Composite membranes Nafion/PTFE

Yu, T. L., Lin, H., Shen, K., Huang, L., Chang, Y., Jung, G. and Huang, J. C. 2004. Nafion/PTFE composite membranes for fuel cell applications. Journal of Polymer Research Taiwan 11 217-224. 

One activity at DuPont s Fuel Cells Business Center is the development of a thinner membrane with sufficient mechanical stabdity. Thinner membranes translate into higher current density, which in turn means a higher electrical efficiency. The tradeoff is a less mechanically robust membrane. Nafion membranes are nonreinforced films based on Nafion resin, a PFSA/PTFE copolymer in the acid (H+) form. DuPont is especially marketing Nafion PFSA NR-111 and NR-112 membranes as nonreinforced dispersion-cast films for that purpose. These membranes are dehvered as a composite with the membrane positioned between a backing film and a coversheet. This composite is wound on a 6 in. i.d plastic core, with the backing film facing out, as shown in Figure 27.21. 

An appropriate amount of Nafion ionomer (5% wt/wt, Aldrich) was mixed with 3% wt/wt Si02 (Aerosil 200, Degussa) in an ultrasonic bath for 30 min. This solution was cast [19,20] in a Petri dish and heated at 80°C for 30 min. The recast composite Nafion film was detached from the Petri dish by addition of distilled water and allowed to dry for 15 hrs at room temperature. Afterwards, it was cut to obtain a regular shape and then hot pressed between two PTFE foils at a few bars and increasing temperatures. The final treatment was 160°C for 10 min. The latter step allowed to increase the crystalline fraction inside the composite membrane with consequent improvement of the mechanical properties. The... 

The molecular structure of a conventional polymer used for a PFSA membrane is shown in Fig. 1. Membranes registered as Nafion (DuPont), Flemion , (Asahi Glass), and Aciplex (Asahi Chemical) have been commercialized for brine electrolysis and they are used in the form of alkali metal salt. Figure 4 shows a schematic illustration of a membrane for chlor-alkali electrolysis. The PFSA layer is laminated with a thin perfluorocarboxylic acid layer, and both sides of the composite membrane are hydrophilized to avoid the sticking of evolved hydrogen and chlorine. The membrane is reinforced with PTFE cloth. The technology was applied to PEFC membranes with thickness of over 50 xm [14]. 

The acid-base Nafion composite membranes include blends of Nafion with polypyrrole (PPy) [98-104], polybenzimidazole (PBI) [105-107], poly (propyleneoxide) (PPO) [108, 109], polyfurfuryl alcohol (PFA) [110], poly(vinyl alcohol) (PVA) [111-115], sulfonated phenol-formaldehyde (sPF) [116], polyvinylidene fluoride (PVdF) [117-122], poly(p-phenylene vinylene) (PPV) [123], poly(vinyl pyrrolidone) (PVP) [124] polyanifine (PANI) [125-128], polyethylene (PE) [129], poly(ethylene-terephtalate) [130], sulfated p-cyclodextrin (sCD) [131], sulfonated poly(ether ether ketone) (sPEEK) [132-135], sulfonated poly(aryl ether ketone) (sPAEK) [136], poly(arylene ether sulfone) (PAES) [137], poly(vinylimidazole) (PVl) [138], poly(vinyl pyridine) (PVPy) [139], poly (tetrafluoroethylene) (PTFE) [140-142], poly(fluorinated ethylene-propylene) [143], sulfonated polyhedral oligomeric silsesquioxane (sPOSS) [144], poly (3,4-ethylenedioxythiophene) (PEDT) [145, 146], polyrotaxanes (PR) [147], purple membrane [148], sulfonated polystyrene (PSSA) [149, 150], polystyrene-b-poly(ethylene-ran-butylene)-bpolystyrene (SEES) [151], poly(2-acrylamido-2-methyl-l-propanesulphonic acid-co-l,6-hexanediol propoxylate diacrylate-co-ethyl methacrylate) (AMPS) [152], and chitosan [31]. A binary PVA/chitosan [153] and a ternary Nafion composite with PVA, polyimide (PI) and 8-trimethoxy silylpropyl glycerin ether-1,3,6-pyrenetrisulfonic acid (TSPS) has also been reported [154]. 

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]. 

Chen LC, Yu TL, Lin HL, Yeh SH (2008) Nafion/PTFE and zirconium phosphate modified Nafion/PTFE composite membranes for direct methanol fuel cells. J Membr Sci 307 10-20... 

Table 12.1 Membrane thickness (L), conductivity (tr), and resistance (L/cr) of Nafion and Nafion/PTFE composite membranes...

The main advantage of Nafion/PTFE composite membranes is their low fuel gas crossover and high mechanical strength despite the low thickness L, and thus a low Lla as summarized in Table 12.1. Nafion/PTFE composite membranes can thus be made considerably thinner (<25 pm) than the commercially available Nafion-117,... 

Nafion-115, Nafion-112, and Nafion-212 which have thickness of 175 pm, 125 pm, 50 pm, and 50 pm, respectively. These thiimer composite PEMs contain significantly less amounts of the expensive Nafion resin than the thicker neat Nafion membranes. Thus, another advantage of Nafion/PTFE composite PEMs is the fact that they are inexpensive. Besides porous PTFE films, porous films such as polyethylene (PE) [21, 22] and electro-sptm polymer nanofiber films such as those of poly(vinyhdene fluoride) (PVdF) [23, 24], poly(vinyhdene fluoride-co-hexafluoropropylene) (PVdF-co-HFP) [25], and poly(vinyl alcohol) (PVA) [26-31] have also been used as supporting films for impregnating Nafion ionomer solutions to prepare Nafion/fiber composite PEMs for PEMFC and DMFC applications. 

Table 12.2 Tensile stress of Nation and Nafion/PTFE composite membranes...

Fig. 12.14 SEM micrographs of PBI/PTFE-nc composite membranes, (a) Plane surface of PBI/PTFE-nc (x5000 scale bar 5 pm), which fabricated using PTFE support pretreated with Nafion coupling agent (b) cross...

Compared with Nafion/porous PTFE and Nalion/ electrospun nano-fiber composite membranes, there are few reports of PBl/porous PTFE and PBl/electrospun nano-fiber composite membrane for high temperature PEMFC appUcatiOTi. The PBl/porous PTFE composite membrane was shown to exhibit excellent mechanical strength and good durability, which allowed researchers to reduce the membrane thickness and thus reduce the area specific resistance and ultimately improve fuel cell performance. After the report of PBl/porous PTFE composite membrane, two... 

Yu TL, Lin HL, Shen KS et al (2004) Nafion/PTFE composite membranes for fuel cell applications. J PolymRes 11 217-224... 

Wang L, Xing DM, Liu YH et al (2006) Pt/SiOz catalyst as an addition to Nafion/PTFE self-humidifying composite membrane. J Power Sources 161 61-67... 

Nafion/PTFE composite membranes were fabricated [17] by impregnation of a porons PI LE film with a solution of Nafion in a 2-propanol/water mixture. After solvent evaporation, the impregnated film was annealed at 120°C for 1 h. The resultant membrane was 20 pm in thickness, with a proton conductivity of 0.033 S cm at 25°C (30% that of Nafion 117), while the methanol flux was 4.43x10 mol cm s (as compared with 1.62x10 mol cm s for Nafion 117 and 6.20x10 mol cm s for Nafion 112). DMEC performance of the composite membrane, measured at 70 C with 2.0 M methanol feed and pure oxygen, was superior to that of both Nafion 112 and 117. 

However, as the PTFE content is increased, the methanol permeability decreases but ion conductivity also decreases fairly. A membrane with highest value of the ratio of ion conductivity to the methanol permeability (<1> = ion conductivity/methanol permeability) is regarded as the best. It is evident from Fig. 15.3, that ion conductivity to methanol permeability ratio (<1>) is higher in the SPS/PTFE composite membranes than in Nafion membranes. It increases with decreasing Styrene/PTFE ratio. This indicates that desired SPS/PTFE composite membranes with high potential for DMFC application can be prepared. 

Fig. 15.3 Comparison of the O (S cm ) parameter of Nafion and the SPS/PTFE composite membranes [41]. (This figure was published in Journal of Membrane Science, Vol. 251, J-P. Shin, B-J. Chang, J-H. Kim, S-B. Lee, D. H. Suh, Sulfonated Polystyrene/Composite Membranes, page 253, Copyright Elsevier, 2005.)...

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