Study of new glass-ceramic sealants and protective coatings for SOFC application: processing, characterization and performances in relevant conditions

Solid oxide fuel cells (SOFC) offer potential for efficient and clean power generation obtained from a wide range of fuels (hydrocarbons, renewables and coal derived fuels). Advanced system configurations based on SOFC are currently being developed for applications in both centralized and distributed stationary generation. In a SOFC system, many cells are connected in a stack by an interconnect (ferritic stainless steel). In the planar stack design, the use of a sealant is necessary to prevent gas mixing. Furthermore, the use of Cr-containing metallic alloys as metallic interconnects leads to Cr-poisoning of the cathode. This PhD thesis focuses on two main topics: glass-ceramic sealants and protective coatings for interconnects (to prevent Cr-poisoning) for SOFC application. Two different Ba-free compositions were designed, produced and characterized as glass-ceramic sealants. The sintering-crystallization behaviour was studied in order to choose proper joining thermal treatments. The compatibility between these sealants and metallic interconnect materials was tested in different relevant conditions and the produced joined samples were morphologically and chemically characterized. AISI 441/glass-ceramic/AISI 441 joined samples were tested in dual atmosphere for 1100 h at 800 °C. Crofer 22 H, bare AISI 441 and Mn-Co spinel based coated AISI 441, coupled with the seal, were tested up to 3500 h at 800 °C in static air. The results of post-mortem analyses have been discussed in order to evaluate the integrity of the joining as well as possible interactions between the glass-ceramic and the metallic interconnects. Pre-oxidised Crofer 22 APU/glass-ceramic/pre-oxidised Crofer 22 APU “sandwich-like” samples were tested in dual atmosphere with the application of a DC voltage, for 100 h at 800 °C, by applying two different voltages, 0.7 V and 1.3 V; the resistivity was measured for all the duration of the test. The obtained values were compared with the resistivity of the glass-ceramic. Post-mortem morphological and chemical analyses were conducted in order to evaluate possible interactions between the glass-ceramic and the steel during the tests. The compatibility of one of the glass-ceramics was also tested with yttria-stabilized-zirconia electrolyte, after the joining process and after an aging at 800 °C for 250 h. The joined samples demonstrated low He leak rates values, 10-10 - 10-11 mbar l s-1, below the acceptable limit for SOFC application. Mn1.5Co1.5O4 protective coating was deposited on Crofer 22 APU with three different techniques: radio frequency (RF)-sputtering, thermal co-evaporation and electrophoretic deposition (EPD). The as-prepared coatings were characterized from the morphological and chemical point of view, making a comparison between the different techniques. Mn-Co coated samples were electrically tested for 5000 hours at 800 °C under a 500 mA cm-2 current load to determine their Area Specific Resistance. After tests, the samples were morphologically analysed. The relationships between coating thickness, homogeneity, and effectiveness of its protective nature are reviewed and discussed. The electrophoretic deposition was used also to dope Mn1.5Co1.5O4 with Cu with the aim to further improve the electrical conductivity and the coating densification. Coatings doped with different amounts of Cu, have been deposited and sintered. Samples were characterized in order to investigate the presence of Cu in the protective layer and their morphology, and subsequently submitted to oxidation resistance tests as well as to long term ASR test. Both the glass-ceramics studied here showed good performance in terms of sintering ability and thermo-mechanical properties. The interfacial compatibility was found to be very good with different substrates. Corrosion phenomena (unlikely due to interactions with the glass-ceramic) were detected in the case of one glass-ceramic coupled to pre-oxidised AISI 441 exposed to dual atmosphere. Furthermore, the application of the voltage in the case of the second sealant seems to induce interactions with the pre-oxidised Crofer 22 APU, decreasing the resistivity of the joining. The two sealants were subjected to slightly different tests. Even if one of them demonstrated better performance in terms of CTE and compatibility with Crofer 22 APU (the most used interconnect steel at the moment), further studies are needed in order to attest the best choice. Out of the three Mn-Co coatings compared in this thesis, the one deposited by EPD presents the best protection against Cr diffusion as well as the best behaviour during long term ASR test. The rate of increase of the ASR, calculated by linear fitting of the curves between 2000-5000 hours, gives a value of 3.9 mΩ cm2/1000h for thermal co-evaporation, sputtering and not-coated samples and a value of 1.2 mΩ cm2/1000h for EPD coated sample. Assuming a simple linear extrapolation of the values up to 40000 hours, the EPD coated sample would have a total ASR of about 60 mΩ cm2, so below the acceptable limit.