Solid Oxide Reversible Cells Using Redox-Tolerant Fuel Electrodes

Abstract

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