The development of SOFC with perovskite electrodes by co-sintering and co-casting method

Abstract

The Solid Oxide Fuel Cells (SOFCs) were produced with various geometries by tape casting and co-sintering. Tape casting is a cheap and easily scalable method employed on a large scale for thin layers preparation. In the project, a sequential casting technique was developed where all the function layers in SOFC assembly were cast on each other; co-casting gave a strong interface and reduced electrolyte thickness. Co-sintering further simplified cells manufacturing and reduced processing time and energy demand, making fuel cells more profitable and suitable for mass production. The crucial factor for co-sintering of various-material tapes is to ensure that they have close shrinkage profiles during the sintering step, which usually differ due to individual properties of ceramic materials and pore former’s concentration in the slurry. The minor differences create stress between layers, leading to delamination and cracks in the cell’s structure. By controlling the composition of the slurry, particle size distribution and sintering temperature, it was possible to produce a cell without any internal structural defects.

The developed method was used to produce SOFC with alternative SOFCRoll geometry and the small tubular cells. SOFCRoll gives the possibility to extend the surface area while keeping the volume low. Up to 27.75 cm² of surface area was incorporated in a spiral structure, with a volume of about 2.5 cm³, thus giving better applicability where size reduction is required and a more robust structure; however, the cell was suffered a performance loss due to complications with the current collection and gas distribution. The structure modification and co-sintering of the current collector with high Ni content into otherwise unavailable part of the cell allowed for optimal use of 12 cm² surface area in the smaller version of the cell. Tubular cells are known for their high mechanical and thermal resistance. Tubular cell's surface area was up to 7 cm², much lower than in SOFCRoll but more accessible for gas and current collector. The combination of a small tubular design with thermally and redox stable alternative perovskite fuel electrode gave a highly durable device; without noticeable degradation after testing in extreme conditions.

Through the project, composite electrodes were mainly used, co-cast, and co-sintered with electrolyte at 1350 °C. The state-of-art yttria-stabilised zirconia (Zr₀.₈₄Y₀.₁₆O₁.₉₂, YSZ) was used as the electrolyte, which offers good performance and commercial availability. The co-sintered active fuel electrode contained the nickel-doped lanthanum calcium titanate (La₀.₄₃Ca₀.₃₇Ni₀.₀₆Ti₀.₉₄O[sub](3-γ), LCNT) and YSZ. The co-sintered oxygen electrode was a composite of lanthanum strontium manganate ((La₀.₈Sr₀.₂)₀.₉₅MnO₃, LSM) and YSZ. LCNT belongs to the family of new alternative materials proposed to replace highly active but prone to degradation state-of-the-art Ni/YSZ composite. Thanks to its mixed ionic and electronic properties (MIEC) and the possibility of exsolving nickel nanoparticles on its surface, LCNT offers a high activity for hydrogen oxidation whilst possessing great thermal shock and redox resistance as a fully ceramic electrode.

Despite the successful co-sintering of the LCNT/YSZ fuel electrode and LSM/YSZ oxygen electrode into a SOFCRoll and tubular structure, due to limitations related to the sintering temperature and composition, electrodes showed a significant ohmic and polarisation resistance.

In the following experiment, the composite electrodes were replaced. For the development of the next generation of tubular cells, the active material was impregnated into a co-sintered porous YSZ backbone. For impregnated electrodes, LCNT was impregnated on a porous YSZ backbone for the fuel hydrogen side, while on the air electrode, a lanthanum strontium ferrite (La₀.₈Sr₀.₂FeO₃, LSF). The impregnated electrodes were sintered at a much lower temperature than in state-of-the-art methods, giving more active spatial microstructures with a large surface area. Using this technique in co-sintered cells simplified manufacturing and made a broader range of materials available.

This work contains complete procedures for producing SOFC with various designs, including planar, tubular, and SOFCRoll, by the outlined methods. In addition, it seeks to determine a mechanism of their functionality based on electrochemical tests and DRT analysis.