The impact of anode microstructure on the power generating characteristics of SOFC

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Abstract

It is normally required that the anode materials should have high electrical conductivity and gas permeability to reduce the polarization loss of the cell. In this study, we made anode substrates of SOFC with two different methods, which resulted in different anode microstructures, especially different pore structures. We investigated the effect of microstructure of anode substrate on the unit cell performance in terms of its electrical conductivity, gas permeability and power-generating characteristics. According to the analysis, the anode substrate with higher inactive pore volume resulted in significantly reduced cell performance by decreasing the number of electrical conduction paths, the effective gas diffusion paths and the electrochemically active sites for the anodic reaction.

Introduction

It is well known that there are two basic designs, electrolyte-supported and electrode-supported types, in the solid oxide fuel cells [1]. Between these two designs, the former has rather simple fabrication procedure, but higher ohmic resistance from thick electrolytes on the order of 150–200 μm required higher operating temperature around 1000 °C. Such a high-operating temperature was advantageous for cogeneration applications but normally induced some adverse influences on the cell and/or stack stability due to degradation of its constituent components for long-term operation. Thus, reducing the operating temperature of SOFC has been one of the hottest issues for the commercialization of SOFC in the last 10 years. With reducing the thickness of solid electrolyte in electrode-supported type SOFC, one can lower the operating temperature of SOFC to below 800 °C, which is proper not only for reducing the degradation of cell components but also for allowing the use of readily available and cheap ferritic steel for the interconnector [2].

Multiple functions are required for anode substrate in the anode-supported type SOFC as compared to the single electrodic function in the electrolyte-supported type SOFC. In anode-supported type SOFC, the major role of the anode is of course to provide the proper sites for an electrochemical oxidation of the fuel and to deliver the produced electrons to the interconnector. Additionally, anode substrate in anode-supported type SOFC should also play as a mechanical support for the thin electrolyte and cathode films and as a gas diffusion path for the supply of the fuel to reaction sites [1].

The electrical property and/or electrochemical property of SOFC anode are normally governed not only by the electrical conductivity of the metal–ceramic composite itself but also by the appropriate connection of the gas diffusion path, in order to reduce the ohmic- and diffusional-polarization loss of the cell performance, respectively. It is well known that both electrical conductivity and gas permeability of SOFC anode are strongly related to the microstructural parameters such as particle size, composition and spatial distribution of each constituent phase [3], [4], [5], [6], [7]. Thus, characterization of electrical and/or electrochemical properties of anode substrate and their dependence on the microstructural parameters is very important for the performance optimization of the fuel cells.

Currently used Ni–YSZ cermet is known to have many desirable properties for SOFC anode because it has both electronic conduction path through Ni metal and ionic conduction path through YSZ. In addition, it also has enough porosity to give proper delivering path for the fuel by which the initial NiO phase is reduced to Ni during the cell operation in reducing atmosphere. Nonetheless, because porosity introduced from NiO reduction is not enough to fulfill the generally accepted levels of porosity around 40%, we used to make additional porosity with artificial pore former like graphite. Thus, the microstructure and/or pore structure of anode substrate is greatly influenced by the size, shape and amount of pore former and consequently, the anodic properties are as well.

In this study, our effort to develop anode substrate for the practical application of the low or intermediate temperature anode-supported type SOFC is reported. We made anode substrate using two different methods, which give different anode microstructures, especially different pore structure from each other. We investigated the effect of pore former on the microstructure and/or pore structure of anode substrate and an attempt was made to quantify the microstructural parameters of anode substrate based on the quantitative stereological measurements. The effect of microstructure of anode substrate on the unit cell performance in terms of its electrical conductivity and gas permeability will also be discussed.

Section snippets

Experimental

Fine yttria-stabilized zirconia (Tosoh, Japan) and coarse yttria-stabilized zirconia (Unitec, UK) and nickel oxide (J.T. Baker, USA) were used to prepare NiO-YSZ composite. The average particle size of fine and coarse YSZ and NiO was 0.25, 1.8 and 0.8 μm respectively. We used two different sizes of YSZ in order to maintain structural integrity during the operation at high temperature by forming YSZ skeleton with coarse particles. The pre-mixed powders were ball-milled in proper solvents for 24

Microstructural analysis

SEM micrographs of NiO–YSZ substrate from two different fabrication methods were given in Fig. 1, Fig. 2, which included also the micrographs from two different direction of observation, A and B for each substrate. In this case, directions A and B represent the directions parallel and vertical to the direction of current flow during the fuel cell operation, respectively. As shown in Fig. 1, the NiO–YSZ substrate from spray drying method (SDM) with artificial pore former shows tremendously

Conclusion

According to the investigation of microstructural and anodic properties of Ni-YSZ cermet, the undesirable microstructural properties of anode substrate, such as anisotropic packing or pore structures, lower number density of pores, lower connectivity of conducting or pore phases, resulted in significantly reduced cell performance by decreasing the number of electrical conduction paths, the effective gas diffusion paths and the electrochemically active sites for the anodic reaction. Thus, in

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