Coplanar Fuel Cell Design Strip Cells

3 COPLANAR FUEL CELL DESIGN STRIP CELLS 

The coplanar fuel ceU design is used primarily for fuel cells with a mixed-reactant supply. In this design both selective electrodes (anodes and cathodes) are situated on the same surface of the electrolyte (ion-conducting membrane, matrix filled with liquid electrolyte, or solid electrolyte). This surface also contacts the reactant mixture. Such an electrolyte is said to be single-faced. This is in contrast to the conventional MEAs used for almost all varieties of fuel cells, in which the electrolyte is dual-faced, contacting the anode and the fuel on one side and the cathode and the oxidizer on the other side. 

The earliest references connected with strip cells ( surface migration cells ) seem to be in a report by Van Gool in 1965 and in a patent issued to Louis et al. in 1979. The terms surface-diffusion and surface-migration cells sometimes used for strip cells are not quite accurate since ion migration in the gap between anodes 

The main advantage of the coplanar fuel cell design is the possibility of considerably reducing the internal ohmic resistance of the cell, which is due primarily to the ohmic resistauce of the electrolyte. To decrease the ohmic resistance of conventional MEAs it is necessary to reduce the thickness of the electrolyte. An excessive reduction of this thickness can have detrimental consequences the formation of cracks and pinholes and the loss of stability and gas-tighmess. In strip cells the ohmic resistance is determined by the width of the gap between cathodes and anodes and can be decreased simply by narrowing this gap. If the electrodes are micropattemed to be close together, the cell s power density can be increased considerably. 

Another advantage of coplanar fuel cell design is that the electrolyte (which is no longer used to prevent mixing of the reactants) does not have to be a pore-free thin film. This simplifies preparation of the electrolyte and makes the cell more shock resistant against both mechanical and thermal stresses. 

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In the flat mini-fuel cell battery design described in Section 17.2, strip-type electrodes are also often used (see Figure 17.1), but there, in contrast to the coplanar fuel cells described in this section, they are used in batteries with segregated reactant supply and dual-faced electrolyte membranes, with both reactants on different sides of the membrane. These two cases of using strip electrodes must not be confused. 

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