진보된 정전분무코팅 기술에 의한 박막의 구조 제어 및 이의 연료전지로의 적용

Abstract

Since the Kyoto Protocol to the United Nations Framework Convention on Climate Change was firstly announced all over the world at 1992, many countries have invested on developing new-renewable energies which could be used instead of fossil fuels. Solid oxide fuel cells (SOFCs) are one of the possible renewable energies, and have been also actively studied by many research groups from 1990’s. The SOFCs are consisted with representative three kinds of ceramic layers which are an anode, an electrolyte, and a cathode. Each ceramic layer can be fabricated with various coating techniques such as sputtering, chemical vapor deposition, screen printing, spray coating, and so on. Among the various coating techniques, the spray coating method using pre-synthesized powder slurry has been widely used due to the fact that the powder technique is cost effective and easily controllable the process parameters than the technique using vacuum system. The electrostatic spray deposition (ESD) using electric force, in particular, is the most effective to fabricate thin ceramic films among the coating technique using powder because it has various advantages compared to other method. The effectiveness of this technique is already demonstrated in many previous researches. Therefore, in this study, the objective is to develop advanced electrostatic slurry spray deposition (ESSD) and is deposition of the thin films with various microstructures. The final goal is to apply this novel ESSD coating technique to fabrication of the SOFC single cell and analyze the electrochemical performance of the fabricated cell. More detail explain, in the first part, the advanced electrostatic spray deposition using slurry, so-called electrostatic slurry spray deposition (ESSD), was developed and the various microstructures of thin films fabricated by the ESSD were investigated in terms of dispersion stabilization of prepared slurry, influence of additives in the slurry, and by processing parameter control. In the second part, the developed microstructure control technique by the ESSD was applied to fabricate the SOFC single cell with a thin and dense electrolyte and a nano/micro-porous cathode. Also, the optimization of composite cathode fabricated by the ESSD was investigated, and the various cathodes with double layer and honeycomb structure were developed and applied to the single cell. Finally, the proton conducting-SOFC (PC-SOFC) was also fabricated and developed, then the electrochemical performance was evaluated in the third research part. In order to enhance dispersion stabilization of yttria-stabilized zirconia (YSZ) nanopower for the ESSD, poly vinybutyral (PVB) was used as a dispersion agent and the dispersion condition was examined as a function of the amount of PVB in the first research part. The agglomerated particle size showed the smallest with 3 wt.% of PVB concentration and the size was remarkably increase with increasing the PVB addition. It is demonstrated that this enhanced dispersion stabilization is by polymer adsorption of PVB onto the surface of the YSZ ceramic particles. This polymer additive affected to the microstructures of deposited thin films by the ESSD. Furthermore, as control of the processing parameters such as applied voltage and flow-rate of the slurry, we developed ceramic powder granulation technique. The size of granules showed 1–5 μm in diameter. In the second research part, the SSC-SDC composite cathodes were successfully fabricated using the electrostatic slurry spray deposition (ESSD). It is demonstrated that the ESSD system is the promising technique for fabrication of porous and composite films. The most promising performance was obtained from the SSC-SDC composite cathode with about 20 μm in thickness sintered at 1000 oC, which was composed of 60 wt.% of the SSC and 40 wt.% of the SDC. Its polarization resistance was 0.23 Ωcm2 and the activation energy was 62.9 kJ/mol, respectively. Also, not only the single layer but also multilayer porous cathode films regardless of nano-porous or micro-porous microstructure were fabricated. It was confirmed that the electrochemical performance could be improved by this double layered cathode. The SSC-SDC composite cathode with dual porosity honeycomb structure was successfully fabricated by the electrostatic slurry spray deposition (ESSD). The macro-pores distribute in the whole film with size range of 5 ~ 20 μm and micro-pores were formed throughout the skeleton with the size under 1 μm. The honeycomb cathode showed lower polarization resistance than the conventional cathode and the dominant rate-determining process shifted from the oxygen gas diffusion to the surface diffusion of adsorbed oxygen ions. In the last part of research, Nano crystalline samarium-doped barium cerates (BCS) have been synthesized via citric-nitrate method with various molar ratio of CA to MN and the nanopower was used to fabricate the PC-SOFC. The nanopowder with CA/MN molar ratio of 5 showed the largest crystalline size, smallest aggregated particle size and the best sintering behavior. Also, nano-sized BCS and Cu-doped BCS proton conducting electrolyte powders were synthesized by citric-nitrate method. The saturation temperature of linear shrinkage was decreased from over 1400 oC for the un-doped BCS to about 1300 oC for the small amount of Cu-doped BCS. Cu doping was absolutely increased the sinterability of BCS at 1400 oC by following the mechanism of liquid phase sintering. Also, 1 and 2 mol% Cu-doped BCS showed the similar conductivity measured at 600 oC with 0.0067, 0.0068 Scm-1, while the conductivity of 3 mol% Cu-doped BCS was slightly lower than that of the others. The activation energies for all samples were 0.41 ~ 0.42 eV similar to pure BCS. In order to get more enhancements in the performance, the BaCeO3-BCS bi-layer electrolyte based fuel cells were developed. The insertion of the BaCeO3 layer was effectively suppressed to the formation of BaSm2NiO5 impurity phase on the surface of the BCS electrolyte. Also, the maximum power density of the cell with bi-layer electrolyte was 416 mW/cm2 at 700 oC which was much higher than that of the single BCS electrolyte-based cell. Furthermore, no obvious cell degradation was observed for about 24h stability test at a constant current loading of 450 mA/cm2 at 700 oC. Also, the performance was not decreased even after one thermal cycling.