Polybenzimidazole modification

Functionalisation of MWNTs via a microwave-induced polymerisation modification route to produce polybenzimidazole (PBI) nanocomposite films has been developed by Zhang et al These PBI-MWNT demonstrated an increase in the Young s modulus of approximately 43.9% at 2 wt% CNT loading, and further modulus growth was observed at higher filler loading. " ... 

Hu M, Pearce EM, Kwei TK (1993) Modification of polybenzimidazole synthesis and thermal stability of poly(Nl-methylbenzimidazole) and poly(Nl, N3-dimethylbenzimidazolium) salt. J Polym Sci A 31 553-561... 

The typical MEA constmction has in its original form a symmetric layout. The core unit consists of the membrane. For HT-PEM MEAs, usually polybenzimidazole in several pol5mieric modifications like sulfonation, blending of the polymer with other polymers or additives to... 

Because of good thermal and hydrolytic stability, excellent mechanical and chemical stability, low cost, and commercial availability of sulfonated aromatic hydrocarbon polymers, recent research has focused on the synthesis and development of sulfonated aromatic hydrocarbon polymers specifically for high-temperature PEMFCs. Typical examples include sulfonated poly(ether ether ketone) (SPEEK) or poly(ether ketone ketone) (SPEKK) [1,2], sulfonated poly(ether sulfone) (SPSE) [3], alkyl sulfonated polybenzimidazole (PBI), sulfonated naphthalenic polyinrides (sNPl) [4-6], sulfonated polyCphenylene sulfide) [7,8]. Both post- and pre-sulfona-tion methods have been used in the past. Other than the post-sulfonation modification of aromatic polymers, recently, efforts have been dedicated to direct polycondensation from sulfonic acid containing monomers to synthesize sulfonated polymers [9]. The latter approach, namely pre-sulfonation, is widely applied because of the ease of controlling sulfonation degree and deactivated sites in the arylene backbones, which further avoid side reactions such as decomposition and hydrolysis of polymers resulted from the post-sulfonation method. 

Abstract There have been numerous studies on modifying DuPont s Nafion (a perfluorosulfonic acid polymer) in order to improve the performance of this membrane material in a direct methanol fuel cell. Modifications focused on making Nafion a better methanol barrier, without sacrificing proton conductivity, so that methanol crossover during fuel cell operation is minimized. In this chapter, a brief literature survey of such modifications is presented, along with recent experimental results (membrane properties and fuel cell performance curves) for (1) thick Nafion films, (2) Nafion blended with Teflon-FEP or Teflon-PFA, and (3) Nafion doped with polybenzimidazole. 

Trogadas and Ramani summarized the modification of PEM membranes, including Nafion modified by zirconium phosphates, heteropolyacids, hydrogen sulfates, metal oxides, and silica. Membranes with sulfonated non-fluorinated backbones were also described. The base polymers polysulfone, poly(ether sulfone), poly(ether ether ketone), polybenzimidazole, and polyimide. Another interesting category is acid-base polymer blend membranes. This review also paid special attention to electrode designs based on catalyst particles bound by a hydrophobic poly-tetrafluoroethylene (PTFE) structure or hydrophilic Nafion, vacuum deposition, and electrodeposition method. Issues related to the MEA were presented. In then-study on composite membranes, the effects of particle sizes, cation sizes, number of protons, etc., of HPA were correlated with the fuel cell performance. To promote stability of the PTA within the membrane matrix, the investigators have employed PTA supported on metal oxides such as silicon dioxide as additives to Nafion. 

Chapter 14 presents a brief hterature survey of such modifications, along with recent experimental results (membrane properties and fuel cell performance curves) for (i) thick Nafion films, (ii) Nafion blended with Teflon -FEP or Teflon -PFA, and (iii) Nafion doped with polybenzimidazole. 

Since the polymers to be activated need to have at least one acidic proton in the main chain and must be soluble in suitable solvents, the number of polymers which can be modified by this particular method is limited. So far only the modification of poly(arylene ether sulfone)s and polybenzimidazoles as suitable materials for fuel cell membranes has been reported in the literature [4,5,32,102,136-146,148-151]. The attempt to lithiate poly(ether sulfone) under homogeneous conditions failed so far, due to poor solubility and the alternating electron-donating and electron-withdrawing linkages present in poly(ether sulfone), which result in an overall unfavorable balance in the polymer chain. 

FIGURE 8.11 Synthesis of polybenzimidazoles from the AB-type monomers. (Reprinted with modification from Twieg, R. et al., Macromolecules, 29, 7335, 1996. With permission.)... 

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. 

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