Model Nitrogen-Carbon and Iron-Nitrogen-Carbon Materials for Investigating the Oxygen Reduction Reaction
Authors: Ngo Chilan , Dzara Michael J. , Strand Matthew B , Hagen Jaime , Artyushkova Kateryna , Pylypenko Svitlana 
Published: February 2020

The effect of human development on the environment necessitates the advancement of sustainable and environmentally benign energy sources. Polymer electrolyte membrane fuel cells (PEMFCs) are a promising technology to transform the energy sector, however their cost is currently a substantial limiting factor to commercial adoption [1]. The use of platinum (Pt) and other precious metals, which catalyze the oxygen reduction reaction (ORR) at the cathode, is the costliest component of PEMFCs. It is thus critical to develop inexpensive, earth-abundant Pt-group-metal-free (PGM-free) catalysts. PGM-free ORR catalysts based on transition metals (TMs) and nitrogen doped carbon (N-C) have shown promising performance. While significant effort has been put into investigating the chemical and structural properties of TM-N-C catalysts, the exact nature of these materials’ active site(s) is still debated due to their high degree of heterogeneity [2,3]. Therefore, it is important to develop clear correlations between morphological and chemical properties, and the electrocatalytic performance of TM-N-C electrocatalysts. Our approach is to synthesize a set of model N-C and TM-N-C materials with appropriate surface areas and controlled properties, to enable characterization of this class of materials with novel methods in order to elucidate the nature of the most active sites.

Model high-surface area N-C nanospheres were synthesized by a hydrothermal treatment of resorcinol, formaldehyde, and ethylenediamine, followed by pyrolyzation under flowing nitrogen, producing materials with different nitrogen concentration and varied relative distribution of nitrogen functionalities. Iron (Fe) was incorporated into N-C nanospheres to form Fe-N-C model catalyst materials, either by addition of an Fe precursor during the N-C nanosphere synthesis, or by wet-impregnation and subsequent pyrolysis in flowing nitrogen following synthesis of the N-C nanospheres. Properties of N-C and Fe-N-C nanospheres were controlled by altering parameters of the synthesis, resulting in a set of model catalyst materials with varied nitrogen and iron content and functionality, and varying degrees of Fe-N coordination. Multiple characterization techniques were used to understand the chemistry and morphology of the N-C and Fe-N-C nanospheres. Scanning transmission electron microscopy (STEM) and energy dispersive x-ray spectroscopy (EDS) were used to determine the morphology and distribution of elements within the materials. Further insights into the 3-D distribution of elements were made via atom probe tomography (APT), providing a unique view of the composition and structure of these model materials. X-ray photoelectron spectroscopy (XPS) and ambient-pressure XPS (AP-XPS) were employed to quantify the elemental concentrations and speciation both under ultra-high vacuum and in the presence of a few hundred millitorr of oxygen and water at elevated temperature. The state of iron in Fe-N-C nanospheres was further characterized by Fe57 Mossbauer spectroscopy, and by measuring the Fe L-edge both under ultra-high vacuum and in-situ using near-edge x-ray absorption fine structure (NEXAFS) spectroscopy.

Figure 1. High angle annular dark field (HAADF) STEM images and EDS elemental maps of Fe-N-C nanospheres produced with different Fe precursors: a) iron (III) chloride, incorporated post NC synthesis, b) iron (II) acetate, incorporated post NC synthesis, and c) iron (II) acetate, incorporated during NC synthesis. The distribution of iron is highly varied and occurs in the form of both Fe nanoparticles and atomically dispersed sites.

Figure 1