Fabrication and characterization of improved PFSA/ePTFE composite polymer electrolyte membranes
Introduction
The development of polymer electrolyte membrane fuel cells (PEMFCs) with high performance, high durability and lower cost for power applications has been intensively interested [1], [2], [3], [4]. Polymer electrolyte membrane (PEM) is one of the key components of a PEMFC system. Perfluorosulfonic acid (PFSA) membranes, such as Nafion® (DuPont) membranes, are the state-of-the-art PEMs because of their high proton conductivity and excellent chemical stability [5], [6], [7], [8]. However, freestanding pure ionomeric films are usually weak and susceptible to swelling due to the significant reduction of the membrane strength in hydrated state [9], [10].
Previous reports have shown that cost reduction and performance improvement could be reached by replacing PFSA membrane with a PTFE-based composite membrane [11], [12], [13], [14], which is prepared by impregnating PFSA ionomers into porous poly(tetrafluoroethylene) (expanded PTFE, ePTFE) membranes. Due to the virtue of higher strength, the composite membranes can be used in fuel cell at thinner thickness, resulting in a higher area conductivity and the reduction of the expensive PFSA resin needed [15], [16]. This is significant since the large reduction of the output voltage compared with the theoretical value for a H2/air PEMFC is mainly due to the IR loss of the higher resistance membrane.
PFSA/ePTFE membrane can be prepared by placing PFSA films on both side of ePTFE membrane, and then hot-pressing the sandwich at a temperature above Tg (glass-transition temperature) of the PFSA resin. PEM prepared from this method have good mechanical and hydrolytic properties [17]. However, much of voids and bubbles can be clearly observed across the membrane cross-section. The proton conductance and performance of the PFSA/ePTFE are strongly dependent on the amount of PFSA in the ePTFE matrix. To increase the membrane compactness, most of researchers selected PFSA solution as material source since the polymer molecules were more dispersed at this state, hence can permeate the matrix and reduce voids or pinholes in the PTFE microspores. Shim et al. prepared PFSA/ePTFE membrane by spraying PFSA alcohol solution on ePTFE surface and then drying at an elevated temperature [18]. However, the solution permeation into the matrix would be largely limited since the solvent is very volatilizable. As a result, solution impregnation method was usually used to prepare PFSA/ePTFE membrane. The solution impregnation method was firstly reported by Gore [12], [13] and developed by many of researchers in the recent years. To improve the impregnation, Lin et al. [19] and Yu et al. [20] dissociated the Nafion® aggregations in the solutions by using 1–3 wt.% surfactant and solvent with solubility parameter close to Nafion® backbone. The re-dispersion of the aggregations makes the ionomers easy to impregnate into the porous structure of the ePTFE, resulting in a better ion conductivity and lower gas permeability. Ramya et al. improved the PFSA resin in the ePTFE membrane by choosing solvents with solubility parameters closing to PFSA side chains and increasing the wetting efficiency [21]. However, preparing PFSA/ePTFE composite PEMs without little voids or pinholes is still very difficult because it is difficult to impregnate the hydrophilic PFSA solution into the hydrophobic pores of the ePTFE [17], [22].
The existence of voids and pinholes would decrease the membrane proton conductance, and give the PEMs hazards of interface break and gas crossover. Our recent studies revealed that little voids and pinhole in the PEM can be enlarged by the RH-induced stress during the membrane operation, and then make the PEM high permeation to reactant gas and failure [23], [24].
Impregnation of the solution into a porous medium can always be realized as a capillary behavior [25], [26]. The impregnation processes will be affected by both the component of the solution and the properties of the porous matrix. This work aims to improve the PFSA/ePTFE composite PEM based on the investigation of the capillary behavior of the Nafion® solution and porous ePTFE. The effects of the pore size of the ePTFE, the surfactant content in the impregnating solutions and the gas pressure on the impregnation efficiency are investigated experimentally and theoretically.
Section snippets
The capillary behaviors of the Nafion® solution in the PTFE capillary
PTFE modified glass capillaries were used to investigate the capillary behavior of the Nafion® solution. For preparing the samples, glass capillaries with inner diameter of 0.1–5 mm were irrigated with 10 wt.% diluted PTFE emulsions. Then the capillaries were heat-treated at 345 °C for 60 s. After that, one of the ends of the capillaries was sealed by using of alcohol blast burner. The capillaries were finally cut to 5.5 cm length for use.
The capillary-rise method [27] was used to monitor the
The capillary behavior of the Nafion® solution in the PTFE capillary
Pure water does not penetrate spontaneously into hydrophobic capillaries [28]. However, the Nafion® 520 dispersion consisted of 48 ± 3 wt.% 1-propanol can slightly wet the PTFE wall and results in a little capillary height of 0.40 ± 0.01 cm (see Fig. 2) in the end-sealed tube. Fig. 2 compares the capillary height of the Nafion® solution as a function of Triton® X-100 surfactant concentration. The results reveal that the capillary height increases with the increase in the surfactant concentration at
Conclusions
The highly impregnated PFSA/ePTFE composite PEMs were prepared based on the capillary experimental results. Capillary experimental results showed that it is very difficult to improve the solution filling degree into the capillary tube by only changing the conditions of surfactant concentration and capillary radius at normal pressure. However, the decrease of the gas pressure in the capillary (initial Pinner) is very effective. The Nafion® solution can occupy 98.2% (4.91 cm vs. 5 cm) of the
Acknowledgements
This work is financially supported by the National High Technology Research and Development Program (“863” Program) of China (2006AA11A139). The authors would like to thank WUT New Energy Co., Ltd. for the durability experiment support.
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