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).
Understanding the phenomena which occur inside a solid oxide cell in operation is important in the development of more efficient devices. However, it is difficult to experimentally visualize the distribution of the internal power generation state due to the very high temperature operation. In this study, the performance of a reversible solid oxide cell (r-SOC) was simulated to visualize current-voltage (I-V) characteristics and internal temperature distribution. The validity of the model was verified by comparing with the I-V characteristics and temperature distribution experimentally measured by an actual cell. The establishment of this technique will eventually enable the simulation of cell stacks and systems.
Reversible Solid Oxide Cells (r-SOCs) are an attractive electrochemical energy conversion technology that can act as both a Solid Oxide Fuel Cell (SOFC) for power generation, and a Solid Oxide Electrolysis Cell (SOEC) for steam electrolysis. Unfortunately, Ni-zirconia cermet, which is widely used in SOFCs, has the problem that the electronically conductive phases, Ni, agglomerates due to redox reactions and breaks the conductive path, resulting in performance degradation. In this study, we fabricated an alternative fuel electrode with a stable conductive structure using both an ionic conductor Ce0.9Gd0.1O2 (GDC) and an electronic conductor Sr0.9La0.1TiO3 (LST), and we co-impregnate Ni and GDC to the fuel electrode using only Ni as an electrocatalyst. The durability of the conventional fuel electrode and our alternative fuel electrode was compared and evaluated, and the potential of the alternative fuel electrode material was demonstrated.
Different fuel electrodes for reversible solid oxide cells (r-SOC) are investigated with the aim of improving performance in both solid oxide electrolysis cell (SOEC) and solid oxide fuel cell (SOFC) modes, and durability in reversible operation mode. Electrodes based on gadolinium-doped ceria (GDC) as a mixed ionic electronic conductor, and lanthanum-doped strontium titanate (LST) as an electronic conductor are selected. The current-voltage characteristics of r-SOC single cells, and their cycling durability up to 1000 cycles are evaluated. LST-GDC co-impregnated with Ni and GDC prove to be highly durable in reversible operation, as a suitable fuel electrode material for r-SOCs.
Introduction In recent years, environmentally-compatible renewable energy becomes increasingly important as major power resources. However, there exist several issues to overcome including energy storage due to the fluctuating nature. Solid oxide reversible cells (SORCs), able to act as both solid oxide fuel cells (SOFCs) and solid oxide electrolyzer cells (SOECs), may enable power generation in an SOFC mode and hydrogen production in an SOEC mode [1-3]. Therefore SORCs are of scientific and technological interest towards low-carbon and carbon-free energy society. However, since SORC operation may be associated with redox cycling of their fuel electrodes, the commonly-used fuel electrode material, Ni-zirconia cermet, has a difficulty in stability against such redox cycling. For SOFCs, alternative catalyst-impregnated fuel electrodes [3-5] are demonstrated to be applicable with long-term durability under high water vapor pressure and against redox cycling. Here, the aim of this study is to investigate the electrochemical properties of such redox-tolerant fuel electrode materials for SOECs and SORCs. Experimental In this study, the electrochemical characteristics of three types of cells were evaluated. First, for comparison, (i) conventional Ni-ScSZ cermet fuel electrode was used as a reference electrode material. As alternative fuel electrodes, (ii) Ni-GDC co-impregnated fuel electrode cell (Ni: 0.167 mg cm-2) and (iii) Rh-GDC co-impregnated fuel electrode (Rh: 0.178 mg cm-2) with electron-conducting backbone (porous composite of La-Sr-Ti oxide (LST) and Gd-doped ceria (GDC)) were applied, for which catalytic metals (Ni or Rh) were co-impregnated with additional GDC, respectively [5]. Electrochemical impedance spectroscopy (EIS, Solatron) was applied to separate and evaluate ohmic and non-ohmic overvoltages. The materials stability against high water vapor pressure was evaluated by durability tests up to 80 h in an SOEC mode, where 80%-humidified hydrogen was supplied to the fuel electrode with the applied current density of -1.2 A cm-2. The durability against reversible SOEC / SOFC cycling was evaluated by varying current density and by switching the current between positive (1.2 A cm-2) and negative (-1.2 A cm-2) at 50%-humidified hydrogen supplied to the fuel electrodes. Results and discussion Figure 1 shows the fuel electrode voltage, measured against the Pt reference electrode on the air-electrode side, of two types of the co-impregnated cells kept at a constant current density during the 80h durability test in the SOEC mode. The co-impregnated cells (ii) and (iii) exhibited a stable fuel electrode voltage. Therefore, performance deterioration was almost negligible. This is probably because the cells of (ii) and (iii) have the redox-stable LST-GDC electrode backbone, which has sufficient durability under high water vapor pressure. Figure 2 shows the fuel electrode voltage measured in the reversible SOEC / SOFC cycling tests. An increase in fuel electrode voltage in the SOEC mode (upper-side) and a decrease in fuel electrode voltage in the SOFC mode (lower-side) correspond to certain performance degradation. However, the increase in fuel electrode voltage was much smaller for the co-impregnated cells of (ii) and (iii), compared to that for the cell (i) using the Ni-cermet electrode. These results reveal that, the co-impregnated fuel electrodes stable in the SOFC operation can also exhibit high durability in the SOEC operation at high water vapor pressure, and thus sufficient durability in the reversible SOEC / SOFC operation. These co-impregnated fuel electrodes are therefore promising for SOFCs, SOECs, and SORCs, with sufficient durability under high water vapor pressure and in reversible operation. References Q. Minh and M. B. Mogensen, Electrochem. Soc. Interface, 22, 55 (2013). N. Q. Minh, MRS Bulletin, 44 (9), 682 (2019). T. S. Irvine, D. Neagu, M. C. Verbraeken, C. Chatzichristodoulou, C. Graves, M.B. Mogensen, Nature Energy, 1, 15014 (2016). Futamura, Y. Tachikawa, J. Matsuda, S. M. Lyth, Y, Shiratori, S. Taniguchi, and K. Sasaki, J. Electrochem. Soc., 164 (10), F3055 (2017). 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). P. Jiang, Mater. Sci. Eng. A, 418, 199 (2006). 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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