Effects of Wet Film Application Parameters on the Structure and Performance of Fuel Cell Catalyst Layers Prepared Using Scalable Methods
Authors: Stoll Jonas , Kjeang Erik , Huynh Philip 
Published: November 2022

The use of high-volume manufacturing processes for polymer electrolyte fuel cells is obligatory to bring the manufacturing cost closer to 80USD/kW fuel cell system cost target for long-Haul Trucks [1] and thereby make this technology an economical competitor in the carbon neutral transportation sector. For the membrane electrode assembly, thin film roll good materials are therefore the norm in the industry. On the lab scale however, catalyst layers and catalyst coated membranes (CCMs) are commonly prepared with low throughput multi-sub-layer coating application techniques, such as ultrasonic spray coating. Overall, each catalyst sub-layer is dried before the next sub-layer is applied via another spraying pass, until the desired catalyst loading and therewith the overall catalyst layer is created. This multi-sub-layer coating approach is therefore time consuming.
However, to simulate high volume manufacturing at the lab scale, direct layer coating techniques such as film applicator or Mayer bar coating can be utilized. These coating methods are designed to form a one pass final catalyst wet film thickness, similar to what transpires in a continuous high throughput roll to roll (R-2-R) manufacturing process. As opposed to the multi-layer coating techniques, the entire wet thickness of the catalyst layer is dried in one step, which allows greater throughput, but also influences the catalyst layer formation. Because when a catalyst wet film dries, there are forces related to evaporation, diffusion, and sedimentation which influence the distribution of materials and the ultimate catalyst layer structure [2]. Furthermore, catalyst inks used for Mayer-rod, film applicator, and R-2-R coating methods generally have higher solids content than the inks used in spray coating. With the one pass techniques, CCMs are typically fabricated by coating catalyst ink on a decal film, such as polytetrafluoroethylene (PTFE), followed by a hot lamination process, such as decal transfer, of the catalyst layer to the membrane [3]. Due to the interaction of the catalyst ink with the coating substrate and drying process, there are many factors that may influence the quality of the catalyst layer obtained with direct layer coating, considering both ink formulation and coating parameters.
To understand the interaction of these influences, we investigate the effects of wet film application parameters on the structure and performance of fuel cell catalyst layers, prepared using scalable methods. The same drying technique of hot air drying is used to obtain a closer representation of the direct layer R-2-R process. In more detail, we are determining the influence of the following factors on the catalyst layer formation, when coated on a PTFE substrate: drying temperature, ionomer-to-carbon support ratio, ink water-to-alcohol ratio, and wet film thickness for each of the two-lab scale direct layer coating methods. Finally, we will discuss the influence of these factors on the catalyst structure via microscopy. As well as performance and electrochemical analysis data, of selected catalyst layers, after being decal transferred onto a membrane and tested.
Figure 1 shows an example of the variance of dried catalyst layers coated via two direct coating methods on PTFE substrates from the alterations of Ink Water-to-Alcohol (2-Propanol, 1-Propanol, Ethanol) ratio and coating wet thicknesses. As can be seen the dried catalyst layer structure vary depending on the Ink Water-to-Alcohol ratio, wet thickness and application method used.

Acknowledgments

This research was supported by the Simon Fraser University Community Trust Endowment Fund, Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, British Columbia Knowledge Development Fund, Western Economic Diversification Canada, and Canada Research Chairs.

Reference

[1] Menezes, Mark W., et al. “"US Department of Energy Hydrogen Program Plan” https://www.hydrogen.energy.gov/roadmaps_vision.html (2020)
[2] C.M. Cardinal, Y.D. Jung, K.H. Ahn, L.F. Francis, Drying regime maps for particulate coatings, AIChE J. 56 (2010) 2769–2780, https://doi.org/10.1002/ aic.12190.
[3] Wei, Zhaoxu, et al., High performance polymer electrolyte membrane fuel cells (PEMFCs) with gradient Pt nanowire cathodes prepared by decal transfer method. International Journal of Hydrogen Energy 40.7 (2015): 3068-3074.

Figure 1