Water and Vehicular Hydronium Diffusivities

The classical MD simulations performed in task I provide self-diffusion coefficients for water and also for hydronium ions, which is strictly the vehicular component of the proton diffusivity. These diffusion coefficients are calculated from the mean square displacement of H2O and HsO using the Einstein relation. The numerical values for Nation and SSC membranes at the four hydration levels are hsted in Table 5 along with the experimental values.  

both experiment and simulation agree that the diffusivity of water increases with increasing water content. This can be attributed to the facts that 

Diffusion Coefficients for Water and Hydronium Ions in Hydrated Nation and SSC PFSA Membranes at Various Water 

The simulated and experimental hydronium diffusivities both increase with increasing water contents. The simulated values are lower than the experimental values, presumably, due to the fact that the simulations report only the vehicular contribution to the proton diffusivity, whereas the experiment measures the total proton diffusivity. Experimentally, Nafion has higher proton diffusivity than SSC at low water contents and lower proton diffusivity than SSC at high water contents. The vehicular diffusion coefficients of the hydronium ion measured from simulation are higher for Nafion than in SSC PFSA at all water contents. Clearly a detailed understanding of the total proton diffusivity as a function of polymer architecture requires a model capable of structural diffusion. 

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