(278d) Durability of Perfluorosulfonic Acid Membranes for PEM Fuel Cells

One of the key challenges
facing the commercialization of fuel cells is developing membrane electrode
assemblies (MEAs) that can meet industry durability targets. Polymer
electrolyte membranes (PEMs) are the most promising membranes for automotive
applications. These membranes serve to conduct protons from the anode electrode
to the cathode electrode of the fuel cell while preventing crossover of the
reactant gases, H₂ and O₂. State of the art PEM fuel
cells utilize perfluorosulfonic acid membranes that are typically no more than
25 microns thick. Fuel cells cannot operate effectively if even small amounts
of gas are able to permeate the membrane through microscopic pinholes.
Ultimately, fuel cells fail because such pinholes develop within the polymer
membranes. It is critical that these membranes are mechanically durable over
the range of conditions experienced during fuel cell operation.

This study describes the methods
that have been developed to characterize the mechanical durability of PFSA
membranes in fuel cell applications. PEM fuel cells will see temperatures
ranging from ambient to 100^oC and a variety of humidification levels
including exposure to liquid water. Complicating matters are the facts that (1)
perfluorosulfonic acid membranes absorb up to 50 wt% water and experience
significant volumetric swelling upon water adsorption and (2) the glass
transition temperature of perfluorosulfonic acid membranes such as Nafion
occurs within the temperature range of fuel cell operation, 60-100^oC.
Additionally, the membranes experience various stresses during fuel cell
operation. For example, the membranes are compressed between sheets of carbon
based diffusion media at pressures up to 500 psig. The membranes will also
experience tensile and shear stresses as the dimensions of these membranes
change in a constrained architecture with changes in temperature and humidity
level. Furthermore, perfluorosulfonic acid membranes are susceptible to
chemical attack by peroxide radicals, which subsequently impacts the mechanical
integrity of the membrane.

An in situ test has
been developed to study the mechanical durability of fuel cell membranes by
cycling the humidity of the membrane in the absence of electric potential or
reactive gases.   Results show that the stresses imposed solely by cycling
between wet and dry operating conditions can create membrane failure.  The
impact of the severity of the humidity swing and the compounding effects of
chemical degradation on mechanical durability will also be discussed.