Mechanical behavior of PEM fuel cell membrane electrode assembly.

摘要

Durability has been a critical barrier for widespread commercialization of Proton Exchange Membrane Fuel Cells (PEMFCs). Chemical degradation and mechanical damage in the membrane electrode assembly (MEA) are major sources of failure. Mechanical stresses developed in the MEA are primarily responsible for the mechanical damage. Therefore, investigating the mechanical behavior of the MEA is an important step toward understanding the fuel cell failure mechanisms and providing a science base for increasing the durability of PEMFCs. This dissertation work is aimed at investigating the mechanical behavior of the MEA to further develop that understanding.;A 2-D finite element model of a representative unit of fuel cell is developed to investigate the effects of gas diffusion layer (GDL) modulus and land-groove geometry on the mechanical stresses developed in the membrane during a simplified hygro-thermal loading/unloading cycle. The results suggest that the in-plane stress in the membrane from clamping is due to two factors: the effect from the through-the-thickness deformation gradient (bending-like rotational deformation) in the GDL and the Poisson's effect in the membrane. The results of the geometric studies provide a science base for optimizing fuel cell land-groove geometry to improve the durability of PEMFCs.;To better understand the mechanical behavior of the MEA, the time-dependent material properties of a PFSA membrane (NafionRTM 211 membrane) and ePTFE-reinforced NafionRTM 211 membrane are measured experimentally at various strain rates, temperatures and humidities. These experimental results characterize the relationship between the mechanical properties of the membranes and the strain rate, temperature as well as humidity. The results also show that the reinforced membrane has significantly higher Young's modulus and proportional limit stress as compared to the unreinforced NafionRTM 211 membrane under same environmental and load condition.;The time-dependent material properties of the electrodes are also studied. The electrodes are typically sprayed or painted onto the membrane during manufacturing and therefore do not exist as independent solid materials. Consequently, it is difficult to directly characterize the mechanical behavior of the electrodes. A numerical-experimental hybrid reverse analysis technique is devised to extract the electrode properties from the experimentally measured properties of the NafionRTM 211 membrane and the MEA based on the Nafion RTM 211 membrane at various temperatures, humidities, and strain rates. The results suggest that the electrode behaves similarly to the Nafion RTM 211 membrane but has lower elastic modulus. The mechanical damage mechanisms in the MEA during tensile loading are also investigated through interrupted tension tests.;Lastly, the time-dependent material properties of the membrane and electrodes are characterized by a two-layer viscoplastic constitutive model and then incorporated into finite element models of a typical fuel cell unit to study the mechanical stresses developed in the MEA and opening of pre-existing cracks in the electrodes during hygro-thermal cycling. The results suggest that the mechanical stresses developed in the MEA are significantly affected by the hydration-dehydration feed rate and hydration-dehydration hold time. The results of the crack studies indicate under what conditions the pre-existing cracks in the electrodes will open during hygro-thermal cycling and the resulting effects on the mechanical stresses developed in the MEA.