Reversible Solid Oxide Cells: Silica Poisoning of Ni-GDC Fuel Electrode during Electrolysis Operation

１． Introduction Water (steam) electrolysis using electricity derived from renewable energy enables green hydrogen production to realize a hydrogen society. Steam electrolysis is the most efficient hydrogen production technology by high temperature operation, and reversible Solid Oxide Cells (r-SOCs) can switch between steam electrolysis (SOEC) and power generation (SOFC) in a reversible manner, for energy management with renewable electricity. The durability of r-SOCs during SOEC operation is an important issue for their practical application. One of the reasons for this is that silica (SiO 2 ), abundant in the system components, sublimates as hydroxide and re-deposits on the electrode reaction sites and inhibits the electrode reaction. This silica poisoning may be associated with the following equilibrium reaction. SiO 2 (s)+2H 2 O(g)⇔Si(OH) 4 (g) (1) Near the silica source, the equilibrium equation Eq. (1) shifts to the right with water vapor in the fuel gas, and silica migrates to the gas phase as Si(OH) 4 . It is considered that the Si-containing species migrate to the gas phase mainly as Si(OH) 4 [1] . During SOEC operation, water vapor is consumed by steam electrolysis, which shifts the equilibrium equation Eq. (1) to the left and re-deposits as silica at the electrode reaction sites. In the commonly-used Ni-YSZ cermet fuel electrode, it is reported that silica precipitates from the electrolyte side [2] . Recently, Ni-GDC is attracted attention as an alternative electrode material because it exhibits higher performance than Ni-YSZ due to the mixed conductivity and higher ionic conductivity of GDC even at lower temperatures [3–5] . It is also reported that silica poisoning occurs on the Ni-GDC cermet fuel electrode after long-term SOEC operation. However, silica precipitates from the electrode surface side. Here in this study, we evaluate silica poisoning and degradation phenomena of r-SOC with Ni-GDC cermet fuel electrodes, especially during SOEC operation, by systematically varying the operating conditions and time to evaluate silica poisoning mechanisms. ２． Experimental In this study, a standard cell was fabricated using Ni-GDC cermet fuel electrode, LSCF/GDC for the air electrode, and YSZ for the supporting electrolyte plate. In the electrochemical testing setup, Pyrex glass was used as the gas sealing. The following electrolysis conditions were used as the standard conditions: the operating temperature was set at 800°C; 50% -humidified H 2 was supplied to the fuel electrode; and a constant current with a current density of -0.2 A/cm² was applied during the test. The influence of silica poisoning on each operating condition was verified by systematically varying the operating temperature, ratio of H 2 O to H 2 in fuel gas, and current density. The operating time was set to be 100 h as the standard condition, and silica deposition over time was evaluated by varying the operating time in the range of 20 to 1000 h. SEM-EDS and STEM-EDS were applied to observe the microstructure and analyze the trend of silica deposition. In addition, thermochemical calculations were made using software, HSC Chemistry, to evaluate Si(OH) 4 concentration in the fuel gas, depending on electrolysis conditions. ３． Results and discussion Thermochemical calculations indicate that the concentration of Si(OH) 4 increased with increasing temperature and H 2 O concentration. Since many of the constituent materials contain silica, it is difficult to identify the poisoning source, but the contribution from the glass seal, which locates near the cell with the highest temperature in the setup, is considered to be most probable. Regarding silica deposition over time, it was confirmed that silica precipitates from the surface side of the electrode and spreads into the interior of the porous electrode with time. Figure 1 shows the results of elemental analysis of the fuel electrode surface observed by STEM-EDS. The precipitated silica formed a complex oxide with GDC and existed as a Ce-Si-O phase. On the electrode surface side, this Ce-Si-O phase is formed on the GDC and Ni surfaces, and it was found that this phase becomes thicker with time. In contrast, on the electrolyte side, precipitation at the three-phase boundary (TPB) of Ni, GDC, and pore was more noticeable than on the GDC surface. At 800℃, 50%H 2 O-50%H 2 , -0.2 A/cm 2 , a significant performance degradation in the SOEC mode was observed compared to the SOFC mode after 1000 h, which was attributed to silica precipitation. In addition, silica poisoning was found to be potentially accelerated by higher temperature operation, lower H 2 O concentration, and higher current density. It should be noted that the silica precipitation was also confirmed in the test without current application (800℃, 50%H 2 O-50%H 2 , 0 A/cm 2 (OCV), up to 100 h), that is, without electrochemical reactions, as shown in Figure 2. This result indicates that the mechanism of silica poisoning of Ni-GDC fuel electrode includes not only the equilibrium shift shown in Eq. (1), but also a direct chemical reaction between GDC and Si(OH) 4 . Acknowledgement This work was supported by Japan Science and Technology Agency (JST) as part of Adopting Sustainable Partnerships for Innovative Research Ecosystem (ASPIRE), Grant Number JPMJAP2307. We would like to express our gratitude for their invaluable contribution. References N. Jacobson, D. Myers, E. Opila, and E. Copland, J . Phys . Chem . Solids , 66 , 471 (2005). A. Hauch, S. H. Jensen, J. 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