Transport Resistances in Fuel-Cell Catalyst Layers

Catalyst layers (CLs) in polymer-electrolyte fuel cells have garnered the attention of researchers due to their association with high local mass-transport resistance at low catalyst loadings [1, 2]. Nevertheless, the genesis and nature of CL resistance(s) is still under debate. CLs are only a few micrometers thick with complex geometry and heterogeneous phases making in-situ examination of CL transport processes extremely difficult. In this study, we probe the nature of CL resistances experimentally and theoretically. Using a hydrogen-pump limiting-current modality, we deconvolute the various gas-transport-related resistances without the impact of those associated with the oxygen-reduction reaction (e.g., water production) [3]. An analytical model to quantify the experimental measurements is developed to analyze the data and to elucidate limiting regimes. Our new results are in agreement with literature limiting-current measurements. Similar to previously reported studies [4], the model demonstrates that the transport of the reactant to the surface through the ionomer film tends to be the dominant resistance in CLs with a linear dependence on the inverse of Pt loading (i.e., roughness factor) [5]. Further, we study the dependence of this resistance on operational parameters including pressure, RH, temperature, gas molecular mass, and ionomer type. The garnered information exposes the controlling resistances, especially in comparison to ex-situ ionomer thin-film properties. Our efforts can be utilized to understand and develop mitigation strategies for local CL resistances. Acknowledgements We would like to thank helpful discussions and data provided by KC Neyerlin at NREL. 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: A. Weber and A. Kusoglu, Journal of Material Chemistry A, 2,17207–17211 (2014). A. Kongkanand and M. F. Mathias, The Journal of Physical Chemistry Letters, 7, 1127 (2016). F. Spingler, A. Phillips, T. Schuler, M. Tucker and A. Weber, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.01.036 N. Nonoyama, S. Okazaki, A. Weber, Y. Ikogi and T. Yoshida, Journal of The Electrochemical Society, 158 (5), B416 (2011). T. Greszler, D. Caulk and P. Sinha, Journal of The Electrochemical Society, 159, F831 (2012)