(282h) Atomic Iron-Dispersed Electrocatalysts Derived from Metal-Organic Framework for Oxygen Reduction

To significantly reduce the cost of proton exchange membrane (PEM) fuel cells, current Pt must be replaced by platinum-metal-group (PGM)-free catalysts for the oxygen reduction reaction (ORR) in acid. We report here a new class of high-performance atomic iron-rich catalyst via chemical doping of iron ions into zinc-zeolitic imidazolate framework (ZIF), a type of metal-organic framework (MOF). The novel synthetic chemistry enables accurate size control of Fe-doped ZIF catalyst particles with a wide range from 20 to 1000 nm without changing chemical property. This provides a great opportunity to increase the density of active sites. An optimal size at 50 nm exhibited the highest activity; further reducing size to 20 nm results in particle agglomeration with decreased active sites. In addition to the size effect, we carefully explored the thermal activation mechanism to provide insight into the origin of active site formation. A temperature of 800oC was identified as the critical point to generate active sties with new bond formation associated with nitrogen doping, carbon, and iron. 1100 oC was found for the best activity with maximum density of active sites along with optimal porosity, graphitic nitrogen doping, and carbon structures. We also found the control of optimal iron doping is crucial for performance enhancement without the formation of inactive iron cluster. Overall, the best performing catalyst is enriched in atomic iron sites embedded into carbon structures, achieving a new performance milestone for the ORR including a half-wave potential of 0.84 V vs RHE in 0.5 M H2SO4 and only 20 mV loss after 10,000 cycles in O2 saturated electrolyte, which has been approaching to Pt in the challenging acidic media. Thus, the chemically Fe doped MOF catalysts are able to further narrow down the performance gap with Pt catalysts in more challenging acidic media, holding great promise to advance PGM-free catalyst as a replacement of Pt for PEM fuel cells.