Oxygen Reduction Mechanism on Fe Macrocycles: is charge transfer always coupled to adsorption?

Sivia Favero^b, Alexander Bagger^b, Ruixuan Chen^b, Reshma Rao^a, Luke Higgins^c, James Durrant^d, Ifan Stephens^a, Magda Titirici^b

^aDepartment of Materials, Imperial College London; London, UK

^bDepartment of Chemical Engineering, Imperial College London, SW7 2AZ, UK, Imperial College Road, London, United Kingdom

^cSchool of Chemical and Process Engineering & School of Chemistry, University of Leeds; Leeds, UK

^dDepartment of Chemistry, Imperial College London, London W12 0BZ, UK

Materials for Sustainable Development Conference (MATSUS)
Proceedings of Materials for Sustainable Development Conference (MAT-SUS) (NFM22)

#SusEnergy - Sustainable materials for energy storage and conversion

Barcelona, Spain, 2022 October 24th - 28th

Organizers: Tim-Patrick Fellinger and Magda Titirici

Contributed talk, Sivia Favero, presentation 058
DOI: https://doi.org/10.29363/nanoge.nfm.2022.058
Publication date: 11th July 2022

Iron-based catalysts represent one of the most promising alternatives to platinum for oxygen reduction. However, the mechanism of oxygen reduction on Fe-based catalysts is still debated and the challenge for pyrolyzed catalysts is to unambiguously identifying the active sites.

Iron macrocycles can assist in understanding the mechanism of oxygen reduction by providing well-defined and reproducible active sites. A first indication of the difference in ORR kinetics among different macrocycles can be obtained by observing the cyclic voltammetry (CV) peaks. Most Fe macrocycles present two peaks in the cyclic voltammetry curve. The conventional wisdom is that the peak at more positive potentials is due to the desorption of *OH and consequent reduction of iron from Fe^III to Fe^II, and the peak at more negative potentials is due to the further reduction of iron to Fe^I.¹ The position of the peak due to *OH adsorption provides a measure of the binding energy of this species; which, in turn, is controlled by the electron-withdrawing or donating nature of the ring.² This causes a previously reported correlation between the position of the OH adsorption peak and the ORR activity.^3,4 However, other macrocycles only present one potential peak in the CV and the activity is inconsistent with its position

In this work we provide a systematic study of oxygen reduction on selected Fe-macrocycles, combining DFT calculations, electrochemical measurements, and in-situ characterization, including operando UV-Vis spectroscopy and x-ray absorption spectroscopy. Our investigations shows that the OH adsorption peak is not accompanied by a change in Fe oxidation state, while the peak at more negative potentials results from the reduction of iron from Fe^III to Fe^II. Furthermore, our analysis indicates the reason why some macrocycles do not show a second voltametric peak in the experimental CV. The insight gained by studying these model molecules will allow the interpretation of more commercially relevant pyrolyzed catalysts and to understand the nature or redox activity in molecular catalysts.

References:

[1] Alsudairi, A. et al. Resolving the Iron Phthalocyanine Redox Transitions for ORR Catalysis in Aqueous Media. J. Phys. Chem. Lett. 8, 2881–2886 (2017).

[2] Ramaswamy, N., Tylus, U., Jia, Q. & Mukerjee, S. Activity descriptor identification for oxygen reduction on nonprecious electrocatalysts: Linking surface science to coordination chemistry. J. Am. Chem. Soc. 135, 15443–15449 (2013).

[3] Zagal, J. H. & Koper, M. T. M. Reactivity Descriptors for the Activity of Molecular MN4 Catalysts for the Oxygen Reduction Reaction. Angew. Chemie - Int. Ed. 55, 14510–14521 (2016).

[4] Zagal, J. H. et al. Towards a unified way of comparing the electrocatalytic activity MN4 macrocyclic metal catalysts for O2 reduction on the basis of the reversible potential of the reaction. Electrochem. commun. 41, 24–26 (2014).

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