Predicted Impacts of Graded Catalyst Layer Ionomer and Pt Distributions on PEMFC Performance
Authors: Randall Corey R. , DeCaluwe Steven C. 
Published: July 2022

As the world moves away from a dependence on fossil fuels, it is crucial to develop cleaner replacement technologies for transportation, electricity production, and more. In regards to the transportation sector, fuel cell electric vehicles (FCEVs) made with proton exchange membrane fuel cells (PEMFCs) provide much promise. These FCEVs offer high energy densities, long-distance ranges, and quick refueling times. Although companies like Toyota, Honda, and Hyundai already have some FCEVs in production, maintaining high levels of performance and durability in low-cost PEMFCs continue to limit the technology. Addressing this challenge has proven difficult due to complex coupled physiochemical resistances in the heterogeneous catalyst layer (CL).

One method to further our understanding of PEMFC performance limitations is to model the device. These models should incorporate known physical processes and structures that occur in the CL, such as ionic and molecular transport, and electrochemical reactions. One complication of such a model is providing accurate parameters for species transport through nano-thin films of Nafion ionomer. Due to confinement, these thin-film polymers have been shown to have unique interfacial structuring and reduced water uptake with decreasing thickness [1,2]. Furthermore, the conductivity of these films has been shown to vary with local water content, temperature, and ionomer thickness – as the water absorption and composition change [2,3]. To capture these structure-property relationships, we developed numerical approximations for ionic conductivity and oxygen diffusion in our previous work [4]. The relationships are derived from quantitative measured structures from neutron reflectometry experiments along with separate conductivity measurements with thin-film Nafion.

In this presentation, we extend our previous modeling efforts to incorporate two-phase transport. This allows for an analysis on the impacts of CL flooding. Additionally, the model has been updated to allow for novel CL microstructures – e.g. graded ionomer and Pt loadings. The graded designs concentrate higher loadings near the membrane and lower loadings near the gas diffusion layer in an attempt to lower molecular and ionic transport resistances. After validating the model against PEMFC data with low Pt loadings [5], physical processes are investigated to identify limiting phenomena. A parametric study on the performance of the graded microstructures is also completed. Preliminary results suggest that individually these graded CLs lead to reduced performance caused by higher levels of localized flooding or reduced ionomer conductivities. A microstructure with combined graded ionomer and graded Pt distributions however leads to improved performance over a uniformly loaded CL by simultaneously lowering ohmic overpotentials and reducing flooding. These predictions suggest modifications for future PEMFC designs that could accelerate the development and adoption of high-performance and low-cost FCEVs.

[1] J.A. Dura, V.S. Murthi, M. Hartman, S.K. Satija, and C.F. Majkrzak, “Multilamellar Interface Structures in Nafion,” Macromolecules, vol. 42, no. 13, pp. 4769–4774, 2009.

[2] S.C. DeCaluwe, A.M. Baker, P. Bhargava, J.E. Fischer, and J.A. Dura, “Structure-property Relationships at Nafion Thin-film Interfaces: Thickness Effects on Hydration and Anisotropic Ion Transport,” Nano Energy, vol. 46, pp. 91–100, 2018.

[3] D.K. Paul, R. McCreery, and K. Karan, “Proton Transport Property in Supported Nafion Nanothin Films by Electrochemical Impedance Spectroscopy,” Journal of The Electrochemical Society, vol. 161, no. 14, 2014.

[4] C.R. Randall and S.C. DeCaluwe, “Physically Based Modeling of PEMFC Cathode Catalyst Layers: Effective Microstructure and Ionomer Structure-Property Relationship Impacts,” Journal of Electrochemical Energy Conversion and Storage, vol. 17, no. 4, 2020.

[5] J.P. Owejan, J.E. Owegan, and W. Gu, “Impact of Platinum Loading and Catalyst Layer Structure on PEMFC Performance,” Journal of The Electrochemical Society, vol. 160, no. 8, 2013.