SOFC, anode resistivity

The current state-of-the-art SOFC anode-supported cells based on doped zircona ceramic electrolytes, ceramic LSM cathodes, and Ni/YSZ cermet anodes are operated in the temperature range 700-800°C with a cell area specific resistance (ASR) of about 0.5 O/cm at 750°C. Using the more active ceramic lanthanum strontium cobalt ferrite (LSFC)-based cathodes, the ASR is decreased to about 0.25 Q/cm at this temperature, which is a more favorable value regarding overall stack power density and cost-effectiveness. 

Suzuki et have developed Ru-YSZ anodes which show significantly lower polarization compared with the conventiorral anodes using Ni-YSZ cermets. Moreover, tests using Ru/AljOj have revealed that imder SOFC anode operational conditions, Ru metal has a high-steam reforming reaction activity, carbon deposition resistance, and sintering resistance. Dees et al. ° have prepared Ni-YSZ cermets by mixing NiO and zirconia. Then NiO is reduced to... 

From the literature it is clear that nano-structured ceria has several advantages over similar microcrystalline materials. In SOFC applications, resistance to impurities such as sulfur, and resistance to carbon formation when hydrocarbons are used as fuel, have been reported. The high density of surface defects in nanostructured materials provides a large number of active sites for various surface interactions and surface exchange processes. There are clearly interesting features in nano-sized ceria compounds that need further exploration. The possibility of minimizing pretreatment and water vapor partial pressure in e.g. the natural gas feed due to lower susceptibility to coke formation of ceria containing fuel electrodes (anodes), may simplify the fuel cell system. 

This chapter first considers the complex mix of attributes required of SOFC anodes, including matching of thermal expansion coefficients, chemical compatibility with the electrolyte and the interconnect, porous structure to allow gas permeation, and corrosion resistance to the fuel and impurities therein. Then the nickel cermet anode is described in detail, especially its fabrication processes. Steady-state anode reactions of hydrogen and carbon monoxide are analysed, followed by a description of transient effects. Finally, behaviour under current load and operation on different fuels are discussed. The details of the anode reactions and polarisations are described in Chapter 9. 

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. 

The introduction of such a layer can dramatically improve the fuel cell performance. For example, in the SOFC with bilayered anode shown in Figure 6.4, the area-specific polarization resistance for a full cell was reduced to 0.48 Hem2 at 800°C from a value of 1.07 Qcm2 with no anode functional layer [24], Use of an immiscible metal oxide phase (Sn()2) as a sacrificial pore former phase has also been demonstrated as a method to introduce different amounts of porosity in a bilayered anode support, and high electrochemical performance was reported for a cell produced from that anode support (0.54 W/cm2 at 650°C) [25], Use of a separate CFL and current collector layer to improve cathode performance has also been frequently reported (see for example reference [23]). 

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