Cermet anodes fabrication

Misono T, Murat, K, Fukui T, Chaichanawong J, Sato K, Abe H et al. Ni-SDC cermet anode fabricated from NiO-SDC composite powder for intermediate temperature SOFC. J. Power Sources 2006 157 754—757. 

There are a number of informative reviews on anodes for SOFCs [1-5], providing details on processing, fabrication, characterization, and electrochemical behavior of anode materials, especially the nickel-yttria stabilized zirconia (Ni-YSZ) cermet anodes. There are also several reviews dedicated to specific topics such as oxide anode materials [6], carbon-tolerant anode materials [7-9], sulfur-tolerant anode materials [10], and the redox cycling behavior of Ni-YSZ cermet anodes [11], In this chapter, we do not attempt to offer a comprehensive survey of the literature on SOFC anode research instead, we focus primarily on some critical issues in the preparation and testing of SOFC anodes, including the processing-property relationships that are well accepted in the SOFC community as well as some apparently contradictory observations reported in the literature. We will also briefly review some recent advancement in the development of alternative anode materials for improved tolerance to sulfur poisoning and carbon deposition. 

Jiang SP, Callus PJ, and Badwal SPS. Fabrication and performance of Ni/3% mol% Y203-Zr02 cermet anodes for solid oxide fuel cells. Solid State Ionics 2000 132 1-14. 

Ni-YSZ (yttria stabilized zirconia) is the state-of-the-art anode for SOFC devices owing to its high electrcmic conductivity, physical and chemical compatibility with most of the common electrolyte materials, and good performance in H2 and reformed fuel environments. Generally, a mixture of NiO and YSZ is used for the fabrication of the anode. In situ reduction of NiO to Ni (Eq. 1) during SOFC operation forms the Ni-YSZ cermet anode ... 

Jiang, S.P., Duan, YY Love, J.G. Fabrication of high-performance Ni0/Y203-Zi02 cermet anodes of solid oxide fuel-cells by ion impregnation. J. Electrochem. Soc. 149 (2002), pp. A1175-Al 183. 

Recently efforts have been made on oxide anodes. The main reason for such investigations is to overcome the demerits of Ni cermet anodes as just described. Although the oxide anodes should be in service under a reducing atmosphere, the fabrication is usually performed in air so that oxide anodes should be stable at both oxidative and reductive atmospheres. This requirement is similar to those for oxide interconnects, implicitly indicating that material selection becomes severe to meet the chemical stability requirement. 

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. 

Empirical development of the nickel-zirconia anode over several decades has led to solid oxide fuel cells with adequate service life and performance, but fuel reforming is still required to operate with commercially available hydrocarbon fuels. It has become evident that the anode reactions are dominated by the three-phase boundary and that the microstructure of the composite cermet anodes is pivotal. Consequently, the processing methods used for making the anode powders, and the fabrication techniques used for deposition on the electrolyte are critical in making high performance anodes. Anode-supported cells with very thin electrolyte films are becoming interesting for operation at lower temperatures. 

Overpotential of Ni-YSZ cermet anode is significantly less compared to other ceramic fuel cell components. Normally, planar SOFCs having standard cell components is capable of producing more than 500 mW/cm of power. However, a careful control of the anode substrate fabrication could even deliver very high power densities, even up to 1.8 W/cm ( 3.5 A/cm at 0.5 V) at 800°C (Kim et al. 1999). The detailed information and fundamental studies on this aspect is available in numerous articles—calculations on the optimal morphology, microstructure, porosity and thickness of such cermet anodes have been reported by Minh et al. (1995), Costamagna et. al. (1998),... 

The anode is usually a thin layer of Ni-YSZ cermet. At temperatures over 1173K, the kinetics of steam reforming reactions are faster than the electrochemical oxidation of hydrogen or carbon monoxide. The other principal component is the bipolar plate, which is commonly fabricated from LaCrOa (Mg-doped) which is predominantly an electronic conductor and is compatible with other cell components at 1273 K. It is however relatively brittle and expensive, and there is a continuous effort to replace it with other materials preferably with a metallic system. 

Metal Supported-Solid Oxide Fuel Cells (MS-SOFC) represent a promising new design for fuel cells which may overcome the limitations of anode-supported cells (such as poor thermal cycling resistance and brittleness. Nickel phase re-oxidation upon exposure to transient uncontrolled conditions) due to the much better mechanical properties of the support that is represented by a porous thick metal substrate, the thickness of the ceramic layers (anode/electrolyte/cathode) being in the order of 10-50 pm, only. In addition, in this design (Fig. 1), the replacement of the thick Ni/YSZ cermet with ferritic stainless steel leads to several benefits in term of fabrication cost and safety. 

The SOFC anode is traditionally fabricated by mechanically mixing NiO with the electrol)de. Taking the Ni-YSZ cermet as an example, the mixture of NiO and YSZ powders is firstly... 

NiAl or NiCr metals have been employed as MCFC anodes. These materials are used because Ni metal anodes are not stable enough under MCFC operating conditions as Ni creeps out [18,20]. Cermet (ceramic metal) materials avoid sintering, pore growth, and shrinkage of the Ni metal so that a loss of surface area does not occur. A low- cost process needs to be found, however, as these materials are still expensive to fabricate. 

Planar SOFC also uses the same common materials for component fabrication as used for other designs of SOFC YSZ is used as an electrolyte, NiA"SZ cermet as an anode, and lanthanum strontium manganite as the cathode. If the electrolyte thickness is 5-20 pm instead of 100 pm then the cell can operate at reduced temperatures, which allows a wide choice of materials along with the reduction in thermal stress. Anyhow, the planar geometries offer some drawbacks including ... 

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