The primary focus of ongoing research is to improve the performance of the cell and lower its cost. The principal areas of development are improving cell membranes, handling the CO in the fuel stream, and refining electrode design. There has been an effort to incorporate system requirements into the fuel cell stack in order to simplify the overall system. This work has included a move toward operation with zero humidification at ambient pressure and direct fuel use.
The Dow Chemical Company has developed the XUS 13204.10 membrane, which has been reported to achieve higher performance than that obtained with Nafion membranes (Figure 3-3). The Dow membrane, also a perfluorinated sulfonic acid, has a lower equivalent weight than Nafion and is prepared with shorter anion-anion distances. Because of these characteristics, the membrane has a slight increase in conductivity and water retention capability. Most of the improvement in performance can be attributed to the Dow membrane being supplied at a thickness of 2 mils, while the Nafion membrane is supplied at 7 mils thickness. DuPont is now producing a membrane of 2 mils thickness that achieves the same performance as the top curve in Figure 3-3 (30). Both the Nafion 117 and the Dow XUS 13204.10 membranes are, at present, expensive and available only in limited ranges of thickness and specific ionic conductivity. There is ongoing work to investigate alternative membranes that not only exhibit durability and high performance, but also can be manufactured inexpensively at high volume. Work at Ballard Advanced Materials Corporation has concentrated on developing low-cost membranes using trifluorostyrene and substituted trifluorostyrene copolymeric compositions (17).
Cells were originally made with an unimpregnated electrode/Nafion electrolyte interface. This was later replaced by a method where the proton conductor was impregnated into the active layer of the electrode. This allowed reduced loading to 0.4 mg/cm2 while obtaining high power density (16). The standard "Prototech" electrodes contained 10% Pt on carbon supports. Using higher surface area carbon supported catalysts, researchers have tested electrodes with even lower platinum loading, but having performance comparable to conventional electrodes. Los Alamos National Laboratory has tested a cathode with a 0.12 mg Pt/cm2 loading, and Texas A&M University has tested a cathode with a 0.05 mg Pt/cm loading. PSI Technology has developed its own fabrication method that has achieved platinum loading also as low as 0.05 mg/cm (22). These laboratory scale tests have used electrodes produced manually. Work continues to develop high-volume manufacturing techniques.
VOLTS PER CELL
VOLTS PER CELL
MK 4 HARDWARE ELECTRODE AREA 0.054 FT2 TEMPERATURE 185 F HYDROGEN PRESSURE 50 PSIG OXYGEN PRESSURE 50 PSIG
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CURRENT DENSITY (mA/cm2)
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