Abstract
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