A New Generation of Protective Coating Materials Based on CuMn1.7Fe0.3O4-Me2O3 (Me=Y, Gd) Ceramic Bilayers for Solid Oxide Cells Interconnect
Authors: Lemieszek Bartłomiej , Ignaczak Justyna , Lankauf Krystian , Błaszczak Patryk , Bik Maciej , Zając Marcin , Sitarz Maciej , Jasiński Piotr , Molin Sebastian 
Published: December 2024

In SOCs (solid oxide cells) technology, coating of interconnects is one of the main processes that determine the long-term operation of the stack. Currently, the main material used for interconnects are steels with high chromium content (e.g. Crofer 22 APU). They are known for their high resistance towards high-temperature corrosion due to the formation of passivation layer composed of chromium oxide. This oxide is characterized by relatively high conductivity compared to other single metal oxides, but at the same time, chromium evaporation causes cathode poisoning, what causes a significant decrease in the cell efficiency. Coating the interconnects reduces the thickness of chromium oxide layer and also protects the cathode. Materials used as protective coatings must be characterized by an appropriate coefficient of thermal expansion (suitable for steel, sealant and cathode) and high electrical conductivity, because target value of the area specific resistance cannot exceed 0.1 Ω cm2. Currently, the state of the art coating material is MCO (MnCo2O4), which is characterized by high conductivity and very good protection against chromium diffusion. Due to the fact that cobalt is a critical element for many areas, it was immediately necessary to find cobalt-free alternatives.
This work will present the research on the spinel material CuMn1.7Fe0.3O4 in combination with a single metal oxide – Y2O3 or Gd2O3. These combinations were made due to high affinity of the spinel material for chromium, and thin base layers of single metal oxides were intended to reduce the diffusion of chromium into the spinel layer. The layers were deposited using two techniques – electrolytic and electrophoretic deposition. The prepared samples with the layers fabricated in three configurations were subjected to oxidation tests at a temperature of 700 °C for 4000 hours. The reaction kinetics was determined by measuring weight gain and determining the Kp coefficient and instantaneous Kp. Electrical resistance measurements were also performed at various stages of oxidation to demonstrate the changes occurring during the oxidation. SEM-EDS measurements allowed for the assessment of the quality of the layer, the distribution of the elements, the thickness of chromium oxide layer and the efficiency for suppression chromium diffusion. Confocal Raman imaging, X-ray adsorption spectroscopy and X-ray diffraction measurements allowed to demonstrate potential changes occurring in the material. The results indicated very good corrosion protection properties with good chromium diffusion blocking properties. A very low value of the area specific resistance was obtained, which after 4000 hours of oxidation ranged from 8 to 20 mΩ cm2, depending on the type of a sample.

Acknowledgement

This research has been supported by National Science Centre (NCN) Harmonia 9 project number UMO-2017/26/M/ST8/00438: ‘Quest for novel materials for solid oxide cell interconnect coatings’.