Ti-Doped LSCF for Long-Term Stability Improvement of Solid Oxide Cells Operating Under SOEC Conditions

High Temperature Steam Water Electrolysis (HTSWE) is a promising way to produce low cost green hydrogen for industrial applications. However, significant efforts are still needed to reduce operating temperature and to improve initial performances and durability. Among the materials for the Solid Oxide Cell (SOC) conception, Lanthanum Strontium Cobalt Ferrite (LSCF) perovskite is often used as oxygen electrode due to its good mixed ionic and electronic conductivity (MIEC) properties at intermediate temperature around 800 °C (1). However, strontium diffusion and its high reactivity with the Yttria-Stabilized Zirconia (YSZ) electrolyte leads to the formation of a mixed oxide SrZrO 3 which significantly increases the cell overall resistance. SrZrO 3 can appear as soon as the LSCF layer is sintered, or during cell operation. Then, this directly affects the initial performances and long-term durability of the cells. The classic strategy to limit the diffusion and reactivity of strontium is to deposit a barrier layer of GDC (gadolinium-doped ceria) on YSZ but its effect is not totally efficient because the GDC densification is uneasy. In order to improve the long-term stability, a complementary way could be the introduction of cations with appropriate valence and size to stabilize the perovskite structure. Indeed, it has been recently shown that titanium introduced in Sr 2 Fe 1.4 Mo 0.6 O 6- δ (SFM) structure in B site enhances the material stability but decreases the conductivity (2). So, in this work, titanium was introduced as a substitution for Co at the B site of the LSCF structure. The powder was synthesized by an organic gel-assisted citrate process (3) because it allows an easy and accurate control of the perovskite composition, i.e. La 0.60 Sr 0.40 Co 0.2-x Ti x Fe 0.80 O 3- δ , where 0<x<0.2 (called LSCF-Ti x ). The crystal structure and physical properties of the powders were studied as a function of Ti content. Oxygen diffusion coefficient (D*) and surface exchange coefficient (k) were also measured by O -18 /O -16 Isotope Exchange Depth Profile technique (IEDP) (4). The thermal and chemical stability of LSCF-Ti x layers screen-printed on a substrate composed of YSZ electrolyte and a GDC barrier layer was also evaluated. After thermal aging at 800 °C for 1, 3 and 6 weeks, cross sections of Scanning Electronic Microscopy (SEM) observations and Energy Dispersive Spectroscopy (EDS) analysis allow the observation of the strontium and cobalt diffusion or agglomeration. Following this previous study, a complete cell of 9 cm² was prepared with a powder presenting the best balance between physical properties and thermal stability. The initial performances of the SOFC and SOEC were measured at 700, 750 and 800 °C at various steam flow rates followed by an aging study for 1000 hours carried out under SOEC conditions at 750°C, 1.0 A/cm² and with a steam conversion of 64 % (see figure 2). After a transient period up to the first 750 hours, a very low degradation rate about 3%/1000h was measured. SEM observation and EDX analysis of cells before and after aging confirm the effect of Ti on LSCF stability. The results will be detailed in the presentation. Acknowledgements: This work benefited from state support managed by the Agence Nationale de la Recherche under the France 2030 programme, referenced ANR-22-PEHY-0008 and is carried out in collaboration with GENVIA. References Jiang, S. P., Development of lanthanum strontium cobalt ferrite perovskite electrodes of solid oxide fuel cells – A review. Int. J. Hydrogen Energy, 44 (2019), 7448 7493. Zheng K., Ti-doped Sr 2 Fe 4-x Ti x Mo 0.6 O 6- δ double perovskites with improved stability as anode materials for Solid Oxide Fuel CellsMaterials Research Bulletin 128 (2020) 11087. M. Neacsa, K. Abbassi, H. Guesmi, P. L. Coddet, J. Vulliet, M. El Amrani, S. Dealmeida-Didry, S. Roger, V. Ta Phuoc, R. Sopracase, F. Gervais and C. Autret-Lambert, Nb and Cu co-doped (La,Sr)(Co,Fe)O 3 : a stable electrode for solid oxide cells, RSC Adv., 2021, 11, 10479–10488. Guillodo, J.M. Bassat, J. Fouletier, L. Dessemond, P. Del Gallo, Oxygen diffusion coefficient and oxygen exchange coefficient of BIMEVOX.10 (ME=Cu, Co) ceramic membranes, Solid State Ionics 164 (2003) 87 – 96. Figure 1 Figure 2