Oxygen transport in La0.6Sr0.4Co1−y Fe y O3−δ

Bouwmeester, Henny J.M.; Otter, M.W. den; Boukamp, Bernard A.

doi:10.1007/s10008-003-0488-3

Cited by 214 publications

(212 citation statements)

References 17 publications

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“…Many publications have assessed the D and k values of MIEC materials, most commonly by; electrical conductivity relaxation (ECR) measurements for dense bulks [4][5][6][7][8] and temperature for LSCF cathodes. 17 Here it should be pointed out that the well-known ALS model can only be applied if the measured impedance shows a Gerischer impedance, which indicates that the surface-exchange and the oxygen bulk diffusion are strongly coupled.…”

Section: -3mentioning

confidence: 99%

Oxygen Transport Kinetics of Mixed Ionic-Electronic Conductors by Coupling Focused Ion Beam Tomography and Electrochemical Impedance Spectroscopy

Almar

Szász

Weber

et al. 2017

J. Electrochem. Soc.

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The oxygen reduction reaction of mixed ionic-electronic conducting (MIEC) cathodes Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF) and La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) is investigated by detailed electrochemical impedance spectroscopy and focused ion beam tomography. The oxygen transport parameters of these materials are usually determined for model systems, such as dense bulks or thin films. However in the present study, probable differences regarding the time and thermal history of the samples (e.g. ambient poisoning gases, grain coarsening, secondary phase or surface segregation, etc.) were avoided by the in situ sintering of electrodes with nominal stoichiometry under synthetic air. The microstructural parameters of the electrodes are obtained by 3D FIB-SEM reconstruction tomography and subsequently used in combination with the Adler-Lane-Steele analytical model to calculate a simulated cathode resistance. Large discrepancies are observed compared to the electrochemical impedance measurements. In particular, the electrochemical impedance measurements do not show a Gerischer behavior, as expected for MIEC materials controlled by a coupled surface-exchange and bulk diffusion. Analysis by distribution function of relaxation times (DRT) reveals four individual processes taking place, indicating a surface-exchange controlled behavior with a reaction zone similar to the particle size. Bulk diffusion (D) and surface-exchange (k) coefficients from literature are critically discussed and tentative surface-exchange coefficients (k) for both MIECs are given. High-temperature devices such as solid oxide fuel cells (SOFCs) and oxygen transport membranes (OTMs) are promising systems for working towards zero-emissions power generation. The selected materials require high chemical and microstructural stability, as well as high catalytic activity during the oxygen reduction reaction (ORR), under normal operational conditions (i.e. operational temperature and oxygen partial pressure). Particularly interesting are the mixed ionic-electronic conducting (MIEC) materials, based on the ABO 3 perovskite structure. Their material properties can be flexibly customized, giving solid solutions with high ionic-electronic conductivity and hence good transport properties. More specifically, La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF) has become established as the stateof-the-art SOFC cathode, while Ba 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3-δ (BSCF) has presented outstanding oxygen permeation flux in its cubic phase. 1-3The surface-exchange coefficient (k) and the oxygen diffusion coefficient (D) provide key information about the oxygen transport mechanism that dominates the cathodic ORR. Many publications have assessed the D and k values of MIEC materials, most commonly by; electrical conductivity relaxation (ECR) measurements for dense bulks [4][5][6][7][8] and temperature for LSCF cathodes. 17 Here it should be pointed out that the well-known ALS model can only be applied if the measured impedance shows a Gerischer impedance, which indicates tha...

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Section: -3mentioning

confidence: 99%

Oxygen Transport Kinetics of Mixed Ionic-Electronic Conductors by Coupling Focused Ion Beam Tomography and Electrochemical Impedance Spectroscopy

Almar

Szász

Weber

et al. 2017

J. Electrochem. Soc.

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“…However, when CO 2 (50 ml/min) was introduced, the flux suddenly dropped to 7.2×10 -7 mol/cm 2 /s, followed by a decrease to almost zero after 57 hours of CO 2 exposure at the sweep side of the membrane. For SCFTa, it took almost 30 hours to achieve a steady oxygen permeation flux in helium, which has also been found by Zhu and Yi in their work [25,26]. This could be due to readjusting of the lattice structure from the as-prepared state to a steady state under permeation conditions or to a reduction of the initial surface oxygen desorption rate.…”

Section: Methodssupporting

confidence: 65%

“…Chen et al [26] have also found that ~4 mol% Zr-doping at the B-site can stabilize SCF to ~700 o C in a For membranes, to be used in the 4-end mode, one of the options is to use CO 2, which is known as an acid gas, as the sweep gas. As most of the MIEC oxides contain alkaline-earth elements (Ca, Sr or Ba) a carbonation reaction occurs when these membranes are exposed to a CO 2 -containing atmosphere.…”

Section: State Of the Art Membranes For Oxy-fuel Combustionmentioning

confidence: 99%

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Oxygen transport membranes: a material science & process engineering approach

Chen

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“…[2][3][4] Electrochemical measurements on LSCF-6428, including conductivity relaxation, 3,5 coulometry, 4,6 and impedance techniques 2,7,8 have underscored the importance of bulk oxygen vacancies in governing rates of O 2 reduction and evolution. However, a clear understanding of how these vacancies contribute to the mechanism and rate-limiting step(s) of O 2 exchange remains elusive.…”

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confidence: 99%

Nonlinear Impedance Analysis of La_0.4Sr_0.6Co_0.2Fe_0.8O_3-δThin Film Oxygen Electrodes

Geary

Lee

Shao‐Horn

et al. 2016

J. Electrochem. Soc.

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Linear and nonlinear electrochemical impedance spectroscopy (EIS, NLEIS) were used to study 20 nm thin film La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF-6428) electrodes at 600 • C in oxygen environments. LSCF films were epitaxially deposited on single crystal yttria-stabilized zirconia (YSZ) with a 5 nm gadolinium-doped ceria (GDC) protective interlayer. Impedance measurements reveal an oxygen storage capacity similar to independent thermogravimetry measurements on semi-porous pellets. However, the impedance data fail to obey a homogeneous semiconductor point-defect model. Two consistent scenarios were considered: a homogeneous film with non-ideal thermodynamics (constrained by thermogravimetry measurements), or an inhomogeneous film (constrained by a semiconductor point-defect model with a Sr maldistribution). The latter interpretation suggests that gradients in Sr composition would have to extend beyond the space-charge region of the gas-electrode interface. While there is growing evidence supporting an equilibrium Sr segregation at the LSCF surface monolayer, a long-range, non-equilibrium Sr stratification caused by electrode processing conditions offers a possible explanation for the large volume of highly reducible LSCF. Additionally, all thin films exhibited fluctuations in both linear and nonlinear impedance over the hundred-hour measurement period. This behavior is inconsistent with changes solely in the surface rate coefficient and possibly caused by variations in the surface thermodynamics over exposure time. The rate of oxygen reduction/evolution on oxide surfaces continues to limit the performance of intermediate temperature (500-800• C) solid oxide fuel cells (SOFCs) and electrolysis cells (SOECs).1 La 1-x Sr x Co 1-y Fe y O 3-δ (LSCF), and particularly the composition La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ (LSCF-6428), exhibits a good balance of kinetic activity, ionic and electronic transport properties, and chemical stability, resulting in its prevalence as an oxygen electrode. 2-4Electrochemical measurements on LSCF-6428, including conductivity relaxation, 3,5 coulometry, 4,6 and impedance techniques 2,7,8 have underscored the importance of bulk oxygen vacancies in governing rates of O 2 reduction and evolution. However, a clear understanding of how these vacancies contribute to the mechanism and rate-limiting step(s) of O 2 exchange remains elusive.The thermodynamic understanding of La 1-x Sr x Co 1-y Fe y O 3-δ derives largely from measurements of equilibrium oxygen nonstoichiometry (δ 0 ) upon exposure to different oxygen environments and temperatures. 4,[9][10][11] In the limit of high B-site iron content (y > 0.6), workers have interpreted this δ 0 − p O2 − T relationship using a ptype point-defect model originally developed for LSF (LSCF, with y = 1), 12 where electrons are localized on iron centers of discrete charge and defects are considered dilute.4,9 For Co-rich compositions (y < 0.4), a metallic model used to describe LSC (LSCF, with y = 0) yields better agreement.4 While La 0.6 Sr 0.4 Co 0.2...

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Oxygen transport in La0.6Sr0.4Co1−y Fe y O3−δ

Cited by 214 publications

References 17 publications

Oxygen Transport Kinetics of Mixed Ionic-Electronic Conductors by Coupling Focused Ion Beam Tomography and Electrochemical Impedance Spectroscopy

Oxygen Transport Kinetics of Mixed Ionic-Electronic Conductors by Coupling Focused Ion Beam Tomography and Electrochemical Impedance Spectroscopy

Oxygen transport membranes: a material science & process engineering approach

Nonlinear Impedance Analysis of La_0.4Sr_0.6Co_0.2Fe_0.8O_3-δThin Film Oxygen Electrodes

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Oxygen transport in La0.6Sr0.4Co1−y Fe y O3−δ

Cited by 214 publications

References 17 publications

Oxygen Transport Kinetics of Mixed Ionic-Electronic Conductors by Coupling Focused Ion Beam Tomography and Electrochemical Impedance Spectroscopy

Oxygen Transport Kinetics of Mixed Ionic-Electronic Conductors by Coupling Focused Ion Beam Tomography and Electrochemical Impedance Spectroscopy

Oxygen transport membranes: a material science & process engineering approach

Nonlinear Impedance Analysis of La0.4Sr0.6Co0.2Fe0.8O3-δThin Film Oxygen Electrodes

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Nonlinear Impedance Analysis of La_0.4Sr_0.6Co_0.2Fe_0.8O_3-δThin Film Oxygen Electrodes