Post-Operational Characterization of a Long-Term Operated Solid Oxide Fuel Cell Stack

Abstract

A two layer solid oxide cell stack based on anode-supported cells was operated galvanostatically at 0.5 A / cm² and 700°C for nearly 100,000 h in fuel cell mode. The overall degradation rate during the operation was approx. 0.5 %1000 h but the stack shows different degradation rates over different time periods. In contrast to our typical stack material choice this stack was mainly assembled with another type of metal: ITM from Plansee SE, Austria instead of Crofer22APU. Crofer 22 APU was merely used for a 200 µm thick foils inserted in the manifold areas at the gas in- and outlet of the cells. After controlled shut-down, the stack was post-test analyzed. After leakage test by pressure drop method, part of the stack was embedded as a whole and part was dissected plane-by-plane. All stack components, the cell, the metallic interconnects, frames and the glass-ceramic sealing were afterwards characterized by means of SEM, TEM and wet chemical analysis. Some analysis were made on whole stack parts including the top and bottom plates and some were performed on dissected individual parts.In summary, most of the components look optically quite good despite the 11 years operation. The measured gas-tightness at room temperature was below the threshold of 1 % gas loss on the fuel side but in an acceptable range regarding the air side. The microstructure of the glass-ceramic sealant showed a high crystallization state of the material with moderate porosity. The triple phase boundary to the furnace atmosphere had a thin layer of barium chromate indicating an early crystallization equilibrium achieved after short terms not being effected by the long-term exposition afterwards. An antiquated application method was used for the sealant resulting in an infiltration of the protective layer up to 100µm depth by the excess glass-ceramic squeezing out of the joining gap. Regarding the interfacial reactions to the YSZ electrolyte, zirconium containing silicate needles are formed as a thin layer. The reaction zone to the steel ITM is very similar to the phenomena observed for Crofer22APU: a chromium manganese spinel layer is dissolved by the glass during joining process resulting in an interfacial layer of 1-2 µm thickness.The interconnect steel ITM formed a thin protective chromia surface scale. The relatively high contents of chromium (ca. 25%) and molybdenum (ca. 3%) in the steel resulted in extensive s-phase formation in the large areas of the interconnect. Gas distributing steel foils of 200 µm thickness made from Crofer 22 APU showed only in some locations nodules of Fe-rich oxide. Substantial parts of these foils formed a protective surface oxide scale consisting of an outer Mn, Cr- spinel layer on top of an inner chromia scale. Interdiffusion phenomena were observed at the spot-welded joints between nickel contact wires and steel components. Ni diffusion from the wire into the ITM interconnect and Crofer 22 APU foils resulted in local transformation of ferritic into austenite structure. On the other hand diffusion of Cr from the interconnect into the Ni-wires resulted in minor Cr-oxide formation on the surfaces of the Ni wires.The cell itself and the air-side contacting reveal no obvious macroscopic changes like delamination, cracking etc. A closer look into the cell shows the formation of an interface zone between the LSCF cathode and the GDC barrier layer. This zone looks fragmented and being composed of various tiny crystals which in turn can be traced back to LSCF and GDC. But in many areas additionally Cr as foreign element could be detected. This Cr is also present in the first micrometers of the GDC barrier layer forming also tiny crystals on the inner surface of pores within the GDC. In the fuel electrode, composed of Ni and 8YSZ, also very small foreign phases containing Mn and Al were found. The amount is very low as the ITM contains only traces of Mn. But an enrichment in Ni in the fuel electrode could be detected. This is opposite to the effect in electrolysis mode were a Ni depletion takes place.To sum up the cell-related findings: i) an enrichment in Nickel in the fuel electrode has been observed in this stack for the first time; ii) a foreign phase composed of Mn-Al-(O) is present in the fuel electrode; iii) the LSCF cathode is, closed to the barrier layer, fragmented and chemically disintegrated; iv) the GDC barrier is also fragmented at the border towards the cathode and lastly v) an interaction between Cr and GDC has been observed also for the first time.