AB-PBI

There is much work in the literature which is devoted to modifications made on the main chain of The properties of 

Poly(2,5-benzmiidazole) or ABPBI, is another polybenzimidazole derivative that has been investigated as an alternative fuel cell membrane material. ABPBI can result from the polymerization of a single monomer (3,4-diaminobenzoic acid, DABA) in polyphosphoric acid (Fig. 5) or in P2O5-MSA mixture. The 

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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]. 

A variety of high-temperature-resistant PBIs were synthesized and reported in the Uterature. The most widely used PBI is poly[2,2 -(l,3-phenylene)-5,5-benzimidazole] (known as m-PBI) [22]. The other variations are poly[2,2 -( 1,4- phenylene)-5,5 -benzimidazole] (known as p-PBI) [13], poly(4,4 -diphenylether-5,5 -bibenzimidazole) (OPBI) [23], poly(2,5-benzimidazole) (AB-PBI) [24], pyridine-based PBI [25], sulfonated PBI [26], hyperbranched polybenzimidazole (HPBI) [27], naphthalene-based PBI [28], fluorinated PBI [29], PBI block copolymer [30], sulfonated PBI copolymer with sulfone or sulfonic acid groups in the backbone [31,32], PBFnano-composite [33], cross-linked PBI [34], and many others. Different approaches have been taken by the researchers to modify the structure and properties of the fluorinated PBI. A detailed summary of these works is discussed below. 

Table 13.2 Percent composition, acid doping level, and proton conductivity data for various p-PBI-block-AB-PBI membranes [25]...

Fara-PBI/AB-PBI (mole ratio, x/y) Acid doping level (PA/2 benzimidazole) Proton conductivity (S/cm 160°C) Membrane composition %) ... 

Pyridine polybenzimidazoles (py-PBIs, Fig. 5d) have been investigated for their use in fuel cells because of their high concentration of basic sites (amine and imine groups). Similar to AB-PBI, the high concentration of basic sites allow these polymers to have a high affinity to acids. The pyridine moiety is commonly combined with the traditional PBI stmcture by including it as part of the backbone structure. 

Asensio JA, Gomez-Romero P (2005) Recent developments on proton conducting poly (2,5-benzimidazole) (AB-PBI) membranes for high temperature polymer electrolyte membrane... 

High working temperatures benefit PEMFC performances because of faster electrode kinetics, higher CO tolerance and the possibility to use the residual heat for energy cogeneration [5, 6]. Among the polybenzimidazole derivatives, the most widely studied are the commercially available poly[2,2 -(m-phenylene)-5,5 -bibenzimidazole] with the acronyms w-PBI and poly(2,5-benzimidazole), AB-PBI (see Fig. 8.1). The polymers contain a basic imidazole functionality that allows the uptake of the acid protic electrolyte, which is responsible, and required, for the proton... 

Fig. 8.1 Structxire of the repeating units of m-PBI and AB-PBI left). Tentative interactions of phosphoric acid with m-PBI (two repeating units are depicted -1-5.5 H3PO4 molecules per repeating unit). The imidazole...

In this chapter we will firstly present a literature survey on vapour pressure, viscosity and conductivity properties of phosphoric acid with an emphasis on the temperature and composition range relevant for fuel cell applications. In a second part we want to elucidate the physico-chemical interactions of a protic electrolyte like phosphoric acid as a doping agent with polybenzimidazole-type polymer membranes. Literature data on m-PBI and AB-PBI as well as own measurements on Fumapem AM-55, a commercial PBI derivative, will be cOTisidered. On the basis of the observed doping behaviour a model describing the thermodynamics of the adsorption process is presented. 

In Fig. 8.1 (left) the chemical structures of w-PBl and AB-PBl are depicted. The polymer w-PBl has two benzimidazole structural units per repeating unit and thus two protonable sites (z = 2). In case of AB-PBI the repeating unit consists of only one benzimidazole stmctural... 

For m-PBI + H3PO4, AB-PBI + H3PO4 and ffi-PBI + HQO4 the obtained values of the fit parameter z correlate well with the known numbers of basic groups per repeating unit. The parameter z is obtained by fitting with an... 

Fig. 8.10 Adsorption isotherms of m-PBI (25 °C), AB-PBI (25 °C) and Fumapem AM-55 (110 °C) doped with various protic electrolytes H A. The curves are fitted by non-linear regression using (8.22). The...

Fig. 9.1 Chemical structure of mPBI (top), pPBI middle), and AB-PBI bottom)...

The simplest PBI chemistry is AB-PBI with a repeat unit consisting solely of one 2,5-benzimidazole ring. AB-PBI is synthesized using 3,4-diaminobenzoic acid as the only... 

Fig. 10.9 Synthetic scheme for (-AB-PBI. Reproduced from [20] with permission of John Wiley and Sons...

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