Determination of Iron Active Sites in Pyrolyzed Iron-Based Catalysts for the Oxygen Reduction Reaction

Abstract

Fe-based oxygen reduction reaction (ORR) catalyst materials are considered promising nonprecious alternatives to traditional platinum-based catalysts. These catalyst materials are generally produced by high-temperature pyrolysis treatments of readily available carbon, nitrogen, and iron sources. Adequate control of the structure and active site formation during pyrolysis methods is nearly impossible. Thus, the chemical nature, structure, and ORR mechanism of catalytically active sites in these materials is a subject of significant debate. We have proposed a method, utilizing CN– ions as ORR inhibitors on Fe-based catalysts, to provide insight into the exact nature and chemistry of the catalytically active sites. Moreover, we propose two possible catalytically active site formation mechanisms occurring during high-temperature pyrolysis treatments, dependent on the specific type of precursor and synthesis methods utilized. We have further provided direct evidence of our proposed active site formations using ToF-SIMS negative and positive ion imaging. This knowledge will be beneficial to future work directed at the development of Fe-based catalysts with improved ORR activity and operational stabilities for fuel cell and battery applications.

Promotion of oxygen reduction by a bio-inspired tethered iron phthalocyanine carbon nanotube-based catalyst

“…In particular, macrocycle compounds have been demonstrated for a long time to suffer from severe degradation in fuel cell environments due to demetalation and/or degradation of iron phthalocyanine (FePc) by ORR intermediates 20,21 . Traditional strategy to enhance the activity and durability of the Fe-N-C type of non-precious metal catalysts involves high temperature pyrolysis process, which resulted in complicated chemical and electronic structure of metal ions [22][23][24] . Although the pyrolysed catalysts offer better performance toward ORR than those without pyrolysis, their chemical structures undergo a complicated and unpredictable transformation during the heattreatment 25 .…”

Recent Progress on Fe/N/C Electrocatalysts for the Oxygen Reduction Reaction in Fuel Cells

“…The current proposed active sites, containing edge plane FeN2/C and FeN4/C [60] species as well as basal plane macrocyclic FeN4/C [19,20] species, are mainly speculated by data obtained from X-ray photoelectron spectroscopy (XPS) [19,100], time-of-flight secondary ion mass spectroscopy (TOF-SIMS) [45,60], X-ray absorption fine structure [18,60], and Mossbauer spectroscopy [17,19]. In 2008, Dodelet's group [61] claimed that the majority of active sites consist of a Fe-N4/C (labeled by the authors as FeN2+2/C) configuration bridging two adjacent graphene crystallites.…”

“…These findings reveal that when focusing on the development of Fe-based catalysts with improved active site densities, it is possible to tune the electronic and structure properties of these active site structures, or develop Fe-based catalysts with higher ORR-activity by developing ways to make a larger fraction of the available Fe-atoms form more of the most ORR-active composite N-FeN2+2…Nprot/C (D3) sites. Recently, Chen et al [45] proposed two possible formation mechanisms for the catalytically active sites occurring during high-temperature pyrolysis treatments through CN − ions poisoning experiments, dependent on the specific type of precursor and synthesis methods utilized. The proposed structures of high-temperature-treated Fe-based catalysts are depicted in Scheme 1.…”

“…Along with the achievement of the excellent ORR activity of diverse Fe-based catalysts, the ORR mechanisms on Fe-based catalysts were also widely studied by many groups due to its importance in research and development of high-performance Fe-based ORR catalysts [21,41,[44][45][46][47][48]. However, due to different preparation protocols used for Fe-based catalysts, there is still an ongoing debate about the active sites of these materials [45,[48][49][50].…”

“…However, due to different preparation protocols used for Fe-based catalysts, there is still an ongoing debate about the active sites of these materials [45,[48][49][50]. Therefore, there is still a long way to go in order to reach the practical usage and understanding of Fe-Based catalysts in fuel cells applications.…”