (sPATPO) with various degrees of sulfonation was prepared by polycondensation of 4,4'-thiobisbenzenethiol with the mixture of sBFPPO and BFPPO at different ratios. The effect of IEC on the properties of the products was studied. The investigations demonstrated that the Td5 of the products decreased with increasing IEC and that the molecular weight, Tg, water uptake and swelling as well as proton conductivity of the products enhanced with the increase of IEC. The membranes with the sulfonation degrees of 90% and 100% show an abrupt swelling at 70 oC while the membrane with the sulfonation degrees of 80% indicates a sharp swelling at 80 oC, losing most of the mechanical strength. The membrane with a sulfonation degree of 70% exhibits no abrupt swelling even at 90 oC but a low conductivity of 0.041 S/cm. Therefore, sulfonated poly(arylene thioether phosphine oxide)s with high IEC denote excellent oxidative stability but high swelling.A series of sulfonated poly(phthalazinone ether phosphine oxide)s (sPPEPO) were prepared via polycondensation of 4-(4-hydroxyphenyl) phthalazinone with sBFPPO and BFPPO. The incorporation of phthalazinone units into the backbone leads to forming the powerful intermolecular H-bonds, decreasing the swelling of the products. sPPEPO show low swelling as well as other excellent properties. sPPEPO with a sulfonation degree of 100% exhibit a IEC of 1.69 meq/g and a swelling of 19.5% at 80oC, which are close to that (20%) of Nafion 117. Moreover, sPPEPO indicates a conductivity of 0.19 S/cm at 80oC, two times higher than that of Nafion 117. On the contrary, sulfonated poly(arylene thioether phosphine oxide)s, with a IEC of 1.60 meq/g, even denote a swelling of 70.1% at 80oC and thus lose most of the mechanical strength. Thus sPPEPO shows much lower swelling than sPATPO.In order to combine the excellent oxidative stability of thioether units and the dimensional stability of phthalainone units, bis(4-phenyl-2,3-phthalinone)thioether with both the thioether group and the phthalainone group was designed and synthesized, and sulfonated poly(diphthalazinone thioether phosphine oxide)s (sPDTPO) were prepared by direct polycondensation of bis(4-phenyl-2,3-phthalinone) -thioether and sBFPPO. For comparison, bis(4-phenyl-2,3-phthalinone)ether and sulfonated poly(diphthalazinone ether phosphine oxide)s (sPDEPO) were also prepared. The difference of the oxidative stability between sPDEPO and sPDTPO was studied, the results demonstrated that the latter show better oxidation stability than the former.The swelling of sPDTPO and sPDEPO increases with increasing temperature at the temperatures lower than 50 oC, then decreases slightly and remains steady. They show a low swelling of 6.6% and 9.8%, respectively. This is due to the fact that the phthalainone groups and sulfonic acid groups form the powerful intermolecular H-bonds. As expected, sPDTPO exhibit better oxidative stability than sPDEPO because the thioether unit was oxidized into the sulfone unit, which inactivates the adjacent benzene rings. From the viewpoint of molecular design, the thioether group and phthalainone group were introduced into the main chain of sPDTPO, endowing them with excellent oxidative stability and low swelling. This provides an idea for preparation of PEM with oxidative stability as well as low swelling. sPDTPO and sPDEPO have a IEC of 1.21 and 1.23 meq/g, while they indicate a conductivity of 2.4×10-3 and 7.0×10-3 S/cm, respectively. The low conductivity of sPDTPO could be improved by increasing IEC in the future work.In order to decrease the swelling of the family of sPATPO and still remain their oxidative stability, sPATPO containing benzoxazole groups (sPATPO-bo) were prepared by polycondensation of 2,2'-bis(2-(4-fulorophenyl)benzoxazole) -hexafuloropropane and sBFPPO with the dimercapto-type monomers . The benzoxazole unit was incorporated into the backbone of the products by polycondensation in order to produce the intermolecular H-bonds between the benzoxazole and sulfonic acid group, which made the products possess low swelling. Similarly, sPATPO containing nitrile groups (sPATPO-cn) were prepared by polycondensation of 2,6-difluorobenzonitrile and sBFPPO with 4,4'-thiobisbenzenethiol. The nitrile group was attached onto the main chain of the products, leading to form the powerful intermolecular interaction in the products and provide the products with low swelling.sPATPO-bo have excellent overall properties. sPATPO-bo with a IEC of 1.68 meq/g shows a T5d of 399 oC, a Tg of 277 oC, while it indicates a swelling of 12.5% and a conductivity of 0.13 S/cm at 80oC. Moreover, its membrane exhibits excellent oxidative stability, which starts to break into pieces after immersing in Fenton reagent for 240 min and completely dissolved after immersing in Fenton reagent for 602 min.sPATPO-cn exhibit excellent overall properties. sPATPO-cn with a IEC of 1.56 meq/g exhibits a T5d of 374 oC, a Tg of 241 oC and a swelling of 11.2% as well as a conductivity of 0.13 S/cm at 80oC. Besides, its membrane indicates excellent oxidative stability, which starts to break into pieces after immersing in Fenton reagent for 255 min and completely dissolved after immersing in Fenton reagent for 730 min.The results of TEM and AFM demonstrated that the above PEM show a nanophase separation morphology, which is composed of the hydrophilic and hydrophobic domains. The nanophase separation becomes more and more obvious with increasing IEC. The hydrophilic domains aggregate to form large domains and even to produce well connected proton channels as the IEC rises, often leading to high conductivity but high swelling. However, for the above-mentioned PEM containing phthalainone groups, benzoxazole groups, or nitrile groups, the swelling is still low at high IEC due to the powerful intermolecular interaction.Besides, the triphenyl phosphine oxide-containing PEM show excellent water retention at high temperatures and strong adhesion with inorganic particles, suggesting a good prospect in high temperature PEM and inorganic doped PEM.