X-ray photoelectron spectroscopy investigation of surfaces and interfaces in fuel cell applications

Abstract

Advanced characterization methods play a central role in materials development, from initial exploratory synthesis to the testing and validation stage. A particularly powerful technique widely employed to identify synthesis-property-processing-performance correlations is X-ray photoelectron spectroscopy (XPS). XPS yields surface sensitive, element-specific information which can be analyzed to reveal differences in composition, oxidation state, and chemical interactions within a sample set. While typical XPS measurements are conducted under ultra-high vacuum (UHV), its applicability can be expanded towards in situ measurements through specialized XPS instruments that operate under near-ambient pressure (nAP-XPS). The core focus of my thesis is the application of both UHV and nAP-XPS to characterize the surfaces and interfaces within the cathode of polymer electrolyte membrane (PEM) fuel cell systems, where the electrocatalytic oxygen reduction reaction (ORR) occurs. Materials investigated in this work include both state-of-the art Pt/C and Pt-group metal free (PGM-free) ORR catalysts using earth abundant elements. Contributions within my thesis fall under three generalized research arcs: i) investigation of surface properties of PGM-free catalysts using UHV XPS, ii) analysis of gas-solid interfaces relevant to the ORR in PGM-free catalysts by nAP-XPS, and iii) studies of the catalyst-ionomer interface in Pt/C cathodes, with both UHV and nAP-XPS. This thesis features an overview of UHV and nAP-XPS techniques, highlighting both the direction of the field and possible applications. Next, results of the UHV XPS studies of PGM-free catalysts are discussed, which yielded the identification of surface properties linking variations in synthetic parameters with trends in ORR activity. The gas-solid interface studies first focused on methodology development towards adsorption studies, which was then applied towards identification of O2-adsorption sites in a set of PGM-free catalysts with various ORR activities. Finally, efforts towards characterizing the catalyst-ionomer interface will be discussed. This includes tracking the stability of the Nafion ionomer species during XPS measurement, proposes protocols that enable reliable XPS studies, and presents initial findings, including evidence of re-orientation of Nafion within an electrode in response to humidification. Methodologies developed herein enable further studies of both the gas-solid interface and the catalyst-ionomer interfaces in PEM electrodes with various state of the art and novel catalyst, support, and ionomer chemistries as part of the over-arching efforts towards wide-spread commercialization of PEM devices.