Raman spectroscopic studies of Polymer Derived Ceramics in the form of protective coatings for SOFCs’ interconnects

Nowadays, with an exponentially increasing amount of people over the world along witha rising demand for the energy, an emphasis is put on the development of the low-emission energy production technologies such as Solid Oxide Fuel Cells (SOFCs). However, despite high electrical efficiency, easiness in manufacturing and low costs of their production, the problem of high-temperature corrosion of SOFCs’ interconnects casts a shadow on such a promising idea [1].

            Among abundant solutions, the application of protective coatings on the ferritic steels seems to be the most interesting one [1]. Thus, in this work, Polymer Derived Ceramics (PDCs) in the form of SiAlOC glasses’ coatings were proposed as a very innovative protection measure. Due to the duality of carbon form (Si-C covalent strong bonds and so called “free carbon phase”), the necessary balance between high thermomechanical stability and electrical conductivity is provided, respectively. Moreover, sol-gel method gives an opportunity to form PDCs in numerous shapes i.e. layers [2].

            It is crucial to report that vibrational spectroscopy, including especially Raman spectroscopy is the powerful tool when investigating material’s structure, but at the same time very underestimated while determining high-temperature corrosion phenomena and mechanisms of diffusive processes, particularly for the ferritic steels.  

            The main aim of this study was to present the possibilities of Raman spectroscopy as a method of the evaluation of corrosion rate of Crofer 22APU ferritic steel used for SOFCs’ interconnects in two systems: uncoated and coated with layers based on SiAlOC glasses.

            The corrosion resistance of the protective layers was investigated using thermogravimetric analysis within isothermal (500 h) and cyclic (500 1h-cycles) conditions at 800°C in the laboratory air. The application of Raman spectroscopy allowed for the determination of:

a)      bulk material’s (gel and glasses) structure used as reference for coatings – point measurements,

b)     structure of coating along with coating/steel interface (MnCr₂O₄ spinel) – linear depth profiles,

c)      distribution of oxidation products on the surface and across the multi-layered scale - imaging.

The Raman studies supported with additional structural (XRD, FTIR) and microstructural (SEM+EPMA) investigation, revealed the outbreak improvement in the oxidation resistance of the steel due to the application of SiAlOC glasses coatings.