Ni-YSZ cermet

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). 

The extent to which anode polarization affects the catalytic properties of the Ni surface for the methane-steam reforming reaction via NEMCA is of considerable practical interest. In a recent investigation62 a 70 wt% Ni-YSZ cermet was used at temperatures 800° to 900°C with low steam to methane ratios, i.e., 0.2 to 0.35. At 900°C the anode characteristics were i<>=0.2 mA/cm2, Oa=2 and ac=1.5. Under these conditions spontaneously generated currents were of the order of 60 mA/cm2 and catalyst overpotentials were as high as 250 mV. It was found that the rate of CH4 consumption due to the reforming reaction increases with increasing catalyst potential, i.e., the reaction exhibits overall electrophobic NEMCA behaviour with a 0.13. Measured A and p values were of the order of 12 and 2 respectively.62 These results show that NEMCA can play an important role in anode performance even when the anode-solid electrolyte interface is non-polarizable (high Io values) as is the case in fuel cell applications. 

The kinetics of H2 oxidation has been investigated on a Ni/YSZ cermet nsing impedance spectroscopy at zero dc polarization. The hydrogen reaction appears to be very complex. The electrode response appears as two semicircles. The one in the high-freqnency range is assumed to arise partly from the transfer of ions across the TPB and partly from the resistance inside the electrode particles. The semicircle observed at low freqnencies is attributed to a chemical reaction resistance. The following reaction mechanism is suggested ... 

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... 

FIGURE 2.1 Change of electrical conductivity with respect to Ni content in the Ni-YSZ cermets at 1,000°C. Two types of YSZ were used to prepare the cermets one was Toyo Soda powder with a specific surface area of 23 m2/g and an agglomerate size of -0.3 pm, and the other was Zircar powder with a specific surface area of 47 m2/g and an agglomerate size of -0.1 pm the NiO used has a specific surface area of 3.5 m2/g. (From Dees, D.W. et al., J. Electrochem. Soc., 134 2141-2146, 1987. Reproduced by permission of ECS-The Electrochemical Society.)... 

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],... 

FIGURE 2.2 (a) Particle size distribution for three different NiO powders used in the Ni-YSZ cermets the circle is for a commercial NiO powder with 0 h milling, the diamond is for the commercial NiO milled for 138 h, and the square is for the NiO powder prepared by the GNP process, and (b) resistivity versus Ni volume percent for the Ni-YSZ cermets made with the three different NiO powders at 1,000°C in reducing atmosphere. The YSZ powder has grain size of 0.1 to 0.2 pm, and the cermets were all sintered at 1,400°C for 2 h. (From Huebner, W. et al., Proceedings of the Sixth International Symposium on Solid Oxide Fuel Cells, 95(l) 696-705, 1995. Reproduced by permission of ECS-The Electrochemical Society.)... 

FIGURE 2.5 Electrical conductivity versus Ni volume fraction for Ni-YSZ cermets fabricated using different starting materials, as labeled in the graph. (From Ivers-Tiffee, E. et al., Berichte der Bunsen-Gesellschaft fur Physikalische Chemie, 94 978-981, 1990. Permission pending.)... 

FIGURE 2.6 Fracture strength of Ni-YSZ cermets as a function of porosity. Standard deviation is superimposed on each average value. The starting NiO and YSZ particle sizes for FF1-13 and FF2-13 are both 0.8 pm, for FC1-13 and FC2-40 they are 0.8 and 6 pm, for CF1-13, CF2-13, and CF2-40 they are 8 and 0.8 pm. The suffixes 13 and 40 represent the volume fraction of carbon black pore former added. (From Yu, J.H. et al., J. Power Sources, 163 926-932, 2007. Copyright by Elsevier, reproduced with permission.)... 

Some researchers explored Ni-YSZ cermets that used both fine (average size of 0.6 pm) and coarse (average size of 27.0 pm) YSZ as the starting materials. It was hoped that the coarse YSZ particles would form a frame that keeps the total volume unchanged while the fine YSZ particles would sustain the network of Ni and pore, providing good electrical conductivity and microstructural stability [14]. However, it is not clear how such an anode compares with conventional anodes made from both fine NiO and YSZ in terms of strength, electrical conductivity, and electrochemical activity. 

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 2.7 Change of electrical conductivity at 1000°C with respect to Ni volume content for Ni-YSZ cermets sintered for 2 h at 1200, 1250, 1300, and 1350°C, respectively. (From Pratihar, S.K. et al., Proceedings of the Sixth International Symposium on Solid Oxide Fuel Cells, 99(19) 513—521. Reproduced by permission of ECS-The Electrochemical Society.)... 

FIGURE 2.13 (a) Maximum power and (b) cell total ohmic resistance (labeled as IR resistance ) and interfacial resistance (labeled as polarization ) at constant current density of 0.3 A/cm2 versus the volume percent of Ni in the Ni-YSZ cermet for electrolyte-supported cells with an active area of 2 cm2 operated at 1000°C. (From Koide, H. et al., Solid State Ionics, 132 253-260, 2000. Copyright by Elsevier, reproduced with permission.)... 

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 infiltrated by Mo orW precursors [116] 2005 Still poisoned by sulfur (C4H4S) but to a lesser extent and showed gradual recovery. No Pt current collector layer. Pmax °f 300 mW/cm2 at 750°C in H2. 

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. 

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