# Fe-based Single Atom Electrocatalyst Structure Probed by X-ray Emission Spectroscopy

Viktoriia Saveleva, Kavita Kumar, Pascal Theis, Nicole Segura Salas, Ulrike

Kramm, Frederic Jaouen, Frédéric Maillard, Pieter Glatzel

► To cite this version:

Viktoriia Saveleva, Kavita Kumar, Pascal Theis, Nicole Segura Salas, Ulrike Kramm, et al. Fe-based Single Atom Electrocatalyst Structure Probed by X-ray Emission Spectroscopy. Gordon Research Conference, Feb 2023, Lucca, Italy. , Reactions at Surface Active Sites: From Single Atoms and 2D Materials to Electrified Interfaces, 10.15151/ESRF-ES-799089889 . hal-04005436

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Submitted on 26 Feb 2023

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Distributed under a Creative Commons Attribution 4.0 International License
# Fe-based Single Atom Electrocatalyst Structure Probed by X-ray Emission Spectroscopy

Viktoriia A. Saveleva¹, Kavita Kumar², Pascal Theis³, Nicole Segura Salas³, Ulrike I. Kramm³, Frédéric Jaouen⁴, Frédéric Maillard², Pieter Glatzel¹

¹ ESRF, The European Synchrotron, 71 Avenue des Martyrs, CS40220, 38043 Grenoble Cedex 9, France

² Univ. Grenoble Alpes, Univ. Savoie Mont Blanc, CNRS, Grenoble INP, LEPMI, 38000 Grenoble, France

³ TU Darmstadt, Department of Chemistry, Catalysts and Electrocatalysts Group, Technical University of Darmstadt, Otto-Berndt-Str. 3, 64287 Darmstadt, Germany

⁴ICGM, Univ. Montpellier, CNRS, ENSCM, 34293 Montpellier, France

viktoriia.saveleva@esrf.fr

Evaluation of the electrocatalyst performance data includes an electrode preparation step. In this work, we compare the structural composition of several Fe-N-C materials prepared via different synthesis routes and used to electrocatalyze the oxygen reduction reaction in proton-exchange membrane fuel cells, before and after catalyst layer preparation using high energy resolution fluorescence detected X-ray absorption (HERFD XANES) and X-ray emission spectroscopies (XES). The measurements were performed at beamline ID26 of the European synchrotron (ESRF).

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## Overview

<figure><img src="image_3.png" /></figure>

## Powder vs Electrode: Kβ HERFD XANES & XES

<figure><img src="image_4.png" /></figure>

## Fe-N-C Catalysts Overview

<figure><img src="image_5.png" /><figcaption>### C-PPy
* polypyrrole-derived
* ⁵⁷Fe-N-C
* Reference [3]</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>### Porph/C<sub>prec</sub>
* Porphyrin-based
* FeTMPPCI / Carbon
* Reference [4]</figcaption></figure>

<figure><img src="image_7.png" /></figure>

## Role of Nafion ionomer

<figure><img src="image_8.png" /></figure>

**Analyzed features:**
Pre-edge intensity and position
Fe K edge 1st inflection point
White line intensity
Kβ<sub>1,3</sub> - Kβ' intensity and position

*NCR = Nafion-to-Catalyst Ratio

## X-ray Absorption & Emission Spectroscopies

<figure><img src="image_9.png" /></figure>

Credits photo: S. CANDE / ESRF

## Conclusions

* FeNₓ sites structure partially evolves from square planar (or square pyramidal) to octahedral by adding -O₂ / -OH bond(s)

* This change is universal for Fe-N-C materials, the magnitude of change varies with catalyst initial composition

* Nafion presence enhances the formation of oxygenated ligands in the ink.

This aspect should be taken into account when discussing the catalyst structure – activity – stability/durability correlation(s) as well as analyzing *in situ/operando* measurements performed on these materials, where one should consider the electronic state of Fe in the prepared electrode and not in the catalyst powder.

<figure><img src="image_10.png" /></figure>

## References

1. Asset, T. and Atanassov, P. Iron-Nitrogen-Carbon Catalysts for Proton Exchange Membrane Fuel Cells, *Joule*, **2020**, *4*, 33-44.

2. Kumar, K.; et al. Physical and Chemical Considerations for Improving Catalytic Activity and Stability of Non-Precious Metal Oxygen Reduction Reaction Catalysts, *ACS Catal.* **2018**, *8* (12), 11264-11276.

3. Scharf, J.; et al. Relation between half-cell and fuel cell activity and stability of FeNC catalysts for the oxygen reduction reaction, *SusMat* **2022**, *2* (5), 630-645.

4. Paul, S.; et al. Influence of the metal center in M-N-C catalysts on the CO₂ reduction reaction on gas diffusion electrodes, *ACS Catal.* **2021**, *11* (9), 5850-5864.

5. Ducotté, L.; et al. Mechanical aspects of the ID26 emission spectrometer II: improving stability for a large instrument by the use of multiple air pad supports, *Diamond Light Source Proceedings 2010*, *1* (Issue MEDSI-6), 26.

## Acknowledgment

We acknowledge the European Synchrotron Radiation Facility for provision of beamtime at ID26 beamline (DOI: 10.15151/ESRF-ES-799089889). P.T. and U.I.K. acknowledge financial support by the BMBF young researcher group FeNC-StRedO (funding number 03XP0092). F.M., F.J. and K.K. gratefully acknowledge financial support from the French National Research Agency through the ANIMA (grant number ANR-19-CE05-0039) and the DEEP (grant number ANR-21-CE05-0021) projects.