The Effect of Freeze-Thaw Cycles on the Mechancial Properties of Proton Exchange Membranes
Date
2011-05
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
Fuel cells could potentially provide a clean source of power for automotive
uses. However, their durability needs to be increased in order to become
commercially viable. A key component of increasing the durability of Proton
Exchange Membrane Fuel Cells (PEMFCs) is characterizing the behavior of the
proton exchange membrane at the heart of the fuel cell. This research investigates how
the mechanical properties of these membranes are influenced by repeated freezing and
thawing in conditions approximating those inside a fuel cell.
A stack consisting of Nafion® 211 membrane held between Toray Carbon
paper and bipolar plates was alternatingly placed in -20°C and room temperature
conditions. After 50, 75, 100 freeze-thaw cycles the membranes were removed from
the simulated stack set-up and a tensile test was performed on them. Membranes were
tested in conditions of 25°C, 30% relative humidity and 80°C, 90% relative humidity.
After testing the membranes the properties such as Young’s modulus, proportional
limit stress, break strain, and swelling strain were analyzed and compared with results
from previous work on membranes that had not undergone freeze thaw cycles. The results showed little change in the mechanical properties at conditions of
25°C and 30% RH. There appeared to be some effect on break strain, however,
coming to any conclusions is difficult due to a large scatter and low sample size.
Results for tests at 80°C and 90% RH show a slight decrease in stiffness, but the low
magnitude of the change and no discernible trend with the number of freeze thaw
cycles suggest this is more likely due to experimental scatter than the freezing
treatment. Swelling results similarly show little impact of freeze thaw cycling. A
small decrease in swelling due to changes in temperatures is noted, but swelling with
changes in humidity seemed unaffected.