ORIGINAL PAPER

Impedance studies on bismuth-ruthenate-based electrodes

Abhishek Jaiswal & Eric D. Wachsman

Received: 14 July 2008 /Accepted: 24 October 2008 / Published online: 13 November 2008# Springer-Verlag 2008

Abstract Doped bismuth ruthenates and bismuth ruthenate-stabilized bismuth oxide composites were studied as prospec-tive cathode material for solid oxide fuel cells. Symmetriccells were fabricated on gadolinium-doped ceria electrolytesand studied by electrochemical impedance spectroscopy. Ca-and Ag-doped bismuth ruthenate electrodes (5–10 mol%)showed the same characteristic frequency as undoped bismuthruthenate but with higher activation energy and slightly betterperformance above ∼550 °C. At 700 °C, area-specificresistance (ASR) of undoped, 5 mol% Ca and 5 mol% Sr-doped bismuth ruthenate electrode was 1.45, 1.24, and 1.41Ωcm2, respectively. The change in ASR as a function ofoxygen partial pressure and current bias suggests that therate-limiting steps for oxygen reduction in bismuth ruthenatesystems are charge transfer and surface diffusion ofdissociatively adsorbed oxygen to triple phase boundaries.Introduction of the erbia-stabilized bismuth oxide (ESB)phase reduced both the rate-limiting steps resulting in muchimproved electrode performance. At 700 °C, compositeelectrodes containing 31.25–43.75 wt% ESB exhibited anASR of 0.08–0.11 Ωcm2.

Keywords Bismuth ruthenate . Bismuth oxide . Cathodes .

Electrochemical impedance spectroscopy . IT-SOFC

Introduction

Solid oxide fuel cells (SOFCs) are electrochemical devicesthat convert the chemical energy of a fuel into electricalenergy in a clean and efficient way. State-of-the-art SOFCsemploy yttria-stabilized zirconia (YSZ) as the electrolytematerial operating at 700–1,000 °C. High operation temper-atures are required to overcome the resistive loss across theYSZ electrolyte. There is considerable driving force to reducethe operating temperatures of SOFCs to 500–700 °C.Advantages include use of cheap ferritic stainless steel as theinterconnect material, lower operating cost, and faster startingtimes for mobile applications [1–4].

For operation at intermediate temperatures, better per-formance electrolytes and electrodes are required. Pyro-chlores based on bismuth ruthenate, lead ruthenate, andyttrium ruthenate have been recently studied for applicationas cathodes in SOFC [5–10]. Bae and Steele [5] studiedBi2Ru2O7.3, Pb2Ru2O6.5, and Y2Ru2O7 as cathode materialsfor intermediate-temperature-SOFC based on ceria electro-lytes. They showed that Y2Ru2O7 electrode has an area-specific resistance (ASR) of 4,000 Ωcm2 at 627 °C, andafter doping with 5 mol% Sr, the ASR reduced to 47 Ωcm2.This much-improved performance of the electrode wasexplained in terms of enhanced ionic conductivity ofY2Ru2O7 with Sr doping. Recently, we have reported theperformance of Bi2Ru2O7 and Pb2Ru2O6.5 pyrochlore-based electrodes [7–10].

A2B2O7 pyrochlore structure is essentially derived froman oxygen-deficient cubic fluorite structure with bothordered cation and anion sub-lattice [11]. The cationordering provides three distinguishable tetrahedral sites forthe oxygen ions: 8a-sites surrounded by four A3+ cations,8b-sites surrounded by four B4+ cations, and 48f-sitessurrounded by two A3+ and two B4+ cations. 8a and 48f-

Ionics (2009) 15:1–9DOI 10.1007/s11581-008-0289-x

A. Jaiswal : E. D. Wachsman (*)Department of Materials Science and Engineering,University of Florida,Gainesville, FL 32611, USAe-mail: ewach@mse.ufl.edu

Present address:A. JaiswalNanoGram Corporation,Milpitas, CA 95035, USA

sites are occupied, while 8b-sites are vacant resulting in anordered oxygen ion sub-lattice but still with an ionicconductivity larger than that of undoped fluorites. As inthe case of fluorites, oxygen ion conductivity can beincreased in the pyrochlore structure by generating addi-tional oxygen ion vacancies by doping with lower-valentcations. These additional oxygen ion vacancies areexpected to primarily occupy 8a- and 48f-sites and, hence,could contribute to oxygen ion conduction. However,depending on temperature and pO2, charge compensationcan also take by valence change of Ru. Further, apart fromthe concentration of mobile oxygen vacancies, the oxygenion conductivity also depends on structure factors such asjump directions, jump paths, activation barrier for theoxygen ion motion, and on dopant–vacancy interactions.

In this work, we studied the effect of Bi-site doping withaliovalent cations on the performance of bismuth ruthenateelectrodes. Bismuth ruthenate forms solid solutions, withlarge solubility, with a number of dopants on the A-site, andthe doping is not reported to significantly affect theelectronic conductivity, at least at room temperature [9,10]. Ca2+ (r=1.12 Å), Sr2+ (r=1.26 Å), and Ag+ (r=1.28 Å) with fixed valence and comparable ionic radii withthe host Bi3+ (r=1.17 Å) were studied as dopants on A-site.The solubility limit in Bi1xMxð Þ2Ru2O7d for Ca2+, Sr2+,and Ag+ was estimated to be 50, 30, and 20 mol%,respectively [12, 13]. This strategy was used, keeping inmind that, possibly as in fluorite systems, the dopant withbetter-matched ionic radii with the host would generateless elastic strain in the lattice and would show higheroxygen ion conductivity. The following nomenclature hasbeen used to identify the oxide compositions: Bi2Ru2O7≡BRO7 and Bi1xMxð Þ2Ru2O7d BMRx; M=C for Ca2+,S for Sr2+, and A for Ag+; x=dopant mol%. Performanceof composite electrodes consisting of bismuth ruthenateand stabilized bismuth oxide electrolyte is also reported.Bismuth oxides stabilized by doping in the fluorite δ-Bi2O3 structure are one of the highest known oxygen ionconductors [14–16]. The idea is to enhance the ionicconductivity of the electrode and the concentration oftriple-phase boundaries (TPBs) to improve electrodeperformance.

Experimental work

Standard solid state synthesis was used to fabricate thebismuth ruthenate powders. Bi2O3 (99.9995%, Alfa Aesar),RuO2·XH2O (99.99%, Alfa Aesar), CaCO3 (99%, FisherScientific), Sr(NO3)2 (99.97%, Alfa Aesar), and Ag(99.9%, Alfa Aesar) were mixed in stoichiometric amountsin an agate mortar and pestle and calcined at 900 °C for10 h to achieve undoped and doped Bi2Ru2O7 pyrochlore

phase. The calcined powder was leached with dilute HNO3

at room temperature, filtered, and dried to remove thesillenite-type impurity phase [7]. For composite electrodes,bismuth ruthenate powders were vibration milled for 72 husing zirconia media to reduce particle size. Erbia-stabilizedbismuth oxide (ESB; 20 mol%) powder was prepared usingan amorphous citrate route; Bi(NO3)3·5H2O (99.999 %, AlfaAesar) and Er(NO3)3·5H2O (99.9 %, Alfa Aesar) in desiredweight ratios were first dissolved in dilute nitric acidsolution, and then citric acid was added in a metal cation/citric acid molar ratio of 1:1.5. The solution was gelled andfoamed at 80–100 °C. The precursor was then calcined at500 °C. Electrode paste was made by mixing the desiredweight ratios of the powders with Heraeus V006 binder.Electrolyte pellets were made from 11 mol% gadolinium-doped ceria powders (GDC, Rhodia) uniaxially pressed andsintered at 1,450 °C. The electrode paste was brush-painted onGDC electrolyte pellets and dried at 150 °C. Symmetricalcells were prepared by sintering doped and compositeelectrodes with Pt lead wires at 850 and 800 °C for 2 h,respectively. The samples were characterized using X-raydiffraction (XRD; APD-3720) and scanning electron micros-copy (JEOL-6400). Lattice parameters of the bismuthruthenate powders were calculated by JADE (MDI) softwareusing whole-pattern fitting and Rietveld refinement. Imped-ance spectroscopy was done using a Solartron 1260 withoscillation voltage of 50mVin the frequency range of 0.1 Hz–32 MHz at temperatures between 350 and 700 °C. Oxygenpartial pressure was varied between 0.04 and 1 atm by usingO2/air–N2 gas mixtures.

Results

XRD patterns of calcined and leached powders: BRO7,BCRx with x between 5 and 30 mol%, and BARx with xbetween 5 and 20 mol% are shown in Fig. 1a and b,respectively. Leaching with dilute HNO3 has earlier beenfound to be effective in removing the sillenite-type impurityphase (Bi12RuO20) [7], which results in predominant single-phase undoped and doped bismuth ruthenate powders. BSRxwith x at 10 and 20 mol% were also synthesized and foundto be single-phase pyrochlore. Energy-dispersive X-rayanalysis showed that the compositions are reasonably closeto intended, apart from BAR20 which has Ag content lowerthan intended possibly due to leaching or Ag volatizationduring calcination. Lattice parameters of undoped and dopedbismuth ruthenate powders, after calcination and leaching,are shown in Fig. 2; for comparison, lattice parametersfrom the study by Kemmler-sack and co-workers are alsoshown [12, 13]. Lattice parameter for undoped bismuthruthenate matched well between the two studies; 10.292(±0.002) Å in this study and 10.292 (±0.007) Å reported

2 Ionics (2009) 15:1–9

previously [12, 13]. The trends for change in latticeparameter with dopant type and concentration also matchedwell between the two data sets, although with some differencein magnitudes of Ag- and Sr-doped bismuth ruthenatepowders. As expected on the basis of ionic radii, the latticeparameter increased and decreased on doping with Sr and Ca,respectively. To account for the unexpected decrease in thelattice parameter for Ag-doped system (Ag+ has a larger ionicradius than Bi3+), it was suggested that Ag exists in 2+valence state in the lattice [13]. Alternative explanation couldbe that the Ag loss during processing resulted in A-sitecation deficiency and/or Ru-incorporation on A-site.

Cross-sectional micrographs of undoped, 5 mol% Ca-and Ag-doped bismuth ruthentate electrodes on GDCelectrolyte are shown in Fig. 3. After sintering at 850 °C,the undoped and Ca-, Ag-, and Sr-doped electrodes are100–115 μm thick.

Impedance plots of Bi2Ru2O7 and Ca-doped Bi2Ru2O7

(BCRx; x=5–30 mol%) at 500 and 700 °C are shown inFig. 4. The electrode ASR was calculated by multiplying theelectrode resistance by the electrode area (∼0.7 cm2) anddividing by 2 to account for the symmetric cell. Bismuthruthenate electrode showed an area-specific resistance of1.45 Ωcm2 at 700 °C and 55.64 Ωcm2 at 500 °C in air. Theelectrode impedance consisted of at least two arcs. At500 °C, the Bi2Ru2O7 electrode showed a dominant lowfrequency arc with characteristic frequency at ∼40 Hz and ahigh frequency arc with characteristic frequency at ∼800 Hz.At 700 °C, the high-frequency arc at ∼3,200 Hz becomesdominant with a smaller low frequency arc at ∼8 Hz.Arrhenius plot of the electrode ASR in air is shown in Fig. 5.At 500 °C, all dopant levels led to increase in the electrodepolarization, while at 700 °C, 5 and 10 mol% dopingresulted in a slight decrease in the electrode polarization ascompared to undoped bismuth ruthenate. At 700 °C, ASRvalues for undoped, 5 mol%, and 10 mol% Ca-dopedbismuth ruthenate electrodes are 1.45, 1.24, and1.38 Ωcm2, respectively. The electrode characteristic fre-quency, at which the magnitude of imaginary part ofimpedance (Z″) is maximum, was same for undoped, 5 mol%, and 10 mol% Ca-doped bismuth ruthenate, suggestingthat the rate-limiting steps in the electrode reaction has notchanged with doping. However, with x≥20 mol%, the

(a)

(b)

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1000

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20 30 40 50 60 70 80

Cou

nts

BRO7

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2000

2500

3000

3500

20 30 40 50 60 70 80

Cou

nts

BRO7

BCR5

BCR20

BCR10

2θ

θ

BCR30

Fig. 1 XRD patterns for a Ca-doped bismuth ruthenate and b Ag-doped bismuth ruthenate

10.20

10.22

10.24

10.26

10.28

10.30

10.32

10.34

0 20 40 60 80 100

BCRx

BARx

BSRx

BARx*

BSRx*

a (A

)

Dopant Concentration (mol%)

BCRx*

Fig. 2 Lattice constants as a function of dopant concentration for Ca-,Ag-, and Sr-doped bismuth ruthenate powders (asterisks from [9, 10])

Ionics (2009) 15:1–9 3

electrode polarization is an order of magnitude higher alongwith additional electrode arcs at lower frequencies. As themicrostructure (particle size, porosity, and thickness) ofelectrodes are comparable, the large increase in polarizationat high dopant levels is attributed primarily to compositional

effects. Doping resulted in the increase in activation energyfrom ∼1.26 eV for undoped Bi2Ru2O7 to ∼1.37 eV for Ca-doped Bi2Ru2O7, though 5 and 10 mol% Ca-dopedBi2Ru2O7 showed slightly better performance at highertemperatures.

The performance of Ag-doped Bi2Ru2O7 (BARx; x=5-20 mol%) was similar to that of Ca-doped systems [17]:same characteristic frequency for undoped and dopedelectrodes, higher activation energy compared to undopedBi2Ru2O7, and better performance at temperatures higherthan ∼550 °C with 5 and 10 mol% Ag-doped Bi2Ru2O7 asshown in Fig. 6. At 700 °C, ASR values for 5 and 10 mol%Ag-doped bismuth ruthenate electrodes are 1.41 and1.31 Ωcm2, respectively. 10 mol% Sr-doped Bi2Ru2O7

(BSR10) showed inferior performance than undoped,10 mol% Ca-, and 10 mol% Ag-doped Bi2Ru2O7, andhence, systems with Sr as a dopant were not studiedfurther [17].

Electrode polarization of 5 mol% Ca- and 5 mol% Ag-doped Bi2Ru2O7 was studied as a function of pO2 in orderto understand the rate-limiting steps with these electrodes.For both systems, electrode impedance decreased andcharacteristic frequency increased with increase in pO2[17]. In general, the electrode ASR varies with the oxygenpartial pressure according to the following equation

ASR½ ¼ ASR½ o pO2ð Þm

The magnitude of m provides an insight into the rate-limiting step in the oxygen reduction reaction at thecathode. Plots of ln(ASR) vs. ln(pO2) at temperaturesbetween 400 and 700 °C for 5 mol% Ca- and Ag-dopedsystems are shown in Figs. 7 and 8, respectively. The valueof m for 5 mol% Ca-doped Bi2Ru2O7 ranged between 0.6and 0.8 at 0.11≤pO2 (atm)≤1, and at a lower pO2 of0.04 atm, the value of m increased, indicating a change inthe rate-limiting step. On the other hand, for 5 mol% Ag-doped Bi2Ru2O7, the value of m ranged consistentlybetween 0.5 and 0.6 in the complete pO2 range of study.In an earlier study on undoped bismuth ruthenate electro-des, the value of m was found to be between 0.5 and 0.6 inthe same temperature and pO2 range [7]. A magnitude of0.5 for m has been related in the literature to surfacediffusion of the dissociatively adsorbed oxygen at theelectrodes to the TPBs [18–21], which appears to be one ofthe rate-limiting steps for undoped, 5 mol% Ca-, and 5 mol%Ag-doped bismuth ruthenate electrodes.

Impedance measurements under direct current bias on5 mol% Ca- and Ag-doped Bi2Ru2O7 also support theargument at least at high temperatures. Impedance plots for5 mol% Ca-doped system as a function of current bias at500 and 700 °C are shown in Fig. 9. At 500 °C, on theapplication of current bias, the electrode polarization

Bi2Ru2O7

GDC

(a)

(Bi0.95Ca0.05)2Ru2O7

GDC

(b)

GDC

(Bi0.95Ag0.05)2Ru2 7O

(c)

Fig. 3 Cross-section micrographs of a Bi2Ru2O7, b (Bi0.95Ca0.05)2Ru2O7,and c (Bi0.95Ag0.05)2Ru2O7 electrodes on GDC electrolyte sintered at850 °C

4 Ionics (2009) 15:1–9

1

10

100

1 1.1 1.2 1.3 1.4

BRO7 (1.26eV)

BAR5 (1.38eV)

BAR10 (1.32eV)

BAR20 (1.40 eV)

1000/T (K-1)

AS

R (

Ωcm

2 )

Fig. 6 Arrhenius plot of Ag-doped Bi2Ru2O7 electrode ASR (Ωcm2)in air

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1 1.1 1.2 1.3 1.4

BRO7 (1.26eV)

BCR5 (1.37eV)

BCR10 (1.36eV)

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BCR30 (1.40eV)

AS

R (

Ωcm

2)

1000/T (K-1

)

Fig. 5 Arrhenius plot of Ca-doped Bi2Ru2O7 electrode ASR (Ωcm2)in air

(a) (b)

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700 oC

Z''

( )

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60 H z

1 Hz4030 Hz

Z''

( Ω)

Ω

60 H z

1 Hz4030 Hz

Fig. 4 Impedance plots of Ca–doped Bi2Ru2O7 electrodes onGDC at 500 °C (a, b) and at700 °C (c, d)

Ionics (2009) 15:1–9 5

decreased, which indicated that the rate-limiting step ischarge transfer. The electrode polarization behavior wasmore complex at 700 °C; electrode polarization increasedwith current bias up to 15 mA, decreased slightly between15–25 mA, and then increased beyond 25 mA along withadditional electrode arcs. Moreover, on application of bias,the characteristic frequency of the dominant arc shifted tohigher values from ∼40 to ∼100 Hz at 500 °C and to lower

values from ∼2,500 to 40 Hz at 700 °C. At highertemperatures, the charge transfer is fast, and hence, acomparatively slower step in the electrode reaction could berate limiting, which in the present case is diffusion related.Similar results on application of bias were obtained for5 mol% Ag-doped Bi2Ru2O7 electrode.

Bi2Ru2O7–ESB composite electrodes were fabricatedusing vibration-milled Bi2Ru2O7 powders. Vibration mill-ing reduced the particle size of Bi2Ru2O7 powders, calcinedat 900 °C, from ∼3 μm to ∼1 μm. Amorphous citrate routeproduced fine sub-micron ESB particles. It was expectedthat fine particle size would ensure homogenous mixing,

(a)

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15 mA

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35 mA

40 mAZ

'' (Ω

)

Z' (Ω)

Z' (Ω)

700 C

2500 Hz

40 Hz

0.8 Hz

Fig. 9 Impedance plots of 5 mol% Ca-doped Bi2Ru2O7 electrodes onGDC as a function of direct current bias at 500 °C (a) and at 700 °C (b)

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ln(A

SR

)

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)

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C (0.62)

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C (0.59)

500o

C (0.57)

550o

C (0.54)

600o

C (0.54)

650 oC (0.60)

700o

C (0.69)

Fig. 8 ln (ASR) vs. ln(pO2) of 5 mol% Ag-doped Bi2Ru2O7 electrodeat different temperatures with m in parenthesis T(m)

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T (m)400 oC (3.71)

450 oC (3.73)

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o

o

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SR

)

(0.85)

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(0.69)

(0.65)

(0.63)

(0.67)

(0.72)

2

o

650 C (3.61)

700 C (3.82)

)ln(pO2

Fig. 7 ln (ASR) vs. ln(pO2) of 5 mol% Ca-doped Bi2Ru2O7 electrodeat different temperatures with m in parenthesis T(m)

6 Ionics (2009) 15:1–9

high concentration of TPBs, and good sinterability of thecomposite electrode. Bismuth ruthenate and ESB phase didnot show any secondary phase formation at 800 °C, andafter sintering, the electrodes are 10–20 μm thick [9].

Impedance plots of Bi2Ru2O7–ESB composite electrodesat 500 and 700 °C are shown in Fig. 10. The performance, interms of electrode ASR, characteristic frequency, andactivation energy, of the vibration-milled Bi2Ru2O7 electro-des (∼1 μm particle size and ∼10 μm thick) is comparable toas-calcined Bi2Ru2O7 electrodes (∼3 μm particle size and∼100 μm thick). This indicates that the rate-limiting steps inthe oxygen reduction reaction for the single-phase pyro-chlore electrode is not influenced to a great degree byparticle size and electrode thickness. Introduction of the ESBphase in the electrode resulted in significant reduction inimpedance of both high- and low-frequency electrode arcs ateach temperature, indicating that the ESB phase is influenc-ing both rate-limiting steps in the electrode reaction.Moreover, the characteristic frequency of both electrode arcsdecreased on addition of the ESB phase. At 500 °C, the

characteristic frequency of the dominant low frequency arcdecreased from ∼10 to ∼2 Hz with increasing ESB content.Similarly, at 700 °C, the characteristic frequency of thedominant high-frequency arc decreased from ∼1,260 Hz to∼200 Hz with increasing ESB content [9].

As shown in Fig. 11, composite cathodes with 31.25–43.75 wt% ESB showed the lowest ASR values of 3.47–5.55 Ωcm2 and 0.08–0.11 Ωcm2 at 500 and 700 °C,respectively. In comparison, the single-phase Bi2Ru2O7

electrode (vibration milled) showed ASR values of 74.15and 1.78 Ωcm2 at 500 and 700 °C, respectively. Arrheniusplots of the composite electrodes are shown in Fig. 12 withactivation energies between 1.20 and 1.34 eV.

Discussion

The electronic properties of ruthenate pyrochlores has beencorrelated with the Ru–O bond length and Ru–O–Ru bondangle [22–24]. As the Ru–O–Ru bond angle decreases, the

(b) (a)

(c) (d)

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Bi2Ru2O7-ESBBi2Ru2O7-ESB

wt% ESB

Z"

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Z"

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100

025

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62.5wt% ESB

025

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62.5wt% ESB

-4

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0

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700 oC700 oC1260 Hz

5 H5 Hz

-0.4

-0.2

0

0.2

13.6 13.8 14.0 14.2

315 Hz

3 Hz

Fig. 10 Impedance plots of Bi2Ru2O7–ESB electrodes on GDC at 500 °C (a, b) and at 700 °C (c, d)

Ionics (2009) 15:1–9 7

orbital overlap decreases resulting in narrowed band anddecreased mobility. Recently, it has been observed thatapart from these two factors, the interaction of A-ionorbitals with Ru-4d and O-2p orbitals near EF plays asignificant role in the observed drift in electronic propertiesof various ruthenate pyrochlores which are iso-structuraland iso-electronic; Bi2Ru2O7 and Pb2Ru2O6.5 are weaklymetallic (temperature-independent resistivity over broadtemperature range), and Y2Ru2O7 and lanthanide ruthenate

pyrochlores are semi-conductors [25–27]. It is possible thatthe introduction of dopants in Bi2Ru2O7 affects the Bi-6porbital overlap along with the Ru–O–Ru bond angle andconsequently the electrical conductivity. It is interesting tonote that electrical conductivity of perovskite CaRuO3 andSrRuO3, although metallic, is lower than that of Bi2Ru2O7;at 727 °C, conductivity values of Bi2Ru2O7, CaRuO3, andSrRuO3 are ∼316, 70, and 28 S/cm, respectively [6].Whether doping improves the ionic conductivity or not isunclear in this study, as doped Bi2Ru2O7 pyrochlore did notimprove the electrode performance as significantly asobserved by Bae and Steele [5] with 5 mol% Sr-dopedY2Ru2O7. In the case of Sr-doped Y2Ru2O7, doping couldhave improved the electronic and/or ionic conductivity ofsemi-conductive Y2Ru2O7.

Linquette-Mailley et al. [19], by using redox potentio-metric measurements, showed that Bi2Ru2O7 reduces andbecomes a mixed ionic electronic conductor only at cathodepolarizations greater than 700 mV/air at 373 °C. Therefore,with single-phase Bi2Ru2O7 electrodes, the active TPBs forthe electrode reaction to take place are largely limited to theinterface between the electrode and the electrolyte. Intro-duction of the ESB phase in the electrode increases theconcentration of TPBs and ionic conductivity, whichsignificantly improves the rate-limiting steps in the oxygenreduction reaction, namely charge transfer at lower temper-atures and surface diffusion of dissociatively adsorbedoxygen at higher temperatures. The optimum compositionin the vicinity of 37.5 wt% (37–38 vol%) ESB for bothcomposite systems could be explained on the basis ofpercolation theory, so as to provide contiguous pathwaysfor both electrons and oxygen ions in the electrode. Thedensity of Bi2Ru2O7.3, Bi2Ru2O7, and ESB is 9.12, 8.92,and 8.96 gm/cm3, respectively, so vol% is approximatelyequal to wt%.

TPBs in single-phase electrode act as bottle necks forcharge transfer, and the current has to be drawn from largesurface diffusion lengths, while for composite systems, thecharge transfer resistance and the surface diffusion lengthsis much smaller as parallel paths are present due to highconcentration of TPBs and percolation of ESB phase. Themuch-improved performance of the composite on additionof ESB phase is also partly due to higher sinterability ofESB phase which provides better contact resistance withinthe electrode and with GDC electrolyte.

Conclusion

Doping with lower valent cations on Bi-site, in order toimprove the ionic conductivity, was not found to be veryeffective in improving the performance of bismuth ruth-enate cathodes. Ca and Ag doping (5 mol%) were found to

0.001

0.01

0.1

1

10

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1 1.05 1.1 1.15 1.2 1.25 1.3

0 (1.28 eV)25 (1.34 eV)31.25 (1.28 eV)37.5 (1.34 eV)43.75 (1.20 eV)50 (1.21 eV)62.5 (1.31 eV)

1000/T (K-1)

Bi 2Ru2O7-ES B

ASR

(Ω

cm2 )

wt% ESB (Ea)

Fig. 12 Arrhenius plot of Bi2Ru2O7–ESB composite electrode ASR(Ωcm2)

0.1

1

10

100

0 10 20 30 40 50 60

wt% ESB

ASR

(Ωcm

2 )

500 oC

550 oC

600 oC

650 oC

700 oC

Fig. 11 ASR (Ωcm2) of Bi2Ru2O7–ESB composite electrodes as afunction of wt% ESB. Lines as a guide to eye

8 Ionics (2009) 15:1–9

slightly improve the electrode performance over undopedbismuth ruthenate pyrochlores above ∼550 °C. At 700 °C,area-specific resistance of undoped, 5 mol% Ca-, and 5 mol%Sr-doped bismuth ruthenate electrode was 1.45, 1.24, and1.41 Ωcm2, respectively. Rate-limiting steps for the oxygenreduction reaction for 5 mol% Ca- and Ag-doped bismuthruthenate electrode include charge transfer and surfacediffusion of dissociatively adsorbed oxygen. Introduction ofthe ESB phase increased the ionic conductivity andconcentration of TPBs in the electrode and in effect reducedboth rate-limiting steps resulting in significantly improvedelectrode performance. At 700 °C, composite electrodescontaining 31.25-43.75 wt% ESB exhibited an ASR of0.08–0.11 Ωcm2.

Acknowledgment The authors wish to acknowledge the support ofthe Department of Energy under grant no. DE-FC26-03NT41959 forthis work.

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