Background:

Hydrogen fuel cells work by the reverse of electrolysis. Electrolysis is the electrification of water to split the water into hydrogen and oxygen. A hydrogen fuel cell creates a controlled reaction between hydrogen and oxygen that produces electricity, water and heat.

Hydrogen molecules enter the anode side of the Proton Exchange Membrane (PEM) fuel cell where they are stripped of their electrons and become positively charged hydrogen ions (which are just protons). These protons pass through an electrolyte between the anode and cathode sides. Once the protons get to the cathode side they combine with oxygen and electrons from the cathode electrode to produce water. The freeing of electrons on the anode side and the recapturing on the cathode side produces an electric current. The reactions are controlled by a catalyst (usually platinum) that is coated on the electrodes (usually carbon) and the electrolyte which only allows the hydrogen ions (protons) through..

The fuel cell electrochemically combines air with fuel and converts it directly into electricity. The conversion is similar to a conventional battery, except the reductant and oxidant are continuously supplied to the cell instead of being contained in the cell. In addition, fuel cells are 'recharged' by filling up the fuel supply. A fuel cell is analogous to a heat engine because its refillable fuel supply is converted into energy.

Objectives:

Current annode/cathode plates are produced by machining and densifying graphite or by adding conductive elements to polymers. Graphite plates can have poor fracture toughness, whereas polymers increase strength by sacrificing electrical properties. The objective of this project is to develop a low-cost, slurry molded carbon fiber material with a carbon chemical vapor infiltrated (CVI) sealed surface as a bipolar plate for proton exchange membrane (PEM) Fuel cells. This carbon/carbon composite should have increased toughness over graphite and better conductivity than polymers. Slurry molded fibers can also be cast to shape or stamped to eliminate costly machining.

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Status:

The fiberous component preform for the bipolar plate is being produced by slurry molding techniques using carbon fibers of appropriate lengths and particulates. A binder is used to provide green strength, and also assists In providing geometric stability after it is carbonized.

The surface of the preform is sealed using a CVI technique in which carbon is deposited on the near-surface fibers sufficient to make the surface hermetic. This is accomplished by placing the preforms in a furnace which is heated 1400-1500°C and through which a hydrocarbon-containing gas at reduced pressure is allowed to flow over the component. The hydrocarbon reacts and deposits carbon on the exposed fibers of the preform, and when sufficient deposition has occurred, the surface becomes sealed. Thus the infiltrated carbon provides both an impermeable surface and the necessary electrical conductivity so that power can be obtained from the cell.

 

Single sided plates have been sucessfully machined using a low cost milling station. A two sided stamp has been created and used to make double sided plates.

Future work will involve optimizing sealing times and conditions. Process variables are being identified to allow economical mass production of parts.