Water sorption in Nafion® membranes analyzed with an improved dual-mode sorption model—Structure/property relationships

• Modeling of sorption isotherms in ionomer membranes.
• Model parameter values of different sigmoid-shaped isotherms for water and alcohols.
• One parameter is water molecule number of the first hydration shell of sulfonic site.
• Second parameter reflects clustered water molecules on the first hydration shell.
• Polymer segment rigidity in hydrophilic domains greatly affects sorption isotherms.

Sigmoid-shaped isotherms for water vapor sorption in fuel cell membranes were analyzed with Feng's new dual-mode model for two series of ionomer systems, Nafion® 117 membranes with different counter-ions and protonated Nafion® membranes aged under different degrees of humidity. Feng's model appeared to be an excellent sorption model for these sigmoid isotherms and the values of its parameters were used for an analysis of the membrane structure/property relationships. The first model parameter, Cp, corresponds to the number of water molecules strongly sorbed in the first hydration shell of the membrane sulfonic sites. The upward curvature in the isotherms at high water activities, reflected by the A′ parameter, appeared to be mainly due to the clustering of new water molecules on those of the first hydration shell. The number of water molecules per ionic site hydration depends on several structural parameters. The counter-ion of high electrostatic power attracts more water molecules per ion site, both in the first hydration shell and in subsequent sorption layers. The qualitative relationships between the membrane structure and its sorption properties obtained by analyzing the New Dual Mode Sorption (NDMS) model parameters make possible different interpretations of several data sets reported in the literature, such as the data on the sorption of different alcohols and the water sorption in Nafion® membranes during ageing.