Coatings for SOFC Interconnects in Fuel Side Environments

Bipolar plates in solid oxide fuel cells operate under a significant oxygen potential gradient (0.2 to 10^-18 bar pO₂). For air side conditions (pO₂ = 0.2 bar), a PVD technique has successfully been used to apply combination reactive element/Co coatings to inhibit the rapid oxidation of and Cr volatilization from Fe-Cr alloys such as Crofer22 APU and Sanergy HT. However, relatively little work has been done on developing similar coating strategies for fuel side conditions.

In this work, the ferritic steel Sanergy HT was coated with single element (Ce,La, Co and Cu) and duplex (Ce/Co, La/Co) coatings of varying thicknesses and exposed at 850°C in Ar-5%H₂ charged with 3% H₂O in a tubular furnace up to 500 h. Additionally, the effects of a ‘sealing’ step on corrosion mechanisms were also investigated by pre-oxidising the coated steel in air, prior to exposure in the fuel side environment. Chemical analysis on the samples was subsequently performed with SEM/EDX and XRD.

 It was established that:

(i)                 reactive element (Ce and La) coatings brought about a 2-4x reduction in oxidation rate

(ii)                Co and Cu coatings negatively affected oxidation performance due to their instability in fuel side conditions.