Current Density mAcm Figure Performance of Low Platinum Loading Electrodes

Most PEFCs currently use machined graphite plates for current collection and distribution, gas distribution, and thermal management. Cooling is accomplished by using a heat transfer fluid, usually water, which is pumped through integrated coolers within the stack. The temperature rise across the cell is kept to less than 10oC. Water-cooling and humidification are in series, which results in a need for high quality water. The cooling unit of a cell can be integrated to supply reactants to the membrane electrode assembly (MEA), remove reaction products from the cell, and seal off the various media against each other and the outside (Figure 3-1). The conducting parts of the frames are titanium; non-conducting parts are polysulfone (24).

The primary contaminants of a PEFC are carbon monoxide (CO), carbon dioxide (CO2), and the hydrocarbon fuel. Reformed hydrocarbon fuels typically contain at least 1% CO. Even small amounts of CO in the gas stream, however, will preferentially adsorb on the platinum catalyst surface and block access of the hydrogen to the catalyst sites. Tests indicate that approximately 10 ppm of CO in the gas stream begins to impact cell performance (6, 25). Fuel processing can reduce CO content to several ppm, but there are system costs associated with increased fuel purification. Platinum/ruthenium catalysts that have intrinsic tolerance to CO are being developed. These electrodes have been shown in controlled laboratory experiments to be CO tolerant up to 200 ppm (26). Although much less significant than CO poisoning, CO2 affects anode performance through the reaction of CO2 with adsorbed hydrides on platinum. This reaction is the electrochemical equivalent of the water gas shift reaction.

A number of system approaches can be used to clean up the fuel feed. These include pressure swing adsorption, membrane separation, methanation, and selective oxidation. Although selective oxidation does not remove CO2, it is usually the preferred method for CO removal because of the parasitic system loads and energy required by the other methods. In selective

-Type B cathode, 0.11 mgPt/cmA2 ^Type B cathode, 0.20 mgPt/cmA2

-Type B cathode, 0.11 mgPt/cmA2 ^Type B cathode, 0.20 mgPt/cmA2


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oxidation, the reformed fuel is mixed with air or oxygen either before the fuel is fed into the cell or within the stack itself. Current selective oxidation technology can reduce CO levels to <10 ppm, but this is difficult to maintain under actual operating conditions (26). Another approach involves the use of a selective oxidation catalyst that is placed between the fuel stream inlet and the anode catalyst. Introducing an air bleed to the fuel stream, however, appears to be the most effective way to reduce CO to an acceptable level. Work is continuing to find approaches and materials that are more tolerant of impurities in the fuel feed.

A number of technical and cost issues facing polymer electrolyte fuel cells at the present stage of development have been recognized by managers and researchers (6, 27, 28, 29). These issues concern the cell membrane, cathode performance, and cell heating limits.

The membranes used in the present cells are expensive and available only in limited ranges of thickness and specific ionic conductivity. There is a need to lower the cost of the present membranes and to investigate lower cost membranes that exhibit low resistivity. This is particularly important for transportation applications where high current density operation is needed. Cheaper membranes promote lower cost PEFCs and thinner membranes with lower resistivities could contribute to power density improvement (29). It is estimated that the cost of current membranes could fall (by one order of magnitude) if the market increased significantly (by two orders of magnitude) (22).

There is some question of whether higher utilization of the catalyst is needed even though new research has resulted in the loading being reduced to less than 1 mg/cm . Some researchers cite a need for higher utilization of catalysts, while others state that because only 10% of the cell materials cost is tied up in catalyst, it is better to concentrate on the design of an effective membrane and electrode assembly at this time (27).

Improved performance of the cathode when operating on air at high current densities is needed. At higher current densities, there is a limiting gas permeability and/or ionic conductivity within the catalyst layer. A nitrogen blanket forming on the gas supply side of the cathode is suspected of creating additional limitations (6). There is a need to develop a cathode that lessens the impact of the nitrogen blanket, increases the pressurization of the cell, or increases the ionic conductivity of the cathode catalyst.

Local heating problems limit stack operation with air to a current density of approximately 2 A/cm2. Single cells have shown the capability to operate at higher current densities on pure oxygen. It may be possible to increase current density and power density with better cooling.

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