Degradation of SOFCs by Various Impurities: Impedance Spectroscopy and Microstructural Analysis

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Introduction SOFC is considered as a promising power generation system due to high electric efficiency and fuel flexibility. However, coal gas, for example, contains various impurities. In this study, siloxane, poisoning effect of siloxane to Ni-ScSZ cermet anodes is analyzed by applying Electrochemical Impedance Spectroscopy (EIS). Spectroscopy data are analyzed by the distribution of relaxation times (DRT) method[1] to investigate details of electrode resistance. Furthermore, microstructural analysis was made by electron microscopy. Experimental Electrolyte-supported cells with ScSZ (10 mol% Sc2O3 - 1 mol% CeO2-89 mol% ZrO2) plate (20 mmφ×0.2 mmt) were used in this study. Mixture of 56 wt.% NiO and 44 wt.% ScSZ was used for the anode and was sintered at 1300 oC for 3 h. Mixture of (La0.8Sr0.2)0.98MnO3 (LSM) and ScSZ with a weight ratio of 1:1 was used for the cathode. Electrode area was 8×8 mm2 and Pt mesh was used as the current collector. Figures 1 and 2 describe the experimental setup. Siloxane (decamethylcyclopentasiloxane) was supplied to the fuel cell using the bubbler setup. The fuel consisted of 97% H2 and 3% H2O. Cell temperature was 800oC. Table 1 shows the EIS conditions. EIS data analyzed by DRT enable the identification of electrode processes. FESEM-EDX was applied to investigate changes in the anode microstructure and composition after the cell performance tests. Results and discussion Figure 3 shows EIS spectra measured after 0h and 100h. Figure 4 shows the DRT result of the Fig. 3 data. Impedance semicircles, especially at the lower frequency, DRT peaks became larger with time. The DRT peak with relaxation time around 102~103 sec at 0h became wider, but the DRT peak around the 10-1~100 sec did not change. DRT peak around 10-1~100 sec may originate from the diffusion of H2 gas, that around 102~103 sec for charge transfer reaction and ionic transport[2][3]. Figure 5 shows EDX mapping of zirconium, silicon and oxygen. Ni particle on the anode surface is covered by silica. This reaction is considered as [4]: [(CH3)2SiO]5+25H2O→5Si(OH)4(g)+10CO+30H2 (1)Si(OH)4(g)→SiO2(s)+2H2O(2) Silica was formed under SOFC operating conditions at high temperature and high water vapour partial pressure. Therefore, cell performance degraded by inhibiting gas diffusion and covering by Ni particles with silica in the siloxane poisoning of Ni-based anodes.