X-Ray Photoelectron Spectroscopy Investigation of Catalyst-Ionomer Interactions for PEMFC Electrodes

Abstract

Proton exchange membrane fuel cells (PEMFCs) are an increasingly attractive clean energy technology to meet stationary and transportation energy demands without contributing to greenhouse gases. Further developments to PEMFCs are primarily focused on improving the cathode catalyst layer which uses expensive platinum catalysts to compensate for the sluggish oxygen reduction reaction and a thin film of proton conductive ionomer to aid in mass transport. It is understood that the Pt-based carbon supported catalyst may experience heterogeneity in ionomer coverage motivating further studies of catalyst layers with focus on catalyst-ionomer interface. Specifically, it is important to improve understanding of how support morphology, Pt catalyst loading, amount of ionomer, and method of catalyst layer coating affect the catalyst-ionomer interface to identify potential pathways to further to improve catalyst utilization as the technology increases in scalability.This study focuses on investigating the catalyst-ionomer interface using x-ray photoelectron spectroscopy (XPS) as it is highly surface sensitive (top ~3-10 nm) and can capture subtle differences in the chemistry of the catalyst, ionomer, and support at the catalyst-ionomer interface. Due to the ionomer’s inherent susceptibility to x-ray induced damage, XPS analysis was conducted with a recently developed protocol to mitigate instrumental artefacts. In this work, elemental ratio quantification metrics derived from XPS are used to track surface ionomer content relative to amount of Pt nanoparticles and the amount of carbon support producing two parameters. These parameters are used to show how variations in carbon support graphitization, Pt loading, ionomer chemistry, and ionomer loading affect the catalyst-ionomer interface in unique ways. Additionally, changes to the catalyst-ionomer interface after degradation were also assessed, comparing beginning of life and end of life samples. Further, this presentation will also discuss the evolution of the catalyst-ionomer interface under humidified conditions which was assessed with in situ XPS. This talk highlights the potential of XPS to improve our understanding of relationships between surface properties, processing, performance, and durability and motivates future work in understanding catalyst layer fabrication, performance, and degradation.