B701 Application of In-Situ Diagnostic Methods for the Study of SOFC Operational Behaviour Günter Schiller, Wolfgang Bessler, Caroline Willich, K. Andreas Friedrich Deutsches Zentrum für Luft- und Raumfahrt (DLR) Institut für Technische Thermodynamik Pfaffenwaldring 38-40, D-70569 Stuttgart, Germany Tel.: +49-711-6862635 Fax: +49-711-6862747 guenter.schiller@dlr.de Abstract In order to optimise the operational behaviour of fuel cells and minimise cell degradation it is very helpful to use in-situ and ex-situ analytical methods. This paper gives an overview of in-situ diagnostic methods that are applied at DLR Stuttgart for the study of SOFCs. It includes spatially resolved measurements with an experimental segmented cell configuration where different techniques such as IV characteristics, impedance spectroscopy, gas chromatography and temperature measurement are involved. The investigation by means of segmented cells aims at the determination of local effects and the identification of critical operating conditions during technically relevant SOFC operation. Recently, a new test setup with transparent optical access has been built up allowing for microscopic observation of processes within the cell as well as for application of in-situ laser Raman spectroscopy to determine highly resolved concentrations of gas species along a flow channel. Examples of analytical studies by applying these diagnostic methods are presented and potentials and limitations of the different techniques are discussed. Introduction High electrical performance and long lifetime are key requirements that must be fulfilled for a successful introduction of fuel cells into the market. For achieving a high efficiency, a high fuel utilisation is required which means an electrochemical conversion of fuel into electricity as high as possible. This requirement, on the other hand, results in strong concentration gradients at the anode where the fuel is successively diluted by reaction products during the reaction process and, hence, in an inhomogeneous distribution of electrochemical performance and temperature. Inhomogeneous distributions of electrochemical and thermal properties such as local power density and local temperature might detrimentally affect both efficiency and long-term durability through thermo- mechanical stress and degradation phenomena induced by locally varying operating conditions. In order to optimise the operational behaviour of fuel cells and minimise cell degradation the application of advanced diagnostic methods by monitoring cell characteristics under real operating conditions can provide detailed information about the spatial distribution of the electrical, chemical and thermal cell properties. DLR has developed spatially resolved diagnostic techniques with a segmented cell arrangement where different techniques such as IV characteristics, impedance spectroscopy, gas chromatography and temperature measurement are involved [1, 2]. The segmented cell