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Other contaminants of concern include ammonia (membrane deterioration), alkali metals (catalyst poisoning, membrane degradation), particles, and heavy hydrocarbons (catalyst poisoning and plugging). Both the anode and cathode flows must be carefully filtered for these contaminants, as even ppb-level concentration can lead to premature cell and stack failure. A number of technical and cost issues face polymer electrolyte fuel cells at the present stage of development (35, 38, 39, 40, 41). These concern the cell membrane, cathode performance, and cell heating limits. The membranes used in present cells are expensive, and available only in limited ranges of thickness and specific ionic conductivity. Lower-cost membranes that exhibit low resistivity are needed. This is particularly important for transportation applications characterized by high current density operation. Less expensive membranes promote lower-cost PEFCs, and thinner membranes with lower resistivities could contribute to power density improvement (41). It is estimated that the present cost of membranes could fall (by a factor of 5) if market demand increased significantly (to millions of square meters per year) (33). The DOE has set platinum loading targets at 0.4 mg/cm2 total, a maximum to allow achieving the automotive cost targets. This will require a significantly higher catalyst effectiveness (present loadings are on the order of 1 mg/cm2 total) while achieving the other improvements in performance required. Improved cathode performance, when operating on air at high current densities, is needed. At high current densities, there is a limiting gas permeability and ionic conductivity within the catalyst layer. A nitrogen blanket forming on the gas side of the cathode is suspected of creating additional limitations (1). There is a need to develop a cathode that lessens the impact of the nitrogen blanket, allows an increase in cell pressure, and increases ionic conductivity. Local heat dissipation limits stack operation with air at 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 through better cooling schemes. 3.1.2 ComponentDevelopment The primary focus of ongoing research has been to improve performance and reduce cost. The principal areas of development are improved cell membranes, CO removal from the fuel stream, and improved electrode design. There has been a move toward operation with zero humidification at ambient pressure, increased cell temperature, and direct fuel use. DuPont now produces a membrane of 2 mils or less thickness that performs (at lower current densities) similar to the Dow Chemical Company membrane, the XUS 13204.10 depicted in the top curve of Figure 3-5 (42). There is ongoing work to investigate alternative membranes and MEAs that not only exhibit durability and high performance, but also can be manufactured inexpensively in high volume. 3-11PDF Image | Fuel Cell Handbook (Seventh Edition)
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