Phosphoric acid-doped films

PPS doped with AsFs dissolves readily in AsFs, but cast films are no longer soluble. Frommer has suggested that the solubilization mechanism involves solvation of both reactive radical intermediates and dopant counterions.f In addition, dialkyl esters of phosphoric acid dope PANi and render it soluble in certain solvents such as decalin. The resulting solutions can be mixed with conventional polymers and used to prepare films and fibers. 

Polybenzimidazoles are normally highly soluble in polyphosphoric acid (PPA) which is commonly used as polycondensation solvent. From this solution, phosphoric acid-doped membranes can be obtained directly by casting the crude mixture as a thin film. A sol-gel transition is subsequently induced by hydrolyzing the PPA (good solvent) to orthophosphoric acid (poor solvent) under an atmosphere with carefully controlled temperature and humidity to control the acid content of the membrane [1]. This approach is further reviewed in Chap. 10. 

The PBI/PBI-PBz-f (thickness 75 pm) composite membrane containing 20 wt% of the PBI-PBz electrospun nano-fiber, of which 10 wt % was the PBz crosslinker, was doped with a phosphoric acid aqueous solution followed by MEA preparation to perform high-temperature fuel cell tests. The phosphoric acid doping level and proton conductivity, membrane mechanical properties, and fuel cell performance of this PBI/PBI-PBz-f composite are listed in Tables 12.3, 12.4, and 12.5, respectively. The data for neat PBI are given for comparison. The PBI/PBI-PBz-f composite membrane showed higher PAdop and a, higher mechanical strength and strain at break, and better fuel cell performance than the neat-PBI membrane. Compared to the hydrophobic porous PTFE film, the... 

Phosphoric acid-doped (PA) poly-benzimidazole (PBI) films attain proton conductivities >0.25 S cm at temperatures above 150°C (Xiao et al 2005). The shift to higher temperatures of fuel cell operation, enabled by the use of these membranes, improves the fuel tolerance to carbon monoxide impurities, enhances the electrode kinetics, and alleviates humidification requirements. The problem is that the higher temperature not only accelerates the kinetics of desired electrode reactions, but also those of degradation processes. Moreover, PBI membranes function poorly under ambient conditions. PEFCs using PBI PEMs are suitable for residential applications, but they are currently not envisaged for automotive applications. 

Gomez-Romero et al. [74] revisited the phosphomolybdic acid (PMA)-doped AB-PBI in a recent paper. They cast PMA impregnated films directly from a methanesulfonic acid (MSA) solution, and then doped the films with phosphoric acid. The AB-PBI had an IV of 2.3-2.4 dLg and the films contained up to 60 wt % PMA. It was found that a 60 wt % PMA film could be doped in a bath of up to 68% PA. The AB-PBI films dissolved when placed in higher PA bath concentrations. FTIR spectroscopy showed that the PMA and phosphoric acid were interacting with the polymer. X-ray diffraction of the polymer-acid complex indicated a quasi-amorphous structure, which is consistent with previous reports. TGA showed the membranes to be stable up to 200 °C after phosphoric acid doping, which is well within the temperature range needed to operate a PEM fuel cell. The conductivity of the... 

Another interesting change in PBI morphology was performed by Mecer-reyes et al. [83]. They prepared porous films, which were then doped with phosphoric acid. The films were made by leaching out a low-molecular weight compound using a selective solvent to control porosity up to 75%. Initially,... 

The levels of phosphoric acid doping that occur in m-PBI membranes produced by the PPA process are 14-26 moles PA/PRU. A membrane with 14.4 moles PA/PRU generally corresponds to about 64wt% PA, 14.1 wt% PBI polymer, and 21.5 wt% water in the gel film [91]. The level of phosphoric acid doping for conventionally imbibed membranes has been reported to be 6-10 moles PA/PRU [2]. 

Acid-Doped Polybenzimidazole Phosphoric-acid-doped polybenzimidazole (PBI see Fig. 29.8a) fuel cell membranes were developed some 13 years ago at Case Western Reserve University (Wainright et al., 1995). Researchers found that a significant amount of phosphoric acid could be absorbed into a PBI film without scarifying the mechanical strength of the membrane. The sorbed acid species conduct protons in the absence of... 

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. 

PBI films doped with phosphoric acid were prepared by immersion of PBI films in aqueous solutions of phosphoric acid for at least 16 h [181-185]. Upon equilibration in a 11 M H3PO4 solution a doping level of 5 phosphoric acid molecules per repeating unit of the polymer was achieved. 

Three groups of plasticizing rotonating agents have been used in the preparation of a conductive composite of PANI and cellulose acetate sulfonic acids, phosphonic acids (phenyl phosphonic acid), and aliphatic diesters of phosphoric acid (dipheityl, dioctyl and dibutyl). These plasticizers improved the flexibility of the film but also significantly lowered the percolation threshold. The addition of plasticizer improves the dispersion of PANI in cellulose acetate. In this and other inventions based on the composite polymer it has been foimd to be important that the plasticizer not only helps to dissolve or soften PANI but also plasticizes the matrix polymer. In some inventions, a combination of plasticizers has been used. For example dodecylbenzenesulfonic add acted as a doping... 

The PA-doped /m-PBI fuel cell membrane maintains thermal and physical stability while operating at high temperature. To illuminate the fundamental differences in polymer film architecture, polymers with similar physical characteristics were prepared by the conventional PPA Process (Table 13.1). Even though the ratio of phosphoric acid-to-polymer repeat unit (PA/PRU) achieved by both processes were nearly identical, the PPA Process produces membranes with much higher proton diffusion coefficients and conductivities. The higher protmi diffusion coefficients of membranes produced by the PPA... 

Since the processing method greatly affects the properties of the PBI films, differences in phosphoric acid content and conductivities vary among the PPA process, direct casting, and traditional casting from DMAc even with identical PBI chemistry. When meta-PBI was cast fi om DMAc and doped in PA baths, conductivities ranged from 0.01 to0.05 Scm [11,13]. Savinell et al. [11] found that by casting from... 

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