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286 A K Sahu et al ΔG° = –nFV0, (4) where n is the number of electrons involved in the overall reaction, F the Faraday constant, and V0 the cell voltage at thermodynamic equilibrium in absence of the current flow, i.e. under open-circuit condition. V0 is the diffe- rence of the equilibrium electrode potentials at the cathode and the anode of the cell, and is calculated from V0 = –ΔG/nF = 1⋅23 V. (5) Although substantial improvements have been made in PEFCs, it has been difficult to realize them commercially owing to their cost. The estimated production cost per kW for a PEFC stack is presented as a Pi-chart in figure 2. It is obvious that the major cost of a PEFC stack is from membrane electrolytes, electrodes and bipolar plates (Tsuchiya and Kobayashi 2004). The following sub- sections describe Nafion and modified-Nafion mem- branes as electrolytes for PEFCs. 2. Nafion membranes as electrolytes for PEFCs An effective membrane electrolyte for PEFCs, besides acting as a physical separator for the fuel and oxidant, must possess high ionic conductivity with little electronic component. It should be possible to cast them as thin and large surface-area membranes. The membranes should be stable in oxidizing and reducing environments prevailing during the operation of the PEFCs. The membrane must be easy to manufacture and be able to withstand the stress during electrode processing and attachment, and should be both mechanically and electrically robust to endure the start-up and shutdown schedules of the PEFCs. As early as in 1940s, researchers were engaged in developing organic ion-exchange membranes for fuel cell applications (Grot 1985; Roziere and Jones 2003). These ion-containing polymers were termed as ionomers. It was believed that these ionomeric polymers would possess better properties, as they involve the interaction between polymer and ions. The earliest PEFC systems served as the power plants for Gemini space missions in the early 1960s. But the cells were short-lived due to the oxidative degradation of the membranes comprising a copolymer of sulfonated polystyrene and divinylbenzene. In the late 1960s, DuPont developed membranes from Nafion, a co- polymer of perfluoro-sulfonic acid with hydrophobic fluorocarbon backbone and hydrophilic sulfonic acid pendent side chains. This membrane was primarily used as a perm-selective separator in chlor-alkali electrolysers. In 1980s, the use of Nafion membrane helped realizing the concept of ‘zero-gap’ cell design. With the induction of zero-gap cell concept, the internal resistance of the PEFCs decreased and, as a result, the overall efficiency of the system increased substantially (Shukla and Raman 2003). The chemical structure of Nafion is depicted in figure 3. Nafion is produced by co-polymerization of variable amounts of unsaturated perfluoroalkyl sulfonyl fluoride with tetrafluoroethylene. Nafion is commercially available in varying equivalent weights, viz. 900, 1100, 1200, etc. Equivalent weight (EW) represents grams of dry Nafion per mole of sulfonic acid groups when the material is in the acid form. Nafion membranes employing polymer with EW = 1100 are listed in table 1. These membranes Figure 3. Chemical structure of Nafion. Figure 1. Schematic diagram of a PEFC. Figure 2. Estimated cost per kW for PEFC stack.PDF Image | Nafion and modified-Nafion membranes
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