What can In-situ XPS Tell us About Reaction Mechanisms on Solid Oxide Cell Electrodes?

Andreas Nenning^a, Alexander Opitz^a, Matthäus Siebenhofer^b, Christoph Riedl^a, Raffael Rameshan^c, Christoph Rameshan^c, Jürgen Fleig^a

^aTU Wien, Getreidemarkt 9/BC/02, 1060, Wien

^bMassachusetts Institute of Technology, Vassar Street, 32, Cambridge, United States

^cChair of Physical Chemistry, Montanuniversitaet Leoben, Franz-Josef-Strasse 18, A-8700 Leoben, Austria

Proceedings of 24th International Conference on Solid State Ionics (SSI24)

Advanced characterisation techniques: fundamental and devices

London, United Kingdom, 2024 July 14th - 19th

Organizers: John Kilner and Stephen Skinner

Oral, Andreas Nenning, presentation 168
Publication date: 10th April 2024

Although solid oxide fuel and electrolysis cells are already commercially available, the exact surface chemistry and reaction mechanisms for oxygen exchange at the air and fuel side are still only partly understood. It was realised that slight alterations of the surface chemistry during operation can have substantial impact on the electrochemical activity, which explains the tremendous scatter in the measured electrochemical activity of a given material¹. In-situ ambient pressure XPS measurements on electrochemical model cells reveal the surface chemistry during electrochemical operation and its changes with time², atmosphere and electrochemical polarisation^3–5 and provide a direct link between surface chemical state and electrochemical activity. This talk presents a survey of own experiments that show the complex interplay of operation conditions, surface chemistry and electro-catalytic activity in oxygen and fuel atmospheres.

Specifically, this talk will present advanced designs for two and three electrode model cells⁶ that are optimised for in-situ characterisation in ambient pressure XPS as well as UHV⁷ and corresponding results. In oxidising conditions, primarily degrading factors were observed. Interestingly, the primary source of the fast initial degradation is not A-site cation segregation per se, but the formation of a SO₄^2- adsorbate layer from sulphur trace impurities on various different materials¹. In reducing conditions, mixed oxidation states of Fe²⁺/Fe³⁺ in Fe containing perovskites and Ce³⁺/Ce⁴⁺in doped ceria⁴ are observable, and most likely the mediators for charge transfer to hydroxide⁴ or carbonate^3,8 reaction intermediates in the rate determining step. Interestingly, only ceria exhibits a strongly enhanced Ce³⁺ fraction at the surface, whereas the Fe²⁺ on perovskite-type SrTi_0.6Fe_0.4O_3-δ and La_0.6Sr_0.4FeO_3-δ is not enriched at the surface. Lastly, cathodic polarisation in a reducing atmosphere can trigger the exsolution of catalytically active Fe⁰ metal particles^9–11, on which the H₂ release rate can be greatly enhanced, compared to the bare perovskite.

References:

[1] Riedl, C.; Siebenhofer, M.; Nenning, A.; Schmid, A.; Weiss, M.; Rameshan, C.; Limbeck, A.; Kubicek, M.; Karl Opitz, A.; Fleig, J. In Situ Techniques Reveal the True Capabilities of SOFC Cathode Materials and Their Sudden Degradation Due to Omnipresent Sulfur Trace Impurities. J. Mater. Chem. A 2022, 10 (28), 14838–14848.

[2] Opitz, A. K.; Rameshan, C.; Kubicek, M.; Rupp, G. M.; Nenning, A.; Götsch, T.; Blume, R.; Hävecker, M.; Knop-Gericke, A.; Rupprechter, G.; Klötzer, B.; Fleig, J. The Chemical Evolution of the La0.6Sr0.4CoO3−δ Surface Under SOFC Operating Conditions and Its Implications for Electrochemical Oxygen Exchange Activity. Top. Catal. 2018, 61 (20), 2129–2141.

[3] Nenning, A.; Opitz, A. K.; Rameshan, C.; Rameshan, R.; Blume, R.; Hävecker, M.; Knop-Gericke, A.; Rupprechter, G.; Klötzer, B.; Fleig, J. Ambient Pressure XPS Study of Mixed Conducting Perovskite-Type SOFC Cathode and Anode Materials under Well-Defined Electrochemical Polarization. J. Phys. Chem. C 2016, 120 (3), 1461–1471.

[4] Chueh, W. C.; Hao, Y.; Jung, W.; Haile, S. M. High Electrochemical Activity of the Oxide Phase in Model Ceria-Pt and Ceria-Ni Composite Anodes. Nat. Mater. 2012, 11, 155–161.

[5] Zhang, C.; Grass, M. E.; Yu, Y.; Gaskell, K. J.; DeCaluwe, S. C.; Chang, R.; Jackson, G. S.; Hussain, Z.; Bluhm, H.; Eichhorn, B. W.; Liu, Z. Multielement Activity Mapping and Potential Mapping in Solid Oxide Electrochemical Cells through the Use of Operando XPS. ACS Catal. 2012, 2 (11), 2297–2304.

[6] Nenning, A.; Reuter, S.; Schlesinger, R.; Summerer, H.; Ramehsan, R.; Lindenthal, L.; Holzmann, M.; Huber, T. M.; Rameshan, C.; Fleig, J.; Opitz, A. K. Surface and Defect Chemistry of Porous La0.6Sr0.4FeO3 Electrodes on Polarized Three-Electrode Cells. J. Electrochem. Soc. 2022, 169 (9), 094508.

[7] Nenning, A.; Fleig, J. Electrochemical XPS Investigation of Metal Exsolution on SOFC Electrodes: Controlling the Electrode Oxygen Partial Pressure in Ultra-High-Vacuum. Surf. Sci. 2019, 680, 43–51.

[8] Feng, Z. L. A.; Machala, M. L.; Chueh, W. C. Surface Electrochemistry of CO2 Reduction and CO Oxidation on Sm-Doped CeO2-x: Coupling between Ce3+ and Carbonate Adsorbates. Phys. Chem. Chem. Phys. 2015, 17, 12273–12281.

[9] Summerer, H.; Nenning, A.; Rameshan, C.; Opitz, A. K. Exsolved Catalyst Particles as a Plaything of Atmosphere and Electrochemistry. EES Catal. 2023, 1 (3), 274–289.

[10] Opitz, A. K.; Nenning, A.; Rameshan, C.; Rameshan, R.; Blume, R.; Hävecker, M.; Knop‐Gericke, A.; Rupprechter, G.; Fleig, J.; Klötzer, B. Enhancing Electrochemical Water‐Splitting Kinetics by Polarization‐Driven Formation of Near‐Surface Iron(0): An In Situ XPS Study on Perovskite‐Type Electrodes. Angew. Chem. Int. Ed. 2014, 54, 2628–2632.

[11] Opitz, A. K.; Nenning, A.; Vonk, V.; Volkov, S.; Bertram, F.; Summerer, H.; Schwarz, S.; Steiger-Thirsfeld, A.; Bernardi, J.; Stierle, A.; Fleig, J. Understanding Electrochemical Switchability of Perovskite-Type Exsolution Catalysts. Nat. Commun. 2020, 11 (1), 4801.

Acknowledgements:

We want to thank the Christoph Rameshan, and his ERC Project "TUCAS" for the great collaboration and the granting of extensive APXPS measurement time.

Also we want to thank the FWF, projects W1243 and P35686-N for finantial support.

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