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1 Lawrence Berkeley National Laboratory Lawrence Berkeley National Laboratory Title LSM-YSZ Cathode with Reaction-Infiltrated Nanoparticle Permalink Author Lu, Chun Sholklapper, Tal Z. Jacobon, Craig P. et al. Publication Date escholarhip.org Powered by the California Digital Library Univerity of California

2 LSM-YSZ CATHODES WITH REACTION-INFILTRATED NANOPARTICLES Chun Lu 1, Tal Z. Sholklapper 1, 2, Craig P. Jacobon 1, Steven J. Vico 1, and Lutgard C. De Jonghe 1, 2 1 Material Science Diviion Lawrence Berkeley National Laboratory Berkeley, CA and 2 Material Science and Engineering Department Univerity of California at Berkeley Berkeley CA ABSTRACT To improve the LSM-YSZ cathode performance of intermediate temperature olid ide fuel cell (SOFC), Sm 0.6 Sr 0.4 CoO 3-δ (SSC) perovkite nanoparticle are incorporated into the cathode by a reaction-infiltration proce. The SSC particle are ~20 to 80nm in diameter, and intimately adhere to the pore wall of the pre-formed LSM-YSZ cathode. The SSC particle dramatically enhance ingle-cell performance with a 97%H 2 +3%H 2 O fuel, between 600 o C and 800 o C. Conideration of a implified TPB (triple phae boundary) reaction geometry indicate that the enhancement may be attributed to the high electrocatalytic activity of SSC for electrochemical reduction of ygen in a region that can be located a mall ditance away from the trict triple phae boundarie. The implication of thi work for developing high-performance electrode i alo dicued. 1

3 INTRODUCTION For practical SOFC cathode, the elected material mut be chemically and tructurally table, and be compatible with adjacent cell component, not only under operation condition, but alo at the high temperature where SOFC membrane are fabricated. The cathode mut be catalytically active to promote the electrochemical reduction of ygen, and have a high electronic conductivity to exhibit a low polarization reitance (1, 2). Since La 1-x Sr x MnO 3-δ (LSM)-YSZ (yttria tabilized zirconia) compoite reaonably meet thee criteria, they are the mot popular cathode material ued in SOFC operated at temperature above 800 o C. To reduce cot, lowering of the SOFC operating temperature ha been widely purued to expand the material election, to uppre the degradation of SOFC component, and conequently to extend cell lifetime (3-7). Unfortunately, with decreaing temperature the low ygen ion diffuivity and reduced catalytic activity of LSM tend to limit cell performance (8, 9). Noble metal particle, including Ru and Pt, have been incorporated in cathode to improve performance (8, 10) but, from a practical point of view, their prohibitive cot and tendency to coaren at high temperature make thi approach unattractive. Recently, cobalt ide particle were infiltrated into LSM-YSZ cathode, ignificantly raiing cathode performance (11-13). Thi ugget the general poibility of directly introducing catalytically active material into pre-formed LSM-YSZ cathode to enhance their activity. Compared to LSM, the Sm 0.6 Sr 0.4 CoO 3-δ (SSC) perovkite i a uperior electrocatalyt for ygenreduction reaction becaue of it mixed ionic-electronic conduction propertie (14-16). We developed a reaction-infiltration method that produce nanoize SSC 2

4 particle in the pore of the pre-formed LSM-YSZ cathode. The morphologie of the reulting cathode and the effect of SSC addition on cell performance are reported. EXPERIMENTAL A reaction-infiltration method wa developed to depoit nano particle (SSC, LSM, La 0.8 Sr 0.2 FeO 3- δ (LSF)) into porou LSM-YSZ electrode. A a bae line, aqueou precuror olution for thee material, containing nitrate in correponding ratio a well a urea (CO(NH 2 ) 2 ), were heated at 90 o C for 2hr before firing at 800 o C for 2hr. The decompoition product were then examined uing a (Siemen D-500) diffractometer with CuK α radiation in the 2θ range from 20 o to 80 o. Single SOFC cell were fabricated by tape cating (6). Nickel ide (NiO; J. T. Baker) and YSZ (Tooh-Zirconia; TZ-8Y) powder in a 50:50wt% ratio were thoroughly mixed in water with Duramax D3005 a the diperant. Duramax B1000 and HA-12 binder were then added to form the NiO-YSZ lurry. After evaporation of exceive water, the lurry wa tape-cat onto a Mylar film, and dried at ambient temperature. The green tape wa then cut into circular dic with a diameter of about 4cm, and preheated at 1100 o C for 2hr. A thin layer of YSZ powder wa depoited by an aerool pray method (7), and the reulting bi-layer wa intered at 1400 o C, for 4hr, to obtain a dene YSZ electrolyte. A La 0.65 Sr 0.3 MnO 3-δ (Praxair Specialty Ceramic):YSZ (50:50wt%) cathode with an active area of 1cm 2 wa then depoited on the electrolyte via the aerool pray method, and the ample were fired again, at 1150 o C, for 1hr, in air. Symmetric cell (cathode/electrolyte/cathode) were alo prepared uing the aerool pray method (7). For thee cell, LSM-YSZ electrode were prayed on both ide of a thick YSZ dic (~0.7mm), and ubjected to the identical treatment decribed above. The porou cathode 3

5 were aturated with a 0.7M nitrate/urea precuror olution, and heat-treated uing the ame heating chedule a for the ex-itu preparation of the SSC. In thi proce, the pore may be regarded a micro-reaction veel, in which the pore wall act a the heterogeneou nucleation ite for precipitate formation. Related urea method for coating SiC particle and hollow gla phere with ide, exploiting heterogeneou nucleation, were employed in everal other intance (17, 18). Platinum current collector were placed on the electrode of both ymmetric and ingle cell, and thee ample were fired at 800 o C for 2hr. For the ymmetric cell, the electrode polarization reitance wa meaured in air at open circuit (OCV), uing a Solartron 1260 frequency repone analyzer with a Solartron 1286 electrochemical interface. Completed ingle SOFC membrane were ealed onto an alumina tube with Aremco-516 cement. Cell current/voltage (I-V) for 97%H 2 +3%H 2 O a the fuel, and air a the idant were collected uing LabView oftware, and cell impedance pectra were obtained a well. The cathode morphologie were characterized with a JEOL FEcanning electron microcope and examined in a PHILIPS CM200 tranmiion electron microcope. Baed on the SEM obervation of ingle cell, the cell component thickne wa ~300μm, ~10μm, and ~30μm for the anode, the electrolyte, and the cathode, repectively. RESULTS Urea i added into the aqueou nitrate precuror olution to facilitate the formation of the perovkite SSC phae at 800 o C. At thi temperature, advere reaction between SSC and YSZ that can occur at high temperature (14) are avoided. Fig. 1 how the XRD pattern of the decompoition product from the nitrate precuror olution with and 4

6 without urea, after heating at 800 o C for 2hr. The characteritic x-ray peak in Fig. 1a indicate that the SSC perovkite phae did not form a the decompoition product of the nitrate precuror olution. In contrat, with the addition of urea, the perovkite SSC phae become the predominant decompoition product, a evident in Fig. 1b. The detailed role of urea in thi change ha yet to be identified; it probably erve a a precipitation and complexing agent to form intimately mixed intermediate pecie, only requiring lowtemperature heating to produce the deired phae (19-21). The SSC particle that formed in the pore were examined with EDX (energy diperive x-ray analyi) in the tranmiion electron microcope. The inet in Fig. 2 how a particle of ~60nm in the cathode produced by the reaction-infiltration proce. The EDX pectrum contain O, La, Sm, Sr, Zr, Ar, Mn, Co and Y. Beide the contribution from upporting LSM, YSZ and Ar implanted during ion-mill proceing to prepare the TEM pecimen, the preence of Sm, Co trongly indicate the formation of nanoized SSC particle intead of the individual metal ide, conitent with the ex-itu procedure. Fig. 3 i a SEM image of a porou LSM-YSZ cathode with infiltrated SSC nano particle. The infiltrated SSC particle are ~20 to ~80nm, and appear to be well bonded to the pore wall of the LSM-YSZ network. Interetingly, mot of particle are iolated from each other. The inert in Fig. 3 how the SSC particle after heating at 700 o C for 750hr; the particle appear to be about 40 to 90nm and, a expected, how little if any evidence of coarening. Thi i an encouraging reult, ince morphological tability i an important requirement for the effectivene of the proce. The reaction-infiltrated SSC nanoparticle in LSM-YSZ cathode ubtantially enhance the cell performance, a hown in Fig. 4. At 600 o C the cell OCV with the 5

7 97%H 2 + 3%H 2 O fuel are ~1.1V, indicating that the cell were well ealed. The SSC reaction-infiltration almot double cell performance, and the cell maximum power denity (MPD) increae from 80mW/cm 2 to 153mW/cm 2. Furthermore, the SSC addition noticeably decreae the initial curvature of the I-V plot at low current denitie. Thi decreae can be further ubtantiated by the impedance meaurement data hown in Fig. 5. At 600 o C the cell total reitance (R t ) (the intercept of the impedance pectrum on the real axi at low frequency) decline from ~9.9 to 3.3Ω cm 2 when SSC i included into the LSM-YSZ cathode. The enhancement effect perited up to 800 o C. The performance improvement factor i calculated a the percentage of MPD (maximum power denity) increae and i ~ 91%, 67%, 44% and 38% for 600 o C, 650 o C, 700 o C and 800 o C, repectively. The relative improvement diminihe when the cell operating temperature i raied, which may be attributed the increaing catalytic activity and ygen ion diffuivity of LSM itelf with temperature. However, even at 800 o C the SSC particle in the cathode till enhance the performance. DISCUSSION The cell-performance enhancement originate in the improvement of the LSM-YSZ cathode upon the introduction of the SSC nanoparticle. Fig. 6 diplay the impedance pectra of LSM-YSZ/YSZ/LSM-YSZ ymmetric cell without and with infiltrated catalyt at 600 o C. All AC impedance pectra how a relatively mall high-frequency branch (HFB) and a dominant low-frequency branch (LFB). The cell ohmic reitance in the pectra i truncated for comparion, and the cathode polarization reitance (R p ) i directly determined from the pectra a illutrated (22). With the infiltrated SSC 6

8 nanoparticle in the cathode, both the HFB and LFB are largely diminihed o that R p i reduced from ~19.8 to 8.5Ω cm 2 at 600 o C. Where LSM contact the YSZ, a triple phae boundary (TPB) i formed, in the preence of a ga phae. Important in the functioning of uch junction i the extent to which the relevant reaction can occur ome ditance away from the trict triple junction. Numerou advanced tudie on electrode behavior have been carried out to date (16, 23-26). The ygen reduction procee in fuel cell cathode were recently reviewed by Adler (9), yet undertanding of TPB procee i till incomplete, due to the complex competition of variou tranport path and interface mechanim. A qualitative aement of the relative effect of ome tranport parameter on the effectivene of cathode may be obtained from a very implified model for an extended, modified TPB, provided contancy of the microtructure. The extended TPB i ketched in Fig. 7, and can be related to a model decribed by Kawada et al. (27). It conider an LSM grain, of length 2L, in electronic contact with the current collector, between two grain of the YSZ keleton, with the TPB extending over the LSM grain urface. It i then aumed that thi extended TPB may be divided in two zone, Zone 1 of extent δ, where the unaffected urface exchange limit the reaction rate, and Zone 2, extending to L where, after infiltration, an enhanced urface reaction rate prevail. The total extended TPB current, ignoring all dependence on current denity, could be regarded a the erie reitance of the Zone 1 and 2. It i ueful to define reaction and tranport reitance for uch implified geometry. R (in Ωcm) could tand for the ygen ion diffuion "reitance", ignoring the detail of the tranport geometry, while R 1 and (in Ωcm 2 ) ymbolize the interface reaction reitance in Zone 1 and 2, repectively. R 2 7

9 In uch implified model, the total relative TPB conductivity, ρ, will be proportional to ρ=ρ δ + ρ L Eqn.1 where ρ δ =δ/ R 1 Eqn.2 and ρ L L 1 1 R ln( o L + R = dx 2 = ) δ R2 + Ro Ro Ro δ + R2 Eqn.3 where the total TPB flux can be take to proportional to the total conductivity. With the uual expreion for the conductivitie, to D o R o and R may be taken to be related, the bulk ygen ion elf-diffuion coefficient, and to the urface exchange coefficient, k. The relationhip between D o, k and L c, the critical length above which diffuion reitance dominate, were developed by Steele (28), and have been reviewed by Gelling and Bouwmeeter (29). Thee conideration led to D o / k =L c and conequently to R / o R = L c. In the one extreme, for a fully impeded urface reaction, i.e. R 2, ρ L (Eqn.3) L 2 become imply proportional to ( δ ) / R, a expected, ince R ln[ R o o L + R δ + R correponding to R L 2 o ( δ ) ] 2 R2 R 2 <<. In the other extreme, for very effective catalyi, 1 L R δ, ρ L become proportional to ln( ), i.e. to the ygen δ o diffuion coefficient, a it hould. Enhanced catalyi by dipered nanoparticle adhering R o 8

10 to the pore wall, beginning ome mall ditance δ away from the trict TPB, can thu be effective only between the relative limit L δ 1 L and ln( ) R δ 2 Rx, correponding to the high and the low interface reitance condition in Zone 2, if it i aumed that the urface reaction rate in Zone 1 and the ygen diffuion rate are invariant. The reult i, of coure, enitive to the choice of δ. If δ i maller, then the range of effectivene can become larger, while the oppoite prevail if δ i larger. S S R A plot of o R R Ro ρ = δ + ln[( L + 2 ) /( δ + 2 )] in relative unit veru R1 Ro Ro log[ R S R 2 o (cm)] i hown in Fig. 8, for aumed value of δ=10-3 μm and L=1μm. It may be argued that for LSM δ L c = R1 Ro, o that ρ δ would be about 1 (with the preent definition ρ would have unit of Ω -1 ). It i clear from thi graph that to allow for a factor of 2 increae in the TPB performance, a experimentally oberved here, while keeping R R o contant, the correponding tarting value of 1 = L c would have to R o be ignificantly higher than 10-5 cm, about 2 order of magnitude higher than the 2*10-7 cm - the value of L c derived by Carter et al. (30) for LSM at about 700 o C, uing the meaured ygen bulk elf-diffuion coefficient and urface exchange coefficient. Thi implie an inconitency with the model of more than 2 order of magnitude. The difference in the L c that make the dipered catalyt effective, and the L c value cited by Carter et al. are not readily reconciled, unle it i aumed that for LSM the actual ygen diffuion rate are ubtantially higher than ygen ion elf diffuion rate 9

11 derived from tracer meaurement, or that the urface reaction reitance are much larger than predicted from the k for LSM. Recent work by Horita et al. (31, 32) indeed ugget that the ygen tranport for LSM i more extenive than uggeted by the ygen elf diffuion, leading to active urface of about 0.5μm away from the trict TPB. It i alo clear that to enhance further the TPB performance, enhanced ygen ion tranport hould accompany enhanced urface reaction rate, ince nothing more can by gained by further enhancing catalyi in Zone 2. Lowering the urface reaction reitance in Zone 1, uch a by urface alloying, however, can remain effective, up to the ga tranport limit. Thi tudy demontrate that the performance of LSM-YSZ cathode at intermediate temperature can be improved ubtantially by decorating the pore wall with nanoparticle providing upplemental catalytic function. Thee nanoparticle catalyt need not be confined to the trict TPB to be effective. The critical length concept, however, by itelf i not a ueful parameter for predicting the effectivene of an extended TPB, ince a large L c can derive from either a high ygen diffuion rate or a low urface reaction rate, and thu doe not relate directly to TPB reitance. The total TPB reitance i, however, within limit, apprimately proportional to the ygen diffuion rate, and to a function of the L c, a follow form Eqn. 3. The choice of nanoparticle catalyt i not limited to SSC but hould extend to other catalyt ide that poe ubtantially higher catalytic activity. For example, a hown in Fig.6, replacing SSC with LSF nanoparticle in the LSM-YSZ cathode lead to a imilar beneficial reult that i abent for LSM nanoparticle infiltration. It i therefore plauible that the reaction-infiltration concept can be exploited broadly in the engineering of high-performance electrode. 10

12 CONCLUSIONS It ha been demontrated that compoite cathode can be made to operate more effectively by reaction-infiltration of dipered nanocatalyt that need not be fully confined to the trict TPB. Specifically, in thi tudy conventional LSM-YSZ cathode have been modified by the reaction-infiltration of SSC nanoparticle, uing a proce that doe not exceed 800 o C. Microtructural examination howed extended tability of the SSC particle at leat at 700 o C. Electrochemical characterization of ingle cell how that the SSC nanoparticle dramatically increae the cell power denitie between 600 o C and 800 o C. A implified extended TPB model ugget that the enhancement by the dipered nanocatalyt involve an ygen ion tranport rate for LSM that i ignificantly higher than expected from the bulk ygen ion elf diffuion in the LSM. ACKNOWLEDGEMENTS Thi work wa upported by the U. S. Department of Energy, through the National Energy Technology Laboratory. Dr. Xiao-Feng Zhang i acknowledged for the TEM analyi, carried out at the LBNL NCEM. REFERENCES 1. N. Q. Minh and T. Takahahi, Science and Technology of Ceramic Fuel Cell, p. 117, Elevier Science Pub. Co. (1995). 2. H. Yokokawa, T. Horida, in High Temperature Solid Oxide Fuel Cell, S. C. Singhal and K. Kendall editor, p.136, Elevier Advanced Technology, Oxford, UK (2003). 11

13 3. K. Huang, R. Tichy, J. B. Goodenough and C. Milliken, J. Am. Ceram. Soc., 81 (10) 2581 (1998). 4. R. Dohi, V. Richard, J. D. Carter, X. Wang, and M. Krumpelt, J. Electrochem. Soc., 146 (4), 1273 (1999). 5. C. Xia, F. Chen and M. Liu, Electrochemical and Solid-State Letter, 4 (5) A52 (2001). 6. C. Wang, W. L. Worrell, S. Park, J. M. Voh, R. J. Gorte, J. Electrochem. Soc., 148 (8), A864 (2001). 7. S. J. Vico, C. P. Jacobon, and L. C. De Jonghe, U.S. Pat. No , (October 1, 2002). 8. H. Uchida, S. Ariaka, and M. Watanabe, J. Electrochem. Soc., 149 (1), A13 (2002). 9. S. B. Adler, Chem. Rev., 104, 4791 (2004) 10. M. Watanabe, H. Uchida, M. Shibata, N. Mochizuki, and K. Amikura, J. Electrochem. Soc., 141, 342 (1994). 11. S. J. Vico, C. P. Jacobon, and L. C. De Jonghe, U.S. Pat , (January 27, 2004). 12. C. P. Jacobon, X. Chen, L. C. De Jonghe, and S. J. Vico, Ionic and Mixed Conducting Ceramic V, The Electrochemical Society Proceeding, Pennington, NJ (2004). 13. K. Yamahara, C. P. Jacobon, S. J. Vico, and L. C. De Jonghe, Solid State Ionic, 176, (3-4) 275 (2005). 14. J. M. Ralph, C. Roignol, and R. Kumar, J. Electrochem. Soc., 150 (11), A1518 (2003). 12

14 15. R. A. De Souza, J. A. Kilner, Solid State Ionic, 106, 175 (1998). 16. R. A. De Souza, J. A. Kilner, Solid State Ionic, 126, 153 (1999). 17. T. Shihido, Y. Yamamoto, H. Morioka, K. Takaki, and K. Takehira, Applied Catalyi A: General, 263, 249 (2004). 18. H. Zhao, D. J. Draelant, and G. V. Baron, Catalyi Today, 56, 229 (2000). 19. S. Jung, C. Lu, H. He, K. Ahn, R. J. Gorte, J. M. Voh, J. of Power Source, In pre. 20. T. D. Mitchell and L. C. De Jonghe, 78 (1), 199 (1995). 21. S. J. Wu and L. C. De Jonghe, J. Mater. Sci., 32, 6075 (1997). 22. C. Xia, Y. Zhang, and M. Liu, Electrochemical and Solid-State Letter, 6 (12), A290 (2003). 23. C. Tanner, K. Fung, and A. Virkar, J. Electrochem. Soc., 144, 21 (1997). 24. S. Jiang, Y. Leng, S. Chan, and K. Khor, Electrochemical and Solid-State Letter, 6 (4), A67 (2003). 25. B. C. H. Steele, K. M. Hori, and S. Uchino, Solid State Ionic, 135, 445 (2000). 26. S. B. Adler, J. A. Lane, B. C. H. Steele, J. Electrochem. Soc., 143, 3554 (1996). 27. T. Kawada, N. Sakai, H. Yokokawa, and M. Dokiya, Solid State Ionic, 40/41, 402 (1990). 28. B. C. H. Steele, Solid State Ionic, 75, 157 (1995). 29. P. J. Gelling and H. J. M. Bouwmeeter, The CRC Handbook of Solid State Electrochemitry, CRC Pre, Boca Raton, P.481 (1996). 30. S. Carter, A. Selcuk, R. J. Chater, J. Kajda, J. A. Kilner and B. C. H. Steele, Solid State Ionic, 53-56, 597 (1992). 13

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16 Figure caption Fig. 1 XRD pattern of the decompoition product from (a) nitrate precuror (b) urea+nitrate precuror heated at 800 o C for 2hr. (P) Peak correponding to perovkite phae. Fig. 2 The EDX pectrum of an infiltrated particle hown in the embedded TEM image. Fig. 3 SEM image of a porou LSM-YSZ cathode with infiltrated nano SSC particle and after heated at 700 o C for 750hr (the embedded). Fig. 4 Performance curve for the cell without ( ) and with ( ) nano SSC particle in an LSM-YSZ cathode at 600 o C (fuel: 97%H 2 + 3%H 2 O). Fig. 5 Impedance plot for the cell without ( ) and with ( ) nano SSC particle in an LSM-YSZ cathode at 600 o C (fuel: 97%H 2 + 3%H 2 O). Fig. 6 Impedance pectra of ymmetric LSM-YSZ/YSZ/LSM-YSZ cell (quare) with infiltrated nano LSM (circle), SSC (triangle) and LSF (tar) at 600 o C. Fig. 7 Simplified extended TPB between YSZ and LSM. Fig. 8 Total ygen conductivity multiplied by the ygen ion diffuion reitance, per unit TPB, in arbitrary unit veru log[ R S R 2 o (cm)].. 15

17 b P Intenity (a.u) a P P P P P P θ (deg) Fig. 1 16

18 200 Count O 150 LSM-YSZ 100 Zr Particle La Sm Sr Zr Zr Ar Co Sm La Mn Sm La La Sm Co Mn 5 Energy (kev) nm Sr Y 15 Zr Zr Fig. 2 17

19 Nano particle 500nm 100nm Fig. 3 18

20 Voltage (V) Power Denity (mw/cm 2 ) Current Denity (ma/cm 2 ) Fig. 4 19

21 10 8 -ImZ (ohm*cm 2 ) 6 4 R t ReZ (ohm*cm 2 ) Fig. 5 20

22 ImZ (ohm*cm 2 ) R p ReZ (ohm*cm 2 ) Fig. 6 21

23 YSZ TPB pore 2L LSM δ x L TPB YSZ Fig. 7 22

24 L= 1 micron; δ=10-3 micron; R o. ρ x log[ 2 R / R o (cm)] Fig. 8 23