Composite cathode fabrication by flash light sintering process for solid oxide cells

Abstract

Solid oxide fuel cells (SOFCs) have been interested as an energy conversion device that directly converts chemical energy to electrical energy because of their energy efficiency, fuel flexibility, low pollutant emissions, and less environment restrictions. However, the component materials of SOFCs are ceramics and a high-temperature thermal sintering process is required to obtain the material properties. That process cause various fabrication issues such as interfacial side reaction at interface and excessive grain growth, which result in decreasing electrochemical reaction kinetics and performance degradation of cells. Therefore, it is essential to develop the alternative novel sintering process to overcome that fabrication issues. The one of the MIEC cathode material La0.6Sr0.4Co0.2Fe0.8O3+δ used in this study was fabricated directly on the YSZ electrolyte without generating secondary phases at the LSCF/YSZ interface using the flash light sintering (FLS). The FLS process completes sintering in a few seconds, suppressing the inter-diffusion of strontium and zirconium and the formation of the SrZrO3 layer at the LSCF/YSZ interface. The LSCF cathode fabricated on YSZ electrolyte using the FLS process achieved high performance of 1.02 W/cm2 at 750 °C, which is significantly higher than the conventional thermal sintered LSCF cathode cell of 0.04 W/cm2. The FLS process simplifies the manufacturing process of solid oxide fuel cells by eliminating the need for buffer layer deposition processes such as GDC and SDC electrolytes and thermal sintering processes. Moreover, it has the potential to replace conventional thermal sintering process to solve a variety of manufacturing challenges, including secondary phase formation. Additionally, solution infiltration methods have been studied to improve the performance of solid oxide cells by increasing reaction sites. Infiltration was accomplished by dropping the precursor solution onto the cathode surface and then was proceeded flash light sintering. The FLS process showed the effects of suppressing excessive grain growth due to a short process time. The impregnated Nano-particles in the porous structure were able to maintain their Nano-sized characteristics. In contrast, in the conventional thermal sintering process, the size of the infiltrated particles increases, which reduces the triple phase boundary (TPB) length and reaction site. As a result, performance improvements of approximately 40% and 70% were achieved using SDC solution infiltration into the LSCF backbone and FLS process compared with thermal sintering, respectively. The effectiveness of the infiltration and FLS process was excellent for improving performance without any side effects. Moreover, the lanthanum nickelate material (LNO) was considered to secure long-term stability because of the excellent metarials and chemical stability. It has ABO3 structure basically same as MIEC material such as LSCF, LSC, LSM. Lanthanum nickelate material has excellent electrical conductivity in case of perovskite structure and has high oxygen surface exchange coefficient in case of La2NiO4 structure (layered perovskite, Ruddlesden- Popper(RP structure)) due to phase transition, resulting in high performance in ORR reaction and OER reaction. The LNO cathode was fabricated using the structural characteristics, and previously developed infiltration and flash light sintering process were used. The maximum power density of the GDC infiltrated LNO cathode cells was 1.7 W/cm2 at 750 °C and it showed 0.83 W/cm2 at 650 °C which is the relatively high performance compared with various cathode materials. Consequently, the research in the dissertation contributes to develop the novel flash light sintering process to fabricate ceramic functional cathode for solid oxide cells. The flash light sintering process was investigated as an alternative sintering process to replace the conventional thermal sintering process, which cause side reaction and unwanted grain growth in manufacturing process. The high-performance functional cathode solid oxide cells were fabricated by flash light sintering process with suppression of side reaction at electrode/electrolyte interface and excessive grain growth of infiltrated Nano-particles.