Degradation of SOFCs By Phosphorus Impuriteis: Impedance Spectroscopy and Microstructural Analysis

Abstract

Introduction SOFC is considered as a promising power generation system due to high electric efficiency and fuel flexibility. But, coal gas, for example, contains various impurities. In this study, phosphorus poisoning effect to Ni-ScSZ cermet anodes is analyzed by measuring Electrochemical Impedance Spectroscopy (EIS). Spectroscopy data are analyzed by the distribution of relaxation times (DRT) method1to 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. Figure 1 describes the experimental setup. P2O5 container is locating at the fuel upstream to mix phosphorus impurity which is evaporated as PH3. PH3 concentration was specified as 2ppm by a TG-DTA measurement. The fuel consisted of 67% H2 and 33% H2O. Cell temperature was 800oC. Table 1 shows EIS condition. EIS data analyzed by DRT enable identification of electrode processes. FESEM-EDX was applied to investigate changes in the anode microstructure and composition by the cell performance tests. Results and discussion Figure 2 shows EIS spectra measured after 1h, 50h, and 100h. Figure 3 shows the DRT result of the Fig. 1 data. DRT peak becomes gradually visible due to the increase in impedance. DRT peak around 10-1~10-2 sec may originate from the diffusion of H2 gas2. Figure 3 shows SEM-WDX mapping of nickel and phosphorus. Ni particle is condensed and Ni-P compounds are formed on the anode surface. Therefore, cell performance degradation by inhibiting gas diffusion and forming Ni-P compound in the phosphorus poisoning of Ni-based anodes. References (1) A. Leonide et al., Journal of The Electrochemical Society, 155(1), B36-B41 (2008). (2) T.H. Wan et al., Electrochimica Acta, 184, 483-499 (2015). Figure 1