Journal of The Electrochemical Society 162 (3) F317-F326 (2015) F317

Comparison of Electrochemical Properties of La06Sr04FeO3-δ ThinFilm Electrodes Oxidizing vs Reducing ConditionsSandra Kogler Andreas Nenning Ghislain M Rupp Alexander K Opitz and Jurgen Fleiglowastz

Institute of Chemical Technologies and Analytics Vienna University of Technology 1060 Vienna Austria

Owing to its mixed ionic and electronic conductivity and high thermochemical stability La06Sr04FeO3-δ (LSF64) is an attractiveelectrode material in solid oxide fuelelectrolysis cells (SOFCsSOECs) Well defined thin film microelectrodes are used to comparethe electrochemical properties of LSF64 in oxidizing and reducing conditions The high electronic sheet resistance in hydrogen canbe overcome by the use of an additional metallic current collector With the sheet resistance being compensated the area specificelectrode resistance is similar in humidified hydrogen and oxygen containing atmospheres Analysis of the chemical capacitance andthe electrode resistance for current collectors on top and beneath the LSF64 thin film allow mechanistic conclusions on active zonesand bulk defect chemistry Cyclic gas changes between reducing and oxidizing conditions performed on macroscopic LSF64 thinfilm electrodes with top current collector reveal a strong degradation of the surface kinetics in synthetic air with very fast recoveryin reducing atmosphere Additional in-situ high-temperature powder XRD on LSF64 demonstrates the formation of small amountsof iron oxides in humidified hydrogen at elevated temperaturescopy The Author(s) 2015 Published by ECS This is an open access article distributed under the terms of the Creative CommonsAttribution Non-Commercial No Derivatives 40 License (CC BY-NC-ND httpcreativecommonsorglicensesby-nc-nd40)which permits non-commercial reuse distribution and reproduction in any medium provided the original work is not changed in anyway and is properly cited For permission for commercial reuse please email oaelectrochemorg [DOI 10114920731503jes]All rights reserved

Manuscript submitted September 29 2014 revised manuscript received December 23 2014 Published January 6 2015

Solid oxide fuel cells (SOFCs) are in the process of gaining moreand more commercial success as highly efficient power generation sys-tems They transform the chemically bound energy of a fuel to electri-cal energy Solid oxide electrolysis cells (SOECs) are the counter partsof SOFCs as they use electrical energy eg excess energy from thegrid to form fuel by electrolysis Owing to their very high efficienciesalso SOECs are promising devices in future energy technologies1

Currently NiYSZ cermet electrodes are the standard electrodes inSOFECs for reducing conditions but they are known to suffer fromseveral problems like sulfur poisoning (in SOFC operation) sinter-ing redox cycle stability etc2 To find an alternative to NiYSZ couldtherefore be favorable for both SOFCs and SOECs

Electrodes in SOEFCs have to meet numerous requirementsthermochemical stability over a wide oxygen partial pressure rangehigh catalytic activity for oxygen exchange reactions compatibil-ity with adjacent cell components sufficiently high electronic andionic conductivity etc Perovskite-type oxides such as LaMnO3LaCoO3 and LaFeO3 based materials are used in state-of-the-artSOFEC electrodes in oxygen atmosphere Often they are mixedionic and electronic conductors (MIECs) which is advantageous inSOFECs as the whole electrode surface area may become activein the oxygen exchange reaction Employing such MIECs also inreducing atmosphere could be highly attractive However Sr-dopedLaMnO3 and LaCoO3 are only stable under comparatively high oxy-gen partial pressures and thus not suited for the use in hydrogen3ndash5

Therefore generally other compositions such as (LaSr)(CrMn)O36ndash9

(LaSr)(CrV)O31011 (LaSr)(CrRu)O31213 or La and Fe dopedSrTiO3

1415 are tested under reducing conditions16

Although Sr-doped LaFeO3-δ (LSF) is a typical SOFC cathodematerial it is also rather stable in reducing conditions17ndash19 withLa06Sr04FeO3-δ decomposing below 10minus27 bar pO2 at 600C19 More-over its thermal expansion coefficient matches acceptably well withtypical electrolytes in SOFECs1920 Several studies were performedon the defect chemistry and transport properties of LSF 21ndash31 and leadto the following defect chemical picture Substitution of La3+ by Sr2+

acts as acceptor doping and causes both a partial valence change ofiron from Fe3+ to Fe4+ and the formation of oxygen vacancies1932 InKroger-Vink notation this is written as

Sr FeO3LaFeO3minusminusminusminusminusrarr Sr prime

La + 3OxO + Febull

Fe [1]

lowastElectrochemical Society Active MemberzE-mail juergenfleigtuwienacat

1

2O2 + 2Fex

Fe + V bullbullO harr Ox

O + 2FebullFe [2]

Fe4+ states can be interpreted as electron holes (hbull) and those deter-mine the electronic conductivity at high oxygen partial pressure asthey are the majority mobile charge carrier20ndash22

Towards lower oxygen partial pressures (andor higher tempera-tures) the equilibrium of Eq 2 is shifted to the left hand side increasingthe amount of oxygen vacancies and thus the ionic conductivity2324

Concurrently oxygen release causes a reduction of the iron whichdecreases the concentration of electron holes This also increases theelectron concentration (ie Fe2+)19 in accordance with

2FexFe harr Febull

Fe + FeprimeFe [3]

In humidified hydrogen FeprimeFe becomes the predominant electronic

charge carrier Besides concentration and type of electronic chargecarriers also their mobility is affected by the higher amount of oxy-gen vacancies Since the electronic transfer occurs along Fe-O-Fechains a disruption of these chains by oxygen vacancies hinders thehopping of the electronic charge carriers2533ndash35 As a result in reduc-ing atmospheres the electronic conductivity is much smaller than inair2

The decreased electronic conductivity of LSF and other acceptordoped perovskite-type oxides in reducing atmosphere can be a prob-lem for their use as electrode materials as it causes resistive lossesfrom the outer current collector to the reaction site However the lackof electronic conductivity could be compensated by additional currentcollectors In analogy to NiYSZ where YSZ supplies the ionic con-ductivity a metal may be used in combination with an acceptor-dopedoxide such as LSF to provide sufficient electronic conductivity in acomposite electrode

This approach of applying a cermet with an electrochemically ac-tive mixed conducting oxide and a current collecting metal is muchless common than composites of MIECs with a pure ion conductor Arealization with ceria-based thin films and metal current collectors canbe found in Ref 36 and an in-situ XPS study of electrochemically ac-tive zones in ceria films with gold current collectors was presented inRef 37 Also in a recent study on Sr(TiFe)O3 in H2H2O such thinfilm model composites were employed38 A novel electrode geometrywith two interdigital Pt current collectors within a single microelec-trode allowed detailed mechanistic conclusions on the kinetic lossesfrom electronic and ionic charge transport and the H2 oxidationH2Osplitting reaction on such electrodes The impedance response ofmixed conducting thin film electrodes with well-defined patterned

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F318 Journal of The Electrochemical Society 162 (3) F317-F326 (2015)

metal current collectors in a symmetrical cell was also numericallymodeled39 Those studies revealed that such model type experimentsbased on MIEC thin films and current collectors are highly valuablein order to better understand properties and limitations of mixed con-ducting SOFC anodes SOEC cathodes However mechanistic studieson LSF thin film electrodes in H2H2O are rare one study reports itsperformance in a SOEC cell40 A systematic investigation of its elec-trochemical properties in reducing conditions and a comparison withthose in air is still missing

In the present study we therefore investigate the effect of humid-ified H2 and gas changes between reducing and oxidizing conditionson the electrochemical properties of La06Sr04FeO3-δ (LSF64) thinfilms on yttria stabilized zirconia (YSZ) electrolytes Impedance spec-troscopy was used to analyze the resistance related to the surface reac-tion kinetics and the chemical capacitance of thin film microelectrodeswith and without a micro-patterned current collector Additional mea-surements on macroscopic LSF64 thin films showed the impact ofcyclic gas changes on the polarization resistance and the associatedchemical capacitance In-situ high-temperature XRD diffraction pat-terns recorded under humidified H2 at elevated temperatures revealedinformation on the phase stability

Experimental

LSF64 powder was synthesized by Pecchinirsquos41 method Fe2O3

(9999 Alfa Aesar) SrCO3 (ge9999 Aldrich) and La2O3

(ge9999 Sigma-Aldrich) were weighed according to the desiredstoichiometry the compounds were dissolved in HNO3 (65 extrapure Merck) Citric acid (ge999998 Fluka) was added to the solu-tion with 111 molar-ratio of cations The solution was concentratedto form a gel The gel was further heated until self-ignition took placeThe attained powder was calcined for 3 h at 900C and then annealedfor 5 h at 1200C After calcination the powder was grinded andpressed into a pellet using a cold isostatic press The pellet was thensintered at 1250C for 5 h Phase purity was confirmed by powderX-ray diffraction (XRD) (Philips XrsquoPert)

The LSF64 pellet was used as target for pulsed laser deposi-tion (PLD) (100) oriented single crystals of yttria stabilized zirconia(YSZ) (95 mol Y2O3 Crystec Germany) were employed as sub-strate Ablation was done by a KrF excimer laser (Compex Pro 201 F)with 248 nm wavelength and an intensity of 400 mJpulse for 30 min

Figure 1 a) SEM image of a LSF64 thin film after PLD deposition on YSZb) breaking edge of a 420 nm thick LSF64 thin film c) LSF64 thin film afterheating in humidified hydrogen for 70 h at Tcalc = 622C d) a light microscopeimage of a LSF microelectrode with top current collector

Figure 2 Left sketch of a microelectrode with bottom current collector andzoom-in illustrating the current collector geometry right sketch of microelec-trodes and a macroscopic counter electrode on a single crystalline substrate

with 5 Hz pulse repetition rate for microelectrode samples and 45 minfor macroelectrode samples The distance between oxide target andsubstrate during ablation was 6 cm The substrate temperature wasapproximately 650C measured by a Heitronics KT 1999 pyrome-ter The oxygen pressure was 4 middot 10minus2 mbar within the PLD chamberAfter deposition the substrate with the thin film was cooled at a rateof 15Cmin After deposition SEM pictures (FEI TECNAI F20) ofthe surface and a cross section of the thin film showed a pore-freeand dense polycrystalline film of typically 200ndash400 nm thickness (seeFig 1a and 1b) 220 nm and 330 nm thin films were used in theimpedance measurements After heating in humidified hydrogen for70 h at Tcalc = 622C the thin films are still dense however smallcrystallites grow on top of the thin film (see Fig 1c)

For current collection titanium (BAL-TEC) (15 nm) and plat-inum (9995 pure OGUSSA) (100 nm) films were sputter deposited(MED 020 Coating System BAL-TEC Germany) with the titaniumacting as adhesion layer The sputter current was 100 mA with thedistance between sample and target being 6 cm The argon pressurewas set to 7 middot 10minus3 (Ti) and 2 middot 10minus2 mbar (Pt) yielding a sputter rateof 018 nmsec (Ti) and 075 nmsec (Pt) respectively

Two types of samples were prepared Circular LSF64 microelec-trodes (95 and 195 μm diameter 220 nm thickness) with currentcollector on top (see Fig 1d) or beneath (85 μm distance betweenstripes of 115 μm width see Fig 2) and macroscopic LSF64 elec-trodes (5 times 5 mm2 330 nm thickness) with current collecting fingerson top (Fig 3) For the microelectrodes (Fig 2) micro-patterning ofthe LSF64 thin film including current collector was carried out bystandard photolithography and Ar ion-beam etching (tectra GmbHionEtch Sputter Gun) An approximately 15 times 5 mm2 sized LSF64thin film stripe with current collector was used as the counter elec-trode (CE) in impedance measurements Additionally for contactingreasons platinum paste was brushed onto the back side of these sam-ples and connected to the CE stripe Owing to its much bigger sizecompared to the microelectrodes the contribution of the CE to theimpedance spectrum can safely be assumed to be negligible42

In case of the macroscopic samples (Fig 3) porous LSF64 withplatinum paste on top was deposited on the back side of the YSZsingle crystal and used as counter electrode The polarization resis-tance of such a counter electrode was additionally measured in aconventional macroscopic setup on a symmetrical YSZ sample withtwo extended porous LSF64 electrodes A value of ca 11 cm2 inair and 15 cm2 in reducing conditions was found at 683C This is

Figure 3 Sketch of a macroscopic electrode with metallic fingers on top anda porous counter electrode on the back side also a zoom-in with geometricdimensions of the metallic fingers is shown

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Journal of The Electrochemical Society 162 (3) F317-F326 (2015) F319

significantly smaller than the polarization resistance of the macro-scopic LSF64 thin film electrode (gt6 cm2) but is still visible inthe impedance spectra (see section Results and Discussion) It can beidentified by the relaxation frequency which differs from that of thethin film electrode

For electrochemical measurements on microelectrodes the sam-ples were placed on a heating stage (Linkam UK) in a vacuumchamber43 Impedance measurements were carried out under ambi-ent or synthetic air (indicated for each measurement) and H2H2OArHumidification of the 25 H2Ar (Air liquide ARCAL 10) gas wasachieved by slowly bubbling the gas through distilled water at roomtemperature The resulting partial pressure of H2O in the gas was as-sumed to be equal to the vapor pressure of saturated water at ca 22C(sim26 mbar) The electrodes were contacted using PtIr alloy needleswith a typical tip radius of 3ndash8 μm Contact needles were accuratelypositioned by micromanipulators (Karl Suss) under an optical mi-croscope Impedance measurements on macroscopic electrodes werecarried out in a tube furnace (Gero SR 40-20012) under syntheticair and humidified H2H2OAr (see above) Sample contacting wasachieved by Pt wires

Impedance spectra on both sample types were measured in a fre-quency range of 106 to 10minus1 or 10minus2 Hz with an rms of 10 mV (AlphaA impedance analyzer Novocontrol) In case of macroscopic samplesthe temperature was measured by a thermocouple close to the sam-ple In microelectrode measurements43 the asymmetric heating andthe local cooling by the contact tip cause a substantial deviation oftrue electrode temperatures from set temperatures A reasonable es-timate (Tcalc) of the true electrode temperature can be obtained fromthe measured YSZ electrolyte resistance if electrode geometry and theYSZ conductivity vs temperature are known44 Here approximately610Cndash625C are found for a set temperature of 700C

Diffraction patterns were recorded using Cu-Kα radiation with0008 steps and 3 hour dwell from 20 to 80 2 (PANalytical XrsquoPertPro) The powder (approximately 1 g) was placed in the XRK 900reaction chamber (Anton Paar) which allows high-temperature in-situX-ray diffraction In the chamber the powder was placed on a frit witha flux of humidified hydrogen (same as for impedance measurements)The gas flow was already applied 1 h before starting the measurementA total of 29 XRD diffraction patterns in 2 scan geometry wasmeasured while heating the powder to the maximum temperature of800C and cooling it down again in multiple steps A full 2 scan(20ndash80 step size 0008 PANalytical XrsquoPert Pro) was collected ateach step for 1 h or 3 h (at 50C 800C 50C) The long dwell timeand the small angular steps increased sensitivity to compensate thehigh background of the Fe containing samples Peaks were comparedto ICDD-database 01-082-1961 for LSF64 (La06Sr04FeO2796) 00-001-1111 for Magnetite (Fe3O4) 00-002-1180 for Wustite (FeO)00-029-1305 for SrFeLaO4 (A2BO4 Ruddlesten Popper) and 00-057-0088 for LaSrFe2O55 (A2B2O55 Brownmillerite)

Results and Discussion

Electrode impedance under dry oxidizing conditionsmdash Typicalimpedance spectra of LSF64 microelectrodes (195 μm diameter) withand without additional current collector measured under ambient airat a set temperature of 700C (Tcalc = 619C) are shown in Fig 4 Thespectra exhibit a high frequency non-zero axis intercept Ra At inter-mediate frequencies microelectrodes with additional current collectorshow a small depressed semicircle with resistance Rb while micro-electrodes without current collector only exhibit a slight shoulder seeinset of Fig 4a For both types of microelectrodes a dominant semicir-cle is observed at low frequencies with resistance Rc and capacitanceCc

In accordance with literature44ndash46 the high frequency axis interceptRa is assigned to the ohmic resistance of the electrolyte (and used tocalculate Tcalc)42 At elevated temperatures the electrolyte only causesan intercept instead of a semicircle due to its low capacitance45 Ra

is almost identical for both types of microelectrodes which indicateshigh in-plane electronic conductivity in air see below and Ref 47

Figure 4 a) Impedance spectra of LSF64 microelectrodes (195 μm diame-ter) in ambient air with bottom (open triangles) and without (filled triangles)current collector The electrode temperature Tcalc calculated from the highfrequency offset Ra is 619C for both measurements Right inset zoom ofthe high frequency range of the impedance spectra Left inset simple equiv-alent circuit used to fit the low frequency arc in the impedance spectra b)Schematic picture of active and inactive electrode surface areas for electrodeswith (bottom) current collector

According to literature45 the intermediate frequency feature iscaused by the small interfacial resistance Rb between the LSF64 elec-trode and the YSZ electrolyte Its capacitance Cb describes stoichio-metric changes at the electrodeelectrolyte interface andor chargingof the electrostatic double layer capacitance45 In case of the additionalbottom current collector the interfacial impedance is much more pro-nounced than for the microelectrode without current collector Thismight be a simple consequence of geometry The current collectorcovers approximately 80 of the electrodeelectrolyte interface there-fore the interfacial resistance strongly increases However owing toits minor impact on the DC resistance a more detailed analysis of thiseffect is beyond the scope of this paper Also in the equivalent circuitused to parameterize the spectra the intermediate frequency featureis only considered in terms of Rb while neglecting its capacitance (seeFig 4) The main resistive process at low frequencies (Rc) reflectedby the dominant second arc is attributed to the oxygen exchange atthe surface of the electrode45 In accordance with Ref 45 the parallelcapacitor (represented by CPEc see below) is caused by changes ofthe oxygen content in the electrode bulk (stoichiometry change) andis usually referred to as chemical capacitance

This chemical capacitance Cchem can with the assumption of di-luted charge carriers and the absence of traps be expressed by4849

Cchem = e2

kTAL

(1

zion2ceq

ion

+ 1

z2eonceq

eon

)minus1

[4]

with e denoting the elementary charge k Boltzmannrsquos constant T tem-perature z charge number (zion = 2 zeon = 1) and ceq the concentrationof ionic and electronic charge carriers in equilibrium respectively Inthis paper the chemical capacitance is mostly normalized to the MIEC

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F320 Journal of The Electrochemical Society 162 (3) F317-F326 (2015)

Figure 5 Electrode resistance per area over time for a strongly degrad-ing circular microelectrode without current collector in synthetic air atTcalc = 561C (red) and a circular microelectrode with top current collec-tor (resistance normalized to free surface area) in humidified hydrogen atTcalc = 616C (black)

volume involved given by area A and film thickness L Accordingto Eq 4 the minority charge carrier determines the chemical capaci-tance In case of LSF64 in air the minority charge carriers are oxygenvacancies while electronic charge carriers should determine Cchem un-der reducing conditions19 In the equivalent circuit a constant phaseelement (CPEc) is used instead of an ideal capacitance accounting fornon-ideal semicircles in the spectra Its impedance ZCPE is defined by

ZC P Ec = 1

[Tc( j middot ω)Pc ][5]

with j and ω denoting imaginary unit and angular frequency respec-tively The relation

Cc = Cchem = (R1minusPc

c middot Tc

) 1Pc [6]

is used to calculate the corresponding capacitance where Pc and Tc

are fit parameters50

After the normalization to the entire film volume the chemicalcapacitances of the spectra in Fig 4 have values of 1293 Fcm3 and289 Fcm3 for the sample without and with current collector respec-tively These values are in the order of magnitude expected for Cchem

of MIEC thin film electrodes51 The Pc values of 098 and 094 with-out and with current collector respectively show that the capacitoris almost ideal as expected for a chemical capacitance The areaspecific electrode resistance (ASR) in air deduced from Rc and thetotal electrode area (Atot) is 136 cm2 without current collector and682 cm2 with current collector These values are somewhat lowerthan those reported in Ref 52 (200 cm2 at 610C) possibly due todifferent deposition parameters or thermal history

Several values of electrodes with and without current collectorscale by similar factors inverse ASR (021) chemical capacitance(0221) and free electrode area without current collector (0181) Theratio of the ASR should be taken with some caution since absolutevalues of ASR can be strongly affected by degradation especially inthe case of microelectrodes measured in the asymmetrically heatedset-up43 As an example of a strong degradation process Fig 5 showsthe increase of the main electrode polarization resistance Rc with timeExact reasons for this fast degradation are unknown yet but might berelated to silicon poisoning53 in the microcontact set-up In absence ofgas cycling such a fast degradation was not found in the experiments onmacroscopic LSF64 thin film electrodes performed in a different set-up (see below) In this contribution only freshly prepared samples wereconsidered and we therefore exclude degradation as major cause of

the measured resistance difference The chemical capacitance is muchless prone to time dependent changes and scatter between differentelectrodes

Similarity of all these factors indicates that the MIEC volume abovethe current collector is not polarized and does neither contribute tooxygen exchange nor to the chemical capacitance (see Fig 4b) Thisis not surprising taking into account that in air oxygen vacancies arethe minority charge carrier accordingly ionic conductivity may betoo low to polarize the MIEC volume above the current collectorsee also below When normalizing the ASR of the microelectrodewith current collector to the free area we obtain 123 cm2 whichis in accordance with the 136 cm2 found for the microelectrodewithout current collector A good agreement is also found for thearea corrected chemical capacitance 1606 Fcm3 and 1293 Fcm3 forthe microelectrodes with and without current collector respectivelyThe similarity of the area corrected capacitances indicates a similarpolarized volume

These values of Cchem can further be compared to values calcu-lated from defect chemical literature data and Eq 4 Defect chemicalproperties of thin films may deviate from those of bulk samples54 butthe authors are not aware of any detailed data set describing defectthermodynamics of LSF thin films Therefore comparison is madewith the accurate bulk defect chemical data of LSF64 given by Kuhnet al19 From the defect model of LSF64 (Eqns 1ndash3) the correlationbetween oxygen partial pressure pO2 and nonstoichiometry (3-δ) canbe derived19 It reads

pO2 = 1

16

(minus 1

K 12ox

(2δ minus 04)(3 minus δ)12

(2δ + 06)δ12+

(1

Kox

(3 minus δ)(2δ minus 04)2

δ(2δ + 06)2

+ Ki

Kox

4(3 minus δ)(14 minus 2δ)

δ(2δ + 06)

)12)4

[7]

with

Ki = [FeprimeFe][Febull

Fe][Fex

Fe

]2 2FexFe Feprime

Fe + FebullFe [8]

and

Kox =[Ox

O

] [Febull

Fe

]2

p12O2

[Fex

Fe

]2 [V bullbull

O

] 1

2O2 + 2Fex

Fe + V bullbullO Ox

O + 2FebullFe

[9]

These equations describe the oxygen nonstoichiometry of bulk LSF64from 1 bar to the decomposition oxygen partial pressure measuredby thermogravimetry and coulometric titration Bucher and Sitte usedthe same defect chemical model to quantify electronic conductivityrelaxation experiments of LSF463355 In the following Eq 4 is appliedto estimate chemical capacitances of our LSF64 thin films Ki and Kox

were calculated using the values for standard enthalpy and entropyevaluated by Kuhn et al19 Then Eq 7 was solved numerically for02 bar oxygen partial pressure at 620C giving δ = 00048 Theequilibrium concentration of ionic charge carriers (vacancies) ceq

ion canbe calculated by

ceqion = c0 middot δ

3[10]

where c0 = 507 times 1022 cmminus3 is the concentration of oxygen sitesin LSF64 determined from the lattice parameters given in 01-082-1961 (ICDD database) For the equilibrium concentration of electroniccharge carriers the charge neutrality equation

ceqh minus ceq

e = (04 minus 2δ)c0

3[11]

is used where 04 is the Sr fraction on the A site ceqh and ceq

e are holeand electron equilibrium concentrations We obtain 812 times 1019 and660 times 1021 cmminus3 for oxygen vacancies and electron holes A similarestimation of charge carrier density in cerium oxide thin films was

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Journal of The Electrochemical Society 162 (3) F317-F326 (2015) F321

Figure 6 Impedance spectra of LSF64 microelectrodes (95 μm diameter)in humidified hydrogen with bottom current collector (open squares) andwithout current collector (filled squares) Both spectra were recorded at 700Cset temperature Tcalc for the electrode with current collector is equal to 605CFor the sample without current collector a meaningful temperature could notbe calculated due to the sheet resistance

conducted by Chueh and Haile49 The calculated concentrations wereused in Eq 4 (ceq

eon asymp ceqh ) and result in a chemical capacitance at pO2

= 02 bar of 644 Fcm3 This value agrees acceptably well with thechemical capacitance found in the measurement keeping in mind thatthermodynamic bulk data rather than thin film data were used in thecalculation

Electrode impedance under reducing conditionsmdash Importanceof current collectormdash An impedance spectrum of a circular 95 μmdiameter LSF64 microelectrode without additional current collectorin reducing conditions is shown in Fig 6 Compared to oxidizingconditions (Fig 4) the main arc of the spectrum becomes distortedHowever even more important is the drastic increase of both thesize of the electrode-related arc and the high frequency interceptRa In principle two contributions could increase Ra for microelec-trodes without current collector First a contact resistance betweenthe contact tip and the microelectrode and second an electronic sheetresistance within the thin film electrode We exclude the contact re-sistance between the tip and the sample to be high enough to havean impact on the impedance spectra also due to its absence in airThe second reason however a sheet resistance in LSF64 under re-ducing conditions is even expected as LSF64 loses electronic con-ductivity when changing from oxidizing to reducing conditions seeIntroduction A high sheet resistance reduces the electrochemicallyactive area to the vicinity of the contact tip and thus also increasesthe YSZ bulk resistance due to geometrical reasons In contrast formicroelectrodes with additional current collector the value of Ra cor-responds to that expected for the ohmic resistance of YSZ at thegiven temperature and the electrode impedance is drastically reduced(Fig 6) A similar reduction of the electrochemically active zone ofLSF64 without current collecting grid could be visualized by 18Otracer exchange experiments on polarized thin film samples56 wherea high cathodic bias lowered the chemical potential of oxygen inLSF64 in analogy to our experiments in H2H2O

In case of the additional bottom current collector a shoulder inthe intermediate frequency range is followed by a ca 45 slope seeFig 6 Such a slope often indicates relevance of a transmission lineeg due to a diffusion limited process As the amount of oxygen vacan-

Figure 7 Possible transmission lines describing a MIEC with metallic currentcollector grid for the case of a) limited lateral ionic conductivity resulting inan ionic transmission line in the MIEC b) limited electronic conductivity withionic short circuit in the electrolyte and electronic transmission line Lateralcharge transport lines (given by Rtrans-ion or Rtrans-eon) are set at the correctlocation (in the MIEC) while the surface resistance Rsur has been movedinside the MIEC for reasons of clarity of the equivalent circuit as Rsur and thedifferential Cchem have to be in parallel However the actual surface resistanceRsur is located at the gas-MIEC interface

cies in LSF64 under reducing conditions is high the ionic conductivityis expected to be high This was also proven by additional 18O tracerexperiments using a two step approach for analyzing tracer diffusionunder reducing conditions57 Across-plane transport of oxygen canthus hardly cause the 45 feature of a thin film electrode However thein-plane transport of oxide ions in LSF64 above the current collectinggrid after surface oxygen exchange via H2 + O2minus H2 O + 2eminuscanbe modeled by a transmission line circuit (Fig 7a) which can explainslopes close to 4538 Moreover the resistive contribution of in-planeelectron transport in LSF64 can lead to similar 45 impedance features(Fig 7b) and of course a combination of both is conceivable whichis discussed in detail in Ref 38 for Sr(TiFe)O3 thin film electrodes

For the analysis of the chemical capacitance of the sample withcurrent collector we again used the circuit shown in Fig 4 and ex-cluded the slope region of the spectrum from the fit We obtain 103Fcm3 when normalizing to the total LSF64 volume This is muchless than Cchem in air and a defect chemical interpretation is possiblebased on the data reported by Kuhn et al19 The corresponding cal-culation is in analogy to that for oxidizing conditions with 10minus23 barpO2 at 620C This oxygen partial pressure in reducing conditionsis estimated from thermodynamic data58 assuming approximately 25mbar of H2O in the H2 25Ar gas giving a ratio of H2OH2 of 1Accordingly the equilibrium concentration of electrons and oxygenvacancies are 267 times 1020 and 351 times 1021 cmminus3 respectively (thehole concentration is 200 times 1017 cmminus3) This clearly shows that thecharge carrier determining Cchem is now electronic (electrons) ratherthan ionic in contrast to air From that the chemical capacitance inreducing conditions is calculated to be 545 Fcm3 This is within theorder of magnitude of the chemical capacitance evaluated from ourmeasurements but the deviation between model and experiment islarger than for Cchem in oxygen This might be caused by differencesbetween defect chemical data of bulk samples and our thin filmsbut most probably also indicates that not the entire LSF64 film ispolarized due to sheet resistancesTop vs bottom current collector under reducing conditionsmdashThe ad-ditional current collecting grid influences the electrode impedancedifferently when it is on top of the LSF64 thin film or buried beneathit (bottom current collector) With top current collector we observea non-zero high frequency axis intercept (Ra) a small intermediatefrequency shoulder and a low frequency arc (Rc Cc) see Fig 8 Inanalogy to oxidizing conditions the small intermediate frequency fea-ture is assigned to the interfacial resistance and capacitance betweenelectrode and electrolyte In contrast for bottom current collector a

) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 12813119721Downloaded on 2015-01-27 to IP

F322 Journal of The Electrochemical Society 162 (3) F317-F326 (2015)

Figure 8 Impedance spectra of LSF64 microelectrodes (95 μm diameter) inhumidified hydrogen with top (filled triangles Tcalc = 623C) and bottom(open squares Tcalc = 605C) current collector

pronounced shoulder is found at intermediate frequencies followed byan approximately 45 slope For the high frequency axis intercept itdoes not make a significant difference whether the current collector isbeneath or on top of the electrode The slight difference of Ra in Fig 8corresponds to approximately 18C difference in temperature whichcan be caused for example by the positioning of the sample on theheating stage or by different mechanical pressures of the contact tip

In addition to the shape change the top current collector leads toa substantially larger total electrode resistance see Fig 8 This indi-cates a strongly reduced electrochemically active surface area due tocovering approximately 80 of the surface by the current collectorthe inactive area is denoted by A3 in Fig 9b Areas A1 and A2 inFig 9 might be active but oxygen exchange requires in-plane chargetransport of either electrons (A2) or ions (A1) An estimate of thisactive area (a fraction or even 100 of A1+A2) from the two mea-sured surface resistances in Fig 8 is difficult since those are proneto statistical variations and also some time-dependent changes (see

Figure 9 Schematic picture of ionic and electronic current and reaction path-ways for bottom (a) and top (b) current collector A2 represents the free elec-trode surface area without any current collector beneath or above while A1and A3 describe the areas with bottom and top current collector respectively

Figure 10 Comparison of impedance spectra of LSF64 microelectrodes(195 μm diameter) in humidified hydrogen with top current collector (blacksquares Tcalc = 609C) and in ambient air without current collector (red tri-angles Tcalc = 619C) Both spectra are normalized to the total electrodearea Atot The top current collector covers large parts of the surface and theactual surface resistance in reducing conditions normalized to the free surfaceis therefore 45 cm2 (green star)

Fig 5) However additional information can be gained from thechemical capacitance which is very reproducibly measurable It wasdetermined by using the equivalent circuit in Fig 4 to fit the low fre-quency arc In case of the top current collector definitely the wholeLSF64 volume beneath the metal grid (A3) ( = 082lowastAtot) and possiblylarge parts of A2 (=018lowastAtot) contribute to Cchem giving a value of129 Fcm3 (normalized to Atot) For the bottom current collector wefind 103 Fcm3 (normalized to Atot see above) This relatively minordifference clearly shows that a substantial part of the LSF64 surfaceabove a bottom current collector is stoichiometrically polarized andtherefore electrochemically active and that considerable lateral iontransport indeed takes place Activity of A2 only without polarizationof substantial parts of A1 cannot explain the value of 103 Fcm3 Thisis different to oxidizing conditions see above

The substantial polarization of LSF64 above the current collec-tor is also in accordance with the very different DC resistances inFig 9 A more precise quantification of the fraction of A1 and A2 be-ing active would require fitting by means of transition line models38

This could then reveal the two decay lengths of the MIEC polariza-tion with increasing distance from the metal grid edge (along A1 andA2) It may also provide further information on the origin of the pro-nounced shoulder for bottom current collectors This detailed analysishowever is beyond the scope of this paperComparison of impedance data in both atmospheresmdashSo far mainlythe effect of a current collecting grid and its influence on the impedanceof the LSF64 microelectrodes was discussed We now compare theimpedance of a LSF64 microelectrode without current collector inair and a LSF64 microelectrode with top current collector in reducingconditions (Fig 10) both 195 μm in diameter This pair has been cho-sen since it allows a reasonable comparison of electrode resistancesWe already concluded that in air the total electrode surface area iselectrochemically active even without a current collector In reducingconditions the current collector is necessary to compensate the sheetresistance and most probably large parts of the free LSF64 surface(A2 in Fig 9) are electrochemically active in our case

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Journal of The Electrochemical Society 162 (3) F317-F326 (2015) F323

As the conductivity of YSZ is essentially independent of the oxy-gen partial pressure60 the ohmic resistance is expected to be equal atthe same temperature The contribution from the contact resistance isnegligible as mentioned above and the sheet resistance of the LSF64in reducing conditions is accounted for by the current collector Thesmall difference in Ra observed in the spectra is therefore most likelydue to the positioning of the sample on the hot stage or different tippressures ie caused by a slight temperature difference Calculatingthe ASR with the total free electrode area (subtracting the area coveredby the current collector for the sample in reducing atmosphere) de-livers a remarkable result namely that the very different atmospheresare not reflected in very different resistances Rather those are prettysimilar 45 cm2 (green star in Fig 10) in reducing and 136 cm2 inoxidizing conditions A part of the remaining difference might evenbe caused by statistical variations and time dependences of Rc cfFig 5 This similarity is rather astonishing taking into account thatnot only electron electron hole and vacancy concentration are verydifferent in the two measurements but even the species participatingin the electrochemical reaction are not the same

1

2O2 + V middotmiddot

O harr OxO + 2hmiddot oxidi zing conditions [12]

H2 + OxO harr H2 O + 2eprime + V middotmiddot

O reducing conditions [13]

An interpretation of this fact cannot be given yet It might have avery fundamental mechanistic reason but could also be a coincidentalresult for the given material gas composition and temperature

Comparing the chemical capacitances (normalized to Atot) we ob-serve that Cchem under oxidizing conditions where it is proportionalto the vacancy concentration (1293 Fcm3) is approximately one or-der of magnitude bigger than under reducing conditions where it isproportional to the concentration of electronic charge carriers (149Fcm3 for this specific electrode with top current collector) This in-dicates that more oxygen vacancies are present in air than electronsin H2H2O A more detailed analysis in terms of the defect chemicalmodel of Ref 19 was already performed aboveCyclic change of gaseous atmosphere investigated on macroscopicelectrodesmdashIn order to obtain further information on similarities anddifferences of LSF64 in oxidizing and reducing conditions cyclic gaschanges were performed on macroscopic 5 times 5 mm2 samples with atop current collector (Fig 3) In the case of these macroscopic thinfilm electrodes much less of the electrode area than before is coveredby Pt (approximately 20 vs 80 for microelectrodes) Preparationof working and counter electrodes is described in the Experimentalsection The gas flow was changed every two hours from humidifiedhydrogen to dry synthetic air or back while the temperature was keptconstant at 668C

During the gas cycling series impedance spectra were continu-ously recorded measuring one spectrum took approximately 12 minExamples of these spectra are shown in Fig 11 The shapes of thespectra are similar to those observed for microelectrodes under therespective atmosphere (Figs 4 and 8) The contribution of the counterelectrode to the measured spectra was identified by comparison withimpedance spectra from samples with symmetrical porous LSF64Ptelectrodes A high frequency intercept with a small depressed in-termediate frequency arc and a low frequency semicircle is found inoxidizing conditions Here the intermediate frequency shoulder couldbe assigned mainly to the counter electrode rather than to the thin filmelectrode Its small additional resistance is within the Ra + Rb partof the equivalent circuit (Fig 4) and therefore does not influence theanalysis of the main working electrode arc at low frequencies

In reducing conditions a close to 45 slope of the impedance atintermediate frequencies is followed by two semicircles In this casethe counter electrode causes the second arc at low frequencies Itsimpedance contribution is larger than in air which is probably due tothe increased electronic resistance of LSF64 in reducing atmosphereBecause of the different absolute chemical capacitances of the thin filmelectrode and the porous counter electrode the impedance features are

Figure 11 Spectra of samples with macroscopic LSF64 thin film electrodesrecorded at 668C with top current collector and porous LSF64Pt counterelectrode (see Fig 3) in synthetic air (red) and humidified hydrogen (black)The arrow indicates the change of subsequently measured spectra in syntheticair

still rather well separated In reducing conditions only the main arcwas considered in the fit procedure using the circuit of Fig 4 Dueto the overlap of this main arc with the slope at higher frequenciesand the counter electrode arc at low frequencies this fit procedureleads to some inaccuracies but meaningful comparison of data is stillpossible

The high frequency offset under reducing conditions is shifted tohigher resistances compared to oxidizing conditions This indicatesdifferent capacitively active areas In air the electronic sheet resistanceof the LSF64 film is low therefore the parts of the film with substantialdistances from the current collectors (see Fig 3) are still largelyinvolved in the charging and discharging of Cchem Under reducingconditions mainly the area between the fingers and beneath the currentcollector is reached by the electronic charge carriers This is confirmedby the numerical analysis of the chemical capacitances In air Cchem

has a value of 792 Fcm3 if normalized to the entire electrode areaand this is acceptably close to 1293 Fcm3 found for microelectrodeswithout current collector at somewhat lower temperatures

Normalizing Cchem for reducing conditions to the entire area leadsto ca 45 Fcmminus3 and thus much less than the values of ca 100ndash150 Fcmminus3 found for microelectrodes However normalization tothe area covered by the current collector plus the area between thefingers gives very reasonable 90 Fcm3 Hence due to the highsheet resistance the area outside the current collectors is inactive (cfFig 6) Moreover the macroscopic sample has larger spacing betweenthe current collecting fingers (23 μm) than the microelectrodes (85μm see Figs 2 and 3) and it is likely that the center between thecurrent collecting fingers is only partly polarized The smaller capac-itively active area in reducing conditions causes the larger electrolyteresistance Ra compared to air However an exact analysis of this Ra in-crease would require numerical simulations due to the different activeelectrode areas on the two sides of the sample (in H2H2O)

These considerations also suggest the following normalization ofthe measured electrode resistances For the samples in air the electroderesistance is related to the total surface area minus the area coveredby the current collector since almost the entire free surface can beassumed to exchange oxygen For reducing conditions the resistanceis related to the area between the metal fingers as this area is assumedto be electrochemically active The area of a supposedly small activeregion outside the finger region was neglected This normalization mayslightly overestimate the active surface areas cf Cchem analysis givenabove However all main messages deduced from the experiments arenot affected by this approximation Fig 12 shows the change of thesenormalized electrode resistances Rc with time and gas phase

First of all the finding from the microelectrode measurements isconfirmed the area specific electrode resistances in both atmospheresare similar In the first gas cycle under reducing conditions the ASR is5 cm2 and for the first measurement in oxidizing conditions it is ap-proximately the same (Fig 12) The electrode resistance rises steeply

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F324 Journal of The Electrochemical Society 162 (3) F317-F326 (2015)

Figure 12 Electrode resistance per area (normalized to the total free LSFsurface in oxidizing conditions and the free LSF surface between currentcollector fingers in reducing atmosphere) over time at 668C Red trianglesare measured in synthetic air while black squares are measured under H2H2OOpen symbols indicate that the measurement was taken while the atmospherein the setup was not yet defined The inset shows degradation of macroscopicthin film electrodes in the absence of gas cycling in humidified hydrogen(black T = 600C) and dry synthetic air (red T = 591C)

in dry synthetic air while its stability under reducing conditions iscomparatively high Stability in H2H2O is in accordance with the factthat the humidified hydrogen used here has an oxygen partial pressureof ca 14 middot 10minus22 bar at 668C which is still in the range where LSF64is supposed to be stable19 Additional measurements on macroscopicelectrodes in humidified hydrogen without gas change confirmed thestability see Fig 12 inset The strong changes in air upon gas cyclingare in contrast to the high phase stability of the LSF64 bulk mate-rial under oxidizing conditions19 and to the much slower degradationrate observed in the absence of gas cycling (Fig 12 inset) Degrada-tion in air could be caused by the formation of a less active surfacetermination or Sr segregation as was found for LSC6162

Interestingly by changing back from oxidizing to reducing con-ditions the initial electrode resistance is again almost reached Thismeans that the variation of the electrode resistance under oxidizingconditions is not an irreversible degradation Rather exposure to re-ducing conditions again undoes the strong changes in air Only on alonger time scale there is a slow degradation of the electrode resis-tance with time that affects both atmospheres At present we can onlyspeculate about the mechanisms behind this reversibility of degrada-tion in synthetic air Possibly a drastic but fast chemicalstructuralreorganization of the LSF64 surface takes place in H2H2O irrespec-tive of the degradation state in air This might even be the formationof a second phase Below we present XRD results showing the fastformation of small amounts of Ruddlesten Popper like phases and ironoxides on LSF64 powder samples in H2H2O without decompositionof the main part of the bulk Also SEM images show additional parti-cles on the surface after (long time) exposure to H2H2O see Fig 1cAssuming such a severe change of LSF64 film surfaces in H2H2Owould explain the undoing of the degradation in air This novel sur-face structure formed in reducing atmosphere seems to be less proneto degradation than the LSF64 surface in air Also humidity presentin H2H2O may play a role here

In the first cycle the chemical capacitance is almost one order ofmagnitude higher in oxidizing conditions than in reducing conditionsin accordance with the findings from the microelectrode measure-ments Fig 13 shows that in air the chemical capacitance is almostconstant over the entire measurement it is not affected by the gascycling or temporal resistance degradation The chemical capacitancein reducing atmosphere slowly decreases from ca 90 to 38 Fcm3

within 48 hours This process cannot be reversed by gas change tosynthetic air The chemical capacitance is a bulk property (in contrast

Figure 13 Chemical capacitance per volume (normalized to the total elec-trode area for oxidizing conditions and to the sum of the areas below the entirePt current collector and between the current collector fingers in reducing con-ditions) over time at 668C Red triangles are measured in synthetic air whileblack squares are measured under H2H2O

to Rc) and the change indicates a variation on the level of the elec-tronic charge carriers of LSF64 in H2H2O A lower concentration ofelectrons would decrease Cchem and a lower electronic conductivitycould also reduce the effective area which contributes to Cchem thuslowering the absolute value of Cchem Interestingly this change doesnot significantly affect the oxygen vacancy concentration in air whichcan be concluded from the almost constant chemical capacitance inoxidizing conditions Moreover comparing Figs 12 and 13 indicatesthat the change in the bulk Cchem and the slow overall degradationof the surface resistance (Rc) in reducing atmosphere are possiblylinked Accordingly the change of Cchem in H2H2O might be relatedwith a slow phase change or second phase formation affecting both theelectronic conductivity in the bulk of our film and the electrochemicalsurface reaction

In both graphs (Figs 11 and 12) data points immediately aftergas changes marked with open symbols seem to deviate from thegeneral trend However from the peak frequencies in the impedancespectra (approximately 05 Hz in oxidizing and 10 Hz in reducingconditions Fig 11) we can conclude that the equilibration to thedifferent atmospheres is fast Hence the deviation is most probablysimply due to the slow gas change in the setup which leads to anundefined gas mixture in the first minutes of the measurementHigh-temperature in-situ X-ray diffraction on LSF64 powdermdashIn-situhigh-temperature powder XRD was performed to investigate possiblephase changes in reducing conditions XRD can detect a phase changeif sufficient material does undergo the change A phase change maybe restricted to the surface or only occurs to a minor degree whichis why it can be difficult to detect on thin films even with gracingincidence XRD Therefore we used LSF64 powder through which aconstant flow of humidified hydrogen was applied The powder wasexposed to a temperature profile (see Fig 14 inset) and the measureddiffraction patterns were compared to those from the ICDD-database(see Experimental)

Fig 14 shows the diffraction patterns of LSF64 powder in humid-ified hydrogen a zoom of the 20ndash50 region is provided in Fig 15The first diffraction patterns measured below 200C show only theLSF64 peaks Within the first hours of measurement between 50Cand 600C the area of the LSF64 peaks increases Additional peaksappear (at 238 246 267 289 292 298 304 312 331352 360 and 418) starting from 400C These peaks do notvanish upon cooling but their intensity remains rather low over theentire measurement The peaks emerging around 550C at 2 = 298

and 352 (Fig 15 black diamonds) can be assigned to Fe3O4 Whenheating to temperatures above 700C these two peaks disappear whiletwo other peaks (black stars at 360 and 418) appear indicating a

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Journal of The Electrochemical Society 162 (3) F317-F326 (2015) F325

Figure 14 Diffraction pattern (Cu-Kα) of LSF64 powder in humidified hy-drogen Top Reflections of LSF are given with their respective (hkl) indexesInset Temperature profile at each dwell temperature diffraction patterns werecollected Bottom 3D plot of measured diffraction patterns with time

transition from Fe3O4 to FeO The different transition temperatures(Fe3O4 to FeO at around 700C FeO to Fe3O4 at around 550C)indicate that at least one transition is kinetically hindered After theformation of the iron oxides their peaks do not further grow in inten-sity over the entire measurement This suggests that an equilibriumis reached after the precipitation of a small amount of iron oxide andlarge parts of the material do not demix A comparison of peak areasof the peaks with highest intensity of LSF64 (2 = 322) and Fe3O4

(2 = 352 ) at 550C (cooling) indicates the formation of 17magnetite

Besides the formation of the iron oxides other peaks arise and re-main present even after cooling the powder back to 50C The major

Figure 15 Diffraction pattern (Cu-Kα) of LSF64 in humidified hydrogen inthe angular range of 20ndash50 Peaks due to different crystalline phases aremarked as follows Fe3O4 (black diamond) FeO (star) Ruddlesten-Popperlike phase (LaSr)FeO4 (open triangle) and unassigned peaks (x)

peak of a Ruddlesten Popper phase is at 31249 and indeed a shoul-der can be seen for the (110) LSF64 peak around 31 which growswith increasing temperature Furthermore the peak forming at 238 isalso assigned to this Ruddlesten Popper phase (Fig 15 open triangle)Other peaks appeared between 25ndash33 but could not be assigned Theformation of a Ruddlesten Popper like phase is in agreement with theloss of iron from the perovskite phase ie the formation of iron ox-ide Ruddlesten Popper like phases in strontium ferrites have alreadybeen investigated in Refs 63ndash65 and the ternary phase diagram ofLa2O3-SrO-Fe2O3 in air at 1100C shows multiple Ruddlesten Pop-per phases66 Further the loss of iron from the perovskite and theformation of a Ruddlesten Popper phase were observed in Refs 19and 67 XRD measurements in H2 (5)He26 probably have not shownthese secondary phases due to the use of pressed and sintered LSF pel-lets where the gas LSF interphase is limited to the pellet surface Totaldecomposition of the LSF64 powder was observed in dry hydrogenwhich is in accordance with thermogravimetric measurements19

In principle the oxygen vacancies could also order to form aBrownmillerite-like phase Bahteeva et al26 observed the formation ofBrownmillerite-like structures in La1-xSrxFeO3-δ for x gt 05 at 700Cin a hydrogen (5)helium mixture they speculated that the oxygenvacancies are randomized in a composition range of x le 05 Still aBrownmillerite-like phase may form in LSF64 at lower temperatureswere oxygen vacancy ordering is more likely From the diffraction pat-terns we cannot confirm the formation of Brownmillerite-like phasesin LSF64 as their respective peaks superimpose with those of LSF64and the Ruddlesten-Popper phase However the peak found at 331

would fit to a Brownmillerite-like phaseThese XRD results indicate that in the temperature range of our

thin film studies LSF64 quickly forms small amounts of secondaryphases in reducing conditions despite nominally remaining within thestability limit found in Ref 19 Irrespective of these phase formationsthe main LSF64 peaks remain by far the largest Possibly surfaceswith their different energetics are already unstable in our reducingatmosphere and demix while the bulk stays perovskite-type

Transferring this interpretation to our thin film measurements sug-gests that iron oxide and a Ruddlesten Popper like phase might beformed on the electrode surface under reducing conditions This couldalso happen for a surface degraded in air and thus might be the originof the reactivation of the LSF64 surface after exposure to oxidizingconditions as suggested above If for example degradation in air iscaused by segregation of a Sr-rich phase or formation of a Sr-rich sur-face termination (cf LSC)6162 the surface phases evolving in H2H2Ocan repeatedly undo these processes by forming the surface being moststable in the reducing atmosphere

Conclusions

Impedance measurements of well-defined LSF64 thin film mi-croelectrodes with and without additional current collector could besuccessfully carried out under oxidizing as well as reducing condi-tions The low electronic conductivity of LSF64 thin film electrodesunder reducing conditions leads to a high sheet resistance which canbe overcome by the use of an additional current collector Chemi-cal capacitances (Cchem) are about one order of magnitude larger inair compared to H2H2O and in acceptable agreement with valuespredicted from defect chemical models in literature In both casesCchem reflects the concentration of the minority charge carriers whichare oxygen vacancies in air and electrons in H2H2O The chemicalcapacitance even allows an estimation of the electrochemically ac-tive area and thus determination of area-specific electrode resistancesfor oxidizing and reducing conditions Surprisingly similar specificresistances are found in both atmospheres

These findings were also reproduced with macroscopic LSF64electrodes that were exposed to a cyclic gas change Further the cyclicgas change shows that there is a continuous and severe increase in theelectrode resistance under oxidizing conditions after each exposureto reducing conditions In H2H2O the previous degradation uponair could be reversed As a reason we suggest reversible demixing

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F326 Journal of The Electrochemical Society 162 (3) F317-F326 (2015)

of near-surface regions in H2H2O with formation of a new stablesurface irrespective of previous surface degradation processes in airThis is in line with high-temperature in-situ XRD measurementswhere the formation of small amounts of iron oxides were observedin reducing conditions together with the formation of another phasewhich is probably Ruddlesten Popper-like All together the resultsshow that LSF64 might be used in SOFCSOEC electrodes underreducing conditions if prepared with an additional current collector

Acknowledgments

The XRD measurements were carried out within the X-Ray Centerof the Vienna University of Technology SEM images were recordedat the Universitare Service-Einrichtung fur Transmissionselektro-nenmikroskopielsquo (USTEM) TU Vienna by Elisabeth EitenbergerThe authors gratefully acknowledge financial support by AustrianScience Foundation (FWF) Projects F4509-N16 P21960-N17 andW1243-N16

References

1 M A Laguna-Bercero J Power Sources 203 4 (2012)2 N Q Minh J Am Ceram Soc 76 563 (1993)3 E Konysheva and J T S Irvine ECS Transactions 13 p 115 (2008)4 T Nakamura G Petzow and L J Gauckler Mater Res Bull 14 649 (1979)5 E Konysheva and J T S Irvine Chem Mater 21 1514 (2009)6 E Lay G Gauthier S Rosini C Savaniu and J T S Irvine Solid State Ionics 179

1562 (2008)7 S Tao and J T S Irvine Nat Mater 2 320 (2003)8 S P Jiang L Zhang and Y Zhang J Mater Chem 17 2627 (2007)9 I Jung D Lee S O Lee D Kim J Kim S-H Hyun and J Moon Ceram Int

39 9753 (2013)10 P Vernoux Solid State Ionics 135 425 (2000)11 S Primdahl J R Hansen L Grahl-Madsen and P H Larsen J Electrochem Soc

148 A74 (2001)12 W Kobsiriphat B D Madsen Y Wang L D Marks and S A Barnett Solid State

Ionics 180 257 (2009)13 P Vernoux E Djurado and M Guillodo J Am Ceram Soc 84 2289 (2004)14 C D Savaniu and J T S Irvine Solid State Ionics 192 491 (2011)15 S Cho D E Fowler E C Miller J Scott Cronin K R Poeppelmeier and

S A Barnett Energy Environ Sci 6 1850 (2013)16 C Sun and U Stimming J Power Sources 171 247 (2007)17 M Mosleh M Sooslashgaard and P V Hendriksen J Electrochem Soc 156 B441

(2009)18 S-i Hashimoto Y Fukuda M Kuhn K Sato K Yashiro and J Mizusaki Solid

State Ionics 181 1713 (2010)19 M Kuhn S Hashimoto K Sato K Yashiro and J Mizusaki Solid State Ionics

195 7 (2011)20 P Plonczak M Gazda B Kusz and P Jasinski J Power Sources 181 1 (2008)21 J Mizusaki M Yoshihiro S Yamauchi and K Fueki J Solid State Chem 58 257

(1985)22 J Mizusaki T Sasamoto W R Cannon and H K Bowen J Am Ceram Soc 66

247 (1983)23 M V Patrakeev J A Bahteeva E B Mitberg I A Leonidov V L Kozhevnikov

and K R Poeppelmeier J Solid State Chem 172 219 (2003)24 M Soslashgaard P Vang Hendriksen and M Mogensen J Solid State Chem 180 1489

(2007)25 M V V Patrakeev I A Leonidov V L L Kozhevnikov and

K R R Poeppelmeier J Solid State Chem 178 921 (2005)26 J A Bahteeva I A Leonidov M V Patrakeev E B Mitberg V L Kozhevnikov

and K R Poeppelmeier J Solid State Electrochem 8 578 (2004)

27 E V Bongio H Black F C Raszewski D Edwards C J McConville andV R W Amarakoon J Electroceramics 14 193 (2005)

28 M C Kim S J Park H Haneda J Tanaka and S Shirasaki Solid State Ionics40ndash41 239 (1990)

29 J Mizusaki M Yoshihiro S Yamauchi and K Fueki J Solid State Chem 67 1(1987)

30 J E ten Elshof R Bellur and A I Kingon J Electrochem Soc 144 1060(1997)

31 J Yoo A Verma S Wang and A J Jacobson J Electrochem Soc 152 A497(2005)

32 S Adler X Chen and J Wilson J Catal 245 91 (2007)33 E Bucher and W Sitte Solid State Ionics 173 23 (2004)34 J Goodenough and J Stickler Phys Rev 164 768 (1967)35 P A Cox Transition Metal Oxides An Introduction to Their Electronic Structure

and Properties OUP Oxford 2010 (1992) p 9436 W C Chueh Y Hao and W S M Jungand Haile Nat Mater 11 155 (2012)37 C Zhang M E Grass A H McDaniel S C DeCaluwe F El Gabaly Z Liu

K F McCarty R L Farrow M A Linne Z Hussain G S Jackson H Bluhm andB W Eichhorn Nat Mater 9 944 (2010)

38 A Nenning A K Opitz T M Huber and J Fleig Phys Chem Chem Phys 1622321ndash22336 (2014)

39 C Chen D Chen W C Chueh and F Ciucci Phys Chem Chem Phys 16 11573(2014)

40 G Walch A K Opitz S Kogler and J Fleig Monatshefte fur Chemie - ChemMon 145 1055 (2014)

41 M P Pechini and N Adams (1967)42 J Fleig F S Baumann V Brichzin H-R Kim J Jamnik G Cristiani

H-U Habermeier and J Maier Fuel Cells 6 284 (2006)43 T M Huber A K Opitz M Kubicek H Hutter and J Fleig Solid State Ionics

268 82 (2014)44 A K Opitz and J Fleig Solid State Ionics 181 684 (2010)45 F S Baumann J Fleig H-U Habermeier and J Maier Solid State Ionics 177

1071 (2006)46 F S Baumann J Maier and J Fleig Solid State Ionics 179 1198 (2008)47 A Wedig M E Lynch R Merkle J Maier and M Liu ECS Trans 45 213 (2012)48 J Jamnik and J Maier J Electrochem Soc 146 4183 (1999)49 W C Chueh and S M Haile Phys Chem Chem Phys 11 8144 (2009)50 V F Lvovich Impedance Spectroscopy - Applications to Electrochemical and Di-

electric Phenomena A John Wiley amp Sons Inc (1967) p 3951 F S Baumann thesis (2006) httpelibuni-stuttgartdeopusvolltexte2006270552 E Bucher and W Sitte J Electroceramics 13 779 (2004)53 E Bucher C Gspan F Hofer and W Sitte Solid State Ionics 238 15 (2013)54 T Kawada J Suzuki M Sase A Kaimai K YashiroY Nigara J Mizusaki

K Kawamura and H Yugami J Electrochem Soc 149 E252 (2002)55 A K Opitz M Kubicek S Huber T Huber G Holzlechner H Hutter and J Fleig

J Mater Res 28 2085 (2013)56 K Langer-Hansel S Kogler W Hetaba H Hutter and J Fleig in preparation57 L V Gurvich V S Iorish V S Yungman and O V Dorofeeva in CRC Handbook

of Chemistry and Physics CRC Press (2009)58 V V Kharton F M B Marques and A Atkinson Solid State Ionics 174 135

(2004)59 M Kubicek Z Cai W Ma B Yildiz H Hutter and J Fleig ACS Nano 7 3276

(2013)60 Z Cai M Kubicek J Fleig and B Yildiz Chem Mater 24 1116 (2012)61 M Kubicek A Limbeck T Froomling H Hutter and J Fleig J Electrochem Soc

158 B727 (2011)62 M V Patrakeev I A Leonidov V L Kozhevnikov and V V Kharton Solid State

Sci 6 907 (2004)63 L Mogni J Fouletier F Prado and A Caneiro J Solid State Chem 178 2715

(2005)64 B V Beznosikov and K S Aleksandrov Crystallogr Reports 45 792 (2000)65 A Fossdal M-A Einarsrud and T Grande J Am Ceram Soc 88 1988 (2005)66 D Bayraktar S Diethelm T Graule J Van herle and P Holtappels J Electroce-

ramics 22 55 (2008)67 T Horita T Shimonosono H Kishimoto K Yamaji M E Brito and H Yokokawa

Solid State Ionics 225 141 (2012)

) unless CC License in place (see abstract) ecsdlorgsiteterms_use address Redistribution subject to ECS terms of use (see 12813119721Downloaded on 2015-01-27 to IP