Procedure of fuel cell preparation with graphite bipolar plates

 

The monopolar plates include a serpentine flowfield to provide increased Reynolds number for the flow, thus enhancing mass and heat transfer. It is this plate component of the cell that will be changed in testing. Currently, graphite plates will be used in testing as a baseline model. The performance of the new materials will be compared with the results of the baseline model. More information on these materials and the testing procedures will be covered later.

To finish assembling the cell, the gas diffusion layer, mesoporous media, and MEA must all be handled with extreme care so as not to contaminate the materials with unwanted debris. Therefore, latex goloves and plastic tongs are used in handling the materials and double deionized water is used to hydrate the MEA. The sizes of the GDL, mesoporous media, and MEA correspond to three different hole punches; the smallest is 1-inch diameter, and the largest is 1.5-inches diameter. The three materials are layered atop each other making certain that each is separate from the one beneath. Therefore, the final “sandwich” of components is monopolar plate, graphitized GDL, mesoporous media, and then MEA.

Prior to assembling the fuel cell, the MEA is hydrated in a precise process to ensure optimum performance within the cell. The stored MEA is placed on a cutting board that has been thoroughly cleansed with double deionized water. A clean punch is used to cut a 1.5-inch diameter portion of MEA.

The fuel cell components line up on both sides of the assembled MEA and are placed together carefully. The eight bolts slide through both aluminum endplates as well as the two Teflon plates. The cell uses compression to prevent the internal material from slipping by inserting three Belleville washers on each bolt. Nuts are screwed finger-tight onto the blots in a criss-cross manner so that pressure is distributed evenly on all components of the cell. The nuts are further tightened using a torque wrench in the same pattern. The torque setting maybe varied to provide different contact pressures in the cell. These contact pressures affect the observed cell resistance, with an upper limit set by the crush strength of the gas diffusion media. In practice, the best compression load has to be determined experimentally with the aid of pressure sensitive film and repeated cell tests. The optimal pressure applied throughout the cell was achieved by finger-tightening the bolts and then applying one full turn with a wrench for further compression.

The Teflon plates are both equipped with a plastic fitting on each side. Each plate is used for a specific reactant. Since the MEA is bipolar and has equal anode and cathode catalyst loadings, it does not matter which reactant flows on which side of the MEA. Therefore, hydrogen will flow through one fitting into the cell and air will flow through the other. The fittings feed into a small channel that allows gas to pass through the Teflon plate, and through a hole that is lined up in the electrode and monopolar plate. The gas continues to flow along the flowfield, and the products are released through another tiny hole at the opposite end of the flowfield. These products continue once again through another hole in the electrode and out through an opposite channel in the Teflon. A tube on the second fitting can distribute the gas into a ventilated area, which expels the gas into the atmosphere.

 

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