Water in a polymeric electrolyte membrane: Sorption/desorption and freezing phenomena

Abstract

Nafion® is a perfluorosulphonated 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 water-filled channels and cavities appear, in which the acidic protons are released to form a solution of hydronium ions in water embedded in the polymer matrix. Below 273K, 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 180K. Upon cooling, we observe that ice forms outside of the membrane and crystallises in the hexagonal form (Ih). Simultaneously, the membrane shrinks and dehydrates, leading to an increase of the hydronium ions concentration inside the matrix. Reversibly, the ice melts and the membrane re-hydrates 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 behaviour 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.