Oxygen permeation of La Sr CoO

Transcription

1 Solid State Ionis 98 (1997) 7 13 Oxygen permeation of La Sr CoO.3.7 3d 1 * C.H. Chen, H.J.M. Bouwmeester, R.H.E. van Doorn, H. Kruidhof, A.J. Burggraaf Laboratory for Inorgani Materials Siene, Faulty of Chemial Tehnology, University of Twente, P.O. Box 17, 75 AE Enshede, The Netherlands Reeived 8 July 1996; aepted 1 February 1997 Abstrat The oxygen permeability of dense La.3Sr.7CoO3d membranes has been measured in the range C under various oxygen partial pressure gradients. A sweep gas method was employed. Results indiate that in the range of thikness m used in the present study, the oxygen flux is predominantly ontrolled by bulk diffusion aross the 1 membrane. The measured ativation energy is 6 kj mol. By fitting the permeation data for various thiknesses to the transport equation obtained upon assuming linear kinetis for the surfae hange reations and bulk ioni transport, we ould derive the oxygen ioni ondutivity and the harateristi membrane thikness. The latter quantity determines the 1 transition from predominant ontrol by diffusion to that by surfae hange. The ioni ondutivity is about.5 S m at 18C. The harateristi thikness is trapolated at a value of about 8 mm. Keywords: Oxygen permeation; Perovskite oxide; Membrane 1. Introdution ber for seleted ompositions in this series remains below a value of.1, the ioni ondutivity may be Dense erami oxides hibiting high oxide ion 1 orders of magnitude higher than that of stabiand eletroni ondutivity are of great interest for lized zironia. potential appliations in oxygen separation and novel An obvious onsideration to the design of the atalyti reators [1]. In reent years, oxygen per- aforementioned appliations is the funtional depenmeation studies of many mixed-onduting oxides dene of the oxygen flux on membrane thikness and have been reported. Amongst them perovskite oxides oxygen partial pressure differene aross the mem- (La,Sr)(Co,Fe)O3d have reeived partiular atten- brane. Attention has already been drawn to the tion. Teraoka and oworkers [ 4] were the first to ontribution of the surfae hange kinetis to the report high oxygen fluxes through the obalt-rih overall permeation rate [5]. A harateristi memompositions. Although the ioni transferene num- brane thikness L has been defined, at whih point the oxygen flux is under mixed ontrol of the surfae * hange kinetis and bulk diffusion. No signifiant Corresponding author. 1 Present address: Laboratory for Applied Inorgani Chemistry, gain in the oxygen flux an be realized by making Delft University of Tehnology, Julianalaan 136, 68 BL Delft, membranes thinner than about.1l. In the The Netherlands. present study, the oxygen permeation through / 97/ $ Elsevier Siene B.V. All rights reserved PII S (97)97-

2 8 C.H. Chen et al. / Solid State Ionis 98 (1997) 7 13 La.3Sr.7CoO3d is investigated under various - hemial potential differene drop DmO perimental onditions. The data obtained are used to (5RT ln P O /P 9O ) is distributed aross the mem- alulate the oxygen ioni ondutivity of the materi- brane suh that the rate determining proess reeives al and its harateristi membrane thikness. the greater proportion. For thik membranes bulk diffusion dominates, but as the thikness dereases the limited transfer aross the interfaes beomes. Theory rate determining. Assuming linear kinetis for both bulk diffusion and surfae hange, the oxygen flux.1. Wagner equation an be written as [1,5] ]] Dm 1 telsion O The rate at whih oxygen permeates through a jo 5 ]]]]]]]], (3) non-porous erami membrane is essentially on- 11(L /L) 4 F L trolled by two fators: the rate of oxygen diffusion where ]] telsion is the mean value of the produt over aross the membrane and that of interfaial oxygen the applied oxygen pressure interval, and L the hange. When the flux is governed by bulk oxygen harateristi membrane thikness, defined by [7] diffusion it is generally desribed by Wagner s ]] equation [6], RT telsion L5]] 3 ]]. (4) 4 F j ln PO RT 1 j 5]] E t s dlnp. (1) Here the quantity j (mol O m s ) denotes O el ion O 4 F L the balaned hange rate at the gas solid interfae ln P9O in the absene of oxygen potential gradients. Com- Here d is the ioni ondutivity, t the eleion el paring Eq. (3) with the Wagner equation (Eq. (1)), troni transferene number, L the membrane thik- we see that the diffusional flux of oxygen aross the 1 ness, while P9 and P are the oxygen partial membrane is redued by (11L /L), relative to O O pressures at the high and low pressure side of the that in the absene of transfer limitations aross the membrane, respetively. Other parameters have their interfaes. The fator in this pression takes into usual signifiane. For mixed ondutors in whih aount the fat that there are two interfaes with the eletroni ondution predominates, i.e. t 1, idential properties, representing the general ase of el integration tends only over s. Differentiating a symmetrial membrane. The driving fore ion Eq. (1) with respet to the lower integration limit DmO is equally used for bulk diffusion and for the yields, surfae hange when the thikness of the mem- brane equals L. j O RT The influene of the membrane thikness on S D ion ]]] 5 s 3 ]]. () ln P9O PO 4 F L oxygen flux alulated from Eq. (3) is shown in Fig. 1. When L 4L, the oxygen flux varies inversely Hene, the ioni ondutivity at a given pressure g with L where g 51, whih is in agreement with of P9O an be obtained perimentally from the Wagner s theory. Departures from this inverse relaslope of jo versus ln P9 at that P 9, provided that O O tionship are observed when the flux is partly gov- PO is kept onstant during the periment. Similar- erned by the surfae hange kinetis. The value of ly, sion an be evaluated from permeation data g, at a given L, orresponds with the negative slope measured by varying P O, keeping P O9 at a fixed in the double logarithmi plot of the oxygen flux value. versus membrane thikness at that L. Taking the logarithm of Eq. (3), partial differentiation with.. Mixed ontrolled kinetis respet to log L shows that g is equal to the In the ase where the oxygen flux is also partly governed by the surfae hange reations, the 1 redution fator (1 1 L /L), the value of whih gradually dereases with dereasing thikness to beome zero for L<L, as is shown in Fig. 1. The

3 C.H. Chen et al. / Solid State Ionis 98 (1997) into disks ([51. m) and polished with 1 MESH SiC to final thiknesses.15,.115, and.57 m. Oxygen permeation measurements were performed in the temperature range of C. Supremax glass rings (Shott, Nederland B.V.) were used to seal the dis speimen into the quartz permeation reator at 158C. Prior to sealing, the side wall of the dis was overed with a paste based on fine Duran glass powder (Shott, Nederland B.V.) to avoid radial ontributions to the oxygen flux. Oxygen/ nitrogen mixtures and helium were passed along opposite membrane sides. The oxygen partial pressures at the Fig. 1. Thikness dependene of the dimensionless oxygen flux j9o immediate surfaes were taken to be similar to those alulated from Eq. (3). The quantity j9o is defined by the ratio of measured down-stream, assuming ideal gas mixing 1 the oxygen flux over the maximum ahievable oxygen flux (] j? onditions to hold in the reator volume. Unless Dm O ) in the surfae hange limited regime. Only if L4L, g the oxygen flux beomes proportional to 1/L with g 51 in stated otherwise, the partial pressure at the oxygen- agreement with the Wagner theory. For smaller thiknesses, g ranges between 1 and (right hand sale). rih side was similar to that in ambient air. The one at the lean side of the membrane was ontrolled by adjusting the helium flow rate. Gas flow rates were ontrolled by Brooks 58 mass flow ontrollers. latter situation orresponds with the maximum ahievable flux, whih for a symmetrial membrane is Gas leakage, if present, ould be deteted gas 1 given by ] j Dm. It should be noted that, due to hromatographially by the presene of nitrogen in O the linearised kinetis for bulk diffusion and surfae the helium stream, and was found normally to be hange in its derivation, Eq. (3) neglets hanges below 1%. Both the permeation reator and the in material parameters with oxygen partial pressure. apparatus have been desribed in detail elsewhere Without the knowledge thereof it should stritly be [1,11]. used only if small P -gradients appear aross the Oxygen fluxes were normalized on the surfae O membrane. In the general ase, L will be a funtion area posed to the helium permeate stream. Corre- of proess parameters P and temperature. tions were applied for that portion of the membrane O surfae overed by the glass seal ring. Table 1 lists orretion fators G for these edge-effets, whih were obtained from a numerial proedure to solve 3. Experimental Powders of La.3Sr.7CoO3d were prepared by adapting the aqueous sol gel route originally pro- the steady-state diffusion equation on the basis of Fik s seond law (with a onstant diffusion oeffiient) in ylindrial oordinates [11]. The oxygen flux was alulated from posed by Bilger et al. [8] for the preparation of La.86Sr.14MnO 3. Details of the synthesis are de- 1 F permeate O ] ]]] sribed elsewhere [9]. Chemial analysis of the j O 5? (5) G A powder obtained after alining at 88C showed a slight obalt ess. The real omposition was La.3Sr.68Co1.5O 3d. X-ray powder diffration indiated a single ubi perovskite phase. No evi- Table 1 Sample dimensions and flux orretion fators dene was found for any ontamination or phase Thikness Radius at P9 side Radius at P O O side G inhomogeneity. Powders were isostatially pressed at (m) (m) (m) (m) 4 MPa for 5 min into pellets ([51.6 m), whih were sintered at 118C for 1 h. The relative density was about 9%. These pellets were shaped

4 1 C.H. Chen et al. / Solid State Ionis 98 (1997) where F (m s (STP)) is the flow rate at the outlet Table permeate 3 of the reator, (mol m ) the oxygen Experimental onditions and alulated ativation energies O onentration in the effluent stream and A (m ) the Thikness P9 P O O E at 1 geometri surfae at the helium side of the membrane. (m) (atm) (atm) (kj mol ) Results and disussion 4.1. Temperature dependene 4.. Oxygen pressure dependene Arrhenius plots of oxygen permeation through La.3Sr.7CoO3d speimens of different thikness are given in Fig.. The oxygen partial pressure differene aross the membrane was fixed during measurement. Values for the oxygen partial pressures at both sides of the membrane and the measured ativation energy are given in Table. The lose math between the ativation energies obtained upon varying the membrane thikness suggests that permeation through the speimens is ontrolled by a ommon rate determining step. As desribed below, this turns out to be bulk diffusion aross the mem- Fig. 3 shows perimental permeation data at 18C as a funtion of oxygen partial pressure. During measurement the oxygen partial pressure at the opposite side of the membrane was kept at a fixed value. Under these perimental onditions, the oxygen flux varies almost linearly with the logarithm of oxygen partial pressure, even though some small divergene is seen for the thinnest speimen of.57 m at the lowest oxygen partial pressures in Fig. 3b. In view of Eq. () a onstant slope would indiate that the ioni ondutivity remains invariant under hanges in the oxygen partial pressure. Average values of the ioni ondutivity of La Sr CoO evaluated from the slopes of the urves in Fig. 3a and b are listed in Table 3. The above observations agree well with isting data of oxygen non-stoihiometry of La.3Sr.7CoO3d obtained from a thermogravimetri study performed by Mizusaki et al. [1], assuming the vaany diffusion oeffiient in the range of oxygen pressure to adopt a onstant value. The authors showed that the nonstoihiometry, at 88C and oxygen partial pressures similar to those used in brane d Fig.. Temperature dependene of the oxygen permeation rate of La.3Sr.7CoO 3d. Thiknessses are (j).57 m, (.).115 m and (m).15 m. Experimental onditions are listed in Table. 1/16 O the present study, varies approximately with P. The absolute power was found to derease even further with inreasing temperature. With the additional assumption that all oxygen vaanies ontribute to transport one would thus predit the ioni ondutivity of La.3Sr.7CoO3d to be almost invariant with P. Yet, a disagreement is apparent O from Table 3 between sion evaluated from jo versus ln P 9 (Fig. 3a) and that from j versus ln P (Fig. O O O 3b). Sine the analysis in terms of Eq. () is based upon ioni diffusion through the bulk as the rate limiting step in oxygen permeation, we attribute the observed disrepany to partial rate ontrol by the surfae hange reations disussed below.

5 C.H. Chen et al. / Solid State Ionis 98 (1997) Fig. 3. Oxygen pressure dependene of the oxygen permeation rate of La.3Sr.7CoO3d at l8c. (a) high-pressure side; (j).57 m, P 5.53 atm; (.).115 m, P atm. (b) low-pressure side; (j).57 m; (.).115 m: (m).15 m. Air (P 9 O O O 5.1 atm) is on the high-pressure side. Table 3 Ioni ondutivity of La Sr CoO at 18C.3.7 3d 1 1 Membrane thikness s ion (S m ) evaluated from s ion (S m ) evaluated from (m) j versus ln P 9 (Fig. 3a) j versus ln P (Fig. 3b) O O O O Thikness dependene overed by the periments bulk ioni diffusion determines the overall performane of the mem- A onstant ioni ondutivity of brane. The deviation from the ideal inverse relation- La.3Sr.7CoO 3d, as a funtion of oxygen partial ship between the oxygen flux and membrane thikpressure as one would predit from data of oxygen ness as predited by Wagner s theory (Eq. (1)) is non-stoihiometry, tends the appliability of Eq. interpreted to reflet the partial rate limitation by the (3) to investigate possible ontributions of the sur- surfae hange kinetis. fae hange kinetis to the overall permeation rate. Parameters obtained from fitting of Eq. (3) to the In Fig. 4, we show the log log plot of the oxygen flux data are listed in Table 4, indiating that the flux versus membrane thikness for three different harateristi thikness L at whih point the transi- temperatures 1, 15 and 118C, respetively, tion from predominant ontrol by bulk diffusion to normalized to a value of PO of.5 atm. The that by surfae hange would our at a thikness g oxygen flux is found to vary with 1/L with g 5 just below 1 mm. The latter value has also been Sine g is lose to unity, this onfirms quoted reently by Kilner [13] based upon analysis that in the range of thiknesses and temperatures of data of isotopi hange for a wide variety of

6 1 C.H. Chen et al. / Solid State Ionis 98 (1997) mol O m s at 18C. An obvious onsideration is that suh a membrane needs to be supported by a porous substrate in order to provide suffiient mehanial integrity. 5. Conlusions We have shown that the results from oxygen permeation measurements through dense La.3Sr.7CoO3d membranes may be interpreted in terms of a simple model based upon linearised kinetis of bulk ioni transport and the surfae hange reations. In the range of thikness m used in the present study of bulk ioni transport mainly governs the overall rate of oxygen permeation. The ioni ondutivity at 1 18C is estimated at about.5 S m. It is, however, evident that partial rate limitations our Fig. 4. Thikness dependene of the oxygen permeation rate of by the surfae hange kinetis, sine the permea- La.3Sr.7CoO 3d. Data from Fig., normalized to PO 5.5 tion rate measured as a funtion of thikness departs atm. Drawn lines indiate the fit to Eq. (3). from that peted for purely bulk-ontrolled kinetis. Calulations show that in the range of tempera- Table 4 ture 1 118C the membranes operate under Charateristi thiknesses L ioni ondutivity sion and surfae hange rate j at different temperatures mixed ontrol of the surfae hange kinetis and bulk diffusion at a harateristi thikness L esti- Temperature L s j ion 1 1 (8C) (mm) (S m ) (mol m s ) mated to be about 8 mm ( ) ( ) (9.56.3)31 Aknowledgments This study has been arried out in the framework mixed-onduting perovskites. As onerns the mag- of a ollaboration programme between Aademia nitude of the ioni ondutivity, it may be noted that Sinia and Duth Aademy of Sienes (KNAW) this favourably ompares with the orresponding and has been finanially supported by (KNAW). Prof data given in Table 3. Meng Guangyao (Department of Materials Siene The maximum obtainable flux is determined by and Engineering, University of Siene and Tehnol- the surfae hange rate. Values for j alulated ogy of China) is gratefully aknowledged for from Eq. (4) are listed as well in Table 4. As noted stimulating this projet. before, no further advantage will be gained from making membranes thinner than O.1L. If trapola- 4 tion of the present data of values of PO of 1 atm Referenes is allowed, provided that suh a low value of P O an be maintained under atual membrane operating [1] H.J.M. Bouwmeester, A.J. Burggraaf, in: Fundamentals of onditions, it is easily alulated from Eq. () that Inorgani Membrane Siene and Tehnology, A.J. the oxygen flux passing from the air side of a Burggraaf and L. Cot (Eds.), Elseviers Siene, Amsterdam, membrane of thikness of 1 mm an be as high as Ch. 1, pp

7 C.H. Chen et al. / Solid State Ionis 98 (1997) [] Y. Teraoka, H.M. Zhang, S. Furukawa, N. Yalnazoe, Chem. [8] S. Bilger, E. Syskakis, A. Naoumidis, H. Nikel, J. Am. Lett. (1985) Ceram. So. 75 (199) 964. [3] Y. Teraoka, T. Nobunaga, N. Yamazoe, Chem. Lett. (1988) [9] C.H. Chen, H. Kruidhof, H.J.M. Bouwmeester, A.J. 53. Burggraaf, Mater. Si. and Eng. B. 39 (1996) 19. [4] Y. Teraoka, T. Nobunaga, K. Okamoto, N. Miura, N. [1] H.J.M. Bouwmeester, H. Kruidhof, A.J. Burggraaf, P.J. Yamazoe, Solid State Ionis. 48 (1991) 7. Gellings, Solid State Ionis (199) 46. [5] H.J.M. Bouwmeester, H. Kruidhof, A.J. Burggraaf, Solid [11] J.E. ten Elshof, H.J.M. Bouwmeester, H. Verweij, Solid State State Ionis. 7 (1994) 185. Ionis. 81 (1995) 97. [6] C. Wagner, Progr. Solid State Chem. 1(1) (1975) 3. [1] J. Mizusaki, Y. Mima, S. Yamauhi, K. Fueki, J. Solid State [7] Here we would like to note that a fator is missing in Eq. Chem. 8 (1989) 1. (4) in Ref. 5. With the present definition of L, while using [13] J. Kilner, in: T.A. Ramanarayan, W.L. Worrell, H.L. Tuller the Nernst Einstein equation, it an be shown that L5D*/ (Eds.), Pro. nd Int. Symp. Ioni and Mixed Conduting ks when tel51, where D* is the traer diffusion oeffiient Oxide Ceramis, The Eletrohemial Soiety, Pennington, and ks the surfae hange oeffiient. Both parameters are NJ, 1994, p aessible from O O isotopi tehniques where the ratio D*/ks appears to be the fundamental parameter governing traer diffusion bounded by a limiting surfae hange rate 18 between lattie oxygen and oxygen from the O-enrihed gas phase.