Current density as a function of time during fuel cell operation with a pre-aged membrane. At the beginning, no power can be extracted from the cell. After 800 h of operation, the cell performance of the “aged” membrane are rejuvenated to the level obtained with a fresh membrane.
Understanding and controlling the aging of the proton-conducting Nafion® membrane is one of the key points for the development of fuel cells. In collaboration with the LITEN and ENSAM Paris, we have shown that the mechanism of hygrothermal aging, previously observed ex situ with a nude membrane, does not occur during fuel cell operation. Better yet, an aged Nafion® membrane can be rejuvenated by fuel cell operation.
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.
Last update : 04/29 2014 (1027)