The high cost platinum (Pt) catalyst used for the electrocatalysis of oxygen reduction reaction (ORR) is one of the bottlenecks for the widespread deployment of polymer membrane electrolyte fuel cells (PEMFCs). Fe-N-C catalysts, produced by the pyrolysis of iron, nitrogen and carbon precursors together, have been found as one of the promising alternatives to replace Pt with abundant elements and low cost.^1-3 The Fe-N coordination structure bonded with carbon, generated during high-temperature pyrolysis, has been identified as the active site for the high ORR activity of Fe-N-C catalysts. However, it remains a grand challenge to prepare Fe-N-C catalysts with abundant Fe-N active sites to achieve high ORR activity, since undesired inactive species (e.g., Fe/Fe₃C) are easily obtained during high-temperature pyrolysis due to the improper design and the poor chemistry control in the integration of all precursors, lowering the active sites density of catalysts.

We have developed an approach to synthesize Fe-N-C catalysts with exclusively atomic Fe-N active sites instead of inactive Fe agglomeration by using well-defined Fe-containing metal-organic frameworks (MOFs) precursors.⁴ The morphology of MOF precursors can be directly transferred to Fe-N-C catalysts with the retained and homogenous morphology after pyrolysis. Such well-defined precursors and resulting homogenous catalysts allow us to precisely control and tune the composition and morphology of final Fe-N-C catalysts to understand how the property change of catalysts impact their ORR activity of catalysts.

In this presentation, we will discuss the effect of particle size of catalysts on their ORR activity and the critical role of pyrolysis temperature on the formation of Fe-N active sites. The Fe-N-C catalysts with size from 20 nm to 1000 nm are prepared by adjusting the size of MOF crystals in the precursor synthesis. Similar to Pt nanoparticles, the unique size control of the Fe-N-C catalysts enables us to increase the accessible number of Fe-N active sites for ORR. The 50 nm catalyst 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 of catalyst is reduced to 20 nm, significant agglomeration of particles are found in the catalyst, resulting in ORR activity decrease of the catalyst. Using our homogenous model catalysts, the formation of active sites during pyrolysis is investigated by correlated the measured-ORR activity with the bonds change of precursors at various pyrolysis temperature. 800 ^oC is found to be the critical temperature to form the Fe-N active sites with notable ORR activity, which is related to the generation of new Fe species likely bonded with pyridinic N in the carbon structure. These high performance Fe-N-C catalysts exhibit a promising potential to replace Pt for ORR in future PEMFCs.

(1) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. Science 2011, 332, 443.

(2) Zhang, H.; Osgood, H.; Xie, X.; Shao, Y.; Wu, G., Nano Energy, 2017, 31, 331-350.

(3) Wu, G.; Santandreu, A.; Kellogg, W.; Gupta, S.; Ogoke, O.; Zhang, H.; Wang, H.-L.; Dai, L., Nano Energy, 2016, 29, 83–110.

(4) Zhang, H.; Hwang, S.; Wang, M.; Feng, Z.; Karakalos, S.; Luo, L.; Qiao, Z.; Xie, X.; Wang, C.; Su, D.; Shao, Y.; Wu, G. Journal of the American Chemical Society, 2017, 139, 14143–14149.