  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.
