The development of an inexpensive proton exchange membrane (PEM) that has a high proton conductivity at low relative humidity (RH) conditions along low fuel/O_2 permeability, high chemical, thermal and mechanical stability and low degree of swelling is crucial to the commercialization and large scale deployment of PEM fuel cells. The design of such a PEM would be aided by a quantitative understanding of the sorption and transport characteristics of Nafion, which is the objective of this paper. The design parameters that affect PEM performance are the nature of polymer backbone, the nature of the side chain, the nature and strength of the acid group, the equivalent weight, the degree of cross-linking, and thermo-mechanical properties of the membranes. The key operating parameter is RH, which determines the number of water molecules absorbed per site and, hence, the conductivity. In this paper, the sorption model of Nafion~(~R) is first developed, followed by the transport model.
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