Exchange Current Density of Solid Oxide Electrolysis Cell Electrodes

Abstract

Introduction Numerical simulation is a powerful technique in evaluating solid oxide fuel cell (SOFC) performance. Such method enables to visualize SOFC performance e.g. in using new materials. It is also desirable to develop such method that visualizes internal phenomena in solid oxide electrolysis cell (SOEC). While various parameters are important in simulating phenomena in SOEC stacks, we focus on exchange current density (1,2). Here, exchange current density is experimentally measured and evaluated using single SOEC / SOFC reversible cells. Furthermore, the exchange current density for different electrode materials in both SOEC and SOFC operation modes is examined. Exchange current density is defined as the current exchanged at the electrode/electrolyte interfaces in both directions in equilibrium and thus , so that an identical value for a given electrode has to be obtained in both SOFC and SOEC modes. However, as such values are practically measured by applying certain current in opposite directions, apparent exchange current density measured could be somehow different due to various irreversible processes at the electrode/electrolyte interfaces. In this study, such apparent exchange current density values are obtained and discussed. Experimental In this study, we evaluated the exchange current densities of solid oxide cells with three types of fuel electrodes (A-1, 2, 3) and two types of air electrodes (B-1, 2). The exchange current densities were measured in the SOFC and SOEC modes using following materials: (A-1) Ni-ScSZ (NiO-ScSZ (ScSZ: 10mol%Sc2O3-1mol%CeO2-89mol%ZrO2)), (A-2) Rh-GDC (La0.1Sr0.9TiO3, Gd0.1Ce0.9O2, volume ratio 50:50 (LST-GDC), Rh 0.178 mg/cm2 supported on LST-GDC) co-impregnated material, and (A-3) Ni-GDC (Ni 0.167 mg/cm2 supported on LST-GDC) co-impregnated material for the fuel electrode; (B-1) LSCF/GDC ((La0.6 Sr0.4)(Co0.2 Fe0.8)O3/GDC), and (B-2) LSM ((La0.8Sr0.2)0.98MnO3) for the air electrode. The exchange current density was derived using the Butler-Volmer-type equation. The exchange current density obtained was fitted by the commonly used phenomenological formula of the exchange current density. We compared the obtained parameters and the dependencies on gas partial pressure. Results and Discussion Here, the differences in exchange current density at the fuel electrode are examined. Figure 1 shows the exchange current density as a function of humidity in the fuel, and fitted curves. The fitted curves of exchange current densities are almost similar in the SOFC and SOEC modes. However, the maximum value measured in the SOFC mode shown in Fig. 1 (b) was somehow higher than that measured in the SOEC mode shown in Fig. 1 (a). The difference in using the Ni-ScSZ fuel electrode was more remarkable than that using the Rh-GDC co-impregnated fuel electrode shown in Figs. 1 (c) and (d). It may be possible that the apparent exchange current density at the fuel electrode could include influences of irreversible processes, associated with steam electrolysis. The dependence of exchange current density on gas partial pressure (3) will be examined to discuss electrode processes in steam electrolysis. The exchange current density obtained in this study enables various simulations for SOECs, besides SOFCs. References T. Yonekura, Y. Tachikawa, T. Yoshizumi, Y. Shiratori, K. Ito, and K. Sasaki, ECS Trans., 35 (1), 1007-1014 (2011).K. Takino, Y. Tachikawa, K. Mori, S. M. Lyth, Y. Shiratori, S. Taniguchi, and K. Sasaki, Int. J. Hydrogen Energy, 45 (11), 6912-6925 (2020).T. Hosoi, T. Yonekura, K. Sunada, and K. Sasaki, J Electrochem Soc., 162 (1), F136-F152 (2015). Figure 1