Charakterisierung und Optimierung der Grenzfläche Elektrolyt/Kathode in metallgestützten Festelektrolyt-Brennstoffzellen

Metal-supported solid oxide fuel cells (MSC) offer various advantages compared to full ceramic cells. Low-cost materials and high mechanical ruggedness make MSCs the fuel cell of choice for mobile applications, such as auxiliary power units for heavy duty vehicles or range extender modules for battery electric passenger cars. However, MSC-specific degradation phenomena occur, as the processing has to be adapted to the porous metal substrate. For example, the La$_{0,58}$Sr$_{0,4}$Co$_{0,2}$Fe$_{0,8}$O$_{3-\delta}$(LSCF) cathode of the state-of-the-art Plansee MSC is in situ activated at 850 °C,which differs considerably from the established cathode sintering at 1040 °C for anode supported cells. As a result, the cathode adherence on the Ce$_{0.8}$Gd$_{0.2}$O$_{2-\delta}$ diffusion barrier and long-term stability during operation are insufficient. The aim of the present work is to increase the long-term stability of the LSCF cathode by improving the adherence strength of the cathode layer. Moreover, an increase of the cell performance is of interest from an industrial point of view, in order to lower system weight and volume. Three approaches were considered: i) development of an ex situ sintering procedure for complete MSCs under controlled atmosphere in combination with an increased sintering temperature; ii) improvement of the cathode adherence after in situ activation by optimizing the activation conditions or cathode raw material; and iii) implementation of alternative cathode materials like La$_{0,5}$8Sr$_{0,4}$CoO$_{3-\delta}$ (LSC) in order to increase cell performance. Increased sintering activity and adherence strength were observed by dilatometry and adhesive tape test, when increasing the sintering temperature to T ≥ 950 °C. Ex situ sintering of MSCs under argon atmosphere caused phase decomposition of the cathode material. The reversibility of this phase decomposition was confirmed by ambient temperature as well as high-temperature XRD. Full re-oxidation to single phase perovskite takes place at T ≥ 750 °C during the heat-up and sealing procedure prior to cell operation, without damaging the cathode layer. Cells utilizing Ni/YSZ anode and LSCF cathode sintered ex situ delivered improved cell performance of 1.4 A/cm$^{2}$ at 785 °C and 0.7 V. 1500 h of continuous operation (300 mA/cm²,700 °C), without any degradation, confirmed the long-term stability. Implementationof LSC cathodes resulted in increased cell performance. 700 h of operation at 300 mA/cm$^{2}$ and 700 °C did not reveal any degradation of a cell consisting of Ni/YSZanode and LSC cathode activated in situ at 850 °C. Promising low-temperature performance of 0.8 A/cm$^{2}$ at 600 °C and 0.7 V was achieved by utilizing LSC cathodeon cells with Ni/GDC anode. As a further development, LSC/GDC dual-phase cathodes were applied using the ex situ sintering approach. This cathode type not only revealed improved layer stability during storage but also provided high electrochemical performance of 1.3 A/cm$^{2}$ at 750 °C and 0.7 V, despite nonoptimized microstructure. The overarching conclusion is that cathodes sintered ex situ provide significantly improved long-term stability as well as high electrochemical performance during MSC operation. Optimization of the microstructure of dual-phase cathodes offers further potential to improve cell performance.