Cermet Anode

One can only admire the insight of the first researchers who used Ni as the active electrode material in the Ni/YSZ cermet anodes In addition to being a good electrocatalyst for the charge transfer reaction (3.8), Ni is also an excellent catalyst for the steam or C02-reforming of methane ... 

I.V. Yentekakis, Y. Jiang, S. Neophytides, S. Bebelis, and C.G. Vayenas, Catalysis, Electrocatalysis and Electrochemical Promotion of the Steam Reforming of Methane over Ni Film and Ni-YSZ cermet Anodes, Ionics 1, 491-498 (1995). 

Sol-gel technique has also been applied to modify the anode/electrolyte interface for SOFC running on hydrocarbon fuel [16]. ANiA SZ cermet anode was modified by coating with SDC sol within the pores of the anode. The surface modification of Ni/YSZ anode resulted in an increase of structural stability and enlargement of the TPB area, which can serve as a catalytic reaction site for oxidation of carbon or carbon monoxide. Consequently, the SDC coating on the pores of anode leads to higher stability of the cell in long-term operation due to the reduction of carbon deposition and nickel sintering. 

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. 

In the following sections, the electrical conductivity, electrochemical activity toward hydrogen oxidation, and the sulfur poisoning behavior of Ni-YSZ cermet anodes will be discussed in detail, together with the effects of various processing procedures and testing conditions. 

The electrical conductivity of a Ni-YSZ cermet anode depends on the composition (i.e., Ni to YSZ volume ratio), the microscopic features of the starting materials (e.g., particle size and distribution of NiO and YSZ powders), and the sintering and reduction conditions (e.g., temperature and atmosphere), as will be discussed in detail in the following sections. 

Since the conductivity of Ni is more than 5 orders of magnitude greater than that of YSZ under the fuel cell operating conditions, the electrical conductivity of a porous Ni-YSZ cermet anode changes several orders of magnitude, usually from -0.1 S/cm... 

Similar to the percolation threshold, the effective electrical conductivity of a porous Ni-YSZ cermet anode depends on the morphology, particle size, and distribution of the starting materials as well. In general, the effective conductivity increases as the NiO particle size is reduced when other parameters are kept constant. As shown in Figure 2.4 (samples 1 and 2), the cermet conductivity increased from -10 S/cm to 103 S/cm as the NiO particle size was decreased from 16 to 1.8 pm while using the same YSZ powder (primary particle size of -0.3 pm) and the same Ni to YSZ volume fraction [30],... 

The bulk conductivity of a fully dense cermet, oh, can be estimated from the effective conductivity, oe, and the porosity,, of a porous cermet anode using the Bruggeman equation [12]... 

In addition to composition and starting materials particle size, the conductivity of the Ni-YSZ cermet anode is strongly influenced by processing procedures including the sintering and the reduction conditions of the cermet, which will be discussed in detail in this section. 

FIGURE 2.12 Effect of Ni content on the electrode/electrolyte interface conductivity (aE) and the ohmic resistance (Wohm) for Ni-YSZ cermet anodes prepared using NiO-YSZ powder mixtures precalcinated at 1400°C and sintered at 1500°C. (From Kawada, T. et al.,./. Electrochem. Soc., 137 3042-3047, 1990. Reproduced by permission of ECS-The Electrochemical Society.)... 

The influence of porosity on the electrochemical activity has not been studied much for electrolyte-supported cells because anode pastes for electrolyte-supported cells are made for screen printing, and thus contain significant amounts of organics, which almost guarantees sufficient porosity. In addition, since the anode thickness for electrolyte-supported cells is only on the order of 50 pm, the concentration polarization itself becomes much less of an issue. In fact, Jiang et al. [44] showed that anode overpotential for cermet anodes prepared with extra graphite pore formers... 

The sulfur poisoning of SOFCs with Ni-YSZ cermet anodes has been studied extensively and, in this section, the previous studies will be briefly summarized. 

Dees et al. [66, 67] reported that the sulfur poisoning was due to a large increase in anode interfacial polarization resistance (Rp). They found that total Rp for an Ni-YSZ cermet anode/electrolyte/anode symmetrical cell in 97% H2/3% H2 increased from 0.27 to 0.45 fl/cm2 (an -67% increase) when 100 ppm H2S was introduced into the... 

Ni-YSZ cermet anodes satisfy most of the basic requirements for SOFC anodes. The effective conductivity of a Ni-YSZ cermet anode increases with the Ni to YSZ volume ratio, relative density, and decreasing the particle size ratio of NiO to YSZ. While coarse YSZ powders may result in poor mechanical strength and low stability, coarse NiO powders may lead to poor effective conductivity. The effective conductivity increases with the temperature at which the NiO is reduced to Ni metal in a reducing atmosphere. Further, very low reduction temperatures (e.g., below 400°C) may result in not only low electrical conductivity, but also poor mechanical strength. 

The development of new, alternative anode materials has recently attracted considerable interest. Several new materials show improved tolerance to sulfur poisoning and carbon deposition. However, critical issues associated with each candidate material are yet to be overcome. The traditional Ni-YSZ cermet anode still offers the best performance when clean hydrogen is used as the fuel and will continue to play an important role in SOFCs. 

Primdahl S and Mogensen M. Oxidation of hydrogen on Ni/yttria-stabilized zirconia cermet anodes. J Electrochem Soc 1997 144 3409-3419. 

Kawada T, Sakai N, Yokokawa H, Dokiya M, Mori M, and Iwata T. Characteristics of slurry-coated nickel zirconia cermet anodes for solid oxide fuel cells. J Electrochem Soc 1990 137 3042-3047. 

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. 

Wen C, Kato R, Fukunaga H, Ishitani H, and Yamada K. The overpotential of nickel/ yttria-stabilized zirconia cermet anodes used in solid oxide fuel cells. J Electrochem Soc 2000 147 2076-2080. 

Smith M and McEvoy AJ. Sulfur-tolerant cermet anodes. In Singhal SC, Mizusaki J, editors. Proceedings of the Ninth International Symposium on Solid Oxide Fuel Cells (SOFC IX), Pennington, NJ The Electrochemical Society, 2005 2005(7) 1437-1444. 

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. 

Kim H, Lu C, Worrell WL, Vohs JM, and Gorte RJ. Cu-Ni cermet anodes for direct oxidation of methane in solid-oxide fuel cells. J. Electrochem. Soc. 2002 149 A247-A250. 

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