The Influence of Grain Size on La0.6Sr0.4Co0.2Fe0.8O3-d ThinFilm Electrode Impedance

M. Ali Haidera and Steven McIntosha,b,*,z

aDepartment of Chemical Engineering, University of Virginia, Charlottesville, Virginia, 22904-4741, USAbDepartment of Chemical Engineering, Lehigh University Bethlehem, Pennsylvania 18015-4791, USA

Dense thin-film La0.6Sr0.4Co0.2Fe0.8O3-d (LSCF6428) electrodes of thickness 450 nm were symmetrically deposited onto eitherside of gadolinium doped ceria (CGO) electrolyte pellets by spray pyrolysis. Three film electrodes were fabricated with three dif-ferent average grain sizes; 285, 370 and 535 nm. The grain size was controlled by changing the electrode sintering duration from2 to 12 h and 36 h at 1373 K. The polarization resistance (Rp) of the electrode was observed to decrease with decreasing grain sizein both fuel cell and electrolysis cell modes. The impedance curves of the electrodes were dominated by a large low frequency arc,attributed to a surface electrocatalytic process. The electrode performance was enhanced initially on the application of 0.25 and0.5 V potential bias for 5 min.VC 2011 The Electrochemical Society. [DOI: 10.1149/1.3609002] All rights reserved.

Manuscript submitted May 18, 2011; revised manuscript received June 15, 2011. Published July 11, 2011.

Mixed ion-electron conductors (MIECs) of the series LaxSr1-x

CoyFe1-yO3-d (LSCF) are commonly utilized as the cathode ofsolid oxide fuel cells (SOFCs).1–6 The majority of prior studies onLSCF based materials have employed porous electrodes as theseprovide high surface area for reaction and a high concentration oftriple phase boundaries (TPB), leading to low electrode polariza-tion resistance, for example.7–9 Beckel et al utilized spray pyroly-sis to prepare thin film La0.6Sr0.4Co0.2Fe0.8O3-d (LSCF6428) elec-trodes, and note a significant improvement in ASR upon thetransition from more dense to more porous film microstructuresdue to an increase in triple phase boundary (TPB) contactsbetween the gas, film electrode, and underlying electrolyte.9 Whilesuch results on porous films provide useful insights and representthe most active electrode structures for eventual application, theuse of porous electrodes can complicate the analysis of the electro-chemical response due to competition between direct incorporationat the TPB, and surface oxygen incorporation followed by bulk ox-ygen transport in the MIEC electrode.10 As such, researchers areincreasingly employing dense film electrodes in order to simplifythis analysis.11–18 Our own prior work on both porous and denseLa0.8Sr0.2MnO36d (LSM) electrodes illustrates this difficulty inanalysis.16,17 Dense thin electrodes simplify analysis by providingan almost 2-D electrode, requiring a bulk oxygen anion diffusionpathway through the MIEC layer.

It has been suggested that the texture of dense MEIC films influ-ences the surface oxygen exchange rate,19 and it is well establishedthat oxygen ion conductivity differs between grain bulk and grainboundaries,20–22 however, we are unaware of any prior study exam-ining the influence of grain structure on the performance of MIECthin film electrodes.

In addition to SOFCs, there is growing interest in utilizing thesame materials set for application as Solid Oxide Electrolysis Cells(SOECs).6,23–25 The SOEC mode is the reverse of the SOFC: oxy-gen ions formed by water electrolysis at the water electrode aretransported to the oxygen electrode through the electrolyte. Theelectrodes are polarized in the opposite direction of the SOFCmode. The realization of a reversible SOFC/SOEC holds promise asa device for large scale intermittent electrical energy storage. Previ-ous studies have suggested that the area specific resistance of a cellwith an LSCF6428 air electrode is the same for the SOFC andSOEC mode of operation at 850�C, despite the reversal of operatingdirection.23,7

Finally, freshly prepared SOFC cathode materials showimproved performance on initial cathode polarization.16,17,26–29 Theinitial activation process in the La0.8Sr0.2MnO3-d (LSM) constitutesa necessary step to improve the performance of the electrode, and is

related to changes in both surface composition and bulk microstruc-ture. Similar activation has been reported for LSCF.5

There are a number of techniques available to produce thin elec-trode films, including pulse laser deposition, RF-magnetron sputter-ing, or spray pyrolysis.15,30–32 We have utilized spray pyrolysis as itis relatively simple to implement and is quite flexible with regard tothe material composition. While porous continuous LSCF films aresimple to deposit by spray pyrolysis, dense thin film fabricationrequires optimization of several deposition parameters.32

The focus of this study is the influence of grain size and filmmicrostructure on the electrochemical performance and initial activa-tion of LSCF6428 electrodes operating in SOFC and SOEC mode.

Experimental

Electrolyte pellets of Gd0.2Ce0.8O2-d (CGO) were fabricated bypressing 2.25 g of CGO powder (GDC 20-M, Fuel Cell Materials,Ohio, USA) in a uniaxial press at 140 bar in a 25 mm diameter die.CGO pellets were sintered in air at 1723 K for 6 h, resulting in densepellets of 20 mm diameter. The electrolyte pellets were ground to aplanar surface using a diamond polishing paste of 0.05 �m (Master-Prep Polishing Suspension, Buehler, USA) on a P400 and P4000 sil-icon carbide polishing pads [Buehler, Lake Bluff, IL, USA]. Theelectrolyte thickness after polishing was 0.7 mm.

Spray pyrolysis system was employed to deposit LSCF film onthe CGO substrate. Aqueous solutions of La, Sr, Co and Fe nitratesalts (Alfa Aeasar, > 99.98%, Ward Hill, MA, USA) were preparedand the metal concentration was determined by complexometric ti-tration with EDTA. A precursor solution was prepared by mixingthe La, Sr, Co and Fe nitrate solutions in the ratio 6:4:2:8. The pre-cursor metal nitrate solution, diethylene glycol monoethyl ether, andethanol were mixed in the volumetric ratio of 1:2:2 to form a 0.04M solution. The solution was sprayed on the CGO substrate usingan airbrush (Badger Model 200-20, IL, USA) and syringe pump(New Era Pump Systems, Inc., NY, USA) with flow rate 3 ml/h andair pressure of 4 psi for 12 min. The substrate electrolyte was heatedto 545 K and placed at a distance of 15 cm from the tip of the air-brush. A stainless steel mask was utilized to deposit circular electro-des with an area of 0.4 cm2 aligned at the center of the CGO sub-strate surfaces. The deposition procedure was repeated three timesand after every deposition cycle the electrode was sintered to 1373K for 2 h with a heating and cooling rate of 3 K/min. In order toobtain different grain size and surface microstructure, the thin-filmelectrode was finally sintered in air for 2, 12 or 36 h at 1373 K. Elec-trodes were symmetrically deposited on each side of the electrolytefollowing the same procedure. LSCF reference electrode with anarea of approximately 0.01 cm2 was painted onto one side of thecell using the mixture of glycerin and LSCF6428 powder. The refer-ence electrode was added to the free electrolyte surface at a distance

* Electrochemical Society Active Member.z E-mail: mcintosh@lehigh.edu

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greater than 3 times the electrolyte thickness from the electrode.33,34

Silver wire was attached with silver ink to make electrical contact tothe electrodes.

X-Ray diffraction (Scintag X-ray diffraction, XDS 2000,Cupertino, CA, USA) patterns were recorded in the 2h range of20–65� using Cu Ka radiation with a step size of 0.01� step size.Field Emission SEM (JEOL 6700F, Waterford, VA, USA) wasused to analyze the film surface morphology and fracture crosssection. Before analyzing in SEM, samples were coated with a Au/Pd conductive layer of � 50 nm in the precision etching coatingsystem.

Samples were characterized for their electrochemical perform-ance by impedance spectroscopy (Reference 600, Gamry Instru-ments, Malvern, PA, USA). Impedance spectra of the symmetriccell were recorded in galvanostatic mode with an AC perturbationof 1 mA root-mean-square rms between 1 MHz and 0.01 Hz, withten points collected per decade of frequency. Measurements wereperformed both during application of DC bias and at open circuit af-ter application of DC bias in a three-electrode configurations tocharacterize individual electrodes. All the samples were tested at973 K in laboratory air. Impedance spectra were fitted in model RCcircuits using a Levenberg Marquardt method (Gamry Echem Ana-lyst, Version 5.5, Malvern, PA, USA).

Results

Figure 1 shows the XRD spectra of LSCF6428 thin-film electro-des deposited on CGO substrates and sintered at 1373 K for (a) 2 h,(b) 12 h and (c) 36 h. The LSCF6428 phase was indexed to a rhom-bohedral perovskite structure, space group R3c, with refined latticeparameters a¼ b¼ 5.442 A and c¼ 13.302 A. The peaks of theCGO phase were indexed to the cubic Fm3m structure with refinedlattice parameter a¼ b¼ c¼ 5.423 A. A small impurity peak isobserved in the 2 and 36 h samples, at 2h¼ 35.56� and 2h¼ 27.92�

respectively. Beckel et al. also reported small impurities inLSCF6428 thin-films on sintering at temperatures above 1123 K,30

However, these higher sintering temperatures are required to obtaindense film microstructures.

The structure of the LSCF6428 thin-film was analyzed usingSEM. Figure 2a shows the surface morphology and fine grainedmicrostructure of the film sintered for 2 h at 1373 K. The averagegrain size measured along the longest diagonal and calculated fromat least 50 grains is 285 nm. The mean thickness of the film was 450nm measured from the electron micrograph of the fractured cross-section, Fig. 2b. Figures 2c and 2d are SEM images of the surfacemicrostructure of the films sintered for 12 and 36 h respectively.The average grain size was 370 and 535 nm for the 12 and 36 h sin-tered films, respectively. The grain size increases on increasing thesintering time. The fractured cross sections of all the films are denseand similar to the one shown in Fig. 2b.

A nominally identical counter electrode was deposited symmetri-cally at the center of the opposite side of the electrolyte disc. Such asymmetric system and correct reference electrode placement isrequired to accurately isolate the electrochemical response of a singleelectrode. The electrode impedance was measured between each elec-trode and the reference electrode at zero DC bias upon initial heatingof the cell to 973 K. This was done to verify the reproducibility of thefilm electrodes, and the geometric accuracy of aligning the two elec-trodes and placing the reference electrode. The two impedanceresponses should be identical for symmetrically placed identical elec-trodes. Experiments were only performed after confirming the twoelectrode impedances to be identical within 10% of the polarizationresistance. This often required testing several cells of each type priorto selecting an accurately aligned system. We have previously imple-mented this technique with other thin film electrode systems.16–18

Initial Performance.— Figure 3 shows the open circuit singleelectrode impedance response measured in air at 973 K. The polar-

ization resistances, Rp, of the electrodes, calculated as the span ofthe semicircular arcs, were 0.9, 7.6 and 10.9 X cm2 for the 2 h(microstructure A), 12 h (microstructure B) and 36 h (microstructureC) sintered thin-film. The cell ohmic resistance, RX was 1.48 X cm2

for all of the electrodes, compares well with the predicted value ofCGO at 973 K (1.49 X cm2 for a 0.7 mm thick electrolyte),35 indi-cating negligible contribution of the electrode and electrical con-tacts. The ohmic resistances are subtracted from the spectra in Fig. 3for clarity. The suppressed semicircular impedance spectra were de-convoluted by an equivalent RC circuit. In order to correctly repre-sent the suppressed nature of the semicircular arc, a constant phaseelement (Q) was used in place of the capacitor (C). We utilized twoparallel RQ elements in series to represent the impedance responseof most of the spectra in this paper; however, as clearly observed in

Figure 1. XRD patterns of La0.6Sr0.4Co0.2Fe0.8O3-d film (!) deposited onCGO and sintered at 1373 K for (a) 2 h, (b) 12 h and (c) 36 h. The impuritypeak is indexed as (^) and all other relatively large truncated peaks representthe CGO phase.

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Figs. 6a and 7c, the large lower frequency arc is most likely a com-bination of two overlapping arcs. Fitting of three arcs to these spec-tra resulted in very poor reliability of the fit and correspondinglypoor matching of trends between spectra. Therefore, we restrict ourfitting of most data sets to two arcs and report only the total spanand apparent peak frequency of these convoluted lower frequency

arcs. This convolution of the data restricts our ability to assign spe-cific processes to each ‘arc’. We report the data for two lower fre-quency arcs (to give a total of three arcs) when these arcs are clearlyseparable in the data.

For the initial performance data in Fig. 3, the low frequency arcwith peak frequency of order 1 Hz was dominant, with span of 0.7,6.7 and 9.5 X cm2 for microstructures A, B and C, respectively. Thesmaller arcs, with peak frequency of order 100 Hz, had spans of 0.2,0.9 and 1.4 X cm2 for microstructures A, B and C, respectively.This smaller, higher frequency arc did not change during the experi-ments and thus the data for this arc is not included in the rest of thediscussion.

Initial activation.— The three different microstructures (A, Band C) of the thin-film LSCF6428 electrode were tested for initialactivation under cathodic polarization. EIS measurements were per-formed at open circuit after applying cathodic potentials of 0.25 and0.5 V in air at 973 K for 5 min across the electrode. Figures 4a–4cshow the impedance responses. The total Rp decreased from the ini-tial un-activated value of 0.9 to 0.7 X cm2 after polarization at 0.25V and then to 0.64 X cm2 after polarization at 0.5 V, Fig. 4a. Thesechanges occurred in the span of the low frequency arc and wereaccompanied by a slight decrease in peak frequency. The data fitsfor this arc are detailed in Table I. The ohmic resistance remainedconstant. Similar reductions in Rp were observed for the othermicrostructures, although the percentage change increased withincreasing grain size from microstructure A through C. The parame-ters extracted from the impedance spectra are detailed in Table I.

The observed initial- activation occurs in a short-time interval.In contrast, the relaxation process back to the initial state was not

Figure 2. (Color online) (a) Surface mor-phology and (b) fractured cross section ofLSCF6428 thin-film electrode depositedon CGO substrate and sintered at 1373 Kfor 2 h. Surface morphology of similarlydeposited electrode sintered for (c) 12 hand (d) 36 h.

Figure 3. (Color online) Impedance spectra measured between reference! electrode at open circuit in air at 973 K for a symmetrical cell withLSCF6428 thin-film electrode electrodes sintered for 2 h (�), 12 h (n), and36 h (h) at 1373 K. Numbers indicate frequency in 10n Hz.

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complete even after 1 h. Figure 5 shows the impedance response ofthe thin-film electrode measured at zero DC bias in air, after 60 minof the initial activation test. Upon initial cathodic polarization, Rp

decreased from 0.9 to 0.64 X cm2 and then increased to 0.75 X cm2

for microstructure A after 1 h of relaxation, Fig. 5a. However, Rp

did not relax to its original un-polarized state indicating the processto be irreversible. Similar trend was observed for microstructure Band C as well, Table I. Rp originally at 7.6 X cm2 was reduced to5 X cm2 and then increased to 6.1 X cm2 after relaxation for micro-

structure B, Fig. 5b, and was measured 7.4 X cm2 as compared tothe initial un-activated value of 10.6 X cm2 for microstructure C,Fig. 5c. Again, the extracted parameters for the low frequency arcare reported in Table I.

Operation under DC cathodic bias.— The impedance responseof the thin-film electrodes was also measured under the applicationof DC cathodic current, Fig. 6. Rp was found to decrease non-line-arly on increasing the applied current density for all microstructures,again associated with the low frequency arc, Table II. On applyingDC cathodic current densities of 10, 20 and 30 mA/cm2, Rp for thisarc decreased from 0.77 to 0.67, and then split into two discernablearcs with spans of 0.15 and 0.5 X cm2 for microstructure A, Fig. 6a.Similarly for microstructures B and C, the low frequency arc Rp

reduced to 3.3 and 4.0 X cm2 respectively on applying current up to30 mA/cm2, Table II. The high and low frequency components aremore clearly observed in Bode plots of modulus and phase angle ofthe measured spectra, Fig. 7.

Operation under DC anodic bias.— In order to test the influenceof microstructure on SOEC anodes, the impedance response wasmeasured upon applying anodic currents of 10, 20 and 30 mA/cm2.Rp of the low frequency arc decreased from 0.55 to 0.44, to 0.38X cm2 on applying current of magnitude 10, 20 and 30 mA/cm2,Fig. 8a, and Table III. While the peak frequencies remained nearlythe same for the microstructure A and C, it was observed to be sig-nificantly increased from 12.4 to 44.7 Hz for the microstructure Bon increasing the anodic current density from 10 to 30 mA/cm2.The impedance spectra measured on applying anodic current den-sities of 10, 20 and 30 mA/cm2 on microstructure C, show splittingof the one low frequency arc into two nearly equal suppressedsemicircular arcs, Fig. 8c. The lowest frequency impedance arcwas split into two arcs of span 0.6 and 0.45 X cm2 after applying30 mA/cm2 anodic current, Table III. The corresponding Bodeplots are shown in Fig. 9.

In general, the performance of the electrode was better in termsof measured polarization resistance under the DC anodic current ascompared to the DC cathodic current. In order to directly comparethe SOFC and SOEC mode of operation, an I-V curve was plottedfor the 12 h sintered LSCF6428 thin-film electrode corresponding tomicrostructure B, and is shown in Figure 10. The slope of the curvewas significantly greater under the SOFC mode of operation as

Figure 4. (Color online) Impedance spectra of LSCF6428 thin-film electro-des sintered for 2 h (a), 12 h (b), and 36 h (c) at 1373 K, measured at opencircuit after application of cathodic potential across the cell of 0 (h), 0.25 V(n) and 0.5 V (�) for 5 min. Numbers indicate frequency in 10n Hz.

Table I. Parameters extracted from the open circuit impedance

measurement of the 2 h (A), 12 h (B) and 36 h (C) sintered

LSCF6428 thin-film electrode before and after applying initial

DC bias of 0.25 and 0.5 V for 5 min and relaxation for 60 min at

973 K in air.

Microstructure(sinteringtime)

Applied potentialfor 5 min

beforemeasurement (V)

Rp(X cm2)

Peakfrequency

of thelow frequency

arc (Hz)

Span ofthe low

frequencyarc (X cm2)

A (2 h) 0 0.9 5 0.7

0.25 0.7 5 0.5

0.5 0.64 3.95 0.44

0.5 (after 60 min) 0.75 3.95 0.53

B (12 h) 0 7.6 3.95 6.7

0.25 6.4 5 5.4

0.5 5 6.3 4.2

0.5 (after 60 min) 6.1 5 5.2

C (36 h) 0 10.9 1 9.5

0.25 7.8 1.26 6.7

0.5 5.6 1.58 4.6

0.5 (after 60 min) 7.4 1.26 6.3

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compared to the SOEC mode of operation of the same thin-filmelectrode, indicating higher resistance in the SOFC operation.

Discussion

Spray pyrolysis has previously been shown to be effective forthe deposition of porous LSCF6428 thin-film electrodes,9,30 with theresulting porosity attributed to solid state dewetting.30 In this study,we have obtained dense thin-film LSCF6428 electrodes by changing

the deposition parameters,32 and by repeated depositions steps withintermediate annealing at relatively high temperature (1373 K)between each deposition step. We suggest that the partially formed,and likely porous, dewetted, film formed by the first pair of deposi-tion and annealing steps, is filled in during subsequent depositionsteps to yield a dense microstructure. This technique of repeateddeposition has been shown successful in the fabrication of denseLa0.8Sr0.2MnO3 and La0.75Sr0.25Cr0.5Mn0.5O3-d thin-film electro-des.16,18 This procedure also lends itself to accurate geometric

Figure 5. (Color online) Impedance spectra of LSCF6428 thin-film elec-trode electrodes sintered for 2 h (a), 12 h (b), and 36 h (c) at 1373 K, meas-ured at open circuit before (h) and after 60 min (n) of initial activation.Numbers indicate frequency in 10n Hz.

Figure 6. (Color online) Impedance spectra of LSCF6428 thin-film electro-des sintered for 2 h (a), 12 h (b), and 36 h (c) at 1373 K, measured in airduring application of DC cathodic currents of 10 (h), 20 (n) and 30 (�)mA/cm2.

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placement of the electrodes, enabling separation of electrode imped-ance response via a reference electrode on the free electrolytesurface.

We utilized different annealing times with the goal of producingdifferent electrode microstructures in order to understand the effectof surface microstructure on the electrochemical performance. Theaverage grain size did increase substantially when changing the finalsintering duration from 2 to 36 h, Fig. 2. This was accompanied bysignificant changes in electrode activation, cathodic performance,and anodic performance. The measured polarization resistances, Rp

were consistently lowest for the 2 h sintered film, microstructure A,corresponding to an average grain size of 285 nm. Rp increasedacross all measured experimental conditions with increasing averagegrain size from 285 to 370 and to 535 nm, microstructures A, B, andC respectively.

Direct comparison with other experimental work and models forthin film LSCF cathodes is complicated by the fact that we studied alarge dense film (a 0.4 cm2 circle), where oxygen incorporation atthe triple phase boundary along the electrode edge will be negligi-ble. Our test geometry forces a bulk oxygen pathway.

The critical thickness Lc, calculated as the ratio of bulk diffusiv-ity coefficient and surface exchange rate coefficient,36 was estimatedto be around 22 �m for bulk LSCF6428 material at 973 K.37 The450 nm average thickness of the electrodes used in this study is wellbelow this value, indicating that the impedance response will bedominated by a rate-limiting surface process. A significant problemin clearly identifying the nature of this process is that the apparentsingle low frequency impedance arc most likely consists of twooverlapping, convoluted arcs, as evidenced in Figs. 6–9. While wecan fit single capacitance and resistance values to this arc, furtheranalysis to assign a single process would be misguided. Therefore,we must simply state that these convoluted arcs are most likelydominated a surface reaction process, and limit our discussion to rel-ative changes in the total span. This assignment is supported by theprevious work of Baumann et al who reported that the resistance ofLSCF film electrodes is dominated by the surface oxygen incorpora-tion resistance.12 Qualitative comparison with the work of Baumanet al, reveals a similarity in peak frequency of the dominant arc,Fig. 8. This low frequency peak is assigned to a surface process, fur-ther confirming that our electrodes are limited by the surface oxygenincorporation reaction. While the magnitude of this arc varies sub-stantially between the various electrode microstructures, the peakfrequency does not change substantially.

In addition to changing the microstructure, altering the annealingtime may also lead to compositional changes in the electrode. Weattempted to probe for differences in composition across the grainsusing energy-dispersive X-ray spectroscopy (EDX) coupled withscanning electron microscopy (SEM); however, the EDX samplespot size is significantly larger than the grains and no compositionalvariation could be observed. Previous studies on similar and related

materials all indicate that Sr is enriched on the surface of these elec-trodes12,38–40 and we thus expect the initial surface of our samplesto be Sr-enriched. The electrodes in this study show the same activa-tion behavior as has demonstrated for La0.8Sr0.2MnO3 6 d electrodes.One proposed explanation for the activation of the electrodes uponinitial cathodic polarization is the removal of these Sr-rich species.However, these changes are relatively small compared to the largedifference in polarization with changing grain size. Other possibleexplanations include a decrease in surface area due to increased

Figure 7. (Color online) Bode plots of impedance modulus (closed symbols)and phase angle (open symbols), of LSCF6428 thin-film electrodes sinteredfor 2 h (a), 12 h (b), and 36 h (c) at 1373 K, measured in air during applica-tion of DC cathodic current of 10 (4), 20 (n) and 30 (�) mA/cm2.

Table II. Parameters extracted from the impedance spectra of

the 2 h (A), 12 h (B) and 36 h (C) sintered LSCF6428 thin-film

electrode under DC cathodic current at 973 K in air.

Microstructure(sintering time)

DC Current(mA/cm2)

Rp

(X cm2)

Peak frequencyof the lowfrequencyarc(s) (Hz)

Span of thelow frequencyarc(s) (X cm2)

A (2 h) 10 1 3.15 0.77

20 0.87 6.3 0.67

30 0.8 100, 5 0.15, 0.5

B (12 h) 10 6 3.15 4.9

20 4.7 3.15 4

30 3.6 3.15 3.3

C (36 h) 10 6.4 1 5.9

20 5.4 1 5

30 4.3 1 4

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sintering time or possible the influence of the small impurity phase.It seems unlikely that these changes in surface composition are agreater influence on the observed polarization resistances than adoubling of the grain size from microstructure A to C.

The primary difference between the samples is the changingratio of grain boundary to grain bulk as the gran size changes. Kishi-moto et al. measured faster B-site ion diffusion at the grain bound-ary of the La0.8Sr0.2CoO3-d (LSC) and the La0.8Sr0.2FeO3-d (LSF)SOFC cathode materials as compared to the bulk,41 and studies onother nanostructured materials demonstrate a significant increase in

conductivity with decreasing grain size.42 In terms of surfacekinetics, in addition to Baumann et al.,12 Lee et al. have suggestedthat a large low frequency impedance arc, similar to that observedin this study, represents the impedance to the oxygen surface reac-tion at the surface for a 200 nm dense LSCF6428 electrode fabri-cated by RF-magnetron sputtering.15 Chen et al. demonstrated astrong dependence of surface exchange rate upon morphologicalchanges in La0.5Sr0.5CoO3-d thin films43 and Kim et al. havereported related changes in surface exchange rates measured on apulse laser deposited La2NiO4þ d thin-film due to two independentregions on the same film corresponding to two different microstruc-tures.19 Based on these findings, we suggest that the changes in elec-trode polarization observed in this study are due to the enhancedsurface exchange rate and oxygen ion transport at grain boundarieswhen compared to the grain bulk. Thus the lowest grain size elec-trode shows the lowest polarization resistance. This suggests thatsignificant enhancements in SOFC cathode and SOEC anode per-formance could be achieved by controlling the grain size of theactive electrode material. Further detailed measurements of surfaceexchange as a function of morphology are required to confirm thishypothesis.

The non-linear reduction in Rp, on the application of cathodiccurrent is likely due to the reduction of the transition metal ion atthe B-site and the concomitant formation of oxygen vacancies. Thisfinding is in agreement with the previous study by Baumann et al,who report significant reduction in the surface exchange resistanceof LSCF films upon DC cathodic bias.12

The changes observed upon anodic polarization (SOEC mode)are harder to reconcile at this time. If we consider that a cathodiccurrent will reduce the material, we must consider that an anodiccurrent will oxidize the material. Our research group has previouslyreported increased surface activity for oxidation reactions withincreasing oxygen stoichiometry in SOFC anode materials.44,45

However, in this case we are preforming oxygen reduction andwould expect a decrease in oxygen vacancy concentration todecrease rates. We can speculatively consider the results put for-ward for water electrolysis by Zhang et al. have shown a large shiftin the surface Ce3þ /Ce4þ concentrations relative to equilibrium inCeO2 electrodes.46 They indicate that, it is the oxidation state of thecation itself that dominates the activity. For the LSCF6428 elec-trode, on increasing the local pO2 in the oxidizing environment thetransition metal at site-B is oxidized.47 Since the overall reaction issurface limited, a shift in the surface concentrations of B-site cationswith different oxidation states could possibly enhance the surfaceexchange rate and reduce the impedance. For the LSCF6428 bulkmaterial, Lane et al. have measured surface exchange rate coeffi-cient (k) and observed it to be directly proportional to the pO2.48

The increase in surface exchange coefficient leads to faster surfaceexchange kinetics and thus explain the enhanced performance in theSOEC mode of operation. In-situ measurements, such as those by

Figure 8. (Color online) Impedance spectra of LSCF6428 thin-film electro-des sintered for 2 h (a), 12 h (b), and 36 h (c) at 1373 K, measured in air dur-ing application of DC anodic current (electrolysis) of 10 (h), 20 (n) and 30(�) mA/cm2.

Table III. Parameters extracted from the impedance spectra of

the 2 h (A), 12 h (B) and 36 h (C) sintered LSCF6428 thin-film

electrode under DC anodic current at 973 K in air.

Microstructure(sintering time)

DC current(mA/cm2)

Rp

(X cm2)

Peak frequencyof the lowfrequencyarc(s) (Hz)

Span of the lowfrequency arcs

(X cm2)

A (2 h) 10 0.75 3.15 0.55

20 0.64 3.15 0.44

30 0.57 3.15 0.38

B (12 h) 10 2.9 12.4 2.2

20 2.1 24.9 1.6

30 1.2 44.7 0.9

C (36 h) 10 2.5 125.5, 1.26 0.5, 1.9

20 1.9 158.3, 1 0.8, 1

30 1.15 158.3, 1 0.6, 0.45

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Zhang et al,46 would be invaluable in unraveling this complexinteraction.

Conclusions

The grain structure of SOFC electrodes plays an important rolein determining electrochemical performance, with a higher densityof grain boundaries leading to lower electrode impedance. This ismost likely due to more rapid oxygen incorporation and transport atgrain boundaries. The electrode impedance is decreased upon bothcathodic and anodic bias. This change with cathodic bias is likelydue to increased oxygen vacancy concentration leading to more

rapid oxygen transport. The case of anodic polarization is less clearbut may be directly related to differences in catalytic activitybetween different oxidation states of the transition metal B-site cati-ons. LSCF6428 film electrodes show similar initial activation toLa0.8Sr0.2MnO36d electrodes.

Acknowledgment

This work was supported by the National Science Foundationunder the Faculty Early Career Development Program (CAREER)grant CBET-0643931 and the University of Virginia Energy SeedFund.

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Figure 9. (Color online) Bode plots of impedance modulus (closed symbols)and phase angle (open symbols), of LSCF6428 thin-film electrodes sinteredfor 2 h (a), 12 h (b), and 36 h (c) at 1373 K, measured in air during applica-tion of DC cathodic current of 10 (4), 20 (n) and 30 (�) mA/cm2.

Figure 10. Potential versus current density curve of LSCF6428 thin-filmelectrode microstructure B measured between reference! electrode at opencircuit in air at 973 K for a symmetrical cell.

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