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.