PBI membranes

Benicewicz, B. C., Yu, S., Xiao, L. and Perry, K. 2007. Advances in polybenzimidazole (PBI) membrane materials. In Advances in materials for proton exchange membrane fuel cell systems, Pacific Grove, CA, Feb. 18-21. 

Membranes. Flat sheets of CAc, PA, and PBI membranes were cast at the National Research Council of Canada by using published procedures (24) and were selected to obtain two different porosities as determined by percent NaCl rejection. The membrane sheets (ca. 400 /xm thickness) were cut into circles approximately 7.5 cm in diameter. 

Table IV presents the comparative data on the permeation rates of the three types of membranes with two different porosities for various aqueous organic solutions and for pure water as measured over the duration of the study. The data shown here represent the relative chronological order in which the samples were tested. In the beginning, even though the percent rejection of NaCl is high for PA and CAc (indicating small-size pores), the rates of permeation of pure water are higher for denser membranes than for membranes having lower percent rejection of NaCl. In the case of the PBI membrane, the reverse of this phenomenon is observed.

Also, the dense PBI membrane is slow (99%). Although this membrane exhibited, in general, the best rejection characteristics, the very low flux observed is a definite drawback of such dense membranes. 

Ma Y,Wainright J, Savinell R, (2004). Conductivity of PBI Membranes for high-temperature polymer electrolyte fuel cells. Journal of Electrochemical Society, 151 8-16... 

PBI (see chemical structure above) is a hydrocarbon membrane that has been commercially available for decades. Free PBI has a very low proton conductivity ( 10 S/cm) and is not suitable for PEM fuel cell applications. However, the proton conductivity can be greatly improved by doping PBI with acids such as phosphoric, sulfuric, nitric, hydrochloric, and perchloric acids. The PA-doped PBI membrane is the most popular one in PEM fuel cell applications because H3PO4 is a nonoxidative acid with very low vapor pressure at elevated temperature. Savinell et al. and Wainright et al. first demonstrated the use of PBI-PA for HT fuel cells in 1994.270 272 since then, there has been a significant amount of research on the PBI-based membrane because of its low cost and good thermal and chemical stabil-... 

PEMEAS, a 2004 spin-off from Celanese AG, has developed a membrane made from the heat-resistant polymer PBI. The PBI membrane marketed by PEMEAS under the brand name Celtec enables a fuel cell to operate at temperatures of up to 200°C (392°F), while more conventional technologies allow PEMFC-operating temperatures of up to 100°C (212°F). Due... 

An electro-osmotic drag due to proton migration is defined as the number of water molecules moved with each proton in the absence of a concentration gradient For comparison, the electro-osmotic drag coefficient for vapor or liquid-equilibrated Nafion membranes ranges from 0.9 to 3.2 at room temperature [146]. For phosphoric acid-doped PBI membranes, however, the water drag coefficient is dose to zero [149,... 

In H2/O2 fuel ceU tests with phosphoric add-doped PBI membranes [156], power densities of 0.28 and 0.55 Won were reported for a H3PO4/PBI membrane operating at atmospheric pressure using dry H2 and O2 at temperatures of 125 °C and 200 °C, respectively. The CO tolerance was assessed by supplying different amounts of CO in the hydrogen feed gas [157] a 3% CO admixture was found to be permissible when the operating temperature was 200 °C. 

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. 

Pu et al. [197] have prepared and tested N-methyl (PNMBI) and N-ethyl (PNEBI) derivates of PBI, and Chuan and Hsu [198] reported fluorine-containing PBI. A number of PBI membranes modified by ionic or covalent cross-linking and composite PBI membranes have been reviewed by Li et al. [199]. 

After polymerization of TAB and isophtalic acid in polyphosphoric acid (PPA) at 200 °C the PBI solution in PPA (polycondensation agent and polymerization solvent) is cast at directly from the hot mixture. The water uptake by the mixture hydrolyzes the PA to phosphoric acid, which along with the decrease in temperature induce a transition to a gel state. The PBI membranes formed by this method retain... 

A commercial phosphoric acid doped PBI membrane, Celtec V by BASF Fuel Cells, tailored for DMFC is based in a blend of PBI and poly(vinyl phosphonic acid) (PVPA). The PVPA poly acid is immobilized in the PBI matrix by interpenetration, crosslinking and covalent bonding [201]. 

The water uptake from the vapor phase of PBI membranes depends not only on the water activity but also on the degree of doping. Usually the water uptake in PBI is expressed as 1 , the number of water molecules per imidazole group, and it is lower than the corresponding (molecules of water per sulfonic group) for Nafion... 

H3PO4 doped PBI membranes (A = 2.5) 100 pm thick exhibit methanol crossover current densities less than 10 mA.cm [403, 410], while the PBI/PVPA composite commercial membrane Celtec-V shows crossover current densities higher than 100 mA.cm at 90 °C in 1 M methanol [201]. 

Water-methanol uptake from the vapor phase by PBI was determined in thin membranes (100 nm thick) using the quartz crystal microbalance method [280]. The results, expressed as moles of water plus methanol sorbed per imidazole ring, are shown in Fig. 6.31. It was assumed that the partition constant of methanol is Kx = 1.29, as determined by NMR analysis of a PBI membrane in equilibria with a methanol aqueous solution (20 wt%) [280]. 

The behavior observed in Fig. 6.32 is different from that observed Nafion in Fig. 6.17. In the case of PBI, and also for the modified ABPBI membrane, the pme water sorption almost doubles pure methanol sorption, that is, these membranes have a clear preference for water uptake over methanol, while no differences in the uptake of water and methanol was observed in Nafion. The water-methanol uptake from the liquid phase (methanol 20 wt%) of a thick PBI membrane (50-100 pm) shows the same behavior, that is water uptake (A = 3.48) is much higher than methanol uptake (2 = 0.63), which is desirable behaviour for a membrane which intend to be a good barrier for methanol crossover. 

The permeability coefficients of PBI membranes, even those with high doping degrees, indicate than they are more efficient methanol barrier than Nafion. For instance, Pr = 0.01 for undoped PBI, and = 0.040—0.036 for PBI with 2a =1.9 in the range of temperature 30-90 °C. This is not only a result of the lower methanol uptake of methanol by PBI, but it is certainly due to a reduced diffusion coefficient of methanol in this polymer. 

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. 

Dry PBI membranes a =0) have very low conductivities (doping levels 2a < 1-5, and moderate conductivities ( 3.0 and temperatures close to 200 °C. A small increment in humidity, by exposing the membrane to the ambient [408] results in an important increase in conductivity, particularly at high doping levels [418]. Proton conductivities close to that found for Nafion membranes (around 100 mS.cm ) are found for PBI membrane with 2a > 3.0 in high humidity conditions, but the solvent used to prepare the membrane by casting seems to have a great influence, probably due to the formation of different polymer microstructures. On the other hand, the temperatures at which PBI membranes reach such conductivity levels are above 150 °C. 

Table 6.6 Conductivity of H3P04-doped PBI membranes prepared by casting...

Crosslinked PBI membranes have been proposed for improved mechanical strength and chemical stability. Li et al. [428] used p-xylene dibromine as cross linker agent and obtained 13 % crosslinked PBI with conductivities close to 100 mS.cm at 180 °C and high acid doping (2a = 7.75). Xu et al. [429], synthesized crosslinked PBI by condensation of 1,3,5 benzenetricarboxylic acid (BTA) and 3,3 -diaminobenzidine (DAB). The membranes exhibited high acid uptake and a conductivity of 64 mS.cm at 170 °C under dry conditions. 

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