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Electrochimica Acta 52 (2007) 7217–7225

Improvement of the electrochemical properties of novel solid oxidefuel cell anodes, La0.75Sr0.25Cr0.5Mn0.5O3−δ and

La4Sr8Ti11Mn0.5Ga0.5O37.5−δ, usingCu–YSZ-based cermets

J.C. Ruiz-Morales a,∗, J. Canales-Vazquez b, D. Marrero-Lopez a, J.T.S. Irvine c, P. Nunez a

a Departamento de Quımica Inorganica, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spainb Renewable Energy Research Institute, Universidad de Castilla la Mancha, 02006 Albacete, Spainc School of Chemistry, University of St Andrews, North Haugh, St Andrews, Scotland KY16 9ST, UK

Received 11 May 2007; accepted 21 May 2007Available online 2 June 2007

bstract

A Cu–metal-based cermet was used to improve the electrochemical properties of two novel oxide-based systems with intrinsic low electroniconductivity such as La0.75Sr0.25Cr0.5Mn0.5O3−δ (LSCM) and La4Sr8Ti11Mn0.5Ga0.5O37.5−δ (LSTMG). The introduction of Cu results in a markedmprovement of the polarisation resistance values and hence in the performance. The best results correspond to the addition of ∼15% of CuO. Inoth systems, the polarisation resistances were improved by a least a factor of 2. Despite there are reports claiming that the CuO–zirconia-based

ystems exhibit catalytic activity, such an improvement seems to be mainly related to the capability of CuO as a sintering agent, helping to bridgelectrode particles together, creating new electronic paths and thus effectively increasing the triple phase boundary through the whole electrodeaterial.2007 Published by Elsevier Ltd.

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eywords: LSCM; LSTMG; Anode; Solid oxide fuel cell; Polarisation

. Introduction

Solid oxide fuel cells (SOFCs) are devices that can producelectricity, heat and water from the electrochemical combina-ion of a fuel and an oxidant. The efficiency of these ratherimple devices depends largely on the adequate properties ofhe three main elements in a single cell, e.g. anode, cathode andlectrolyte.

The anode is perhaps one of the most crucial elementsn a SOFC. In addition to some general requirements [1,2]t must exhibit an adequate porosity, high catalytic activitynd high electronic conductivity, although conductivity values

s low as 1 S/cm may be acceptable in thin and flat SOFConfigurations. The state of the art SOFC anode material is thei/YSZ cermet [3] which offers excellent catalytic properties,

∗ Corresponding author. Tel.: +34 922 318464; fax: +34 922 318461.E-mail address: jcruiz@ull.es (J.C. Ruiz-Morales).

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013-4686/$ – see front matter © 2007 Published by Elsevier Ltd.oi:10.1016/j.electacta.2007.05.060

ixed conductivity and good current collection. However, suchermets present some disadvantages related to the low toleranceo sulfur, carbon build up when operating under hydrocarbonuelling and volume instability upon redox cycling [4,5].nother important and frequently forgotten issue concernsiO toxicity, as there exists a number of studies reporting that

nhalation/exposure to nickel aerosols may result in the devel-pment of asthma. Furthermore the exposure to nickel oxiden animals shows clear indications of carcinogenic effects andon-cancerous lesions [6–8]. Some very interesting approachesave been recently proposed to tackle such physical–chemicalroblems [9]. However, the key to overcome all the drawbacksssociated to the use of Ni–YSZ cermets, especially thoseegarding health issues, lies on the use of alternative materialsdopting two strategies. The first is the use of new cermets

omposed of two or three single-phase oxides as is the case ofu–ceria anode cermets [10] that allow the direct oxidation ofydrocarbon fuels rendering high performance and no carboneposition. The second is the use of mixed oxides, especially

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218 J.C. Ruiz-Morales et al. / Electr

erovskite-type materials such as chromium–manganitesa0.75Sr0.25Cr0.5Mn0.5O3−δ (LSCM) [11,12], or substitutedrTiO3 [13–18] like La4Sr8Ti11Mn0.5Ga0.5O37.5−δ (LSTMG)17,18] or more recently double perovskites as Sr2MgMoO6−δ

19]. Unfortunately, most of these alternative materials requirehe use of an adequate current collector as their electroniconductivity is rather low. Consequently, the development ofurrent collectors is crucial to use them as components of novelnodes alternative to Ni-based cermets.

If the material under study shows high electrical conductiv-ty, is a mixed conductor and/or is prepared as a composite withSZ or CGO, one may consider that the triple phase region is thehole electrode region, from the interface electrolyte–electrode

o the electrode-outer current collector interface. On the otherand, the triple phase boundary (TPB) will be restricted to thenterface close to the current collector if the electronic con-uctivity is low. In these cases, the addition of an electroniconductor could help to extend the TPB through the electrode,hus improving the overall performance.

The use of Cu–YSZ-based cermets may be considered asne of the simplest and cost-effective options to effectivelychieve the aforementioned improvement. Furthermore, an addi-ional potential benefit is the intrinsic catalytic effects ofu–ziconia-based systems [20,21] extensively reported in the

iterature for several reactions involving hydrocarbon-derivedpecies such as CO oxidation [22,23], methanol steam reforming24–28], methanol synthesis [29,30], methanol decomposition31], methane oxidation [32], propene and toluene oxidation33] and carbon black oxidation [34].

Herein, we report the improvement of the polarisationesistances in LSCM and LSTMG-based anodes using Cu aslectronic conductor.

. Experimental

X-ray powder diffraction (XRD) patterns were obtained inPhilips X’Pert Pro diffractometer, equipped with a primaryonochromator (Cu K�1) and a X’Celerator detector. The pat-

erns were performed in 0.0085◦ steps (30 s/step) in the 20–90◦θ range. Several composites were prepared to test the com-atibility in oxidising conditions. LSCM:(YSZ + CuO) powdersere mixed in a ratio 1:1, using three different YSZ:CuO ratio,

.e. 75:25, 50:50 and 25:75. The powders were mixed in angate mortar with acetone, left to dry and fired at 1000 ◦Cor 3 h. The same procedure was applied to the LSTMG pow-ers. Finally, the powders were fired on a platinum foil torevent potential reactions with ceramic substrates. The pow-ers were also treated under humidified 5% H2 at the sameemperature for 20 h to test the compatibility under reducingonditions.

The morphology (microstructure) of the anode material afterhe electrochemical tests was monitored using a scanning elec-ron microscope (Jeol JSM-6300). All samples were covered

ith a thin film of gold, deposited by sputtering to avoid chargingroblems.

LSCM and LSTMG powders were prepared by conventionalolid state reaction. La2O3 was fired at 1000 ◦C for over 3 h

aTfd

ica Acta 52 (2007) 7217–7225

efore weighing to remove any remaining water and/or CO2.toichiometric amounts of the corresponding pre-dried highurity oxides were mixed and ground in acetone, using zirco-ia ball mills for 30 min. The purity of the oxides was: La2O3Alfa Aesar, 99.99%), SrCO3 (Aldrich, 99.9%), TiO2 (Aldrich,9.9%), Cr2O3 (Merck, 99%), Mn2O3 (Aldrich 99%) and Ga2O3Alfa Aesar, 99.999%).

LSCM was obtained after milling the stoichiometric amountsf the corresponding oxides and then uniaxially pressed into aellet and fired at 1500 ◦C for 10 h. Similarly LSTMG pow-ers were obtained after milling, calcining the correspondingrecursors at 1200 ◦C for 6 h. This step was repeated twice.he resulting powder was subsequently ground, mixed andniaxially pressed into pellets and fired in a Pt crucible at200–1400 ◦C for 24–48 h.

About 8 mol% YSZ (Pikem) was used as electrolyte and ascomposite element. Dense YSZ pellets, for symmetrical celleasurements, 1.1–1.3 mm thick were obtained after uniaxially

ressing YSZ powders at 1 ton for 1.5 min and further sinteringt 1500 ◦C for 10 h.

CuO used in the composites was from Aldrich (>99%) andhe grain size was <5 �m.

Several LSCM–YSZ–CuO and LSTMG–YSZ–CuO com-osites were prepared for the polarisation measurements. In allhe composites, the amount of anode material (A) was fixednd the YSZ:CuO (Y:C) ratio was gradually modified, i.e.:Y:C = 50:50:0, 50:40:10, 50:25:25 and 50:10:40 (%, w/w).

n the case of LSTMG an extra composite was also prepared,.e. 50:32.5:17.5.

The powders were mixed in an agate mortar, milled withcetone and left to dry in air. After that, a binder – DecofluxWB41, Zschimmer and Schwartz) – was added to obtain alurry, which was used to paint symmetrical electrodes onto ahick YSZ pellet for symmetrical measurements under the sameas composition and fired at 1000 ◦C for 3 h. Finally, a Pt-basednk was used to paint the outer current collector on both sidesnd fired at 950 ◦C for 30 min. The final electrode thickness waspproximately 60 �m.

The sample was fixed in a setup for symmetrical measure-ents. The cell has been designed following the Primdahl’s

ecommendations [35] to minimise the problems associated withire inductances at high frequency, which could largely affect

he estimation of the polarisation values. A flow of humidified% H2 was passed through the cell to obtain the reduction of theample in situ. The water content was fixed by bubbling the gashrough a humidifier thermostated at 20 ◦C. The concentrationf water at that temperature was 2.3%.

The polarisation measurements were performed on a two-lectrode arrangement. The area specific resistances (ASRs)ere obtained under symmetric atmospheres. a.c. impedanceeasurements of the electrochemical cell was carried out using a

requency response analyser (Solartron 1260) in the 106–0.1 Hzrequency range, at open circuit voltage (OCV). A 50 mV

mplitude a.c. signal was used, rendering reproducible spectra.he impedance data were analysed using the Zview software

rom Scribner [36]. The corresponding polarisation values wereivided by two because of the symmetrical configuration.

J.C. Ruiz-Morales et al. / Electrochimica Acta 52 (2007) 7217–7225 7219

F TPBc layers lectro

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ig. 1. Schemes showing different triple phase boundaries (TPB). (a) Typicallose to the electrolyte. (b) For a pure ionic material the TPB is restricted to theuch as Cu, could extend the TPB from the outer-current collector towards the e

. Results and discussion

It is generally accepted that when the anode is a purelectronic material, the active layer where the electrochemicaleactions take place, the so-called triple phase boundary (TPB),s restricted to a ∼20-�m-layer surrounding the electrolyte [37],ig. 1(a), with an estimated maximum of 50 �m when usingighly resistive electrolytes [35], although some other authorseport smaller regions of approximately 1 �m on both electrolytend electrode when a Ni electrode is used [38,39]. In this sit-ation the only truly dynamic electroactive regions are thoseituated close to the electrolyte.

Considering the anode as a pure ionic material, Fig. 1(b), thePB would be restricted to the layer close to the outer-currentollector, as this will be the only region where electrons, oxy-en ions and gas molecules would meet. The rest of the materialroduces very high polarisation due to the very high resistanceampering electron transport through the whole electrode thick-ess. The lower the thickness, the better will be the apparent

esponse of the material under study. Therefore, the addition ofn electronic conductor such as Cu to an electrolyte will producecermet where the TPB will be extended from the outer-currentollector towards the electrolyte, Fig. 1(c). This should be rel-

muom

where electrochemical active layer made of electrolyte, electrode and gas areclose to the outer-current collector. (c) The addition of an electronic conductor,lyte.

vant for samples with fairly low electrical conductivity, thusemoving the main source of the polarisation values.

As mentioned before, most of the novel alternative anodesLSCM and LSTGM in the present case) proposed in the lit-rature show rather modest electronic conductivities, hence theddition of CuO as a composite might enhance the polarisationalues under reducing conditions. Despite their promising per-ormances, these materials show conductivities of ∼1.6 S/cm11,12] and ∼0.1 S/cm [17,18] for LSCM and LSTMG at00 ◦C, respectively, under humidified pure hydrogen, whichestrict their applications as SOFC anodes to the use of an ade-uate current collector. These values are in the limit of theeported as acceptable values, i.e. 1 S/cm [40]. Consequently,hese systems are ideal to improve their overall electrochemicalesponse using the proposed approach.

The starting point of the present work was a 1:1 elec-rode:YSZ, which allows an improvement of the interfacialontact, an enlargement of the TPB through the whole electrodeaterial and a decrease of the thermal expansion coefficient mis-

atch in comparison to the raw electrode material. However the

se of YSZ in the composite will produce an increase in thehmic losses and consequently will hamper the fuel cell perfor-ance. Therefore a partial replacement of YSZ for Cu should

7220 J.C. Ruiz-Morales et al. / Electrochimica Acta 52 (2007) 7217–7225

F sed ci spond

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ig. 2. Impedance spectra for (a) LSCM-based composites and (c) LSTMG-banformation in 3D to allow a clearer view of the overlapped spectra in the corre

ave a positive effect as one would be approaching the situationhown in Fig. 1(c).

.1. Reactivity

Evidence of chemical reaction between LSCM and LSTMGith YSZ and/or CuO was observed by XRD, after firing inxidising conditions. Additional XRD-studies [41] reveal thathe aforementioned reaction is due to the interaction of Cund YSZ. The samples, previously fired at 1000 ◦C for 3 hn air, were annealed at 950 ◦C under wet 5% H2 for 20 h.o extra reflections were found compared to the XRD pat-

erns in air. However, in LSCM-based composites a weak neweak at 2θ–30.9◦ is observed. This peak has been occasionallyetected under slightly more reducing conditions for LSCM ands thought to be either associated with decomposition to form a

2NiF4-type phase or with some sort of ordering in reducedSCM. In either case it would seem that CuO slightly facili-

ates the reduction process. The intensity of this extra peak wasery low for a reaction of 20 h and it does not affect the overallerformance. In the case of LSTGM-based composites no extraeaks were observed upon reduction.

.2. Electrochemical characterisation

The polarisation resistance related to any process occurringhrough the TPB is coupled in parallel to the double layer capac-tance between the electrochemical active electrode material and

f

C

omposites, measured at 950 ◦C, under humidified 5% H2. (b and d) The sameing 2D representation.

he YSZ. This double layer capacitance exhibit a typical valuef 10 �F/cm2 in Ni–YSZ systems [42]. At least two processesay be expected due to the participation of two possible active

lectrode materials for fuel oxidation, i.e. LSCM/LSTMG andu–YSZ. The kinetics toward this reaction in both materials willontrol the relaxation frequencies between both processes.

.2.1. Morphology of the impedance spectraThe impedance plots obtained under humidified 5%H2,

ig. 2(a) and (c), show at least two rate limiting processes,lthough for the LSTMG composite a new process was observedor the composites with lower Cu content. Thus, an equivalentircuit as LRs(RQ)1(RQ)2(RQ)3 was used to describe the exper-mental data from 950 to 700 ◦C for all the composites. L is annductance element, Rs includes all the ohmic resistances and isermed as series resistances and Ri is the polarisation resistanceorresponding to each limiting process. The total polarisationesistance (Rp) is given by the addition of all the polarisationontributions Ri. Q is a constant phase element with admittance* = Y0(jω)n. The dimension of Y0 is Ssn (�−1 sn), hence the

deal C associated to each constant phase element was calcu-ated to properly compare values. Eq. (2) was used to calculatedeal capacitance values:

summit = 1

2πRC= 1

2π(RY0)1/n(1)

= R(1−n)/n(Y0)1/n (2)

J.C. Ruiz-Morales et al. / Electrochimica Acta 52 (2007) 7217–7225 7221

Fig. 3. Temperature dependence of the parameters obtained from the analysis of the equivalent circuit: LRs(RQ)1(RQ)2(RQ)3, using the Zplot software. The resultsfor the composites with ratio LSCM:YSZ:CuO (%, w/w) = 50:40:10, 50:25:25 and 50:10:40 are shown in columns 1, 2 and 3, respectively. (a–c) Single and totalp –i) thf

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olarisation values. (d–f) The ideal capacitance associated to each CPE and (grequency arcs, respectively).

SCM-based composites, Fig. 2(a), show one process at veryigh frequencies (100 kHz to 1 MHz), that slightly decrease withncreasing the Cu content, Fig. 3(g)–(i). This contribution isot usually very important except in lower temperature rangeT < 750 ◦C), Fig. 3(a)–(c). The main process, which is control-ing the morphology of the spectra, appears at low frequencies0.5–3 Hz), with associated values of approximately 0.5 F/cm2

nd remains almost constant with the temperature. This valueould indicate absorbed charged species as occurs in the Ni–YSZermets [43].

LSTMG-composites, Fig. 2(b), shows one process at very

igh frequencies ∼1 MHz and another at intermediate frequen-ies (10–60 kHz), Fig. 4(g)–(i). Both processes are slightlyemperature-dependent and control the morphology of thepectra with almost the same contribution for each process,

bcte

e corresponding relaxation frequencies (where HF and LF denotes high a low

ig. 4(a)–(c). The third process exhibits relaxation frequen-ies ranging between 15 and 80 Hz and is only important foromposites with CuO content ≤25%. The polarisation is approx-mately one or two orders of magnitude lower than the overallne and therefore its contribution is not very important. Indeed,his third arc disappears in composites with Cu-contents above0%. In this composite, the data suggest a possible microstruc-ural change. The capacitance of the second process notablyncreases and there is a significant change in the relaxation fre-uencies as a function of the temperature in both processes. Inll cases, the bode plot consists of a single peak with a very

road distribution, which might indicate a very low interfacialontact. The lower the contact surface, the higher the polarisa-ion resistances will be as occurs in LSTMG-based composites,specially for the composites containing 40% of CuO.

7222 J.C. Ruiz-Morales et al. / Electrochimica Acta 52 (2007) 7217–7225

Fig. 4. Temperature dependence of the parameters obtained from the analysis of the equivalent circuit: LRs(RQ)1(RQ)2(RQ)3, using the Zplot software. The resultsf and 5p g–i) ti

3

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or the composites with ratio LSCM:YSZ:CuO (%, w/w) = 50:40:10, 50:25:25olarisation values. (d–f) The ideal capacitance associated to each CPE and (ntermediate and low frequency arcs, respectively).

.2.2. Series resistances, Rs

The series resistances for LSCM and LSTMG-based com-osites are shown in Fig. 5(a) and (b), respectively. Despitehe rather poor microstructure, Fig. 6(a)–(d), the LSCM-based

aterial exhibits a good adherence between electrode and elec-rolyte. The same conclusion can be obtained from the matchetween the experimental Rs with the resistance of a blank YSZellet (dash line), which also indicates that the electrode con-ribution to the ohmic losses is negligible even at low CuOontents.

The activation energy of a blank YSZ in the high temperature

ange, i.e. 750–950 ◦C, is 0.74 eV, which is equivalent to thealues obtained for all the composites (0.74–0.76 eV), indicatinghat the nature of the charge carrier controlling the Rs value ishe same in all cases.

sr2w

0:10:40 are shown in columns 1, 2 and 3, respectively. (a–c) Single and totalhe corresponding relaxation frequencies (where HF, IF and LF denotes high,

For LSTMG-based composites, the contact between elec-rode and electrolyte is rather poor as can be extracted fromhe series resistance values, Fig. 5(b) and SEM images,ig. 6(e)–(h). The ohmic drop associated to materials with an

ntrinsic low electrical conductivity is overcome only for com-osites containing 25% of CuO. The main problem in this caseelates to the low firing temperature (1000 ◦C), which is notigh enough to produce a good contact between grains (in thelectrode) and to properly attach the electrode to the electrolytend therefore, the addition of more CuO could help due to itsell-known capability as sintering aid [44]. Indeed the very

mall particle size in Fig. 6(e) (lower than 0.5 �m) becomesapidly much larger with the addition of 10% CuO, Fig. 6(f) and5%CuO, Fig. 6(g). The apparent activation energy is 0.74 eV,hich indicates that the nature of the charge carrier is the same

J.C. Ruiz-Morales et al. / Electrochimica Acta 52 (2007) 7217–7225 7223

M-ba

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3

ca

actaovsrac

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Fig. 5. Temperature dependence of the series resistances (Rs). (a) LSC

han for the LSCM-based composites. The activation energyemains almost unchanged at higher CuO contents (40%), e.g..75 eV, although the series resistances are much larger. Thisan be explained in terms of the contact among YSZ particlesYSZcomposite–YSZelectrolyte), known as bridge-YSZ particles.his contact remains stable, although Cu agglomerates as wells the YSZ particles (due to the sintering effect of the Cu), pro-ucing larger particles, Fig. 6(h), decreasing the real surfaceontact and hence increasing the series resistances.

.2.3. Polarisation resistances, Rp

The polarisation resistances for LSCM and LSTMG-basedomposites under 5% H2 feed are shown in Fig. 7(a) and (b). Inll cases the temperature dependence is the same, with apparent

F

Ch

ig. 6. SEM pictures of cross-sections of four anode composites after testing. For LSChe ratio Anode:YSZ:CuO (%, w/w) was: (a and e) 50:50:0; (b and f) 50:40:10; (c an

sed and (b) LSTMG-based composites. Measured under wet 5% H2.

ctivation energies of approximately 1.05–1.14 eV. This indi-ates that the nature of the main limiting process is the same in allhe composites investigated, independent of the microstructurend anode composition. Nevertheless, one should note that theverall behaviour in LSTGM is controlled by two processes withery similar parameters. In both cases, these limiting processeshow typical capacitance values of 1–10 �F/cm2, similar to thoseeported for Ni-YSZ cermets [42], as also occurs for the apparentctivation energies, i.e. 1.0–1.5 eV [45,46]. The LSTMG-basedomposite with 40%CuO shows a higher activation, i.e. ∼1.5 eV,

ig. 7(b), that may be due to the poor microstructure, Fig. 6(h).

For LSCM, Fig. 8(a), it is very clear that the addition of 10%uO improves the polarisation values by a factor of two underumidified 5% H2, i.e. from ∼2 to ∼1 � cm2 at 950 ◦C. The opti-

M-based composites (a–d) and LSTMG-based composites (e–h). In both casesd g) 50:25:25; (d and h) 50:10:40.

7224 J.C. Ruiz-Morales et al. / Electrochimica Acta 52 (2007) 7217–7225

SCM

mftAtbaiF

tv

tecctFd

ClrarCtacpwadtbS

FH

Fig. 7. Temperature dependence of the polarisation resistances (Rp). (a) L

um value is around 15% of CuO. Increasing the CuO contenturther does not produce any benefit and, indeed, for 40% CuOhe polarisations are three times larger than the previous values.lthough the series resistances improve due to the increase in

he electronic conductivity, the polarisations get worse, proba-ly due to Cu forming large particles and in many cases the YSZnd LSCM grains will be enclosed in a Cu matrix, thus decreas-ng the TPB and producing a situation close to that shown in theig. 1(a).

For LSTMG, Fig. 8(b), the results are similar and the addi-ion of 10% CuO produces the same benefits in the polarisationalues by a factor of two, at 950 ◦C, under humidified 5% H2.

Consequently, the addition of certain amounts of CuO helpso improve the polarisation values. Two possible factors couldxplain such an improvement. On one hand, the sinteringapability of CuO helps to bridge electrode particles together,

reating new electronic paths and hence effectively increasinghe triple phase boundary through the whole electrode material,ig. 1(c). On other hand, one may assume that the Rp valuesepend on the catalytic activity of the material under study.

taai

ig. 8. Dependence of Rp with the CuO-content in the corresponding composite, for

2.

-based and (b) LSTMG-based composites. Measured under wet 5% H2.

u has been considered as an inert and excellent current col-ector and YSZ as an ionic conductor. However, there existseported evidence of the catalytic properties of ZrO2–Cu cat-lysts for several reactions [20–34] as mentioned before. Veryecently, Itome and Nelson [32] have shown the potential ofo–, Cu–, and Ag–YSZ cermets as alternative to replace the

raditional Ni–YSZ cermets, due to their good catalytic activitynd moreover resistance to carbon deposition and high methaneonversion and therefore one would suggest that Cu–YSZ maylay an important role in the electrocatalytic activity. In all theseorks [20–34] the main conclusion is that the catalytic effects

nd reactivity of the combination Cu–zirconia-based materialepends on the particle size of the current collector, surface area,he microstructure of the oxide used as support and the distri-ution of the oxygen vacancies in the oxide particles [20,21].imultaneously, the oxide support helps to expand the distribu-

ion of the current collector particles by increasing the surfacerea. Consequently, the catalytic effects largely depend on thebility to produce nanoparticles and a well-dispersed Cu metaln the oxide used as support. However, in our case, it is very

(a) LSCM and (b) LSTMG-based ones, respectively. Measured under wet 5%

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ikely that the solid state route and the relatively high tempera-ures used will not produce nanoparticles, thereby the main rolef Cu is restricted to act as an sintering agent, helping to bridgelectrode particles and additionally good current collector.

. Conclusions

In summary, it has been shown that the addition of an elec-ronic conductor like Cu effectively helps to extend the TPB fromhe outer current collector towards the electrolyte; similarly, anonic conductor extends the TPB from the electrolyte towardshe outer current collector. In this case, the TPB enlargement isroduced due to the sintering effect of Cu that links electrodearticles to create electronic percolation paths.

The use of Cu promotes a marked improvement of the polar-sation values in materials with low electrical conductivities,s is the case of the recently proposed perovskites LSCM andSTGM. In both cases, the Rp values were enhanced by a factorf two with just an addition of 15% of CuO. Further additionroduces agglomerated structures causing larger ohmic lossesnd hence worse performances, despite the series resistancesmprove. Nevertheless, recent studies indicate that the combi-ation of Cu–YSZ have intrinsic catalytic effects, which in turnight explain such improvement. However the solid state route

nd the relatively high temperatures used to prepare the com-osites are likely to produce an inhibition of this effect, givenhe impossibility to obtain nanoparticles under the conditionsxplored which seems to be the key issue. Therefore, cermetsrepared by impregnation are very likely to take full advantagef this reported catalytic activity and hence they should exhibituch better performances.

cknowledgements

J.C.R.-M. and J.C.-V. acknowledge the Spanish “Ministe-io de Educacion y Ciencia” for “Ramon y Cajal” Fellowships.his work was supported by the Spanish Research pro-ram (MAT2004-3856) and the Canary Islands GovernmentPI2004/093) J.T.S.I acknowledges EPSRC for support.

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J.K. Dunnicka, C.H. Hobbs, Fundam. Appl. Toxicol. 15 (1990) 476.

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