Understanding the Mechanisms of Solvent Transport in Acidic Membranes

The study of solvent transport in solvated acidic membranes, such as those used in proton exchange membrane (PEM) fuel cells, reveals a complex interaction between diffusional processes and viscous flow. As the interaction between the solvent and polymer decreases, the contribution of viscous flow to solvent permeation becomes more pronounced. This dynamic is influenced by the width of the channels formed within the membrane, which is determined by the degree of hydration and the separation of hydrophilic and hydrophobic regions.

Two prominent types of acidic membranes are Nafion and solvated sulfonated polyarylenes. These materials exhibit distinct differences in their solvent transport coefficients due to their varying degrees of separation and polymer-solvent interactions. In general, Nafion membranes tend to show a greater separation and weaker interactions compared to sulfonated polyarylenes, leading to differing permeation behaviors.

An intriguing aspect of solvent transport in these membranes is the phenomenon known as electro-osmotic drag. Even though solvents like water and methanol are electrically neutral, they can be influenced by an electric field when in contact with an acidic ionomer. This effect is particularly significant for protons, as their interaction with the solvent molecules can induce a drift velocity, facilitating the movement of the solvent through the membrane.

Historically, the theory of electro-osmosis has its roots in the works of scientists like Helmholtz and Smoluchowski, who initially proposed models based on electrical double layers. However, these classical models are not entirely applicable to modern PEM materials like Nafion, where the hydrated channels are significantly smaller than the theoretical dimensions predicted by earlier theories. Researchers Breslau and Miller have since developed a more relevant hydrodynamic model that treats ions as spherical particles in a viscous medium, providing a more accurate representation of ion and solvent dynamics within these membranes.

Despite the simplifications inherent in their model, Breslau and Miller's approach successfully explains variations in electro-osmotic drag across different model membranes. Experimental data indicate that typical electro-osmotic drag coefficients range from 1 to 2.5, reflecting the number of water molecules transported per protonic charge carrier. Recent advancements, including electrophoretic NMR techniques, have further clarified the hydrodynamic nature of electro-osmosis, particularly at higher levels of solvation, enhancing our understanding of solvent transport in these critical materials.