Abstract
Membrane stability is an important consideration for the overall durability and lifetime of polymer electrolyte fuel cells. During operating conditions, the ionomer membrane is subjected to chemical degradation due to the formation of radicals and their subsequent attack on the chemical bonds of the polymer [1,2]. The membrane is also subjected to mechanical degradation due to the hygrothermal cycles during operation, which causes the membrane to expand and contract repeatedly leading to the development of cyclic stresses [3,4]. Recent studies show that the mechanical and chemical degradation processes are connected and, once combined, cause accelerated degradation and reduced integrity of the membrane [3–6].
On one hand, the chemical degradation causes a change in the ionomer membrane mechanical properties, transforming it from a ductile material to a brittle material [3]. This transformation makes the membrane less resistant to crack formation and propagation and increases the mechanical damage that can be caused by typical hydrothermal cycles. On the other hand, it was observed that applying compression to the membrane increases the rate of chemical decomposition [7]. Understanding the interaction between the chemical and mechanical degradation processes is therefore an important step in developing more durable membranes and improving the performance and lifetime of the fuel cell.
We propose a statistical model to establish a correlation between the chemical degradation at the level of the polymer backbone chains and the initiation of microcracks due to mechanical stress. The representation of the morphology of the ionomer is based on the fibrillary structure described in [6,8]. In this representation a bundle of backbone chains is considered as a basic building block of the structure. We generate a network of bundles representative of a microcrack initiation site.
A crack initiation in the membrane structure is the tipping point that leads to an accelerated degradation in terms of hydrogen leaks and ultimate failure of the fuel cell under regular operating conditions. Once a crack is initiated, it increases the amount of reactant crossover leading to an increasing chemical decomposition [4]. The crack also constitutes a stress concentration site where the cyclic mechanical load participates further in its propagation. The combination of these processes accelerates the overall rate degradation leading to a complete failure of the ionomer.
Clearly, the crack initiation process is a critical root cause for the accelerated cycle of degradation of the ionomer. Our model allows for the estimation of the time of the crack initiation under relevant mechanical stresses and follows a realistic chemical degradation pattern. The statistical analysis provides a comprehensive understanding of the interaction between the local amounts and distribution of chemical decomposition sites and the mechanical stressors causing the initiation of the physical damage in the ionomer structure.
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