(518b) Impact of Ionomer Content on Fuel-Cell Catalyst Layer Structure

Catalyst layers (CLs) in polymer-electrolyte fuel-cells are the micron-thick porous structures fabricated using inks consisting of carbon particles with Pt nanoparticles and ionomer dispersed in a solvent. The various phases are essential to transport reactant gases, protons and electrons to/from the Pt catalyst site¹. Facile transport processes are thus essential for high-performing CLs. While the overall CL structural properties such as agglomerate size, ionomer coverage etc. is known to greatly influence various transport processes, very little is understood about the interactions in ink phase dictating the final CL structure^2,3.

In this study, we probe the impact of Pt/carbon (Pt/C) weight ratio on gas-transport resistance. We utilize a hydrogen-pump limiting-current setup to measure CL gas-transport resistance and observe that the CL resistance increases with increasing primary particle Pt loading. These results agree with previous literature data^4,5. Additionally, the trends in CL resistance qualitatively coincide with ink zeta-potential measurements with high Pt/C weight-ratio ink samples demonstrating lower zeta-potential magnitudes. Zeta potential is a well-accepted indicator of colloidal stability, with low zeta-potential indicating lower-repulsion between ink particles and greater degree of aggregation⁶. The results hence suggest that the observed trends are due to different CL structures arising due to varying ink-interactions due to use of different primary particle loading. This hypothesis is reaffirmed from SEM imaging, where larger agglomerates and ionomer clumps are observed in CL samples fabricated using high Pt/C weight ratio primary particles.

To elucidate the observed trends further and understand ionomer-catalyst ink interactions, we study zeta potential trends with bare carbon inks while varying the ionomer content. Bare-carbon zeta potential measurements demonstrate a V-shaped curve. The behavior can be explained by segregating the plot into saturated and unsaturated regions. Ionomer is highly acidic and typically dissociates in solvent³. At low ionomer concentrations, the ionomer continues to adsorb on carbon surface via hydrophobic interactions resulting in increasing magnitude of zeta potential. Once the carbon surface is saturated, additional ionomer added contributes to the solvent ionic strength resulting in decreasing magnitude of zeta potential. The explanation is supported by ink pH and particle size measurements.

Acknowledgements

We would like to thank Sarah Berlinger for helpful discussions and insights. This work was funded under the Fuel Cell Performance and Durability Consortium (FC PAD) funded by the Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U. S. Department of Energy under contract number DE-AC02-05CH11231.

References

1 Weber, A. Z. & Kusoglu, A. Unexplained transport resistances for low-loaded fuel-cell catalyst layers. Journal of Materials Chemistry A 2, 17207-17211, doi:10.1039/c4ta02952f (2014).

2 Hatzell, K. B., Dixit, M. B., Berlinger, S. A. & Weber, A. Z. Understanding inks for porous-electrode formation. Journal of Materials Chemistry A 5, 20527-20533 (2017).

3 Berlinger, S. A., McCloskey, B. D. & Weber, A. Z. Inherent Acidity of Perfluorosulfonic Acid Ionomer Dispersions and Implications for Ink Aggregation. The Journal of Physical Chemistry B 122, 7790-7796 (2018).

4 Schuler, T. et al. Fuel-Cell Catalyst-Layer Resistance via Hydrogen Limiting-Current Measurements. Journal of the Electrochemical Society 166, F3020-F3031, doi:10.1149/2.0031907jes (2019).

5 Owejan, J. P., Owejan, J. E. & Gu, W. Impact of Platinum Loading and Catalyst Layer Structure on PEMFC Performance. Journal of the Electrochemical Society 160, F824-F833, doi:10.1149/2.072308jes (2013).

6 Berlinger, S. A., McCloskey, B. D. & Weber, A. Z. Understanding Binary Interactions in Fuel-Cell Catalyst-Layer Inks. ECS Transactions 80, 309 (2017).