Case Study – Design Concepts and Fabrication of SOFCs
...ual cells can be put into a module, as well as showing how the input and exhaust gasses are controlled. As a module the cells are connected in parallel and series using a nickel felt. In series, cells are connected by attaching the anode to the nickel-plating on the interconnect of the next cell, in parallel cells are connected anode to anode, as in Figure 4. In this method of connecting in parallel, current and voltage can be controlled as desired. ii) How the Tubular design is fabricated. Here I will go through a step-by-step process of how it is done. 1. A support tube is fabricated by an extrusion process. Extrusion is used as it is a cheap method for producing large numbers of units. The material used for the support tube must be both porous and have adequate strength. Currently a plastic mixture of CaO stabilised ZrO2, with cellulose and starch as pore formers is used, and a porosity of 35% is generally used to allow for the oxidant access to the cathode. After extrusion the tube is air sintered at 1550oC. 2. A slurry of the cathode material is then deposited on the support tube and sintered at 1400 oC. 3. The interconnect is then added to the cathode using electrochemical vapour deposition (EVD). To ensure a thin strip of interconnect material is deposited on the length of the cathode only, an easily removable masking material is used. 4. The electrolyte is again deposited by EVD on the entire cathode surface and also a 0.5mmoverlap onto the interconnect. Masking materials are again used to protect the interconnect during the EVD. 5. The interconnect is then plated with nickel to provide electrical contact. 6. The anode is formed by first covering the cell in a nickel slurry (obviously masking the interconnect to avoid short circuiting) and the anode is then fixed by EVD of the anode material into the nickel matrix. The individual cells are now ready to be put into fuel cell modules. Flat Plate i) How the flat plate design works: The flat plate design, common to other types of fuel cell, consists of thin plates of each of the components making up a cell, the interconnect generally doubles up as the gas channels by being ribbed on both side as well as acting as a bipolar gas separator between the anode and the cathode. The general way in which gasses are ducted in and out of the cell are as in the diagram above, going in one face of the cell and out of the other, however because of the flat plate designs, this can be altered to a radial or axial flow as required. As with the tubular design the method of producing electricity is the same, with the oxidant and the fuel being passed through the grooved interconnect, the oxygen ions crossing the electrolyte and reacting with the fuel to give the potential difference and the waste products which will be taken out at the opposite side to the input. The flat plate design needs high temperature gas seals at the edges of the plates, and due to the compressive nature of the seal, these have proven very difficult to fabricate reliably, as non-uniform stress can build up on the ceramic causing fatal cracking. Also at these high operating temperatures (>900 oC) cements and glasses tend to react with the cell materials, and due to the probability of mismatches in tolerance with larger numbers of cells in a stack, the problem with gas seals can also limit cell stack height. On a plus side, due to in-plane conduction, resistive losses can be minimised by making the components very thin, as the internal resistive loses are independent of cell area. ii) How the flat plate design if fabricated: The fabrication of a flat plate fuel cell is very much easier than that of the tubular cell due to it being possible to fabricate the electrolyte and the interconnect independently. In the cell one of the components must be thick enough to act as the supporting part. 1) Electrolyte and interconnect fabricated using tape casting or hot pressing techniques (50μm - 120μm) and sintering. 2) The electrodes are applied to the electrolyte using slurry methods, screen-printing, plasma spray or EVD. Then cofired with the electrolyte. 3) Interconnect attached to rest of cell and sealed. Comparison of Tubular vs. Flat Plate Tubular Flat Plate · Seamless · Big problems with seals due to non-uniform stress concentrations – can lead to ceramic failure. Seals also limit cell size and possible stack height. · Single cell unit structure – allows some freedom of thermal expansion · Multi cell unit structure – leads to cracking from thermally induced stresses · Exhaust gasses expelled into common manifold – eliminates need for leak-free gas manifolding. · Separate exhaust gas manifolds – need complex gas manifolding · Lower power density · High power density · Long current path increases resistive losses · Low internal resistive losses · Support tube limits oxygen diffusion to cathode – sets lower than necessary limiting current · No support tube – limiting factors are the component materials themselves · Difficult and expensive to fabricate · Quick and relatively cheap to fabricate · Fixed fabrication techniques – therefore fixed layer thickness, little potential to reduce ohmic losses. · Design allows for variety of fabrication options – spray pyrolosis, rf sputtering and plasma spraying as examples. Possibility of producing micrometer thick layers, reducing resistive losses. May also allow for lower temperature operation. From this it may seem that the flat plate design has more going for it, however the tubular design being seamless is a major factor as one of the main reasons SOFCs do not have a wide usage is their lack of durability. The sealing of the cells being probably the area where most damage is done, sealing products are one of the most secretive areas of current SOFC research. The tubular design SOFC is the only current design to have reasonable durability. Obviously if components of the cell are having to be replaced at frequent intervals then it will make the cell economically unviable. Component Materials 1) Electrolyte – Requirements: High ionic conductivity 0.1Scm-1 – 0.01Scm-1 depending on cell type. Gas tight – gas cannot flow between anode and cathode, as this will reduce efficiency. Very low electrical conductivity, any conductivity will “short circuit” the cell internally. Chemical stability – thermodynamic stability and stability under the oxygen potential gradient. Mechanical strength – level required depends on cell design and heating-cooling rates. Choice Yttria stabilized Zirconia is the most commonly used electrolyte, as it possesses high oxygen-ion conductivity, is stable in both reduction in oxidising atmospheres. The Yttria not only stabilises the Zirconia’s high temperature cubic phase, but also increases ionic conductivity due to extra oxygen vacancies. 8-10mol% doping with Y2o3 is generally used. Other stabilisers do have superior effects, however they are too expensive for general use. Stabilised bismuth oxide, ceria, partially stabilised Zirconia and Zirconia with matrix strengthening alumina are also being investigated. 2) Cathode – Requirements Stable in highly oxidising conditions Hig...