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Impact of elevated temperature on structural changes of perfluorosulfonated ionomer for polymer electrolyte membrane fuel cells

The electrochemical reactions in low- temperature polymer electrolyte membrane (PEM) fuel cells take place on the catalyst surface at the so-called triple phase boundary. These reactions, particularly on the cathode side, result in significant heat release. The temperature at this surface will correspond to an equilibrium between the heat release and heat removal rates, and can reach up to 20°C above the coolant temperature. Insufficient local heat removal may result in local high temperatures which may cause morphological changes in polymer electrolyte surrounding the reaction sites, which in turn may affect its proton conductivity thus resulting in fuel cell performance degradation. In order to better understand the fuel cell degradation phenomena, there is a strong need to fully understand thermal behaviour of the fuel cell materials, primarily the polymer electrolyte. The thermal behaviour of NafionTM, perfluorosulfonate ionomer, was studied by differential scanning calorimetry (DSC). The sample was NafionTM 112, a 50 µm thick membrane Due to the ionic nature of the membrane, NafionTM stable structure can turn to an unstable state by heating, which should be visible on DSC curves in the form of an endothermic peak. Interchange of formation and disruption of the ionic clusters is caused by the thermoreversible characteristic of the intramolecular ionic interactions. Therefore, it is important after each heat treatment to leave a sample for a certain time while the bond is formed again. Otherwise, the peak on DSC curves would not have been visible if the DSC analysis had been repeated immediately after each treatment. In order to prove that, three measurements were performed on the same sample at previously set time intervals (initial, 3 days after and 7 days after). The obtained measurements showed that the first endothermic peak is apparent in the temperature range of 60-140 °C. While the most of the authors have overlapped this peak to the glass transition or evaporation of the residual water, we have attributed that peak to the ionic bonding. The second heat run, performed immediately after the first heat run, indicates suppression of the first peak. Remaining measurements were performed again after 3 and 7 days. Gradual peak increase was noticed, shifting peak to a higher temperature. Increasing the time interval between measurements, the ionic clusters arrange themselves, demanding more energy for their translation into the unstable state. It means higher endothermic peak and higher heat of transition, which is evident through the increase of enthalpy. The membrane structure, during the heat release, will not be affected by irreversible degradation due to its thermoreversible ionic interactions. Future work concerns the deeper analysis of thermal properties with focus on temperature distribution through the triple phase boundary of the PEM fuel cells.

Polymer electrolyte membrane fuel cell ; Elevated temperature ; Perfluorosulfonated ionomer ; Differential scanning calorimetry

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