Sulfonated PBI membranes

In addition, Mader and Benicewicz [106,107] prepared sulfonated pPBI-based copolymer membranes using diacid monomers of TPA and the monosodium salt of 2-sulfoterephthalic acid (s-TPA) with TAB in PPA. The rigidpPBI repeat units provided the copolymer membranes excellent mechanical strength even bearing excess PA molecules. These membranes exhibited greatly improved properties compared with the post-sulfonated PBI membrane. 

PBI-based polymer electrolytes with pendant functional groups were proposed by Gieselman and Reynolds [133, 134], by producing N-substituted PBI with sulfonates, which could be traced back to Sansone s work [130]. In the following years, more efforts were devoted to prepare sulfonated PBI membranes by reacting PBIs with sodium (4-bromomethyl)benzenesulfonate [135, 136], arylsulfonates, or alkylsulfonates [137]. The introduction of benzylsulfonic, arylsulfonic, or alkylsulfonic acids was found to create proton conductivity in the presence of water. Using A-substitution, the sulfonation degree of the membranes could be accurately controlled [133]. 

Staid, R, Lufrano, F., Arico, A.S., Passalacqua, E., and Antonucci, V. (2001) Sulfonated PBI membranes-preparation and physico-chemical characterization, J. Membr. Sci., 188,71-78. 

The water uptake from the vapor phase of PBI membranes depends not only on the water activity but also on the degree of doping. Usually the water uptake in PBI is expressed as 1 , the number of water molecules per imidazole group, and it is lower than the corresponding (molecules of water per sulfonic group) for Nafion... 

Other membranes, such as sulfonated polyimide [102] or cross-linked sulfonated PVA [103] exhibit a major amoimt, although no quantified, of non-freezable water as compared to Nafion. An interesting behavior was observed in the case of PBl and ABPBI membranes doped with phosphoric acid [104], where the amount of frozen water is 2.2 % for PBI and between 1.1 % and 9.1 % at 100 % relative humidity, much lower than that observed for Nafion under similar conditions. When equilibrated with aqueous methanol, PBI membranes exhibit a maximum of 10 wt% of frozen water, while ABPBI membranes, particularly those prepared by high temperature casting, presented very low percentages of frozen water in all methanol concentrations studied, with a maximum value of 0.6 wt% in methanol 25 w/w%. This result is relevant for the application of ABPBI in DMFC because, the fuel cell start up at low temperatures would not be affected. 

A partially sulfonated PBI can be prepared from 3,3 -diaminobenzidine, isophthalic acid, and 5-sulfoisophthalic acid with poly(phosphoric acid) [8]. The reaction takes place at 220 °C for 25 h. In the course of the reaction, the color of the solution changes from ocher to dark brown. Afterwards the polymer is precipitated in water and dried in vacuo. In the course of the preparation of a fuel cell membrane, the poly(phosphoric acid) is hydrolyzed into phosphoric acid due to moisture in air so that the polymer membrane has an acid doping level of 2000. 

By using a partially sulfonated PBI-based polymer, a membrane for a fuel cell can be prepared [8]. Acid doping can be carried out after making a membrane. Also, acid doping can be carried out while making a membrane in situ, a method that is useful for improving the dimensional stability and the performance of the fuel ceU. 

Block copolymers combing PBI with other types of macromolecular units have also been developed for superior membrane properties, as shown in Fig. 7.6. Two types of copolymers have been prepared, characterized, and evaluated as fuel cell electrolytes. One is the sulfcmated copolymer containing PBI and snUrmated polymer moieties for low temperature PEMs in both PEM fuel cells and direct methanol fuel cells (DMFCs) [123-126]. The other is the random copolymer containing PBI and poly(imine/ amide) moieties [127, 128]. For the sulfonated PBI copolymers, benzimidazole monomers... 

Pasupathi et al. fabricated an acid-base polymer blend membrane based on sulfonated poly(etheretherketone) and poly(benzimidazole) for direct methanol fuel cells. A SPEEK/PBI membrane demonstrated a noticeable enhancement in the DMFC performance compared with Nafion 117. The maximum power densities (45 mW cm ) obtained with SPEEK/PBI membranes were twice that of Nafion 117 at 60 °C. This membrane maintained high power densities for more than 50 days of operation and therefore is seen as a potential candidate for portable DMFC applications [89]. 

Specifically, the conductivity of m-PBI was investigated as a function of doping electrolyte [9-11,15-18], and was nicely discussed by Schuster and Meyer [19]. Overall, it was found that m-PBI with a doping level between 2-8 moles PA/PRU typically has a conductivity between 10 and 10 S cm in low humidification or nonhumidified conditions at high temperatures (>120 °C). hi general, for the m-PBI/sulfuric acid complex, conductivities were similar to the m-PBI/PA complex or slightly lower [10,11,16,17]. The higher values are comparable to the current state of the art perfluorinated sulfonic acid membrane (Nafion) at atmospheric pressure and full hydration. However, the m-PBI/PA complex is the most widely studied, because of its conductivity and thermal stability. 

Acid-doped sulfonated PBIs (sPBI) have been synthesized by multiple research groups [12,15,40-42,50-54]. Typical approaches to sulfonation include direct sulfonation of the PBl backbone, chemical grafting of functionalized monomers onto the chain, or copolycondensation of sulfonated monomers. The last approach is highly favored, because side reactions can be avoided and degree of sulfonation easily controlled. A recent review by Rikukawa and Sanui [54] thoroughly describes the preparation of sulfonated hydrocarbon polymers and the properties of the sulfonated membranes. 

Sulfonated PBIs, other sulfonated polymers, and their blends show great potential for use as membranes in high-temperature fuel cells. The synthesis, conductivity, mechanical properties, and performance still require further development, but results so far are promising. Further investigation remains to determine whether these problems can be overcome and useful chemistries developed to meet the needs of high-temperature membranes with performance characteristics comparable to lower-temperature membranes. 

---