Electric Transport and Oxygen Permeation Properties of Lanthanum

Dense perovskite-structured membranes with desired composition of La0.8Sr0.2Co0.6Fe0.4O3-δ (LSCF) were prepared from powders produced by four ...
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Ind. Eng. Chem. Res. 2000, 39, 646-653

Electric Transport and Oxygen Permeation Properties of Lanthanum Cobaltite Membranes Synthesized by Different Methods Xiwang Qi,† Y. S. Lin,*,† and S. L. Swartz‡ Department of Chemical Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0171, and NexTech Materials, Ltd., Worthington, Ohio 43085

Dense perovskite-structured membranes with desired composition of La0.8Sr0.2Co0.6Fe0.4O3-δ (LSCF) were prepared from powders produced by four different methods. LSCF powders prepared by citrate, solid-state, and spray-pyrolysis methods had compositions close to the desired stoichiometry with a slight difference in cobalt concentration, whereas coprecipitated powders had a large strontium deficiency. The membrane composition was a determining factor that affected the electronic conductivity and therefore oxygen permeability. The membrane with a large strontium deficiency had much lower electronic conductivity and oxygen permeability (ionic conductivity) than the other three membranes with compositions close to the desired stoichiometry. The electronic conductivity of membranes prepared from citrate, solid-state, and spraypyrolysis methods increases with the cobalt concentration of the membrane. For the three membranes with similar composition, the activation energy of oxygen flux decreases with increasing grain size. Oxygen pressure dependency of oxygen vacancy concentration is also influenced by the membrane microstructure and composition. LSCF membranes with same composition and similar microstructure should have similar electric and oxygen transport properties. 1. Introduction Mixed-oxygen ion-electron conducting membranes are oxygen semipermeable with appreciable oxygen permeability at elevated temperatures and have potential applications in clean, efficient, and flexible membrane processes for producing oxygen from air at high temperatures.1-4 It has been suggested that this mixed conducting membrane technique is commercially attractive for air separation when integrated with a hot turbine system.5 Another promising application of mixedconducting ceramic membrane is as membrane reactors for partial oxidative reactions because of their oxygen semipermeability and catalytic activity.6,7 The focus has been on the natural gas upgrading into ethane/ ethylene8-10 or to syngas.11,12 Compared with the fixed bed reactor, mixed-conducting ceramic membrane reactor not only provides controlled oxygen supply at the membrane surface to eliminate gas-phase combustion, but also generates oxygen species (O2-, O-) that may be more selective to partial oxidation reactions, thus resulting in a great improvement of the selectivity for many catalytic selective oxidation processes.1,13 Teraoka et al.14 first demonstrated that La1-xSrxCo1-yFeyO3-δ oxide based perovskite-type ceramic membranes have appreciably high oxygen permeation fluxes at high temperatures. The partial substitution of A and B site cations by other metal cations with lower valences brings about the formation of oxygen vacancies and the appearance of oxygen ionic conduction. It was found that the oxygen flux tends to increase with the increase of substitution of Sr2+ for La3+ and Co3+ for Fe3+, but a * To whom correspondence should be addressed. E-mail: [email protected]. Telephone: (513) 556-2769. Fax: (513) 556-3473. † University of Cincinnati. ‡ NexTech Materials, Ltd.

small amount of Fe is necessary to preserve perovskitetype structure at high Sr contents.15 Following Teraoka et al.’s work, many researchers studied the La-Sr-CoFe series as oxygen permeable membranes.16-20 The ionic transference number obtained was in the range of 10-5 to 10-3, depending on the temperature and ambient atmosphere, and the activation energy for ionic transport was 64-125 kJ/mol. However, no authors had ever achieved an oxygen flux exceeding the results reported by Teraoka et al.,14 and inconsistencies (up to 1 order of magnitude) are frequently reported on the oxygen permeation rates even with nominally identical membrane materials. For example, Teraoka et al.14 showed the oxygen permeation flux as high as 2.31 × 10-6 mol/cm2‚s at 850 °C for the SrCo0.8Fe0.2O3-δ membrane; while the oxygen permeation fluxes for SrCo0.8Fe0.2O3-δ membrane of the same membrane thickness from Kruidhof et al.16 and Qiu et al.17 are 1.8 × 10-7 and 6.3 × 10-7 mol/cm2‚s, respectively, under same experimental conditions. Conflicting results were also reported concerning the effects of A-site substitution by different elements on the oxygen permeation flux. Teraoka et al.15 and Tsai et al.19 showed that the oxygen permeation flux of A-site substituted LaCo1-yFeyO3-δ based membrane decreases in the order of Ba > Ca > Sr, but Stevenson et al.18 obtained a different result with the oxygen permeation flux decreasing in the order of Sr > Ba > Ca. Different experimental conditions, such as unsteady-state, surface modification of membrane by CO2, and different seals used have been suggested for the discrepancies. Little attention was focused on the effects of preparation methods on oxygen permeation properties of the mixedconducting ceramic membranes. Dense ceramic membranes usually are prepared by the powder-pressing/sintering method. Several approaches can be employed to synthesize powders, such

10.1021/ie990675e CCC: $19.00 © 2000 American Chemical Society Published on Web 01/27/2000

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as the most widely used solid-state method and coprecipitation method. Different methods produce powders with different sizes and distributions, which further affect the sintering behavior and lead to different membrane microstructures. Zhang et al.21 recently reported that the grain size has a strong effect on the oxygen permeation flux for SrCo0.8Fe0.2O3-δ membrane. More importantly, the compositions of membranes prepared by different methods may not be the same as desired from the amounts of precursors used in preparing the samples. The deviation of composition will lead to different defect concentrations of the resulting membrane. Subsequently, the electrical conductivity and the oxygen permeation flux (depending on the electronic and ionic conductivity) will be different. Therefore, studies on electrical conduction and oxygen permeation properties of dense membranes prepared by different approaches are essential to the understanding of the oxygen transport and practical applications of the dense ceramic membranes. This paper reports the electrical conductivity and oxygen permeation properties of La0.8Sr0.2Co0.6Fe0.4O3-δ (abbreviated as LSCF) membranes prepared from powders synthesized by four different methods: the citrate, solid-state, spray-pyrolysis and coprecipitation methods. The coprecipitation and citrate methods are wetchemical approaches that are expected to give a more uniform mixing of the metal elements at a molecular level and therefore a more uniform microstructure of the resulting membranes. The electrical conductivity and oxygen permeation rates of the membranes were studied at various temperatures and oxygen partial pressures with the aim to clarify the influence of preparation process on the membrane properties. 2. Experimental Section 2.1. Membrane Preparation. The following four methods were used to prepare LSCF membranes. The precursor chemicals used in the preparation were La(NO3)3‚H2O, Sr(NO3)2, Co(NO3)3‚5H2O, Fe(NO3)3‚6H2O, SrCO3, CoCO3, La2O3, and Fe2O3 (99.9%, all from Alpha). Citrate Method. La, Sr, Co, and Fe nitrates in stoichiometric amounts were dissolved into water to form a 0.2 M solution of total metal ions. Citric acid with an amount of 1.2 times of the total metal ions was added into the solution, which was then heated to 96-100 °C for polymerization for 3.5 h with stirring. The water was finally evaporated to allow the solution condensed to a sticky gel. After drying, the gel was heated to 400 °C in an oven for self-ignition to take place to burn out the organic components. The resulting powder was calcined at 800 °C and pressed into disks at 160 MPa, which were sintered at 1275 °C for 10 h to form final membranes. Solid-State Method. This is also known as ball milling and calcination method. Raw materials La2O3, SrCO3, CoCO3, and Fe2O3 were mixed and ball-milled. After drying, the mixed powders were calcined at 1000 °C to achieve single-phase perovskite powder. Then the powder was pressed by cold isostatic method at 330 MPa and sintered at 1300 °C for 2 h to obtain the final LSCF membranes. Spray-Pyrolysis Method. Aqueous solutions of La, Sr, Co, and Fe nitrates in stoichiometric amounts were spray-pyrolyzed in a heated chamber at 800 °C. The resulting fine oxide powders were collected and calcined

Figure 1. Schematic diagram of oxygen permeation measurement system.

at 800 °C for 4 h and then pressed into disks at 330 MPa. The final LSCF disks were sintered at 1275 °C for 2 h. Coprecipitation Method. The corresponding La, Sr, Co, and Fe nitrates in stoichiometric amounts were dissolved into water, and the metal ions were coprecipitated when KOH solution was added slowly with stirring. The coprecipitated gel was filtrated and washed several times with distilled water to remove the potassium salt impurities. The solid material was then dried and calcined at 800 °C for 4 h to form powder, which was pressed into disks at 330 MPa and sintered at 1275 °C for 2 h. 2.2. Membrane Characterization. The phase structures of LSCF disks were determined by X-ray diffraction (XRD) using Cu ΚR radiation (Siemens) with 2θ varying from 20° to 70°. Scanning electron microscopy (SEM) (Hitachi 4000) was employed to observe the microstructures of the membranes. The elemental analysis of the membranes was carried out by the inductively coupled plasma (ICP) method (Thermo Jarrel Ash ICAP 1100). The sample for ICP measurement was a solution with concentration below 200 ppm, prepared by dissolving 30 mg of membrane powder into nitric acid and diluting with 2% nitric acid solution. The gastightness of the sintered membranes was checked by roomtemperature nitrogen permeation experiments. The four-point dc method was used to measure the total conductivities of these LSCF membranes. Bar-shape samples with dimension of 1.4 × 6.0 × 20 mm were used for the measurements. The experimental apparatus and detail procedures were shown elsewhere.22 2.3. Oxygen Permeation Measurement. The oxygen permeation experiments were performed in a vertical high-temperature permeation system shown in Figure 1. The gastight LSCF membranes with a diameter of 25 mm were sealed by a ceramic sealant (50% strontium cerate, 40% Pyrex glass, and 10% sodium aluminate) on the top of the inner dense alumina tube (2.54 cm in o.d., from Coors Ceramics) which was inserted into a larger dense alumina tube (3.81 cm in o.d., from Coors Ceramics). Air was introduced into the inner tube (serving as an upstream chamber) and helium was swept through the space between two alumina tubes (serving as the downstream chamber). The system was first heated to 950 °C at which the membrane was sealed by the melted sealant. The nitrogen in the effluent of downstream was checked by a gas chromatograph (Perkin-Elmer) with a molecular sieve column (1/8 o.d., 10 ft, 80/100 mesh, from Alltech) during the annealing step. Until no nitrogen was detected in the helium flow, the system was cooled at a rate of 5 °C/min to 600 °C and then the oxygen permeation experiment began.

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Table 1. Powder Properties and Densities of LSCF Membranes Prepared by Different Methods method abbreviation microstructure membrane density (g/cm3)

citrate CA bulk solid with bubbles trapped (very large grain size) 6.54

solid state SS average grain size: 5 µm, loosely compacted 6.55

spray-pyrolysis SP average grain size: 1 µm, interconnected with pores 6.53

coprecipitation CP average grain size: 3 µm, closely compacted 6.70

Figure 2. SEM Micrograph of cross section of LSCF membrane prepared by different methods: (a) citrate method, (b) solid-state method, (c) spray-pyrolysis method, and (d) coprecipitation.

An O2/N2 mixture was used to adjust the oxygen concentration of feed gas (oxygen partial pressure). The oxygen concentration in the helium flow was measured by an oxygen analyzer (series 6000, Illinois Instruments) with a resolution of 10-8 atm. The oxygen permeation fluxes through the membrane were calculated from the oxygen concentration and flow rate of the downstream effluent. In all cases, the gas flow rates passing through both chambers were kept at 50 mL/ min. In this experiment, it took about 3 h to reach the steady-state oxygen permeation after air and helium were introduced into the permeation chambers, and 1 h after temperature change.

3. Results and Discussion 3.1. Membrane Characteristics and Electronic Conductivity. The LSCF membranes with a good mechanical strength, all black in color, were prepared by the four methods. Membranes derived from starting powders prepared by the citrate, solid-state, spraypyrolysis, and coprecipitation methods are abbreviated as CA, SS, SP, and CP membranes, respectively, as summarized with other major characteristics in Table 1. Figure 2 shows the cross-section SEM micrographs of the four membranes. The CA membrane (Figure 2a) shows well-sintered morphology with no clear grain boundary. The SS, SP, and CP membranes (Figure

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Figure 3. XRD patterns of LSCF membranes prepared by different methods.

Figure 4. Total conductivity of LSCF membranes prepared by different methods (in air).

Table 2. Element Analysis Results of LSCF Membranes by ICP Method membranes

La

Sr

Co

Fe

∑ni

CA SS SP CP

0.800 0.800 0.800 0.800

0.194 0.196 0.183 0.030

0.597 0.581 0.613 0.636

0.405 0.413 0.412 0.431

1.996 1.990 2.008 1.897

2b,c,d) have a typical intergranular fractured microstructure with grains clearly visible from the SEM pictures. The average grain sizes for the three membranes are estimated at about 5, 3, and 1 µm in the decreasing order: SS > CP > SP. All the four membranes have similar relative densities of about 97% of the theoretical values. They are also gastight to nitrogen. Figure 3 shows the XRD patterns of the four membranes at the 2θ range from 20 to 70°. The peaks of each sample can be clearly indexed at d spacings of 2.731, 2.704, 1.922, 1.572, 2.222, 3.834, 1.366, 1.355, 1.559, 1.724, and 2.199 Å and no impurity peaks are observed, indicating the membranes prepared by the four methods are of single-phase orthorhombic perovskite structure and compositional homogeneity. The results of the elemental analyses of these four membranes are listed in Table 2. The molar ratios of the metal elements were based on lanthanum (0.8 according to the stoichiometry of La0.8Sr0.2Co0.6Fe0.4O3-δ). As seen from Table 2 the strontium content in CP membrane is surprisingly low, only 0.03 compared the desired value of 0.2. This indicates that a large amount of strontium was removed by the filtration and washing steps in the preparation process due to the considerable solubility of strontium hydroxide in aqueous solution. The processing conditions of CP method need to be further optimized to achieve the desired stoichiometry. As indicated in Table 2, the LSCF membranes prepared by the CA, SS, and SP methods have desired composition, suggesting a good control of the material stoichiometry of these three methods. Figures 4 and 5 show the total conductivities of the four membranes in air and helium atmospheres respectively in the temperature range from 600 to 950 °C. The total conductivity measures the transport rate of all the charged carriers in the membrane. For the LSCF membranes studied in this work, electronic-hole transport dominates the electric conduction.18,23 Therefore the conductivity measured is predominantly the electronic conductivity for these membranes. The temperature

Figure 5. Total conductivity of LSCF membranes by different methods (in helium).

dependence of the electronic conductivity is similar for the CA, SS, and SP membranes. It increases with temperature and decreases after a maximum point. The electronic conductivity of the CP membrane, increasing monotonically with temperature, is nearly 1 order of magnitude lower than those of the other three membranes. The maximum behavior of conductivity versus temperature was also shown in the work of Stevenson et al.18 for LSCF series with higher Sr contents (x ) 0.4, 0.6, and 0.8). This can be explained by two types of charge compensation mechanisms (p-type electronic compensation and oxygen ionic compensation) when trivalent La is substituted by divalent acceptor Sr. At low temperatures, the charge compensation for LSCF is primarily electronic,18 so the conductivity increases with the temperature. At higher temperature, more oxygen vacancies are created due to the release of oxygen from the oxide and the ionic compensation becomes significant. In this case, the concentration of electronic holes decreases and results in the decrease of electronic conductivity. In contrast, the CP membrane has very low Sr content, so the oxygen vacancy concentration is very small. Therefore, a relatively constant concentration of electronic holes can be maintained, leading to increasing conductivity with temperature. The electronic conductivity for the four membranes in helium (PO2 ) 1 × 10-4 atm) has a similar trend as that in air. But the onset temperature from which the conductivity begins to decrease with temperature moved from 900 °C in air to 700 °C in helium for CA, SS, and

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Figure 6. Temperature dependence of oxygen permeation fluxes of LSCF membranes.

SP membranes. This is a typical p-type conduction in which the electronic conductivity decreases with decreasing oxygen partial pressure. Since oxygen vacancy concentration is higher in helium than in air, ionic compensation tends to be more favorable, resulting in a decrease of the onset temperature. This further supports that the electronic conductivity is dominant in the LSCF materials. The CP membrane shows much lower electronic conductivity due to its serious deficiency in Sr element (about x ) 0.03). Tai et al.24,25 also reported a decrease in electronic conductivity with decreasing Sr for La1-xSrxCo0.2Fe0.8O3 and La0.8Sr0.2Co1-yFeyO3. For the three membranes with similar Sr concentration, there appears to be no correlation between the electronic conductivity and microstructure of the three membranes (CA, SS, and SP). However, it is known that electronic conductivity increases with Co concentration.18,23 Therefore, the difference in electronic conductivity of these three membranes is caused by the small difference in Co concentration. The electric conductivity data given in Figures 4 and 5 show that the composition of the LSCF membrane is the major factor determining the electronic conductivity. 3.2. Oxygen Permeation. The oxygen permeation fluxes of the four membranes in the range of 650-950 °C are shown in Figure 6. Air and helium were used as the feed and sweeping gases, respectively. The oxygen permeation fluxes of the CA, SS, and SP membranes are about 1 order of magnitude higher than that of the CP membrane, obviously due to the difference in Sr concentration in the membranes. Zeng et al.26 reported a higher oxygen permeation for a different membrane prepared by the same coprecipitation method. The composition of that CP membrane was not analyzed. It is obvious that the composition (Sr concentration) of the membranes prepared by the coprecipitation method strongly depended on the processing conditions. A careful control of synthesis conditions, such as pH and precipitation temperature, is required to obtain the material with desired composition. The oxygen permeation fluxes of all membranes in the studied temperature range (650-950 °C) can be well described by the Arrhenius relationship. Similar temperature dependency was reported by several other researchers for the LSCF membranes with similar compositions.14,15,18,20,27 These data do not show the order-disorder transition phenomenon with different activation energies in the lower (order state) and higher (disorder state) temperature ranges observed in par-

Figure 7. Influences of oxygen partial pressure of feed gas on the oxygen permeation flux for CA membrane.

Figure 8. Influences of oxygen partial pressure of feed gas on the oxygen permeation flux for SS membrane.

Figure 9. Influences of oxygen partial pressure of feed gas on the oxygen permeation flux for SP membrane.

ticular for LSCF with higher Sr concentration (e.g., SrCo0.8Fe0.2O3).16,17,26 Obviously, the composition is again the determining factor affecting the temperature dependency of the oxygen permeation flux. Figures 7-10 show the influences of the oxygen partial pressure of the feed gas on the oxygen permeation fluxes for CA, SS, SP, and CP membranes, respectively. The oxygen permeation fluxes were measured at two temperatures (800 and 900 °C) with various upstream oxygen partial pressures of 0.05, 0.1, 0.2, 0.5, and 1.0 atm. The experimental errors are estimated to be less than 8%. The curves of the CA, SS, and SP membranes show that the oxygen permeation fluxes increase with increasing oxygen partial pressure in the feed gas. On the other hand, the CP membrane

Ind. Eng. Chem. Res., Vol. 39, No. 3, 2000 651 Table 4. Parameters Regressed from Model Equation and Ionic Transference Number for LSCF Membranes membranes exponent n σoi (S/cm) ti (in air)

Figure 10. Influences of oxygen partial pressure of feed gas on the oxygen permeation flux for CP membrane. Table 3. Average Activation Energies and Preexponential Constants for Oxygen Permeation through Membranes Prepared by Different Methods method CA activation energy: E (kJ/mol) preexponential constant: Ko (mol/cm2‚s)a a

SS

SP

CP

90.7

92.7

127.8

81.9

4.85 × 10-4

7.91 × 10-4

3.70 × 10-2

2.81 × 10-5

exhibits a negligible influence of oxygen partial pressure on the permeation flux at both temperatures. By assuming that the bulk diffusion is the ratelimiting step, the oxygen permeation flux through the thick LSCF membrane can be described by the following equation:

RTσoi 8F2L

(P′′O2-n - P′O2-n)

SS

SP

0.17 0.12 0.036 0.015 10 × 10-5 4.0 × 10-5

0.46 0.33 0.0055 0.0027 2.8 × 10-5 1.1 × 10-5

0.41 0.38 0.010 0.0027 3.8 × 10-5 1.0 × 10-5

Values of the two parameters n and σoi for each membranes at two temperatures were determined by fitting the experimental oxygen permeation flux data at four or five different sets of P′O2/P′′O2 by MarquardtLeverberg nonlinear regression. Because the permeation flux of the CP membrane is essentially constant over the P′O2 range studied in this work, the parameters were not regressed for this membrane. The regressed permeation flux versus P′O2 are also presented in Figures 7-9. As shown, this model describes quite well the experimental data and allows prediction of JO2 at different P′O2/P′′O2. The regressed values of n and σoi are summarized in Table 4. The oxygen ion conductivity can be correlated to oxygen partial pressure as (see the Appendix)

σi ) σoiPO2-n

Arrhenius equation: JO2 ) Ko exp(-E/RT).

JO2 )

900 °C 800 °C 900 °C 800 °C 900 °C 800 °C

CA

(1)

where P′O2 and P′′O2 are oxygen partial pressures at upstream and downstream sides, σoi is the oxygen ion conductivity at PO2 of 1 atm, F is the Faraday’s constant, L is membrane thickness, n is an exponent. The derivation of eq 1 on the basis of ambipolar theory is given in the Appendix. Regression of the oxygen permeation flux data (at fixed P′O2) shown in Figure 6 by the Arrhenius equation gives average activation energy for oxygen ionic transport, according to eq 1. The values of the activation energy and preexponential constant for these four membranes are listed in Table 3. It should be noted that the P′′O2 also varies with temperature and therefore the activation energy estimated should be considered as an average value. The activation energy for the CP-derived membrane is much smaller than the other three membranes, reflecting the strong effects of Sr concentration on oxygen ion transport in the membrane. Among the three membranes with similar composition (CA, SS, and SP), the activation energy increases in the order CA < SS < SP with decreasing grain size. The grain boundary may have a positive or negative effect on ionic transport in oxide ceramic, depending on the relative diffusion rate of oxygen ions through the grain boundary and grain bulk.1 The results indicate that for these three membranes of the particular composition, the transport of oxygen ions through the bulk grain requires lower energy than through the grain boundary.

(2)

A positive value for n indicates that the oxygen vacancy concentration, and hence its ion conductivity, increases with decreasing oxygen partial pressure. For the CP membrane, because of the large deficiency of Sr, the oxygen vacancy concentration is very small. In this case, it is understandable that the upstream oxygen partial pressure has little effect on the permeation flux. As shown in Table 4, n increases with temperature. This is consistent with the equilibrium data of oxygen vacancy versus oxygen partial pressure reported by Mizusaki et al.,30 who showed a stronger oxygen partial pressure dependency of the oxygen vacancy concentration at higher temperatures for most LSCF samples studied. The values of n for the three membranes with similar composition are different. Such differences could be caused by the differences in both microstructure and Co concentration in the samples. As shown in Table 4, σoi is in the order of magnitude of 10-3-10-2 S/cm for the CA, SS, and SP membranes, several orders of magnitude smaller than that of the electronic conductivity. With the parameters in Table 4, the oxygen ionic conductivity and oxygen ion transference numbers ti of the three membranes under different oxygen partial pressures were calculated. The results are given in Figure 11 and Table 4. It can be found that ti of the CA, SS, and SP membranes increases with increasing temperature and decreasing oxygen partial pressure. The transference numbers of LSCF membranes (20% Sr doping) obtained in this work (about 10-5 in air and 10-3 in He at 900 °C) are in the same range as the values of 20% Sr-doped LSCF ceramics measured by Teraoka et al.22 using electronicblocking electrode method. But they are smaller than the averaged values (over an oxygen gradient) reported by Stevenson et al.18 for LSCF membranes with higher Sr contents (40% Sr, 9 × 10-4; 60% Sr, 2 × 10-3; and 80% Sr, 5 × 10-3). These results are quite consistent and prove that larger oxygen vacancy concentration

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Appendix Following the ambipolar diffusion approach,28 the oxygen permeation flux through LSCF membranes is related to the concentrations and diffusion coefficients of oxygen vacancy and electronic hole in the membrane by

JO2 )

Figure 11. Relationship between oxygen ion transference number and oxygen partial pressure.

1 2L

Dense ceramic membranes with desired composition of La0.8Sr0.2Co0.6Fe0.4O3-δ (LSCF) were prepared from powders produced by four different methods: the citrate, solid-state, spray-pyrolysis, and coprecipitation methods. All the membranes have perovskite-type crystalline structure. However, the actual compositions of the prepared LSCF membranes differ considerably. The membranes prepared by the citrate, solid-state, and spray-pyrolysis methods have compositions close to the desired stoichiometry with a slight difference in cobalt concentration. The membrane sample prepared by the coprecipitation method has a large strontium deficiency due to the filtration and aqueous washing steps involved in this method. The membrane composition is the determining factor affecting the electronic conductivity and oxygen permeability. The membrane prepared by the coprecipitation with a large Sr deficiency has much lower electronic conductivity and oxygen permeability (ionic conductivity) than the other three membranes with desired stoichiometry. The electronic conductivity of the three membranes prepared by the citrate, solid-state, and spray-pyrolysis methods increases with the Co concentration of the membrane. Membrane microstructure appears to have some effects on the activation for oxygen ion transport. For the three membranes with similar composition, the activation energy decreases with increasing grain size. Furthermore, oxygen pressure dependency of oxygen vacancy concentration is also influenced by the membrane microstructure and membrane composition. The results show that actual composition with a substantial deviation from the desired stoichiometry could be a major reason for the discrepancies in the oxygen permeation data for the membranes of seemingly the same composition. The difference in microstructure of the membranes could also cause differences in the transport properties. But this effect is less important as compared to the membrane composition. Acknowledgment We are grateful to the support of NSF (CTS-9502437) and Ohio’s Edison Materials Technology Center on this project.

v

v

(Ce + 4Cv) DeDv dCv CeDe + 4CvDv

(A1)

where subscript e and v refer to electronic species and oxygen vacancy, single and double primes (′ and ′′) represent upstream surface and downstream surface, respectively, and L is the membrane thickness. The concentration and diffusion coefficient can be correlated to conductivity σ by Einstein’s equation:

(larger Sr doping or higher temperature) always results in larger ionic transference number. 4. Conclusions

∫C′C′′

σi )

Zi2F2 CD RT i i

(A2)

where T is temperature, R is universal gas constant, F is Faraday’s constant, and Zi is the charge number. Because the ionic conductivity σv is much smaller than electronic conductivity σe for LSCF membranes, that means CvDv , CeDe, also with small oxygen partial pressure gradient, we assume Ce.Cv, then equation A1 is integrated into

JO2 )

Dv (C′′v - C′v) 2L

(A3)

In this work, the membrane thickness is 1.4 mm, considerably greater than the “characteristic thickness” Lc (