X-ray photoelectron spectroscopy investigation of iridium oxide catalyst layers: Insights into the catalyst-ionomer interface

X-ray photoelectron spectroscopy (XPS) is a commonly used technique for investigating the surface properties and composition of catalysts used in polymer electrolyte membrane fuel cells and electrolyzers. XPS analysis of catalyst layers (CLs) is becoming increasingly utilized to provide greater understanding of CL properties and relationships between catalyst and support composition and structure, catalyst ink composition, CL fabrication methods and parameters, and their performance and durability. Characterization of Ir-based CLs is challenging due to several factors including interpretation of Ir 4f spectra, deconvolution of catalyst and ionomer species in O 1s spectra, and ionomer susceptibility to X-ray damage that leads to changes at the catalyst-ionomer interface often more significant than differences between samples. This study reports an approach for detailed XPS characterization of Ir-based CLs, establishes quantitative metrics, and provides insights into the catalyst-ionomer interface that can be correlated to wide variety of processing and performance metrics. Specifically, we have evaluated surface compositional differences in CLs prepared with several common CL coating methods. We also investigated CLs prepared with different catalyst loadings and selected samples after electrochemical testing. In general, we found good agreements in trends observed from elemental ratios and those derived from detailed analysis of the O 1s spectra. Additionally, O 1s analysis revealed differences in the catalyst composition, addressing some of the challenges and limitations related to the interpretation of the Ir 4f spectra.