Tracking Nafion Evolution in a Humidified Environment through Near-Ambient Pressure X-Ray Photoelectron Spectroscopy

Abstract

Near ambient pressure X-ray photoelectron spectroscopy (nAP-XPS) affords unparalleled insight into the physicochemical processes that drive electrocatalytic devices.1 Studies featuring nAP-XPS span a broad range of materials and reactions, with many focused on thin films or other well-defined materials. Experiments featuring a water vapor atmosphere, or a humidified atmosphere (often with O2 or H2 as the other vapor component) have been effectively used to study the fundamentals of the electrocatalytic reactions occurring in polymer electrolyte membrane fuel cells and water electrolyzers (PEMFCs and PEMWEs).2,3 However, many of these studies have solely focused on the interaction between gas and catalyst, even if ionomer was also present within the catalyst layer. This talk focuses on the investigation of the evolution of ionomer species, and the catalyst-ionomer interface under conditions relevant to the operation of PEM devices. Analyzing the catalytically relevant surfaces and interfaces present in PEMFC and PEMWE electrodes with nAP-XPS is a complex challenge, due in part to the subtlety of the changes induced in nAP-XP spectra by interactions between the catalyst, ionomer, and gas. Adsorption of a gaseous reactant species onto a catalyst’s surface results in a weak interaction and a small chemical shift in the adsorbent species,5 while ionomer may undergo protonation, re-orientation or degradation upon exposure to reactants, also altering the spectra. Indeed, few studies feature XPS measurements of Nafion, as it has been shown to degrade over the course of standard XPS operation.6 This issue is likely exacerbated through the use of much more intense synchrotron radiation sources most commonly used for nAP-XPS experiments in the literature. Within this work, we present an evaluation of the degree of Nafion degradation during our measurements using a unique Scienta Omicron HiPP-3 nAP-XPS system, and the results of an approach to data collection developed to mitigate the induced damage. This enables the reliable study of both Nafion films and Nafion present within platinum-catalyst based PEMFC cathodes in a water vapor atmosphere using a laboratory-based, commercially available nAP-XPS system. This work lays the foundation for future study of different classes of electrocatalysts at the electrode scale, with the goal of deconvoluting complex changes at the electrode surfaces and interfaces, both in inert and catalytically relevant environments. (1) Starr, D. E.; Liu, Z.; Hävecker, M.; Knop-Gericke, A.; Bluhm, H. Investigation of Solid/Vapor Interfaces Using Ambient Pressure X-Ray Photoelectron Spectroscopy. Chem. Soc. Rev. 2013, 42, 5833–5857. (2) Yamamoto, S.; Bluhm, H.; Andersson, K.; Ketteler, G.; Ogasawara, H.; Salmeron, M.; Nilsson, A. In Situ X-Ray Photoelectron Spectroscopy Studies of Water on Metals and Oxides at Ambient Conditions. J. Phys. Condens. Matter 2008, 20 (18). (3) Casalongue, H. S.; Kaya, S.; Viswanathan, V.; Miller, D. J.; Friebel, D.; Hansen, H. A.; Nørskov, J. K.; Nilsson, A.; Ogasawara, H. Direct Observation of the Oxygenated Species during Oxygen Reduction on a Platinum Fuel Cell Cathode. Nat. Commun. 2013, 4. (4) Saveleva, V. A.; Savinova, E. R. Insights into Electrocatalysis from Ambient Pressure Photoelectron Spectroscopy. Curr. Opin. Electrochem. 2019, 17, 79–89. (5) Dzara, M. J.; Artyushkova, K.; Shulda, S.; Strand, M. B.; Ngo, C.; Crumlin, E. J.; Gennett, T.; Pylypenko, S. Characterization of Complex Interactions at the Gas − Solid Interface with in Situ Spectroscopy : The Case of Nitrogen-Functionalized Carbon. J. Phys. Chem. C 2019, 123 (14), 9074–9086. (6) Paul, D. K.; Giorgi, J. B.; Karan, K. Chemical and Ionic Conductivity Degradation of Ultra-Thin Ionomer Film by X-Ray Beam Exposure. J. Electrochem. Soc. 2013, 160 (4), F464–F469.