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High Oxygen Exchange Activity of Pristine La0.6Sr0.4FeO3–δ  Films andIts DegradationTo cite this article: Alexander Schmid et al 2020 J. Electrochem. Soc. 167 124509

 

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High Oxygen Exchange Activity of Pristine La0.6Sr0.4FeO3–δFilmsand Its DegradationAlexander Schmid,z Andreas Nenning, Alexander Opitz,* Markus Kubicek, andJürgen Fleig**

Institute of Chemical Technologies and Analytics, TU Wien, Vienna, A-1060, Austria

The polarization resistance of La0.6Sr0.4FeO3–δ (LSF) thin film electrodes was investigated by electrochemical impedancespectroscopy. Different states were considered, ranging from pristine films right after deposition to films degraded under variousconditions. Impedance measurements performed inside the pulsed laser deposition chamber (in-situ) revealed the existence of avery facile, low resistive state of as-deposited thin films with up to two orders of magnitude lower resistances compared toconventional ex-situ measurements and literature data. Ex-situ and in-situ measurements show different pO2

dependencies ofpolarization resistances, suggesting a different oxygen exchange reaction mechanism on the as-deposited films. The timedependence of the polarization resistance indicates that two degradation mechanisms with different timescales are responsible forthe much higher resistances found in ex-situ measurements. Degradation mechanisms are discussed and conclusions with respect tothe rate determining step of oxygen exchange on different surfaces are drawn.© 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open accessarticle distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/1945-7111/abac2b]

Manuscript received July 28, 2020. Published August 14, 2020.

Supplementary material for this article is available online

Solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells(SOECs) are promising devices for energy conversion, due to theirhigh efficiency and power density as well as fuel flexibility. Thelimiting factor for the performance of SOFCs is often the oxygenreduction at the cathode, and current research aims at finding materialsthat exhibit sufficiently fast oxygen reduction kinetics even atintermediate temperatures (500 to 600 °C °C) while also exhibitinglong term stability. Mixed ionic and electronic conducting (MIEC)perovskite-type oxides are among the most promising candidates forhigh performance electrode materials in SOFCs and SOECs.1–10

Various compositions, for example (La,Sr)MnO3–δ (LSM),11–14

(La,Sr)(Co,Fe)O3–δ (LSCF)15–39 and (Ba,Sr)(Co,Fe)O3–δ (BSCF)

40–45

are studied. Oxygen exchange kinetics of such materials are ofteninvestigated by electrochemical methods such as impedancespectroscopy20,25,46 or conductivity relaxation47 and isotope exchangeexperiments.26 Despite extensive research, reported values for kineticparameters (i.e. oxygen exchange coefficients) vary widely, often byseveral orders of magnitude. Additionally, many of those materialsalso suffer from limited long term stability, i.e. degradation of theelectrode kinetics, which not only hampers industrial application butalso complicates basic research as it convolutes the material’s inherentoxygen exchange kinetics with the influence of thermal and chemicalhistory.

The surface chemistry of such perovskites has been studied byvarious surface sensitive analytical techniques such as X-rayphotoelectron spectroscopy (XPS),30,48,49 secondary ion mass spec-trometry (SIMS)26,50 or low energy ion scattering (LEIS).33 Srenrichment on the surface was found to be responsible for a decreaseof electrode kinetics over time, and recent studies could directlyobserve the adverse effect of Sr decoration on MIEC surfaces.51

Furthermore, various catalyst poisons such as Si, SO2 or Cr werefound to cause increased degradation of the surface resistance,especially in the presence of humidity.47,49,52,53

In-situ impedance spectroscopy during electrode depositionoffers great potential for an accurate investigation of MIEC thinfilm electrochemical properties.51 Impedance measurements insidethe pulsed laser deposition vacuum chamber, immediately after thinfilm deposition give access to the kinetics of the electrode in its

pristine state, and thus reflect the inherent material kinetics. Byexposing the freshly deposited sample to potentially degradingenvironments inside the deposition chamber and monitoring theimpedance, their effect can be investigated in a selective andcontrolled way. Direct and real time monitoring of the impedanceresponse to potential degradation sources also allows separating theirtrue effect on the oxygen exchange kinetics from effects due to theoften quite substantial sample variation found in ex-situ studies.

This approach of in-situ impedance spectroscopy during pulsedlaser deposition (IPLD) was used to investigate the oxygen exchangeon La0.6Sr0.4FeO3–δ thin films. Exposing the pristine films todifferent conditions and comparing the results with ex-situ measuredproperties revealed the inherent LSF properties in the pristine stateand their striking variation depending on the sample history.Additional surface chemistry analysis (XPS) indicated chemicalreasons behind the kinetic changes.

Experimental

Sample preparation.—Yttria stabilized zirconia (YSZ) singlecrystals (5× 5× 0.5 mm3 in (100) orientation), polished on the flatsides, were used as electrolyte substrates. Platinum current collectorgrids (35/15 μm mesh/strip width, 100 nm thickness) were preparedon both sides by lift-off photolithography and magnetron sputtering(BAL-TEC MED 020). A 5 nm Ti layer was deposited between YSZand Pt to improve adhesion of the Pt grid. On one square surface, alow resistive, porous La0.6Sr0.4CoO3–δ (LSC) thin film (approxi-mately 300 nm) counter electrode was deposited on top of the Pt gridby pulsed laser deposition (PLD).54 Pt and LSC deposited on thesamples edges were removed by grinding to eliminate parasiticcurrent paths. A dense La0.6Sr0.4FeO3–δ (LSF) working electrodewas deposited on the other side on top of the Pt grid and immediatelyafter this deposition the in-situ EIS measurements were started, seeIn-situ impedance spectroscopy. Additionally, symmetrical sampleswith LSF thin films deposited on both sides were prepared for ex-situ characterization.

PLD targets were prepared by Pechini syntheses. La2O3, SrCO3

and Co/Fe (all Sigma Aldrich, 99.995%) were dissolved in nitricacid and citric acid was added in a molar ratio of 1 with respect tothe total amount of cations. The solution was then heated andevaporated until self ignition and combustion took place, and theobtained powder was calcined in air at 850 °C for 12 h. Targets werepressed isostatically (150 MPa) and sintered in air at 1200 °C forzE-mail: alex.schmid91@gmail.com

*Electrochemical Society Member.**Electrochemical Society Fellow.

Journal of The Electrochemical Society, 2020 167 124509

12 h. Phase purity of the targets was confirmed by X-ray diffraction(XRD). Thin film deposition was done using a KrF excimer laser(Complex Pro 201F, 248 nm). Deposition parameters for the LSCcounter electrodes and LSF working electrodes are listed in Table I.Figure 1 shows a sketch of the sample geometry and a cross-sectionof a dense LSF film. The resulting LSF thin films are polycrystallineand exhibit columnar grain growth, with a preferential orientation ofthe (100) direction perpendicular to the substrate. Average grainsizes are in the range of 20 to 40 nm. Similarly produced thin filmswere already described in Ref. 25.

In-situ impedance spectroscopy.—Samples with Pt current col-lectors on both sides and a porous LSC counter electrode on one sidewere prepared as described in Sample preparation. These were thenmounted in the in-situ PLD setup displayed in Fig. 2a. A thin fused silicadisk was placed between sample and heater to isolate the sample fromthe heater. The sample was placed on a platinum sheet to contact thecounter electrode, and an alumina disk with a cut-out was used to holdthe sample in position. A second thin alumina disk with a smaller cut-outwas used as a mask to ensure that LSF is only deposited on a definedarea on the flat surface of the substrate and to prevent deposition at theedges of the sample. The top side of the substrate was contacted on thePt current collector grid via a platinum needle. Dense LSF thin filmworking electrodes were deposited onto the samples with the parametersdescribed in Table I. Immediately after deposition the sample wascharacterized by impedance spectroscopy either at deposition pO2

(0.04 mbar) or at 0.25 mbar or 1 mbar. If the pO2 was adjusted, achange in temperature resulted due to modified heat transfer conditions.Hence, also the temperature was adjusted to reach 600 °C again. An acvoltage of 10 mV was used in impedance measurements with afrequency range of 1MHz to 0.1 Hz and each frequency point wasmeasured for at least one second or one period.

Time dependency—To investigate the time dependency of theresistance degradation inside and outside the PLD chamber, LSFthin film electrodes were prepared and characterized by in-situimpedance spectroscopy as described above. Samples were thenmeasured in an ex-situ setup (see Ex-situ impedance spectroscopy)for several hours. Afterwards, samples were transferred back to thedeposition chamber, the PLD chamber was evacuated to 10−5 mbar,the sample was heated to measurement temperature, the pO2 andtemperature were adjusted and the impedance was again measured.

Investigating potential degradation sources—After film deposi-tion, impedance spectra were measured for 2 h to determine theelectrode polarization resistance of the freshly deposited LSF filmsand estimate their degradation rate. Samples were then exposed tothe different potentially degrading sources listed in Table II for 2 hand afterwards impedance spectra were again measured for 2 h atdeposition conditions. All these impedance measurements wereperformed in 0.04 mbar O2 at 600 °C, except for the humidity test,which was done in 0.25 mbar O2.

Ex-situ impedance spectroscopy.—Ex-situ impedance measure-ments were done in the setup displayed in Fig. 2b. Electrodes werecontacted by clamping the sample between two platinum sheets withadditional Pt meshes in between for better contacting. The spring

loaded middle tube ensured good contact by pressing the sample/contacting sheet stack against the fixed inner tube. The inner tubealso served as a gas inlet close to the sample. An S-type thermo-couple was placed close to the sample for temperature measurement.These setups were placed in an outer tube and sealed by a KF flangewith a Viton O-ring sufficiently far from the hot zone of the tubefurnace to remain at room temperature. Different measurementsetups made from either fused silica or alumina were used.

XPS measurements.—The surface chemistry of LSF films wasinvestigated by X-ray photoelectron spectroscopy. Measurements weredone on as-deposited samples as well as after 24 h of ex-situ impedancemeasurement at 600 °C in 0.25 mbar O2. Additionally, samples weretransferred back to the PLD chamber after ex-situ measurements,annealed in the PLD chamber for 4 h at 600 °C and then measured byXPS. Samples were transferred to the XPS measurement setup under Aratmosphere. XPS measurements were carried out at room temperature ina UHV chamber by SPECS, Germany, using a monochromated Al K-alpha source (XRC-125 MF, SPECS) operated at 80W, and an angleresolved photoelectron analyser (SPECS POHIBOS WAL), collectingphotoelectrons at emission angles from 20° to 80° from the surfacenormal. Survey spectra were recorded at 100 eV analyser pass energy,

Table I. Deposition parameters for porous LSC and dense LSF thinfilms.

LSC LSF

Substrate temperature 450 °C 600 °COxygen pressure 0.4 mbar 0.04 mbarSubstrate to target distance 5 cm 6 cmLaser pulses 9000 9000Laser fluence 2 J cm−2 2 J cm−2

Pulse frequency 5 Hz 5 HzFilm thickness 300 nm 150 nm

Figure 1. (a) Sketch of the sample geometry used for impedance spectro-scopy. (b) Cross section secondary electron microscopy image of an LSFfilm on top of the YSZ substrate.

Journal of The Electrochemical Society, 2020 167 124509

and detailed spectra for peak fitting were recorded at 50 eV pass energy.Peak fitting and quantification was carried out with CasaXPS software,using S-shaped “Shirley” background functions, and compositionalanalysis was based on the peak areas and Scofield cross sections.55

Mixed Gaussian-Lorentzian peak shapes were used for the Sr surfaceand Sr bulk components. The Sr3d 3/2 and Sr3d 5/2 multiplet peakswere constrained to an energy difference of 1.7 eV, an area ratio of 2:3,and equal FWHM.

Results

Impedance analysis.—Figure 3 displays impedance spectrameasured inside the PLD chamber (in-situ) and in the ex-situ setup

for different oxygen partial pressures. All spectra exhibit a highfrequency offset and a large semicircle at low frequency, addition-ally a small shoulder is visible in the intermediate frequency range.The high frequency intercept is mainly caused by the ionic transportresistance in the YSZ electrolyte and a small contribution from thewiring. Due to the strong temperature dependence of the YSZconductivity the electrolyte resistance measurement enables accuratedetermination of the sample temperature, cf. section 1 of thesupporting information (available online at stacks.iop.org/JES/167/124509/mmedia).

The large semicircle is attributed to the oxygen exchangereactions at the working and counter electrode in parallel to theirchemical capacitances, respectively.25,56 Since the porous LSCcounter electrode has much faster oxygen exchange kinetics, andthus a much lower polarization resistance, the low frequencyimpedance response of the cell is largely determined by theimpedance of the dense LSF working electrode film.51 The inter-mediate frequency shoulder can be attributed to an interfacialresistance and capacitance at the LSF∣YSZ interface.45 The resis-tance of this intermediate frequency feature contributes only margin-ally to the total electrode resistance. In the following, we thereforeconsider the entire electrode resistance, i.e. the sum of the smallintermediate and the dominant low frequency resistance. The LSFsurface above the current collector grid is inactive with respect tooxygen exchange due to the high in-plane oxygen ion transportresistances.25,57 The electrode resistance was thus normalized to theactive LSF surface area (above the YSZ), which is approximately51% of the total LSF surface area. This is true regardless of weatherthe electrodes are contacted by clamping the sample between two

Figure 2. (a) In-situ setup for impedance spectroscopy inside the PLDchamber, consisting of: ablation target (1), sample (2), alignment pins (3),alumina shadow mask (4), sample holder (5), Pt contact sheet (6), silicainsulation disk (7), resistive heater (8), contacting needle (9), impedanceanalyser (10). (b) Ex-situ setup consisting of: Sealed outer tube (1), springloaded middle tube (2), thermocouple (3), gas inlet (4), inner tube (5),impedance analyser (6), spacer (7), Pt contacting sheet (8), sample (9).

Table II. Potential sources of resistance degradation investigated by exposing a freshly deposited LSF film inside the deposition chamber.

Abbreviation Description

High pO2Fill chamber with 50 mbar O2 (Alphagaz, 99.995%)

Amb. air Fill chamber with 50 mbar ambient airT-cycle Cool sample to 25 °C and reheat in 0.04 mbar O2 (15 °C min−1)T-cycle in air Cool sample to 25 °C and reheat in 1 bar ambient air (15 °C min−1)Humidity Add 6.25 μbar H2O to the chamber, measured in 0.25 mbar O2

Figure 3. Impedance spectra of LSF thin films at 600 °C measured insidethe PLD chamber (“in-situ”) and in an “ex-situ” impedance measurementsetup. Markers indicate measured data points, lines are fits to an equivalentcircuit consisting of two R∣∣CPE elements and a resistor in series. The insetsshow magnifications of the high frequency region.

Journal of The Electrochemical Society, 2020 167 124509

platinum sheets, or by placing the sample on a platinum sheet andcontacting the other electrode by a needle. This is described in moredetail in section 2 of the supporting information.

In-situ and ex-situ polarization resistances.—At the depositionpressure of 0.04 mbar O2 an initial electrode resistance of17.5 ± 3.1 Ω cm2 is found for five samples. Hence, the measuredresistance is surprisingly reproducible and very low, keeping in mindthe low oxygen partial pressure, see also further comparison below.Unfortunately, this low pO2 was not reached in the ex-situ setup. Adirect comparison was thus performed at a somewhat higher oxygenpartial pressure. Figure 4a displays the area specific electroderesistances in 0.25 mbar O2 measured in-situ on seven differentsamples and ex-situ on ten samples. These resistances are again the

initial values, i.e. measured immediately after deposition for in-situmeasurements, or right after reaching a stable temperature in thecase of ex-situ measurements. Most obvious is the almost two ordersof magnitude lower polarization resistance found in-situ. For reasonsyet unknown, these in-situ values scatter more than those at0.04 mbar O2.

In the following we compare these in-situ and ex-situ results withliterature data on surface exchange coefficients of LSF and similarmaterials (all ex-situ), see also Table III. Unfortunately, experi-mental data at our measurement conditions of 600 °C and 0.04 to1 mbar O2 are scarce. Thus, for the sake of comparison we may takesurface exchange coefficients measured at slightly higher tempera-tures or partial pressures and extrapolate them to our conditionsusing the activation energies and pO2 dependencies given in therespective papers.17,25,36,46,58,59 Such a comparison shows that theex-situ measured surface exchange coefficients of our thin films arehigher than those of bulk samples by a factor of 7.5, and higher thanthose of LSF thin films by a factor of 101 to 5× 106, respectively.Experimental data at our conditions, however, are available forLa0.6Sr0.4CoO3–δ (LSC) or La0.6Ba0.4CoO3–δ (LBC) thin films.These show comparable (LSC) and slightly higher (LBC) surfaceexchange coefficients, despite LSC and LBC commonly beingcommonly considered much more catalytically active than LSFwith regards to oxygen exchange.34 Our in-situ measured surfaceexchange coefficients are even higher than those measured ex-situand consequently they far exceed the ex-situ measured exchangecoefficients reported in literature (by factors between 3× 101 and2× 106). Therefore, we do not face a problem of low performanceex-situ films, but rather of outstandingly fast in-situ films.

A further peculiarity of our pristine films becomes obvious fromthe pO2 dependency. Under ex-situ conditions the polarizationresistance of LSF shows a distinct decrease with increasing pO2.Figure 4b displays the pO2 dependence for one individual film andthe initial ex-situ resistances obtained for ten samples in 0.25 mbarand six samples in 1 mbar O2. These pO2 dependencies are also inaccordance with those reported in literature for LSF and otherperovskites.34,36,46,60 However, when measured in-situ no clear pO2

dependence is found for 5 samples in 0.04 mbar, seven samples in0.25 mbar and four samples in 1 mbar O2. Deviations betweendifferent oxygen partial pressures are within the variability betweendifferent samples. This change of the pO2 dependence might indicatea switch of the oxygen incorporation mechanism upon transferringthe sample to the ex-situ setup.

Altogether, these results strongly indicate, that the electroche-mical properties of pristine, as-deposited thin film MIEC electrodesare drastically different compared to the properties investigated incommon ex-situ measurements, not only in our laboratory but ingeneral. The highly active state found on pristine, as-depositedsamples far surpasses, to the best of our knowledge, any exchangecoefficients reported in literature for comparable LSF thin filmelectrodes. Without the possibility of such in-situ impedancemeasurements one might consider the initial ex-situ value as theoptimum of surface exchange and does not realize that it alreadyrefers to a severely degraded surface. In the following we examinemore closely the drastic performance decrease upon transfer of thesamples from the deposition chamber to the ex-situ setup andinvestigate potential causes of this effect.

Time dependency.—Figure 5 shows the time dependency of theLSF polarization resistance degradation in-situ and ex-situ; thestriking increase in resistance upon transfer from in-situ to ex-situis clearly visible. Both in-situ and ex-situ the polarization resistancedegrades with time at a moderate rate. However, between the in-situand ex-situ measurements, i.e. upon transfer of the sample to the ex-situ setup, there is a drastic increase of the resistance at a much fasterrate than the continuous degradation mentioned before. Please notethat the sample shown in Fig. 5 is the outlier in Fig. 4. In contrast toall other data points in Fig. 4 this ex-situ resistance was measuredafter the sample had already been investigated in the PLD for 2 h

Figure 4. Area specific LSF thin film electrode resistances at 600 °Cdetermined by impedance spectroscopy. Measurements were done insidethe deposition chamber immediately after thin film deposition (“in-situ”) andin an ex-situ impedance spectroscopy setup. (a) Polarization resistances ofseveral different samples at 0.25 mbar O2. (b) Area specific resistances at600 °C and in different pO2

. The filled boxes represent the first and thirdquartiles, the median is represented by a solid line. Whiskers indicatemaximum and minimum resistances measured, outliers are indicated bycircles and the numbers in/below the boxes represent the number ofinvestigated films. Markers indicate a pO2

sweep measurement performedon a single sample, the line represents a fit of these resistances to a powerlaw.

Journal of The Electrochemical Society, 2020 167 124509

which partly explains the higher resistance. However, its polariza-tion resistance is still lower than many reported in literature. Uponsample transfer back to the in-situ setup in the PLD chamber adecrease in polarization resistance could be observed. However, theinitial low resistances were not reached, i.e. the fast degradation ispartially but not completely reversible on this time scale.

Based on the data we assume that two distinct degradationmechanisms with different characteristic time scales are at workhere: a continuous (slow) one and a fast one associated with thetransfer to ex-situ measurement conditions. Resistance degradationof LSF and related perovskites on the time scale of hours and days isoften linked to Sr segregation to the surface.33,38,48,49,51,54 This isalso consistent with our XPS measurements (see below), where wefind an increase of surface Sr with time. We thus suggest that theslower, continuous degradation observed both in-situ and ex-situ iscaused by Sr segregating to the LSF surface. The much faster, large

resistance increase upon transfer from in-situ to ex-situ setups,however, needs further consideration also because the comparisonwith literature values revealed that such a degradation is not specificto our samples but a common feature of essentially all studies we areaware of. Several potential causes were investigated.

Potential degradation sources.—The effects of different potentialdegradation sources on the LSF polarization resistance are shown inFig. 6. All experiments are performed in the pulsed laser depositionchamber, i.e. in the in-situ setup and resistances refer to 600 °C and0.04 mbar O2 (deposition conditions), except the humidity test whichwas conducted in 0.25 mbar O2. For each experiment the resistancebefore and after exposure to a potential degradation source is shown,measured both initially and after 2 h, respectively. One can clearly seean increase of the polarization resistance with time before, during andafter the exposure period. This increase is, however, within the rangeexpected from extrapolating the resistance degradation of the asdeposited samples. More importantly, it is much less than the drasticdifference in resistance between in-situ and ex-situ setup, and in somecases the exposure even slightly slows down the resistance degradation.Therefore, the discrepancy between in-situ and ex-situ measurementscannot be explained by a surface modification of the as-deposited filmsthrough exposure to ambient air, high pO2, temperature cycles, humidityor a combination thereof. These measurements also reveal that highlyactive LSF surfaces not only exist for pristine films under depositionconditions but can sustain several thermal and chemical treatments,except transfer into the ex-situ setup.

Additionally, silicon contamination from the ex-situ setup (fusedsilica) was considered as a degradation source, since silicon isknown to be a potent catalyst poison for mixed conducting oxygenelectrodes.61–63 To investigate the influence of the material used inthe ex-situ sample holder, two ex-situ measurement setups (seeFig. 2b) were fabricated that differ only by the tubes and spacersmade either of fused silica or alumina. Both ex-situ setups led todrastically higher polarization resistances compared to in-situmeasurements, and differences between samples measured in eitherof them were within the sample variance. Moreover, XPS measure-ments showed absence of Si signals on all samples. Siliconpoisoning from the ex-situ setup can thus also be ruled out as fastdegradation source.

Lastly, the effect of total pressure and the presence of nitrogenwas investigated. Measurements were performed on as-depositedfilms inside the PLD chamber in an atmosphere consisting of 1 mbarO2 and 900 mbar N2. These also showed high surface activity anddid not differ significantly from measurements in pure O2 at 1 mbar.Accordingly, all these potential degradation sources do not affect thepolarization resistance in the drastic manner found for the exposureto ex-situ measurement conditions.

Table III. Surface exchange coefficients kO of La0.6Sr0.4FeO3–δ bulk (B) and thin film (TF) samples. For comparison surface exchange coefficientsare also extrapolated to our measurement conditions of 600 °C and 1 mbar with the activation energies and partial pressure dependencies given inthe sources. LSC and LBC exchange coefficients are included to further highlight the exceptional surface exchange coefficient of our in-situ films.

T pO2kO kO extrapolated Sample

(°C) (mbar) (cm s−1) (cm s−1)

ten Elshof et al.36 650 1 1 × 10−8 4 × 10−9 BSøgaard et al.58 800 1 2 × 10−7 2 × 10−9 BSøgaard et al.59 600 40 1 × 10−10 3 × 10−11 TFMosleh et al.46 725 25 3 × 10−10 6 × 10−14 TFKogler et al.25 600 210 2 × 10−7 3 × 10−9a) TFBaumann et al.17 600 210 5 × 10−9 1 × 10−10a) TFRupp et al. LSC34 600 1 2 × 10−8 2 × 10−8 TFRupp et al. LBC34 600 1 1 × 10−7 1 × 10−7 TFThis study ex-situ 600 1 3 × 10−8 3 × 10−8 TFThis study in-situ 600 1 1 × 10−7 1 × 10−7 TF

Note.a) No pO2dependency was reported in the paper, thus the extrapolated exchange coefficients were estimated with our (ex-situ) measured pO2

dependence, i.e. k pO O0.73

2µ .

Figure 5. Time dependence of the polarization resistance of an LSF thinfilm at 600 °C and 0.25 mbar O2. Measurements were done inside the PLDchamber after deposition and in an ex-situ setup. After the ex-situmeasurement the sample was transferred back into the PLD chamber andmeasured again. The inset shows a magnification of the first in-situmeasurement phase.

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Surface composition characterization.—The surface composi-tion of LSF thin films was determined by XPS. The Sr region of thespectra was examined more closely (see Fig. 7a) as Sr is known toplay a critical role in the resistance degradation of perovskites. Inaccordance with literature, two different species (surface and bulk)can be identified in the Sr 3d binding energy range.30,48 An increaseof surface Sr was found on samples with thermal history (at least24 h at 600 °C in the ex-situ setup) compared to the as-depositedfilms, indicating a segregation of Sr to the LSF surface duringannealing, see Fig. 8, bottom. Similar Sr segregation is known forrelated perovskites and is often linked to resistance degradation ofperovskite electrodes.33,48,54 Additional annealing of the samples inthe PLD chamber for four hours after the ex-situ measurement leadsto a slight increase in the total Sr signal, but not in the fraction ofsurface to bulk Sr. The polarization resistance became even lower inthe in-situ setup. Hence, this Sr segregation can only explain thestandard slow degradation but not the huge resistance increase fromin-situ to ex-situ setups.

However, on samples exposed to the ex-situ setup, considerableamounts of sulfur were found, see Fig. 7b, and the binding energy ofthe S 2p peak corresponds to an oxidation state of SVI, suggestingSO4

2- ions.64 Samples that were transferred back to PLD chamberand annealed there showed a reduced amount of sulfur. Accordingly,the presence and amount of sulfur on the surface strongly correlatewith the electrochemical measurements. No sulfur is found on theas-deposited samples and correspondingly these exhibit very lowpolarization resistances. Samples measured in the ex-situ setup showa large amount of sulfur on the surface and correspondingly, theirpolarization resistance is drastically increased. Upon transferringback to the PLD chamber and annealing there, both the sulfurcontent on the surface and the polarization resistance decrease.Besides sulfur, no other surface contaminants (e.g. chromium orsilicon) could be detected by XPS.

Mechanistic Discussion of Degradation and pO2 Dependency

Based on the electrochemical and surface composition analysis ofLSF thin film electrodes in-situ and ex-situ, we propose that also

chemically two degradation mechanisms with different time con-stants are at work. First, there is a segregation of Sr to the surface,either as a SrO phase or simply as a SrO terminating layer on LSF.The Sr segregation causes the slower degradation observed both in-situ and ex-situ. This is a continuous process, possibly accelerated byhigh pO2, and takes place on the time scale of hours and days at 600 °C. Second, there is a much faster degradation caused by the rapidformation of a sulfate (most likely SrSO4) at the surface under ex-situ conditions. The latter is partially reversible by transferring thesample back to the PLD chamber and annealing there. Sulfurpoisoning of perovskite oxide electrodes or catalysts is also reportedin literature.47,52,53,65 However, such poisoning experiments are

Figure 6. Polarization resistance of freshly deposited LSF thin filmelectrodes at 600 °C measured inside the PLD chamber before and afterexposure to different potentially degrading conditions. Sample “Humid” wasmeasured in 0.25 mbar O2, all other samples were measured at the depositionpressure of 0.04 mbar O2. The values above the bars indicate the resistancerelative to the initial (pristine) resistance.

Figure 7. Sr 3d (a) and S 2p (b) regions of X-ray photoelectron spectra ofLSF thin films with different histories. The “ex-situ” film was exposed to ex-situ measurement conditions for 24 h (in 0.25 mbar O2). “returned to in-situ”indicates the same sample after another 4 h in the PLD setup at 0.25 mbar O2

and 600 °C.

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often conducted by enriching the gas phase with appreciableamounts of a sulfur source (often SO2, 1 to 100 ppm), whereas inour nominally sulfur free ex-situ setup high purity gases (99.999%)are used and great care is taken to ensure a high purity measurementsetup. Inductively coupled plasma mass spectrometry (ICP-MS)measurements showed the sulfur concentration in our gases to bebelow 0.5 ppm. This indicates, that even minute traces of sulfur inthe measurement atmosphere can lead to considerable formation ofsulfur on the MIEC surface, leading to drastically increasedpolarization resistances.

It is also worth again emphasizing that the sulfur poisonedelectrodes of our study still exhibit polarization resistances that areamong the most active LSF films reported so far. Hence, we areconfident that we do not face a specific problem of our laboratory,but a rather common effect and thus a general problem (seepolarization resistances above). Without sulfur poisoning (achievedin-situ) electrodes can exhibit extremely low polarization resis-tances. It is yet unclear why the sulfur does not affect samples in thedeposition chamber. The surface of the deposition chamber remainsat room temperature and covered with a porous mix of many oxides(e.g. CaO, SrO, La2O3, ...), which strongly bind to sulfur compoundssuch as SO2 or H2S and thus may act as a getter for sulfurcompounds. The absence of the sulfur related degradation in thePLD chamber (in-situ) might thus be due this “getter” effect of thechamber. Great experimental efforts seem to be required to ensurethat a pristine MIEC surface is probed and inherent materialproperties are observed.

Finally, we briefly discuss the mechanistic conclusions that maybe drawn from the different pO2 dependencies found for pristine (in-situ) and conventional (ex-situ) LSF surfaces. In Ref. 66 wediscussed in detail that pO2 dependencies of oxygen surfaceexchange coefficients are an interplay between defect and adsorbateconcentrations which themselves both depend on pO2. The strongdecrease of the polarization resistance with increasing pO2 found inour ex-situ measurements and in many papers is a strong hint thatadsorbates (possibly their dissociation) play a decisive role in therate determining step. For the in-situ measurements we find the veryuncommon case that the polarization resistance is either pO2

independent or exhibits a minimum (could not be resolved due to

data scattering). In any case the increase of pO2 does not trulyaccelerate the oxygen exchange. This points toward involvement ofa defect the concentration of which decreases with increasing pO2,i.e. to oxygen vacancies as decisive species. Possibly the accel-erating effect of more adsorbates is counter balanced by the decreaseof oxygen vacancy concentration and thus it is oxygen incorporationinto a vacancy rather than oxygen dissociation which is rate limitingfor pristine films. This would mean that the (sulfur) degradationultimately deactivates oxygen dissociation sites and thus changes therate limiting step from oxygen incorporation to oxygen dissociation.However, further measurements are needed to support this model.

Conclusion

The polarization resistance of LSF thin film electrodes wasstudied by electrochemical impedance spectroscopy, both in-situinside the PLD chamber immediately after deposition and ex-situ ina conventional measurement setup. These measurements revealedexceptionally low polarization resistances of the pristine, in-situmeasured films, not only compared to our ex-situ data but alsocompared to literature. To the best of our knowledge, the pristine, in-situ measured samples far outperform any comparable LSF thin filmelectrodes reported in literature. Moreover, a different pO2 depen-dence of the polarization resistance was found under in-situconditions, with very little pO2 dependence, suggesting that theoxygen exchange reaction mechanism is different on as-depositedfilms compared to ex-situ measured samples. Possibly, in-situsamples still have highly active oxygen dissociation sites and oxygenincorporation into vacancies is rate limiting while for ex-situsamples oxygen dissociation is decisive.

Time dependent degradation measurements revealed two distinctdegradation phenomena with different timescales. A continuousslow degradation was observed both in-situ and ex-situ. Thiscontinuous degradation correlates with an enrichment of Sr on thesurface as determined by XPS and is most likely caused by Srsegregation. Additionally, a fast degradation occurred upon transferto the ex-situ setup. Several potential sources of this secondphenomenon, such as exposure to high pO2, humidity or ambientair, could be excluded as potential reasons. However, XPS studiesrevealed the formation of sulfates under ex-situ conditions, despiteall measures to avoid sulfur contamination sources. Hence, the fastdegradation upon transfer to the ex-situ setup seems to be linked totraces of sulfur contamination and might also be the reason behindincreased (ex-situ) polarization resistances of many other studies inliterature.

Acknowledgments

The authors gratefully acknowledge funding by Austrian ScienceFund (FWF) project P4509-N16 and W1243-N16. This project hasreceived funding from the European Union’s Horizon 2020 researchand innovation programme under grant agreement No 824 072.

ORCID

Alexander Schmid https://orcid.org/0000-0002-4457-4730Alexander Opitz https://orcid.org/0000-0002-2567-1885

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