Failure modes chemical degradation

Much recent ongoing work has focused on the chemical degradation mechanisms and revealed that radical attack is the root cause of the membrane decomposition. However, there is still a lack of fundamental understanding of the mechanisms for the degradation of mechanical strength, which is related to the membrane failure mode responsible for the sudden death behavior of fuel cells. 

As mentioned in the introduction to this paper, scientific study has concentrated on the tensile mode. Except for two forms of break in cotton, all the tensile failures discussed in this paper consist of breaks that run transversely aeross the fibre. However, the fibres arc fairly highly oriented, so that the bonding across the fibre is much weaker than along the fibre. Transversely, there are weak intermolecular bonds plus a small component of the covalent bonding. In use, failure is rarely due to a direct tensile overload, unless this is on fibres weakened by chemical degradation. The common forms of wear in use are due to weakness in the transverse direction, related either to shear stresses or to axial compression. There is no detailed structural prediction of the response to shear stresses or axial compression at a molecular or fine-structure level. All that one can say is that at a certain level of shear stress cracks will form and that at a certain level of axial compressive stress the structure will buckle internally. What can be described is how these stresses occur. 

Membrane degradation can result in loss of the electrolyte, loss of separator functionality, and severe fuel cell failure. In following sections, three membrane degradation modes—chemical, mechanical, and thermal—are introduced. 

By increasing experimental variables like temperature, humidity or radiation, chemical processes that lead to certain failure modes can be accelerated (e.g. the chemical degradation of a photovoltaic module can be accelerated by inducing an electric potential). 

PEMFC chemical, thermal and mechanical degradation. These overarching failure categories can contain numerous individual failure modes that a PEMFC can experience. 

The primary failure mode for membranes is hole formation, which can be caused by chemical degradation, material fatigue due to mechanical stresses, or a combination... 

There are various improvements that can be made to the presented model, some improvements could be accomphshed. Foremost among these possible future-work directions is the inclusion of nonisothermal effects. Such effects as ohmic heating could be very important, especially with resistive membranes or under low-humidity conditions. Also, as mentioned, a consensus needs to be reached as to how to model in detail Schroder s paradox and the mode transition region experiments are currently underway to examine this effect. Further detail is also required for understanding the membrane in relation to its properties and role in the catalyst layers. This includes water transport into and out of the membrane, as well as water production and electrochemical reaction. The membrane model can also be adapted to multiple dimensions for use in full 2-D and 3-D models. Finally, the membrane model can be altered to allow for the study of membrane degradation, such as pinhole formation and related failure mechanisms due to membrane mechanical effects, as well as chemical attack due to peroxide formation and gas crossover. 

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