Polymer Electrolyte Membrane Fuel Cells (PEMFCs) are an important component in the portfolio of renewable energy technologies that are important for our society. While being green technology, PEMFCs rely on precious metals to catalyze the oxygen reduction reaction (ORR) on the cathode side. Several approaches have been explored in order to reduce the amount of precious metals, including core-shell structures, extended thin films, and various alloys with non-precious metals. Potential solutions also include PGM-free catalysts made with earth abundant, inexpensive elements. One promising area of PGM-free catalyst development is the integration of nitrogen and iron into high surface area carbon nanostructures.^1,2 These materials have significantly improved in activities over the past decade, yet their heterogeneous nature motivates further study into the nature of the active sites, with the goal of improving activity, active site density, and stability.

Multi-technique, multi-scale characterization of these materials, under ex situ and in situ conditions, provides opportunity to better understand surfaces of the materials under relevant conditions and to optimize various interfaces in these systems for optimum performance. In this work near-ambient pressure x-ray photoelectron spectroscopy (nAP-XPS) is used to investigate adsorption and desorption behavior of a diverse set of catalysts and supports, which contain nitrogen and iron sites of interest. Challenges associated with analysis of nAP-XPS data from adsorption and desorption studies include: i) presence of different types of potential sites and the multitude of binding energy shifts that could be happening upon adsorption on these various sites, ii) subtle changes in the spectra associated with adsorption/desorption due to low number of potential sites for adsorption, and iii) sampling depth limitations of the technique.³ To overcome these challenges, we explore less conventional approaches to analysis of the XPS N 1s and O 1s spectra, and complemented XPS with other in situ techniques such as diffuse reflectance infrared fourier transform spectroscopy (DRIFTS).

Figure 1. XPS N1s difference spectra comparing adsorption and desorption processes on a PGM-free FeNC material.

1. Jaouen, F., Proietti, E., Lefèvre, M., Chenitz, R., Dodelet, J.P., Wu, G., Chung, H.T., Johnston, C.M., and Zelenay, P., Energy Environ. Sci. 4, 114 (2011).
2. Wood, K. N., O’Hayre, R., and Pylypenko, S. Energy Environ. Sci. 7, 1212 (2014).
3. Matanovic, I., Artyushkova, K.; Strand, M. B., Dzara, M. J., Pylypenko, S., Atanassov, P., Core Level Shifts of Hydrogenated Pyridinic and Pyrrolic Nitrogen in the Nitrogen-Containing Graphene-Based Electrocatalysts: In-Plane vs Edge Defects. J. Phys. Chem. C 120, 51 (2016)