Solid oxide electrolysis cells (SOECs), which enable steam electrolysis by operating solid oxide fuel cells (SOFCs) in reverse, are capable of highly efficient hydrogen production. While SOFCs and SOECs are using similar materials and cell structures, electrode reactions in SOEC operation have to be investigated in more detail. In this study, we compared and discussed conventional cells and cells fabricated by an impregnation method by characterizing the electrode reactions through measurements of electrochemical impedance spectra (EIS) and subsequent analysis of distribution of relaxation times (DRT).
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
Introduction Solid oxide electrolysis cell (SOEC), which enables steam electrolysis by reversely operating solid oxide fuel cell (SOFC), is capable of highly efficient hydrogen production. While SOFC and SOEC are using similar materials and cell structures, electrode reactions in SOEC operation have to be studied in more details. Here in this study, we aim to establish experimental evaluation method for the development of solid oxide cells with high performance and durability by characterizing the electrode reactions through electrochemical impedance measurements and subsequent analysis of distribution of relaxation times (DRT). Such analysis is made to reveal similarities and differences of electrode reactions between SOFC and SOEC. Experimental Three types of cells were prepared with different fuel electrodes: Ni-ScSZ cermet fuel electrode; Ni-GDC co-impregnated fuel electrode; and Rh-GDC co-impregnated fuel electrode. Ni-cermet is widely used as the fuel electrode for SOFC. (Ni, Rh)-GDC co-impregnated fuel electrodes, made by impregnating the catalysts (Ni, Rh) and the Gd-doped ceria on the composite of La-Sr-Ti oxide (LST) and GDC, exhibit high durability at high water vapor partial pressure and against redox cycling (1,2). The electrochemical impedance under the open-circuit condition and at given current densities was measured in the frequency range between 0.1 Hz and 1 MHz with a signal amplitude of 0.02 A/cm2. Negative bias current density means a measurement in the SOEC mode, while positive one means a measurement in the SOFC mode. Measurements were mainly performed for the cells with the Ni-ScSZ cermet fuel electrode at low and high current density up to ± 1.2 A/cm2, in every 0.2 A/cm2. In addition, the cells with (Ni, Rh)-GDC co-impregnated fuel electrodes were also characterized up to high current density. Impedance measurements were made by using an impedance analyzer (Solartron 1255WB). 50%-humidified hydrogen was supplied to the fuel electrode. The operating temperature was 800℃. Results and discussion Figure 1 shows typical DRT peaks of the cell using the Ni-ScSZ cermet fuel electrode. A few DRT peaks are distinguished, which can be used to analyze individual electrode processes and their dependencies. Figure 2 shows the polarization resistance of the cell with the Ni-GDC and Rh-GDC co-impregnated fuel electrodes. Whilst the cell with the Ni-cermet fuel electrode exhibited an increase in polarization resistance in the SOEC mode, the Ni-GDC co-impregnated fuel electrode cell exhibited identical polarization resistance within a wide current density region. A similar trend was also found for the Rh-GDC co-impregnated fuel electrode cell. It was suggested that electrode reactions in the SOEC mode could be more complicated, compared to those in the SOFC mode. Various factors could affect polarization resistance, including the difference in the microstructure of these fuel electrodes, and catalytic activity of GDC. Detailed DRT analysis of the polarization resistance for various fuel electrodes will be presented. References Futamura, A. Muramoto, Y. Tachikawa, J. Matsuda, S. M. Lyth, Y, Shiratori, S. Taniguchi, and K. Sasaki, International J. Hydrogen Energy, 44 (16), 8502 (2019). S. P. Jiang, Mater. Sci. Eng. A, 418, 199 (2006). Figure 1
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