Mechanical characterization of PFSA membrane in fuel cell
Date
2019
Authors
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Publisher
University of Delaware
Abstract
The durability of Proton Exchange Membrane Fuel Cells (PEMFCs) has been a critical obstacle that inhibits the commercialization of fuel cells while the durability of the PFSA membrane often determines the lifetime of the fuel cell. Both mechanical and chemical factors contribute to the failure of the fuel cell unit. Mechanical stresses developed due to changes in the temperature and relative humidity causing deformation during the fuel cell operation which in turn contributes to the mechanical damage. This work is aimed at understanding the mechanical response of the membrane under hygrothermal loading. Fatigue testing was performed to characterize and understand the fatigue behavior of the membrane and its dependence on the environmental conditions. Also, the out-of-plane hygrothermal swelling of the membrane was measured and found to be different than the in-plane swelling. The anisotropy of the swelling coefficients is incorporated in a numerical model to investigate the development of stresses in the membrane in an operating fuel cell stack. ☐ Fatigue tests were performed on Nafion® 211 membrane to understand the failure mechanism and to predict the lifetime of the membrane. Both mechanical and hygrothermal load have an influence on the fatigue life of PFSA membrane. These tests showed that fatigue life of the membrane decreases with an increase in stress. The fatigue life decreases exponentially with the relative humidity at given temperature and stress. When the humidity and stress are constant, the fatigue life decreases substantially with increasing the temperature. Moreover, when the temperature and relative humidity are below a certain limit (i.e. T=25C RH=50%) for a selected mechanical load, the fatigue crack in the membrane stops propagating or propagates at a significantly small rate, which make the membrane seem to have a fatigue limit under these conditions. ☐ The out-of-plane swelling behavior of the membrane was measured as a function of hygrothermal loading. The results show that the swelling of the membrane under hygrothermal loading is not isotropic as previously assumed. In the out-of-plane direction, the swelling of the membrane increases with increasing relative humidity but decreases with increasing temperature. The swelling coefficient in this direction is larger than that in the in-plane direction for almost all temperatures, but the difference is more significant when the temperature is low. The out-of-plane and in-plane swelling behaviors are described using separate polynomial equations and incorporated into a fuel cell numerical model. Considering the anisotropy of the swelling of the membrane relative to the isotropic model, the stress distribution and stress history in the fuel cell changes significantly but the spot where maximum compressive and tensile stress occur remains the same. However, the magnitude of the stress increases. For the GoreTM cycle, the stress range increases by 13.2% and the maximum residual tensile stress increases by 11.2%. For DoE RH cycle, the stress range increases by 12.4% and the maximum residual tensile stress increases by 9.2%. This result indicates that the lifetime prediction from the isotropic model might be longer than when the swelling anisotropy in the membrane is considered.