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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.

Solid Oxide Fuel Cell systems for stationary applications should exhibit low degradation (<< 1%/1,000h voltage loss) to ensure long-term operation (> 40,000h). The rather high operation temperature of SOFCs (≥ 700°C) is on the one hand beneficial for oxygen conduction through the ceramic-based electrolyte, but on the other hand all diffusion-related, energetically-driven mechanisms are also increased. This counts less for gas diffusion processes but is more pronounced for solid-state diffusion (surface, bulk, grain boundary). For gas diffusion processes other parameters like gas stream, individual partial pressures and partial pressure gradients are dominating. Within two different stack tests with four-plane short stacks and their intensive post-test characterization two varying diffusion-related degradation mechanisms have been investigated. One is a short-term (~1250h) test with two different chromium evaporation protection layers on the air side metallic interconnect and frame and the other one is a long-term endurance test (~ 35,000h). For the first stack two planes were coated with a manganese oxide layer applied by wet powder spraying (WPS) while the other two planes were coated by atmospheric plasma spraying (APS) with a manganese-cobalt-iron-spinel layer. Thus the protection layers do not only differ by coating technology but also by material. The voltage loss in the planes with WPS-coated interconnect was markedly higher than in those coated by APS. Finally it turned out that the layers microstructure plays the key role in Cr evaporation minimization. In this stack, gas-phase diffusion is dominating degradation. In the long-term operated stack, the influence of a diffusing element from the cathodic contact layer towards and through the electrolyte has been characterized. Manganese, presumably originating from the contact layer (but it’s additionally also part of the protection layer), diffuses via solid-state diffusion and reacts while reaching reducing conditions in the fuel compartment with the zirconia-based electrolyte. This interaction forms a secondary sponge-like porous phase at the boundary electrolyte-anode. This secondary phase causes stresses and leads, after growing to big islands of some ten micrometers in diameter, to electrolyte-anode delamination and finally to complete cell failure. For the Mn-related degradation solid state diffusion has been supposed, as the electrolyte is coated by a dense thin-film diffusion barrier layer which suppresses gas diffusion. Both results indicate that volatile elements (which could either evaporate via gas phase or diffuse via solid state diffusion) should be avoided as they can lead to severe cell, stack and system failure even after thousands of hours of high-temperature operation.

We report on the systematic examination of the redox behavior of a novel metal-supported SOFC cell type based on a porous Cr/Fe support and a thin YSZ-electrolyte layer applied by PVD methods. The oxidation of the Ni catalyst at the anode and the subsequent damage to the fuel cell microstructure is one of the main contributions to degradation of an SOFC. Our cell concept was tested for tolerance to cyclic oxidation and reduction of the anode, employing gravimetric and electron microscopy methods. The effects of redox cycling on the microstructure are examined and compared to corresponding results from state-of-the-art anode-supported SOFCs.

One of the current R&D projects on solid oxide fuel cells concerns the reduction of the operation temperature. Nowadays stacks with anode-supported cells and an LSCF-cathode operate typically at a mean temperature of 700°C. The minimum temperature for SOFC using natural gas is approx. 650°C due to reforming limitations. Reducing temperatures below 600°C leads to lower costs, shorter start-up time and thus in-time power delivery by the system (e.g. for auxiliary power units). Forschungszentrum Jülich is also working on the development of an SOFC operating at 400°C for special applications, which can be fueled either with hydrogen (produced by, for example, electrolysis using renewable energy) or with reformate gas based on, for instance, ethanol/methanol. The main challenge for using the SOFC at such low temperatures is, besides electrode activities, overcoming the resistances, especially the ohmic resistance of the electrolyte. This can be achieved in two ways: by introducing materials which show higher oxygen diffusivity or by using the classical materials and minimizing the electrolyte thickness down to values below 2µm.

Physical vapor deposition (PVD) technologies allow for a variety of microstructural morphologies. PVD is especially qualified for the production of dense ceramic layers at comparatively moderate substrate temperatures. This paper discusses the physical vapor deposition of SOFC functional layers, comprising perovskites, NiO/8YSZ and composite electrolyte layers, with a thickness between 300 nanometres and several micrometres and lateral dimensions of 100 mm⋅100 mm. Magnetron sputtering and electron beam evaporation were used as PVD techniques, with and without additional ion bombardment of the layers during deposition. Scanning electron microscopy analysis revealed that an additional ion bombardment during layer growth increasingly promotes the creation of a continuous and dense layer morphology instead of a columnar microstructure.

The effects of both temperature and applied bias power during the sputtering of gadolinia-doped ceria (GDC) interlayers used as diffusion barriers in anode-supported solid oxide fuel cells (SOFC) were investigated. Scanning electron microscopy analysis revealed that increasing the applied bias power, in the 0-300W range, progressively inhibits the columnar structure typically observed in sputtered films, favoring the deposition of dense interlayers. Such feature was mirrored in the electrochemical tests of single cells that demonstrated enhanced power density for SOFC with bias-assisted sputtered interlayers. In addition, similar electrochemical performances of fuel cells having interlayers deposited in the 400-800 {degree sign}C temperature range indicated that the applied bias allows for the sputtering of GDC protective interlayers at relatively lower temperatures than unbiased depositions. The presented results evidenced that bias-assisted sputtering is an effective technique for the fabrication of high-performance anode supported SOFC.

This paper reports a study on the preparation of thin nano-structured 8YSZ membrane films, which can be sintered to full density at a lower temperature (~ 1200{degree sign}C) than currently applied in SOFC manufacturing (> 1400{degree sign}C). For the deposition process, four different coating liquids with varying particle size, covering the range from 85 nm to 6 nm, were prepared. In the first part, it is demonstrated that nano-dispersions with a particle size of e.g. 85 or 60 nm can be used for the formation of dense YSZ membranes with a thickness of 1 - 2 µm on a regular macroporous SOFC anode, but the layers can only be sintered to full density at 1400{degree sign}C. In the second part, ultra-thin YSZ membranes are prepared by sol-gel coating (particle size 35 and 6 nm). These layers show a thickness < 500 nm and a very tight mesoporous or microporous structure in the calcined state (pore size < 5 nm), which allows a sintering treatment in the desired temperature range (~ 1200{degree sign}C).

As part of the Real-SOFC Integrated Project of the European Commission, four SOFC stacks each with 2 planar anode-supported cells were operated for durations of up to 3000 h under varying fuel and electrical load conditions. The variations comprised the external applied current density (0.3 or 0.5 A/cm²) and the fuel gas (hydrogen or methane). All stacks were operated galvanostatically at a temperature of 800 {degree sign}C and a fuel utilization of 40%. A pronounced difference in degradation behaviour was observed between the two stacks operated at 0.5 A/cm² and those operated at 0.3 A/cm². The latter displayed linear behaviour of the voltage with time over the full duration of operation. The degradation rate was always below 1%/kh. From the start, the two stacks operated at 0.5 A/cm² showed an increased degradation rate (> 2%/kh). Post-test characterization of the cathode with respect to chromium poisoning was performed on cells from all four stacks. Segments of the cells were cut by sawing and examined. On the one hand, polished samples for SEM characterization were prepared for microstructural and EDX investigations, and, on the other hand, the cathode was removed and chemically characterized with respect to compositional changes due to the incorporation of species from the surrounding components.