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HYDROGEN ECONOMY Fuel cells
MANUFACTURING SOLID OXIDE FUEL CELLS WITH AN AXIAL-INJECTION PLASMA SPRAY SYSTEM
Z. Tang1, I. Yaroslavsky1, A. Burgess1, O. Kesler2 3, B. White2, N. Ben-Oved2
1 Northwest Mettech Corp., North Vancouver, BC, Canada 2Dept of Mechanical Engineering, University of British Columbia, Vancouver, BC, Canada 3 National Research Council — Institute for Fuel Cell Innovation, Vancouver, BC, Canada
Atmospheric plasma spraying (APS) has emerged as a cost-effective alternative to traditional sintering processes for solid oxide fuel cell (SOFC) manufacturing. However, the use of plasma spraying for SOFCs presents unique challenges, mainly due to the high porosity required for the electrodes and fully dense coatings required for the electrolytes. By using optimized spray conditions combined with appropriate feedstocks, SOFC electrolytes and electrodes with required composition and microstructure could be deposited with an axial plasma spray system. In this paper, the challenges for manufacturing SOFC anodes, electrolytes, and cathodes are addressed. The effects of plasma parameters and different feedstocks on coating microstructure are discussed, and examples of optimized coating microstructures are given. Process was provided with APS Northwest Mettech Co. plasma system "Axial IIItm". Northwest Mettech's Axial Plasma Technology has been demonstrated worldwide to have the highest production capability of any thermal spray system on the market today. The same technology can be use also for the production of the catalyst reactors for the Gas-To-Liquids, Fischer-Tropsch Processing.
Introduction
Plasma spraying is a widely used process to deposit metallic and ceramic coatings for thermal barrier and wear- and corrosion-resistant coatings. In recent years, an increasing number of research and development efforts have been devoted to manufacturing solid oxide fuel cells (SOFCs) by plasma spraying [1]. SOFCs are very highly efficient energy conversion devices that convert fuel electrochem-ically to electricity with negligible pollution emissions. Typically, SOFCs are produced using wet ceramic techniques based on tape casting, screen printing, and subsequent sintering processes [2]. The use of plasma spraying for SOFC manufacturing presents many advantages over wet ceramic processing with regards to both performance and cost. One obvious advantage is the speed of processing that results from elimination of the sintering steps, so that the cell layers can be processed in rapid succession, with the possibility of introducing functional gradients in both composition and microstructure to improve thermo-mechanical and electrochemical performance. The elimination of sintering also facilitates the use of robust, inexpensive metallic substrates as the mechanical support and electrical interconnects of the cells, with the more expensive active cell layers produced in thin layers.
The plasma spraying process is also rapid and easy to automate, making the process potentially very well suited for mass production [3].
However, the use of plasma spraying for SOFCs presents significant challenges, mainly due to the fully dense coatings required for electrolytes and the high porosity required for the electrode layers. Plasma sprayed coatings currently manufactured for common applications such as thermal barrier coatings typically exhibit porosities in the range of 5-15 % [4]. However, for the successful production of SOFCs, this range must be extended down to 0 % open porosity for electrolytes, and up to 40 % open porosity for optimal electrode coatings. This significantly wider range of coating porosities requires substantial modification to the spraying process from procedures that have been established previously for other applications. Significant work has been aimed at overcoming these challenges and extending the range of porosities with considerable success [5-9].
In this paper, efforts are focused on SOFC deposition using a plasma torch with axial powder feeding, which is capable of dense electrolyte deposition with fine powders (<10 microns), and porous anode and cathode deposition with powder mixtures of very different particle sizes.
Статья поступила в редакцию 21.06.2007 г. на конкурс «Водородная энергетика и транспорт». The article has entered in publishing office 21.06.2007 for contest "Hydrogen energy and transport"
Experimental Procedure
The atmospheric plasma spray system used for these experiments was an Axial III Series 600 Torch (Northwest Mettech Corp., Vancouver, Canada). As its name implies, this torch injects powder axially, between 3 electrodes, ensuring that virtually all of the powder injected passes through the hottest part of the jet [10]. The Axial III torch allows for the feeding of fine powders (<10 microns) in combination with an MPF powder feeder (Thermico, Dortmund, Germany).
Yttria-stabilized zirconia (YSZ) powders (8 mol. % Y2O3) with two different particle size ranges of 5-25 and 3-5 microns were used for electrolyte deposition. Both powders are fused and crushed with irregular particle shapes. It is interesting to note that the 3-5 micron fine powders exhibited good flowability with the MPF powder feeder. For composite anode and cathode deposition, two different powders were dry mixed and fed from a single hopper. YSZ and lanthanum strontium manganite (LSM) powders were mixed and used for cathode deposition, and YSZ and Ni-C, or samaria-doped ceria (SDC) and CuO powders were used for anode deposition.
During spraying, the particle temperatures and velocities were characterized with SprayWatch (Os-eir, Finland). Deposition was carried out onto sandblasted stainless steel substrates mounted on a rotating turntable. The surface topography and cross section of as sprayed coatings were examined by optical microscopy and scanning electron microscopy (SEM).
Results and Discussion
Most efforts were spent on the deposition of the dense electrolyte with different feedstock and plasma parameters. Fig. 1 shows the cross section microstructures of sprayed coatings. Both coatings were sprayed with the same high power plasma (up to 140 kW), but with different particle size ranges: (a) 5-25 microns, and (b) 3-5 microns. It is seen that at this spray condition, the coating with powder of 5-25 microns is much more porous than that with 3-5 microns. Fig. 2 shows the surface morphologies of both coatings, which indicate that the finer powders resulted in finer spray splats than those of the coarser ones.
The average in-flight particle temperature and velocity were measured with SprayWatch, and the results are summarized in Fig. 3. Both the average particle temperature and velocity decreased significantly at increasing stand-off distance (SOD). The particle temperature profiles are very similar for both powders. At a SOD of 100mm, which is the spraying distance for both coatings, the mean particle temperature is up to 2700 °C, which is above the meting point of YSZ. However, the average particle velocity is much higher for finer powder (420 m/s) than that for the coarse one (345 m/s).
Therefore, the process optimization was focused on the feedstock of 3-5 micron powder, and a dense electrolyte was achieved. A typical coating structure is shown in Fig. 4.
Fig. 1. Cross section microstructures of plasma sprayed electrolyte using YSZ powders with the particle size range of (a) 5—25 microns, and (b) 3—5 microns
Fig. 2. Surface morphologies of plasma sprayed electrolyte using YSZ powders with the particle size range of (a) 5—25 microns, and (b) 3—5 microns
Z. Tang, I. Yaroslavsky, A. Burgess, O. Kesler, B. White, N. Ben-Oved Manufacturing solid oxide fuel cells with an Axial-injection Plasma Spray System
Fig. 3. In-flight particle temperature and velocity with different stand off distance (SOD)
Depositing the dense electrolyte is the biggest challenge for plasma spraying processesing of SOFCs. With plasma spraying, a dense coating is only achievable with completely molten particles, which are accelerated to a high velocity in the plasma jet and therefore flatten into dense lamellae on impact with the substrate. As shown above, particle size is a very important factor affecting the coating microstructure. It is seen that finer powders result in finer splats and lamellae, and
exhibit higher particle velocities, which are seen in Figs. 1 and 3, to benefit in the deposition of a dense coating.
The principle challenge in producing LSM/YSZ composite cathodes by plasma spraying is achieving the desired cathode composition (50/50 vol. %) and porosity (~40 %). YSZ usually requires a very high temperature plasma with hydrogen gas in order to melt the powder feedstock. However, plasma gas compositions with even as little as 5 % hydrogen gas led to extensive reduction and decomposition of the LSM. The use of large LSM and small YSZ powders can be used to partially compensate for the DE differences of the two materials. The coatings produced using plasmas with Ar-N2 gas mixtures at short standoff distances tend to have both LSM and YSZ particles remaining partially solid during deposition, causing the coating to have much higher levels of porosity, while still containing a sufficient amount of YSZ (Fig. 5). This combination of high open porosity and reasonably good YSZ content is most desirable for SOFC cathodes [6].
A composite consisting of YSZ and Ni is the most common material combination for the anode. Similarly to the co-deposition of YSZ and LSM, a big challenge for Ni and YSZ co-deposition is addressing the large difference of their melting temperatures, and the required high porosity (~40 %). By adjusting plasma parameters, and using nickel-graphite as feedstock, in which graphite serves
Fig. 4. SEM micrograph of optimized electrolyte coatings with 3—5 micron YSZ powders
Fig. 5. Coating Surface of YSZ-LSM cathode with high porosity and surface area
Fig. 6. Coating surface of well-mixed, high surface area anodes, (a) Ni-YSZ, and (b) Cu-SDC
as a pore former, a desired Ni-YSZ microstructure was achieved (Fig. 6a). Furthermore, a CuO-SDC composite was successfully co-deposited for application in direct-oxidation SOFC anodes [8], with a microstructure resulting in good anode performance after reduction of the CuO to Cu, as shown in Fig. 6b. This process will greatly simplify the current multi-step complex processing procedure of Cu based anodes.
As shown above, finer YSZ powder is preferred for the deposition of dense electrolytes, and powder mixtures with optimal combinations of particle sizes result in anode and cathode layers with sufficient porosity and desired compositions. Plasma spray processing, traditionally, uses feedstock with powder sizes in the range of 10-150 microns that is injected into the plasma radially. In this case, the coating quality and deposition efficiency is sensitive to the injection position, angle, and velocity in addition to the particle size and size distribution. Axial powder feeding has advantages over radial powder injection in the deposition efficiency and coating quality [11]. Combined with MPF powder feed technology, the axial powder injection torch allows for the feeding of fine powders up to 3-5 microns, which is not achievable by radial injection plasma systems. As the powders are immediately and completely entrained in the plasma jet, the deposition efficiency and coating quality are not so sensitive to injection conditions, and to particle size and size distribution. This makes the configuration ideal for composite electrode deposition using powder mixtures with very different particle sizes, as well as for single materials with a small particle size.
Conclusions
Axial injection plasma spraying was employed for deposition of SOFC electrolytes and electrodes. By using optimized spray conditions combined with appropriate feedstocks, SOFC electrolytes and electrodes were deposited with the required compositions and microstructure. The axial injection plasma system demonstrates the capability of dense YSZ deposition using fine powders ranging from 3-5 microns in size, and composite electrode deposition with powder mixture of very different particle sizes. In cooperation with several SOFC companies, process optimization is being refined in relation to specific SOFC design and substrate specifications.
Acknowledgements
The authors would like to acknowledge the financial support of The Natural Sciences and Engineering Research Council of Canada (NSERC), and National Research Council-Industrial Research Assistance Program (NRC-IRAP).
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