(392l) Ex-Situ Tests Based Correlations to Predict Chemical and Mechanical Durability of Polymer Electrolyte Membranes

Ex-situ Tests Based Correlations to
Predict Chemical and Mechanical Durability of Polymer Electrolyte Membranes

R. Yadav, G. DiLeo, N. Dale, and K. Adjemian

Zero Emission ? Research

Nissan Technical Center North America, Farmington Hills,
MI-48331

Polymer electrolyte membrane (PEM)
fuel cells are a high-potential technology to allow for zero-emission
transportation. The durability of the PEM is a vital aspect that must be
understood for the successful commercialization of PEM fuel cell vehicles. The
ability to forecast the chemical and mechanical durability or lifetime of a
membrane material in a fuel cell environment would be a valuable screening or
development tool. This work proposes a chemical stability factor (CSF) and expands
on a previously reported humidity stability factor (HSF), based on ex-situ
testing, to predict the in-situ lifetime of PEM materials.

Chemical degradation of PEMs
occurs through free-radical attacks (OH^., OOH^.) on weak
and vulnerable sites (-COOH, side group) in the PEM[1-3]. These free-radicals
are generated from H₂O₂ that can be formed through a
2-electron O₂ reduction process at multiple locations in the
membrane electrode assembly [4-5]. Open Circuit conditions (high potential, low
RH) are favorable for the formation of free-radicals leading to severe chemical
degradation of PEM. Variables such as temperature, O₂ concentration,
water activity (relative humidity), membrane thickness, polymer type (PFSA,
hydrocarbon), and processing techniques (reinforcement or chemical stabilizers)
affect the chemical durability  of a membrane [6-7]. In automotive applications,
PEMs are also exposed to numerous cycles of wet and dry conditions in which
swelling and shrinking, stress, creep, and fatigue of the membrane lead to
membrane cracking and performance loss [8-9].  

In-situ evaluation of PEM chemical
durability and mechanical durability is lengthy in time even with accelerated
stress tests (i.e. OCV hold test: up to 4 weeks and dry/wet cycling: up to 10
weeks). Correlations based on quicker ex-situ tests to predict chemical
durability and mechanical durability can help save significant in-situ testing
times and resources. This work is aimed at developing such correlations defined
as a chemical stability factor (CSF) for chemical durability and a humidity
stability factor (HSF) for mechanical durability of PEMs. The CSF is a
correlation based on oxygen permeation through the membrane and the fluoride
emission from exposing the membrane to a solution of Fenton reagents. We have
observed that the relative trend of PEM chemical durability obtained from OCV
hold tests matches with the relative trend predicted by CSF. The correlation
for HSF is based on the tensile strength of the membrane along with its
swelling in liquid water. This factor has been previously reported by MacKinnon
et al. [10] and in this work is applied to a wide variety of membranes. The
relative trend of PEM mechanical durability from in-situ testing follows the
relative trend predicted by HSF.

This work will discuss the
fundamentals of these ex-situ-test-based correlations to predict in-situ
chemical durability and mechanical durability. Results from OCV hold test
hours, CSF, dry/wet cycling test, and HSF will be presented to show the reliability
of these correlations.

Acknowledgement:

We thank the Composite Materials and Structure
Centers, Michigan State University for measurement of oxygen permeation rate
across PEMs.

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