Preparation and Characterization of Nanocrystalline Mixed Proton

Jun 25, 2009 - The influence on conduction properties and the hydrogen separation properties are presented as well as the stability in CO2-rich atmosp...
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Chem. Mater. 2009, 21, 3079–3089 3079 DOI:10.1021/cm900067k

Preparation and Characterization of Nanocrystalline Mixed Proton-Electronic Conducting Materials Based on the System Ln6WO12 Sonia Escol astico, Vicente B. Vert, and Jose M. Serra* Instituto de Tecnologı´a Quı´mica (UPV-CSIC), E-46022 Valencia, Spain Received January 9, 2009. Revised Manuscript Received May 28, 2009

The tungstates Ln6WO12 are proton-conducting materials exhibiting sufficient electronic conductivity to consider them as potential candidates for the separation of hydrogen at high temperature. Hydrogen-permeable membranes will find application in power plants applying precombustion strategies, process intensification using high-temperature catalytic membrane reactors, and in components for electrochemical systems as proton conducting fuel cells (PCFCs) and electrolyzers. This work presents the preparation and characterization of nanocrystalline mixed conducting materials with three different nominal compositions (Nd6WO12-Eu6WO12-Er6WO12) using a sol-gel complexation synthesis method. The evolution of the crystalline structure and crystallite size is studied as a function of the sintering temperature. Generally, the nanosized oxides show a (pseudo)-cubic crystalline fluorite structure which evolves into the most stable fluorite symmetry (tetragonal for Nd and rhombohedral for Er) with increasing sintering temperatures, i.e., crystallite sizes. Shrinkage behavior was analyzed for the three compositions in the range from 1000 to 1500 °C and the nanosized Nd-based oxide showed a very high sintering activity even at relatively low temperatures (1100-1200 °C). In addition, the total conductivity in different environments has been studied systematically for samples sintered at different temperatures and the highest total conductivity was obtained for the Eu-based compound having structure with tetragonal symmetry (0.009 S/ cm at 850 °C). Hydrogen permeation was studied for a disk-shaped Nd6WO12 membrane in the range of 700-1000 °C. Finally, stability of these materials at 700 and 800 °C has been evaluated in contact with a CO2-rich gas stream (dry or humidified) as well as thermochemical compatibility with yttriastabilized zirconia in the range 1200-1500 °C. 1. Introduction New proton conducting materials offer new opportunities in energy production and process engineering. Indeed, hydrogen separation at high temperature will allow the implementation of precombustion strategies in power plants,1,2 wherein CO2 and H2 are separated after fuel gasification, resulting in a final exhaust steam consisting of humid CO2, which can readily be liquefied and stored. Moreover, process intensification can be achieved by integrating protonic membranes in new catalytic reactors, allowing for controlled hydrogen removal or feeding and making possible the equilibrium displacement and improvement in the process selectivity. Among the different proton conducting oxides, Ln6WO12 compounds are one of the most promising solid materials for hydrogen permeable membranes at high temperature *To whom correspondence should be addressed. Fax: 0034.963.877.809. E-mail: [email protected].

(1) Jordal, K.; Bredesen, R.; Kvamsdal, H. M.; Bolland, O. Energy 2004, 29, 1269. (2) Fontaine, M. L.; Larring, Y.; Norby, T.; Grande, T.; Bredesen, R. Ann. Chim. 2007, 32(2), 197. (3) Haugsrud, R. Solid State Ionics 2007, 178, 555–560. (4) Shimura, T.; Fujimoto, S.; Iwahara, H. Solid State Ionics 2001 143(1), 117. (5) Hausgrud, R.; Fjeld, H.; Haug, K. R.; Norby, T. J. Electrochem. Soc. 2007, 154(1), B77. r 2009 American Chemical Society

because of their reported3-5 mixed conductivity (protonic and electronic) and stability in moist CO2 environments. For instance, undoped La6WO12 is the most conductive compound reported up to now, showing both proton conductivity and electronic conductivity around 0.005 S/cm at 900 °C in wet hydrogen. A membrane made of such material with a thickness of 10 μm will allow obtaining a hydrogen flux of 4 mL min-1 cm-2 at 900 °C for a differential hydrogen partial pressure of 10 atm. The tungstates Ln6WO12 have a defect fluorite structure and they can be formulated as Ln6WO12κ2 or A4O6.85κ1.15 (κ=ν. ) O for a fluorite formulation. The symmetry of these Ln6WO12 oxides depends6,7 on the lanthanide element: cubic from La to Pr, tetragonal from Nd to Gd, and rhombohedral from Tb to Lu and for Y, Sc and In. In other words, the structure symmetry changes gradually as a function of the ionic radii of the trivalent lanthanide cation. In Table 1, the ionic radii of the different lanthanide-type cations (for trivalent cations and selected lanthanide cations having other common oxidation states) (6) McCarthy, G. J.; Fisher, R. D.; Johnson, G. G.; Gooden, C. E. In Proceedings of the 5th Materials Research Symposium, Solid State Chemistry; National Institute of Standards: Washington, D.C., 1972; p 397. (7) Diot, N.; Bernard-Rocherulle, P.; Marchand, R. Powder Diffr. 2000, 15(4), 220.

Published on Web 06/25/2009

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Table 1. Shannon Radii for Seven- and Eight-Coordinated Cations and the Corresponding Crystal Symmetry in the Ln6WO12 Structure; Atoms Ordered by the Ionic Shannon Radii for the Trivalent Cation ionic radii Shannon coordination element

atomic weight

valence

7

8

reported structure symmetry

La Ce

138.91 140.12 140.907

Nd Sm

144.24 150.35

Eu

151.96

Gd Tb

157.25 158.924

Dy Y Ho Er Tm Yb

162.50 88.905 164.93 167.26 168.934 173.04

Lu Sc

174.97 44.956

1.100 1.070 0.920a 1.058a 0.905a 1.046a 1.220 1.020 1.200 1.010 1.000 0.980 0.820a 0.970 0.960 0.958a 0.945 0.937a 1.080 0.925 0.919a 0.808a

1.160 1.143 0.970 1.126 0.960 1.109 1.270 1.079 1.250 1.066 1.053 1.040 0.880 1.027 1.019 1.015 1.004 0.994 1.140 0.985 0.977 0.870

cubic cubic

Pr

3 3 4 3 4 3 2 3 2 3 3 3 4 3 3 3 3 3 2 3 3 3

a

tetragonal tetragonal/cubic tetragonal/cubic tetragonal rhombohedral rhombohedral rhombohedral rhombohedral rhombohedral rhombohedral rhombohedral rhombohedral rhombohedral

iInterpolated from tabulated data.

in 7-fold and 8-fold coordination are displayed, as calculated using Shannon tables,8 together with the expected Ln6WO12 symmetry. The crystalline structure can be described9 as seven cubes sharing edges, in where the central cube is occupied by a hexavalent tungsten cation and has two oxygen vacancies located at opposite vertex positions. The six neighboring cubes of this central cube are occupied by trivalent lanthanide cations and have uniquely one oxygen vacancy. Therefore, tungsten cations are six-coordinated, whereas lanthanide cations are sevencoordinated. Isostructural crystalline compounds with rhombohedral symmetry are for instance Pr7O126 and Y6UO12.10 Concerning the implication of the structure symmetry on the proton conduction in this class of matrials, it is still unknown whether the rhombohedral symmetry, distorted and with small unit-cell dimensions, favors or hinders the oxide hydration and, hence, the solubility of the proton in the structure. The hydration mechanism of the oxide11-13 in the presence of water involves the reaction of oxygen vacancies and water, according to H2 OðgÞþv::O þOxO S2OH:O :: ðaccording to the Kroger-Vink notationÞ namely, the compound La6WO1202 hydrates at 600 °C in contact with humidified gas (2.5% water) to form the hydrated oxide La6WO11.95(OH)0.101.95 as stated by thermogravimetry.14 On the other hand, the cubic structure, more ordered and with bigger unit-cell dimension, (8) (9) (10) (11) (12) (13) (14)

cubic

Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751. Ray, S. P.; Coix, D. E. J. Solid State Chem. 1975, 15, 333. Bartram, S. F. Inorg. Chem. 1966, 5(5), 749. Norby, T.; Larring, Y. Solid State Ionics 1995, 77, 147. Bonanos, N. Solid State Ionics 2001, 145, 265. Kreuer, K. D. Annu. Rev. Mater. Res. 2003, 33, 333. Haugsrud, R.; Kjølseth, Ch. J. Phys. Chem. Sol. 2008, 69, 1758.

promotes the proton mobility, i.e., rotational diffusion around the lattice oxygen and proton hop to a neighboring lattice oxygen atom. One would expect that there exists a compound Ln6WO12 with appropriate crystal symmetry and composition to properly combine proton solubility and mobility, therefore maximizing proton conduction. In this line, Haugsrud et al.3,5 have studied the nature of the lanthanide cation (La, Nd, Gd, and Er) and acceptor doping (Ca2þ) in Ln6WO12 materials prepared by solid-state reaction, aiming to understand transport properties, i.e., proton and electron conduction, whereas Shimura et al.4 studied the partial substitution of the lanthanide, i.e., La, by Nd and Zr. The conventional preparation of this class of proton conducting materials typically involves successive firing steps at very high temperatures in the range of 15001600 °C, starting from a finely grinded mixture of the corresponding metal oxides. Moreover, the preparation of high-density pellets, for permeation or electrochemical characterization, requires even higher sintering temperatures. These high-temperature treatments are very inconvenient because of the high volatility of WOx species in oxidizing conditions, leading to progressive and undetermined changes in the stoichiometry of the mixed oxide and the lack of homogeneity of the sample. For the targeted application of currentless hydrogen separation, the preparation of supported thin films of these materials is a key issue. The relatively low sintering activity and the involved temperatures make difficult the choice of a stable (while manufacturing) substrate material with sufficient shrinkage to promote the thin-layer densification. One of the possible solutions to this entails the preparation of Ln6WO12 material having crystallite sizes below 50 nm and consequently offering a high sintering activity. In addition, this nanocrystalline mixed electronicprotonic conducting material could be used as catalytic

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layers for hydrogen oxidation/reduction on top of the already-sintered gastight electrolyte. This layer may contain the Ln6WO12 material impregnated/mixed with a low amount of a catalytic metal, i.e., Ni or Pt, and would require a relatively mild sintering treatment. This work presents the preparation of nanocrystalline mixed conducting materials with three different nominal compositions (Nd6WO12 -Eu6WO12-Er6WO12) using a sol-gel complexation synthesis method. The evolution of the crystalline phase and crystallite size is studied as a function of the sintering temperature. Shrinkage behavior was analyzed using uniaxially pressed pellets ranging from 1000 to 1500 °C. In addition, the total conductivity in different environments has been studied systematically for samples sintered at different temperatures using DC and AC methods. Hydrogen permeation was studied for a disk-shaped Nd6WO12 membrane in the range of 7001000 °C. Finally, stability of these materials at 700 and 800 °C has been evaluated in contact with a CO2-rich gas stream as well as chemical compatibility with yttriastabilized zirconia at 1200 and 1400 °C. 2. Experimental Section The preparation method is based on the citrate-complexation route modified in order to stabilize W- and Ln-containing ions in the solution, and has been adapted from those reported elsewhere.15-17 Nd2O3 (Aldrich, 99.9%), Eu2O3 (ABCR, 99.9%) and Er2O3 (Aldrich, 99.9%) were dissolved in concentrated hot nitric acid (65% vol.) in stoichiometric proportion and the resulting nitrate was complexed using citric acid at a molar ratio 1:2 cation charge to citric acid. Care should be taken to avoid sudden nitrate decomposition. Another solution was prepared using ammonium tungstate (Fluka, >99%) and complexing it with citric acid (Fluka, 99.5%) at the same ratio. Metal complexation in both cases was promoted by heat treatment at 120 °C for 1 h. Both solutions were neutralized by controlled addition of ammonium hydroxide (32% wt.) and mixed at room temperature. The resulting solution is gradually concentrated by stepwise heating under stirring up to 150 °C and followed by foaming. The resulting thick foam product is subsequently calcined in air to eliminate carbonaceous matter and favor the mixed oxide crystallization. The minimum calcination temperature to attain this was around 700 °C for 10 h. Preparation of bar samples was performed using the Ln6WO12 material as calcined at 900 °C for 10 h and uniaxially pressed at 100 MPa. The green geometry of the bar was 40  5  4 mm3. The samples were sintered in air at either 1150 or 1350 °C for 4 h. XRD was carried out on a Philips X0 Pert Pro equipped with a X0 celerator detector using monochromatic Cu KR radiation. XRD patterns were recorded in the 2θ range from 20 to 90° and analyzed using X’Pert Highscore Plus software (PANalytical). Electrical conductivity measurements were conducted by standard four-point DC technique on the sintered rectangular bars. Silver paste and wire were used for contacting. The measurements were carried out under different atmospheres,

(15) Tessier, F.; Marchand, R. J. Solid State Chem. 2003, 171, 143. (16) Serra, J. M.; Vert, V. B.; Betz, M.; Haanappel, V. A. C.; Meulenberg, W. A.; Tietz, F. J. Electrochem. Soc. 2008, 155(2), B207. (17) Yoshimura, M.; Ma, J.; Kakohana, M. J. Am. Ceram. Soc. 1998, 81(10), 2721.

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i.e., argon and hydrogen saturated with water at 20 °C. The constant current was supplied by a programmable current source (Keithley 2601), whereas the voltage drop through the sample was detected by a multimeter (Keithley 3706). To eliminate the thermal effect and avoid nonohmic responses, we measured the voltage with the current in both forward and reverse directions. Electrochemical impedance spectroscopy (EIS) analysis was carried out using a disk-shaped sample 15 mm in diameter and a frequency response analyzer (Solartron 1455A) coupled with a Solartron CellTest 1470E system. Permeation measurements were performed on 15 mm diameter disks. The sample consisted of a gastight 510 μm thick Nd6WO12 disk sintered at 1550 °C. Both disk sides were coated by screen printing with a 20 μm layer of a Pt ink (Mateck, Germany), aiming to improve surface hydrogen exchange. Sealing was done using gold gaskets. Hydrogen was separated from a mixture H2-He (1/9 mol/mol and 2/8 mol/mol150 mL/min) saturated with water at room temperature using 150 mL/min argon as sweep gas. Permeate was analyzed using micro-GC Varian CP-4900 equipped with Molsieve5A, PoraPlot-Q glass capillary, and CP-Sil modules. Chemical stability against CO2 was carried out using the Ln6WO12 oxide as powder and contacting it with a dry and humidified (2.5% water) continuous gas flow (10% CO2 and 90% CH4) at 700 and 800 °C for 72 h. On the other hand, chemical compatibility test with YSZ was studied as follows: Nd6WO12 sintered at 1150 °C and 8% mol yttria-stabilized zirconia (Aldrich, 99.9%) were mixed in equal weight proportion and grinded together in acetone; the resulting powder was sintered in stagnant air for 24 h at 1200, 1400, and 1500 °C.

3. Results and Discussion 3.1. Structural Characterization. The preparation method involves the formation of a citrate-based polyester having homogeneously distributed the chelated lanthanide and tungsten cations in the organic framework. This organic-inorganic gel is pyrolyzed at 250 °C for 15 h and subsequently calcined in air at the final desired temperature, ranging from 900 to 1500 °C. Figure 1 shows the DTA-TG study on a Nd6WO12 sample previously treated at 250 °C. At the lowest temperatures

Figure 1. TG-DTA measurement of the intermediate material obtained after gelation and pyrolysis at 250 °C. The intended nominal composition of the targeted mixed oxide is Nd6WO12.

Figure 2. Structural characterization of Nd6WO12, Eu6WO12 and Er6WO12 compounds: XRD patterns as a function of the final sintering temperature.

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Figure 3. Structural characterization of Nd6WO12, Eu6WO12 and Er6WO12 compounds: phase symmetry transition diagram as a function of the final sintering temperature.

Figure 4. Evolution of the crystallite size of Nd6WO12, Eu6WO12, and Er6WO12 compounds as a function of the final sintering temperature. Note: Crystallite sizes are calculated applying the Scherrer equation using the diffractions peaks at 28-30 and 56-67° 2θ angles and LaB6 as standard.

up to 400 °C, there is a continuous mass loss that is ascribed to the gradual polymer decomposition and partial oxidation. The highest mass loss takes place between 450 and 500 °C and involves the loss of the 30% of the sample weight. This mass loss is ascribed to the burn-up of the major part of the citrate polymer and is associated with a pronounced exothermic peak, with an ignition point around 450 °C. On the other hand, at slightly higher temperatures (∼600 °C), another mass loss is observed and this is again associated with another smaller exothermic peak. In this case, the mass loss is attributed to the final burn-up of the organic matter intimately attached to the inorganic matrix. At those temperatures (600-650 °C), the inorganic matter is

predominantly amorphous and has not detectable XR diffraction peaks (Figure 2). However, at 700 °C, broad diffraction peaks corresponding to the defect fluorite structure start appearing, although no crystallization heat can be stated through the DTA measurement. Figure 2 displays a summary of the XRD patterns of the three tungstate compositions sintered at different temperatures ranging from 900 to 1480 °C as powders. The first indication of the early crystallization of the fluorite phase from the amorphous matrix is observed around 700 °C (see the Supporting Information, Figure S1), albeit part of this inorganic matrix might still remain amorphous. All three compounds show cubic crystal symmetry in the first crystallization stages when existing

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as nanocrystals with averaged crystal size below 20 nm and consequently a rather broad peak width. However, each tungstate compound shows a particular phase symmetry development. Figure 3 summarizes the symmetry evolution for the three tungstates as function of the sintering temperature, when treated as powders. Ndbased tungstate presents a cubic symmetry when the sample is sintered in the range from 700 to 1200 °C, whereas very weak diffraction peaks corresponding to the tetragonal symmetry start appearing at higher temperatures. The samples treated at temperatures ranging from 1300 to 1480 °C present mostly cubic phase symmetry and only minor diffraction peaks (the arising peaks corresponding to the tetragonal phase are at 24.5, 26.3, 30.7, 33.0, 47.0, 50.1, 51.8, 53.5, and 74.1° according to ref 19) corresponding to the tetragonal phase can be stated. Indeed, for this crystalline compound, the symmetry

Figure 5. Shrinkage behavior of the Nd6WO12, Eu6WO12 and Er6WO12 compounds. Pellets of 10 mm in diameter were uniaxially pressed at 300 MPa using powder previously calcined at 900 °C. Shrinkage values show the averaged of identical samples with the corresponding error bar.

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transition uniquely involves a minor change in the c unit cell parameter from 5.481 (cubic) to 5.453 A˚ (tetragonal). This final phase symmetry is the one expected by preparation through direct firing of a mixture of the single oxides at 1480 °C. Er-based tungstate shows a cubic symmetry when the sample is sintered in the range from 900 to 1100 °C and at higher temperatures the symmetry gradually changes from cubic to rhombohedral. At temperatures higher than 1350 °C, the rhombohedral symmetry is practically pure. The observed phase symmetry transformation at high temperature should involve the ordering of cation (Er and W) and anion (oxygen ion and vacancies) sublattices into the most thermodynamically stable state. This behavior is in line with the symmetry transition observed for Y6WO12 by Yoshimura et al.17 This cation arrangement process may strongly influence the water solubility and the location of structural oxygen vacancies, and consequently, the proton conduction. The structural phase evolution of Eu-based compound is a case between the ones observed for Nd and Er compounds. The cubic symmetry observed at low sintering temperatures, e.g., 900 °C, evolves with increasing temperatures into a symmetry mix between rhombohedral and tetragonal, even after sintering at high temperatures. This might be related to the coexistence of trivalent and divalent europium cations and the averaged europium ionic radius increased, whereas divalent europium presence would lead to the generation of new oxygen vacancies. Table 1 displays the Shannon ionic radius for both europium cations in 7- and 8-coordination. However, when the europium tungstate calcined at 900 °C is pressed into bars or pellets and sintered at 1150 and 1350 °C for total conductivity measurement, the observed phase symmetry was tetragonal with minor traces of the rhombohedral symmetry for the one sintered at 1350 °C. This behavior could be ascribed to the tunable

Figure 6. Total conductivity measurements of Nd6WO12, Eu6WO12, and Er6WO12 compounds sintered at 1150 and 1350 °C for 4 h under humidified (2.5% H2O) hydrogen gas flow.

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Figure 7. Total conductivity measurements of Nd6WO12, Eu6WO12, and Er6WO12 compounds sintered at 1150 and 1350 °C for 4 h under humidified (2.5% H2O) argon gas flow. Table 2. Total Conductivity and Apparent Activation Energy of Nd6WO12, Eu6WO12, and Er6WO12 Compounds lanthanide

Tsint (°C)

environment

σ800 (S/cm)

σ 900a (S/cm)

EAb (eV)

R2b

Nd Nd Nd Nd Er Er Er Er Eu Eu Eu Eu

1150 1150 1350 1350 1150 1150 1350 1350 1150 1150 1350 1350

Ar H2 Ar H2 Ar H2 Ar H2 Ar H2 Ar H2

1.25  10-4 1.73  10-4 2.90  10-4 2.00  10-4 4.33  10-6 2.65  10-4 3.60  10-6 1.05  10-4 1.60  10-5 5.10  10-03 1.90  10-6 4.60  10-3

2.08  10-4 5.18  10-4 5.04  10-4 6.05  10-4 1.14  10-5 8.16  10-4 8.56  10-6 3.35  10-6 4.45  10-5 1.41  10-2 5.34  10-6 1.01  10-2

0.55 1.19 0.60 1.20 1.05 1.22 0.94 0.87 1.11 1.10 1.12 0.85

0.998 0.999 0.997 0.998 0.999 1.000 0.997 0.999 1.000 1.000 0.997 1.000

a

Extrapolated from experimental data at 850 °C. b Calculated in the range 850-600 °C.

reduction/oxidation of europium cations, affected by the applied pressure and the lower accessibility of the air into the pressed sample during the subsequent heat treatment. Figure 4 shows the evolution of the crystallite size for the three compounds as calculated from XRD patterns. The samples calcined at 700 °C show a very small crystallite size, which depends on the lanthanide nature. The lowest sizes are obtained for Er6WO12 with values of 12 nm at 900 °C. Eu- and Nd-based compounds present similar crystallite sizes in the range 900-1000 °C, e.g., 21 nm at 900 °C, although their behaviors diverge when symmetry phase transition start taking place. The crystal size grows with increasing temperatures following an exponential model and relatively large crystallites are obtained for Nd (356 nm) and Er (114 nm) at 1300 °C. Certainly, the crystallite growth is accompanied by the symmetry transformation, as indicated in Figure 4. The crystallite growth behavior is in line with the shrinking trends showed below. The shrinkage behavior was studied because the sintering activity is a key issue in the manufacturing of both supported and bulk membranes. Figure 5 shows the shrinking curves for pressed pellets (301 MPa for 1 min) of all three tungstate compositions previously treated at

900 °C. Nd6WO12 showed the highest sintering activity achieving a shrinkage value of 20% already at ∼1350 °C. This compound allows obtaining high density components at relatively low firing temperatures. Europium and erbium compounds show a much lower sintering activity and generally require sintering temperatures 200 °C higher, with respect to Nd6WO12, to achieve the same densification degree. 3.2. Electrochemical Characterization and Permeation. The total conductivity of three tungstate compositions was studied using bars sintered at 1150 and 1350 °C in humidified hydrogen and argon and these results are displayed in Figures 6 and 7, whereas the apparent activation energy at high temperatures is displayed in Table 2. The highest conductivity is obtained for Eu6WO12 at the highest tested temperature in hydrogen for both sintering temperatures, although the sintering temperature has a huge impact on the conductivity and its thermal evolution. The Eu6WO12 sample treated at 1350 °C shows a higher conductivity at any tested temperature and it is less thermally activated than the sample treated at 1150 °C, which showed a lower conductivity except for the highest tested temperature (850 °C). The reason for the change in the conduction behavior between

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Figure 9. Hydrogen permeation flux of the Nd6WO12 membrane as a function of temperature. Feed composition was moist H2 (20%) in helium and argon as sweep gas (both sides at atmospheric pressure).

Figure 8. XR diffraction patterns of the pressed samples employed in the total conductivity measurement. Note: The XRD patterns remained unchanged after conductivity measurements.

both samples is very complex and several aspects are influencing. Three main factors have been varied through the sintering at higher temperature: (1) crystallite size grew from 44 to 100 nm; (2) phase symmetry changed from cubic to tetragonal, i.e., the treatment at high temperature has promoted the ordering of cation and anion (oxygen ion and vacancies) sublattices; and (3) the sample density increased. The nanocrystalline sample calcined at 1150 °C should have in general a major contribution to the conduction mechanisms from the grain boundary effects on diffusion of protons and electronic conduction. An additional argument is the influence of the nanosize on the electronic crystal properties and local lattice ordering, including oxygen vacancies and cation distribution, and the consequent changes in the water uptake and proton mobility. Certainly, the nanoscale should have a strong impact on structural and electronic properties. Considering the averaged phase symmetry derived from powder XRD, the symmetry change from cubic to tetragonal might result in slight changes in the proton conduction, i.e., water uptake and proton mobility. The fact is that the apparent activation energy of the sample calcined at lower temperature is higher (1.10 eV, see Table 2) than the one sintered at 1350 °C (0.85 eV), both values constant at all tested temperatures for both samples, and this leads to a higher total conductivity at temperatures above 800 °C for the sample calcined at 1150 °C. For instance, at 900 °C the conductivity for the first one is 0.014 S/cm, whereas for the latter is 0.010 S/cm. As a general outcome, nanocrystalline Eu6WO12 is a promising candidate for hydrogen separation membranes at high temperature because of its high total conductivity and activation energy.

The gas environment shift from wet hydrogen to wet argon has a strong detrimental impact on the total conductivity and it decreases in two to 3 orders of magnitude depending on the sample. This indicates that Eu6WO12 reduces in presence of hydrogen, very likely, through partial reduction of Eu3þ to Eu2þ and therefore, Eu2þ has an extra electron. The material in reduced state may have potentially a higher electronic (n-type) conduction because of the higher concentration of electrons (Eu2þ) and a higher ionic conductivity because of generation of new oxygen vacancies, as for the case of acceptor doping using Ca2þ. Summarizing, the hydrogen reduction can be expressed as 1 Euþ3 þ H2 fEuþ2 þOH• 2 Nd6WO12 shows a conductivity roughly 1 order of magnitude lower than Eu6WO12. At high temperature, Nd samples calcined at both temperatures show similar total conductivity values although the behavior differs drastically at temperatures below 600 °C. For the sample calcined at 1350 °C, a change in the apparent activation energy is observed around 600 °C, from 1.2 to 0.5 eV with decreasing temperatures. The sample calcined at 1150 °C presents an unexpected and reproducible marked change in the conductivity at 600 °C, showing very low activation energy at lower temperatures in both wet hydrogen and argon (typical values for bulk proton conduction in cubic perovskites). In both cases, the change of the activation energy indicates the change of the transport limiting mechanism, much more pronounced for the sample sintered at lower temperature. This behavior at low temperature of the sample calcined at 1150 °C may be related to the small crystal size (70 nm) and the cubic symmetry of the cell, which allows adequate proton mobility and may increase the hydration rate of the oxide at low temperature, when the equilibration of the water concentration needs longer times. Er6WO12 appears to be a clear example of the influence of the structure symmetry on the conduction properties in hydrogen. In this case, the sample calcined at 1150 °C has

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Figure 10. SEM pictures of the fracture cross-section of a Nd6WO12 membrane sintered at 1550 °C after permeation measurement.

Figure 11. Structural stability test for Nd6WO12 treated in both dry and humidified (2.5% H2O) gas flow (90% CH4 and 10% CO2) at 700: XRD patterns as a function of the final sintering temperature and treatment conditions.

(pseudo)cubic symmetry, whereas the one calcined at 1350 °C has pure rhombohedral symmetry (see also Figure 8). The cubic symmetry sample has a higher total conductivity in hydrogen practically in the whole temperature range and also higher activation energy (1.22 eV, see Table 2) as for the case of Eu6WO12. This behavior is also observed in argon although the conductivity is more than 1 order of magnitude lower. The activation energy

remains constant in the whole temperature range as for the case of Eu6WO12, whereas the conductivity values at the highest temperatures are very close to those exhibited by Nd6WO12 in hydrogen. Figure 8 shows the XRD patterns of sintered bar samples used in the conductivity measurements. The XRD patterns remained unchanged after conductivity measurements. One should note the aforementioned case of the Eu-based samples because the sample sintered at 1350 °C presented only minor amounts of the rhombohedral symmetry, whereas the sintered powder (Figure 2) has transformed fully into the rhombohedral symmetry phase. Hydrogen permeation was measured using a 510 μm thick Nd6WO12 disk membrane coated on both polished sides with a 20 μm thick porous Pt layer. From the hydrogen content measured in the argon sweep side of the permeation assembly and the argon flow rate, the total hydrogen permeation rate (mL/min cm2) was calculated. Figure 9 shows the temperature dependence of the hydrogen flux for the membrane of Nd6WO12. The hydrogen flux increases with the partial pressure of hydrogen in the feed stream and with temperature with a constant activation energy of 0.38 eV. The relatively low permeability of Nd6WO12 (0.023 mL/min cm-2 at 1000 °C with 20% v/v H2 in the feed stream) and the low activation energy, in contrast to the high activation energy of the total conductivity, suggest that the limiting factor is the ionic conductivity in the permeation process while the electronic transport prevails at high temperature in agreement with previous works.3 The hydrogen flux could be considerably increased by using more conductive tungstate compositions and supported thin membranes; and, for instance, a 10 μm thick membrane made of Nd6WO12 would allow achieving a flux value of 1.2 mL/min cm2, assuming the bulk conduction is limiting the permeation in this thickness range (500-10 μm).

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Figure 12. Thermal evolution of the reaction between Nd6WO12 and 8% yttria-stabilized zirconia.

Impedance spectroscopy analysis (two-point symmetrical configuration) of a sample identical to the one tested for hydrogen permeation corroborated the proton contribution to transport through the membrane (see the Supporting Information, Figure S4). When the sample environment was shifted from helium to D2O-saturated helium and then to H2O-saturated helium, both membrane resistivity and electrode impedance decreased following this order in the temperature range-from 700 to 900 °C. Figure 10 shows two scanning electron micrographs at two magnifications of the fracture cross-section of a representative Nd6WO12 membrane after permeation testing. The sample is well sintered after treatment at 1550 °C and presents only closed porosity. The small particles on the large grains are platinum particles coming from the catalytic coating of both sides during sample preparation. 3.3. Stability Assessment. Figure 11 shows the XRD patterns of the Nd6WO12 samples, previously calcined at 900, 1150, and 1350 °C, before and after the stability treatment at 700 °C for 72 h under continuous gas flow (10% CO2 and 90% CH4) saturated at room temperature with water. These conditions were chosen to achieve humid reducing conditions and a CO2-rich environment while having a temperature high enough for the evolution of carbonation or other reactions but below the limit of decomposition of carbonates18. The whole set of materials remained unchanged after the treatment at 700 and 800 °C, regardless of its original sintering temperature and lanthanide nature. Moreover, the addition of water to CO2-rich gas stream had not effect on the material (18) Jeevanadam, P.; Koltypin, Y.; Palchik, O.; Gedanken, A. J. Mater. Chem. 2001, 11, 869.

stability (see the Supporting Information, Figure S3). Therefore, the materials are stable in these conditions, even for nanocrystalline Ln6WO12 materials calcined at 900 °C. In addition, the structural stability in reducing conditions (wet hydrogen flow) at 900 °C was confirmed after one week of treatment. Another interesting aspect concerning the manufacturing of supported thin Ln6WO12 layers is the chemical compatibility with potential substrate materials. The linear thermal expansion coefficient for La6WO12 is around 12.5  10-5 K-1 and matches properly with yttria-stabilized zirconia (8YSZ), widely applied in the standard manufacturing process of anode-supported solid oxide fuel cells. A preliminary step is therefore the assessment of the chemical compatibility of both oxide materials. This study was done by preparing pellets using a physical mixture (50:50 weight proportion) of particles of a Nd6WO12 sample previously calcined at 1150 °C and commercial nanocrystalline 8YSZ (Aldrich) and treating them at 1200, 1400, and 1550 °C for 24 h in air, mimicking the manufacturing conditions for supported films. Figure 12 shows the evolution of the XRD patterns of the pristine oxide mixture and the treated samples. It is noticeable that Nd6WO12 and 8YSZ react very fast to form a solid solution phase (SSP-1), with a high molar content of Nd, and it formed a solid solution with higher Zr content to a lower extent. In fact, already at 1200 °C, the diffraction peaks of phase Nd6WO12 are not detectable, indicating the total conversion of this phase into two solid-solution phases: (1) new solid-solution phase SSP-1 enriched in Nd, which is the major material component at 1200 °C; and (2) a Nd-Zr balanced solid solution (SSP2), although 8YSZ phase is still present in a lower amount. At 1400 °C, the Nd-rich phase SSP-1 reacts with

Article

Chem. Mater., Vol. 21, No. 14, 2009

8YSZ to form Nd-Zr balanced solid-solution (SSP-2) crystals. The crystallinity of the solid-solution SSP-2 has increased while SSP-1 and 8YSZ diffraction peaks are still detected. Gradually, the SSP-2 phase grows through the reaction of the remaining YSZ and SSP-1 phase. At 1550 °C, the crystallinity of the phase SSP-2 has developed strongly and only minor amount of the phase SSP-1 is detected. The new phase has a fluorite structure and the nominal composition Nd2.6Y0.6Zr3.4W0.4O12.8. The cell size was calculated applying Treor Indexing Method. The crystal system is tetragonal and the parameters are: a = b = 5.2044 A˚, c = 2.9579 A˚ and R = β = γ = 90° with a cell volume of 80.0170 A˚3. The crystal system of Nd6WO1219 is tetragonal, and its parameters are a= b=5.4810 A˚ and c=5.4530 A˚, and R=β=γ=90°, according to the databases. The crystal system of Zr0.85Y0.15O2 (JCPDS card 82-1246) is cubic (a=5.1390 A˚). 4. Conclusions A preparation method based on sol-gel and controlled pyrolysis has been developed, making possible the preparation of nanoscrytalline mixed electronic-protonic conducting materials with nominal compositions Nd6WO12Eu6WO12-Er6WO12 with (pseudo)cubic fluorite symmetry. The structural evolution of these three fluorite compounds has been studied as a function of the sintering temperature. The Nd-based compound transformed into the tetragonal symmetry at temperatures above 1200 °C, whereas the Er-based compound transformed into rhombohedral symmetry above 1200 °C. The Eu-based compound is an intermediate case, in where the final symmetry tetragonal or rhombohedral depends on the exact thermal and manufacturing treatment. The shrinking behavior was very dependent on the fluorite composition and NdWO12 showed a very high sintering activity, whereas Eu and Er compounds showed a significantly lower sintering activity (in decreasing order Eu>Er). (19) Trunov, V. K.; Russ, J. Inorg. Chem. 1968, 13, 491.

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The total conductivity was studied taking into account the variables: (i) lanthanide nature (Nd, Eu, Er); (ii) sintering temperature (1150 and 1350 °C); and (iii) gas environment (moist hydrogen and argon). The highest conductivity values were obtained for Eu6WO12, achieving values of 0.009 S/cm at 850 °C in humid hydrogen. For this compound, the activation energy remained constant, albeit it was dependent on the sintering temperature and gas environment, i.e., 1.10 eV at 1150 °C and 0.85 eV at 1350 °C, both in humid hydrogen. Er6WO12 sintered at 1150 °C with cubic symmetry presented a higher total conductivity and activation energy than the same compound sintered at 1350 °C with pure rhombohedral symmetry. As for the other compounds, the sample sintered at lower temperature exhibited higher activation energy around 0.91 eV. The hydrogen permeability was measured in a Nd6WO12 dense membrane and the flux at 1000 °C was 0.012 mL/(min cm2) mm. The stability in humid CO2-rich reducing environment was studied for the three compounds sintered at 900, 1150, and 1350 °C. The whole set of materials remained stable after treatment for three days at 700 and 800 °C. On the contrary, Nd6WO12 reacted very fast with 8YSZ to form a solid-state solution with the same fluorite structure. Acknowledgment. Financial support by the Universitat Politecnica de Valencia (Grant UPV-2007-06), the Spanish Ministry for Science and Innovation (Grant JAE-Pre 080058 and Project ENE2008-06302/CON), and the Helmholtz Association of German Research Centres through the Helmholtz Alliance MEM-BRAIN (Initiative and Networking Fund) is kindly acknowledged. Supporting Information Available: Additional SEM images, XRD patterns of the samples treated at increasing temperatures starting from 700 °C, XRD patterns of the whole set of treatment conditions during stability testing, impedance spectroscopy analysis of a Nd-based membrane under different atmospheres (dry helium and D2O/H2O-saturated helium), and photographs of samples (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.