Thin-Film Coated Steel Foils as Interconnects for Solid Oxide Fuel Cells (SOFC)

Doktorsavhandling, 2015

Solid Oxide Fuel Cells (SOFCs) are electrochemical energy conversion devices that have the potential to achieve high efficiencies while using a broad range of fuels. The advancement of the technology has however been hindered by a number of technical and economic barriers, one of which is the durability and cost of the bipolar plate or interconnect that connects individual cells in series to form a so-called ‘stack’. The ferritic steel based interconnect experiences oxidising (pO2≈ 0.2 bar) and reducing (pO2<10-15 bar) conditions on its ‘air’ and ‘fuel’ sides respectively. In air side environments, the oxide scale volatilises and deposits on the cathode, significantly degrading cell performance. In addition, the interconnect itself can experience accelerated degradation due to the extraneous loss of Cr resulting from the volatilisation process. This thesis focusses primarily on the Cr evaporation and oxidation properties of PVD coated ferritic steel foils in air side environments at 850°C. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) coupled with Energy Dispersive X-ray (EDX) analysis have been used to investigate oxidation mechanisms and establish a quantitative framework to evaluate the influence of coatings on the oxidation of the interconnect material. It is shown using long term oxidation (>3000 h) data that the application of a 10nm thick layer of Ce on the steel surface significantly improves the oxidation resistance of the steel while time resolved Cr evaporation data over 1000 h establishes the fact that a 640nm thick Co coating reduces the evaporation rate by one order of magnitude in air side environments. A duplex coating consisting of 10nm of Ce underneath 640nm of Co successfully minimises both the oxidation and evaporation rate. Additionally, the effect of PVD coatings on the oxidation performance of interconnects in simulated fuel side environments are examined It is established that Ce and La coatings brings about a factor 2-3 reduction in the oxidation rate while the presence of a La coating results in the formation of a continuous perovskite layer by reaction with the thermally grown oxide; a property that could be leveraged to improve the otherwise poor conductivity of the oxide scale in low oxygen partial pressure environments.