Evidence for Micro-Porous Layer Degradation Under Accelerated stress test Conditions
Authors: Andisheh-Tadbir Mehdi , Dutta Monica , Orfino Frank , Kjeang Erik 
Published: February 2020

Durability is important for high-volume fuel cell production and commercialization. Different fuel cell components are prone to various types of degradation. The ionomer inside the membrane and catalyst layer undergoes degradation chemically and/or mechanically and Pt may dissolve and coalesce resulting in a loss of active surface area. The carbon in the catalyst support and gas diffusion layer (GDL), meanwhile, may degrade through corrosion and/or erosion [13]. GDL degradation is mainly characterized by the changes in its properties. The micro-porous layer (MPL), which is the most recently added component of modern seven-layer membrane electrode assemblies (MEAs), has a soft and delicate structure comprising of carbon nanoparticles and PTFE that may also be susceptible to degradation. Although MPL is key for high fuel cell performance, particularly at high current densities where water management is a concern, there is no investigation in the literature to date that accounts for its possible degradation behavior during fuel cell operation.
The methods used previously to study degradation of the macro-porous GDL substrate could also be applied in general to evaluate MPL degradation, albeit a higher resolution may be required. As an example, erosion of PTFE from the MPL would lead to changes in hydrophobicity, and any structural changes due to electrochemical degradation in the form of carbon corrosion would likely alter the surface characteristics and morphology [1,2], as shown previously for the GDL substrate.
In this work, the cathode MPL of a highly corroded MEA is analyzed using nano-scale X-ray computed tomography (NXCT). Samples for this study are obtained from a cathode corrosion accelerated stress test, in which the MEA is subjected to voltage cycling at high temperature and relative humidity. The goal is to compare the structure and properties of the MPL at the beginning of life (BOL) and degraded states. A ZEISS Xradia 810 Ultra NXCT scanner is used to visualize and reconstruct the 3D structure of the MPL with 50 nm resolution. Cylindrical samples of 350 µm diameter are punched from the MEAs and trimmed under microscope in preparation for NXCT scanning.
Presented in Fig. 1 are the obtained 3D structures for the BOL and degraded MPLs. The degraded sample is observed to contain smaller agglomerates than the BOL material, which could be caused by material loss from carbon corrosion; additionally, the structure appears more compact with smaller pores which suggests that the structure is collapsed. Table 1 lists the calculated porosity and average pore size obtained using an in-house MATLAB code [3,4]. The porosity is constant for the two samples; however, the average pore size is decreased for the degraded sample. The thickness of the BOL and degraded MPLs are examined using cross-sectional scanning electron microscopy (SEM), revealing a 30% thickness reduction for the degraded sample. The reduction in average pore size along with the reduction in thickness evidences that the MPL experienced degradation through carbon corrosion, resulting in structural collapse.

Acknowledgments
Funding for this research was provided by the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, and Ballard Power Systems through an Automotive Partnership Canada grant.

References

[1]W. Schmittinger, A. Vahidi, Journal of Power Sources 180 (2008) 1.
[2]R.L. Borup, J.R. Davey, F.H. Garzon, D.L. Wood, M. a. Inbody, Journal of Power Sources 163 (2006) 76.
[3]M. El Hannach, R. Singh, N. Djilali, E. Kjeang, Journal of Power Sources 282 (2015) 58.
[4]A. Nanjundappa, A.S. Alavijeh, M. El Hannach, D. Harvey, E. Kjeang, Electrochimica Acta 110 (2013) 349.

Figure 1