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To protect solid oxide fuel cells ͑SOFCs͒ from chromium poisoning and improve metallic interconnect stability, manganese cobaltite spinel protection layers with a nominal composition of Mn 1.5 Co 1.5 O 4 were thermally grown on Crofer22 APU, a ferritic stainless steel. Thermal, electrical, and electrochemical investigations indicated that the spinel protection layers not only significantly decreased the contact area specific resistance ͑ASR͒ between a LSF cathode and the stainless steel interconnect, but also inhibited the subscale growth on the stainless steel by acting as a barrier to the inward diffusion of oxygen. A long-term thermal cycling test demonstrated excellent structural and thermomechanical stability of these spinel protection layers, which also acted as a barrier to outward chromium cation diffusion to the interconnect surface. The reduction in the contact ASR and prevention of Cr migration achieved by application of the spinel protection layers on the cathode side of ferritic stainless steel interconnects resulted in improved stability and electrochemical performance of SOFCs.

Concentration polarization is important because it determines the maximum power output of a solid oxide fuel cell (SOFC) at high fuel utilization. Anodic concentration polarization occurs when the demand for reactants exceeds the capacity of the porous cermet anode to supply them by gas diffusion mechanisms. High tortuosities (bulk diffusion resistances) are often assumed to explain this behavior. However, recent experiments show that anodic concentration polarization originates in the immediate vicinity of the reactive triple phase boundary (TPB) sites near the anode/electrolyte interface. A model is proposed to describe how concentration polarization is controlled by two localized phenomena: competetive adsorption of reactants in areas adjacent to the reactive TPB sites, followed by relatively slow surface diffusion to the reactive sites. Results suggest that future SOFC anode design improvements should focus on optimization of the reactive area, adsorption, and surface diffusion at the anode/electrolyte interface. © 2003 The Electrochemical Society. All rights reserved.

With the development of solid oxide fuel cells ͑SOFCs͒ that operate in the intermediate temperature range of 650-800°C, ferritic stainless steels have become promising candidate materials for interconnects in SOFC stacks. The SOFC interconnect requires that the alloy possess not only excellent surface stability, but also high electrical conductivity through the oxide scale that forms at elevated temperatures and contributes to the alloy's surface stability. It appears that ferritic Fe-Cr-Mn alloys may be potential candidates due to the formation of an electrically conductive scale containing (Mn, Cr) 3 O 4 spinel. To improve the understanding of scale growth on manganese-containing ferritic stainless steels and evaluate their suitability for use in SOFC interconnects, the oxidation behavior ͑i.e., growth kinetics, composition, and structure of the oxide scale͒ and the scale electrical conductivity of a commercially available Fe-Cr-Mn steel developed specifically for SOFC applications were investigated. The results are reported and compared with those of conventional ferritic stainless steel compositions.Solid oxide fuel cells ͑SOFCs͒ are solid-state energy conversion devices that produce electricity by electrochemically combining an incoming fuel ͑such as hydrogen or natural gas͒ and oxidant gas ͑typically air͒ across an ionically conducting oxide membrane. To build up a useful voltage, several cells ͑comprising positive cathode, electrolyte, and negative anode͒ are electrically connected in series in a ''stack'' via bipolar plates, also known as interconnects. These interconnects must demonstrate satisfactory stability and low electrical resistance over the operating lifetime of the SOFC stack. In recent years, progress in materials and fabrication techniques has allowed for a reduction in SOFC operating temperatures to a range ͑e.g., 650-800°C͒ where oxidation-resistant, high-temperature alloys can be considered as replacement materials for the traditional ceramic interconnect materials used in high-temperature ͑900-1000°C͒ SOFC stacks. 1-9 Chromia-forming ferritic stainless steels are among the most promising alloy candidates, due to their electrically conducting oxide scale, appropriate thermal expansion behavior, and low cost. 4-6,10 However, even at these reduced temperatures, the SOFC interconnect application remains challenging for traditional chromia-forming ferritic stainless steels. The chromia scales on the steels can grow to lengths of micrometers or even tens of micrometers after thousands of hours in the SOFC environment, leading to high electrical resistance, which causes unacceptably high degradation rates in stack performance. 5,[11][12][13][14][15][16][17] The chromia-forming alloys present another challenge in the evaporation of chromium species from the scale 16,18-22 and subsequent deposition of chromium oxides at the cathode/electrolyte interface, which, by increasing diffusion and charge-transfer resistances at the interface, degrade cell performance. 16,18,[23][24][25] Furthermore, the re...