The Effect of Electrical Polarization on Electronic Structure in LSM Electrodes: An Operando XAS, RIXS and XES Study

M L Traulsen,P Zielke,H W P De Carvalho,J -D Grunwaldt

doi:10.1149/2.0091710jes

Abstract

The influence of electrical polarization on Mn in La0.5Sr0.5MnO3±δ electrodes has been investigated by operando High Energy Resolved Fluorescence Detected X-Ray Absorption Near-Edge Structure (HERFD-XANES) spectroscopy, Kβ X-ray Emission Spectroscopy (XES) and Resonant Inelastic X-ray Scattering (RIXS) at the Mn K-edge. The study of polarization induced changes in the electronic properties and structure has been carried out at 500°C in 10–20% O2 with electrical polarization applied in the range from −850 mV to 800 mV. Cathodic polarizations in the range −600 mV to −850 mV induced a shift in the Mn K edge energy towards lower energies. The shift is assigned to a decrease in the average Mn oxidation state, which based on Kβ XES changes from 3.4 at open circuit voltage to 3.2 at −800 mV applied potential. Furthermore, RIXS rendered pronounced changes in the population of the Mn 3d orbitals, due to filling of the Mn d-orbitals during the cathodic polarization. Overall, the study experimentally links the electrical polarization of LSM electrodes to the structural and electronic properties of Mn - these properties are expected to be of major importance for the electrocatalytic performance of LSM electrode towards the oxygen reduction reaction.

Highlights

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Chronoamperometry and potential distribution in the electrochemical cell.—During the operando HERFD-X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopies (XES) characterization potentials in the range −850 mV to 800 mV were applied over the cell by recording chronoamperometry curves
The application of novel hard X-ray spectroscopic techniques (HERFD-XAS, XES, RIXS) for determining structure-function relationships has been demonstrated to be a powerful approach for studying electrocatalysts like LSM used for the oxygen evolution reaction (OER) and oxygen reduction reactions (ORR) in air electrodes

Summary

Introduction

General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. The highest electrocatalytic activity was observed for LSM50, which had the maximum level of Sr dopant, and the highest level of oxygen vacancies, of the investigated compounds.[3] Lee et al.[4] used in situ XPS and electrochemical studies to prove that the cathodic polarization decreased the Mn oxidation state and led to formation of oxygen vacancies. These surface oxygen vacancy sites together with triple phase boundary sites were assumed to be active in the oxygen reduction.[4] The experimental findings summarized so far all point in the direction that the oxygen reduction reaction on LSM may be strongly enhanced during cathodic polarization due to the formation of oxygen vacancies and the correlated reduction in manganese oxidation state

Results

Discussion

Conclusion