Mogens Mogensen

Peter Vang Hendriksen

Petru Gordes

Rasmus Barfod

Severine Ramousse

Steen Brian Schougaard

Wei Guo Wang

Williford

Yi-Lin Liu

Anode-supported cells were fabricated with optimized cathodes showing high power density of 1.2 W/cm(2) at 800 C under a cell voltage of 0.7 V and an active area of 4 x 4 cm. A microstructure study was performed on such cell using a field-emission gun scanning electron microscope, which revealed that the (La1-xSrx)(y)MnO3 +/-delta (LSM) composite cathodes consist of a network of homogenously distributed LSM, yttria-stabilized zirconia (YSZ), and pores. The individual grain size of LSM or YSZ is approximately 100 nm. The degree of contact between cathode and electrolyte is 39% on average. (c) 2005 The Electrochemical Society

Wang, Wei Guo

Liu, Yi-Lin

Barfod, Rasmus

Schougaard, Steen Brian

Gordes, Petru

Ramousse, Severine

Hendriksen, Peter Vang

Mogensen, Mogens Bjerg

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Dec 17, 2017Nanostructured lanthanum  manganate composite  cathodeWang, Wei Guo; Liu, Yi-Lin; Barfod, Rasmus Gottrup; Schougaard, Steen Brian; Gordes, Petru;Ramousse, Severine; Hendriksen, Peter Vang; Mogensen, Mogens BjergPublished in:Electrochemical and Solid-State LettersLink to article, DOI:10.1149/1.2081890Publication date:2005Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Wang, W. G., Liu, Y-L., Barfod, R., Schougaard, S. B., Gordes, P., Ramousse, S., ... Mogensen, M. B. (2005).Nanostructured lanthanum  manganate composite  cathode. Electrochemical and Solid-State Letters, 8(12),A619-A621. DOI: 10.1149/1.2081890Nanostructured Lanthanum Manganate Composite CathodeWei Guo Wang,a,*,z Yi-Lin Liu,a Rasmus Barfod,a Steen Brian Schougaard,aPetru Gordes,b Severine Ramousse,a Peter Vang Hendriksen,a andMogens Mogensena,*aDepartment of Materials Research, Risø National Laboratory, DK-4000 Roskilde, DenmarkbTopsoe Fuel Cell A/S, Material and Stack Development, DK-2800 Lyngby, DenmarkAnode-supported cells were fabricated with optimized cathodes showing high power density of 1.2 W/cm2 at 800°C under a cellvoltage of 0.7 V and an active area of 4  4 cm. A microstructure study was performed on such cell using a field-emission gunscanning electron microscope, which revealed that the La1−xSrxyMnO3± LSM composite cathodes consist of a network ofhomogenously distributed LSM, yttria-stabilized zirconia YSZ, and pores. The individual grain size of LSM or YSZ is approxi-mately 100 nm. The degree of contact between cathode and electrolyte is 39% on average.© 2005 The Electrochemical Society. DOI: 10.1149/1.2081890 All rights reserved.Manuscript submitted June 14, 2005; revised manuscript received August 10, 2005. Available electronically October 7, 2005.Planar anode-supported cells have shown power density of morethan 1 W/cm2 at 800°C under 0.7 V.1-5 This led to a reduction ofoperation temperature from 1000 to 750°C in solid oxide fuel cellSOFC systems, which has resulted in a significant cost reductionand improvement of the long-term stability. Such successes havebeen realized by the cell design as LSM + YSZ/YSZ/Ni+ YSZ using state of art materials, namely, yttria-stabilized zirconiaYSZ for the electrolyte, Ni + YSZ for the anode, andLa1−xSrxyMnO3±LSM + YSZ as the cathode. This design is byfar the most commonly used. The theoretical and experimental workhas shown that the dominant loss was attributed to the cathode in thecell operating at the range of 600–850°C. To reduce the polarizationresistance in the cathode, many groups have studied the relationshipbetween the cathode microstructure and the electrochemicalproperties.6-10 They indicated that a composite cathode with desir-able microstructure was the key to obtaining good electric andchemical performance of the cathode. In the composite cathode withLSM and YSZ, the electrochemical reduction of oxygen occurs atthe conjunction of the three-phase boundary TPB. The threephases are LSM, YSZ, and gas pores. In a fuel cell using compos-ite cathode, the TPBs extend well into the cathode in areas placedaway from the physically distinct cathode/electrolyte interface, andsubstantially lower the cathodic polarization. Therefore not only themicrostructure at the cathode/electrolyte interface, but also the entirecathode microstructure was considered to be responsible for the po-larization of the cathode. Recently, Williford and Singh reportedtheir computational design analysis of a high performance cathode.11Their calculation illustrated that an optimal cathode structure maybe built by a two-layer structure with a thinner layer with smallergrain diameters at the cathode/electrolyte interface followed by arelatively thicker outer layer with larger grains at the electrode/oxidant interface. However, this report was lacking detailed micro-structure information and a clear relation between performance andmicrostructure. Moreover, little microstructure information was re-ported in the cells with high power densities. At Risø National Labo-ratory, the processing optimization of the LSM/YSZ composite cath-ode was carried out in a systematic way,5 which led to doubledpower density enhancement in comparison with cells from a previ-ous version. In this work, the properties of the high power densitycell are reported and the cathode microstructure is investigated usinga high resolution field-emission gun scanning electron microscopeFEGSEM.ExperimentalAnode-supported cells were produced by tape casting the anodesupport, spraying the NiO-YSZ active anode and the YSZ electro-lyte layer, followed by cosintering. Then the cathode was sprayedand sintered.5 The cells with an active electrode area of 4  4 cmwere tested in a setup designed at Risø.12 Humidified hydrogen with5% water was used as the fuel and air was used as the oxidant.Current loads up to 40 A were applied to the cell to obtain thecurrent-voltage I-V measurements. The cell tests were performedin a temperature range from 850 to 650°C. The FEGSEM ZEISSSupera-35 used in this work has a superb image resolution, namely,1.0 nm at 20 kV and 2.5 nm at 1 kV in theory.Results and DiscussionFigure 1 shows the I-V curves of an anode-supported cell pro-duced using the improved cathode processing technique. The cellswith an active area of 4  4 cm were tested using humidified hy-drogen with 5% H2O as fuel and air as oxidant. The hydrogen flowwas 30 L/h and airflow was either 140 or 170 L/h. Figure 2 dem-onstrates the high power density achieved in the anode-supportedcell using the optimized cathode.Table I summarizes the performance of the anode-supported cell.Here the Timax is the measured cell temperature at maximum current.The ASR is the area specific resistance of the anode-supported cellASR = OCV–V/imax, where OCV is the open-circuit voltage, V isthe cell voltage, and imax is the maximum current density. Almosttheoretical open cell voltages were obtained indicating tight electro-lyte, optimal electrical potential through the cell, and tight cell testhouse. Power density as high as 1.2 and 0.5 W/cm2 at 800 and700°C, respectively, have been achieved under a voltage of 0.7 V.The fuel utilization F.U. at 0.7 V is also shown in Table I. This* Electrochemical Society Active Member.z E-mail: wei.guo.wang@risoe.dkFigure 1. I-V curves obtained under a hydrogen flow of 30 L/h with 5%water vapor.Electrochemical and Solid-State Letters, 8 12 A619-A621 20051099-0062/2005/812/A619/3/$7.00 © The Electrochemical Society, Inc.A619Downloaded 28 Jun 2010 to 192.38.67.112. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jspperformance indicates that the anode-supported cell with improvedLSM/YSZ composite cathode has the potential to be operated at650–750°C with an acceptable power density.The polished cross section of the cell was examined in theFEGSEM ZEISS Supra-35 with focus on the cathode and thecathode/electrolyte interface. Figure 3 is a micrograph obtained bythe backscattered detector. Figure 4 was obtained using the highefficiency inlens detector and a very low accelerating voltage. It is ahigh-resolution image of the polished sample surface. As this imageis free of the influence of the depth under the surface, it is excellentfor evaluating the adhesion at the interface.The cathode was a very fine structured composite with uniformlydistributed LSM/YSZ and pores. The LSM and YSZ were sinteredtogether forming a network, and it was incorporated uniformly witha network of pores. Because the backscattered coefficients of LSMand YSZ are very close to each other, the two phases cannot bedistinguished clearly in Fig. 3. However, energy dispersive spec-trometry EDS mapping shows that the distribution of the LSM andYSZ grains are homogeneous. Furthermore, a distinct feature of thiscathode is that the LSM/YSZ network is well adhered to the elec-trolyte as shown in Fig. 4. It can be seen that almost all the available“branches” of the LSM/YSZ network at the interfacial region areconnected to the electrolyte.To quantify the degree of interface adhesion, the linear coverageof the electrolyte interface by the cathode phases, i.e., the lengthfraction of the “connecting bridges,” is measured on 3 inlens im-ages. The measured coverage value varied from 32 to 52% with anaverage value of 39%, shown in Table II. The width of each bridgewas measured as well, the average value of which was 62 nm. TheYSZ network in the composite cathode is expected to grow like atree with strong roots. Such connected network offers large extendedcontacts between ionic conductor electrolyte YSZ and electronicconductor LSM. Porous structure with very small grain size enlargesthe number of the active TPBs. However, such beneficial effect canbe realized only when the adhesion between electrolyte and cathodeis strong, namely, the roots are strong.Figure 5 shows a fractured cross section of a symmetrical cath-ode cell in a configuration of LSM + YSZ/YSZ/LSM + YSZ.The fabrication method of the cathode in the full cell reported in thiswork was implemented from the parameters found in the optimiza-tion of processing with a series of symmetrical cathode samples. Itwas expected that identical processing conditions be applied to thepreparation of the cathodes in the samples shown in Fig. 5 and 3.From Fig. 5, the individual grains either LSM or YSZ are clear andround. The grain size is approximately of 100 nm. Sintering necksbetween the grains indicate good connection between LSM/LSM,LSM/YSZ, or YSZ/YSZ grains. High numbers of active TPB isexpected in such cathode network under the condition of strongadhesion at the cathode/electrolyte interface shown in Fig. 4 andTable II. In this picture, it was expected that the cross section wasthe weakest one because of the fracture nature of the broken sample.We have demonstrated that a nanostractured cathode can be pro-duced in the SOFC processing condition, namely, at high tempera-ture. Our long-term performance evaluation and post-test analysisindicated the nanostructure was kept in the SOFC operationconditions.13,14Table I. Performance of the anode supported cell (4 Ã 4 cm ac-tive area).Temperature°CTimax°COCVVCurrent at0.7 VAPowerdensity at0.7 VW/cm2ASR cm2F.U.a%850 864 1.070 32.9 1.44 0.175 46800 816 1.082 26.7 1.17 0.209 35750 758 1.092 18.3 0.8 0.305 22700 708 1.101 11.7 0.51 0.47 16650 655 1.111 6.6 0.29 0.845 9a Fuel utilization at 0.7 V.Figure 2. Power density obtained in the anode-supported cell 4  4 cmactive area.Figure 3. A backscatter image of the cathode and cathode/electrolyte inter-face.Figure 4. A secondary electron image obtained using the inlens detector onthe cathode/electrolyte interface.A620 Electrochemical and Solid-State Letters, 8 12 A619-A621 2005Downloaded 28 Jun 2010 to 192.38.67.112. Redistribution subject to ECS license or copyright; see http://www.ecsdl.org/terms_use.jspIt’s difficult to draw the direct conclusion that smaller grain sizeleads to larger number of active TPBs and lower cathodepolarization.9 From many studies done in the past, it is clear that theelectrode performance, which was ultimately evaluated by the areaspecific resistance, is determined by multimicrostructure factors, in-cluding grain size, pore size, electrolyte/electrode interface, elec-trode uniformity, as well as the distribution of the network. Theoret-ical calculation has shown that doubled current may be obtained byreducing the grain size from 1.0 to 0.25 m.11 It is also straightfor-ward that smaller grain and pore size lead to larger number of activeTPBs. However, such a hypothesis may be established only underthe condition of well and homogeneously connected ionic, elec-tronic components and pores in the cathode as well as good adhe-sion at electrolyte/electrode. This work has demonstrated that highpower density can be achieved by the nanostructured cathode.ConclusionBy FEGSEM ZEISS Supera-35, a microstructure analysis wasconducted on the cathode of a high power density fuel cell. Themicrostructure of the composite cathode shows a homogeneous po-rous network consisting of three components LSM, YSZ, and pores.The individual grain size of either LSM or YSZ was approximately100 nm. The interface at cathode and electrolyte is well bonded. Thepercentage of contact line at cathode/electrolyte interface was mea-sured to be 39% on average. By engineering the cathode throughprocessing optimization, it was possible to make nanostructuredcathodes, which are believed to increase the length of the activeTPB, thereby enhancing the power density of the cell.AcknowledgmentsThis work was performed as a part of the Danish SOFC programDK-SOFC, which was supported by the Danish Energy Agency.Colleagues at Risø National Laboratory are acknowledged for assis-tance in the sample preparation and testing.Risø National Laboratory assisted in meeting the publication costs of thisarticle.References1. S. Souza, S. J. Visco, and L. C. De Jonghe, J. Electrochem. Soc., 144, L35 1997.2. J.-W. Kim, A. V. Virkar, K.-Z. Fung, K. Mehta, and S. C. Singhal, J. Electrochem.Soc., 146, 69 1999.3. E. Tang, F. Martell, B. Brule, B. Borglum, and D. Ghosh, in Proceedings of FifthEuropean SOFC Forum, J. Huijsmans, Editor, V1, p. 26 2002.4. N. Minh, in Solid Oxide Fuel Cells VIII, S. C. Singhal and M. Dokiya, Editors, PV2003-07, p. 43, The Electrochemical Society Proceedings Series, Pennington, NJ2003.5. W. G. Wang, R. Barfod, P. H. Larsen, K. Kammer, J. J. Bentzen, P. V. Hendriksen,M. Mogensen, in Solid Oxide Fuel Cells VIII, S. C. Singhal and M. Dokiya,Editors, PV 2003-07, p. 400, The Electrochemical Society Proceedings Series,Pennington, NJ 2003.6. T. Kenjo and M. Nishiya, Solid State Ionics, 57, 295 1992.7. M. J. L. Østergård, C. Clausen, C. Bagger, and M. Mogensen, Electrochim. Acta,40, 1971 1995.8. K. Sasaki, J.-P. Wurth, R. Gschwend, M. Gödickemeier, and L. J. Gauckler,J. Electrochem. Soc., 143, 530 1996.9. F. H. van Heuveln, H. J. M. Bouwmeester, and F. P. F. van Berkel, J. Electrochem.Soc., 144, 126 1997.10. M. J. Jørgensen, S. Primdahl, C. Bagger, and M. Mogensen, Solid State Ionics,139, 1 2001.11. R. E. Williford and P. Singh, J. Power Sources, 128, 45 2004.12. R. Barfod, S. Koch, Y.-L. Liu, P. H. Larsen, and P. V. Hendriksen, Solid Oxide FuelCells VIII, S. C. Singhal and M. Dokiya, Editors, PV 2003-07, p. 1158, The Elec-trochemical Society Proceedings Series, Pennington, NJ 2003.13. R. Barford, M. Mogensen, T. Klemensø, A. Hagen, Y.-L. Liu, and P. V. Hendriksen,Solid Oxide Fuel Cells IX, S. C. Singhal and J. Mizusaki, Editors, PV 2005-07,p. 524, The Electrochemical Society Proceedings Series, Pennington, NJ 2005.14. A. Hagen, R. Barford, P. V. Hendriksen, Y.-L. Liu, and S. Ramousse, Solid OxideFuel Cells IX, S. C. Singhal and J. Mizusaki, Editors, PV 2005-07, p. 503,The Electrochemical Society Proceedings Series, Pennington, NJ 2005.Table II. Length fraction of the connecting bridge betweenLSM + YSZ and YSZ.Cell name Bridge min Bridge max AverageAverage size ofthe bridgeC0631 32% 52% 39% 62 nmFigure 5. Fractured cross section of a symmetrical cathode cell.A621Electrochemical and Solid-State Letters, 8 12 A619-A621 2005Downloaded 28 Jun 2010 to 192.38.67.112. 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Nanostructured lanthanum manganate composite cathode

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Nanostructured lanthanum manganate composite cathode

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