Transport Mechanisms of Protons and Water

The dynamics of protons and water molecules in the bulk of pores are very similar to those in free bulk water. Percolation models and random network simulations can be used to study the impact of pore swelling and changes in pore connectivity upon water uptake on proton conductivity. Existing pore network models provide a good agreement with conductivity data. Updates of these models are needed to account for advances in understanding of the membrane morphology. 

Water transport occurs through diffusion of surface water, relative to the polymer matrix, and hydraulic permeation of bulk water. The water content determines the relative contributions of diffusion and hydraulic permeation to the net rate of water transport. 

The electro-osmotic coupling of proton and water transport depends on the molecular mechanism of proton transport. It is useful to distinguish a molecular and a hydro-dynamic contribution to the electro-osmotic drag coefficient. The latter contribution increases strongly with water uptake and temperature. 

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