Polybenzimidazole membranes conductivity

As shown in Fig. 14, PIM-1 and PIM-7 have been found to exhibit substantially higher O /Nj selectivities (a(O2/N2)>3.0) than other polymers of similar permeability [41]. Other thermally rearranged [78] polyimides show excellent CO /CH separation selectivities. These materials were also shown to function as fuel cell membranes when doped with H3PO4 and proton conductivities of 0.15 S cm" were observed at 130°C [78] that is, higher than polybenzimidazole membranes. 

P. Staiti, M. Minutoli, Influence of composition and acid treatment on proton conduction of composite polybenzimidazole membranes. Journal of Power Sources 94 (2001) 9-13. 

For phosphoric acid-doped polybenzimidazole membranes with an intermediate Ap Ta (3.5), hydrogen bonds are likely formed as shown by infrared spectroscopy [61, 62] and molecular dynamic simulation [63]. The overall proton conductivity of the acid-doped membranes is at least an order of magnitude lower than that of pure phosphoric acid due to the presence of the solid polymer phase. The polymer does not seem to interrupt the extensive hydrogen bond network of the phosphoric acid, though it does decrease the percolation within the liqmd-like part of the phosphoric acid domain. As a result, the proton conducting mechanism remains the same, i.e., primarily via the Grotthuss... 

Casting by solvent evaporation is a commonly used procedure for fabrication of membranes based on organic polymers. It is probably the most widely used technique for polybenzimidazole membrane preparation for high-temperature polymer electrolyte membrane fuel cells. After casting, doping with phosphoric acid provides proton conductivity to the membrane. 

Mecerreyes, D., Grande, H., Miguel, O. et al. 2004. Porous polybenzimidazole membranes doped with phosphoric acid Highly proton-conducting solid electrolytes. Chemistry of Materials 16 604-607. 

R.H. He, Q.F. Li, G. Xiao, and N.J. Bjerrum. Proton conductivity of phosphoric acid doped polybenzimidazole and its composites with inorganic proton conductors. Journal of Membrane Science 226, 169-184 2003. 

Another approach was the synthesis of inorganic/organic composite materials to influence the properties of the membrane. An overview on the state of the art of composite perflourinated membranes is given in [15]. Infiltration of a polymer carrier material with various inorganic proton conductors is subject of a patent [16]. For operation at elevated temperature, several materials have been considered, hi an early work, Nafion /Fl3P04 showed better conductivity at temperatures above 100°C compared with blank Nation and also reduced methanol permeabihty [17]. New types of polymers are also under development for better temperature stabihty one of the most advanced examples is the high-temperature material polybenzimidazole, which usually is doped with phosphoric acid [18]. 

Polybenzimidazole films doped with phosphoric acid have also been investigated for direct methanol fuel cells. These membranes, however, only display the requisite conductivities at high temperatures and have only been demonstrated in vapor feed systems operated at 150-200 C. Thus, although these novel membrane applications have been demonstrated to have decreased methanol permeability in fuel cells, none of the systems have been successful in being applied to low temperature liquid-feed direct methanol fuel cells. 

Polybenzimidazole (PBI) (initially manufactured by Hoechst-Celanese, now PE ME A) is one of the few polymers under consideration for high-temperature operation. The application of PBI [206, 207] and the noncommercial AB-PBI [208] in fuel cells was introduced by Savinell and coworkers. For that, the membrane was immersed in concentrated phosphoric acid to reach the needed proton conductivity. Operation up to 200 °C is reported [209]. A disadvantage of this class of membranes is the acid leaching out during operation, particularly problematic for cells directly fed with liquid fuels. Additionally, the phosphoric acid may adsorb on the platinum surface. A review on membranes for fuel cells operating above 100 °C has been recently published [209]. 

M.Y. )ang, Y. Yamazaki, Preparation, characterization and proton conductivity of membrane based on zirconium tricarboxybutylphosphonate and polybenzimidazole for fuel cells. Solid State Ionics 167 (2004) 107-112. 

P. Staiti, Proton conductive membranes constituted of silicotungstic acid anchored to silica-polybenzimidazole matrices, Journal of New Materials for Electrochemical Systems 4 (2001) 181-186. 

In the search for PEMs with lower alcohol permeability than Nafion and other perfuorinated membranes, without degradation of the proton conductivity, a number of new polymeric membranes were synthetized and characterized, such as sulfonated polyimides, poly(arylene ether)s, polysulfones, poly(vinyl alcohol), polystyrenes, and acid-doped polybenzimidazoles. A comprehensive discussion of the properties of these alternative membranes is given in Chap. 6, along with those of Nafion and Nafion composites. 

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