One of a grand challenge for large-scale deployment of proton exchange membrane fuel cells (PEMFCs) is to replace platinum group metal (PGM) catalysts with earth-abundant materials for the oxygen reduction reaction (ORR) in acidic media. In this presentation, we report a high-performance atomically dispersed transitiona metal sites (Fe, Co, or Mn) catalysts derived from chemically metal-doped zeolitic imidazolate frameworks (ZIFs) by directly bonding metal ions to imidazolate ligands within 3D frameworks. Although the ZIF was identified as a promising precursor, the new synthetic chemistry enables the creation of well-dispersed atomic metal sites embedded into porous carbon without the formation of aggregates. The size of catalyst particles is tunable through synthesizing metal-doped ZIF nanocrystal precursors in a wide range from 20 to 1000 nm followed by one-step thermal activation. Similar to Pt nanoparticles, the unique size control without altering chemical properties afforded by this approach is able to increase the number of PGM-free active sites. The best ORR activity is measured with the catalyst at a size of 50 nm. Further size reduction to 20 nm leads to significant particle agglomeration, thus decreasing the activity. Using the homogeneous atomic metal model catalysts, we elucidated the active site formation process through correlating measured ORR activity with the change of chemical bonds in precursors during thermal activation up to 1100 °C. In addition to traditional Fe-based catalysts, we will present our latest resrach on Fe-free catalysts through Co and Mn catalyts approaches. The general rules of catalyst design and synthesis to enhance catalyst activity and stability will be extensively discussed.

Metal-organic framework-derived atomically dispersed metal site catalysts for oxygen reduction in acids

2019 Spring National Meeting

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