Platinum group metal-free (PGM-free) catalysts are under intense development for the oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells. Recent research efforts have focused on catalysts derived from metal organic frameworks doped with transition metals such as Fe, Ni, Co, and Mn. High temperature pyrolysis is a key synthesis step in the formation of the active catalyst, and aims to produce porous, mesographtic carbons with a high density of single transition metal atoms bound to nitrogen.

In this work, correlated in situ characterization techniques have been employed to explore the mechanisms that influence catalytic activity and provide guidance for synthesis protocols. The coordination of advanced synctrotron and electron microscopy methods provide new insight into the atomic-level transformations which occur during pyrolysis.

Micro-electro-mechanical system (MEMS)-based heating devices were utilized to directly observe transformations occurring at the atomic scale as the temperature was ramped to 1100^oC within the electron microscope. It was observed that significant amounts of the transition metal and nitrogen were lost as the pyrolysis temperature increased from 900-1100^oC, which could limit the active site density. However, rotating disk electrode measurements indicate that pyrolysis temperatures beyond 900^oC appear to be required to maximize mass activity. As shown in Figure 1, low voltage scanning transmission electron microscopy observations indicated that the degree of carbon graphitization increased as the temperature was elevated 1100^oC and nitrogen-bound transition metal sites were also more readily observed within the graphene lattice by electron energy loss spectroscopy. Consequently, approaches are being explored to maintain higher Fe and N content while also generating highly graphitized domains.

Beyond active site density, the microporosity of the support can also play a key role in the performance of the catalyst. Ongoing in situ studies will also aim to explore the mechanisms of microporosity formation within the MOF support and its subsequent influence on fuel cell performance.

Research sponsored by the Fuel Cell Technologies Office, Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy, as part of the ElectroCat Consortium, which is part of the Energy Materials Network. Microscopy performed as part of a user project at ORNL’s Center for Nanophase Materials Sciences (CNMS), which is a DOE Office of Science User Facility.