Modified PBI

Commercial PBI is a proprietary product (Celanese) made from 3,3, 4,4 -tetra aminobiphenyl (TAB) and diphenyl isophtalate in a two step, melt/solid polymerization [190]. A modified PBI structure such as poly(2,5)-benzimidazole (ABPBI) (see Fig. 6.10) has been synthesized by condensation of 3,4-diaminobenzoic acid (DABA) monomer in polyphosphoric acid [191]. 

In this section the properties of PBI and modified PBI will be reviewed in relation to their use in DAFC and high-temperature DAFC. 

The proton conductivity of PBI and modified PBI membranes has been studied under different conditirms of membrane preparation, acid doping degree, temperature, and water activity. The results for PBI membranes prepared by casting from dimethyl acetamide (DMA) and other solvents (NMP N-Methyl pyrrolydOTie, DMSO dimethyl sulfoxide, TFA, trifluoroacetic acid) are summarized in Table 6.6. 

Multitudes of other organically modified PBI membranes exist that include, but are not limited to, fluorinated PBI, ionicaUy and covalently cross-linked PBI, PBI blends, and a wide variety of PBI copolymers. Because there are far too many to describe, this subsection wiU highlight select PBI membranes that have not been included in the prior subsections. 

In the early years, extensive work was carried out to synthetically modify PBI derivatives using different chemical structures of the monomers, i.e., different tetraamine and dicarboxylic acid derivatives, as comprehensively reviewed by Neuse et al. [2]. Since the mPBI membrane was successfully demonstrated as the membrane electrolyte in high temperature PEM fuel cells, much effort has been devoted to the synthesis of various novel PBI derivatives in order to improve the physicochemical properties of the membranes and their durability in fuel cells [9, 10]. This section summarizes the recent synthetic efforts on PBI structure analogues, primarily for applications in the high temperature PEM fuel cells. Table 7.1 lists the chemical structures of PBI variants with modified main chains. 

Xue at al. [79] very recently prepared a modified PBI membrane by 3,3 -diaminobenzidine and 5-tert-butyl-isophthalic acid (BuIPBI) with GO-based materials as fillers. The surface of GO was treated with tert-butyl-isocyanate to improve the dispersion of the modified GO (iGO) in both water and organic media. The amount of fillers was varied from 1 to 15 wt%. The phosphoric acid doping level of BuIPBI, GO-BuIPBI, and iGO-BuIPBI membrane decreased with increasing... 

The rigid aromatic backbone chemical structure and the strong intermolecular H-bond interactions between the -NH and -N=C groups of PBI polymers (Figure 1.25) [215] make the PBI membranes brittle, which restricts the H3PO4 doping level and limits the applications of PBI membranes to HT-PEMFCs. Several efforts have been exerted in modifying PBI membranes to improve their proton conductivity and mechanical properties for HT-PEMFC applications, which have been... 

Conductivity ([Pg.37]

Modified PBI membranes. Temp and RH stand for temperature and relative humidity for measurements. 

Furthermore, they chemically grafted the groups of -CH2CH3 onto the PBI backbone by a green and facile route (Figure 11.17b) the obtained AEM exhibited ionic conductivity and ethanol permeability of 22 mS/cm and 5.24 x 10 cmVs, respectively. By a single CH3CH20H/air cell test, it delivered a peak power density of about 11 mW/cm at A handful of reports on alkali-doped structural-modified PBI were also published recently." " ... 

Poly(pyridine-co-sulfone) copolymer, PPycoPSF(1 1) Modified PBI with -8(0)2- nd -C(CF3)2- linking groups Copolymers doped with phosphonic acid... 

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