Multiscale Characterization of N-Functionalized Carbon-Based Catalysts and Supports

Abstract

Nitrogen-functionalized carbon-based materials are an intriguing class of materials finding utility in many energy and environmental applications [1]. These materials have been explored as alternatives to conventional carbon supports for precious and non-precious catalysts in polymer electrolyte fuel cells (PEMFCs), demonstrating improvements in catalyst-support interactions resulting in enhanced activity and stability. Incorporation of atomically-dispersed transition metal into the network of nitrogen-doped carbon leads to significant enhancement in activity of nitrogen sites producing one of the most promising classes of platinum group metal-free (PGM-free) catalysts for oxygen reduction reaction (ORR) at the cathode of the PEMFCs [2]. These catalysts have shown significant improvements in activities over the past decade, offering opportunity to reduce the cost of PEMFCs, eliminate reliance on scarce elements, and motivating further studies to improve their activity, active site density, and stability. Since current synthetic methods are not capable of producing a perfectly homogeneous material with only one dopant configuration, this work aims to better understand the heterogeneities present in N-doped carbons (N-Cs) and PGM-free catalysts based on iron, nitrogen and carbon (Fe-N-Cs) by combining synthesis efforts with multi-technique characterization. High surface area N-C nanospheres are synthesized by a two-step process of solvothermal treatment followed by pyrolysis. A solvothermally treated, aqueous alcohol solution of resorcinol, formaldehyde, and ethylenediamine is pyrolyzed to produce porous carbon nanospheres which contain a diverse set of nitrogen sites [3]. The amount of ethylenediamine added to the solution and the temperature of pyrolysis are modified in order to produce nanospheres with controlled size and relevant compositions. Iron is integrated into the carbon using various iron precursors, either during or after carbon synthesis [4]. Similar to the N-C spheres, alterations in this synthesis allow for a diverse array of materials with different prominent active sites. Initial assessment of a two-dimensional morphology of these materials is made using transmission electron microscopy (TEM) and elemental distributions are acquired with energy dispersive x-ray spectroscopy (EDS). Three-dimensional, atomic distribution of the species, demonstrating differences between surface and bulk are studied with atom probe tomography (APT). Since APT analysis of carbon materials is not developed yet, this work focuses on both, development of sample preparation techniques using a focused ion beam (FIB) to produce specimens with appropriate dimensions and APT data analysis to generate reconstructions of such soft materials such as these N-C and Fe-N-C spheres. Integration of 2D and 3D data at multiple scales provides a pathway to better understand these complex materials and to maximize active site density for higher efficiency of these materials in various electrocatalytic reactions. Figure 1. a) STEM image of several N-C spheres, b) EDS map showing elemental distribution of N (green) and C (red) across the spheres. Wood, K. N., O’Hayre, R., and Pylypenko, S. Energy Environ. Sci. 7, 1212 (2014).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).Wickramaratne, N. P., Xu, J., Wang, M., Zhu, L., Dai, L., and Jaroniec, M. Chemistry of Materials 26, 2820 (2014).Pylypenko, S., Borisevich, A., More, K.L., Corpuz, A.R.., Holme, T., Dameron, A.A., Olson, T.S., Dinh, H.N., Gennett, T., and O' Hayre, R. Energy Environ. Sci. 6, 2957 (2013). Figure 1