Foghmoes, Søren Preben Vagn

Holtappels, Peter

Ramos, Tania

Sudireddy, Bhaskar Reddy

Redox stable solid oxide fuel cells are beneficial in many aspects such as tolerance against system failures e.g fuel cut off and emergency shut down, but also allow for higher fuel utilization, which increases efficiency. State-ofthe-art Ni-cermet based anodes suffer from microstructural changes upon redox cycling, while other properties such as catalytic activity for methane reforming and/or water gas shift, thermal conductivity in addition to electronic conductivity for current pickup are highly wanted for SOFC applications.In order to combine the advantages of a redox stable anode with a multifunctional anode support, the development of a two layer fuel electrode based on a redox stable strontium titanate layer for the electrochemically active layer and a redox stable Ni-YSZ support was pursued. Half-cells with well adhearing strontium titante anode layers on stateof-the-art Ni-YSZ cermet supports have been achieved. Redox tolerance of the half-cell depends could be increased by optimizing the redox stability of the cermet support

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Downloaded from orbit.dtu.dk on: Apr 10, 2018Development of redox stable, multifunctional substrates for anode supported SOFCSSudireddy, Bhaskar Reddy; Foghmoes, Søren Preben Vagn; Ramos, Tania; Holtappels, PeterPublished in:Proceedings of the 7th European Fuel Cell Technology &amp; Applications Piero Lunghi ConferencePublication date:2017Document VersionPublisher's PDF, also known as Version of recordLink back to DTU OrbitCitation (APA):Sudireddy, B. R., Foghmoes, S. P. V., Ramos, T., & Holtappels, P. (2017). Development of redox stable,multifunctional substrates for anode supported SOFCS. In V. Cigolotti (Ed.), Proceedings of the 7th EuropeanFuel Cell Technology & Applications Piero Lunghi Conference (pp. 181-182)181PROCEEDINGS EFC17 /Copyright © 2017Proceedings of EFC2017European Fuel Cell Technology & Applications Conference - Piero Lunghi ConferenceDecember 12-15, 2017, Naples, ItalyEFC17106DEVELOPMENT OF REDOX STABLE, MULTIFUNCTIONAL SUBSTRATES FOR ANODE SUPPORTED SOFCSB. R. Sudireddy, S. P. Foghmoes, T. Ramos, and P. Holtappels Department of Energy Conversion and Storage, Technical University of Denmark, Frederiksborgvej 399, DK-4000 Roskilde (Denmark)Abstract - Redox stable solid oxide fuel cells are beneficial in many aspects such as tolerance against system failures e.g fuel cut off and emergency shut down, but also allow for higher fuel utilization, which increases efficiency. State-of-the-art Ni-cermet based anodes suffer from microstructuralchanges upon redox cycling, while other properties such as catalytic activity for methane reforming and/or water gas shift, thermal conductivity in addition to electronic conductivity for current pickup are highly wanted for SOFC applications.In order to combine the advantages of a redox stable anode with a multifunctional anode support, the development of a two layer fuel electrode based on a redox stable strontium titanate layer for the electrochemically active layer and a redox stable Ni-YSZ support was pursued. Half-cells with well adhearing strontium titante anode layers on state-of-the-art Ni-YSZ cermet supports have been achieved.Redox tolerance of the half-cell depends could be increased by optimizing the redox stability of the cermet support.Index Terms – SOFC, anode supports, redox stability, ceramic anode.I. INTRODUCTIONRedox stable solid oxide fuel cells are beneficial in many aspects such as tolerance against system failures e.g fuel cut off and emergency shut down, but also allow for higher fuel utilization increasing efficiency. Related to redox tolerance, is also overload tolerance.State-of-the-art Ni-YSZ cermet electrodes can be tailored in microstructure so they possess a certain redox stability, however, usually through a coarse grained microstructure on the expense of electrochemical activity.A redox stable anode can be prepared by mixed or electronically conducting ceramics, and promising results have been published for strontium titanate based electrodes infiltrated with electrocatalytically active metal particles [1,2]. However, these ceramic materials suffer from low mechanical strength, low electrical and thermal conductivity and insufficient catalytic activity for internal reforming or water-gas shift reaction. All these properties are known to be important for efficient operation of SOFC cells, and are usually provided by a Ni-cermet based anode support ( see Fig. 1).Fig. 1.  Concept of a redox stable anode supported SOFCIn order to combine the advantages of a redox stable anode with a multifunctional anode support, the development of a two layer fuel electrode based on a redox stable ceramic anode layer for the electrochemically active layer and a redox stable Ni-YSZ support was pursued. II. RESULTSThe concept of the redox stable, multifunctional anode anode support is shown in Fig. 1, indicating the different layers and functionalities. Cells with strontium titanatebased anode layers (Sr0.94Ti0.9Nb0.1O3–?) (STN) and Ni-/ PROCEEDINGS EFC17182Copyright © 2017YSZ (Zr0.84Y0.16O1.92 (8YSZ)) anode supports have been prepared by tape casting the individual layers and subsequent lamination and sinteringFig. 2 shows an STN anode (middle layer) integratedonto a NiO-8YSZ support (bottom) with a Sc-modified zirconia electrolyte layer (top layer). Well adhearing layers can be achieved for various types of Ni-YSZ supports and both yttria and scandia modified zirconia as electrolyte layer. The cells could be successfully reduced in 9% H2 at 850 C without any visible damage to the microstructure.Fig. 2. Microstructure of a STN anode layer integrated between a Sc-stabilized zirconiaelectrolyte (top) and a NiO-YSZ cermet (bottom) layerThe cells were further tested as regards their redox tolerance by two redox cycles between 9% H2 in Ar humidified with 3% water and air at 850C. Fig 3 shows the microstructure of cells with a “standard” Ni-YSZ anode support with an STN anode and 8YSZ electrolyte layer. While the anode support has survived the redoc cycles, cracks have been developed through the electrolyte and anode layer, most probably resulting from dimensional changes in the anode support during re-oxidation.Fig. 3. Microstructure of a “standard” Ni-YSZ anode supported cell with STN anode after redox cyclingThe redox tolerance of anode supports can to a large extend be tailored by the Ni/YSZparticle size distribution and the Ni-volume fraction in the Ni-cermet [3,4].Fig 4 shows the microstructure of an STN layer integrated onto a redox optimized Ni-YSZ support after redox cycling. Significantly reduced number cracks through the anode and electrolyte layer are observed, indicating an increased redox stability of the anode supported half-cell.Fig. 4. Microstructure of a redox optimized Ni-YSZ anode supported cell with STN anode after redox cyclingIII. CONCLUSIONNi-YSZ cermet based anodes and supports are state-of-the-art and have been developed up to industrial fabrication. Redox stable ceramic anode layers can be integrated into standard Ni-cermet anode supports, which allows for the development of a full redox stable anode supported cell, without the need for completely newsubstrate developments. This has been proven on a rather fine-grained Ni-YSZ support, and even better redoxstability is expected by using more coarse, low Ni-containing anode supports.ACKNOWLEDGMENTParts of the research leading to these results has received funding from the Fuel Cells and Hydrogen Joint Undertaking under grant agreement n° 256730 and from the Danish ForskEL project “SOFC4Ret”.REFERENCES[1] P. Holtappels, J. T. S. Irvine, B. Iwanschitz, L. T. Kuhn, L.Y. Lu, Q. Ma, J. Malzbender, A. Mai, T. Ramos, J. Rass-Hansen, B. R. Sudireddy, F. Tietz, V. Vasechko, S. Veltze,M. C. Verbraeken, Full Ceramic Fuel Cells Based OnStrontium Titanate Anodes, An Approach Towards MoreRobust SOFCs, ECS Transact. 57(1) (2013) 1175-1184[2] M. C. Verbraeken, B. Iwanschitz, E. Stefan, M. Cassidy, U.Weissen, A. Mai, J. T. S. Irvine, Short Stack and Full System Test Using a Ceramic A-Site Deficient StrontiumTitanate Anode, Fuel Cells, 15(5) (2015), 682–688[3] Final Report EU Integrated project Real-SOFC[4] T. Ramos et al. Unpublished results DTU Energy (formerRisø National Laboratory), DK.

Development of redox stable, multifunctional substrates for anode supported SOFCS

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Development of redox stable, multifunctional substrates for anode supported SOFCS

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