Understanding N-Functionalized Carbon-Based Catalysts and Supports Via Atom Probe Tomography and Electron Microscopy
Authors: Ngo Chilan , Diercks David R , Strand Matthew B , Dzara Michael J. , Pylypenko Svitlana 
Published: February 2020

Functionalization or doping of carbon is an effective method to tailor material properties towards improving performance of electrocatalysts in a variety of catalytic reactions, including the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs). In particular, N-doping carbon supports has been shown to improve nucleation & growth, durability, and activity of supported Pt and Pt-alloy nanoparticles [1]. Nitrogen is also central towards Pt-group metal-free (PGM-free) catalysts based on nitrogen, carbon, and non-precious metals such as iron. Although nitrogen species present in N-doped carbons (NCs) and transition metal-N-C (MNC) catalysts play a pivotal role in ORR catalysis, the respective contributions and identities of all active species present in these materials are not well known [2]. Significant efforts have been spent to determine accurate correlations between composition, structure, morphology, catalytic activity, and stability of these catalysts [3]. Traditional characterization techniques include X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XPS), and 57Fe Mossbauer spectroscopy, among other methods. Information obtained with these techniques, however, is averaged over large areas. Insights on atomic-level composition, information about 3D distribution of nitrogen and iron, and evolution of species under realistic conditions relevant to operation of the fuel cell remain largely unexplored.

Scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray spectroscopy (EDS) provides valuable information about these materials at the nanoscale, and can be performed under in situ conditions, while atom probe tomography (APT) is a distinctive technique that can provide 3D information with atomic resolution. However, carbon-based materials present significant challenges related to sample/specimen preparation and analysis, in part due to their relatively large dimensions. To address these challenges, model high surface area NC and MNC materials of appropriate size and shape (200-400 nm nanospheres) have been synthesized and protocols for specimen preparation are being developed.

A diverse set of samples with different distributions of nitrogen and iron species based on N-doped carbon nanospheres is synthesized from resorcinol, formaldehyde, and ethylenediamine in aqueous alcohol solution by treatment under hydrothermal conditions, then pyrolyzed under flowing nitrogen.[4] Pyrolization temperatures were adjusted to vary nitrogen concentration and speciation of the synthesized materials. Incorporation of iron was achieved by adding an iron precursor to the solution either prior to hydrothermal treatment or after pyrolysis of NC spheres.

NC and MNC nanospheres were isolated into a workable APT tip by first spin casting them onto silicon substrates, then milling with modified focused-ion beam (FIB) lift-out methods. Prior to the destructive APT process, STEM imaging and EDS mapping were used to confirm the presence of the NC or MNC sample in each tip. Relative distributions of carbon, nitrogen, and iron will be discussed in the context of expected synthesis outcomes based on the precursor, as well as location of iron on the surface vs. bulk and the relation to electrochemical performance. This work suggests that APT indeed can provide high-resolution, 3D insight into the distribution of nitrogen and transition metals in carbon-based materials. The combination of qualitative and quantitative characterization methods in 2D and 3D provides a more thorough understanding of NC and MNC materials guiding synthesis of more efficient PGM-free catalysts.

Figure 1. a) STEM image of several Fe-N-C spheres as-synthesized, b) EDS map showing elemental distribution of N (blue) and Fe (red) across the spheres shown in (a), c) high-resolution STEM image of a single sphere.

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Figure 1