Polymer electrolyte membrane modified Nafion® membranes

One of the most important parts of the fuel cell is the electrolyte. For polymer-electrolyte fuel cells this electrolyte is a single-ion-conducting membrane. Specifically, it is a proton-conducting membrane. Although various membranes have been examined experimentally, most models focus on Nafion. Furthermore. it is usually necessary only to modify property values and not governing equations if one desires to model other membranes. The models presented and the discussion below focus on Nafion. 

The solid polymer electrolyte (SPE) fuel cell makes use of the high stability and the cation selectivity of Nafion, a Teflon-like material that has been modified by the incorporation of sulfonic groups. The membrane is coaled with a porous catalyst on both sides. Hydrogen is oxidized on one side of the membrane and the formed in the process is transported across the membrane to the other side, where it interacts with (OH) ions formed by the reduction of oxygen, to form water. This water is removed from the cell by capillary action with the use of a... 

L.J. Hobson, H. Ozu, M. Yamaguchi and S. Hayase, Modified Nafion 117 as an improved polymer electrolyte membrane for direct methanol fuel cell, J. Electrochem. Soc., 2001, 148, A1185-A1190. 

Recently, Dahr [1], Stonehart [2] and Watanabe [3] have made an attempt to reduce the humidification constraints in solid polymer electrolyte fuel cells (SPEFCs) by using modified perfluorosulfonic membranes. A recast Nafion film sandwiched between the two electrodes was first proposed by Dahr [1] for the realization of an internally humidified SPEFC. Stonehart [2] suggested the inclusion of small amounts of silica powder into the recast film in order to retain the electrochemically produeed water inside the membrane. Watanabe et al [3] have tried to exploit the H2/O2 crossover through the membrane to produce a chemical recombination to water on small Pt clusters inside the membrane. All of these membranes were operated with H2/O2 at 80°C and allowed the development of systems without assisted humidification or with near ambient humidification. 

There is considerable methanol permeation through Nafion, which affects the fuel cell performance in a DMFC. Using doped PBI the same proton conductivity as Nafion can be maintained while virtually eliminating the crossover of methanol. PBI is doped with a conducting solid, usually phosphoric acid, to make it suitable for DMFC applications [55, 56]. In another attempt PBI is modified by sulfonation to make it an intrinsic proton conductor and is deposited onto a layer of Nafion membrane. This gives a composite polymer electrolyte that is a reasonable proton conductor and reduces the crossover of methanol [57]. 

Sahu, A.K., Selvarani, G., Pitchumani, S Sridhar, P and Shukla, A.K. (2007) A sol-gel modified alternative Nafion-silica composite membrane for polymer electrolyte fuel cells. /. Electrochim. Soa, 154 (2), B123-B132. 

For PEMFCs, the solid electrolytes are polymer membranes polymers modified to include ions, usually sulfonic groups. One of the most widely used membranes today is the polymer Nafion , created by the DuPont company. These membranes have aliphatic perfluorinated backbones with ether-linked side chains ending in sulfonate cation exchange groups [6, 7], Nafion is a copolymer of tetrafluoroethylene and sulfonyl fluoride vinyl ether [8] and has a semi-crystalline structure [9], This structure (which resembles Teflon ) gives Nafion long-term stability in oxidative or reductive conditions. The sulfonic groups of the polymers facilitate the transport of protons. The polymers consist of hydrophilic and hydrophobic domains that allow the transport of protons from the anode to the cathode [10, 11],... 

---