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Solid oxide cells (SOCs) would be economically more favorable with lower operating temperature. Currently the target temperature is between 550 °C and 750 °C. Such low operation temperatures would reduce thermally activated degradation phenomena and would allow the use of cheaper materials. However, reduction of the operating temperature increases the ohmic and electrode polarization losses of the cells. These losses can be reduced by using electrode materials with a sufficient high activity at lower temperature. The most promising substitutes for the oxygen electrode material are mixed ionic electronic conductors (MIEC) like La1-xSrxCoO3- d , LaxSrx-1CoyFey-1O3- d and LaCo1-xNixO3- d . The problem with these MIEC cathode materials is the extensive chemical reaction with the YSZ (yttria stabilized zirconia) electrolyte at elevated temperatures and the formation of electrical insulating phases of La2Zr2O7 and/or SrZrO3. This problem can be partly overcome by using an oxygen electrode consisting of a porous backbone infiltrated with nano-particles of the electro-catalyst. The advantage of this concept is that the number of sintering steps in the manufacturing process of the SOCs is reduced and that the high sintering temperature of the oxygen electrode is significantly reduced (to ca. 350 °C). Furthermore, the infiltrated electro-catalysts typically have a high surface area and consequently a high catalytic activity. The important parameters in the infiltration process are the uniformity of the electro-catalyst, its microstructure and crystallinity, which can be drastically improved by controlling the wettability of backbone. However, a nano-sized infiltrated electro-catalyst will have higher reactivity at lower temperature and even long–term operation at low temperatures can lead to formation of insulating phases. Therefore, it is crucial to study and develop efficient barrier layers between the YSZ electrolyte and the electro-catalyst. In this study the development of a homogenous CGO barrier layer by infiltration to prevent direct contact between the YSZ and the infiltrated oxygen electrode material will be presented. CGO precursor solutions containing different surfactants were studied and it was shown that the addition of surfactants strongly influences the wetting angle and uniformity of nanoparticles coating. The concept of CGO barrier layer infiltration was indirectly studied by electrochemical impedance spectroscopy on symmetrical cells with infiltrated electro-catalysts, where a variation in polarization resistance was observed and was correlated with a growth of catalyst particles, phase composition, uniformity of CGO barrier layer and formation of highly resistive phases at high temperatures. Compared to sample without the CGO barrier layer, the total resistivity of the symmetric cell was reduced for 60 %. The optimized CGO barrier layer infiltration process on the symmetric cells was also transferred to anode supported SOCs, where the oxygen electrode contribution to the overall cell resistivity and its time dependence were studied. Comprehensive results of infiltration of the symmetric and SOC with the CGO barrier layers will be presented at the conference. |
