Polybenzimidazole membranes thermal stability

Much attention has been paid to the synthesis of fluorine-containing condensation polymers because of their unique properties (43) and different classes of polymers including polyethers, polyesters, polycarbonates, polyamides, polyurethanes, polyimides, polybenzimidazoles, and epoxy prepolymers containing pendent or backbone-incorporated bis-trifluoromethyl groups have been developed. These polymers exhibit promise as film formers, gas separation membranes, seals, soluble polymers, coatings, adhesives, and in other high temperature applications (103,104). Such polymers show increased solubility, glass-transition temperature, flame resistance, thermal stability, oxidation and environmental stability, decreased color, crystallinity, dielectric constant, and water absorption. 

Sll, Fig. 4.5) are depicted along with B2. Again, all TGAs indicate excellent and similar thermal stabilities of all membranes up to 300 °C. Therefore, it can be concluded that both sulfonated and phosphonated polymers can be selected as acidic cross-linkers for polybenzimidazole membranes. 

Phosphoric acid is of special interest as dopant for fuel cell applications in the 120-200 °C range due to its excellent thermal stability, low vapor pressure, and high proton conductivity at very low water contents [47, 48]. It completely dissolves in the /wPBI matrix to form a material that, at high acid contents, more resembles a gel than a solid [49]. Among the different acid-doped polymer membranes this is also by far the most thoroughly studied system. The phosphoric acid doping chemistry of polybenzimidazoles is, however, rather complex. Phosphoric acid can form a variety of ionic species and depending on the temperature and water content the... 

Chapter 7 emphasizes polymer/inorganic composite membranes to increase thermal stability. More specifically, polymers include perfluoronated polymers, sulfonated poly(arylene ether)s, polybenzimidazoles (PBls), and others. The inorganic proton conductors are silica, heteropolyacids (HPAs), layered zirconium phosphates, and liquid phosphoric acid. 

A sulfonated polybenzimidazole that has all the desired properties of conductivity and thermal stability to be used at 150-200 °C is sulfonated ABPBI prepared by direct sulfonation of a previously cast membrane, followed by phosphoric acid doping as detailed in Fig. 3.34. Sulfonation of ABPBI reduces the phosphoric acid uptake at low doping bath concentrations, but at high bath concentrations the sulfonated ABPBI absorbs more acid than the nonsulfonated ABPBI (Fig. 3.35), and the phosphoric acid uptake in these concentrated baths increases with the sulfonation degree (Fig. 3.36). As it could be expected, this higher doping level leads to an increase in conductivity as shown in Fig. 3.37 [382,396]. 

Nafion-based hybrid membranes are traditionally used electrolytes for both PEM fuel cell and DMFCs. " Besides Nafion, many composite engineering thermoplastic polymers based on poly(etheretherketone) (PEEK), polyvinyl alcohol (PVA), polysulfone, " polybenzimidazole (PBI), polyimide, and other organic-inorganic composite membranes have been employed as alternative membranes for both PEM fuel cells and DMFCs due to their lower cost, comparable conductivity, high mechanical and thermal stabilities, and easy modification as well. 

The low cost and excellent oxidation and thermal stability of phosphoric acid doped polybenzimidazole (PBI) prompted researchers at Case Western University (Samms et a/., 1996 Wang et /., 1996a,b,c) to develop this membrane as a polymer electrolyte for DMFCs. After investigation of the thermal stability of PBI doped with phosphoric acid up to 600°C, it was concluded that this membrane is adequate for use as PEM in a high temperature fuel cell. These studies may also be extended to other polymers like the polybenzimidazobenzophenanthrolines (PBIPAs) which exhibit excellent thermal and mechanical properties (Zhou and Lu, 1994). 

In this area of solid polymer electrolytes, polybenzimidazole (FBI) doped with KOH has to be mentioned, as it (Fig. 5.5) is known for its high thermal and chemical stability, even if it is not water soluble, which is desired for ion-solvating polymers [61-65]. Ionic conductivities between 5 X 10 and 1X 10 S/cm were obtained for FBI using KOH with a concentration of 6 M at 70-90°C [65]. These membranes were assembled with commercial electrodes from E-Teck and the resulting MEA was then incorporated into a single cell. The performance in a H2/O2 fuel cell test was reported and a current of 620x 10 A/cm at 0.6 V was obtained. However, measurements were performed at 50 °C under pressurized H2 and O2, which results in a significant improvement in performance compared with the use of nonpressurized fuels [65]. 

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