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Nafion and modified-Nafion membranes for polymer electrolyte fuel cells 289 Table 3. Conductivity values for solid-acid conductors. Membrane system Comments on ionic conductivity Nafion/silica Nafion–titania Nafion–zirconia Nafion/ZrP Nafion/CNT Nafion/Cesium phosphate Nafion/mordenite Nafion/HPA (HPA: PWA, SWA, PMA) Nafion/imidazole HPA (hetero polyacid); PWA (phospho tungstic acid); SWA (silico tungstic acid); PMA (phospho molybdic acid). Conductivity slightly lower to Nafion in high RH condition, improved conductivity at elevated temperatures and lower RH values Conductivity slightly lower to Nafion in high RH condition, improved conductivity at elevated temperatures and lower RH values Conductivity slightly lower to Nafion in high RH condition, improved conductivity at elevated temperatures and lower RH values Conductivity similar to Nafion in high RH condition, improved conductivity at elevated temperatures and lower RH values Improved conductivity and mechanical strength leading to better PEFC performance Conductivity similar to Nafion in high RH condition, improved conductivity at elevated temperatures and lower RH values Conductivity slightly lower to Nafion in high RH condition, improved conductivity at elevated temperatures and lower RH values Improved conductivity over Nafion. But suffers leaching of HPA. Good conductivity. However, imidazole poisons the Pt catalyst during PEFC operation 3. Modified-Nafion membranes as electrolytes for PEFCs Conductivity of Nafion membranes often varies substan- tially depending on the system, membrane pre-treatment, and prevailing equilibrium parameters. At 100% RH, the conductivity of Nafion is generally about 0⋅1 S cm–1 at 60°C that drops by several orders of magnitude as the humidity is decreased. It is, therefore, mandatory to hy- drate Nafion membranes to keep the ionic inclusions bridged. Transport of water is generally due to protonic defects and occurs through the breaking and forming of bonds. This is caused because the protonic defects weaken the intermolecular interaction affecting large variations in bond lengths combined with rapid breaking and forming of bonds. Hydrophilic ceramic/inorganic fillers, such as SiO2, TiO2, ZrO2, zeolites, are added to Nafion matrix to retain water in the composite membrane at higher temperatures and low humidity (Miyake et al 2001; Song et al 2004, 2006; Jalani et al 2005; Klein et al 2005; Panero et al 2005; Borup et al 2007). These hydrophilic inorganic materials, when incorporated with Nafion, increase the binding energy of water as well as the number and strength of acid sites. The addition of inorganic fillers narrows the hydrophilic channels in Nafion matrix from 7⋅9 to 6⋅5 nm, facilitating proton conduction. Besides, the filler particles have affinity to absorb and retain water in the Nafion matrix. Table 3 summarizes different inorganic additives/fillers that have been added to the Nafion ma- trix for operating PEFCs at high temperatures and low relative humidity values. Solid-acid proton conductors like zirconium phosphate, titanium phosphate, caesium phosphates and heteropolyacids have also been explored as additives to Nafion to facilitate proton transport at reduced or zero hydration levels in the matrix. Although this concept seems promising, the success so far remains limited and the interaction between the inorganic phase and proton conductor stands vaguely understood. The best possible way to test the limitations of these mem- branes is to study them as prototype MEAs in PEFCs. There are many other significant issues that need to be resolved before solid-acid membranes are used in PEFCs. These include standard operating parameters such as mechanical strength, durability, cyclability, synthetic conditions, and design issues including catalyst compati- bility and scale-up. 3.1 Nafion membranes with inorganic filler as electrolytes for PEFCs In order to realize optimum PEFC performance and to make the system simple, it is highly desired to operate PEFCs at elevated temperature (> 80°C) and reduced RH values at atmospheric pressure. To this end, Nafion- composite membranes suitably modified with ceramic/ inorganic fillers, viz. SiO2, TiO2, ZrO2, zeolite etc are widely used to facilitate proton conductivity in stringent environmental conditions (Watanabe et al 1998; Adjemian et al 2002; Jung et al 2002; Aricò et al 2003; Ramani et al 2005; Sacca et al 2005). In the literature, membranes with inorganic fillers have been fabricated by a variety of techniques. One method includes impregnating inorganic oxide particles as fine powders into the Nafion ionomers and fabricating a com- posite membrane (Mauritz et al 1995; Baglio et al 2005). Another method includes addition of a precursor, gene-PDF Image | Nafion and modified-Nafion membranes
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