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The Impacts of Cathode Catalyst Layer Morphology on PEMFC Design
Commercializing polymer electrolyte membrane fuel cells (PEMFC) is ultimately a matter of achieving the necessary cost for market penetration. Cost is a function of materials, manufacturability, performance, and durability. Performance and durability targets have been set by the DOE [1] to provide guidance and focus to the PEMFC industry for specific applications such as the automotive sector.
Since both performance and durability are heavily dependent on operating conditions, PEMFC design will be strongly dependent upon the specific requirements of any given commercial application. Designing PEMFCs for applications with narrow operating windows (eg. low or high RH) is a relatively simple task; however, most commercial applications have performance/durability requirements that must be met across a wide range of operating conditions.
Cathode catalyst layer morphology is one of the dominating factors for achieving the performance and durability required for many applications; therefore, understanding how manufacturing impacts the catalyst layer morphology and the resulting performance under a wide range of operating conditions is important for designing commercial PEMFCs.
Results & Discussion
Aside from catalyst activity the cathode catalyst layer performance is dictated completely by the mass transport of protons (proton conductivity), oxygen (gas diffusivity), and water (gas & liquid permeability) [2-4]. Figure 1 shows the balance between proton conductivity and catalyst layer porosity as a function of ionomer loading.
Figure 1 – Maximum performance obtained at 30wt% ionomer content with a balance between proton conductivity and gas porosity.
Understanding the processes and variables required to control these mass transport parameters is necessary for proper PEMFC design [5-6]. Table 1 outlines several processes and design variables that help control mass transport through the cathode catalyst layer and have therefore had an impact on both performance and durability.
Table 1 – Design variables that affect key functions for PEMFC performance.
A study was conducted investigating several of these variables, resulting in relationships with mass transport and ultimately performance.
These relationships have been used to design a robust catalyst layer that is capable of performing under a wider range of operating conditions compared to previous designs. The goal of this specific design case was to achieve the best attributes of both the top performing MEA over a range of operating conditions and the most durable MEA, in a single design.
Starting with a more corrosion resistant catalyst, the performance was increased under a wider range of operating conditions, by improving proton, oxygen, and water mass transport. This was successfully done by utilizing the relationships described above, specifically regarding the types of catalyst and ionomer, the catalyst-ionomer ratio, mixing and coating methodologies and layer contiguity.
The authors would like to acknowledge National Resources Canada (NRCan), National Research Council of Canada (NRC-IRAP), and the US Department of Energy (DOE) for funding various aspects of this work.
Reference
1.http://www1.eere.energy.gov/hydrogenandfuelcells/mypp/pdfs/fuel_cells.pdf
2. Egushi, M., Baba, K., Onuma, T., Yoshida, K., Iwasawa, K., Kobayashi, Y., Uno, K., Komatsu, K., Kobori, M., Nishitani-Gamo, M., Ando, T., Polymers, 4, p. 1645 (2012)
3. Xie, J., Xu, F., Wood, D.L., More, K.L., Zawodzinski, T.A., Smith, W.H., Electrochimica Acta 55 p. 7404 (2010)
4. Gode, P., Jaouen, F., Lindbergh, G., Lundblad, A., Sundholm, G., Electrochimica Acta 48 p. 4175 (2003)
5. Mehta, V., Cooper, J.S., J. Power Sources 114, p. 32