LSCF-CGO Screen-Printed Gradient as an Oxygen Electrode Scaffold for Rare Earth Oxides (REO) Infiltration

Surface segregation represents critical issues to the long-term stability of oxygen electrodes in Solid Oxide Electrolysis Cells (SOECs) devices. La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3- δ stands as the state-of-the-art oxygen electrode material due to its advantageous properties with its electronic conductivity of ~10 2 S.cm⁻¹ and ionic conductivity of ~10 - 2 S.cm⁻¹ at 800 °C (1). However, at temperatures higher than 700 °C, Strontium and Cobalt tend to migrate, leading to the formation of insulating phase of SrZrO 3 at the interface with YSZ electrolyte, thereby accelerating performance degradation and material aging (2). The common way to limit the strontium diffusion is to intercalate a thin and dense barrier layer of Ce 0.9 Gd 0.1 O 2- δ (CGO) between LSCF and YSZ. However, it introduces a supplementary resistance that reduces the cell performances. Addressing the degradation mechanisms requires innovative electrode architectures to limit diffusion without increasing the total resistance. Diminishing the LSCF content in oxygen electrodes by combining it with CGO takes advantage of the high ionic conductivity of CGO (10 -1 S.cm⁻¹ at 800 °C) (3) and the mixed conductive properties of LSCF to extend the Triple Phase Boundaries (TPBs), essentials for the Oxygen Evolution Reaction (OER) (4). Therefore, this study explores compositional gradient designs of a multilayer LSCF CGO composite. Due to graded composition, ionic conductivity is prioritized near the electrolyte, while electronic conductivity is reinforced towards the upper surface. LSCF-CGO composites layers were deposited using the simple and commonly used screen-printing technique, producing electrodes with varying LSCF-CGO compositional gradients and La 0.99 (Ni 0.6 Fe 0.4 )O 3- δ (LNF) as a (optional) current collecting layer. Two configurations were investigated for each gradient design: with and without the GDC dense barrier layer. Subsequently, surface decoration of MIEC materials can enhance the electrode’s active surface area, thereby improving its catalytic properties by increasing the TPBs (5). As those would be reduced near the electrolyte due to the graded deposition of LSCF content, the obtained electrodes were impregnated with MIEC Rare Earth Oxides (REO). Rare Earth elements exhibit a specific atomic structure; their 4f orbitals enable high ionic and mixed conductivity as well as a great catalytic activity, thereby boosting the electrode performances. Phase identification, compositional analysis and morphological characterization were performed using X-ray diffraction (XRD) and Energy Dispersive Spectroscopy (EDS) coupled with Scanning Electron Microscopy (SEM) on cross-sectioned electrodes. The obtained SEM images and elemental maps confirmed, respectively, the successful formation of the targeted gradient structures (see Figure 1) and a uniform REO distribution. Further analysis using High-Resolution SEM before infiltration revealed a rougher surface texture in regions with a higher LSCF content within the same scaffold. Symmetrical cells were manufactured to conduct the electrochemical impedance spectroscopy (EIS) measurements. These experiments were carried out to evaluate the electrode polarisation resistance under different airflow conditions. A significant reduction of electrode polarization was observed for all gradient designs after REO impregnation, dropping from 0.284 to 0.044 Ω.cm -2 at 750 °C for G1 (see Figure 2). During the air flow sweeping at 800 °C, the G1-REO electrode exhibited better performance with increased air flow, decreasing from 0.036 to 0.024 Ω·cm² in static air atmosphere and 1.2 L.min⁻¹ air flow, respectively. Lastly, the distribution of relaxation times (DRT) analysis was used to provide better insight into the different contributing processes, thus enhancing the interpretation of the overall electrode response. The influence of the gradient design, the infiltration of REO as well as the barrier layer on the electrode performance will be discussed in this presentation. References (1) Ndubuisi, A., Abouali, S., Singh, K., & Thangadurai, V. (2022). Recent advances, practical challenges, and perspectives of intermediate temperature solid oxide fuel cell cathodes. J. Mater. Chem.. A, 10(5), 2196 2227. (2) Jiang, S. P. (2019). Development of lanthanum strontium cobalt ferrite perovskite electrodes of solid oxide fuel cells – A review. Int. J. Hydrogen Energy, 44(14), 7448 7493. (3) Steele, B. (2000). Appraisal of Ce 1−y Gd y O 2−y/2 electrolytes for IT-SOFC operation at 500°C. Solid State Ionics, 129(1 4), 95 110. (4) Laurencin, J., Hubert, M., Couturier, K., Bihan, T. L., Cloetens, P., Lefebvre-Joud, F., & Siebert, E. (2015). Reactive Mechanisms of LSCF Single-Phase and LSCF-CGO Composite Electrodes Operated in Anodic and Cathodic Polarisations. Electrochim. Acta, 174, 1299 1316. (5) Jiang Z., Xia C., Chen F. (2010). Nano-structured composite cathodes for intermediate-temperature solid oxide fuel cells via an infiltration/impregnation technique. Electrochimica Acta, 55 (11), 3595–3605. Figure 1 Figure 2