ABSTRACT

Percec et al., 1995; Bouchet and Siebert, 2000; Fontanella et al., 1998; Glipa et al., 1999; Kawahara et al., 2000; Wainright et al., 1995; Allcock and Fitzpatrick, 1992; Allcock and Kwon, 1989; Allcock et al., 1991; Montoneri et al., 1989), and acid-base composite membranes (Savinell et al., 1994; Samms et al., 1996; Wainright et al., 1995). Membranes based on functionalized non-fluorinated backbones are potentially promising for high-temperature operation. High conductivities have been obtained at temperatures up to 180°C (Bonnet et al., 2000; Nunes et al., 2002; Alberti and Casciola, 2003; Zaidi et al., 2000; Honna et al., 1999, 2001; Staiti et al., 1999, 2000, 2001). In addition to high proton conductivity, polymer electrolyte membrane (PEM) durability is also required for successful PEFC operation. Several studies on PEFC durability have shown that components of the central membrane electrode assembly (MEA), especially the central PEM, deteriorate during long-term operation (LaConti et al., 2003; Luo et al., 2006; Cleghorn et al., 2006; Chen et al., 2006; Liu et al., 2006; Mittal et al., 2007; Inaba et al., 2006; Endoh et al., 2004; Curtin et al., 2004; Schiraldi, 2006). PEM degradation modes can be classified as thermal (desulfonation, solvolysis), mechanical (pin-hole and crack formation), and chemical (free radical induced oxidative degradation) (LaConti et al., 2003); key aspects of PEM durability including degradation modes will be discussed in the following sections. 8.2  Mechanical DegradationMechanical degradation can arise because of (i) improper sealing of the fuel cell resulting in the formation of cracks in the PEM (Collier et al., 2006) and (ii) inadequate (or excessive) humidification and concomitant swelling-deswelling, which transforms the PEM to a fragile membrane with high degradation rate (Huang et al., 2003; LaConti et al., 2003). The most common experiments used to investigate the endurance of the membrane include cyclic stress as well as relative humidity (RH) cycling. Cycle stress experiments are conducted in electromechanical testing equipment by applying different stresses (1-10 MPa) to the membrane every minute (Tang et al., 2007), while for the latter experiments, the MEA is mounted in a single cell hardware at constant cell temperature and various

humidified gas conditions (0-100% RH). During RH cycling, the catalyst layers are replaced every ~900 cycles to eliminate the influence of catalyst degradation on membrane properties (Tang et al., 2007). The formation of cracks in the PEM increases the gas crossover, resulting in cell voltage losses and softening of the PEM (Endoh et al., 2004; Healy et al., 2005; Huang et al., 2003; Knights et al., 2004; LaConti et al., 2003; Liu et al., 2001; Taniguchi et al., 2004; Xie et al., 2005; Yu et al., 2005). 8.3  Thermal DegradationThis mode of PEM degradation is reported to be due to high operating fuel cell temperatures (Knights et al., 2004); the higher the temperature, the higher the water loss in the PEM promoting membrane dryness and degradation (Knights et al., 2004). Thermal degradation of polymer membranes is investigated by thermogravimetric analysis (TGA) in combination with infrared (FT-IR) and mass spectroscopy (MS); the generation of degradation products from polymer during thermal treatment is monitored, and the weight losses are correlated at different temperatures with the molecular composition of the membrane (Chu et al., 1990; Deng et al., 1998; Feldheim et al., 1993; LaConti et al., 2003; McDonald et al., 2004; Samms et al., 1996; Wilkie et al., 1991). Nafion® membranes degrade at temperatures above 150°C, when the breaking of C-F and C-S bonds along with the loss of sulfonated groups occurs (Chu et al., 1990; Deng et al., 1998; Feldheim et al., 1993; LaConti et al., 2003; McDonald et al., 2004; Samms et al., 1996; Wilkie et al., 1991). Chu and coworkers (1990) used infrared reflectance absorption spectroscopy (IRRAS) to investigate the thermal stability of Nafion® for a wide temperature range (22-300°C). No spectral changes were observed when Nafion® samples heated up to 200°C, whereas at 300°C, the intensity of the S-O related bands decreased indicating the loss of sulfonic acid groups from Nafion® samples (Chu et al., 1990). Similar results were reported by Surowiec and Bogoczek (1988); TGA and differential thermal analysis (DTA) along with infrared spectroscopy revealed that Nafion® is stable up to 280°C, and at higher temperatures, sulfonic groups are detached for the main membrane chain accompanied by total PEM weight loss.