Performance Changes in Stratified Fuel Cell Catalyst Layer with Ultra-Thin Membranes
Authors: Macauley Natalia , Borup Rod L. , Mukundan Rangachary , Komini Babu Siddharth , Spernjak Dusan , Neyerlin K.C. , Kocha Shyam S. , Wilson Mahlon S. , Grot Stephen 
Published: February 2020

Stratified catalyst layers can increase fuel cell performance compared to standard flat catalyst layers1,2 due to improved mass transport because of their irregular thickness and porosity. Stratified catalyst layer structures are expected to have enhanced performance in mass transport region due to improvements in water removal from the catalyst layer.
Custom electrodes are fabricated with a custom designed spray coating procedure and catalyst ink recipe. Results from multiple fabrication approaches will be discussed to achieve the electrode structure with the highest performance. One approach used involves a topographical patterning of the catalyst layer, and is based on Ion Power proprietary manufacturing techniques.
The second approach uses glass epoxy masks during the spray coating process to create a patterned electrode on the GDL with varying thickness. The mask dimensions mirror the bipolar plate flow-field at the cathode, exposing either the land or channel regions. This way the two extreme cases were investigated and the resulting performance compared. The application of more catalyst material in the channels was found to be more beneficial than when applied in the land regions, likely due to rapid reaction times via better access of the incoming reactant gases through the channels. However, the presence of a thicker catalyst layer under the channel may be an issue for product water removal and oxygen diffusion at high current densities.
The best performance was observed when in addition to applying the catalyst preferentially in the channels, a carbon-ionomer filler was used in the land region. The filler is used for both mechanical stabilization of the catalyst layer and to improve ionic and electronic conductivity within the catalyst layer. Two ionomer to carbon (I/C) ratios were tested in the carbon-ionomer filler. The high I/C ratio matches the I/C ratio of the Pt/C ink and is equal to 0.9. This case achieved a significant improvement in the kinetics of the catalyst, but had no effect on mass transport. When the I/C ratio was reduced to 0.6 in the carbon-ionomer filler, there was a visible improvement in mass transport. This is likely due to lower ionomer content which results in less water retention in the catalyst later.
In order to better understand the performance mechanisms taking place, the effect of adding hydrophobic agents in the carbon filler is examined. This includes the addition of Polytetrafluoroethylene (PTFE) and Fluorinated ethylene propylene (FEP) to further support water removal at high current densities.
Finally, the effect of using thin membranes will be discussed, as by stratifying the catalyst layer, much of the proton conduction occurs in a limited portion of the membrane. Membrane electrode assemblies (MEAs) using 25 micron thick membranes will be compared to significantly thinner membranes of 5 and 10 microns.

Acknowledgments

This research is supported by DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells program manager: Dimitrios Papageorgopoulos.

References

1. T. E. Springer, M. S. Wilson, and S. Gottesfeld, J. Electrochem. Soc., 140 (12), 3513–3526 (1993).
2. R. Borup and T. Rockward, US Dep. Energy Annu. Merrit Rev., Project ID: FC052 (2015). https://www.hydrogen.energy.gov/pdfs/review15/fc052_rockward_2015_p.pdf