La5.5WO12-δ Characterization of Transport Properties under

ARTICLE pubs.acs.org/JPCC

La5.5WO12-δ Characterization of Transport Properties under Oxidizing Conditions: A Conductivity Relaxation Study Cecilia Solís,† Sonia Escolastico,† Reidar Haugsrud,‡ and Jos
e M. Serra*,† †

Instituto de Tecnología Química, Universidad Polit
ecnica de Valencia
Consejo Superior de Investigaciones Científicas, av. Los naranjos s/n, E-46022, Valencia, Spain ‡ Department of Chemistry, Centre for Materials Science and Nanotechnology, FERMiO, University of Oslo, NO-0349, Oslo, Norway

bS Supporting Information ABSTRACT: The conduction properties of La5.5WO12-δ have been studied by using the electrochemical conductivity relaxation technique. Exchange and diffusion coefficients have been obtained from relaxation experiments including measurements of conductivity transients from (1) oxidation
reduction under dry and wet oxygen and (2) hydration
dehydration for both H2O-saturated and D2O-saturated oxygen at atmospheric pressure. The evolution of the total conductivity as a function of temperature, oxygen partial pressure, and the presence of H2O or D2O has moreover been characterized. The exchange coefficient for oxygen and water incorporation was calculated from the relaxation experiments. It was also possible to determine the coefficients for oxygen vacancy/oxygen ion (in dry and hydrated state) diffusion and for ambipolar diffusion of water. Oxygen vacancy/oxide ion diffusion is apparently not affected by the hydration state within the studied temperature range and water and oxygen partial pressures. The ambipolar water diffusion coefficient was significantly smaller than the one for the oxygen ion.

1. INTRODUCTION Proton-conducting oxides have the potential to serve as key components in future environmentally friendly power generation and process intensification. Proton conduction at high and intermediate temperature allows the operation of proton-conducting solid oxide fuel cells,1
3 whereas mixed proton
electron conduction enables hydrogen transport through membranes4,5 and hydrocarbon upgrading using catalytic membrane reactors. For these applications, materials with chemical stability in CO2and H2O-rich atmospheres are a requisite, besides relatively high proton or ambipolar proton
electron conductivity. La6WO12based compounds are among the emerging materials meeting these requirements. Shimura et al.6 reported proton conductivities in the order of 0.005 S/cm for La5.8WO11.7 at 1073 K in wet air, and others have shown that compounds of Ln6WO12 series7
9 exhibit mixed proton
electron conductivity at high temperature in humid reducing or oxidizing atmospheres. Specifically it has been pointed out that electronic conductivity is predominant in La6WO12 above 1073 K under sufficiently oxidizing or reducing atmospheres, while ionic conduction dominates at intermediate pO2 at those high temperatures and over a broad pO2 range at temperatures below 1073 K.10 Moreover, high stability against carbonation at high temperatures has been reported.9 The lanthanide tungstate Ln6WO12 has been described as an ordered fluorite structure (space group R3).11 The symmetry of the structure depends on the rare earth element: cubic or pseudocubic from La to Pr, via pseudotetragonal from Nd to r 2011 American Chemical Society

Gd, and rhombohedral from Tb to Lu and for Y.12 The structure can be described as seven edge-sharing cubes with oxygen ions in the corners. The central cube is occupied by W6þ cations in the middle and has two vacant positions on its main diagonal, WO6/ 402/4, where 0 denotes a vacant position. The other six cubes surrounding this central one contain the rare-earth cations (Ln3þ) in the middle and one vacancy LnO7/401/4 (each shares one of the vacant corners with the central W-containing cube13,14). The 7-fold coordination of Ln3þ and the 6-fold coordination of W6þ have been confirmed for Ho6WO12 and Y6WO12. There is no clear relationship between the Ln6WO12 structure and transport properties to date, i.e., the nature of the oxygen vacancy, the location of protons in the structure, and the transport mechanism. In the present work, the transport properties of La6WO12 are studied by means of conductivity relaxation measurements at high temperature (923 to 1023 K) under high oxygen partial pressures (1 and 0.21 atm). The kinetics of different processes has been studied: (1) by oxidation and reduction in dry and wet atmospheres and (2) by hydration and dehydration in oxygen using water and heavy water. Additionally, the isobaric conductivity has been measured as a function of temperature in different gas atmospheres. Received: February 15, 2011 Revised: April 29, 2011 Published: May 17, 2011 11124

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2. EXPERIMENTAL SECTION La5.5WO12-δ (LWO) was prepared following the citratecomplexation route.9 This specific stoichiometry was selected to achieve phase-pure materials, since nominal La6WO12 has been shown to segregate La2O3. La2O3 (Aldrich, 99.9%, predried at 1373 K) was dissolved in nitric acid (65% vol.), and then citric acid was added as a complexing agent. Another solution was prepared using ammonium tungstate (Fluka, >99%) also with citric acid (Fluka, 99.5%). Both solutions were heated at 393 K for 1 h. Then ammonia was added to neutralize the solutions after which they were mixed at room temperature. This solution was gradually concentrated by stepwise heating under stirring, which led to gel foaming. The product was calcined in air at 1073 K to oxidize carbonaceous matter and promote crystallization. The powder was then calcined at 1173 for 10 h after which they were crushed and uniaxially pressed to bar-shaped samples at 100 MPa. The resulting green bodies were sintered at 1773 K for 12 h. To identify the crystalline phase, the powders were characterized by X-ray diffraction (XRD) by a PANalytical X’Pert PRO diffractometer, using CuKR1,2 radiation and an X’Celerator detector in Bragg
Brentano geometry. Furthermore, fracture cross sections of graphite sputtered bars were analyzed by scanning electron microscopy (SEM) using a JEOL JSM6300 electron microscope. To analyze the transport properties and the kinetics of LWO, relaxation experiments have been performed by DC conductivity measurements using the four-probe bar configuration (40  4  1 mm3). Parallel silver electrodes were painted on the surface of each sample (voltage probes in the center 1.8 cm away from the bar ends). A constant current ramp (from
20 up to 20 μA, each 4.5 μA) was supplied by a programmable current source (Keithley 2601), while the voltage drop was detected by a multimeter (Keithley 3706). To eliminate the thermal effect and avoid nonohmic responses, the voltage was measured with the current in both forward and reverse directions. The relaxation experiments included the following studies: 1. Oxidation
reduction kinetics in dry (∼30 ppm H2O) and moist (2.5% H2O) atmospheres by changing the oxygen partial pressure, pO2, from 1 to 0.21 atm pO2 at different temperatures. 2. Hydration
dehydration kinetics by changing from dry to either 2.5% H2O or D2O to observe possible differences in kinetics due to OH•o (protons) and OD•o (deuterons), respectively. Also, these measurements were carried out in oxygen. Impedance spectroscopy, by using a PSM 1735 with an IAI interface from Newtons4th Ltd. in the frequency range from 100 mHz
10 MHz, was employed to analyze contributions from the electrodes and from the bulk and grain boundaries of LWO. Isobaric conductivity measurements were carried out from high to low temperatures after stabilization of the material at the highest testing temperature for 2 h. In all cases, the measurements were performed under a continuous flow of gas where the water level was obtained by saturation at room temperature yielding pH2O = 0.025 atm and pD2O = 0.022 at 293 K. The different pO2 were reached by using calibrated gas mixtures (O2
Ar) provided by Linde. The concentration steps in the relaxation experiments were monitored using a Balzers mass spectrometer (Figure S1 in Supporting Material), and it was concluded that the concentration step time (∼5 s) is negligible with respect to the conductivity relaxation time within the studied temperature range.

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Figure 1. (a) XRD pattern of an LWO powder sintered at 1823 K and (b,c) SEM images of fracture cross sections of the LWO bar after electrochemical testing.

3. RESULTS AND DISCUSSION The XRD pattern of the LWO powder calcined at 1773 K is plotted in Figure 1a. This pattern agrees with the one reported for the cubic structure,15,16 and no secondary phases could be detected to the limit of XRD. Figures 1b,c present SEM images of a fractured cross-section from a sintered sample after the electrochemical testing. The porosity is negligible, and the bonding between the grains is very good. The sample remained stable during the measurements, and the conductivity did not change even after storage for several months. 3.1. Study of Total Conductivity As a Function of Temperature and Gas Environment. Figure 2 shows the total

conductivity of LWO versus inverse temperature under (a) dry 11125

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Figure 2. Total conductivity versus inverse temperature of LWO measured in different atmospheres.

O2 and O2 saturated with H2O and D2O; (b) dry He and He saturated with H2O and D2O; and (c) dry H2, dry D2, and H2 saturated with H2O and D2 saturated with D2O (wet conditions correspond to 2.5% H2O or D2O, respectively). Two major trends can be highlighted from these results: (1) the total conductivity is considerably higher in wet than in dry atmospheres, and (2) a clear isotope effect is observed up to 1100 K when comparing data from H2O and D2O humidification. These effects are clear indications that protonic conduction is predominant in this material under wet conditions at temperatures below 1000 K, regardless of the oxygen partial pressure, as previously reported.10 Above 1000 K, the material becomes a mixed conductor where the relative contribution of the different charge carriers (e
, h 3 , O2
, and Hþ) depends on the oxygen partial pressure (pO2) in addition to the temperature. Above 1150 K, the conductivity eventually becomes independent of water vapor pressure. This is attributed to the exothermic nature of the hydration of the oxide; i.e., the proton concentration decreases as the temperature increases. The relative influence of the protonic charge carriers, consequently, decreases at high temperatures. It is observed that the apparent activation energy under dry conditions and high pO2 is practically constant (see also Figure 3b). Therefore, charge carriers other than protons (likely oxygen ion and/or electron hole transport) prevail in the whole temperature range. The relative influence of these charge carriers with reaction conditions is difficult to ascertain based on these experimental results. However, from general point defect chemistry one may deduce expected pressure dependencies for given electroneutrality conditions. By way of example, since protons are positive and increase with water vapor, the other positive defects will decrease in concentration, provided that their levels are not set by the concentration of disordered intrinsic negative defects or extrinsic acceptors. Under dry conditions, the conductivity is higher in reducing compared to oxidizing atmospheres. This behavior is ascribed to the n-type conductivity in H2/D2 due to partial reduction of the material. At temperatures above 1150 K, the electronic conductivity

seems to dominate, although the remaining isotope effect reveals that protons still contribute significantly to the conductivity. The minor isotope effect observed on the conductivity between dry H2 and dry D2 is due to some residual humidity (∼30 ppm). One should note that the conductivity characteristics presented correspond essentially to those reported elsewhere,14,10 although the sample composition and processing conditions differ substantially. A gradual change in the apparent activation energy for the conductivity is observed at around 800
1000 K in wet atmospheres (H2/D2, He, and O2) as different from in dry atmospheres, in which an Arrhenius behavior is observed for the whole temperature range. This change under wet conditions reflects (i) the aforementioned gradual decrease of the proton concentration in the oxide and the consequent reduction of the proton conductivity and (ii) the fact that oxygen ion and n-type electronic (very low pO2) conductivity are more thermally activated than the protonic conductivity and become the major conduction processes at high temperatures. This is also supported by the fact that the activation energy of the conductivity in dry and wet conditions is very similar at high temperatures, e.g., 1200 K. Study of Conductivity As a Function of the pO2 (High pO2 Range). The effect of the oxygen partial pressure on the electrical conductivity was measured to clarify the influence of p-type conductivity at high temperatures. Figure 3a presents the conductivity as a function of pO2 in a log
log plot under dry and wet conditions at 873 and 973 K. At the lowest temperatures (T < 873 K), the conductivity is essentially independent of pO2 and, as such, dominated by ionic conductivity. When the temperature increases, the conductivity increases slightly with increasing pO2 reflecting that electron holes start to contribute to the total conductivity. This effect is smaller and appears at higher temperature under wet compared to dry conditions, as a consequence of the higher relative ionic conductivity due to the prevailing protonic conductivity under wet conditions. Figure 3b compares, in an Arrhenius representation, the temperature dependence of the conductivity in oxygen partial pressures of 1 and 10
4 atm under dry conditions. From this 11126

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Figure 3. Total conductivity of LWO at 973 and 873 K as a function of pO2 measured in dry and wet atmospheres (a) and Arrhenius representation of the conductivity measured in 1 and 10
4 atm dry oxygen (b).

graph it can be observed that the activation energies are constant in the whole range of temperatures studied and decrease slightly with decreasing pO2, from 1.04 eV in 1 atm to 0.96 eV in 10
4 atm O2. A constant activation energy for a given pO2 indicates that practically the same conduction mechanism prevails in the whole temperature range. Effect of Water Presence on Electrode/Polarization Resistance. The possible contribution of the porous silver electrodes was studied to ensure that the conductivities reported reflect the true transport properties (bulk and grain boundary) of the LWO specimen. The impedance spectra were recorded sweeping the whole frequency range given above in a wide window of temperature and pO2. Figures S2 and S3 (Supporting Information) present the impedance results from humidified and dry atmospheres at different temperatures. The contribution of the electrode processes (low frequency arc: 0.1 and 100 Hz) is in general negligible, and it is only detectable in wet atmospheres and low pO2 at temperatures below 823 K. In dry atmospheres, the electrode contribution is visible only at temperature below 773 K in dry argon. In summary, the DC conductivity measurements (steady and transients studies) in the chosen temperature and pO2 range reflect the material transport properties and not electrode processes. 3.2. Study of Hydration and Oxidation Kinetics by Conductivity Relaxation. The oxidation and hydration kinetics are studied by conductivity relaxation upon stepwise changes in the oxygen or water vapor activity, as has been performed for different protonic conductors such as SrCe0.95Yb0.05O2.975,17 doped SrZrO3,18 and doped BaTiO3.19 The diffusivity of protonic defects involves proton transfer between neighboring oxygen ions (Grotthuss-type mechanism), and the protons are incorporated in the oxide through the hydration of oxygen vacancies.

This process entails the reaction between water, oxygen vacancies, and lattice oxygen, that according to Kr€oger-Vink notation20 can be written as X • H2 O þ V •• O þ OO T 2OHO

ð1Þ

10

It has been assumed that in this material oxide ions in the oxygen sublattice exchange with available interstitial positions in the octahedron around W, getting anti-Frenkel (intrinsic) disorder 00

vXi þ OXO T V •• O þ Oi

ð2Þ

yielding the complete reaction 00

H2 O þ 2OXO þ vXi T 2OH•O þ Oi

ð3Þ

Kreuer et al.21 concluded from mass relaxation experiments that the chemical diffusion of water in an (anhydrous) proton conductor is governed by ambipolar diffusion of protons and oxygen ions, and therefore, the hydration is governed by a unique water diffusion process. Recently, Yoo et al.19 reported that there are three possible patterns of conductivity relaxation, i.e., (1) single-fold monotonic relaxation, (2) 2-fold monotonic, and (3) 2-fold nonmonotonic relaxation, where 2-fold relaxation means that two independent diffusion processes with different relaxation times take place simultaneously. More specifically, the relaxation behavior depends on the participating charge carriers (O2
, Hþ, electrons, or electron holes) and on the given water or oxygen activity stepwise change. In Yoo’s and Maier’s work,22 a 2-fold profile is observed when the electronic conductivity is sufficient to allow for independent proton and oxygen vacancy diffusion. Typically, proton diffusion in oxides is 2 orders of magnitude faster than oxygen diffusion.23 11127

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Figure 5. Diffusion coefficient and exchange coefficient as a function of temperature oxidation (filled symbols) and reduction (empty symbols) and for hydration (squares) and dehydration (spheres).

with lx k Lx ¼ βn, x tan βn, x  D

Figure 4. Conductivity relaxation measurements at around 1023 K from O2 to air and vice versa (a) and from wet O2 to wet air and vice versa (b). Symbols correspond to experimental data and lines to the fitted diffusion model.

To investigate the hydration and oxidation kinetics of LWO, different conductivity relaxation measurements were performed on a single LWO sample by changing pO2 and pH2O/pD2O under high pO2 conditions. By assuming that the chemical diffusivity was constant across the pO2 and pH2O experimental window, the general solution for the spatial average concentration of species can be calculated employing established analytic solutions of Fick’s second law. For a rectangular (2lx  2ly  2lz) bar, the solution to the 3-dimensional diffusion problem can be described with the equation24,25 " ¥ σ
σ0 2L2x ¼ 1
2 2 2 σ¥
σ0 n ¼ 1 βn, x ðβn, x þ Lx þ Lx Þ ! ¥ 2L2y
β2n, x Dt exp 2 2 2 l2x m ¼ 1 βm, y ðβm, y þ Ly þ Ly Þ ! ¥
β2m, y Dt 2L2z exp 2 2 2 l2y m ¼ 1 βm, y ðβm, z þ Lz þ Lz Þ !#
β2m, z Dt exp ð4Þ l2z

∑

∑

∑

ð5Þ

D is the diffusion coefficient; k is the surface reaction constant; and 2lx, 2ly, and 2lz are the dimensions of the sample. βn are the n roots obtained by solving eq 5. It has to be pointed out that k and D have been calculated simultaneously from the experimental fits to eq 4. The sample dimensions are selected to have contributions of both k and D, and none of them is predominantly limiting the relaxation process. Consequently, a systematic error could exist, and k values could be coupled to D values. To avoid this possible error, the fitting has been initialized with a D value obtained from the diffusion limiting case, and then k and D were refined. Oxidation
Reduction Kinetics Study. The relaxation measurements upon oxidation and reduction were performed at high pO2 (O2 T air changes) where the conductivity changes as a function of pO2 are due to the minor contribution of electron holes (cf. Figure 3 and ref 10). Figure 4 shows typical conductivity relaxation upon oxidation (black symbols) and reduction (red symbols) using air and O2, here in (a) at 1023 K in dry gas and in (b) at 1013 K in moist gas. The conductivity relaxation in both cases follows a single-fold monotonic behavior. The curves included represent the best fit of the data to eq 4, and the k and D derived from these fittings are displayed in Figure 5 as a function of inverse of temperature. Both kox,red and Dox,red are similar in dry and wet atmospheres. This indicates that oxygen incorporation and diffusion are not significantly influenced by the presence of protons/water in the lattice and that proton diffusion is much faster than oxygen vacancies since proton motion does not interfere in the oxygen diffusion. Moreover, the amount of water incorporated at 973 K in 0.025 atm H2O is around 0.024 mol H2O per mol LWO which corresponds to that only 1.2% of the total amount of structural oxygen vacancies is occupied by OH•.10 Therefore, protons probably impose only a minor effect on the oxide structure and on the overall vacancy concentration. The activation energies for the surface exchange and diffusion are 1.3 ( 0.2 eV and 1.1 ( 0.2 eV, respectively. The activation energy for the diffusion coefficient is in line with the one observed for the conductivity (1.2 eV) in dry conditions, previously ascribed to 11128

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Figure 6. Conductivity changes when the atmosphere is changed from dry oxygen (pO2 = 1 atm) to wet oxygen (pH2O = 0.025 atm, pO2 = 1 atm) and vice versa, two times.

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Hydration
Dehydration Kinetics Study. Figure 6 shows the effect on the conductivity when the atmosphere surrounding an LWO sample suddenly changes from dry O2 to O2 þ 2.5% H2O. The conductivity change is reproducible, as demonstrated in the experiment of Figure 6 where the dry to wet transient is performed twice. A single-fold monotonic relaxation with just one diffusion coefficient is encountered. As shown in Figures 2 and 3, the conductivity in wet O2 is higher than in dry O2 due to the contribution of the proton conductivity under wet atmospheres. This behavior differs from proton conductors previously investigated by conductivity relaxation measurements,17,19 where an increase in the water vapor partial pressure resulted in a decrease of the total conductivity due to the reduction in the electron hole concentration. Figure 7 shows the conductivity relaxation upon hydration (black square symbols) and dehydration (red round symbols) at 902 and 1018 K with dry O2 and O2 þ 0.25% H2O (Figure 7a,b) and similarly at 923 and 1027 K with O2 and O2 þ 2.5% D2O (Figure 7c,d). The conductivity relaxation shows a single-fold monotonic behavior. Solid lines through the relaxation data represent the best fits to eq 4, which as seen depicts the conductivity relaxation sufficiently precise. Again the predominance of protonic charge carriers is manifested in the ratio of around 1.5 between conductivities in H2O and D2O.

Figure 7. Conductivity relaxation measurements at around 973 and 903 K from O2 to O2 þ H2O and vice versa (a,b) and from O2 to O2 þ D2O and vice versa (c,d). Symbols correspond to experimental data and lines to fitted data to eq 4. 11129

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Figure 8. (a) Exchange and diffusion coefficient as a function of temperature for hydration (solid symbols) and dehydration (empty symbols) and (b) all calculated coefficients for both hydration
dehydration and reduction
oxidation.

Surface exchange coefficients (k) and diffusion coefficients (D) obtained from the fitting of the hydration
dehydration transients (khyd, kdehyd and Dhyd, Ddehyd, respectively) at different temperatures are shown in Figure 8 as a function of the inverse temperature (a) and compared with coefficients derived from the oxidation
reduction transients (kox, kred and Dox, Dred, respectively) (b). It can be observed that the diffusion coefficients in (a) are in the range 10
5
10
6 cm2/s from 913 to 1023 K, which is similar in magnitude to previous results reported for other proton conductors.19 The activation energies of the surface exchange and diffusion coefficients are 0.57 ( 0.1 eV and 0.58 ( 0.2 eV respectively, which are similar to those values reported for protonic defects in proton-conducting perovskites (e.g., 0.54 eV for D).26 Comparing the diffusion coefficients in Figure 8a, they all fall within the uncertainty at the respective temperature showing that there is no clear isotope effect. Consequently, this diffusion coefficient reflects migration of charge carriers not related with protons/deuterons, e.g., oxygen vacancies/oxygen ions through the polycrystalline oxide. From Figure 8b, it can be observed that the surface exchange coefficient in hydration
dehydration experiments is 1 order of magnitude lower than the one determined in the oxidation
reduction

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experiments. The step change in pH2O does not imply any substantial change in pO2, and therefore the change in the materials conductivity is due exclusively to water incorporation in the oxide (overall hydration process). Accordingly, the exchange coefficient may describe the rate-determining step of the water adsorption, dissociation, and incorporation of OH
and Hþ (or alternatively O2
and 2Hþ) into the oxide lattice to occupy structural oxygen vacancies (OH•v) and protonation of lattice oxygen atoms (OH•O or H•i ). This is in contrast to the exchange coefficient calculated through the oxidation
reduction relaxation, which corresponds to the oxygen exchange, comprising dissociative adsorption of oxygen, reduction, and oxygen ion incorporation. Moreover, the diffusion coefficients from the hydration
dehydration experiments are also 1 order of magnitude smaller than that of oxidation
reduction. Since the diffusion coefficients of the two processes differ substantially, it cannot be considered that the diffusion process during the hydration process is simply limited by the oxygen vacancy diffusion, especially with our previous statement that the hydration does not interfere significantly with the oxygen diffusion (Figure 5). Additionally, the relaxation is described by a single-fold monotonic model, and therefore only one overall diffusion process takes place. This overall process can be considered as the ambipolar diffusion of water, i.e., diffusion of protons and oxygen vacancies through the oxide, as has been previously reported for proton conductors.27 In view of the fact that the material is practically a pure ionic conductor (oxygen-ionic and protonic) under the given conditions (high pO2 and 923
1023 K), the rapid proton diffusion is hindered by the low electron hole concentration, and hence the local charge balance during the diffusion process requires the simultaneous diffusion of both charge carriers. In contrast, when a material has sufficient p-type electronic conductivity such as for instance Yb-doped SrCeO3 or Nd6WO12, protons diffuse rapidly since the following reaction is possible H þ h•(þOxO) f H•i , and therefore the decoupled ambipolar diffusion of H and OH takes place. This is also in agreement with the reported28,8 relative ratio of charge carriers of La6WO12 and SrCeO3 or Nd6WO12. Consequently, in the present case it can be concluded that the coupled diffusion (through the LWO polycrystalline specimen) of protons and oxygen ion (or oxygen vacancies) is slower than the net oxygen diffusion.

4. CONCLUSIONS The total conductivity of the proton conductor La5.5WO12-δ has been studied as a function of temperature in a wide range of gas environments. The contribution of the different charge carriers has been analyzed qualitatively in the different environments. Moreover, the kinetics has been studied by conductivity relaxation. Exchange and diffusion coefficients have been obtained from (1) oxidation
reduction experiments in both dry and wet oxygen and (2) hydration
dehydration experiments in both H2O-saturated and D2O-saturated oxygen. Coefficients for oxygen vacancy/oxygen ion (in dry and hydrated states) diffusion and for ambipolar water diffusion were, moreover, determined. Oxygen vacancy/oxygen ion diffusion is not affected by the presence of protonic defects within the studied pH2O, pO2, and temperature range. The ambipolar water diffusion coefficient was significantly smaller than the oxygen ion diffusion, and the reason for this fact remains unanswered. 11130

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’ ASSOCIATED CONTENT

bS Supporting Information. The gas concentration transient step is displayed in Figure S1 as recorded by mass spectrometry. Figures S2 and S3 present impedance spectroscopy measurements (Nyquist and Z’’-Bode plot) of the La5.5WO12 sample measured in wet conditions at different pO2 and temperatures. Finally, additional conductivity results as a function of pO2 and temperature are plotted in Figure S4. This material is available free of charge via the Internet at http://pubs.acs.org.

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(24) Song, C. R.; Yoo, H. I. Solid State Ionics 1999, 124, 289–299. (25) Søgaard, M.; Hendriksen, P. V.; Mogensen, M. J. Solid State Chem. 2007, 180, 1489–1503. (26) Kreuer, K. D.; Dippel, Th.; Baikov, Yu. M.; Maier, J. Solid State Ionics 1996, 86
88, 613–620. (27) Kreuer, K. D.; Munch, W.; Traub, U.; Maier, J. Ber. Bunsen-Ges. 1998, 102 (3), 552–559. (28) Dahl, P. I.; Haugsrud, R.; Lein, H. L.; Grande, T.; Norby, T.; Einarsrud, M.A. J. Eur. Ceram. Soc. 2007, 27, 4461–4471.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: þ34963877809.

’ ACKNOWLEDGMENT Funding from European Union (FP7 Project EFFIPRO Grant Agreement 227560) and the Spanish Ministry for Science and Innovation (Project ENE2008-06302 and Grant JAE-Pre 080058) is kindly acknowledged. Authors are indebted to S. Jim
enez for sample preparation. ’ REFERENCES (1) Bonanos, N. Solid State Ionics 1992, 53, 967–974. (2) Iwahara, H.; Uchida, H.; Ono, K.; Ogaki, K. J. Electrochem. Soc. 1988, 135, 529. (3) Serra, J. M.; Meulenberg, W. A. J. Am. Ceram. Soc. 2007, 90, 2082–2089. (4) Norby, T.; Larring, Y. Solid State Ionics 2000, 136
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