SOFC Analysis through the Distribution of Relaxation Times for Various Preparation and Operation Parameters

Abstract

Introduction In order to extend the lifetime of SOFC systems, it is important to separate and analyze the factors affecting their performance and durability quantitatively. The reactions at both electrodes of fuel cells in power generation are rather complicated. Electrode resistances consist of several factors. Here in this study, we systematically evaluate their dependence on fuel composition, operating temperature, and constituent materials, by measuring various correlations through impedance measurements and the distribution of relaxation times (DRT) analysis [1-4]. The aim of this study is to systematically analyze electrode reaction processes of SOFCs for various preparation and operation parameters. Experimental First, electrochemical performance was measured while lowering the operating temperature by 50°C but keeping the oxygen concentration for the cathode and the hydrogen concentration for the anode constant for various cells. After that, the performance was measured by varying oxygen and hydrogen concentrations with the operating temperature fixed at 800°C. Impedance measurement was performed, and each resistance component was separated by the DRT analysis. In the previous DRT studies [1-4], it is reported that the resistances caused by the transport of charges such as ions appear in the high frequency region, while the resistances caused by the interfacial electrode reactions, gas adsorption / dissociation, gas diffusion, etc. appear in the middle and low frequency regions. The materials dependency was also evaluated by using five types of cells with different materials denoted as Anode|Electrolyte|Cathode: Ni-ScSZ|ScSZ|LSM, Ni-ScSZ|YSZ|LSM, Ni-YSZ|ScSZ|LSM, Ni-YSZ|YSZ|LSM, and Ni-ScSZ|ScSZ|GDC/LSCF. The electrode thickness dependency was measured by using the cells with 40 µm, 60 µm, and 80 µm thick anode and the cells with 40 µm and 60 µm thick cathode. The electrode sintering temperature dependency was measured by using the cells with the anodes sintered at 1250°C, 1300°C, and 1350°C, and the cells with the cathodes sintered at 1150°C, 1200°C, and 1250°C. The impedance measurements and DRT analyses were performed for analyzing the temperature dependence and fuel and air concentration dependence of these cells. Results and Discussion For the cells studied, DRT peaks for the anodes appeared at around 10-0.5 Hz, 102 Hz, and 103 Hz, and those for the cathodes appeared at around 101 Hz, 103.5 Hz, and 104.5 Hz. Figures 1 and 2 show the DRT spectra obtained from the anode impedance measurements for various preparation and operation parameters. To mention a few, regarding the DRT peak at around 103 Hz, the resistance is related to the presence of ScSZ (see Fig. 1(a)), dependent on anode thickness (Fig. 1(b)), but independent of H2 concentration (Fig. 2(b)). DRT analysis of SOFCs under various operational conditions and with different cell materials was systematically conducted for model cells, and the relationship between the reaction processes and peak frequency was also investigated. Various DRT peaks related to different electrode processes will be systematically considered and classilfied. References C. Graves, S. D. Ebbesen, and M. Mogensen , Solid State Ionics, 192, 398 (2011). T. H. Wana, M. Saccoccioa, C. Chena, and F. Ciuccia, Electrochim. Acta, 184, 483 (2015). A. Leonide, V. Sonn, A. Weber, and E. Ivers-Tiffée, J. Electrochem. Soc., 155 (1), B36 (2007). H. Sumi, T. Yamaguchi, K. Hamamoto, T. Suzuki, Y. Fujishiro, T. Matsui, and K. Eguchi, Electrochim. Acta, 67, 159 (2012). Acknowledgment This study was partially supported by the Center of Innovation (COI) program (JPMJCE1318) of the Japan Science and Technology Agency. Figure 1