Decay in Mechanical Properties of Catalyst Coated Membranes Subjected to Combined Chemical and Mechanical Membrane Degradation

Abstract

The mechanical stability of catalyst coated membranes (CCMs) is an important factor for the overall durability and lifetime of polymer electrolyte fuel cells. In this article, the evolution of the mechanical properties of degraded CCMs is comprehensively assessed. A combined chemical and mechanical accelerated stress test (AST) was applied to simulate field operation and rapidly generate partially degraded CCM samples for tensile and expansion experiments under both room and fuel cell conditions. The tensile results indicated significant reductions in ultimate tensile strength, toughness, and fracture strain as a function of AST cycles, accompanied by a mild increase in elastic modulus. The increased brittleness and reduced fracture toughness of the CCM, caused primarily by chemical membrane degradation, is expected to play an important role in the ultimate failure of the fuel cell. The expansion tests revealed a linear decay in hygrothermal expansion, similar in magnitude to the loss of mechanical strength. The decline in CCM sensitivity to environmental changes leads to non‐uniform swelling and contraction that may exacerbate local degradation. Interestingly, the hygrothermal expansion in the late stages of degradation coincided with the fracture strain, which correlates to in situ development of fractures in chemically weakened membranes.

Progression in the Morphology of Fuel Cell Membranes upon Conjoint Chemical and Mechanical Degradation

“…At this stage of degradation, the membrane has already lost a substantial amount of fluoride from chemical degradation initiated from the side chains of the ionomer 17 and hence affecting the functional groups, the loss of which is associated with decay in mechanical strength. 12 After a significant effect of degradation exerted upon the membrane through subsequent AST cycles, the radical attacked regions are believed to have lost a large portion of the sulfonic acid end groups, which can influence the local phase-separated ionomer structure. Interestingly, the membrane subjected to eight AST cycles revealed a relatively large non-ionic feature in the severely degraded region that resembles the presence of a hollow, void space in the membrane slice (Figure 3f).…”

“…12,17,26 Apart from the formation of chemically degraded locations in the membrane discussed in the previous section, other regions were found to undergo long range ordering of ion-rich domains until end-of-life. This could be perceived as mechanical stress induced morphological change 28 which is acting throughout the area of the membrane during the wet/dry cycles of the COCV-AST.…”

“…12 Reduction in molecular weight and disentanglement was inferred to cause the observed changes. 12,17 The combined chemical and mechanical degradation resulted in failure modes based on physical damages such as cracks, delamination, and membrane thinning. 17 The proton conductivity was found to decrease with in situ degradation of PFSA membranes, 20 since the proton transfer occurs through the formation and breaking of hydrogen bonds, which is a strong function of the size and connectivity of the hydrophilic ionic domains.…”

Accelerated Stress Testing of Fuel Cell Membranes Subjected to Combined Mechanical/Chemical Stressors and Cerium Migration

“…An interesting observation from the cross-sectional imaging analyses is the complete lack of membrane mechanical damage in the forms of electrode or ionomer layer cracking, even in segments with the most severe thinning. As previous studies 12,13 report that severe chemical degradation can significantly reduce membrane's mechanical strength, we expected the high mechanical stress in these locations of severe thinning would have led to membrane fracture and thus increasing convective crossovers. The result indicate that the significantly elevated convective crossover at the end of the test is direct H 2 passing through the porous ePTFE layer that is depleted of ionomer due to the severe chemical degradation.…”

Fuel Cell Durability Enhancement with Cerium Oxide under Combined Chemical and Mechanical Membrane Degradation

“…[15][16][17][18] The obtained stress−strain curves of the baseline and CeO 2 -MEAs are depicted in Fig. 3 and the extracted properties are summarized in Table I.…”