Applicability of La 2Mo 2− y W y O 9 materials as solid electrolyte for SOFCs

007) 1366–1378www.elsevier.com/locate/ssi

Solid State Ionics 178 (2

Applicability of La2Mo2−yWyO9 materials as solid electrolyte for SOFCs

D. Marrero-López a, J. Peña-Martínez a, J.C. Ruiz-Morales a, D. Pérez-Coll a,M.C. Martín-Sedeño b, P. Núñez a,⁎

a Departamento de Química Inorgánica, Universidad de La Laguna, 38200-La Laguna, Tenerife, Spainb Departamento de Química Inorgánica, Universidad de Málaga, 29071-Málaga, Spain

Received 12 February 2007; received in revised form 22 June 2007; accepted 25 July 2007

Abstract

Electrolyte materials with composition La2Mo2−yWyO9 (y=0–1.5) exhibit high ionic conductivity ranging from 0.11 S cm−1 (y=0) to 0.05 Scm−1 (y=1.5) at 1023 K, which is comparable to gadolinia doped ceria. The ionic conductivity is predominant in a wide range of oxygen partialpressures from 0.21 to 10−20 atm at 973 K with ionic transport numbers higher than 0.95 under humidified 5%H2–Ar. Above this temperature, asignificant increase of the n-type electronic conductivity and degradation of the ceramic microstructure are observed as a consequence of the cellvolume expansion in addition to the formation of new phases upon reduction. Considering the potential use of these materials as solid electrolytes,the chemical compatibility with most of the typical electrodes commonly used in SOFCs (e.g. cobaltites, ferrites, chromites and manganites) wasinvestigated. Severe chemical reaction was found between La2Mo2−yWyO9 and most of the studied electrodes, mainly due to molybdenummigration. Furthermore, the high thermal expansion coefficients of La2Mo2O9 based materials (∼15 ·10−6 and ∼20 ·10−6 K−1 in the low and hightemperature range, respectively) restrict the choice of compatible materials as cell components. Moreover, electrolyte and electrodes present poorcontact due to the low fixing temperature, necessary to prevent excessive chemical reaction between the materials, and also the large thermalexpansion coefficients of lanthanum molybdate materials, resulting in the separation of both materials after several consecutive thermal cycles.Therefore, the use of these materials as SOFC electrolyte seems to be rather limited.© 2007 Elsevier B.V. All rights reserved.

Keywords: La2Mo2O9; Solid electrolyte; SOFC; Chemical stability

1. Introduction

Solid Oxide Fuel Cells (SOFCs) are devices for theelectrochemical conversion of a fuel directly into electricalpower. Such devices render theoretical efficiencies significantlyhigher than those based on the combustion of fossil fuels [1–3].

An SOFC consists of three main ceramic components: ananode and a cathode separated by a solid electrolyte. Theelectrolyte must be an oxide ion conductor with negligibleelectronic contribution and must be dense to prevent gas mixing.Whilst the anode and cathode should have good electronicconductivity in addition to certain catalytic activity towards fueloxidation and oxygen reduction respectively. Furthermore, theelectrodes must be porous to allow the transit of reagents to theactive sites.

⁎ Corresponding author. Tel.: +34 92 2318501; fax: +34 92 2318461.E-mail address: [email protected] (P. Núñez).

0167-2738/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.ssi.2007.07.008

Each cell component must have a suitable stability (chemical,morphological and dimensional) in the corresponding gas-environment. In other words, the ceramic components must bechemical compatible to avoid undesirable reactions that maylead to the formation of new phases in the interface electrolyte/electrodes, hence decreasing the cell efficiency. The cell compo-nents must also exhibit similar thermal expansion coefficients toavoid separations or fractures during fabrication and operation ofthe cell.

Yttria-stabilised zirconia (YSZ) [1,4] is the most commonlyused SOFC electrolyte as it exhibits good thermal and chemicalstability to operate at high temperature, although the ionicconductivity is insufficient below 1073 K to achieve highefficiency. Alternative oxide-ion conductors have been reportedover the last few years, including doped CeO2 with fluorite-typestructure [5–7], LaGaO3-based perovskites [8,9], apatitesderived from La10−xSi6O26±δ [10–12] and La2Mo2O9 basedmaterials (LAMOX) [13–15]. These phases exhibit higher ionicconductivity than YSZ in the intermediate temperature range,

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1367D. Marrero-López et al. / Solid State Ionics 178 (2007) 1366–1378

but present some limitations that must be overcome before theybecome competitive compared to YSZ. In particular, the partialreduction of Ce4+ to Ce3+, in doped ceria produces an expansionof the lattice which may lead to mechanical failure. Further-more, such reduction increases the n-type electronic contribu-tion to the overall conductivity at temperatures above 873 Kunder reducing atmospheres [6]. The applicability of lanthanumgallate at low oxygen partial pressures is limited by volatilisa-tion of gallium oxide [16]. Apatite phases generally exhibit poorsinterability and undergo moderate degradation of the ionictransport with time under reducing conditions and above1100 K, which is ascribed to silicon volatilisation [11].Nevertheless, these limitations can be minimised by operatingin the intermediate temperature range. Indeed, high perfor-mances have been obtained with ceria based electrolytesoperating in an SOFC at 873 K, and doped lanthanum gallateat 1073 K [17,18]. On the other hand, La2Mo2O9-basedelectrolytes present high conductivity comparable to most ofthe ionic conductors already mentioned, although with a limitedstability range under reducing conditions as Mo6+ is relativelyprone to be reduced, causing an enhancement of the n-typeelectronic conductivity and also promoting its degradation [19–21]. Previous reports have demonstrated that the substitution ofMo6+ by W6+ stabilises the β-polymorph at room temperature[22–24], suppressing the α↔β transformation and extendingthe redox stability range [25–28] with ionic transport numbersgreater than 0.98 at 1073 K and oxygen partial pressure above10−15 atm [26]. Therefore, tungsten-substituted La2Mo2O9

could be possibly used as SOFC electrolytes in the lowtemperature range and under moderate reducing conditions.Nevertheless, electrode materials compatible with LAMOX arenecessary to produce efficient cells. In this context, Corbel et al.have studied the chemical compatibility of undoped α-La2Mo2O9 with some electrodes [29,30] and they concludedthat nickel–ceria cermets can be considered as compatibleanodes.

Ni–YSZ cermets are the most widely-used SOFC anodes,although there are certain disadvantages related to their use, i.e.sulphur poisoning, carbon deposition when using hydrocarbonfuels and volume instability upon redox cycling causing amarked drop of the anode performance [31]. Several alter-natives have been proposed to overcome this issue, as are thecases of copper-based cermets [32] and the development of thenew oxide anodes [33]. Copper-based anodes do not promotecarbon deposition and tolerate a wide range of fuels, such asmethane and other light hydrocarbon fuels. Alternatively, somemixed oxides have been proposed as anode materials, most ofthem based upon perovskite structure and exhibiting sulphurtolerance, negligible formation of carbon deposits and highcatalytic activity towards hydrocarbon oxidation, e.g. titanatesderived from (La,Sr)TiO3−δ [34,35], chromite–manganites (La,Sr)(Cr,Mn)O3−δ [36] and double perovskites Sr2MgMoO6−δ[37].

Lanthanum strontium manganite (La,Sr)MnO3−δ is thestandard SOFC cathode as it shows high electronic conductivity[1,2], although the catalytic activity for oxygen reduction israther modest below 1073 K. A number of alternative cathode

compositions, all of themwith the perovskite structure, have beenproposed, including cobaltites and ferrites with lower areaspecific resistance than manganites at low temperature as873 K, i.e. La0.7Sr0.3CoO3−δ, Ba0.5Sr0.5Co0.8Fe0.2O3−δ,Sm0.5Sr0.5CoO3−δ and La0.7Sr0.3FeO3−δ [18,38–40].

In the work presented herein, we have evaluated the stabilityof dense ceramic La2Mo2−yWyO9 materials under reducingconditions as a function of both temperature and annealing time.Furthermore, the chemical compatibility with different electro-des was also investigated to evaluate the applicability of thesematerials as SOFC electrolytes.

2. Experimental

2.1. Synthesis of the materials

Polycrystalline La2Mo2−yWyO9 powders (y=0–1.5) wereprepared via a modified freeze-drying precursor method asdescribed elsewhere [26,41]. The un-substituted α-La2Mo2O9

polymorph is a superstructure of the high temperature cubic β-polymorph below 830 K [23]. In addition, W-substituted phasespresent cubic structure in the composition range 0.5≤y≤1.5,suppressing the α↔β transformation of undoped La2Mo2O9 atintermediate temperature, whilst at yb0.5 a slight monoclinicdistortion of the unit cell was clearly observed by highresolution X-ray diffraction [24]. In addition, a triclinic phaseisostructural with α-La2W2O9 is stabilised for yN1.5 [22].

Electrode materials were prepared by freeze-drying, sol–gelcitrate method or traditional ceramic route (Table 1). Thestarting materials were: La2O3 (99.99%), Sm2O3 (99.99%),MoO3 (99.5%), WO3 (99.5%), Ba(NO3)2 (99%), Sr(NO3)2(99.9%), Ca(NO3)2·4H2O (99%), Co(NO3)2·6H2O (99%), Fe(NO3)3·9H2O (98%), Mn(NO3)2·6H2O (99.99%), Cr(NO3)3·9H2O (99%), Ni(NO3)2·6H2O (99%), Cu(NO3)2·3H2O(99%) and titanium isopropoxide (97%), all of them supplied byAldrich. Rare earth oxides were precalcined at 1273 K for 2 h inorder to achieve dehydration and decarbonation. Thecorresponding metal nitrates, which are generally hygroscopic,were decomposed up to 1273 K in a TG/DTA (Perkin Elmer,mod. Pyris Diamond) to determine the water content and toensure the correct cation stoichiometry in the final compounds.

2.1.1. Freeze-drying methodThe freeze-drying method was carried out using ethylene-

diaminetetraacetic acid (EDTA) (99.5% Aldrich) as complexagent in a 1:1 molar ratio ligand:metal. Stoichiometricsolutions of the corresponding cations were prepared dissol-ving metal nitrates in distilled water; lanthanide oxides indiluted nitric acid; and molybdenum and tungsten oxide indiluted ammonia. The pH was adjusted to 9 by addingammonia solution. The volume of the resulting stoichiometriccation solution was 100 ml and the cation concentration wasadjusted to obtain approximately 5 g of the final oxide.Droplets of this solution were subsequently flash frozen inliquid nitrogen and then freeze-dried in a Heto Lyolab freeze-dryer for 3 days. The dried precursor powders were initiallyprecalcined at 573 K to produce the pirolysis of the organic

Table 1Composition, code, synthesis method, firing temperature and cell parameters at room temperature are listed for several electrolyte and electrode materials

Composition Code Method Ts (K) s.g. Cell parameters (Å)

La2Mo2O9 LMO FD 1223 P21 a=7.1443(7), b=7.1565(7), c=7.1580(7), β=89.561(5)°La2Mo1.5W0.5O9 W0.5 FD 1273 P213 a=7.1584(4)La2Mo0.5W1.5O9 W1.5 FD 1273 P213 a=7.1395(2)Ba0.5Sr0.5Co0.8Fe0.2O3−δ BSCF FD 1273 P m–3 m a=3.9812(6)Sm0.5Sr0.5CoO3−δ SSC FD 1273 P n m a a=5.400(1), b=7.586(2), c=5.364(1)SrCo0.8Fe0.2O3−δ SCF FD 1273 P m–3 m a=3.8604(4)La0.7Sr0.3CoO3−δ LSC FD 1173 R-3 c a=s5.435(2), c=13.206(7)La0.8Sr0.2MnO3−δ LSM FD 1173 R-3 c a=5.5149(8), c=13.369(2)La0.7Sr0.3FeO3−δ LSF SG 1273 P b n m a=5.510(2), b=5.549(2), c=7.814(3)La0.7Ca0.3CrO3−δ LCC SS 1673 P n m a a=5.4391(9), b=7.691(1), c=5.458(8)La0.75Sr0.25Cr0.5Mn0.5O3−δ LSCM SG 1523 R-3 c a=5.491(2), c=13.313(4)La0.75Sr0.25Cr0.5Fe0.5O3−δ LSCF SG 1473 P m–3 m a=3.8937(7)NiO NiO FD 673 F m–3 m a=4.1813(9)CuO CuO FD 673 C2/c a=4.658(2), b=3.428(1), c=5.134(2), β=99.36(1)°Sr2MgMoO6−δ SMM FD 1273 P 21/c a=5.5876(6), b=7.8804(5), c=5.5858(6), β=90.324(6)°La4Sr8Ti11Mn0.5Ga0.5O37.5−δ LSTGM FD 1473 P m–3 m a=3.9044(3)

Note: FD: freeze-drying, SG: sol–gel citrate, SS: solid state reaction, Ts: firing temperature.

1368 D. Marrero-López et al. / Solid State Ionics 178 (2007) 1366–1378

matter and then calcined at 873 K for 5 h to remove theresidual organic species. All materials were single phases aftercalcination at the temperatures summarised in Table 1 for 5 hwith the exception of Sr2MgMoO6−δ that was fired in air at1273 K for 24 h and then reduced at the same temperature forfurther 24 h, under dry 5%H2–Ar to avoid the segregation ofSrMoO4 impurities.

2.1.2. Sol–gel citrate methodThe sol–gel citrate method was carried out from stoichio-

metric aqueous solutions of reactants. A citric acid solution,1.5 wt.% in excess, was added considering L/M=1 and 2/3ratios for trivalent and divalent cations respectively. Then, thepH was adjusted to 9 by the addition of ammonia solution. A gelwas formed after continuous stirring and mild heating (∼338 K)and then left to dry at room temperature for over 24 h. The driedgel was fired at 573 K for 30 min in a first step and finallycalcined as summarised in Table 1.

2.1.3. Ceramic routeLa0.7Ca0.3CrO3−δ was prepared following the ceramic route

after ball-milling the stoichiometric amounts of reagents inacetone. The resulting mixture was dried and fired in air up to1173 K for 10 h and further ground. A final thermal treatment at1673 K was necessary to obtain a single phase.

2.2. Material characterisation

X-ray powder diffraction (XRD) patterns were recordedusing a Philips X´Pert diffractometer using CuKα radiation. Thescans were performed in the (5–80°) 2θ range with 0.02° stepand at 10 s/step. Structure refinements were carried out usingthe Fullprof program assisted by WinPlotr suite [42,43]. Phaseidentification was performed with X´Pert HighScore Plus v.2.0asoftware [44]. All investigated electrodes were single phasesafter calcined at the temperatures summarised in Table 1 with-out any minor secondary phase. The cell parameters obtained by

the Rietveld refinement at room temperature are consistent withthose reported in ICSD database [45].

XRD studies were also performed to evaluate the chemicalcompatibility of La2Mo2−yWyO9 (y=0.5 and 1.5) electrolyteswith the different electrodes summarised in Table 1. Mixturesof electrolyte and electrodes powders were prepared in a 1:1(wt.%) ratio, ground in an agate mortar and fired at 773–1273 K for 24 h. Additionally, the redox stability of theelectrolytes was studied by XRD after exposing both powdersand sintered pellets to a wet 5%H2–Ar flow for 24 h between873 and 1173 K. The morphology of the samples before andafter reduction was monitored using a scanning electronmicroscope (SEM) (mod. Jeol LTD, JSM-6300) coupled withan energy-dispersive spectroscopy (EDS) detector. The speci-mens were covered with a sputtered thin film of gold for imageacquisition or of carbon for EDS analysis and to avoid chargingproblems.

X-ray photoelectron spectroscopy (XPS) measurementswere carried out to evaluate the oxidation states of Mo and Win the reduced samples. XPS spectra were recorded on aPhysical Electronics PHI-5700 spectrometer equipped withMg–Kα and Al–KαX-ray sources and a hemispherical analyser.The data were analysed using PHI ACCES ESCA-V6.0 Fsoftware. The experimental peaks were decomposed intocomponents using mixed Gaussian–Lorentzian functions. Thebinding energies (BE) were referenced to the adventitiouscarbon C(1s) at 284.4 eV.

A thermomechanical analyser (Perkin Elmer, mod. PyrisDiamond) was used to determine the thermal expansioncoefficients of the sintered samples at a heating rate of 1 Kmin−1 between room temperature and 1273 K.

2.3. Electrical characterisation

Dense ceramic pellets with relative density N98% wereprepared by pressing the oxide powders into disks of 10 mm ofdiameter and 1.5–2 mm of thickness at 100 MPa and then

Fig. 1. Thermal dependence of the overall conductivity of La2Mo1.5W0.5O9

(W0.5) and various solid electrolyte materials: Zr0.84Y0.16O1.92 (YSZ) [1],Ce0.8Gd0.2O1.9 (CGO) [51], La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM) [56]. The insetshows the variation of the overall conductivity as a function of the tungstencontent in La2Mo2−yWyO9 at different temperatures.


sintered in air at 1373 K for 5 h. The overall conductivity of thepellets was determined by impedance spectroscopy using a1260 Solartron Impedance Analyser in the 0.1 to 106 Hzfrequency range from 1223 to 523 K with a cooling rate of 5 Kmin−1 and stabilisation time of 15 min between consecutivemeasurements. An ac signal of 50 mV was applied in the hightemperature range and 200 mV in the low temperature range,below 773 K. Pt-paste electrodes were painted on each side ofthe pellet and then fired at 1223 K for 15 min to ensure goodelectrical contact with the electrochemical cell. Impedancespectra were acquired with ZPlot and analysed with ZViewsoftware [46] using equivalent circuits as described elsewhere[26].

Conductivity measurements as a function of oxygen partialpressure [pO2 from 0.21 atm to ∼10−20 atm] were performed ina closed tube furnace cell at 973 K. The pO2 values weremonitored by using an YSZ oxygen sensor placed close to thesample. The process consisted in flushing the system withhumidified 5%H2–Ar gas mixture for 15 h at 973 K to reach aminimum in oxygen activity inside the furnace. After that, theoxygen partial pressure was slowly raised back to atmosphericpressure by free diffusion as the system is not gas-tight. Theisothermal lasted for over 30 h and the conductivity data wereacquired every 30 min.

The ionic transport numbers were determined by a modifiedelectromotive force method as described in detail elsewhere[47,48]. This modification of the traditional emf techniqueeliminates possible errors in the determination of ion transfer-ence numbers arising from the electrode polarisation [49,50].The ionic transport numbers to were measured under an oxygenpartial pressure gradient of O2/air, Ar/air and wet 5%H2–Ar/air,using a continuous flux of these gases in the temperature range850–1123 K. The water content was fixed by bubbling the gasthrough a humidifier thermostated at 298 K. The waterconcentration at that temperature was estimated as 3%.

Electrode and electrolyte powders were mixed in a 1:1 (wt.%)ratio to prepare electrode composites. A binder (Decoflux) wasadded to obtain a slurry that was used to paint symmetricalelectrodes on dense pellets after calcination at 1023–1073 K for1 h. Pt-paste was used as current collector. The specific arearesistance (ASRs) values were obtained under symmetricalatmospheres in a two electrode configuration using a continuousflow of air or humidified 5%H2–Ar. The series resistance of thecell was obtained from Z´-intercept of the impedance spectra athigh frequency. Polarisation resistance values were determinedfrom the difference of Z´-intercept at high and low frequency. Thecorresponding polarisation values were divided by two to obtainthe ASR values, because of the symmetrical configuration.

3. Results and discussions

3.1. Transport properties

The thermal dependence of the overall conductivity for arepresentative sample with composition La2Mo1.5W0.5O9 isshown in Fig. 1. For comparison the conductivity data ofvarious solid electrolytes are also presented. As can be observed

the conductivity of tungsten substituted La2Mo2O9 is higherthan that of YSZ in the intermediate and high temperature rangeand comparable to doped ceria and doped lanthanum gallate.The variation of conductivity with the tungsten content inLa2Mo2−yWyO9−δ is shown in the inset of Fig. 1 at threedifferent temperatures. The conductivity in the low temperaturerange decreases almost one order of magnitude as the tungstencontent increases. However, in the intermediate temperaturerange the conductivity is less affected by the tungstencomposition with values ranging from 0.11 S cm−1 (y=0) to0.05 S cm−1 (y=1.5) at 1023 K, which is comparable togadolinia doped ceria (e.g. 0.06 for Ce0.8Gd0.2O1.9 at 1023 K[51]). In addition, the conductivity for samples with hightungsten content is nearly independent of the oxygen partialpressure between 0.21 and 10−20 atm at 973 K, suggestingpredominant oxide ion conductivity under these conditions(Fig. 2a). These results are somewhat contradictory with thosereported by Georges et al. for similar compositions [52]. Theseauthors reported a significant decrease of the overall conduc-tivity as the pO2 decreases, although one would expect anincrease of the n-type electronic conductivity below a certainvalue of pO2, when the molybdenum is reduced to loweroxidation states, as reported in some other publications for othermembers of the LAMOX family [26,53,54]. It should be notedthat the different synthetic methods and lower sinteringtemperatures used in the present work affect the microstructureand the overall conductivity [48] and possibly the ionic andelectronic contributions in addition to the redox stability. Indeed,molybdenum segregation produces an enhancement of theelectronic contribution under moderate reducing conditions [20].

Fig. 2. (a) Oxygen partial pressure dependence of the overall conductivity at 973Kof La2Mo0.5W1.5O9 (W1.5) and various solid electrolytes, Ce0.8Gd0.2O1.9 (CGO)[7] and La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM) [55]. (b) Thermal dependence of theionic transport numbers forW1.5 under different gradients: O2/air (full square,▪),Ar/air ("crossed" open square, ⊞) and 3%H2O–5%H2–Ar/air (open square, □)and determined by a modified emf method; for CGO determined under O2/airgradient by an emf method [51] (full triangle, ▴) and at 10−20 atm/air gradientdetermined by ion-blocking technique [57,58] (open triangle,△); and for LSGM(circle, ○) under O2/air gradient evaluated by an emf method [56].

Fig. 3. XPS spectra for La2Mo1.5W0.5O9 sample: (a) Mo3d as-prepared (b)Mo3d after reduction at 923 K under humidified 5%H2–Ar. XPS spectra afterreduction of La2Mo1.5W0.5O9 at 973 K and complete transformation to trigonalLa7Mo5.25W1.75O30 structure, observing two doublets for (c) Mo3d spectraassociated to Mo6+ and Mo4+ atoms, on the contrary only W6+ is observed in (d)W4f spectra. O1s spectra for β-La2Mo1.5W0.5O9 is presented in (e).


The electrolytic domain for La2Mo0.5W1.5O9 (pO2N10−20 atm at

973 K) is lower compared to that of La0.9Sr0.1Ga0.8Mg0.2O1.85

[55], although it seems to be greater when compared to that one ofdoped ceria, Ce0.8Gd0.2O1.9 (pO2N10

−15 atm at 973 K) in theintermediate temperature range [7]. It should be noted that phasedecomposition, occurring for La2Mo0.5W1.5O9 above 973 Kand under flushing 5%H2–Ar, arises from the partial reductionof Mo6+ to lower oxidation states, which is accompanied by asignificant increase on the n-type electronic conductivity. How-ever, the partial reduction of Ce4+ to Ce3+ in doped ceria does notcause a phase transformation as for LAMOX materials.

The ionic transport numbers (Fig. 2b) determined under anoxygen partial pressure gradient of O2/air for La2Mo0.5W1.5O9

(W1.5) remain above 0.99 up to 1200 K and they arecomparable to La0.9Sr0.1Ga0.8Mg0.2O3−δ (LSGM) [56] andlower than those of doped ceria (CGO) [51]. In Ar(10−5 atm)/air (0.21 atm) gradient the ionic transport numbersfor W1.5 decrease slightly to 0.98 at 1173 K. Finally, the ionictransport number under 3%H2O–5%H2–Ar/air gradient is closeto the unity for W1.5 below 850 K and they remain higher thanthose of doped ceria [51,57,58], although, the ionic transportnumbers for W1.5 decrease abruptly above 973 K, when phasedecomposition occurs. Therefore, the electronic conductivity oftungsten-substituted La2Mo2O9 samples is sufficiently small tooperate as SOFC electrolytes, at least under moderate reducingconditions and temperature range 773–973 K, where the ionicconductivity is still high enough compared to most importantsolid electrolytes.

4. Redox stability

The phase stability under reducing conditions forLa2Mo2−yWyO9−δ series was studied in a previous workusing coulometric titration measurements and controlled re-duction of the samples under a constant flux of dry 5%H2–Ar[26]. The un-substituted La2Mo2O9 phase is structurally stableunder diluted hydrogen atmosphere below 923 K. Highertemperature of reduction produces initially a new compoundidentified as La7Mo7O30 [59] and finally amorphisation of the


La2Mo2O9 structure after reduction at 973 K under dry 5%H2–Ar. The presence of W6+ leads to an increase of the stabilityrange and prevents oxygen loss and amorphisation [26]. In thecase of samples with tungsten content, La2Mo2−yWyO9−δ(0.25b yb0.75), the reduction under 5%H2–Ar atmosphere at973 K or higher temperatures produces a compound isostruc-tural with La7Mo7O30. On the other hand, no decomposition orphase change was observed for y=1.5 up to 973 K, but at1023 K a mixture of β-La2Mo2O9 and La7Mo7O30 phasesappeared.

Thus an oxygen deficiency δ equals or higher than 0.5 leads tothe complete decomposition of the β-La2Mo2O9 structureresulting La7Mo7O30 compound as confirmed by XPS in thepresent work. Fig. 3a reveals the characteristic Mo3d spectra inLa2Mo2O9 with two peaks (Mo3d5/2 and Mo3d3/2) assigned toMo6+. A tail appeared at low binding energy (BE) side afterreduction in humidified 5%H2–Ar at 923 K for one night(Fig. 3b). The spectra was fitted with two doublets and the BEvalues obtained∼232.3 and 230.9 eVwere assigned toMo6+ andMo4+, respectively, according to literature data [60]. WhenLa2Mo1.5W0.5O9 is reduced under 5%H2–Ar at 973 K to changein the trigonal La7Mo5.25W1.75O30 structure, two doublets forMo3d5/2–3/2 spectra are observed (Fig. 3c), which could be alsoattributed to Mo6+ and Mo4+ cations. The area ratio of the twopeaks, A(Mo6+)/A(Mo4+) in the reduced compound La7Mo5.25W1.75O30 is 2.55, consequently the molar percentages of Mo6+

and Mo4+ are about 72 and 28%, respectively, hence the averageoxidation state of molybdenum is +5.44. This is in clearagreement with the proposed stoichiometry of the reducedcompound, La7Mo7O30, confirming our previous results, i.e. thecomplete degradation of β-La2Mo2O9 occurs when δ=0.5. Onthe other hand, the spectra of W4f in the reduced compound,

Fig. 4. SEM images showing the microstructure of (a) La2Mo0.5W1.5O9 sample as-973 K for 24 h and (c) at 1173 for one night. Pellet surface of the La2Mo1.5W0.5O9 sa(f) cross-section image after reduction under an oxidant/reducing gradient at 1073 K

La7Mo5.25W1.75O30, (Fig. 3d) shows only the peaks of W6+

cations, thus tungsten is not reduced during the decomposition ofthe β-La2Mo2O9 structure as previously reported by coulometrictitration measurements [26]. Finally, the O1s spectra revealssplitting in different components as oxygen anions in β-La2Mo2O9 structure have three different crystallographic posi-tions [14]. Taking into account that the oxidation state of tungstenis constant before phase decomposition, the electronic conduc-tivity in tungsten substituted-La2Mo2O9 is mainly associated toelectron hopping between molybdenum cations with differentoxidation states. Thus the electronic conductivity is expected todecrease as the tungsten content in La2Mo2−yWyO9 increases aspreviously confirmed by ion-blocking measurements [26].

The microstructure of a representative sintered pellet withcomposition La2Mo0.5W1.5O9 is shown in Fig. 4a. The samplespresent very low porosity with relative density N98% and wellconnected grains. No phase segregations can be observed andconfirmed by EDS analysis. Such samples were exposed to areducing atmosphere using a continuous flux of humidified 5%H2–Ar between 873 and 1173 K during 24 h. The un-substitutedphase presented important fractures in the pellet bulk afterreduction at 973 K due to volume expansion as consequence ofpartial reduction of Mo6+ to Mo4+ and the formation of newphases during this process (Fig. 4b). The reduction is mostsevere at higher temperatures and the microstructure of theceramic pellet is totally degraded after annealing the samples at1173 K for 12 h (Fig. 4c). Thus, La2Mo2O9 exhibits a limitedstability range to operate efficiently in an SOFC. Besides, thephase transition of undoped La2Mo2O9 at 823 K, which isaccompanied by an extra volume increase ∼0.5%, may lead tomechanical failure during several consecutive thermal cycles.Tungsten-containing phases display better redox stability and

prepared, (b) La2Mo2O9 sample after annealing under humidified 5%H2–Ar atmple (d) before and (e) after reduction under humidified 5%H2–Ar at 973 K and.

Table 2Thermal expansion coefficient values (TEC) for some electrolyte and electrodematerials

Sample TEC (·106 K−1) T (K) Reference

LMO 14.7 RT–853 [65]18.1 853–1273 [65]15.7 RT–853 [29]20.1 853–1273 [29]14.1 RT–823 This work18.0 898–1073 This work

W0.5 16.0 RT–773 This work19.4 823–1073 This work

W1.5 13.9 RT–773 This work21.0 823–1073 This work

BSCF 11.7 323–573 [61]24.9 673–1173

SSC 20.5 RT–1273 [39]LSF 16.3 303–1273 [40]LSM 11.3 RT–1273 [2]LCC 10.8 – [2]LSCM 8.9 337–708 [36]

10.1 793–1229SSM 12.7 – [66]NiO 15 – [1]

Fig. 5. XRD patterns for BSCF/W1.5 mixture at room temperature (RT) andfired at 773 and 973 K. The new diffraction peaks associated to the triclinic α-La2W2O9 structure are also indicated (vertical tics and arrows indicate thereflections of β-La2Mo2O9 and BSCF structure, respectively).


moreover no evidence of fractures are observed after reductionbelow 973 K under humidified 5%H2–Ar, despite certain degreeof degradation is evident in the pellet surface. Fig. 4d and e showthe SEM surface images of La2Mo1.5W0.5O9 sample as-preparedand after exposure to a wet hydrogen atmosphere at 973 K for24 h, indicating a clear degradation, although no appreciablecomposition changes were detected by XRD. It should be notedthat no different degradation was observed between the graininterior and grain boundary region of the samples in comparisonto the results published recently by Jin et al. [28]. Nevertheless,the EDS analysis revealed a small decrease of molybdenumconcentration ∼2% on the pellet surface layers after reduction.This effect can be explained by molybdenum volatilisation asmolybdenum oxide presents low melting temperature ∼1074 Kwhen compared to tungsten oxide∼1745 K. The degradation ofthe pellet surface under reducing atmosphere might also explainthe increase of the electrode polarisation at high temperaturesobserved by electromotive-force and ion-blocking measure-ments, producing errors in the determination of the ionictransport numbers [26]. Fig. 4f shows a cross-section of a pelletannealed under a 5%H2–Ar/air gradient at 1073 K with twodifferent regions, corresponding to the oxidised and reduced side(darker region). However, no evidence of cracks can be observedin the bulk of the pellets, which suggests that the degradation islower under an oxidant/reductive gradient as also occurs in otherelectrolytes as doped ceria [5]. Thus tungsten substitutedLa2Mo2O9 phases exhibit better redox stability in comparisonto the un-substituted phase, although their applicability is onlyviable below 973 K and using diluted hydrogen. Nevertheless, itis still necessary to test the stability of these electrolytes afterprolonged annealing periods of time. An approach to solve theseproblems might be the use of multilayer cells having a layer ofLa2Mo2O9-based ionic conductor applied on a buffer layer,which acts as a mechanical support and protection againstreduction. However, the high thermal expansion coefficient of

LAMOX materials (i.e. 15–20 ·10−6 K−1) compared to mosttypical electrolytes (e.g. 10.8 ·10−6 K−1 for YSZ [1]) clearlyhampers the use of buffer layers. Therefore, the applicability ofthese materials as solid electrolytes in a conventional dualchamber configuration seems to be very restricted. On thecontrary, they could possibly operate in single chamber fuel cellsthat use a mixture of fuel and air; this atmosphere would not beso reductive and LAMOX materials are expected to maintain itscomposition operating under it. However, it is necessary to findcompatible electrodes with catalytic selectivity towards oxidantreduction and fuel oxidation. In this context, cobaltites (e.g.BSCF and SSC) are selective cathodes for oxygen reduction asdemonstrated in several reports [18], whereas titanates, NiO andSr2MgMoO6−δ are selective anodes for hydrogen and hydrocar-bon oxidation [33,37].

5. Electrode compatibility with La2Mo2−yWyO9 materials

5.1. Compatibility with cobaltites

Cobaltites are cathode materials with excellent properties foroxygen reduction even at low temperatures as 773 K [18],although their application in SOFC technology is very restricteddue to the high thermal expansion of these materials (Table 2)compared to the state of the art electrolytes (e.g. 10.8 ·10−6 K−1

for YSZ and 12.5 ·10−6 K−1 for CGO [1]). Cobaltite materialspresent thermal expansion coefficients (TEC) values around12 ·10−6 K−1 in the low temperature range and they increase to20–25 ·10−6 K−1 in the high temperature range due to oxygenlosses, which causes an expansion of the unit cell volume [61].The high TEC values of cobaltites are similar to those ofLAMOX materials and they could be physically compatible.The XRD patterns for BSCF/W1.5 mixture do not show

Fig. 6. XRD patterns for (a) LSF/W1.5 and (b) LSF/W0.5 mixtures at roomtemperature (RT) and fired at 973, 1073 and 1173 K. The reaction products thatappear when temperature increases are indicated in the figure (vertical tics andarrows indicate the reflections of β-La2Mo2O9 and LSF structure, respectively).

Fig. 7. XRD patterns for (a) NiO/W1.5 and (b) CuO/W1.5 mixtures at roomtemperature (RT) and fired at 973, 1073, 1173 K and 1273 K. The inset of (a)shows the details of the Rietveld refinement for the mixture of phases in aselected region of the XRD pattern (vertical tics and arrows indicate thereflection of β-La2Mo2O9 and LSF structure, respectively).


apparent reaction below 773 K (Fig. 5). Nevertheless above thistemperature there is a significant reaction between bothmaterials and the diffraction peaks related to W1.5 phasealmost disappear at 973 K, resulting in the formation of two newphases, a perovskite-type similar to BSCF and another oneisostructural with the triclinic α-La2W2O9 and similar cellparameters. One should note that the triclinic phase α-La2W2O9

is stabilised in the La2Mo2−yWyO9 series for yN1.5, thence animportant migration of molybdenum from the LAMOXmaterialto the new perovskite or the other unidentified phases wouldexplain the stabilisation of the triclinic structure after firing themixture above 773 K. A similar trend is observed for the othercobaltites (e.g. SSC, LSC and SCF), showing reaction in theW1.5/cobaltite mixture above 773 K.

5.2. Compatibility with ferrites

Ferrite-based cathodes present somewhat lower TEC valuesthan those of LAMOX materials (e.g. 16.3 ·10−6 K−1 for LSF),

suggesting possible mechanical compatibility between bothmaterials. The XRD patterns for the LSF/W1.5 mixture do notshow any evidence of reaction below 973 K, though a minorfraction of α-La2W2O9 is clearly visible at 1073 K (Fig. 6a). Onthe contrary, the impurity α-La2W2O9 was not observed for theLSF/W0.5 mixture in the overall temperature range studied(Fig. 6b). When the firing temperature exceeds 1073 K asignificant fraction of the scheelite structure SrMoO4 (PDF 01-085-0587) is observed, which fully agrees with Corbel et al.results [29]. In other words, α-La2Mo2O9 is prone to react withSr-containing materials and possibly with electrode materialscontaining alkaline earth elements as previously suggestedCorbel et al. [29]. Therefore, the presence of α-La2W2O9 in theLSF/W1.5 mixture may be explained in terms of molybdenummigration from the β-La2Mo2O9 structure, which results inSrMoO4 formation. Some additional diffraction peaks are

Fig. 8. XRD patterns for (a) NiO/W1.5 and (b) CuO/W1.5 mixtures afterreduction under 5%H2–Ar at 873 and 923 K (vertical tics indicate the reflectionsof β-La2Mo2O9).

Fig. 9. XRD patterns for (a) LCC/W1.5 and (b) LSCF/W1.5 mixtures at roomtemperature (RT) and fired at 973, 1073 and 1173 K (vertical tics and arrowsindicate the reflections of β-La2Mo2O9 and perovskite structures, respectively).


observed at 1173 K at 2θ=25.4 and 34.3°, which seems to beassociated to a new phase identified as La2W1.25O6.75 (PDF 00-032-0503). The formation of the La2(W,Mo)1.25O6.75 phasewith (Mo,W)/La molar ratio smaller than unity could beexplained by a significant migration of molybdenum leading tofurther formation of SrMoO4. Thereby, no significant reactionoccurs between LSF and La2Mo2−yWyO9 below 973 K andconsequently ferrites could be used as cathode, although only inthe low temperature range.

5.3. Compatibility with NiO and CuO

Nickel oxide has been considered chemically compatible withα-La2Mo2O9 up to 1273 K [30]; however we have observedreaction between NiO and W-substituted La2Mo2O9 samplesabove 1073 K (Fig 7a), as the presence of extra diffraction peaksassociated to the triclinic phase α-La2W2O9 suggests. Althoughreaction has been found after firing the NiO/W1.5 mixture at1073 K, there is not a simple explanation for the formation of thetriclinic phase, because no other impurities were detected (insetFig. 7a) and the cell parameters obtained by the Rietveldrefinement did not vary significantly after calcining the powdermixture at different temperatures. The reactivity is lower as thetungsten content decreases, althoughminor reaction products havebeen detected in the NiO/W0.5 mixture fired at 1273 K. Fig. 7bshows the XRD patterns corresponding to a CuO/W1.5 mixtureafter firing between room temperature and 1273K,where there areno extra reflections in the whole temperature range studied,

confirming a good chemical compatibility between both materials.Nonetheless, it is also necessary to study the chemical stability ofthese systems in SOFC atmospheric conditions, i.e. under reducingatmospheres. Fig. 8a shows the XRD patterns for NiO/W1.5mixture after firing under a constant flux of humidified 5%H2–Arat 873 and 923K for 24 h.As can be observed, there is a significantfraction of α-La2W2O9 at 923 K. On the contrary, no reactionproducts are observed after annealing the CuO/W1.5 mixture inthe same atmospheric conditions (Fig. 8b). It should be mentionedthat the triclinic phase exhibits low electrical conductivitycompared to β-La2Mo2O9 and the segregation of this phase maycause a notable increase of the overall cell resistance [26]. Furtherinvestigation is required for a better understanding of the triclinicphase formation in the NiO/W1.5 mixture.

5.4. Compatibility with chromite–manganites

Chromite–manganites with perovskite structure such as LSM,LCC and LSCM (Table 1) are potential electrode materials that

Fig. 11. XRD patterns for (a) SMM/W1.5 and (b) LSTGM/W1.5 mixtures at roomtemperature (RT) and fired at 873, 973, 1073 and 1173 K (vertical tics and verticalarrows indicate the reflections of β-La2Mo2O9 and perovskite structures,respectively).


could operate as anode or cathode depending on the composition.For instance, the strontium-substituted manganite (La,Sr)MnO3−δis a cathode material operating in the high temperature range,although the introduction of Cr in the B-site renders an excellentanode [36]. It is also possible to use some of these materials (e.g.LSCM) simultaneously as both cathode and anode in a symmetricSOFC [62]. These electrodes present low TEC values comparedto La2Mo2O9-based materials (Table 2), being physicallycompatible with standard YSZ, but incompatible withLAMOX. However, it could be possible to prepare anelectrode-electrolyte composite to decrease the TEC mismatchbetween both materials. The XRD patterns revealed certainreactivity for the LCC/W1.5mixture at 973K (Fig. 9a), appearingnew diffraction peaks assigned to the scheelite phase CaWO4

(PDF 00-008-0145). The lattice cell evolution for W1.5electrolyte decreases as the annealing temperature of the W1.5/LCC mixture increases (Fig. 10), which is further evidence of thereaction between both components. The LSCF/W1.5 mixture(Fig. 9b) also shows signs of reactivity at 973 K, revealing thepresence of new diffraction peaks, which could be associated todifferent phases, although the most probable ones are La2O3 andLa6W2O15. It should be noted that the impurity Sr(Mo,W)O4 wasnot observed in this system. The lattice cell for W1.5, in this case,increases with the firing temperature. These results suggest thatalkaline earth metals are the main cations diffusing into the β-La2Mo2O9 structure. Thus Ca

2+ with smaller ionic radii than La3+

produces a contraction of the cell parameter when incorporatedinto the β-La2Mo2O9 structure (Fig. 10), whereas Sr2+ (largerionic radii than La3+) causes an expansion of the unit cell asobserved in the La2−xAxMo2O9−δ (A=Ca, Sr) series [63,64].

5.5. Compatibility with Sr2MgMoO6 and titanates

SMM is an interesting anode material to evaluate itschemical compatibility with LAMOX phases as the presence

Fig. 10. Variation of the cell parameters with the firing temperature for theelectrolyte W1.5 in the LSCF/W1.5 and LCC/W1.5 mixtures.

of molybdenum in both materials might avoid its diffusion toform new phases. However, the XRD patterns for the SMM/W1.5 mixture revealed a significant reaction at rather lowtemperature as 873 K (Fig. 11a). The reaction is more evidentabove 973 K and at least three different phases wereidentified: SrMoO4, α-La2W2O9 and La2W1.25O6.75. Themechanism is again related to molybdenum migration toreact with strontium, leading to the formation of SrMoO4.Thus the loss of molybdenum stoichiometry inβ-La2Mo2−yWyO9

produces initially the appearance of the triclinic phase isostruc-tural with α-La2W2O9 below 1073 K. Then, an excessivemigration of molybdenum from the β-La2Mo2O9 structure toform further SrMoO4 at higher temperatures gives rise to theformation of La2(Mo,W)1.25O6.75 with a (Mo,W)/La ratio smallerthan unity.

Titanates are known to exhibit low reactivity with theelectrolytes commonly used in SOFC [34,35]. However,LSTGM and W1.5 react at 1073 K, leading to the presence ofextra reflections associated to α-La2W2O9 and SrMoO4

(Fig. 11b). The proportion of SrMoO4 increases significantly

Fig. 12. SEM image of the cross-section of the interface electrolyte–electrode of the cell (a) LSF∥W1.5∥LSF, (b) CuO∥W1.5∥CuO and (c) NiO∥W1.5∥NiO beforereduction. All electrodes were fixed at 1073 K for 1 h to avoid excessive reaction between the different materials. (d) Partial separation of the electrode layer in the NiO∥W1.5∥NiO cell is observed after reduction.


after firing at 1173 K, resulting the formation of La2W1.25O6.75

as in the case of SMM/W1.5 mixture.

6. Electrode deposition and polarisation measurements

The chemical compatibility studies carried out with a rangeof electrodes revealed that CuO-cermets and ferrites are themost suitable electrodes for La2Mo2−yWyO9 electrolytes. NiOpresents also an acceptable compatibility, though only at lowtungsten contents. These electrodes were further investigatedto prepare a LAMOX-based fuel cell. For this purpose, 1:1 (w/w) composites were used as electrodes to match the thermalexpansion coefficient between the electrolyte and electrodes.The use of composites may also help to fix the electrodes atlower temperature, typically 1073 K to prevent an excessivereaction between the materials. All the attempts to obtain agood contact between the electrolyte and electrodes at 1073 Kfailed. An alternative strategy was considered to enhance theadherence and the green pellets of pressed powder were coatedwith a slurry of the same electrolyte and sintered at 1073 K.This process was repeated and finally the pellets were sinteredat 1373 K, rendering a more adherent surface layer incomparison to the as-prepared ceramic pellets. Fig. 12 showsthe SEM images corresponding to the electrolyte–electrodeinterfaces of several symmetrical cells. All electrodes presentapparently good contact with the electrolyte after fixing (Fig12a–c) with high porosity and grain sizes below 0.5 μm,except for the CuO-composite (Fig. 12b) that exhibits large

agglomerates of particles possibly due to the sintering effectsof copper.

Preliminary studies of electrode polarisation were performedto determine ASR values under symmetric atmosphere on a twoelectrode configuration. The symmetrical LSF∥W1.5∥LSF cellpresented high ASR values in air, almost one order ofmagnitude larger than Pt∥W1.5∥Pt at 973 K (Fig. 13a). Thiscould be associated with the reaction between the compositeelectrode and the pellet interface or low contact between bothmaterials due to the low firing temperature. On the other hand,the ASR for CuO∥W1.5∥CuO cell could not be determined,because a diffusion process appeared overlapped with theelectrode arc in the impedance spectra hindering the separationof both contributions. A possible explanation for this phenom-enon is the agglomeration of CuO particles that limits the transitof gases towards the electrode. This could be solved by usingless CuO in the composite preparation. NiO∥W1.5∥NiO cellpresented the lowest ASR values ∼2 Ω cm2 under humidified5%H2–Ar at 973 K, although it should be noted that theelectrode polarisation and series resistance values increasedwith the annealing time under reducing atmosphere (Fig. 13b).The SEM image revealed that the electrode layer is partiallyseparated of the electrolyte after reduction, increasing polarisa-tion and series resistance values with the time (Fig. 12d). Thiseffect might be ascribed to mechanical stress caused by the highthermal expansion of the electrolyte and also a poor adherencebetween both materials. Alternatively a nickel-containingsolution was used to impregnate a porous layer of the same

Fig. 13. (a) Thermal dependence of the ASR values for different symmetricalcells. (b) Evolution of the impedance spectra with the time for NiO∥W1.5∥NiOcell under humidified 5%H2–Ar.


electrolyte, previously deposited onto a dense electrolyte pellet.However, the high reactivity of the cation solution increases therate of the reaction products. Therefore, it would be necessary todevelop a protective layer deposited between the electrolyte andelectrodes to avoid molybdenum diffusion or to use alternativeelectrodes to those studied in the present report and preferen-tially without alkaline earth elements. Simultaneously, the highthermal expansion coefficients of LAMOX phases require theuse of composite-graded electrodes in order to reduce thethermal expansion coefficient mismatch between electrolyteand electrode materials.

7. Conclusions

Tungsten-substituted La2Mo2O9 samples exhibit sufficientionic conductivity and stability range (from 0.21 to 10−20 atm at973 K) to be used as SOFC electrolytes, preferably in a single-chamber configuration, where the environment conditions ofoperation are not so reductive. Above 973 K, a degradation of thepellet surface of the electrolyte is observed, which is mainly

associated to the formation of new phases upon reduction. Inaddition, LAMOX materials present high reactivity with most ofthe electrodes typically used in SOFC technology, especially withcobaltites and other electrode materials containing alkaline-earthmetals. The high reactivity of LAMOX materials is associated tomolybdenum diffusion towards the electrodes, leading to theformation of different reaction products. The chemical compat-ibility studies with La2Mo2−yWyO9 (y=0.5 and 1.5) revealed thatCuO is more compatible than NiO to use as anode material.Ferrites seem to be the most physical and chemical compatiblecathodes studied to date, although their polarisation resistanceusing composite is too large to consider their use in an SOFC. Thiscould be related with the low firing temperature used to fix thematerial to the electrolyte. Thus, alternative routes to improve theadherence could result in enhanced responses.

Moreover, the high TECs of LAMOX materials and the lowfixing temperature of the electrodes, necessary to preventexcessive chemical reaction, lead to poor contact between thematerials and consequently their separation after severalconsecutive thermal cycles. Therefore, the practical applicationof La2Mo2−yWyO9 materials is rather limited due to their highchemical reactivity with the state of the art SOFC componentsand high thermal expansion coefficients.

The development of alternative electrodes compatible withLAMOX electrolytes is required or alternatively a protectivebuffer to suppress molybdenum diffusion. Composite-gradedelectrodes would be also necessary tomatch the thermal expansioncoefficients between electrolyte and electrode materials.

Acknowledgements

This work was supported by the Spanish Research program(MAT2004-3856), the Canary Islands Government (PI2004/093). “Programa de Incorporación de doctores y tecnólogos aempresas privadas y otras entidades” (D. M. L. and D. P-C) andthe “Ministerio de Educación y Ciencia” for a Ramón y Cajalfellowship (J. C. R-M). The authors are grateful to LuisHernández (Deparment of Inorganic Chemistry, University ofLa Laguna) for technical assistance and Prof. M. A. G. Aranda(Department of Inorganic Chemistry, University of Málaga) forpermission to use the XPS facility.

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Applicability of La 2Mo 2− y W y O 9 materials as solid electrolyte for SOFCs

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