Experimental Measurement of Stress and Strain in Nafion Membrane during Hydration Cycles

Proton exchange membranes (PEMs) in fuel cell stacks develop biaxial hygrothermal fatigue stresses in response to changing operation conditions. Determination of these stresses is complicated as both constitutive properties of PEM and the hygrothermal strain in PEM are highly dependent on time, temperature and water absorption. In this study, by tracking the spherical bending curvature of a bimaterial strip composed of a 25-μm thick PEM (Nafion) and a 250-μm thick PEEK (polyether ether ketone) substrate using a Digital Image Correlation (DIC) system, the biaxial tensile and compressive stresses in the plane of the PEM during humidity cycles are recorded. Due to the relatively high bending stiffness ratio between PEEK and PEM, the hygral stresses in the PEM are determined from the bimaterial curvature without requiring knowledge of the constitutive properties of the PEM. As the strain in the PEM released due to bending of the bimaterial strip is limited, the stresses measured using the bimaterial strip are very close to that in a fully constrained membrane, mimicking those in a fuel cell. The hygral strain in a freestanding membrane placed next to the bimaterial structure is recorded at the same time using the DIC system. The full stress and strain histories during incremental relative humidity (RH) test between 10%RH and liquid wet as well as during larger RH cycles are presented. The hygral expansion of Nafion during the increasing and decreasing RH step changes exhibits an asymmetry that resembles the behavior of glassy polymers during the classic Kovacs temperature jump test. The stress history also reveals the RH-dependent viscoelasticity of Nafion.