Fuel cells using solid cermet

This reaction is of great technological interest in the area of solid oxide fuel cells (SOFC) since it is catalyzed by the Ni surface of the Ni-stabilized Zr02 cermet used as the anode material in power-producing SOFC units.60,61 The ability of SOFC units to reform methane "internally", i.e. in the anode compartment, permits the direct use of methane or natural gas as the fuel, without a separate external reformer, and thus constitutes a significant advantage of SOFC in relation to low temperature fuel cells. 

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. 

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.)... 

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. 

N. Kiratzis, P Holtappels, C. E. Hatchwell, M. Mogensen, and J. T. S. Irvine, Preparation and characterization of copper/yttria titania zirconia cermets for use as possible solid oxide fuel cell anodes, Fuel Cells 1,211-218 (2001). 

For the solid oxide fuel cells (SOFCs), a number of environmentally critical items have been identified (Zapp, 1996). The carrier sheet electrolyte may be produced from yttrium-stabilised zirconium oxide with added electrodes made of, e.g., LaSrMn-perovskite and NiO-cermet. Nitrates of these substances are used in manufacturing, and metal contamination of wastewater is a concern. The high temperature of operation makes the assembly very difficult to disassemble for decommissioning, and no process for recovering yttrium from the YSZ electrolyte material is currently known. 

Another Ni-based solid oxide fuel cell (SOFC) electrode was developed on which a YSZ (yttria-stabilized zirconia) cermet and Lanthanum chromite were deposited by a slurry coating method. It was also suggested that a plasma spraying process can be used for the cermet deposition on the electrodes. The following reactions are expected to take place in a fuel cell employing a natural gas source, where internal reforming takes place on the Ni-YSZ electrode ... 

The anodes consisting of a nickel catalyst and of cermet mixed with yttria-doped zirconia electrolyte that are used in conventional solid oxide fuel cells also lose their ability to work at lower temperatures because of a loss of conductivity by the ceramic. This suggests that, for the ceramic in the anode, a material having a higher conductivity at intermediate temperatures should be used. It was in fact shown that an anode made with a nickel/samaria-doped ceria cermet has a much lower polarization than the conventional variant. 

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. 

This paper presents a brief review of the literature of nickel-based cermet electrodes for application in solid oxide cells at temperature from 500 to 1000 °C. The applications may be fuel cells or electrolyser cells. Variables that are used for controlling the properties of Ni-cermet-electrodes are (1) Ni/electrolyte volume ratio, (2) additives, e.g. alloying of the Ni or infiltration of the composite with nanoparticles of other elements or compounds, (3) the chemical composition of the electrolyte component and (4) porosity and particle size distribution, which is mainly affected by raw materials morphology, application methods and production parameters such as milling and sintering possibly followed by infiltration of nanosized electrocatalytic active particles. The various electrode properties are deeply related to these parameters, but also much related to the atomic scale structure of the Ni-electrolyte interface, which in turn is affected by segregation of electrolyte components and impurities as well as poisons in the gas phase. 

The basic elements of a SOFC are (1) a cathode, typically a rare earth transition metal perovskite oxide, where oxygen from air is reduced to oxide ions, which then migrate through a solid electrolyte (2) into the anode, (3) where they combine electrochemically with to produce water if hydrogen is the fuel or water and carbon dioxide if methane is used. Carbon monoxide may also be used as a fuel. The solid electrolyte is typically a yttrium or calcium stabilized zirconia fast oxide ion conductor. However, in order to achieve acceptable anion mobility, the cell must be operated at about 1000 °C. This requirement is the main drawback to SOFCs. The standard anode is a Nickel-Zirconia cermet. 

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