2, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
3, University at Buffalo, State University of New York, Buffalo, New York, United States
The search for a suitable platinum group metal-free (PGM-free) catalyst to drive the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells has intensified with the recent arrival of commercial fuel cell electric vehicles. New classes of PGM-free catalysts are constantly emerging, with the latest efforts focused on atomically dispersed transition metal catalysts derived from metal organic frameworks (MOFs). A critical step in the synthesis of these ORR catalysts is the high temperature heat treatment (up to 1100oC) required to convert the Fe, Ni, or Co-doped MOFs into graphitic carbons containing the proposed individual metal-nitrogen active sites. We have utilized low-voltage, aberration-corrected scanning transmission electron microscopy (ac-STEM) coupled with electron energy loss spectroscopy (EELS) to establish a clear link between the dispersion of the single metal atoms and catalyst performance as measured by rotating disk electrode (RDE). In this work, we provide additional insight into the interaction between the metal-nitrogen sites and the graphitic carbon support through the application of in situ electron microscopy. By utilizing advanced micro-electro-mechanical system (MEMS)-based heating devices, it is now possible to reproduce these high temperature heat treatments within the electron microscope to directly observe the transformation of the metal-doped MOFs. In situ and ex situ experiments conducted in tandem with RDE and fuel cell measurements will be employed to elucidate the role of annealing temperature, nitrogen-doping, and metal loading on the formation of graphene-embedded metal-nitrogen sites and their subsequent impact on catalyst activity. This research is sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy and through a user project supported by ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility.