Cathodes interconnect

Figure 11 -7 shows the basic circuit diagram for a tank with two domes. The protection current flows via the two interconnected openers of the cover grounding switch to the cathode connection. If one of the covers is opened, the protection current circuit is broken and the tank grounded via the closing contact. The unconnected cable connection of the tank is without current and can be used for measuring potential. By this method, only one tank at a time is separated from the protection system while the other parts of the installations are still supplied with protection current. 

The electrochemical intercalation/insertion is not a special property of graphite. It is apparent also with many other host/guest pairs, provided that the host lattice is a thermodynamically or kinetically stable system of interconnected vacant lattice sites for transport and location of guest species. Particularly useful are host lattices of inorganic oxides and sulphides with layer or chain-type structures. Figure 5.30 presents an example of the cathodic insertion of Li+ into the TiS2 host lattice, which is practically important in lithium batteries. 

The cathode of a battery or fuel cell must allow good ionic conductivity for the ions arriving from the electrolyte and allow for electron conduction to any interconnects between cells and to external leads. In addition these properties must persist under oxidizing conditions. An important strategy has been to employ layered structure solids in which rapid ionic motion occurs between the layers while electronic conductivity is mainly a function of the layers themselves. 

In this chapter the technological development in cathode materials, particularly the advances being made in the material s composition, fabrication, microstructure optimization, electrocatalytic activity, and stability of perovskite-based cathodes will be reviewed. The emphasis will be on the defect structure, conductivity, thermal expansion coefficient, and electrocatalytic activity of the extensively studied man-ganite-, cobaltite-, and ferrite-based perovskites. Alterative mixed ionic and electronic conducting perovskite-related oxides are discussed in relation to their potential application as cathodes for ITSOFCs. The interfacial reaction and compatibility of the perovskite-based cathode materials with electrolyte and metallic interconnect is also examined. Finally the degradation and performance stability of cathodes under SOFC operating conditions are described. 

The perovskite oxides used for SOFC cathodes can react with other fuel cell components especially with yttria-zirconia electrolyte and chromium-containing interconnect materials at high temperatures. However, the relative reactivity of the cathodes at a particular temperature and the formation of different phases in the fuel cell atmosphere... 

FIGURE 3.15 Polarization curves for the 02 reduction on an LSM cathode in the presence of a Fe-Cr alloy interconnect, measured under a cathodic current passage of 200 mA cnr2 at 900°C. 

Early works were typically carried out using single atmosphere exposure conditions, either air (or moist air) representing the cathode side environment [124-129, 139, 142,144-162] or a reducing atmosphere simulating the anode side environment [124, 125, 127-129, 144, 145], Lately, studies have been also performed to determine the oxidation/corrosion behavior of metal and alloys under dual-atmosphere exposure conditions that closely simulate the interconnect exposure conditions during SOFC operation [154-159], The alloys studied include both Fe-Cr base FSSs and Ni or Ni-Cr base heat-resistant alloys, as well as Cr or Cr base alloys. 

Besides the glass seal interfaces, interactions have also been reported at the interfaces of the metallic interconnect with electrical contact layers, which are inserted between the cathode and the interconnect to minimize interfacial electrical resistance and facilitate stack assembly. For example, perovskites that are typically used for cathodes and considered as potential contact materials have been reported to react with interconnect alloys. Reaction between manganites- and chromia-forming alloys lead to formation of a manganese-containing spinel interlayer that appears to help minimize the contact ASR [219,220], Sr in the perovskite conductive oxides can react with the chromia scale on alloys to form SrCr04 [219,221],... 

Newly developed alloys have improved properties in many aspects over traditional compositions for interconnect applications. The remaining issues that were discussed in the previous sections, however, require further materials modification and optimization for satisfactory durability and lifetime performance. One approach that has proven to be effective is surface modification of metallic interconnects by application of a protection layer to improve surface and electrical stability, to modify compatibility with adjacent components, and also to mitigate or prevent Cr volatility. It is particularly important on the cathode side due to the oxidizing environment and the susceptibility of SOFC cathodes to chromium poisoning. 

Each of the components of an SOFC stack anodes, cathodes, electrolytes, and interconnects must be thermally, chemically, mechanically, and dimensionally stable at the operating conditions and compatible with the other layers with which they come into contact in terms of thermal expansion and chemical inter-reaction. They must also have compatible processing characteristics. In addition to those requirements, the individual layers have additional microstructural, property, and processing target requirements, as summarized in Table 6.1. 

SOFC electrodes are commonly produced in two layers an anode or cathode functional layer (AFL or CFL), and a current collector layer that can also serve as a mechanical or structural support layer or gas diffusion layer. The support layer is often an anode composite plate for planar SOFCs and a cathode composite tube for tubular SOFCs. Typically the functional layers are produced with a higher surface area and finer microstructure to maximize the electrochemical activity of the layer nearest the electrolyte where the reaction takes place. A coarser structure is generally used near the electrode surface in contact with the current collector or interconnect to allow more rapid diffusion of reactant gases to, and product gases from, the reaction sites. A typical microstructure of an SOFC cross-section showing both an anode support layer and an AFL is shown in Figure 6.4 [24],... 

Slurry containing the cathode powder is placed into a cylindrically shaped die with an open end, as shown in Figure 6.7 (a), and then a mandril is inserted into the tube that extrudes the slurry into the shape of a hollow tube with a closed end on one side and an open end on the other side, as shown in Figure 6.7 (b). The electrolyte, anode, and interconnect layers are then deposited onto the cathode support tube, using combinations of processes described in Section 6.2.2. 

In planar SOFCs, individual cathode, anode, and electrolyte layers have been deposited by PS [109-111], as well as coatings on interconnect materials and full cells [108, 110, 112]. In addition to the interconnect layers themselves in tubular SOFCs, dense protective layers with good adhesion have also been deposited to protect planar SOFC interconnects from oxidation [110], and diffusion barriers to inhibit inter-diffusion between the interconnects and anodes have been produced by PS [113]. 

Extensive work has been reported on the deposition of individual cell layers and of full anode-electrolyte-cathode fuel cells on metallic interconnect substrates, much of it by VPS, with no sintering or other post-deposition heat treatments required [112]. However, so far relatively thick YSZ electrolytes, approximately 25 to 35 pm, have been needed to provide sufficient gas tightness [108, 114], so further optimization of the process is required to produce thinner, gas-tight electrolytes. Peak power densities of 300 mW/cm2 have been reported at 750°C for APS single cells [114], with four-cell stacks exhibiting power densities of approximately 200 mW/cm2 at 800°C [55],... 

Summarizing progress in the field thus far, the book describes current materials, future advances in materials, and significant technical problems that remain unresolved. The first three chapters explore materials for the electrochemical cell electrolytes, anodes, and cathodes. The next two chapters discuss interconnects and sealants, which are two supporting components of the fuel cell stack. The final chapter addresses the various issues involved in materials processing for SOFC applications, such as the microstructure of the component layers and the processing methods used to fabricate the microstructure. 

Cathode The primary functions of the cathode are catalyzing the O2 reduction and transporting the O2 ions to the electrolyte. Furthermore, the cathode should possess sufficient electronic conductivity to lead the electrons from the interconnect to the reaction sites. At practical operating conditions, the ionic conductivity of LSM is several orders of magnitude lower than its electronic conductivity. Therefore, in the single-phase and two-phase LSM type cathodes presented in the introduction the reaction sites are essentially at the TPBs of air, YSZ and LSM. Mixing the LSM with YSZ is one means, apart from refining the micro-... 

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