Interconnects oxide ceramic

A high temperature solid electrolyte fuel cell (SOFC) will be considered now. Modern SOFC technology employs calcia-stabilized zirconia as the support tube and yttria-stabilized zirconia as the solid electrolyte. In addition, special oxide ceramic materials are employed as electrodes and interconnection materials. These materials and the solid electrolyte are deposited as thin layers on the support tube by electrochemical vapor deposition. 

In addition to compatibility with the electrolyte, compatibility of the cathode with the interconnect is also important. Both oxide ceramic and metallic materials are used as interconnects in SOFCs. As expected, these two types of interconnects present quite different issues in their compatibility with the cathode. 

Ttvo roles of the interconnect in high-temperature solid oxide fuel cells (SOFCs) are the electrical connection between cells and the gas separation vvithin the cell stack. The fact that the interconnect must be compatible with all of the cell components as well as be stable with respect to both oxidising and reducing gases places very stringent materials requirements on it. These requirements plus the additional constraints of cost and ease of fabrication tend to limit the possible choices to only a few materials. These materials come from either perovskite-type oxide ceramics based on rare earth chromites for operating temperatures in the 900-1000°C range or metallic alloys for lower temperature cell operation. 

In this chapter the requirements of interconnect materials, the characteristics that the leading candidate materials possess, and how well these fulfil the requirements are discussed. The oxide ceramic materials are discussed first followed by a description of several types of metallic interconnection materials. Then, the special protective and contact materials applied as coatings on the interconnects to match them to the electrodes are described. 

In spite of having many favorable characteristics, the metallic interconnects also suffer fi-om certain drawbacks. Some of the pertinent issues are electrical contacts between metallic interconnect and ceramic electrodes (cathode and anode), matching of thermal expansion between the metallic interconnect and adjacent components, oxide scale formation on the metallic surface as well as cathode poisoning. All these issues need drastic improvement. For more than a decade, a number of alloys have been attempted, but the major interest for development of such metallic interconnect started only when SOFC developers started using metallic interconnects for SOFC operation, preferably < 750°C. 

Directed Oxidation of a Molten Metal. Directed oxidation of a molten metal or the Lanxide process (45,68,91) involves the reaction of a molten metal with a gaseous oxidant, eg, A1 with O2 in air, to form a porous three-dimensional oxide that grows outward from the metal/ceramic surface. The process proceeds via capillary action as the molten metal wicks into open pore channels in the oxide scale growth. Reinforced ceramic matrix composites can be formed by positioning inert filler materials, eg, fibers, whiskers, and/or particulates, in the path of the oxide scale growth. The resultant composite is comprised of both interconnected metal and ceramic. Typically 5—30 vol % metal remains after processing. The composite product maintains many of the desirable properties of a ceramic however, the presence of the metal serves to increase the fracture toughness of the composite. 

Nishiyama H, Aizawa M, Sakai N, Yokokawa H, Kawada T, and Dokiya M. Property of (La,Ca)Cr03 for interconnect in solid oxide fuel cell (part 2). Durability. J. Ceram. Soc. Japan 2001 109 527-534. 

Hatch well CE, Sammes NM, Tompsett GA, and Brown IWM. Chemical compatibility of chromium-based interconnect related materials with doped cerium oxide electrolyte. J. Eur. Ceram. Soc. 1999 19 1697-1703. 

Zhou X-L, Ma J-J, Deng F-J, Meng G-Y, and Liu X-Q. A high performance interconnecting ceramic for solid oxide fuel cells (SOFCs). Solid State Ionics 2006 177 3461-3466. 

Zhou X, Deng F, Zhu M, Meng G, and Liu X. Novel composite interconnecting ceramics LaojCao jCrOj j/ Cc02Sm08O 9 for solid oxide fuel cells. Mater. Res. Bull. 2007 42 1582-1588. 

Mori M and Hiei Y. Thermal expansion behavior or titanium-doped La(Sr)Cr03 solid oxide fuel cell interconnects. J. Am. Ceram. Soc. 2001 84 2573-2578. 

The interconnect normally links the anode of one cell to the cathode of the next. It must, of course, be an electronic conductor and also a gas barrier preventing the direct meeting of fuel and oxidant gases. Fig. 4.27 illustrates how the interconnection is achieved in the case of the so-called planar fuel cell stack. In the later discussion of the ceramics-based cells a tubular configuration is described, but the principles are the same. 

Almost all ceramics that contain interconnected porosity exhibit a fall in resistivity when exposed to humid atmospheres at normal ambient temperatures. Water is adsorbed from low-humidity atmospheres by most oxide surfaces when it... 

The beginnings of the SOFC are recorded in an early East German University patent (Mobius and Roland, 1968) which shows awareness of many of the variables still being worked upon today. The oxides of lanthanum, zirconium, yttrium, samarium, europium, terbium, ytterbium, cerium and calcium are mentioned as candidate electrolyte materials. The proposed monolithic planar arrangement has, however, been abandoned by many companies, on the example of Allied Signal. One notable exception is a reversion to a circular planar concept by Ceramic Fuel Cells of Australia, UK (Section 4.7). The Rolls-Royce all-ceramic fuel cell (Section 4.3), which is monolithic and has one compliant feature, namely a gap, is a major exception. One modern trend is towards lower SOFC temperatures, with the intermediate-temperature IT/SOFC allowing the use of cell and stack arrangements with some flexibility and manoeuvrability based on new electrolytes, metallic flow plates, electrodes and interconnects. 

Larring, Y. and Norby, T., Spinel and perovskite functional layers between Plansee metallic interconnect (Cr-5 wt% Fe-1 wt% Y2O3) in ceramic (Lao 85Sro.i5)o.9iMn03 cathode materials for solid oxide fuel cells, J. Electrochem. Soc., 147, 3251-3256 (2000). 

The aforementioned requirements on surface stability are typical for all exposed areas of the metallic interconnect, as well as other metallic components in a SOFC stack (e.g., some designs use metallic frames to support the ceramic cell). In addition, the protection layer for the interconnect, or in particular the active areas that interface with electrodes and are in the path of electric current, must be electrically conductive. This conductivity requirement differentiates the interconnect protection layer from many traditional surface modifications as well as nonactive areas of interconnects and other components in SOFC stacks, where only surface stability is emphasized. While the electrical conductivity is usually dominated by their electronic conductivity, conductive oxides for protection layer applications often demonstrate a nonnegligible oxygen ion conductivity as well, which leads to scale growth beneath the protection layer. With this in mind, a high electrical conductivity is always desirable for the protection layers, along with low chromium cation and oxygen anion diffusivity. 

The microstructures of ceramic matrices grown from two different classes of alloys have been reported. The external growth surface of ceramic matrices grown from an Al-Si-Mg alloy in the absence of a reinforcement was covered by a thin ( 1- to 4-gm) layer of MgO that sometimes contained up to 5% MgAl204 [33]. The external MgO layer typically was separated from the interconnected A1203 matrix by a thin aluminum alloy (1- to 3-gm) layer. Only rarely was an A1203 grain found in direct contact with the external oxide layer. Within the bulk of the composite, the metallic channels typically were 3 to 8 /tm in width. 

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