Quantifying the Impacts of Nafion Interfaces with PEMFC-Relevant Substrates through Neutron Reflectometry and Modeling

Abstract

The critical electrochemical reactions within proton exchange membrane fuel cells (PEMFCs) take place in the heterogeneous catalyst layer (CL). This CL is comprised of three main materials: carbon, platinum, and thin film Nafion. The CL ionomer, Nafion, is particularly important with respect to PEMFC performance and durability. Ionic and charge-neutral reactants and products move through nano-thin Nafion films to reach the carbon-supported platinum catalyst. Increasing the rate of transport through these films reduces cell overpotentials. As the platinum loading of PEMFCs is reduced, to lower costs, the loss of catalyst surface area also decreases the performance; therefore, performance increases obtained from a better understanding of Nafion thin films become more vital in the establishment of low cost and high performance PEMFCs.Neutron reflectometry (NR) is a technique that can be used to simultaneously study the structure and chemical composition of Nafion at length scales relevant to PEMFC CLs. Relating the composition profiles from NR to transport properties across thin-film Nafion gives insight into the relationship between the material’s structure and properties. To date, a majority of NR studies involving Nafion have been performed on the native oxide surface of silicon substrates, where complex layered structures are observed with distinct water-rich and water-poor regions within the polymer [1,2]. Although these studies are useful, their relevance to common PEMFC CL materials (i.e. platinum and carbon) is indirect.In this presentation, we will show and discuss composition depth profiles for Nafion thin films (< 100 nm) on PEMFC-relevant substrates. Experiments performed in both humidified and dry environments allow for robust fits to the data sets. Data from the humidified environment provides insight into Nafion’s water uptake and structure within an operating PEMFC. In contrast to silicon interfaces, Nafion thin films on PEMFC-relevant materials demonstrate much less structuring, with a lower degree of phase segregation in the through-film direction [3,4]. We will provide a detailed look at polymer depth profiles, and discuss the implications of these on PEMFC performance, in particular their influence on ionic conductivity, oxygen diffusivity, and the resulting platinum surface kinetics. Predictions for these relationships and their impacts will also be discussed with the use of a physically based PEMFC model extended from our earlier work [5].[1] 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.[2] S. A. Eastman, S. Kim, K. A. Page, B. W. Rowe, S. Kang, C. L. Soles, and K. G. Yager, “Effect of Confinement on Structure, Water Solubility, and Water Transport in Nafion Thin Films,” Macromolecules, vol. 45, no. 19, pp. 7920–7930, 2012.[3] 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.[4] U. N. Shrivastava, H. Fritzsche, and K. Karan, “Interfacial and Bulk Water in Ultrathin Films of Nafion, 3M PFSA, and 3M PFIA Ionomers on a Polycrystalline Platinum Surface,” Macromolecules, vol. 51, no. 23, pp. 9839–9849, 2018.[5] 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, Jan. 2020.