Interplay between Structure
and Relaxations in Perfluorosulfonic
Acid Proton Conducting Membranes
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Abstract
This study focuses on changes in the structure of ionomer
membranes,
provided by the 3M Fuel Cells Component Group, as a function of the
equivalent weight (EW) and the relationship between the structure
and the properties of the membrane. Wide-angle X-ray diffraction results
showed evidence of both non-crystalline and crystalline ordered hydrophobic
regions in all the EW membranes except the 700 EW membrane. The spectral
changes evident in the vibrational spectra of the 3M membranes can
be associated with two major phenomena: (1) dissociation of the proton
from the sulfonic acid groups even in the presence of small amounts
of water; and (2) changes in the conformation or the degree of crystallinity
of the poly(tetrafluoroethylene) hydrophobic domains both as a function
of EW and membrane water content. All the membranes, regardless of
EW, are thermally stable up to 360 °C. The wet membranes have
conductivities between 7 and 20 mS/cm at 125 °C. In this condition,
the conductivity values follow VTF behavior, which suggests that the
proton migration occurs via proton exchange processes between delocalization
bodies (DBs) that are facilitated by the dynamics of the host polymer.
The conductivity along the interface between the hydrophobic and hydrophilic
domains makes a larger contribution in the smaller EW membranes likely
due to the existence of a greater number of interfaces in the membrane.
The larger crystalline domains present in the higher EW membranes
provide percolation pathways for charge migration between DBs, which
reduces the probability of charge transfer along the interface. Therefore,
at higher EWs although there is charge migration along the interface
within the hydrophobic–hydrophilic domains, the exchange of
protons between different DBs is likely the rate-limiting step of
the overall conduction process