Reversible Solid Oxide Cells: Cycling and Long-Term Durability of Air Electrodes

Abstract

Introduction Reversible solid oxide cell (r-SOC) enables both power generation and electrolysis, when the cell can be operated as a solid oxide fuel cell (SOFC) and as a solid oxide electrolysis cell (SOEC). Many studies are being conducted to improve their performance and durability (1,2).Lanthanum strontium cobalt ferrite (LSCF) is an air electrode material with a perovskite-type crystal structure which exhibits high electronic and ionic conductivity, oxygen diffusivity, and electrocatalytic activity (3). However, performance and durability of the cells with Sr-containing oxide air electrodes have to be carefully analyzed, as Sr ions tend to easily diffuse during sintering and in long-term operation, reacting with Zr ions from the solid electrolyte to form a highly resistance reaction layer such as SrZrO3 (4-6). While the performance and durability of LSCF-based air electrodes and the Sr diffusion in SOFC operation have been studied, there are still limited number of studies on the SOEC operation, and especially on the r-SOC operation. The diffusion of constituent ions such as Co and Fe may not be negligible in SOCs.Here in this study, we fabricate YSZ electrolyte-supported r-SOCs and conduct durability studies in both SOFC and SOEC modes. STEM-EDS observation of the air electrode materials is also made to investigate the durability and diffusion of various elements in the LSCF-based air electrode of r-SOCs. Experimental R-SOCs with yttria stabilized zirconia electrolytes (YSZ, 200 µm thick) were prepared. Ni-GDC co-impregnated fuel electrode was prepared, where a mixture of La0.1Sr0.9TiO3 (LST) and Gd0.1Ce0.9O2 (GDC) at a volume ratio of 50:50 was used as the porous electrode backbone, onto which Ni-containing solution of 1 µL was impregnated for Ni loading of 0.167 mg cm-2 (7). Gd0.1Ce0.9O2 (GDC) buffer layer was prepared between the electrolyte and the air electrode to prevent elemental diffusion (8). La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) was used for the air electrode. In addition, a Pt reference electrode was deposited onto the electrolyte on the fuel electrode side. The voltage measurement terminals of the electrochemical measurement setup were connected between the reference electrode and the air electrode to evaluate the air electrode potential. 50%-humidified hydrogen (100 ml min-1) was supplied to the fuel electrode, and air (150 ml min-1) was supplied to the air electrode. Negative current density means the value in the SOEC mode, while positive current density in the SOFC mode. Results and discussion As shown in Fig. 2 describing one cycle, current density was varied at 800°C for the 1000-cycle durability tests in the r-SOC operation. The range of current density was between -0.2 A cm-2 and 0.2 A cm-2. The air electrode potential at different current densities and the electrode impedance in every 100 cycles were measured. For the 1000-hour electrolysis durability test in the SOEC mode, current density of -0.2 A cm-2 was applied for 1000 hours at 800°C. The air electrode potential and the impedance in every 100 hours were measured (9). It has been found that a gradual degradation of the r-SOCs associated with an increase in air electrode potential in the SOEC mode and a decrease in the potential in the SOFC mode could be distinguished. However, such degradation tended to be stabilized with the number of cycles.STEM-EDS observations were performed to analyze the elemental distribution around the air electrode and the electrolyte in the cell. It was found that such elemental diffusion could occur during sintering and durability experiments.AcknowledgmentsA part of this study was supported by “Research and Development Program for Promoting Innovative Clean Energy Technologies Through International Collaboration” of the New Energy and Industrial Technology Development Organization (NEDO) (Project No. JPNP20005). Collaborative support by Prof. H. L. Tuller, and Prof. B. Yildiz at Massachusetts Institute of Technology (MIT) for their continuous support is gratefully acknowledged. Figure 1