A molecular dynamics simulation model has been developed and used to investigate microscopic transport mechanism and various physical properties In the electrolyte membrane for polymer electrolyte fuel cells. It has been succeeded to quantitatively predict the experimental facts that the diffusion coefficients of positive ions and water molecules in the polymer electrolyte are one-order smaller than those in the aqueous solution and that the O-H vibration spectrum has higher frequencies than that in the liquid water. The very low diffusivity in the polymer electrolyte is due to the fact that polar particles form the disordered heterogeneous structure of hydrophilic region constituting transport paths for ions and water molecules. About 80% of positive lons are bonded with sulfate anions at the end of side chains in the polymer and move sequentially from one sulfate anion to the other in the vicinity within 5Å about two or three times per nanosecond. This transport mechanism of ions implies that the main constraining factors for the ion transport in the polymer electrolyte are the coulomb interaction with sulfate anions and the tortuosity of hydrophilic region. Based on the results, guiding princlples to enhance the ion diffusivity in the polymer electrolyte have been proposed.
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Jinnouchi et al. (2001) studied this question.