Unraveling the Dynamic Structures of Active Sites in ZIF-8 Derived Fe-N-C Single Atom Catalysts for Electrochemical Oxygen Reduction Reactions

Single atom catalysts (SACs) constituting non-noble metal elements offer enormous potential to replacing expensive and rare platinum group metals for electrochemical oxygen reduction reaction (ORR), owing to their superior metal utilization and activity. One promising SAC consists of metal-nitrogen-carbon (M-N-C) motifs where single Fe atom acts as active sites for the adsorption of reactant and the desorption of product. Due to well-defined chemical structure of M-N-C, it is possible to tune the reactivity by modifying a metal center and local environment (e.g., ligands and dopants). The mesoporosity of carbon network provides electrical conductivity and serves as a channel for facilitated mass transport. Despite the great promise, significant barriers exist against the commercialization of Fe-N/C-based fuel cells, including (1) incomplete understanding of active sites and mechanistic pathway, (2) limited site density, and (3) poor electrochemical stability. In this presentation, the synthesis of Fe-N/C SACs derived from a ZIF-8 precursor will be detailed. These Fe-N-C SACs demonstrate excellent ORR activity, comparable to state-of-the-art Pt/C catalyst. The location and distribution of SACs are determined by high-resolution scanning transmission electron microscopy (HR-STEM). Chemical configurations of Fe and N in SACs are probed by X-ray photoelectron spectroscopy (XPS). We further report the detailed analysis of structure and dynamics of Fe active site. Operando confocal-Raman spectroscopy provides valuable information including (i) transient binding of ORR intermediates to active sites, (ii) rate determining step, and (iii) dynamic structure of Fe-N-C active site. Moreover, Nitrite stripping voltammetry was conducted to quantify active site density (SD) and turnover frequency (TOF) in the catalyst. This presentation will elucidate relations between the local environment of active sites and their electrochemical stability.