In-situ study of operating SOFC LSM/YSZ cathodes under polarization by photoelectron microscopy

Lanthanum strontium manganite microelectrodes with the nominal composition of (La 0.75 Sr 0.25 ) 0.95 MnO 3 and a thickness of ca 500 nm was electrochemically characterized in situ at temperatures from 660 to 850 °C using a controlled atmosphere high temperature scanning probe microscope. Impedance spectroscopy and cyclic voltammetry were performed on electrodes with diameters of 20-100 µm in oxygen, air and nitrogen both at open circuit voltage and at anodic and cathodic polarization. In situ conductance mapping, ex situ surface analysis by time-of-flight secondary ion mass spectrometry, and scanning electron microscopy were performed to observe electrical, chemical and structural changes on the microelectrodes.

Section: Discussionsupporting

confidence: 80%

LSM Microelectrodes: Kinetics and Surface Composition

Norrman

Jacobsen

et al. 2015

“…32 In the case of LSM, Huber et al observed the depletion of the LSM surface in both Sr and Mn under cathodic polarization, but also found that cathodic polarization promotes the spreading or diffusion of segregated Sr and Mn onto the electrolyte. 33 The diffusion of manganese oxide out of the LSM electrode onto the YSZ electrolyte surface under the influence of cathodic polarization has also been reported by Backhaus-Ricoult et al 63 We studied the effect of polarization on the chemical reactivity between Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF) and GDC and observed that under cathodic polarization…”

Section: Resultsmentioning

confidence: 64%

Thermally and Electrochemically Induced Electrode/Electrolyte Interfaces in Solid Oxide Fuel Cells: An AFM and EIS Study

Jiang

2015

In high temperature solid oxide fuel cells (SOFCs), electrode/electrolyte interfaces play a critical role in the electrocatalytic activity and durability of the cells. In this study, thermally and electrochemically induced electrode/electrolyte interfaces were investigated on pre-sintered and in situ assembled (La 0.8 Sr 0.2 ) 0.90 MnO 3 (LSM) and La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) electrodes on Y 2 O 3 -ZrO 2 (YSZ) and Gd 0.2 Ce 0.8 O 2 (GDC) electrolytes, using atomic force microscopy (AFM) and electrochemical impedance spectroscopy (EIS). The results indicate that thermally induced interface is characterized by convex contact rings with depth of 100-400 nm and diameter in agreement with the particle size of pre-sintered LSM and LSCF electrodes, while the electrochemically induced interfaces under cathodic polarization conditions on in situ assembled electrodes are characterized by particle-shaped contact marks or clusters (50-100 nm in diameter). The number and distribution of contact clusters depend on the cathodic current density as well as the electrode and electrolyte materials. The contact clusters on the in situ assembled LSCF/GDC interface are substantially smaller than that on the in situ assembled LSM/GDC interface likely due to the high mixed ionic and electronic conductivities of LSCF materials. The results show that the electrochemically induced interface is most likely resulting from the incorporation of oxygen species and cation interdiffusion under cathodic polarization conditions. However, the electrocatalytic activity of electrochemically induced electrode/electrolyte interfaces is comparable to the thermally induced interfaces for the O 2 reduction reaction under SOFC operation conditions. © The Author Solid oxide fuel cell (SOFC) is one of the most efficient technologies for the conversion of chemical energy of fuels such as hydrogen and natural gas directly into electrical power. It employs a solid oxide electrolyte such as yttria-stabilized zirconia (YSZ) that serves as an ionic conductor in the temperature range between 600• C to 1000• C. The electrolyte separates the cathode from the anode and conducts oxygen ions from the cathode to the anode where they react electrochemically with the fuel. Electrons are then released to an external circuit, which provides a useful source of electrical power. The electrochemical reactions such as O 2 reduction at the cathode and H 2 oxidation at the anodes occur at the electrode/electrolyte interface regions.1-3 Thus, in SOFCs, electrode/electrolyte interfaces play a vital role in the mechanism and kinetics of the electrochemical reactions and in the fundamental understanding of the relationship between the electrochemical processes and microstructure.4-10 The cell performance and durability also depend strongly on the microstructural change at the interface under SOFC operation conditions. 11,12Lanthanum strontium manganite (LSM) is one of the most common cathode materials for SOFCs because of high electrical conductivity, good thermal and chem...

“…In summary, this operando study confirms predictions regarding reduction in manganese oxidation state and formation of oxygen vacancies in LSM during cathodic polarization, predictions which to a large extent have been based on indirect measurements (thermogravimetry, electrochemical characterization) 4,6,45 and primarily have been investigated in situ by means of techniques like XPS which have required vacuum environment far from the real operating conditions of LSM electrodes. 4,46 Moreover, the present study gives a quantitative estimation of manganese oxidation state in the bulk LSM, making it possible to compare to predictions from defect models and DFT calculations. Note that the penetration depth of the hard X-rays applied allows real operando studies, but due the high energy of the hard X-rays the whole thickness of the applied LSM electrode (250 nm) is penetrated and thus the Mn oxidation state is probed through the entire LSM thin film in the present study.…”

Section: Changes In Mnmentioning

confidence: 96%

“…In situ XPS has been applied by several groups in order to obtain knowledge on surface composition and oxidation states 4,7,8 and also in situ STM has been employed to study electronic properties on LSM electrodes.…”

mentioning

confidence: 99%

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

Traulsen

Carvalho

Zielke

et al. 2017

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...