Water in a Polymeric Electrolyte Membrane: Sorption/Desorption and Freezing phenomena

TL;DR

This study investigates water sorption/desorption and freezing phenomena in Nafion membranes at low temperatures, combining neutron diffraction experiments with a solution model to explain reversible ice formation and membrane dehydration.

Contribution

It introduces a model linking hydronium concentration to freezing behavior, clarifying the mechanisms behind water transport and phase changes in polymer electrolyte membranes.

Findings

Ice forms outside the membrane and crystallizes in hexagonal Ih form.

Membrane shrinks and dehydrates during cooling, increasing hydronium concentration.

The model accurately predicts ice formation and membrane behavior during temperature cycles.

Abstract

Nafion is a perfluorosulfonated polymer, widely used in Proton Exchange Membrane Fuel Cells. This polymer adopts a complex structural organisation resulting from the microsegregation between hydrophobic backbones and hydrophilic sulfonic acid groups. Upon hydration appear water-filled channels and cavities, in which are released the acidic protons to form a solution of hydronium ions in water embedded in the polymer matrix. Below 273 K, a phenomenon of water sorption/desorption occurs, whose origin is still an open question. Performing neutron diffraction, we monitored the quantity of ice formed during the sorption/desorption as a function of temperature down to 180 K. Upon cooling, we observe that ice forms outside of the membrane and crystallises in the hexagonal Ih form. Simultaneously, the membrane shrinks and dehydrate, leading to an increase of the hydronium ions concentration inside the matrix. Reversibly, the ice melts and the membrane re-hydrate upon heating. A model of solution, whose freezing point varies with the hydronium concentration, is proposed to calculate the quantity of ice formed as a function of temperature. The quantitative agreement between the model and experimental data explains the smooth and reversible behavior observed during the sorption or desorption of water, pointing out the origin of the phenomena. The proposed picture reconciles both confinement and entropic effects. Other examples of water filled electrolyte nano-structures are eventually discussed, in the context of clarifying the conditions for water transport at low temperature.