Proton exchange fuel cells (PEFCs) are a clean technology for efficient conversion of chemical into electrical energy and are specifically promising for the decarbonization of heavy duty vehicles [1]. Currently, the drawback of PEFCs is the high cost of Pt-based catalysts used for cathode and anode, which hinders their commercialization. [2] The rapid development of FeNCs holds promise for replacing Pt-based catalysts for the oxygen reduction reaction (ORR). The nature and characterization of the FeNC active sites is a challenging subject of research, and the exact structure of intrinsic active center for FeNC catalysts is still under debate. [3-6]

⁵⁷Fe Mössbauer Spectroscopy is powerful in obtaining knowledge of iron sites, with respect to structural composition, electronic states as well as magnetic environment [3,7-9]. To solve the debate, ⁵⁷Fe Mössbauer experiments were carried out under ex situ, in situ, or operando conditions to identify iron signatures and their changes induced by different conditions. On the basis of our in situ results of three differently prepared catalysts, two transitions between the oxygenated and deoxygenated state were found and assigned to sites involved in the direct and indirect ORR. [10-11] In order to gain an in-depth understanding of active sites operando conditions (thus during ORR) were performed for the FeNC catalyst that exhibited the strongest change during in situ testing. One iron signature (D4) gets exclusively formed under ORR conditions and its intensity scales with the ORR current. Together with density functional theory calculations the overall set of data enables us to make important conclusions on the ORR mechanism on FeNC catalysts.

Literature:

[1] M. K. Debe, Nature. 486, 2012, 43−51.

[2] C. Sealy, Mater.Today.11, 2008, 65.

[3] S. Wagner, H. Auerbach, et al. Angew. Chem. 131.31, 2019, 10596-10602.

[4] A. Zitolo, V. Goellner, et al. Nat. Mater. 14.9, 2015, 937.

[5] X.,Li, C. Cao, et al. Chem. 6, 2020, 3440–3454.

[6] J. Li, M. T. Sougrati, et al. Nat. Catal. 4, 2021, 10–19.

[7] U.I. Kramm, M. Lefèvre, et al. J. Am. Chem. Soc. 136, 2014, 978-985.

[8] U.I. Kramm, J. Herranz, et al. Phys. Chem.Chem.Phys. 14, 2012,11673-11688.

[9] U.I. Kramm, L. Ni, et al. Adv. Mater.31.31, 2019, 1805623.

[10] L. Ni, C. Gallenkamp, et al. Adv. Energy Sustainability Res. 2, 2021, 2000064.

[11] L. Ni, P. Theis,et al. Electrochim. Acta. 395, 2021, 139200.