Polymer electrolytes for fuel cells perfluorosulphonic acid systems

5 Polymer electrolytes for fuel cells perfluorosulphonic acid systems 

The polymer electrolytes used for low-temperature proton exchange membrane fuel cells (PEMFCs) are fundamentally different from the polymer electrolytes used in batteries. Here, the polymer is a medium for a solvent, normally water, and it is mainly in the solvent that ion transport occurs. The polymer serves several functions, of which the most important is to provide mechanical stability and electrode separation in the fuel cell application. Since the fuel cell needs proton transport from the anode to the cathode, the polymer also contains proton donating groups, often sul-phonic acid (-SO3H). The prototype PEMFC membrane materials have been perfluorosulphonic acids (PFSAs), of which the most established membrane material is Nafion (Fig. 8.8). These consist of hydrophobic teflon -CF2-CF2- backbones, with fluorinated hydrophilic and acidic side-chains for Nafion -0CF2CF(CF3)0CF2Cp2S03H. 

The inhomogeneous multiphase system that a PEMFC membrane such as Nafion comprises makes the system very hard to analyse experimentally.  

Jang et al. were the first to investigate the side-chain sequence on the polymer chain using classical MD. Considering the synthesis methodology 

There has been some criticism of the study by Jang et a/., which address the importance of the equilibration procedure for polymeric systems. Elliott and Paddison have argued that the difference in density between the MD simulated system and the experimentally measured values are due to incomplete chain relaxation or an erratic force field. Since later MD simulations of Nafion using similar force field provided good agreement in density, the latter can probably be ruled out. Jang et al. used a rather complex equilibration process, were the MD boxes were heated, and expanded and compressed several times in the NPT ensemble. The resulting densities (1.60 and 1.67 g/cm ) are 5-10% lower than experimental values, which can partly be explained by the lack of semicrystalline domains in the MD box, which do exist in real membranes and contribute to a somewhat increased density. 

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