Degradation mechanisms and advanced characterization and testing (II) Spatially resolved measurement of SOFC by using segmented cells P. Szabo German Aerospace Center (DLR) Pfaffenwaldring 38-40, D-70569 Stuttgart/Germany * Corresponding author e-mail address: Patric.Szabo@dlr.de The EU project ENDURANCE aims at increasing the reliability and long-term stability of SOFCs by better understanding degradation and lifetime fundamentals which is closely related to the application of sophisticated characterization techniques, accelerated testing strategies and degradation modelling. Based on these procedures early warning output signals (EWOS) will be identified and developed before the stack sustains permanent damage in order to be able to develop counter strategies which can alleviate the degradation effects. Spatially inhomogeneous distributions of current density and temperature in solid oxide fuel cells (SOFC) can contribute significantly to accelerated electrode degradation, thermomechanical stresses, and reduced efficiency. This is particularly the case under technically relevant operating conditions. With spatially resolved measurements using segmented cell technology it is possible to investigate degradation processes locally and identify areas where degradation occurs first. If certain effects can be tied to special local areas the results can be used to improve the cell, the gas distribution or operation envelope. In this respect spatially resolved measurements of anode supported cells from SOLIDpower in a 4 4-segmented cell arrangement with an equivalent area of 80 cm² which are comparable to the cells used in stacks were performed. The measurement setup allows for integral and spatially resolved measurement of current density and voltage, the local and integral determination of impedance data, the local measurement of temperature and temperature distribution and the spatially resolved analysis of the fuel gas concentrations along the flow path. In order to determine the temperature at each segment, thermocouples are introduced in the metallic segments. The tests were performed in co-flow operation for various fuel gas compositions at the anode and air at the cathode. Local and global current-voltage relationships were measured in dependence of gas composition and fuel utilization. Degradation Mechanisms in Solid Oxide Cells and Systems Workshop Proceedings 257
Spatially resolved measurement of SOFC by using segmented cells P. Szabo WORKSHOP PROCEEDINGS DEGRADATION MECHANISMS IN SOLID OXIDE CELLS AND SYSTEMS FEBRUARY 17, 2017 BARCELONA, SPAIN
Spatially resolved measurement of SOFC by using segmented cells Workshop on Degradation Mechanisms in Solid Oxide Cells and Systems Patric Szabo, Günter Schiller Deutsches Zentrum für Luft- und Raumfahrt, Institut für Technische Thermodynamik Pfaffenwaldring 38-40, D-70569 Stuttgart 9th European Forum on Solid Oxide Fuel Cells, June 30 - July 2, 2010, Lucerne
Outline Introduction Motivation Experimental setup with ASC Results of cells Conclusion and outlook
Introduction Insufficient understanding of cell degradation and cell failures in SOFC Extensive experimental experience is not generally available which would allow accurate analysis and improvements Long term experiments are demanding and expensive Only few tools and diagnostic methods available for developers due to the restrictions of the elevated temperatures
Motivation Problems in planar cell technology: Strong local variation of gas composition, temperature, and current density Distribution of electrical and chemical potential dependent on local concentrations This may lead to: Reduced efficiency Thermo mechanical stress Degradation of electrodes Effects are difficult to understand due to the strong interdependence of gas composition, electrochemical performance and temperature
Segmented Cells Anode supported cells (SOLIDPower): Segmented cathode only 16 segments with 3,36 cm² each, 53.76 cm² total active area
Measurement Setup for Segmented Cells 16 galvanically isolated segments Local and global i-v characteristics Local and global impedance measurements Local temperature measurements Local fuel concentrations Flexible design: substrate-, anode-, and electrolyte-supported cells Co- and counter-flow
Test Setup
OCV Voltage Distribution Standard gas flow rates: 20//40 smlpm/cm² H 2 //Air, 750 C OCV is higher in the middle lines (segments 5 to 12) compared to the outside lines => leaks on the rim of cell due to compressive seal
Voltage Distributution under Conditions of High Power Density Even at high power density the voltages show little variation compared to cells from other manufactures
Power Density Distribution Power density is generally lower towards fuel gas exhaust (segment 5) Compared to ASCs from other suppliers more even distribution of power densities over area No real sharp decrease in power density towards fuel gas exhaust Air Flow Fuel Flow
Power Density after 1250 h and 1 Thermal Cycle After long term operation and 1 (unintended) thermal cycle the cell showed some degradation Segment 7 is strongly affected by the thermal cycle, probably contacting since OCV is still good and no deviation in temperature behaviour compared to neigbouring segments
Long Term Testing Cell suffered a failure of segment 14 after 450 h Somewhat high degradation even before segment failure, increases after segment failure Test resumed after a power outage at 1250 h
Investigation of Failure of Segment 14 Drop in voltage for longer than normal oscillation, also corresponds to a drop in current density shortly before failure of segment Short drop in current density and voltage observed about 118 minutes before failure occurs Current density recovers after about 56 minutes but voltage does not Also slight temperature increase can be observed after the initial voltage drop but not before Combination of all three incidents might be an idication of future failure
Conclusion The potential of spatially resolved diagnostics was demonstrated with some exemplary results Strong gradients of gas concentrations and current density particularly at operation with high fuel utilisation may result in locally critical operating behaviour Possible early warning signal (EWOS) identified by monitoring voltage oscillations of cell and combined with an increase in temperature this can possibly be linked to a future failure of cell SolidPower cells show a more even distribution of voltage and current density compared to other cells Outlook More cells will be measured to support this measurement and possibly yield different EWOS
Acknowledgements The leading to these results has received funding from the European Union s 7th Framework Programme (FP7/2007-2013) Fuel Cells and Hydrogen Joint Undertaking (FCH-JU-2013-1) under grant agreement No 621207.
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