Interim report re: component parts for proton-exchange membrane fuel cells
The purpose of the first phase of the grant project is to design, develop and test a simplified fuel cell electrode structure for use in proton-exchange membrane fuel cells (''PEMFC''). By simplifying the structure of the electrode, mass production manufacturing efficiencies can be brought into play which will result in significant cost reductions for this fuel cell component. With a reduction in the cost of this key fuel cell component overall costs for PEMFC's can be brought within the commercialization target range of about US$100 per kilowatt for the fuel cell stack. Fuel cell electrodes are necessarily ''multi-layered'' composites. Multi-layers are required because of the several functions that the electrode must be able to perform in the working PEM fuel cell. The current generation of state-of-the-art porous fuel cell electrodes for PEMFC's is comprised of three primary layers. The first layer is the catalyst layer. Since hydrogen is the fuel used in this project and air is used as the oxidant, the catalyst must be capable of adsorbing hydrogen and oxygen from the air. While work is constantly on-going with respect to new hydrogen or oxygen catalysts, the best available catalyst at present for both of the reactant gases is platinum. To be effective, the catalyst (1) must be exposed to a constant flow of the respective reactant gas; (2) must be in intimate contact with the proton-exchange membrane; and (3) must be a finely divided catalyst and have a large specific surface area, especially on the oxidant side where the electrochemical reaction is slower by several orders of magnitude. The second layer is the substrate layer. The substrate layer provides structural support for the finely divided catalyst. It also functions as an electronic junction for conducting electricity produced by the electrochemical reaction from the catalyst layer to the bipolar plate of the fuel cell. In state-of-the-art PEMFC's, this layer is comprised of carbon particles (onto which the catalyst has been deposited) and a binder material. In Dr. Mahlon Wilson's fuel cell electrode design, the binder material is liquid Nafion. By using liquid Nafion, the membrane is effectively extended into a third spatial dimension. This extension of the membrane serves to increase the effective catalyst surface area per real geometric unit of fuel cell area, which is quite important for the reasons discussed above. In the more traditional Los Alamos design, the binder is liquid Teflon, which is mixed with the catalyzed carbon particles and then sintered to create hydrophobic gas pores in the substrate layer. In order to extend the membrane into a third spatial dimension with this type of electrode, liquid Nafion is then applied to the substrate and allowed to seep through the sintered Teflon pores into the substrate/catalyst layer. The third layer is the backing layer. The backing layer is normally comprised of either carbon cloth or porous carbon paper. The purpose of the backing layer is (1) to conduct electricity generated by the electrochemical reaction; (2) to provide structural support for the substrate layer and (3) to allow the reactant gases to enter and leave the substrate/catalyst layers. Thus, in state-of-the-art fuel cell electrode design, the electrode is a ''triple layer composite'', consisting of the catalyst layer, the substrate layer and the backing layer. The triple layer composite electrode, when hot-pressed to the proton-exchange membrane, is strong enough to prevent the membrane from expanding in the localized area of the fuel cell electrode. This strength is significant because membrane expansion could otherwise damage the electrode and adversely affect its electronic conductivity. While triple layer composite electrodes function well, their structure does not readily lend itself to mass production. Consequently, fuel cell electrodes are extremely expensive to manufacture. For example, E-Tek of Natrick, Massachusetts, the leading manufacturer of fuel cell electrodes in this country, has quoted a mass production price of $0.30 per square centimeter for its fuel cell electrode. Since two electrodes (anode and cathode) are required for the fuel cell, the cost of the electrodes alone for a PEMFC would be about $6000 per square meter. Except in specialized applications where cost is not a significant factor, the projected cost of fuel cell electrodes remains too high for most commercial applications.
- Research Organization:
- Maverick Fuel Cell Research, Western Springs, IL (US)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- FG01-97EE15679
- OSTI ID:
- 761769
- Country of Publication:
- United States
- Language:
- English
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