A dm cer So.87[31-36(2004 journal Low-Temperature Fabrication of Oxide Composites for Solid-Oxide Fuel Cells Hongpeng He, Yingyi Huang, Juleiga Regal, Marta Boaro, John M. Vohs, and Raymond J Gorte Department of Chemical Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104 Composites of yttria-stabilized zirconia (YSZ) with Sr-doped for YSZ is higher than the temperature at which solid-state LaCrO,(LSC)and Sr-doped LaMnO,(LSM) were prepared reactions occur between the two oxides by impregnation of a porous YSZ matrix with aqueous solu- In our laboratory, we have developed a different approach to the tions of the appropriate metal salts, followed by sintering to fabrication of electrode composites to prepare copper-cermet various temperatures. XRD measurements showed that per- anodes for SOFCs that convert hydrocarbons directly, without first ovskite phases formed after sintering at 1073 K, a temperature reforming the fuel to H, and CO. 9-2Because Cu,O and Cuo well below that at which solid-state reactions with YSZ occur. melt at 1508 and 1599 K, respectively, conventional methods for The conductivities of the LSC-YSZ and LSM-YSZ composites"'p synthesize a highly porous YSZ that has been heated to pared in this way were maximized at a sintering tempera- ehre of 1373 K for LSC-YSZ and 1523 K for LSM-YSZ, high temperatures together with the electrolyte and then to add the although reasonable conductivities were achieved at much copper by impregnation of the porous YSZ with soluble salts of lower temperatures. The conductivities of the two composites copper. The metallic phase of the copper can be obtained in increased much more rapidly with the content of the conduc low-temperature sintering and reduction procedures, avoiding any tive oxide than has been found with conventional cor solid-state reactions that can occur between CuO, and YSZ formed by mixing and sintering the oxide powders. The In this article, the results of a study in which a similar approa implications for using this approach to develop novel elec- has been used to prepare composites of YSZ with LSM and with trodes for SOFC applications are discussed Sr-doped LaCrO,(LSC)are reported. LSC has been suggested for use in ceramic anodes, because it is st and electronically conductive under reducing environments LSC-YSZ com I. Introduction ites can potentially combine the electronic properties of the LSC with the ionic conductivity of the YSZ in the same way as electrodes in solid-oxide fuel cells (SOFCs) are typically LSM-YSZ composites. The results indicate that this synthesis ade from a composite of an electronically conductive mate ith an electrolyte oxide. For example. with electrolytes made forms composites with a structure different from that prepared by Ni-YSZ ceramic-metallic(cermet) composite, In addition to conventional methods, which suggests that the method is worth maintaining porosity in the anode and providing a coefficient of further investigation for the synthesis of electrode materials thermal expansion(CTE) match with the electrolyte, the YSZ in the cermet extends the region into which ions can migrate. which increases the length of the three-phase boundary (TPB). A material lL. Experimental Procedure ommonly used for SOFC cathodes is a composite of YSZ with r-doped LaMnO,(LSM). Just as with the nickel cermets, YSZ The LSC-YSZ and LSM-YSZ composites were each prepared in the LSM-YSZ composite provides a path for ion migration to by two methods: method A involved conventional, physical extend the TPB region within the cathode mixing of the oxide powders, while method B involved impreg Great care must be taken in the preparation of oxide composites, YSZ (TZ, 84.8%Y, 0.0.2 um. Tosoh Corp, Tokyo, Japan)and to insulating phases. In general, the two oxide phases are LSM(Laox Sro, MnO,, Praxair Surface Technologies, Woodin fabricated separately, physically mixed, and then heated. 4. l5 The ville, WA)were used as-received. LSC SacrO,8) temperature must be high enough to sinter the ion-conducting synthesized from the nitrate salts of La(NO,), 6H,O (ACS.9% component in the electrode to the electrolyte, "but this temperature and Cr(NO ) 9H,0 (ACS 98.5%. Alfa Aesar). After the salts two oxide phases. In the case of LSM-YSZ compo were dissolved in distilled water. the mixture was dried and above 1523 K is recognized as leading to Laz Zr,0. 16-18 Because calcined at 1073 K in air overnight. This powder was ground in a YSZ powders do not sinter to a significant extent below -1373K mortar and pestle in the presence of isopropyl alcohol, sintered in the conditions for achieving optimal electrode properties from air at 1673 K for 4 h, and then ground again. The resulting powder oxide composites are rather limited. Indeed, it is not possible to was shown to have the correct perovskite structure using X-ray prepare some oxide composites because the sintering temperatur diffractometry (XRD). Finally, the LSM-YSZ and LSC-YSZ composites were prepared by physically mixing the oxide pow ders, uniaxially pressing them into wafers, and sintering the wafers to vanous temperatures. To prepare composites by impregnation, method B, we first prepared a porous YSZ matrix using procedures that have been described elsewhere. The YSZ powder was mixed with distilled water, a dispersant(Duramax 3005, Rohm Haas. Woburn, MA). manuscript No. 186414 Received November 11, 2002: approved October 3, 2003 binders(HA12 and B1000, Rohm Haas), and pore former Supported by the Otice of Naval Research (graphite and poly(methyl methacrylate). This slurry was either cast into tapes that would result in porous ceramic wafers, 600 um
Jounal of the American Ceramic Sociery- He et al Vol 87. No. 3 hick, or formed into rectangular pieces, 2 mm x 2 mm X 10 mm The YSZ wafers and rectangular pieces were sintered to 1823 K. which resulted in a porosity of 60%, as shown by the weight change of the sample after water immersion. Either LSM or LSC was then added to the porous YSZ through impregnation of the YSZ with an aqueous solution containing the appropriate concen trations of the nitrate salts of lanthanum. strontium, and either chromium or manganese (Mn(NO,),xH, O, ACS 99.98%, Alfa Aesar Electrical conductivities were measured using the standard four-probe direct-current method. The samples were placed in a holder, and extermal platinum foils were attached to both ends. Current from an electrochemical interface(Model 1286, Solartron Houston, TX) was passed through the samples while monitoring voltage across the samples using a multimeter(Model 72- 410A. Tenma). The conductivities were typically measured either LSC- in air or in humidified H,. For the LSM-YSZ composites. most of 15 he samples were prepared from the rectangular pieces, while the results for the LSC-YSZ composites were obtained from the 600 um wafers. The phase and microstructure of selected samples were Fig. 2. SEM image of LSC-YSZ composite prepared by sintering of the also investigated using XRD and scanning electron microscopy (SEM: Model JSM-630OLV, JEOL, Tokyo, Japan) IlL. Results mixed powder, with 48 vol%o LSC, after it was sintered to 1373 K. (The volume percent is calculated based on the entire volume of LSC-YSZ and LSM-YSZ. composites, it is important to establish uniform particles, 0.2 um in diameter, with a porosity of -259 that the mixed oxides prepared using method B have the conduct- ing oxide within the porous YSZ matrix. In Fig. 1. the porosities of the mixed oxides are shown for a series of materials with increasing amounts of LSM or LSC. The LSC-YSZ composites in Fig. I were heated to 1373 K and the LSM-YSZ composites were heated to 1523 K. For this data, the porosity of the composites was determined from the mass change after water immersion."while volume of either LSM or LSC was calculated from the mass of oxide that was added to the porous YSZ, using the bulk densities for LSM and LSC. The line in Fig. I is the expected change in porosity of the composite assuming the second oxide fills the res.The fact that the experimental porosities agree with the calculated changes demonstrates that the second oxide remains in structure following the heat treatments. If the LSM or SC had left the YSZ pore structure, one would expect the porosities to change more slowly with the addition of the second oxide (1) LSC-YSZ Composites Figures 2 and 3 provide SEM results for the LSC-YSZ 10,69914m composites made using the two methods. Figure 2 shows the 10 LSC, LSM(vol%) Fig. 1. Porosity of YSZ composites with LSM and LSC, prepared by impregnation((-) expected result assuming the oxides exist in the YSZ Fig 3. SEM images of (a) the initial YSZ matrix and (b) the LSC-YSa pores,(●)LSC.an composite prepared by impregnation
March 2004 Low-Temperature Fabrication of Oxide Composites for Solid-Oxide Fuel Cells indistinguishable. Figure 3 shows the porous YSZ, before and after addition of LSC to a level of 40 vol% by impregnation of the porous YSZ with the lanthanum, strontium, and chromium salts. The porous. YSZ matrix(Fig 3(a)) consists of smooth, relatively uniform pores. "1-2 um in size. After impregnation, 0. 2 um LSC particles are observed coating the YSZ walls, as shown in Fig. 3(b). Obviously, the structure and connectivity of the LSC are very different from the composite sample made using method B Figure 4 shows the XRD patterns following impregnation of the porous YSZ with the lanthanum, strontium, and chromium salts to 8 a loading of 23 vol% LSC, after heating to increasingly higher mperatures. Peaks corresponding to the LSC perovskite phase stably, those peaks at41°,46°,58°,68°,and78°20) become apparent beginning at -1073 K. These peaks become sharper after ntering above 1373 K, but a new peak at 31, due to SrzrO, appears at a sintering temperature of 1473 K. For temperatures 0013001400150016001700 below 1673 K, there are also several overlapping peaks in the Temperature(K) compounds, such as CrO, CrO(OH), and Cr(OH). chromium region near 4120 which are probably associated with Fig. 5. Electrical conductivities at 973 K of LSC-YSZ composites as a To determine the optimal sintering temperature for obtaining function of sintering temperature. Data are shown for composites formed the maximum conductivity from the composites. we measured the from mixed powders (D) in air and (O) in H, and by impregnation ()in onductivities of 25 wt% LSC composites prepared by both air and(●)inH methods as a function of temperature. with the results shown in Fig. 5. For method B, this composition also corresponds to-2 1% LSC; however, the volume percent of LSC changed with emperature to a maximum value at 1373 K, then decreased intering temperature for the sample made using method A, following sintering to higher temperatures. This was consistent because the sample density changed(-22 vol%). The samples with higher conductivity caused by the formation of the LSC phase were heated in air for 2 h at the indicated temperatures before the up to 1373 K. The conductivity decreased at higher temperatures measured in air and humidified(3 mol% H,O) H,. For the conductivity of the composite formed at the highest hases.The temperature was decreased to 973 K, and the conductivities were because of the formation of secondary, insulating composite made using method B, the conductivity increased with emperature was sensitive to the gas-phase composition, while the composite formed at the optimal temperature of 1373 K was not sensitive, Because LSC remained conductive over a wide range of oxygen partial pressure(Po. ) the relatively high conductivity observed in air on the sample sintered to 1673K may not have been due to LSC For the 25 wt% LSC composite prepared using method A th conductivity increased with increased temperature, up to 1673 K, and there was not much difference between the conductivities measured in air and in H,. The reason for increased conductivity with sintering temperature was different in this case, because the conductive LSC phase already had been formed before it was 1673K mixed with YSZ. For this sample, increased temperature increased he density, which in turn improved the connectivity of the conductive phase. The rather small increase in conductivity that was observed between 1373 and 1673 K probably was due to counteracting effects, and the formation of secondary phases 1473K decreased the conductivity and densification increased the conductivit Figure 6 shows the effect of compos the conductivity the LSC-YSZ composites. All the samples were sintered to 1373 K for 2 h before their conductivities were measured in air and humidified H, at 973 K. The samples prepared using method B 1373K showed reasonably high conductivities at relatively low volume fractions of L SC. This was almost certainly due to the structure of the impregnated composites in which the LSC coated the walls of For the samples prepared using method A. the conductivity was 1073K low until the volume fraction of LSC reached%. The fact that such a high weight fraction of LSC was required, much higher than would be expected based on percolation concepts, was due to the low density of these powders after they were sintered at 1373 K. This is shown in Fig. 7. Here, the densities of the samples used for 973K he data in Fig. 6 are plotted as a function of composition, along with a line that shows the theoretical density of a nonporous 20304050607080 composite of LSC-YSZ. Obviously, the composites with <80 20(deg.) vol% LSC are highly porous H厘mm以2 The structure of LSM-YS2 composites prepared by impre (+ SrZro, phases on of the nitrate salts of lanthanum, strontium, and manganese
Journal of the American Ceramic Society- He er al Vol. 87. No. 3 0.0 E058 122 LSC (vol%) Fig. 6. Electrical conductivities at 973 K for LSC-YSZ composites as a function of composition. Data are shown for samples prepared from the mixed powders (D in air and (O) in H, and by impregnation ()in air and Fig 8. SEM image of LSM-YSZ composite prepared by impregnation with the perovskite phase, appeared at 1073K. was very similar to that observed for the related LSC-YSZ Smal ue to the La2Zr2 O, phase(29°.48°,and57°20)were composite. Figure 8 is a micrograph of a 28 vol% LSM-YSZ observed at 1523 K, and these continued to grow with increased YSZ support similar to that shown in Fig 3(b). Again, smal t rous temperature. It seemed likely that the increase in conductivity of composite after it was sintered to 1523 K, prepared using a the sample in going from 1073 to 1523 K was due to increased particles were observed coating the walls of the YSZ matrix. While crystallinity of the LSM phase with sintering; the decreased we do not show micrographs of LSM-YSZ composites prepared conductivity at higher temperatures was due to the formation of the using method A, we observed that the composites prepared by insulating La, Zr,O, phase method A were essentially dense after heating to 1523 K, with Figure I I provides the conductivities of L SM-YSZ com porosities <3% epared by both methods, as a function of LSM content. The Figure 9 shows the electrical conductivity at 973 K, in air, of a samples prepared using method A were sintered to 1523 K for 2h 28 vol% LSM-YSZ composite prepared using method B, as a conductivities were reported for impregnated samples that had function of sintering temperature. Similar to what occurred with been heated to either 1073 or 1523 K. Several interesting obser the LSC-YSZ composites, the conductivity reached a maximum vations could be drawn from this data. First, unlike the LSC-YSZ LSM was now considerably higher and was achieved at-1523 K. The tional, mixed-powder method were reasonably dense, with po absolute change in conductivity was relatively small in this case ities <3%0, and they exhibited high conductivities at reasonable and the conductivity of the composite was already quite high at LSM contents. Indeed, the conductivity of the composites prepared 1073 K. The optimal temperature, 1523 K, was approximately the using physical mixing increased rapidly at an LSM concentration recommended temperature for sintering LSM-YSZ composites, of -28 vol%, which was the expected value for percolation in prepared using traditional methods, for use as SOFC cathodes random media. Second, as we observed with the LSC-YSZ To better understand the structures that resulted in this conduc. composites, the conductivities of the impregnated samples were tivity, XRD measurements were performed on a sample prepare by impregnation of porous YSZ to a loadin responded to particles on the porous, YSZ walls. Third, the conductivities of the 28 vol% LSM, after it was heated to various temperatures. These LSM-YSZ composites formed using method B were already high results are shown in Fig.10. Peaks at23°,33°,41°,47°,and58 after being sintered to only 1073 K. The obvious implication was E a 11001200130014001500 LSC (vol%) Temperature(K) Fig. 7. Densities of LSC-YSZ composites prepared from the mixed Fig. 9. Conductivity of the LSM-YSZ (28 vol% LSM) composite powders after they were sintered at 1373 K Data are plotted along with the prepared by nation. as a function of sintering temperature. Data calculated density assuming no porosity. were obtained at 973 K in ai
March 2004 Low-Temperature Fabrication of Oxide Composites for Solid-Oxide Fuel Cells 1673K 1523K 1373K 1073K 20(deg.) Fig 10. XRD patterns of the LSM-YSZ composite, prepared by impregnation, as a function of sintering temperature((+)LSM (A)La2Zr, O and ()YSZ bases) that reasonable electronic properties could be obtained with very Regarding composition, the work in this article demonstrates low processing temperatures LSC below 1073 K. This temperature is low enough that solid- state reactions should not present problems, Because this temper- IV. Discussion ature is similar to the operating temperature of an SOFC. it may Recent interest in decreasing the operating temperature of not be necessary to have a separate sintering step to form the SOFCs has led to renewed investigations into alternative cathode composite. Furthermore, the LSM and LSC phases are formed materials. For example, Sr-doped LaFeO,(LSF) and Sr-doped below 1073 K without the use of special precursors to promote a Coo, ( LSCo) exhibit superior performance as SOFC cathodes mixing of the metal cations that make up the perovskite struc ompared with LSM. -However, because these materials reac ures: the salts used in this study have been chosen simply for with YSZ at temperatures lower than that necessary for synthesis their solubility and price. Finally, it is likely that the doped LaFeO3 of the cell. it is usually necessary to avoid contact between YSZ and LacoO, phases can be formed by similar procedures nd either LSF or LSCo by incorporating an additional material in Regarding microstructure, the synthesis of the second oxide the cathode as a barrier. Even without changing the material, phase within the preexisting YSZ matrix has several potential the performance of LSM-YSZ composites can be significantl dvantages. First. various sintering temperatures can be used in the enhanced by tailoring the microstructure. The formation of oxide preparation of the two oxide phases. With YSZ, high temperatures composites by impregnation of the components of one oxide into are advantageous for preparing an interconnected network in a porous matrix of the second oxide presents interesting opportu which ions can pass freely from the electrolyte to the YSZ within nities for the optimization of composition and microstructure in the electrode. Obviously, this porous structure can be forme oxide composites and clearly deserves additional investigation along with the dense electrolyte through the use of bilayers, so that very good connectivity can be established between the electrolyte and electrode layers. Second, a wide range of pore sizes and porosities can be established in the YSZ through the use of various pore formers and YSZ particle sizes.22. 5.6 Therefore, it is elatively easy to control the microstructure of the YSZ phase Our results demonstrate that the materials made by impregna ion cannot be modeled as random distributions of the oxide phases. Because the second phase tends to coat the walls of the YSZ matrix, high conductivities can be obtained at low volume fractions of the conductive oxide. Again, for applications in which it is necessary to limit the amount of conductive oxide that is used this property can be advantageous Finally, the LSC-YSZ composite results demonstrate that it ay be possible to form novel, composite oxides for anodes as well as cathodes Ceramic anodes show great promise for SOFCs hydrocarbons to syngas; 4-2737uy, without first reforming those hat convert hydrocarbons dire however these oxides are often LSM(vol%) used in their pure form on electrolyte-supported cells. By impreg Fig. Il. Electrical conductivities at 973 K for LSM-YSZ composites a nating conducting oxides into a porous matrix, it should be a function of co oJ'pregnation followed e s prepared from the possible to make anode-supported cells with thin electrolytes,with a are U)mixed powders ar sintering to()1523 enhanced three-phase boundaries caused by good connection or(●)1073K between the electrolyte and electrode phases
Journal of the American Ceramic Sociery--He er al Vol 87. No. 3 V. Conclusions iJ. H Choi, J, H Jang, and S M. Oh, "Microstructure and Cathodic Performance of Lan Ste MnO Yttria-Stabilized Zirconia Composite Electrodes, Electrochim We have shown that it is possible to fabricate composite oxides 867-74(200 f YSZ with LSM or LSC by impregnation of a porous YsZ by Take oxides for the Cathode in Solid- Oxide Fuel Cells, "Electrochemistry. 68 Y. Tu, M. B. Phillipps, N Imanishi, and O. Yamamoto. 01764-70(200 or LSC. The LSM and LSC perovskite phases are formed at 1A. Mitterdorfer and L J. Gauckler. "La Zr, 0, Formation and Oxygen Reduction temperatures below 1073 K, a temperature low enough to avoid 185-218(1998 he LaossSto is Mn, O, O (g)YSZ system. "Solid State Ionics, 111 [3-41 solid-state reactions with the YsZ. The structure of the LSM-YSZ K. Murata and M. Shimotsu. "Yttrium-Substituted (La, Y Sr)MnO, as a SOFC and LSC-YSZ composites made by impregnation is shown to be Cathode Material. "/. Ceram. Soc. Jpn, 110 [71 618-21(200 very different from composites made by mixing the oxide phases. Park, J. M. Vohs, and R. J. Gorte, "Direct Oxidation of Hydrocarbons in a allowing reasonable electronic conductivities to be observed at Solid-Oxide Fuel Cell, Nature(London), 404 [161 265-67(2000) relatively low concentrations of the conductive oxide. These R. J Gorte, S. Park, J M Vohs, and C. Wang,"Anodes for Direct Oxidation of Dry Hydrocarbons in a Solid-Oxide Fuel Cell. Adv, Mater., 12 [ 19].1465-69(2000) composite oxides show promise for application as SoFC S, Park, R. Craciun, J. M. Vohs, and R. J. Gorte, "Direct Oxidation of electrodes Hydrocarbons in a Solid-Oxide Fuel Cell: I Methane Oxidation. "J. Electrochem Soc,14611013603-605(199 References J. V Here, R. Ihringer, R. V. Cavieres, L. Constantin, and O. Bucheli 2H. Kim, S. D. Park. J. M. 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