Phosphated PBI

Bismayer U, Hensler J, Salje EKH, Guttler B (1994) Renormalization phenomena in Ba-diluted ferroelastic lead phosphate, (Pbi.xBax)3(P04)2. Phase Trans 48 149-168... 

Bismayer U, Hensler J, Salje E, Giittier B (1994) Renormalization phenomena in Ba-diluted ferroelastic lead phosphate, (Pbi j,Baj,)3(P04)2. Phase Trans 48 149-168 Bismayer U, Rower RW, Wruck B (1995) Ferroelastic phase transition and renormalization effect in diluted lead phosphate, (Pbi j,Srj,)3(P04)2 and (Pbi xBax)3(P04)2. Phase Trans 55 169-179 Bismayer U, Salje E (1981) Ferroelastic phases in Pb3(P04)2-Pb3(As04)2 x-ray and optical experiments. Acta CrystallogrA37 145-153... 

C. And, although PBI s resistance to long term exposure in heated air does not match that of some polyimides, new data, which is reported at the end of this paper, indicates that phosphated PBI is very thermo-oxidatively stable. PBI is hydrophilic, which lends to its excellent comfort as a textile and which affords unique chromatographic separation characteristics. For other applications, PBI s hydrophi1icity can be moderated by annealing. 

The improved oxidative stability of phosphated PBI may be due to formation of the benzimidazonium cation (see FIGURE 5). Early work on PBI [8] reported significantly better oxidative stability of PBI films cast from sulfuric acid compared to films cast from organic solvent. Data from other studies fail to support the observation [31]. However, discrepancies may be due to a combined effect of the thermal stability of the PBI salt and the oxidative stability of the PBI cation. PBI sulfate disassociates at temperatures above 350°C whereas PBI phosphate disassociates at temperatures above 850°C. So, a phosphate anion may be able to support a PBI cation to much higher temperatures than a sulfate anion. 

Crytal chemitry. Solid solutions of lead phosphate compounds with structures like that of palmierite ((K,Na)2Pb(S04)2) exhibit transition sequences that are well modeled by Landau-Ginzburg excess free energy expressions. Systems that have been especially heavily studied include Pb3(Pi (As (04)2 (Toledano et al. 1975, Torres 1975, Bismayer and Salje 1981, Bismayer et al. 1982, 1986 Salje and Wruck 1983) and (Pbi jcA c)3(P04)2, where A = Sr or Ba (Bismayer et al. 1994, 1995). The phase diagram for the phosphate-arsenate join is not completely mapped, but Bismayer and collaborators have identified a series of transitions for the P-rich and the As-rich fields. The structure consists of two sheets of isolated (P,As)04-tetrahedra with vertices pointing towards each other and a sheet of Pb atoms in the plane of the apical oxygen atoms. Layers of Pb atoms lie between these tetrahedral sheets (Fig. 18) (Viswanathan and Miehe 1978). 

As for pure phosphoric acid, the transport properties of PBI and phosphoric acid also depend on the water activity, this is on the degree of condensation (polyphosphate formation) and hydrolysis. There is even indication that these reactions do not necessarily lead to thermodynamic equilibrium, and hydrated orthophos-phoric acid may coexist with polyphosphates in heterogeneous gel-like microstructures [99]. There is not much known on the mechanism of proton transport in polymer adducts with polyphosphates and/or low hydrates of orthophosphoric acid. Whether the increased conductivity at high water activities is the result of the plasticizing effect of the water on the phosphate dynamics, thereby assisting proton transfer from one phosphate to the other, or whether the water is directly involved in the conduction mechanism has not been elucidated. 

A few examples of Nafion composite membranes with polymers and inorganic compotmds has been reported, such as poly(ethylene glycol) (PEG)/Si02 [155], PTFE/S102 [156], chitosan/phosphotungstic acid [157], PVI/Pd [158], and PBI/zirconium phosphate [159]. 

Ternary composites of PBI with sPEK + polysulfone (PSU) [437], sPEEK + sulfonated PSU (sPSU) [438], and sPEEK + zirconium phosphate [439] were studied in DMFC. Excellent results were reported for DMFC, at 110 °C, using the first two composite membranes. 

The proton conductivity of SPEEK membrane can be improved by incorporating a fast proton conductor in the polymer matrix as studied by Silva et al. (2005). They utilized zirconium phosphate (ZrPh) pretreated with -propylamine and then with PBI. The pretreatment was to improve the compatibility between ZrPh and SPEEK. Even though the compatibility of the inorganic was good, the proton conductivity and water uptake of the composite membrane were reduced, consequently reducing the methanol permeability. 

Jayakody et al. investigated the dynamical processes and mass transport of phosphoric acid-doped meto-polybenzimidazole (m-PBl) by spin—lattice Tj and spin—spin T2 relaxation time measurements and by PFG NMR measurement. It was concluded that the high proton conductivity is correlated with rapid proton self-diffusion. The P NMR study not only revealed the presence of both phosphoric acid and dimeric pyrophosphoric acid but also indicated strong interaction between the phosphate groups and the m-PBI... 

Electrical properties of PBI films are shown in TABLE XIV. As would be expected, films with higher moisture contents have lower resistivities and higher dielectric constants. PBI plasticized with phosphoric acid, which should be a polycationic polymer with an unbound phosphate anion, has a very low resistivity and a high dielectic constant which is even higher at elevated temperatures. In general, dielectric constants are higher at 100 Hz and lower at 1 MHz. 

He et al. investigated the use of zirconium phosphate (ZrP) in a PA/PBI system [49]. The conductivity of m-PBI with a doping level of 5.6 PA/PRU increased from 0.068 S cm to 0.096 S cm with the addition of 15 wt% ZrP at 200°C and at 5% relative humidity. As seen in Fig. 13.11, the conductivity of the membrane increased as the relative humidity and temperature of its environment increased. Conductivities of other inorganic fillers, such as phosphotungstic acid, sUicotungstic acid, and tricarboxylbutylphosphonate, are comparable or lower than that of ZrP. Unfortunately, there have been no fuel cell performance tests published on these systems. Overall, these inorganic fillers improved the conductivity of m-PBI membranes. 

Javaid Zaidi SM (2005) Preparation and characterization of composite membranes using blends of SPEEK/ PBI with boron phosphate. Electrochim Acta... 

Zirconium and boron phosphate Interesting results have been obtained by using this approach in order to prepare PBI composite... 

The room-temperature structure of lead phosphate is a distorted version of the layered structure adopted by Sr3(P04)2, Ba3(P04)2, Ba3(V04)2, Sr3(Cr04)2, etc. [267, 268]. Pb3(P04)2 itself adopts this more symmetrical structure above 172°C. The high-temperature form consists of sandwiches composed of [PO4] tetrahedra which all point towards the interior. These sandwiches are stacked in a cubic-close-packed manner. The Pb atoms are inserted in the holes between these [PO4] tetrahedra in such a way that we may describe a sandwich in terms of close-packed layers of composition (30 + 1 Pbn)-----(2 0+1 Pbi)---(3 0 + 1 Pbn) re-... 

Membrane prepared by blending sulfonated polybenzimidazole (PBI) with Nafion polymer showed a conductivity of 0.032 S cm The methanol permeability of the composite membrane was found to be 0.82 x 10 cm s as compared to Nafion, which is around 2.21 x 10 cm s [22]. Addressing the problem of methanol permeation, a composite membrane of Nafion with polyvinyl alcohol (PVA) for direct methanol fuel cell has been reported. It is concluded that at the weight ratio of 1 1 in PVA and Nafion, the thin film-coated Nafion membrane exhibited low methanol crossover, and the membrane protonic conductivity could be improved by the sulfonation treatment [23]. Recently, Zaidi et al. [24] prepared composite membranes of PFSA ionomer with boron phosphate and showed the conductivity of 6.2 X 10-2 S cm-i at 120°C. 

With the exception of the inorganic acid doped PBI system, no membrane (such as PEO [119], PVA [122], polyacrylamide (PAAM) [116,117,123], polyethylenimine (PEI) [120], and poly diallyl-dimethyl-anunonium-dihydrogen phosphate, PAMA -H PO " [115] has demonstrated conductivities superior to Nafion at temperatures >100°C and low relative humidities (ambient pressure operating conditions). Most of these acid-polymers blend exhibit proton conductivity <10" S cm" at room temperature. High acid contents result in high conductivity but poor mechanical... 

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