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

Abstract

The influence of electrical polarization on Mn in La 0.5 Sr 0.5 MnO 3±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% O 2 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. In high temperature solid oxide electrolysis and solid oxide fuel cells LSM has for decades been a common choice for application in the oxygen electrode.1 The popularity of LSM based electrodes is mainly due to the LSM's high electronic conductivity, the material's good chemical stability and thermal compatibility with other cell materials and last, but not least, the electrocatalytic activity of LSM towards the oxygen evolution reaction (OER) and the oxygen reduction reactions (ORR).1,2 The complex defect chemistry of LSM and the materials electrocatalytic properties has been investigated and discussed for long, and it is well-established that the interplay between the electrical polarization and the LSM's electronic structure strongly influences the electrocatalytic properties of the material. However, extensive experimental studies of this interplay at realistic operating conditions, i.e operando studies, have been scarce.From earlier experimental work it is known, that the creation of oxygen vacancies and accompanying charge compensation mechanism is relevant for the electrocatalytic properties of LSM electrodes, as oxygen vacancies are formed during the cathodic polarization of the electrodes. Hammouche et al.3 concluded from electrochemical characterization and thermogravimetry that the electrocatalytic properties of LSM for reduction of oxygen were closely related to the generation of oxide vacancies inside the material. Concordingly the highest electrocatalytic activity was observed for LSM50, which had the maximum level of Sr dopant, and thus 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 o...

Understanding the chemistry of manganese fertilizers and glyphosate mixtures by using synchrotron X-ray spectrometry

“…Their spectra presented a slightly reduction of whiteline intensity compared to the pristine and supernatant forms (see also Figure S4). The reduction of whiteline intensity is associated to the increase of electron density on p unoccupied states of Mn 2+ ions, while edge shifts towards lower energies can be caused by increase of Mn-ligand bond length [31]. This corroborates the chemical affinity between Mn-glyphosate which yields a more stable molecule than MnSO 4 and Mn-glycine.…”

Relationship between Mn Oxidation State Changes and Oxygen Reduction Activity in (La,Ca)MnO3 as Probed by In Situ XAS and XES

“…26 Traulsen et al employed these methods to study the polarization-induced changes to the oxidation state of Mn for a La 0.5 Sr 0.5 MnO 3+δ solid-oxide fuel cell electrocatalyst under operando conditions at 500 °C, demonstrating that the average Mn oxidation state decreased from +3.4 at an open circuit to +3.2, corresponding to 34% Mn 4+ , 49% Mn 3+ , and 17% Mn 2+ , at −800 mV applied potential. 27 Here, we implement electrochemical in situ XES and EXAFS to unravel the complex correlation between orbital occupancy of Mn sites and their electrocatalytic activities toward the ORR. For the first time, we demonstrate that electrochemical signatures associated with LaMnO 3 at potential close to ORR formal potential are linked to an increase in the orbital occupancy of Mn 3d sites as well as changes in the Mn−O bond distance and coordination numbers.…”

Molecular Sieving and Current Rectification Properties of Thin Organic Films

“…However, from these sole experiments, such a decrease of current could be explained by the variation of the potential applied to the modified working electrode as the substitution on the ferrocene probe changes not only the size but also its oxidation potential. Indeed, the polarization of the electrode could induce structural changes in the layer and consequently affects the passage of the redox probes through the film and the kinetics of the redox processes . To answer this question, two other ferrocene derivatives were considered: 1,1′-di­(methylphenyl)­ferrocene ( (PhMe) 2 Fc ) and 1-(biphenylethynyl)­ferrocene ( Ph 2 EFc ).…”

“…Indeed, the polarization of the electrode could induce structural changes in the layer and consequently affects the passage of the redox probes through the film and the kinetics of the redox processes. 59 To answer this question, two other ferrocene derivatives were considered: 1,1′-di(methylphenyl)ferrocene ((PhMe) 2 Fc) and 1-(biphenylethynyl)ferrocene (Ph 2 EFc).…”