Predicted Impacts of Catalyst Layer Ionomer Microstructure and Distribution on PEMFC Performance

Abstract

Electric vehicles powered by proton exchange membrane fuel cells (PEMFCs) are a promising means to reduce environmentally harmful emissions. PEMFCs offer low weight, high range (miles traveled before refueling), and quick refueling times; however, persistent technical challenges limit commercialization of fuel cell electric vehicles (FCEVs). In addition to system-level challenges such as an immature refueling infrastructure and H2 production, it remains challenging to manufacture low-cost PEMFCs that meet durability and performance requirements. Addressing the performance issues of low-cost PEMFCs has proven difficult due to a gap in understanding physiochemical resistances within the heterogeneous catalyst layer (CL).Electrochemical reactions occurring in the CL—and therefore PEMFC performance—depend in complex ways upon the CL microstructure. Reactants and products are transported to and from Pt catalysts through nano-thin films of Nafion ionomer, which coat carbon-dispersed Pt catalysts. Through the use of neutron reflectometry (NR) [1,2] and conductivity measurements [3], previous studies show that the structure and material transport properties of Nafion are linked and depend on multiple factors including film thickness, substrate interactions, temperature, and relative humidity. Previously [4], we developed pseudo-2D Newman-type computational models that incorporate novel structure-property relationships derived from these studies. The results concluded that increasing the ionomer film thickness surrounding Pt-covered carbon particles in the CL may lead to a higher PEMFC performance due to an increased conductivity caused by increased ionomer volume fraction and increased Nafion water absorption.In this presentation, we extend our previous model to further advance understanding of how ionomer structure and distribution influence PEMFC performance. The current modeling efforts include two-phase water transport to understand the role of flooding in performance limits under low-Pt loading. Additionally, structure-property relationships from our previous work are updated to include more relevant structures taken from NR experiments performed with Nafion on technologically relevant substrates (Pt and carbon). Presented results will demonstrate the predicted impacts of non-uniform ionomer distribution within the cathode CL, including novel design concepts such as ionomer-rich regions to enhance ion transport for increased catalyst utilization further from the membrane. These predicted impacts provide insight for future PEMFC design modifications to accelerate FCEV commercialization.[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.