Strain accumulation in polymer electrolyte membrane and membrane electrode assembly materials during a single hydration/dehydration cycle

A digital image correlation technique was used to measure drying-induced strains in perfluorosulfonic acid (PFSA) membrane and membrane electrode assembly (MEA) materials which are key components of fuel cells. Circular coupons of each material were constrained in a stainless-steel drum fixture and hydration was achieved by immersing the drum/coupon assembly in 80 degrees C water for 5 min. During air drying at 25 degrees C and 50% relative humidity, a series of 1280 X 960 digital images of each coupon surface was recorded with. a digital camera and image acquisition system for drying periods up to 26 h. Cumulative correlation of an initial image recorded prior to hot water immersion and the final image at the end of drying produced in-plane strain contour maps. Incremental correlation was employed to track strain evolution at the coupon centers and at peripheral points. During the earliest drying stages, where both materials exhibited viscoelastic behavior, peak radial strains of 9.50% and 2.50% were measured in the PFSA membrane and MEA materials, respectively. At longer drying times, peak radial strains reached constant values of 5.70% and 1.25% in the PFSA membrane and MEA materials, respectively. Strain accumulation in the MEA tends to be less uniform than that in the membrane, and corresponding peak strain values in the MEA tend to be less than those in the membrane independent of drying duration. Reinforcement from the electrode layers lowers strain accumulation in the MEA, but it does not preclude crack nucleation in these layers as drying proceeds.