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Table 1. Membrane type Nafion – 112 Nafion – 1135 Nafion – 115 Nafion – 117 *Typical thickness (μm) 51 89 127 178 Basis weight (g/m2) 100 190 250 360 Nafion and modified-Nafion membranes for polymer electrolyte fuel cells 287 are widely used in PEFCs due to their high proton con- ductivity and moderate swelling in water (Hickner et al 2004; Mauritz and Moore 2004). Nafion-117 membrane employs the polymer with equivalent weight of 1100 and has a thickness of 7 mils (1 mil = 1/1000 of an inch = 25⋅4 μm). Other cation exchange membranes with differ- ing EWs of polymer and thicknesses designed for various applications are presented in table 2. Perfluorosulfonate cation exchange membranes with structures akin to Nafion have been developed by Asahi Chemical Com- pany (Aciplex) and Asahi Glass Company (Flemion). Nafion has ∼ 40 Å clusters of sulfonate-ended per- fluoroalkyl ether groups organized as inverted micelles arranged on a lattice (Hsu and Gierke 1983) as shown in figure 4(a). These micelles are connected by pores or channels of ∼ 10 Å in size. These channels with –SO–3 groups invoke inter cluster ion-hopping of positively charged species. A model for proton conduction in Nafion based on three different regions within the poly- mer membrane has been proposed in the literature (Yeo and Eisenberg 1997). These regions are fluorocarbon phase (FC), interfacial region (IF) and ionic clusters (IC). Figure 4(b) shows cluster-network model for the interac- tions between polymer and water in Nafion membrane (Hsu and Gierke 1983). In Nafion membrane, FC region consists of polymer backbone, viz. the PTFE backbone of Nafion, IF region contains side chains comprising water and sulphonate groups and IC region contains absorbed water. A simple structural conceptualization of cross-linked polyelectrolyte and clustered ionomeric system with an- ionic side-chain (Yeo and Eisenberg 1997) is shown in figure 5; they have also studied the diffusion of water in Nafion by sorption from the contacting liquid into dry samples and measuring water uptake as a function of time in the temperature range between 0°C and 100°C. The diffusion coefficient (D) of water in the membrane was found to increase from about 10–6 to 10–5 cm2/s with increasing temperature, and the activation energy was estimated to be 4⋅5 kcal/mol. The structural organization of Nafion membrane under different physiochemical con- ditions by invoking the balance between the elastic deformation of polymer matrix and various molecular interactions existing in the polymer has been discussed in the literature (Mauritz et al 1978). Proton transport in Nafion has also been studied widely in the literature (Miyake et al 1983; Gebel et al 1993; Cappadonia et al 1995; Zadowzinski et al 1995; Wain- right et al 1998; Choi et al 2005). Proton transport in Nafion depends on water content of the membrane. Although the precise mechanism for proton transfer in solvated form of Nafion is not completely understood, a qualitative picture is provided (Zawodzinski et al 1995) in figure 6. Generally, it is assumed that the state of water in Nafion membrane is not fixed. Some of the water is tightly bound to –SO–3 groups and is called chemically- bound water, which has lesser degree of hydrogen bond- ing than the bulk water; the latter is described as physi- cally-bound water present in the central-pore region of Nafion membrane. The transfer of protons near pore sur- (a) Physical parameters of Nafion membranes. Table 1. (b) Physical properties of Nafion membranes. Properties Typicalvalue Water uptake (% watera) 35 Tensile modulus (MPa) 50% RH (25°C) 249 Water soaked (100°C) 64 Tensile strength (max. MPa) 50% RH (25°C) 43b, 32c Water soaked (100°C) 25b, 24c Density (g/cm3) 1H diffusion coefficients (cm2/s) Ion exchange capacity (mequiv./g) 2⋅0 0⋅6 × 10–6 0⋅9 aWater uptake from dry membrane soaked in water at 100°C for 1 h; bmeasured in machine direction; cmeasured in transverse direction, *at 25°C, 50% R.H. Cluster-network model for the interactions between polymer and water in Nafion membrane. Figure 4.PDF Image | Nafion and modified-Nafion membranes
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