PPA sol-gel process

A new process for synthesizing high molecular weight PBI and membrane casting, termed PPA sol-gel process , was developed by Xiao et al. [200] based on the chemical and physical transformations illustrated in Fig. 6.11. 

Fig. 6.11 State diagram of the PPA sol-gel process (Reprinted from Ref [200] with permission Copyright (2005) American Chemical Society)...

More interesting results were obtained by Xiao et al. [200] using the PPA sol- gel process shown in Fig. 6.11. This procedure leads to very high acid doping levels (2a > 10-20), never reached by PBl membranes prepared with standard casting procedures. The conductivity of a membrane prepared with the PA A sol-gel method and 2a 16, raises from 15 mS.cm at 20 °C up to 270 mS.cm at 200 °C. The last value is comparable with those observed for the best proton conducting Nafion/inorganic composite membranes at temperatures above 100 °C. 

An isomer of ABPBI which contains head-to head and tail-to-tail benzimidazole sequences, was recently synthesized [449] and membranes were prepared by using the PPA sol- gel process (see Fig. 6.11). The membranes, like those prepared with PBI using this method PBI [200], have a much higher doping degree and their conductivities are above 200 mS.cm at 180 °C, even without humidihcatimi. Proton conductivities above 200 mS.cm were also reported for commercial crosslinked ABPBI membranes by Fumatech [425, 447], at 120 and 140 °C and partial humidification. ABPBI/MMA membranes exhibit only modest conductivities [419, 445], while an ABPBI/PVPA composite [450], which is the equivalent to the commercial PBI-based Celtec V by BASF Fuel Cells shows a poor conductivity. 

PBI membranes loaded with high levels of phosphoric acid were prepared using a new sol-gel process [190]. This process, termed the PPA process, uses PPA as the condensing agent for the polycyclocondensation and the membrane casting solvent. After casting, absorption of water from the atmosphere causes hydrolysis of the PPA to phosphoric acid. 

In 2005, Benicewicz et al. (in cooperation with BASF Fuel Cell GmbH) reported a new method for producing PBI films imbibed with phosphoric acid [8], This process, termed the PPA process, represents a sol-gel process that utilizes PPA as both the polymerization medium and the casting solvent for PBI polymers. In the PPA Process, dicarboxylic acids and tetramines such as 3,3, 4,4 -tetraaminobiphenyl (TAB) (or AB type monomers) are polymerized in PPA between 195 and 220 °C to produce high molecular weight PBI polymers. After polymerization, the PPA solution containing PBI is directly cast mito... 

Differences in acid loading, conductivity, mechanical properties, inherent viscosity and fuel cell performance have been shown to vary depending on the method through which the PBI was synthesized and processed. Two methods of processing have been shown to produce greater conductivity than the conventional imbibing method. Recent reviews by Savinell, Bjerrum, Li, Benicewicz, and Schmidt discuss PBI membranes for fuel cells prepared by many different methods [5-7], In this review, we will focus on the preparation, properties and fuel cell performance of acid-doped membranes made by a unique sol-gel method termed the PPA process. 

A recently developed imbibing process, PBIs are polymerized and cast in a polyphosphoric acid (PPA) solvent. Under controlled hydrolysis conditions, Polyphosphoric acid, a good solvent for PBI, is converted into phosphoric acid, a poor solvent for PBI. A mechanically stable PBI gel membrane that is highly doped with phosphoric acid is produced by means of a sol-to-gel transition. 

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