(280c) Atomically Dispersed and Nitrogen Coordinated Metal Site Catalysts for Oxygen Reduction in Acids

One of the bottlenecks hindering the wide implementation of polymer electrolyte fuel cells (PEMFCs) is the high cost and massive use of platinum (Pt) catalysts for boosting oxygen reduction reaction (ORR) kinetics at the cathode in PEMFCs. Fe-N-C catalysts, derived from pyrolysis of nitrogen, iron and carbon precursors together, have been the most promising candidates to replace Pt because of the abundance and low cost of such elements as well as the highly intrinsic ORR activity of Fe-N-C sites forming during the pyrolysis¹. However, the major challenge for such Fe-N-C catalysts is to obtain sufficient Fe-N-C active sites catalysts to achieve high ORR activity, since undesired metallic species (i.e. Fe/Fe₃C) are easily produced during high-temperature pyrolysis due to the improper design of synthesis and the poor chemistry control in the preparation of catalysts, lowering the actual Fe-N-C active sites density of catalysts. To address this issue, we have developed an facile approach to prepare Fe-N-C catalysts with exclusively atomically dispersed Fe in Fe-N-C single-sites instead of containing metallic Fe particles by using well-defined Fe-doped metal-organic framework (MOF) precursors. The morphology of MOF precursors can be directly transferred into Fe-N-C catalysts with the retained shape after pyrolysis. Such controlled synthesis of precursors and resulting homogenous catalysts allow us to precisely control and tune the composition and morphology of Fe-N-C catalysts to understand how the structure and composition change of catalysts impact their ORR activity.

This presentation will discuss the critical roles of Fe content in the preparation of catalyst and particle size of catalysts on their ORR activity. The Fe content are controlled at the stage of Fe doped MOF precursor synthesis. Trace amounts of Fe was found to significantly boost ORR activity of catalysts, attributed by the highly-dispersed exclusive Fe-N-C active sites. The ORR activity of such Fe-N-C catalysts enhanced when increasing the density of Fe-N-C active sites at low range of Fe content. On the other hand, decreasing in ORR activity was observed when using high Fe content in the synthesis because of the formation of Fe particles in catalysts at high Fe content. Moreover, it is known that the particle size at nanoscale level critically governs the activity of catalysts. Taking advantages of this facile synthesis from MOF precursors, the Fe-N-C catalysts with size from 20 nm to 1000 nm are able to be prepared by adjusting the size of MOF precursors². As a result, reduction of particle size in Fe-N-C catalysts allows us to increase the utilization of Fe-N-C active sites for ORR. Among studied catalysts, the catalyst with particle size around 50 nm shows the best ORR activity with a half-wave potential of 0.85 V vs. RHE, only leaving 30 mV gap with Pt/C (60 µg_Pt/cm²) in 0.5 M H₂SO₄ along with the excellent stability. When the particle size in catalyst was further reduced to 20 nm, massive agglomeration of particles were found in the catalyst with a decrease in ORR activity. These high-performance Fe-N-C catalysts present a promising potential to replace Pt for ORR in future PEMFCs. In addition to extensively studied Fe-N-C catalysts, atomically dispersed and nitrogen coordinated Co and Mn sites catalysts will be briefly introduced in terms of their synthesis, characterization, electrochemical activity and stability in RDE, and fuel cell performance.