Cermet anodes microstructure

Fig. 28 Two-dimensional schematic of the microstructure of a slurry-coated cermet anode.

Since hydrocarbons are important fuels for SOFC, carbon deposition should be avoided [89]. When carbon deposition once starts, this destroys the microstructure of Ni-cermet anode and deposited carbon acts as catalyst for further deposition so that a large amount of carbon can block the flow of fuels. Since the decomposition temperature of hydrocarbons decreases with increasing carbon number in hydrocarbons, the possibility of carbon deposition increases with carbon number. On the other hand, water vapors are emitted from the electrochemically active sites and this promotes the steam reforming. Thus, under cell operation, the possibility of carbon deposition decreases in the vicinity of active sites. 

So far, the discussion has related largely to the compositional nature of candidate anodes however, the microstructure of the electrode is at least as important as its composition. The optimization of durable efficient nickel cermet anodes in recent... 

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),... 

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. 

Itoh H, Yamamoto T, and Mori M. Configurational and electrical behavior of Ni-YSZ cermet with novel microstructure for solid oxide fuel cell anodes. J Electrochem 5ocl997 144 641-646. 

Lee J-H, Moon H, Lee H-W, Kim J, Kim J-D, and Yoon K-H. Quantitative analysis of microstructure and its related electrical property of SOFC anode, Ni-YSZ cermet. Solid State Ionics 2002 148 15-26. 

The anode (fuel electrode) consists of an approximately 120/rm thick Ni/YSZ cermet layer. This is achieved simply by applying a slurry of nickel and YSZ powders to the electrolyte and sintering. A cross-section of the 3-layer microstructure is shown in Fig. 4.33. 

It is very important for a high-performance electrode to have both a highly active electrocatalytic reaction zone and a sufficient gas-supply network in its microstructure. The conventional anode material used so far is Ni-YSZ cermet prepared from pm-sized NiO and YSZ particles as shown in Fig. 3A. Because all reactants (fuel gas, electrons, and oxide ions) must meet together at the reaction sites, the so-called "three phase boundary (TPB) zone is the effective... 

Microstructures and morphology of the anode cermet before and after reduction were investigated by the S-4700 scanning electronic microscope. Thermogravimetric analysis (TGA) was used to confirm the green tape sintering parameters by TAS-100 TGA. Porosities of the Ni-GDC anode were examined by Archimedes method. 

The deposited carbon not only may block the porosity of the anode but also disrupts the cermet microstructure, breaking Ni-ZSZ linkages, and finally causes degradation of cell performance. 

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