Characterization of Multilayer Anodes for SOFC
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Characterization of Multilayer Anodes for SOFC Axel C. Müller, Albert Krügel, André Weber and Ellen Ivers-Tiffée Institut für Werkstoffe der Elektrotechnik, Universität Karlsruhe (TH) D-76131 Karlsruhe, Germany ABSTRACT SOFC anodes have to combine various tasks. In anode supported single cells a thick anode substrate is used for current collecting and gas distribution whereas a thin functional layer adjacent to the electrolyte is the electrochemically active part of the anode. This functional anode layer is cofired together with the thin film electrolyte to obtain an enhanced interface with low polarisation losses. This multilayer structure was transferred to an electrolyte supported single cell. The electrochemical active Ni/8YSZ anode layer was screen printed onto a 8YSZ electrolyte green tape and subsequently cofired at 1350 °. Mechanical stresses during cofiring due to shrinkage mismatch of anode and electrolyte were avoided by changing the geometry of the anode layer from a continuous layer to a large number of small sized individual areas. Simulations by finite element modeling indicated that a hexagonal pattern similar to honeycombs is preferable. The second layer which adjoins to the fuel gas channels and which is responsible for current collecting and gas distribution was later on screen printed on top and sintered together with the cathode. Single cells with a multilayer anode and different functional layers were electrochemically characterised under realistic operation conditions. The performance and reduction/oxidation stability of this type of anode was investigated. The electrochemically active layer showed only small degradation during redox cycling and long term operation at high fuel utilisation. In contradiction to single layer anodes Nickel agglomeration was not observed in the functional layer. INTRODUCTION The high temperature solid oxide fuel cell (SOFC) is an electrochemical energy converter representing a technology, which allows the economic and environmentally friendly production of electrical energy with high efficiency. To meet the different tasks of a SOFC anode, a multilayer structure is often used in anode supported single cells. A thick anode substrate acts as current collector and gas distributor whereas a thin functional layer adjacent to the electrolyte is the electrochemically active part of the anode. This functional anode layer is usually cofired with the thin film electrolyte to obtain an enhanced interface which exhibits low polarization losses and good mechanical adherence. However, different shrinkage of anode and electrolyte [1] causes bending [2-4] or breaking of the compound. In this work we will show how this multilayer structure was transferred to an electrolyte supported single cell and how the problems of cofiring were encountered. Single cells with a multilayer anode were prepared and electrochemically characterized in respect of performance, redox stability and long term stability. THEORY If the length difference ∆s caused by different sintering shrinkage
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