The Development and Characterization of Aliphatic Sulfonated Polyimide Charged-Transfer Complex Hybrid Film for High Temperature Fuel Cell

Abstract

Polymer electrolyte membrane (PEM) as one of the most important components of fuel cells should satisfy the requirements such as high chemical and thermal stability, high proton conductivity, impermeability against the fuel, and good mechanical properties. In order to achieve the requirements, charged-transfer (CT) complex method was developed and applied to improve the properties of sulfonated polyimide (SPI) for high temperature fuel cell (HT-PEFC) application. The developed aromatic SPI CT films showed high proton conductivity and charged-transfer could control several properties of the films. The obtained aromatic SPI CT films, however, had low flexibility to be applied in a real MEA test. In order to solve this technical problem, an aliphatic monomer consisting of six carbons was added to SPI main chain to improve the flexibility and mechanical strength of the membrane. Aliphatic SPIs were synthesized by polycondensation of 4,4-diamino-2,2-byphenyldisulfanoic acid (DAPS), 1,4,5,8-napthalene tetracarboxilicdianhydride (NTCD), and 1,6-hexanediamine as an aliphatic monomer, at 180 ˚C. The composition and incorporation of aliphatic monomer to all monomer units was 20% and it was confirmed by 1H NMR and FTIR. The obtained aliphatic SPI contained electron-accepting unit (NTCD) in the main chain. To create CT complex in the membranes the electron-donating molecule, 2,6-dihydroylnaphtalene (DHN), was introduced to the aliphatic SPI DMSO solution in some molar ratios. The mixture DMSO solutions were cast on the glass dishes, and DMSO was evaporated at 60 ˚C in vacuo. The obtained films showed darker color than the intact aliphatic SPI film indicating that CT complex was formed between electron-accepting unit (NTCD) in the aliphatic SPI and the electron-donating molecule, 2,6-dihydroylnaphtalene (DHN). Moreover, the further confirmation of CT complex formation in the film was carried out by visible spectroscopy, and the CT absorption peak at 520~530 nm was observed. This result proved that the electron-donating DHN and the electron-accepting NTCD could form CT complex in the films. The introduction of electron donor DHN to the aliphatic SPI resulted in reduced water uptake with increasing donor molecule molar ratio. It proved that CT complex suppress water uptake of the aliphatic SPI CT films. In addition, the aliphatic SPI CT films with different DHN molar ratio exhibited fairly small variation in proton conductivity, which spanned a range of 85 to 140 mS cm-1, measured at 120 ˚C and 100 % relative humidity. The obtained proton conductivities at elevated temperature (120 ˚C) and high relative humidity (100%) were up to 1.5 times higher than that of Nafion 212 . Nevertheless, the obtained results of the proton conductivity indicated a significant dependence on relative humidity. On the other hand, the thermal and mechanical properties of the aliphatic SPI CT films were investigated by TGA, DSC, and tensile measurements. The aliphatic SPI CT films showed high glass transition temperatures (Tg), ~160 °C with higher tensile modulus, and elongation at break than the aromatic SPI CT films. XRD of the aliphatic SPI films shows a broad peak centered around 22.5 ~ 23.1˚ 2θ. And the addition of donor molecule did not affect to the distance between polymer main chains significantly. The aliphatic SPI CT membrane had been demonstrated to be promising candidates for high temperature PEM in fuel cell applications.