Development of advanced interconnects for solid oxide cell stacks

High-temperature solid oxide cells (SOCs) are considered a promising conversion technology (electrolysis or fuel cells) for the transition to more sustainable energy futures. However, achieving their widespread use requires controlling material degradation and lowering capital costs. The auxiliary interconnect, which electrically connects multiple SOCs to form SOC stacks, is a critical component that affects SOC lifetime and cost. In the past, interconnects have widely been made of expensive and scarce ferritic stainless steels (FSSs) like Crofer 22 APU, specifically developed for interconnect applications. Interconnects require protective airside coatings to withstand high-temperature oxidizing  environments. Frequently, SOC stacks use a Ni-mesh to improve the contact at the fuel side.

A promising approach to achieve large-scale manufacturing of interconnects involves lowcost, widely available commodity steels and electroplating of a metallic precursor that can be thermally converted into protective airside coatings. While Co coatings like the (Mn,Co)₃O₄ spinel are common choice for airside protection, Ni-based oxide coatings are considered more scalable due to the higher raw material availability of Ni and existing experience in industrial Ni-electroplating. However, pursuing this route poses challenges due to interdiffusion between Ni and the steel, as well as the lower Cr and higher Si contents in commodity steels, which may affect the stability of protective chromia (Cr2O3) or lead to the formation of electrical insulating silica (SiO₂).

The primary objective of this Ph.D. work is to improve the understanding of the interdiffusion behaviour and its effects on interconnect performance, with the aim of making low-cost, industrially scalable steel-coating combinations more accessible for interconnect applications. The investigation focuses on the role of interdiffusion from a Ni-based oxide coating on airside performance, using AISI 441 as an example steel. Tests include initial SOC stack firing/sealing exposures, high-temperature oxidation resistance, thermal cycling response, and electrical resistance measurements. The effects of interdiffusion are studied at various SOC operating temperatures (750, 780, and 830 °C) and exposure times (up to 1 year), comparing effects at contact areas with SOC stack components and free oxide surfaces. Advanced materials characterization techniques, such as electron backscatter diffraction, in-situ X-ray diffraction, and scanning transmission electron microscopy, are used for detailed analysis. 

The results reveal Ni diffusion into the steel, which enhances a non-detrimental discontinuous silica layer but also increases the importance of steel choice, particularly the demand for a sufficiently high Cr concentration to achieve good interconnect performance. The development of a chromium-upgraded AISI 441 steel using a surface treatment referred to as soft-chromising is a central part of this work. Soft-chromising is a straightforward method that can be applied to existing commodity steels. It enables fast and meaningful performance testing without the need for producing steels through more complex melting processes. Chromium-upgraded AISI 441 offers improved oxidation resistance and thermal
cycling robustness with Ni-based oxide coatings. Regardless of exposure conditions. chromium-upgraded AISI 441 maintains a discontinuous silica layer that does not noticeably contribute to the electrical resistance across interconnects. Benchmarking against highperforming Crofer 22 APU with different coating materials confirms the promising airside performance of the Ni-coated chromium-upgraded AISI 441 steel.

Preliminary investigations in a C-containing fuel gas atmosphere reveal the important role of Ni diffusion from a Ni-mesh contact layer for achieving good electrical efficiency on the fuel side of the chromium-upgraded interconnect material. Finally, electrical resistance measurements from a 75-cell solid oxide electrolysis cell (SOEC) commercial Topsoe Stack Platform version 1 (TSP-1) stack operated in CO2-electrolysis at 750 °C underscore the promise of chromium-upgraded AISI 441 steel for interconnect applications.