We present a combined modeling and experimental study of electrochemical oxygen reduction at mixed-conducting composite LSCF/CGO solid oxide fuel cell (SOFC) cathodes. The developed kinetic model incorporates elementary heterogeneous chemistry and electrochemical charge-transfer processes at two different electrochemical double layers, transport in the porous composite electrode (ionic and electronic conduction, multi-component porous diffusion and convection) as well as gas supply. A full set of thermodynamic and kinetic parameters is developed. Experimentally, La 0.6 Sr 0.4 Co 0.8 Fe 0.2 O 3-δ /Ce 0.9 Gd 0.1 O 2-α composite electrodes embedded into a symmetrical cell with CGO electrolyte were characterized via electrochemical impedance spectroscopy. The model shows good agreement with experimental impedance data over the complete range of investigated conditions (temperature range 775 K-1075 K, frequency range 10 mHz-100 kHz). This allows a mechanistic interpretation of the origin of the three observed impedance features: (i) low frequency: transport in the gas supply (gas conversion), (ii) intermediate frequency: charge transfer and surface double layer at the LSCF/air interface, (iii) high frequency: charge transfer and electrical double layer at the LSCF/CGO interface.
The current paper reports the details of challenges and progress in the joint project on development of metal supported solid oxide fuel cells and stacks by the teams of German Aerospace Center (DLR), ElringKlinger, Sulzer Metco and Plansee. An account of the materials is given followed by the advances in the processing techniques which include alternative plasma spraying and colloidal spraying. Improvement in the cell and stack design is then discussed followed by challenges faced during stack building and operations and solution implemented.
IDEAL-Cell is a new concept of a high temperature fuel cell operating in the range 600-700°C. It is based on the junction between the anode part of a PCFC and the cathode part of a SOFC through a mixed H+ and O2- conducting porous ceramic membrane. This concept, extensively described in the present paper, aims at avoiding all the severe pitfalls connected with the presence of water at the electrodes in both SOFC and PCFC concepts. Spark Plasma Sintering samples were designed specifically for proving the IDEAL-Cell concept. The first electrochemical results obtained at 600°C under hydrogen on millimeter thick samples show that IDEAL-Cell behaves like a high temperature fuel cell. It is estimated that the overall efficiency of this new concept should greatly surpass that of standard SOFCs and PCFCs and that the material constraints, especially in the case of interconnect materials, should significantly decrease
Metal supported solid oxide fuel cells (MSCs) show a high poten- tial predominantly for an application as light-weight planar cells for electricity supply systems in cars and other mobile systems. One of the most promising MSC concepts is the plasma sprayed thin film concept of DLR-Stuttgart in which powder metallurgi-cally manufactured substrates, developed within a collaboration of Plansee and DLR, serves as mechanical cell support. In MSCs gen-erally problems can occur in particular by a mutual diffusion of Ni, Fe and Cr on the substrate/anode interface that causes significant cell degradation. To avoid this mass transport ceramic diffusion barrier layers consisting of Cr2O3 and/or MnCr2O4 or doped LaCrO3-type perovskites are provided. The paper focuses on a ma- terial study of different substrate materials performed with inte-grated diffusion barrier layers. Furthermore, the electrochemical long term behaviour of cells with different plasma sprayed barrier layers is shown demonstrating the barrier effect and the electro-chemical compatibility.
At the German Aerospace Center (DLR) in Stuttgart, a lightweight stack design for mobile applications was developed in cooperation with the automotive industry (BMW, Munich; Elring-Klinger, Dettingen; Rhodius, Weissenburg). This concept is based on the application of stamped metal sheet bipolar plates into which porous metallic substrate-supported cells (MSCs) are integrated. The paper concentrates on the one hand on the investigation of plasma sprayed button cells with a diameter of 48mm on porous metallic substrates during reduction/oxidation and thermal cycling. On the other hand, another focus lies in the electrochemical testing of short stacks in the cassette arrangement. The microstructure of the cells was characterized by optical microscopy, scanning electron microscopy (SEM), X-ray diffraction, and energy dispersive microanalysis (EDX) before and after operation. The cells and short stacks were electrochemically characterized mainly by long-term measurements (life cycle), by current-voltage measurements, and by impedance spectroscopy. In order to understand the nature of degradation mechanisms, the open-circuit voltages (OCV), the ohmic resistances, and the polarization resistances, during dynamic operation are compared and discussed. In order to distinguish between degradation effects due to the dynamic operation and usual stationary effects, these values are compared to values of noncycled cells. All of the cells investigated were able to withstand ten redox and ten thermal cycles without severe failure. Their redox- and thermal-cycling behavior are strongly dependent on their OCVs, which decrease during cycling. This proves that thermomechanical stresses in the electrolyte layer play a major role for the electrochemical performance of the cells during cycling. The improvement of the electrodes during the first 200h of operation and the ohmic resistance of the cells are not significantly influenced by the cycling. The first four-cell short stack with the cassette arrangement shows promising results with an OCV of ∼4V and an overall power of 92W at 800°C. The performances of the single cells are in the range of 180–220mW∕cm2. The differences in cell performance can be attributed to different polarization resistances of the cells in the cassettes, which might be caused by a nonuniform gas supply in the short stack.
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