In Situ X-Ray Photoelectron Spectroscopy Analysis of Catalyst–Ionomer Interactions in Fuel Cell Catalyst Layers

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X-ray photoelectron spectroscopy (XPS) has been widely used to study catalysts and support materials, offering valuable insights about surface composition and contributing to the optimization of their performance. However, investigations of catalyst layers (CLs) using XPS are more limited, due to several challenges—including surface sensitivity of low concentration elements, the ionomer being vulnerable to X-ray damage, and spatial heterogeneity of the CL surface. Our recent studies demonstrated that, although the Nafion ionomer is inherently susceptible to X-ray-induced degradation, XPS can still be effectively employed through a modified acquisition strategy [1]. It has been established that XPS can be used to track ionomer, catalyst and support species which in turn can be utilized to compare both intentional and unintentional variations in ionomer at the catalyst-ionomer interface induced by ink processing, CL fabrication conditions, testing conditions [2, 3]. This talk will present a series of studies focused on the catalyst-ionomer interface under humidified conditions. Initially, a set of Pt/carbon catalyst layers—with variations in support material, Pt loading, and ionomer content—were investigated under ultra-high vacuum (UHV) conditions. The relative amount of ionomer to catalyst and support was quantified using the F/Pt elemental ratio and the C-Fₓ/C-C ratio derived from high-resolution C 1s spectral fitting. The ionomer distribution on the surface was further assessed using the F/S ratio (from elemental analysis) and O 1s-Ionomer/O 1s-Catalyst ratios (from O 1s spectral deconvolution). Selected samples were then evaluated under in situ humidified conditions to detect changes in the catalyst-ionomer interface resulting from water exposure. Following this, the samples were re-analyzed under UHV to assess the reversibility of any observed changes. This study revealed differences in the arrangement and rearrangement of the ionomer as a function of surface ionomer content, Pt nanoparticle loading and dispersion, and the nature of the support material. The parameters and methodology developed here can be applied more broadly to analyze a wide range of catalyst layers and operational conditions relevant to real-world device environments. Dzara, M. J.; Artyushkova, K.; Foster, J.; Eskandari, H.; Chen, Y.; Mauger, S. A.; Atanassov, P.; Karan, K.; Pylypenko, S. X-Ray Photoelectron Spectroscopy Analysis of Nafion-Containing Samples: Pitfalls, Protocols, and Perceptions of Physicochemical Properties. Phys. Chem. C 2024 , 128 (20), 8467–8482. Foster, J.; Lyu, X.; Serov, S.; Mauger, S.; Padgett, E.; Pylypenko, S., X-ray Photoelectron Spectroscopy Analysis of Iridium Oxide Catalyst Layers with a Focus on the Catalyst-Ionomer Interface, Electrochimica Acta. 2025 ,517, 145705. Medina, S.; Foster, J. G.; Dzara, M. J.; Wang, M.; Ulsh, M.; Mauger, S. A.; Pylypenko, S., Multi-technique characterization of spray coated and roll-to-roll coated gas diffusion fuel cell electrodes. Journal of Power Sources 2023 , 560, 232670.

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