Corrosion of Metallic Interconnects for Solid Oxide Fuel Cells - Mitigating chromium evaporation and dual-atmosphere challenges.

Doctoral thesis, 2024

Solid Oxide Cells (SOCs) are high-temperature energy conversion systems that have attracted attention in the last decades due to their high efficiency. Depending on the operating mode, SOCs can act as either Solid Oxide Fuel Cells (SOFCs), producing electricity from various fuels, or Solid Oxide Electrolyser Cells (SOECs), converting electricity to fuel (e.g., H2). However, high system costs and limited life-times have hindered their widespread commercialisation. A factor that limits the life-time of an SOC stack is the interconnects, which electrically connect the individual cells. Nowadays, interconnects are made of Ferritic Stainless Steel (FSS) and suffer from severe corrosion owing to operation at high temperatures (600 ° ‒ 900 °C) and in harsh environments.

Degradation phenomena, such as chromium evaporation (CrVI) from the Cr2O3 scale and continuous scale growth, negatively impact cell performance due to cathode poisoning and increased electrical resistance, respectively. It has been shown that these degradation phenomena are effectively mitigated by the application of a coating, such as (Co,Mn)3O4, sometimes in combination with a “Reactive Element” coating, such as cerium (Ce). This thesis aims primarily to investigate new mitigation strategies that effectively reduce costs and increase the life-span of the SOC stack. The first part of this work investigates the efficiency of using copper (Cu) instead of cobalt (Co) for coating applications in terms of: Cr evaporation, oxide scale growth, oxide microstructure, and area-specific resistance.

Another corrosion phenomenon investigated within the scope of this work, is the dual-atmosphere effect. This effect leads to increased corrosion on the air-side of the interconnect if the FSS is exposed to a dual atmosphere, i.e., with hydrogen on the other side. In this thesis, the corrosion behaviours of a selection of uncoated commercial steels are studied under single-atmosphere and dual-atmosphere conditions at 600 °C. Furthermore, the effect of the “fuel gas” composition is investigated. The results show that the fuel-side of the interconnect must be protected to reduce hydrogen ingress.

These observations lead to the last part of this thesis, which is the development and study of hydrogen barrier coatings, so as to reduce hydrogen permeation from the fuel-side, as well as a combination of coatings on both sides, to address both of the afore-mentioned degradation mechanisms. The main finding is that a combination of coatings is highly effective.