Nafion^® is a good proton conductor when swollen with water. This polymer is based on teflon-like main chains with very hydrophobic perfluorinated pendant chains bearing hydrophilic sulfonic acid groups SO₃H. Proton conduction is located in the ionic phase containing water and ionic groups SO₃^-. At the operating temperature of a stack (typically 80°C), the polymer undergoes a SO₃H condensation reaction very unfavorable to the conduction phenomenon, that is to say a "bridge" between two neighboring acid functions. This is called hygrothermal aging, which is observed in a membrane stored hundreds of days at 80°C in the presence of moisture.

However, during stack operation, the situation is very different: no hygrothermal aging is observed. Why? Because the fuel cell produces an excess of water. In this case, the condensation of SO₃H is balanced by the reverse reaction of hydrolysis, bridges virtually do not exist. We even demonstrated that an "old" membrane can be fully rejuvenated by fuel cell operation! The condensation reaction is reversible, indeed it can regenerate a membrane which was hygrothermally aged ex situ.