SOFC, fabrication

Both wet-ceramic techniques and direct-deposition techniques require preparation of the feedstock, which can consist of dry powders, suspensions of powders in liquid, or solution precursors for the desired phases, such as nitrates of the cations from which the oxides are formed. Section 6.1.3 presented some processing methods utilized to prepare the powder precursors for use in SOFC fabrication. The component fabrication methods are presented here. An overview of the major wet-ceramic and direct-deposition techniques utilized to deposit the thinner fuel cell components onto the thicker structural support layer are presented below. 

Meng G, Song H, Xia C, Liu X, and Peng D. Novel CVD techniques for micro- and IT-SOFC fabrication. Fuel Cells 2004 4 48-55. 

In this text the possibility of application of novel technology, which is a combination of typical powder processing technology and polymer precursor method, for SOFC fabrication was presented. For examples of potential application in SOFC, the following structures were presented ... 

SOFC fabrication normally requires high sintering temperatures (i.e. 1300-1400°C) in order to achieve a perfect gas-tightness of the YSZ electrolyte. In the case of MS-SOFC, in order to prevent steel oxidation, sintering steps have to be performed under inert or protective atmosp-... 

Cost efficient single step co-sintering of metal/anode/electrolyte multilayer laminate is a very attractive route for MS-SOFC fabrication. 

On this basis, doped NiO powder (3 wt%AI-NiO) was selected for MS-SOFC fabrication. The cell was produced by tape casting aqueous powder suspensions, the architecture of the cell being 8YSZ/Ni0-3A +8YSZ/Ce02/steel. A cross-section of this cell is shown in Fig.9. Probably due to the short sintering time, the densification of the electrolyte was not liilly achieved for this test. However, despite the high temperature, a very porous anode microstructure was observed. 

These devices feature the use of micro-fabrication techniques adapted from the micro-electronics industry. These encompass substrate etching, thin-fihn deposition, lithography, and film-etching steps. This field has recently been reviewed by Evans et al. [11], and all devices exhibit the beautiful structural quality resulting from the micro-fabrication techniques. An example of a micro-planar SOFC fabricated on a silicon substrate is illustrated in Fig. 19.4 [12]. Figure 19.4a shows the sequence of fabrication steps used to make the edge-supported SOFC membrane which spans an aperture with dimensions 600 x 600 pm. The yttria stabilized zirconia (YSZ) electrolyte, which is only 70-nm thick, was deposited by... 

Table 21.4 gives an overview of the materials used for SOFC fabrication. The third and fourth columns refer to the raw material and the upstream processes, preparing the final product/material from the raw material. 

Other processes investigated for planar SOFC fabrication include electrostatic-assisted vapour deposition, vapour phase electrolytic deposition, vacuum evaporation, laser spraying, transfer printing, sedimentation method, and plasma metal organic chemical vapour deposition. 

Fig. 28 LT SOFC fabricated by pulsed laser deposition (a) cross-section of anode supported LSGM thin film and (b) output performance of the single cell (after Yan et al. 2005).

Table 4 Advantages and drawbacks of techniques investigated for MS-SOFCs fabrication Technique Estimated Advantages Drawbacks...

Fig. 11 Cross-sectional SEM micrograph of MS-SOFC fabricated by Ceres FSS/Ni-CGO/CGO/CGO-Lanthanum ferrite [25]. Dense CGO is obtained after firing at 1000 °C. Reproduced here with kind permission from ASM International 2004...

Fig. 13 Cross-sectional SEM micrographs of MS-SOFC fabricated (a) by suspension plasma spraying at the NRC Hastelloy X/Ni-SDC/SDC/SSCo-SDC [30, 31], Reproduced here with kind permission from Elsevier 2007. (b) DLR concept [19, 37]. Reproduced here from [44] with kind permission from John Wiley and Sons 2005. (c) and (d) Picture and SEM micrograph of MS-SOFC developed at NRC using HVOF for electrolyte deposition [32]. Reproduced here with kind permission from ASM International 2008...

For high temperature fuel ceUs, there is stiU a strong need to develop lower cost materials for ceU components. In the case of SOFCs, improved fabrication processes and materials that permit acceptable performance in fuel ceUs at lower operating temperatures are also highly desirable. 

Conceptually elegant, the SOFC nonetheless contains inherently expensive materials, such as an electrolyte made from zirconium dioxide stabilized with yttrium oxide, a strontium-doped lanthanum man-gaiiite cathode, and a nickel-doped stabilized zirconia anode. Moreover, no low-cost fabrication methods have yet been devised. 

The slurry process has been enhanced with vacuum to fabricate planar SOFCs [78], This method is of low cost and thus has been widely used to develop low-cost SOFCs. However, together with other liquid precursor methods such as sol-gel and spray pyrolysis, it is time, labor, and energy intensive because the coating-drying-sintering has to be repeated in order to avoid cracking formation. 

Anode-supported SOFCs with LSGM electrolytes (5-fJ.m thick) and SDC interlayers (2-fim thick) were fabricated by pulsed laser deposition [243],... 

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. 

The general requirements for an SOFC anode material include [1-3] good chemical and thermal stability during fuel cell fabrication and operation, high electronic conductivity under fuel cell operating conditions, excellent catalytic activity toward the oxidation of fuels, manageable mismatch in coefficient of thermal expansion (CTE) with adjacent cell components, sufficient mechanical strength and flexibility, ease of fabrication into desired microstructures (e.g., sufficient porosity and surface area), and low cost. Further, ionic conductivity would be beneficial to the extension of... 

Simner SP, Stevenson JW, Meinhardt KD, and Canfield NL. Development of fabrication techniques and electrodes for solid oxide fuel cells. In Yokokawa H, Singhal SC, editors. Proceedings of the Seventh International Symposium on Solid Oxide Fuel cells (SOFC-VII), Pennington, NJ The Electrochemical Society, 2001 2001(16) 1051-1060. 

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